IOWA GEOLOGICAL SURVEY IOWA CITY, IOWA. SAMUEL J. TUTHILL, Director and State Geologist REPORT OF INVESTIGATIONS 10

Transcription

1 IOWA GEOLOGICAL SURVEY IOWA CITY, IOWA SAMUEL J. TUTHILL, Directr and State Gelgist REPORT OF INVESTIGATIONS 10 STRATIGRAPHY OF THE UPPER DEVONIAN SHELL ROCK FORMATION OF NORTOCENTRAL IOWA by DONALD L. KOCH Published by THE STATE OF IOWA 1970

2 State f Iwa 1970 STRATIGRAPHY OF THE UPPER DEVONIAN SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA by DONALD L. KOCH REPORT OF INVESTIGATIONS 10 IOWA GEOLOGICAL SURVEY IOWA CITY, IOWA SAMUEL J. TUTHILL, Directr and State Gelgist PB D-7264

3 CONTENTS Page ABSTRACT 1 INTRODUCTION 2 Lcatin and extent 2 Previus investigatins 2 Stratigraphic data 4 Acknwledgements 5 STRATIGRAPHY 5 Type sectin area. 5 Inadequacy f Belanski's definitin 5 Redefinitin f the Shell Rck Frmatin 8 Masn City Member 9 Rck Grve Member 12 Nra Member 14 Stratigraphy utside the type area 16 Orientatin statement 16 First traverse stratigraphy east and suth f Nra Springs 17 Secnd traverse stratigraphy nrthwest f Nra Springs 24 Third traverse stratigraphy in the regin f Prtland and t the nrthwest alng the Winnebag River 32 DEPOSITIONAL ENVIRONMENT OF SHELL ROCK SEDIMENTS 40 Masn City Member 41 Rck Grve Member 42 Nra Member 43 PETROGRAPHIC NOTES 44 General Statement 44 "Masn City beds" 44 "Cement ledge" 45 Masn City Member 46 Rck Grve Member 46 Nra Member 47 REFERENCES 54 APPENDICES 59 Appendix I. Expsure sectins 60 Appendix II. Well lgs 97 Appendix III. Cre descriptins 113

4 ILLUSTRATIONS Page TABLE 1. Nmenclature f Devnian Frmatins in nrth-central Iwa 7 FIGURE 1. Outcrp area f the Shell Rck Frmatin 3 FIGURE 2. Gegraphy alng the Shell Rck and Winnebag Rivers 8 FIGURE 3. Masn City Cralville cntact 10 FIGURE 4. Ispach f Masn City Member 11 FIGURE 5. Ispach f Rck Grve Member 13 FIGURE 6. Ispach f Nra Member 15 FIGURE 7. Rute f traverse 1 18 FIGURE 8. Nra, Rck Grve, and Masn City Members in Williams' quarry 19 FIGURE 9. Discntinuity surface in Williams' quarry 20 FIGURE 10. Rute f traverse 2 25 FIGURE 11. Subspherical strmatprids and Amphipra in the Nra Member 26 FIGURE 12. Fragmented subspherical strmatprid 27 FIGURE 13. Keidie's Bluff 29 FIGURE 14. Crss-sectin f quarry near Fster's Mill FIGURE 15. Rute f traverse 3 33 FIGURE 16. Crss-sectin f McEachran quarry 34 FIGURE 17. Caverns in Cedar Valley Limestne 35 FIGURE 18. Cntact f Nra Member with Cedar Valley Limestne in drainage cut 37 FIGURE 19. Bistrme f subspherical strmatprids and Amphipra 38 PLATE I. Structural panel diagram - Devnian frmatins f nrth-central Iwa pcket PLATES II - III. Phtmicrgraphs fllwing page 49

5 STRATIGRAPHY OF THE UPPER DEVONIAN SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA by DONALD L. KOCH ABSTRACT The principal expsures f the Upper Devnian Shell Rck Frmatin ccur alng the Shell Rck River in nrth-central Iwa. Three members, the Masn City, Rck Grve and Nra are recgnized. Except fr the Upper Devnian State Quarry Limestne in nrthern Jhnsn Cunty, the Shell Rck Frma tin has the smallest areal extent f any frmatin in Iwa and attains a maximum thickness f nly 65 feet. The Shell Rck Frmatin lies uncnfrmably between the Middle Devnian Cedar Valley Limestne and the Upper De vnian Lime Creek Frmatin. Outliers f Cretaceus sand stnes and shales lie uncnfrmably upn the Shell Rck Frmatin alng its eastern utcrp margin. The frmatin cnsists f a series f argillaceus limestnes, dlmites and shales, and prminent cralline and strmatprid bistrmes. Strmatprid bistrmes predminate in the up per part f the frmatin and indicate successive transgressins f the late Shell Rck sea. During the time f their frmatin these bistrmes extended ver shallw banks r shals and cvered a larger area than the lwer beds. A discussin f the changes in the physical and mineralgical characteristics f each member as bserved in the utcrp sectin and in the subsurface sectin utside the type area is included.

6 2 STRATIGRAPHY OF THE UPPER DEVONIAN Dense limestne (micrite) beds and underlying dlmite beds in the vicinity f Masn City, Iwa, have been crrelated by previus wrkers as facies equivalents f the lwer tw members f the Shell Rck Frmatin. Hwever, cllatin f surface and subsurface data reprted here demnstrates that these beds cr relate with the underlying Cedar Valley Limestne and that the depsitinal basin f the Shell Rck Frmatin was mre restricted than previus wrkers thught. INTRODUCTION The Shell Rck Frmatin f nrth-central Iwa has been a subject f interest t stratigraphers and palentlgists fr many years. Lack f a thrugh study f the surface stratigraphy f the Shell Rck Frmatin and the underlying Cedar Valley Lime stne resulted in miscrrelatins f expsures nly ten miles nrthwest f the type lcality. Furthermre, incrrect crrela tins were made with subsurface strata ver 75 miles suth f the suthern limit f the frmatin. The data reprted here permit a slutin f the prblems in herent in this prtin f the stratigraphic clumn f Iwa. The physical stratigraphy f the utcrpping Shell Rck Frmatin is redescribed and cllated with the subsurface sectin. Lcatin and Extent The Shell Rck Frmatin is limited t the eastern ne-half f nrth-central Iwa and includes the fllwing cunties r parts theref: nrthern Butler Cunty, the western ne-half f Flyd Cunty, suthwest Mitchell Cunty, central and suthwest Wrth Cunty, and the nrtheast ne-half f Cerr Grd Cunty (fig. 1). The frmatin crps ut alng the Shell Rck and Winne bag (frmerly Lime Creek) Rivers and their tributaries, in quarry expsures and in rad cuts. Previus Investigatins Earlier authrs (Owen, 1852; Whitney, 1858; Hall and Whit field, 1872; Calvin, 1896; and Thmas, 1913 and 1919) made passing reference t expsures alng the Shell Rck River in

7 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 3 r WINN EBAGO L. 1 MITCH ELL H HANCOCK I- W R I G H T L. Figure 1. Outcrp area f the Shell Rck Frmatin.

8 4 STRATIGRAPHY OF THE UPPER DEVONIAN western Flyd Cunty, but they cnducted n detailed analysis. Belanski (1927, p ) defined the "Shellrck Stage" and three "substages" n the basis f bistratigraphic studies f ut crps. Definitin f rck units upn bistratigraphic znes was ne accepted apprach t stratigraphic studies during Belanski's time, but such an apprach is cntrary t mdern cncepts f lith-stratigraphy and rck-stratigraphic units as set frth in the Cde f Stratigraphic Nmenclature (1961). Belanski (1928b) cncluded that the macrfauna f the Shell Rck Frmatin was indicative f an Upper Devnian assem blage. That cnclusin is uncntested. He als stated that the fauna had an rigin distinct frm either the underlying Cedar Valley Limestne r the verlying Lime Creek Shale, and that the ancestral fauna f the Shell Rck Frmatin "must be sught in the Devnian f the Mackenzie basin, r mre remtely, in that f the German Eifel." Hwever, Cper and thers (1942, p. 1780) reprted that a fauna similar t that f the Shell Rck Frmatin is widespread in western Nrth America, ccurring in the Jeffersn Limestne (western Wyming, nrtheastern Utah, suthwestern Mntana), Muddy Peak Limestne (Nevada), lwer Devils Gate Limestne (Nevada), lwer Minnewanka Frmatin (Alberta), and parts f the Sultan Limestne and Silverhrn Dlmite (Nevada). An Upper Devnian age assignment fr the Shell Rck Frmatin is substantiated by mre recent studies f its micrfauna. Andersn (1964, p. 19) included a number f sectins f the Shell Rck Frmatin in his study f Upper Devnian and Lwer Mississippian cndnts f nrth-central Iwa. Althugh nly a meager cndnt fauna was recvered the frms identified indicate that the members f the Shell Rck Frmatin are assignable t the Upper Devnian Manticceras Zne. Stratigraphic Data In this study infrmatin frm utcrps was supplemented by subsurface data. Representative hand specimens were cl lected fr examinatin under a bincular micrscpe and, frm a few beds, fr thin sectin preparatin. Drill cuttings that pene trated the Shell Rck Frmatin and adjacent frmatins were examined in detail. Cres that became available during the prgress f this reprt as a part f the Iwa State Highway Cmmissin testing prgram were examined. In additin, spe-

9 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 5 cific cre lcatins were selected and drilled t define the relatinships f the Shell Rck Frmatin and the Cedar Valley Limestne where data was therwise inadequate. In all, 105 expsures, 125 sets f drill cuttings and 14 cre samples were evaluated. Acknwledgements This reprt has been prepared fr the Iwa Gelgical Sur vey. The wrk initially was part f a Master's prgram at The University f Iwa and was directed by H. Garland Hershey, then Directr f the Iwa Gelgical Survey and State Gelgist. F. H. Drheim, Iwa Gelgical Survey, made helpful cm ments. M. C. Parker, als a member f the Survey Staff, gave advice during the preparatin f maps and diagrams. S. J. Tuthill, Directr f the Iwa Gelgical Survey and State Ge lgist, critically reviewed the manuscript. Cre lgs were btained frm the gelgy sectin f the Iwa State Highway Cmmissin. T. L. Welp, frmer chief gelgist, apprved a cperative cring prgram between the Cmmissin and the Iwa Gelgical Survey t btain additinal data. Thanks als are due the many quarry and clay pit peratrs and their emplyees wh allwed access t penings n their prperties. G. A. Cper f the Smithsnian Institutin cnfirmed the identificatin f sme f the fssil specimens. Faculty members f The University f Iwa prvided sug gestins during the preparatin f the riginal thesis. The criticisms f W. M. Furnish and B. F. Glenister are gratefully acknwledged. Furnish als reviewed the final manuscript. A. C. Tester and Keene Swett ffered valuable cmments dur ing the study f thin sectins. STRATIGRAPHY Type Sectin Area Inadequacy f Belanski's Definitin C. H. Belanski was nt a prfessinal gelgist by training. Nevertheless, he accmplished much wrk n the Shell Rck

10 6 STRATIGRAPHY OF THE UPPER DEVONIAN Frmatin, and his publicatins prvided the mst cmprehensive summatin n the stratigraphy and palentlgy f the frmatin available at that time. His fssil cllectin, repsited at The University f Iwa, is unlikely t be surpassed. Belanski (1927, p. 320) divided his "Shellrck Stage" int three "substages" (the Masn City, Rck Grve and Nra) and stated that the strata "lie abve the beds f the Cedar Valley stage and belw the blue shales f the Juniper Hill Shale, being separated frm bth f these frmatins by sharp, persistent, uncnfrmities." The mixing f time-stratigraphic and rckstratigraphic terms in his descriptin f these units des nt cn frm t the existing Cde f Stratigraphic Nmenclature (1961). In additin, Belanski's definitin f the "Shellrck Stage" and its three "substages" upn a bistratigraphic basis is untenable by usage as defined in the Cde, and the nature f his znatin and phase(areal) descriptins is nt valid. Fr instance, the term zne was used by him t refer t "a particular grup f rcks, nt necessarily retaining the same lithlgic character wherever expsed but marked by a species r grup f species which are especially characteristic t them." Subdivisins f his znes did nt strengthen his definitin f the varius "stages" because the term znule was used t refer "t a zne lithlgically distinct frm ther znules f the zne." A particular zne r znule was described by Belanski as "barren" at many expsures and ne f his znules was prefixed by the lithlgic term "dlmite". Because Belanski's definitins are inadequate a redefinitin f these Upper Devnian beds as the Shell Rck Frmatin is given in this reprt. The pririty f Belanski's names "Masn City", "Rck Grve" and "Nra" is recgnized but these names are used with the rank f member. Hwever, the name "Shell rck" is rejected as a single wrd. Instead, the name Shell Rck as used by Wilmarth (1938, p. 1978) is accepted except that the wrd "frmatin" is selected rather than the lithlgic designa tin "limestne". Thus, the Shell Rck Frmatin, as subse quently defined, cntains three members, the Masn City, Rck Grve and Nra. An histrical summary f the nmenclature used fr these Upper Devnian units and the adjining frma tins is given in table 1.

11 Owen 1852 «w J c alvin 897 Thms 1919 Fentn and Fentn 1924 Belanski S tainbrk 1935 IGS Present Reprt 1970 c Owen Owen O Owen Owen Owen c Owen Substage Beds «/> Substge Substage Member Member 0) a >. rr E e m Ul Cerr Cerr Cerr Cerr u. n Grd Grd Grd Grd u Hck- Hack- a. Substage «Substage Member Member w O berry u berry e 3 Substage «E Beds => X X w a> Sheffield Frmatin _ 3 z < Ul rr Divisin _l Ul z Q Suggested a. n a. but nt > 01 s 3 en made UJ Q Beds Masn f the City Shellrck > Substage Valley Cedar «O Valley hi _i a a 2 Cedar Valley Stage rr Ul a. CL Ul -J a 10 u Juniper Hill Frmtin J Juniper Hill Member rr Ul a. n e '3 Juniper Hill Member Nra Nra c Nra c Substge Member Member e Rck Rck 6 Rck cr E Grve Substage Ul Grve Grve a. Member Member u a. Msn City (0 Substage Cedar Valley Stage i Stage 3 CO 1 C «E.J >% a > B e u Masn a: Masn City a City Member «i Member Cralv i Me Member Ul _i Q O 2 > a c a» E» Crlville Member PI r r 53 "J S8 1* y. SI s FJ H 5 > Table I. Nmenclature f Devnian Frmatins in nrth-central Iwa.

12 STRATIGRAPHY OF THE UPPER DEVONIAN [Wrth C.V T Mitchell C. ~1 ynrthwd I I OSt. Antgr H Rudd Chrles City 'Cerr Grd C. I Prnnklin fn I Flyd C. Franklin C. GreenAButler C. :~J Scale I inch =l4 miles Figure 2. Gegraphy alng Shell Rck and Winnebag Rivers. Redefinitin f the Shell Rck Frmatin The name Shell Rck Frmatin is designated fr the interval f Upper Devnian beds in nrth-central Iwa that lies uncn frmably abve the Cedar Valley Limestne (Middle Devnian) and beneath the Lime Creek Frmatin (Upper Devnian). The type area f the Shell Rck Frmatin is alng the Shell Rck River in the vicinity f the twn f Nra Springs (fig. 2). The frmatin is divided int three members, the Masn City, Rck Grve and Nra. A separate lcatin is given fr the type sectin f each member because the entire frmatin is nt expsed at any ne lcatin. In the type area the Shell Rck Frmatin cnsists f a series f mderately argillaceus limestnes, dlmites, and dlmitic

13 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 9 limestnes and shales. Prminent fssiliferus znes include crinidal and cralline limestnes, distinct bistrmes, and strata cntaining many brachipds. The lwer bistrmes are dmi nated lcally by a cral fauna. Thse higher in the sectin are cmpsed either f a mixture f crals and subspherical strma tprids r are dminated by massive, tabular strmatprids. The Shell Rck Frmatin is separated frm the underlying Cedar Valley Limestne by an ersinal uncnfrmity. A slightly undulatry surface is characteristic f the tp f the Cedar Valley Limestne in the type area and is stratigraphically lwer than in expsures t the west, nrth and east. Near the twns f Plymuth and Rck Falls the truncatin f Cedar Valley Limestne beds is cnspicuus. Shale beds f the Lime Creek Frmatin lie uncnfrmably n the Shell Rck Frmatin. Abraded fish teeth are abundant in a pyritic, argillaceus dlmite at the base f the Lime Creek Frmatin (Juniper Hill Member) near the gegraphic center f the ccurrence f the Shell Rck Frmatin. At these lcatins the Lime Creek Frmatin lies n the tp member (Nra Member) f the Shell Rck Frmatin. Farther suth, the Lime Creek Frmatin lies uncnfrmably n a lwer bed f the Nra Member. Beynd the suthern limit f the Shell Rck Frma tin, the Lime Creek Frmatin lies uncnfrmably n the Cedar Valley Limestne. The Shell Rck Frmatin crps ut in an elngate belt alng a nrthwest-sutheast trending line frm nrth-central Wrth Cunty sutheasterly t nrthern Butler Cunty, a distance f abut 45 miles. This belt f utcrp is abut 16 miles wide at its greatest width and the frmatin extends suthwesterly nly abut five mre miles where it is cvered by yunger in durated beds. Maximum thickness f the frmatin is 65 feet. Masn City Member The type sectin f the Masn City Member is alng the east bank f the Shell Rck River in the western part f the twn f Nra Springs (east line f westy2, SEy4, Sec.7, T. 96 N., R. 18 W., Flyd Cunty, Iwa). At this lcatin beds f the Shell Rck Frmatin frm a cliff nearly half a mile in length and abut 35 feet in height. The measured sectin (appendix I, sectin 1) was described near the center f the expsure. The main face f the cliff is frmed by the Masn City Member and the underlying Cedar Valley Limestne is expsed abve river level. Prtins

14 10 STRATIGRAPHY OF THE UPPER DEVONIAN Figure :i. Cntacl f Masn Citj Member "I the shell Rck Frmatin with underlying Cr.ilville Member f Cednr Vnlley Linicstiie(arrw), nrthern part <>1 Nra Springs. (See appendix I, sectin 1). f the Rck Grve and Nra Members are expsed n the slpe abve the cliff. The Masn City Member cnsists dminantly f thick-t massive-bedded limestnes. Crals, strmatprids, brachipds and crinid fragments are cmmn thrughut the upper part and a prminent cral-strmatprid bislrme c curs in the lwer part. The term bislrme refers t beds f predminantly autchthnus bigenic carbnate with hrizntal dimensins greater than vertical dimensins. In the Shell Rck Frmatin these beds average five feet in thickness and generally they extend hundreds f feet laterally. The bistrme in the Masn City Member and thse present in the Nra Member are cnspicuus markers in the surface and subsurface sectins be cause f their bigenic nature. The uncnfrmable cntact f the Masn City Member with the Cedar Valley Limestne is marked by a change in lithlgy frm dlmite (Masn City Member) t lithgraphic limestne (Cedar Valley Limestne). A bradly undulating surface is characteristic f the tp f the Cedar Valley Limestne (fig. 3). Althugh the lwer beds f the Masn City Member parallel this surface they are slightly thicker in the trughs and the bedding planes becme nearly hrizntal in the upper part f the member.

15 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 11 Cerr Grd Flyd Franklin I Butler j Scale I miles Cedar Vailey Bedrck H Rck Grve Cn,0(jr jn,erv(j 5f s-t Outliers Figure 4. Ispach f Masn City Member.

16 12 STRATIGRAPHY OF THE UPPER DEVONIAN The cntact between the Masn City and Rck Grve Mem bers is marked by a distinct change in bedding structure. Thinbedded dense limestne layers separated by shale partings (Rck Grve Member) ccur abve the thick- t massive-bedded fs siliferus limestne f the Masn City Member. The tp surface f the latter usually cntains irregular shallw pits that are stained with irn xides. Maximum thickness f the Masn City Member at this lca tin is 22.5 feet (fig. 4). Cmparable thicknesses ccur at near by expsures, but the thickness gradually decreases suth and east f Nra Springs ver a distance f abut 24 miles t the sutheasterly limit f the frmatin. The decrease in thickness is much mre abrupt nrthwest f Nra Springs. Because f verlap nt a higher surface f the Cedar Valley Limestne, beds f the Masn City Member extend nly abut three miles in this directin. The upper beds f the Rck Grve Member frm the lwest expsed beds in this area and there is n subsurface cntrl t define the nrthwesterly limit f the Masn City Mem ber mre accurately. Hwever, expsures immediately nrth west f this area shw beds f the Nra Member in cntact with the Cedar Valley Limestne. This same cntact relatinship is present at ail ther expsures alng the curse f the Shell Rck River t the nrthwest and at expsures alng the Winnebag River frm the Prtland area nrthwesterly t the twn f Fertile. Rck Grve Member The "type expsure" f Belanski is retained as the type sec tin f the Rck Grve Member (east bank f Shell Rck River, NWV4, SEy4, SE14, NW14, Sec. 17, T. 96 N., R. 18 W., Flyd Cunty; appendix I, sectin 2). An abandned quarry was cut int the Nra Member at the tp f the expsure. Beds f the Rck Grve and Masn City Members are expsed n a steep slpe but they are bscured by talus and vegetatin. Therefre, a reference sectin is designated fr the Rck Grve Member (abandned quarry, center NE^, NEl/4, Sec. 17, T.96N., R. 18 W., Flyd Cunty; appendix I, sectin 4). Dlmitic shales, shaly limestnes and calcareus dlmites are characteristic f the Rck Grve Member in the type area. Cntact relatinships between the Masn City and Rck Grve Members at the reference lcality are identical t thse described fr the type sectin f the Masn City Member. The well defined cntact f the Rck Grve Member with the ver-

17 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 13 yille q?> Scale 3 4 miles Cedar Valley Bedrck f\ Masn City Outliers Cntur interval 5 feet Figure 5. Ispach f Rck Grve Member.

18 14 STRATIGRAPHY OF THE UPPER DEVONIAN lying Nra Member is marked by a change in lithlgy frm massive-bedded dlmite (Rck Grve Member) t a massive limestne bistrme cmpsed f tabular strmatprids (Nra Member). Crss-bedding is cmmn in the dlmite f the Rck Grve Member. Maximum thickness f the Rck Grve Member at this l catin is 22.5 feet (fig. 5). Like the Masn City Member, the reductin in thickness f the Rck Grve Member is gradual suth and east f Nra Springs. Only the upper beds f this Member are expsed alng the Shell Rck River immediately nrthwest f Nra Springs. The stratigraphic relatinships de scribed fr the Masn City Member in the area three miles nrth west f Nra Springs apply t the Rck Grve Member. Nra Member Belanski designated the "type expsure" f the "Nra substage" fr an expsure near the "Junctin" at the suthwestern edge f Nra Springs (center NE»4, NE»4, NW14, Sec. 18, T. 96 N., R. 18 W., Flyd Cunty). "Junctin" referred t the junc tin f tw railrad lines at this lcatin. The "type expsure" has since been cvered with fill -and nly a small part f the Nra is expsed in the nrth-suth radcut east f the "Junctin" (appendix I, sectin 3). An abandned quarry beynd the east city limits f Nra Springs is nw being designated as the reference sectin fr bth the Nra and Rck Grve Members (center NE1^, NE>4, Sec. 17, T. 96 N., R. 18 W., Flyd Cunty; appendix I, sectin 4). This is the mst cmplete sectin f the Nra Member that is expsed in the type area. All f the underlying Rck Grve Member is ex psed and the tp f the Masn City Member frms the bed f a creek that flws past the nrthern edge f the quarry. The Nra Member cntains tw massive strmatprid bistrmes that are separated by a shale interval. Thrughut the reprt these bistrmes are referred t as the lwer and the upper strma tprid bistrmes. The cntact f the Nra Member with the Rck Grve Mem ber is marked by a change frm massive-bedded dlmite (Rck Grve Member) t a limestne bistrme (Nra Member). This cntact is slightly undulatry; the undulatins are nt evidence f an uncnfrmity as interpreted by Belanski. Rather, the clnies f tabular strmatprids began their grwth in shallw

19 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 15 Figure 6. Ispach f Nrn Member. depressins n the surface f the Rck Grve Member and ex panded upward and utward frm these centers. This feature is present t sme degree in all expsures f the Nra-Rck Grve cntact. At the reference lcality the tp f the Nra Member is cn cealed by a thin sil interval. Because the upper strmatprid

20 16 STRATIGRAPHY OF THE UPPER DEVONIAN bistrme has a maximum measured thickness f 6.5 feet in the type area, prbably less than a ft f this bed has been remved by Pleistcene ersin. Expsures f the cntact f the Nra Member with the verlying Lime Creek Frmatin (Juniper Hill Shale member) are referred t later in this reprt. Nne f these utcrps are adequate t be designated as reference sectins because nly a prtin f the upper strmatprid bi strme is expsed beneath the Juniper Hill Shale. The Nra Member is the mst widespread member f the Shell Rck Frmatin. Its maximum thickness in the type area is 19 feet (fig. 6). Three miles nrth f Nra Springs and beynd t the nrthern limits f the Shell Rck Frmatin the Nra Member is the nly unit f the frmatin present. This same relatinship exists near Prtland and Masn City. Only the lwer bistrme f the Nra Member is present near the nrthern and western bundaries. Orientatin Statement Stratigraphy Outside the Type Area The reader is guided n an areal survey by which the strati graphic relatinships f the Shell Rck Frmatin and the Cedar Valley Limestne are demnstrated utside the type area. Vari atins in thickness, facies changes, and changes in the bistrmes that reflect different eclgic cnditins are the factrs empha sized. The areal survey is presented in three traverses that riginate in the type area and radiate t the limits f the Shell Rck Frmatin. In the first traverse, analysis is made f expsures east and suth f the type area f the Shell Rck Frmatin. Facies changes in the Shell Rck Frmatin and in the Cedar Valley Limestne are minr thrughut this area and cntact relatinships between the tw frmatins are clear. A decrease in thickness f the Shell Rck Frmatin because f verlap upn an increasingly higher surface f the Cedar Valley Lime stne is the primary change t be nted in this area. On the secnd traverse expsures alng the Shell Rck River nrthwest f the type area f the Shell Rck Frmatin are dis cussed. Initially, minr facies changes are nted in the Nra Member. Farther nrthwest a change in the mrphlgic frm f the strmatprids in the lwer bistrme f the Nra Mem-

21 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 17 ber is nted. The principal bjective f this traverse is t demnstrate the errr in crrelatins made by earlier stratigraphers in this area. The data shw that beds previusly crrelated with the Masn City and Rck Grve Members f the Shell Rck Frmatin are stratigraphically higher beds f the Cedar Valley Limestne than are present in the type area f the Shell Rck Frmatin. Because the Lime Creek Frmatin is the bedrck west f Nra Springs it is necessary t crss the drainage divide between the Shell Rck and Winnebag Rivers t fllw the third tra verse. This traverse resumes in the vicinity f Prtland where expsures f the Shell Rck Frmatin and adjining units are fllwed nrthwesterly alng the Winnebag River. The many quarries, natural expsures, drill cuttings and cres in the vicinity f Masn City prvide the data t cmplete the areal survey f the Shell Rck Frmatin. The errrs in previus crrelatin f the type nted nrthwest f Nra Springs als apply t this area. In additin, anther facies change in the Nra Member is described. First Traverse Stratigraphy East and Suth f Nra Springs East-sutheast f the type area, the Shell Rck Frmatin is thinner because f verlap upn the Cedar Valley Limestne (fig. 7). A decrease in thickness is evident in the Masn City and Rck Grve Members at the twn f Rudd five miles east f Nra Springs. This decrease is evidenced by thinner beds in bth members and by the absence f the basal dlmite bed f the Masn City Member. A maximum thickness f 15 feet fr the Rck Grve Member (cmpared with 22.5 feet at the type sectin) is btained in a cmpsite sectin f tw expsures in the western part f Rudd (appendix I, sectin 5). Thickness f the Masn City Member is determined frm the subsurface sec tin. Depsits f Pleistcene age lie n the erded surface f the Rck Grve Member at the site f the Rudd twn well (appendix II, W-9845). There is n basal dlmite bed f the Masn City Member in the drill cuttings and there is nly a 14-ft sectin f this member here cmpared with 22.5 feet at the type sectin. The absence f the basal dlmite bed sup prts the interpretatin that beds f the Shell Rck Frmatin verlap upn the Cedar Valley Limestne with a resultant lss f the lwer beds east f the type area. The eastern bundary

22 18 STRATIGRAPHY OF THE UPPER DEVONIAN Map scle miles > a ^wi-""' " r. Masn City Figure 7. Rute f traverse 1; numbers 1-13 refer t sectin descriptins in appendix I; W-I1541, W-9845, etc. refer t well lg descriptins in appendix II; C-l refers t cre 1, appendix III.

23 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 19 Figure S. Nni(N), Rck Grvc(R), and Mnsn City(M) Members in the Williams quarry; arrw marks psitin f discntinuity surface shwn in figure 9. f the Shell Rck Frmatin in this area is bscured by depsits f Pleistcene age. N drill cuttings arc available and the near est expsures are abut nine miles farther east where the Cedar Valley Limestne is the bedrck. Cretaceus sediments presumably were depsited thrughut this part f Iwa, but ersin has remved all but a few rem nants. An utlier f Cretaceus sandstne is expsed ne mile sutheast f Nra Springs (appendix I, sectin 6). Althugh the base f the sandstne is nt expsed it prbably lies n the Nra Member at this lcatin because the lwer strmatprid bistrme f that member is expsed at a slightly lwer elevatin just half a mile farther suth. Expsures f these Cretaceus depsits validate crrelatins at the few lcatins where drill cuttings shw thin intervals f Cretaceus sandstnes and red shales lying uncnfrmably upn the Shell Rck Frmatin. Significant expsures f the Shell Rck Frmatin suth f Nra Springs are limited t utcrps and quarries alng the curses f the Shell Rck and Winnebag Rivers. An excellent sectin is expsed in the Williams quarry apprximately three miles sutheast f Nra Springs (fig. 8). The lwer strmatp rid bistrme f the Nra Member, all f the Rck Grve

24 STRATIGRAPHY OF THE UPPER DEVONIAN V- i. L«T»utt i I'igure 0. Discntinuity surfuec(arrws) in Williams quarrv. (Sec ap pendix I. sectin 7). Member (16.8 feet) and mst f the Masn City Member are ex psed in the quarry (appendix I. sectin 7). Immediately suth west f the quarry, alng the east bank f the Shell Rck River, all f the Masn City Member (27 feet) and the upper beds f the Cedar Valley Limestne are expsed (appendix I, sectin 8). Kch and Strimple (1968) have discussed the unique preser vatin f a bicense in the Masn City Member at the Williams quarry. The fauna f the bicense cnsists f edriasterids, cystids, crals and wrm tubes. These fssils are attached t a discntinuity surface frmed n a dense limestne bed (fig. 9). The knbby limestne surface cntains many brings that are filled with material frm the verlying bed. These brings generally are 5-10mm in length and l-2mm in diameter. Belan ski (p. 337) reprted edriasterids frm the equivalent strati graphic psitin at the expsure immediately suth f the quarry, but there the discntinuity surface is at the tp f a prminent bistrme f subspherical strmatprids. The presence f edriasterids attached t strmatprids at the tp f the bi strme was interpreted crrectly by Belanski t represent de velpment f a new fauna after the bistrme had undergne a perid f ersin. Only a few subspherical strmatprids are present belw the discntinuity surface in the Williams quarry.

25 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 21 This lateral variatin demnstrates the marked change in the cmpsitin f ne bed ver a relatively shrt distance. This change prbably was a result f variatin in the depth f water r the nature f the bttm cnfiguratin, r a cmbinatin f bth during grwth f the bistrme. Tw small quarries recently have been cut int the Nra Mem ber n the western bank f the Winnebag River tw and nehalf miles suthwest f the site discussed abve. Althugh the cntact is nt expsed here the verlying Juniper Hill Shale can be bserved in rad cuts suth f the quarries. The fllwing thicknesses have been measured frm a cre at this lcatin: Nra Member, 18.5 feet; Rck Grve Member, 17.1 feet; Masn City Member, 20.2 feet (appendix III, cre 1). Thus, the thick ness f the Shell Rck Frmatin is abut seven feet less here than the thickness f the frmatin in the type area. The strmatprid bistrme f bed 11 in the abve cre is, by stratigraphic psitin, equivalent t the bistrme f bed 6 in the expsure suth f the Williams quarry. This apparently is the mst sutherly ccurrence f the bistrme as it is nt en cuntered again in the surface r subsurface suth f this l catin. The basal dlmite bed f the Masn City Member is absent here. A minr lithlgic change is nted in the middle f the Nra Member in the cre discussed abve and at an expsure fur miles sutheast (appendix I, sectin 9). At these tw lcatins a thin limestne unit is present within the shale interval that separates the upper and lwer strmatprid bistrmes. This limestne is nt present in the type area. Belanski (p. 169) stated that "the cntact between the tw frmatins [Juniper Hill Shale and Shell Rck Frmatin] is sharply uncnfrmable, made clearer by the sharp lithlgic break. There is n evidence that the uncnfrmity has resulted frm the remval f any great thickness f Shellrck strata, even thugh evidence f an ersinal cntact is clear." Obviusly this is an ambiguus statement. Fr the purpse f clarificatin it is nted that wherever this cntact is expsed, the Juniper Hill Shale lies abve the upper strmatprid bistrme f the Nra Member. There is ther evidence in additin t the "sharp lithlgic break" t define the uncnfrmity between the Shell Rck Frmatin and the Juniper Hill Shale. Subsurface studies

26 22 STRATIGRAPHY OF THE UPPER DEVONIAN demnstrate that, at lcatins east and west f Rckfrd, the Juniper Hill Shale immediately verlies the lwer strmatprid bistrme f the Nra Member (appendix II, W-11541). It is nt knwn whether this cnditin is due t nn-depsitin f the upper bistrme f the Nra Member r pre-juniper Hill Shale ersin (pi. I). Up t ne ft f light-clred limestne is expsed abve water level alng the east bank f the Shell Rck River suth f the dam in Rckfrd. The limestne is lithgraphic (unifrm micrite) and the surface shws numerus brings that are slightly ver 1mm in diameter and average abut 10mm in length. A maximum f 0.2 ft f dark yellwish-brwn, very fine sand t carse silt-sized dlmite is present abve the limestne n the highest part f the expsure. This dlmite fills the brings and ther irregular vids in the limestne. Plished hand specimens and thin sectin studies f the limestne/dlmite cntact re veal fssil rganisms n the limestne surface that appear t be a species f Aulpra. The lack f a mre cmpletely expsed sectin makes crrelatin f this expsure smewhat tenuus. Pssibly, it is the cntact f the Shell Rck Frmatin with the Cedar Valley Limestne. Hwever, the limestne is mre nearly like the lithgraphic limestne f the Masn City Member in bed 2 at the Williams quarry (appendix I, sectin 7). If a crrelatin with the Shell Rck Frmatin is crrect, the dlmite ver lying and filling the brings in the limestne is equivalent t the dlmite that ccurs abve the discntinuity surface in the Williams quarry. This utcrp wuld therefre be the timestratigraphic equivalent f that same discntinuity surface. This secnd expsure differs frm the Williams quarry expsure in that the discntinuity surface ccurs here at a slightly lwer stratigraphic psitin and the echinderm fauna that is attached t the surface in the Williams quarry is absent. Expsures suth and east f the Rckfrd area ffer nly partial sectins f the members f the Shell Rck Frmatin. A significant sectin was btained frm drill cuttings five miles sutheast f Rckfrd. At this lcatin Cretaceus sandstnes and shales lie uncnfrmably abve the Masn City Member. This lcatin als marks the apprximate sutheasterly limit f the basal dlmite bed f the Masn City Member (appendix II, W-17056).

27 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 23 A sectin f the basal Rck Grve Member and five feet f the Masn City Member lying n the Cedar Valley Limestne is expsed in a radcut ne-half mile east f the twn f Rseville (appendix I, sectin 10). Cuttings frm wells in the area generally shw a Pleistcene-erded surface f the Cedar Valley Limestne, but at a few lcatins a veneer f the Masn City Member is preserved. The basal dlmite f the Masn City Member is absent at the radcut expsure and frm the well cut tings. Absence f the basal dlmite supprts the interpretatin that Shell Rck depsitin encrached upn a higher Cedar Valley surface thrugh time. Incmplete sectins f the Shell Rck Frmatin are expsed suth f Marble Rck where nly the lwer part f the Masn City Member is preserved abve the Cedar Valley Limestne. Such an expsure is lcated in an abandned quarry iy2 miles suthwest f Marble Rck. At this lcatin the basal bed f the Masn City Member is fssiliferus, calcareus shale (ap pendix I, sectin 11). Belanski (p. 357) reprted a remnant cap f the lwer str matprid bistrme f the Nra Member half a mile east f the abve described quarry sectin. The bistrme was nt bserved here during this study, but its presence beneath the sil wuld mark the mst sutherly ccurrence f the Nra Member. Suth f this lcatin the Lime Creek Frmatin frms the bedrck. Three and ne-half miles sutheast f the last-described quarry sectin drill cuttings shw the Juniper Hill Shale lying n the strmatprid-bearing Cedar Valley Limestne (appendix II, W-8386; plate I). The suthwestern bundary f the Shell Rck Frmatin therefre is at a pint intermediate t these tw l catins. In supprt f this statement it is necessary t cmment n the identificatin f strmatprids in drill cuttings. When drill chips shw the presence f strmatprids, ne usually can demnstrate whether they are (1) subspherical (glbular) clnies r (2) massive, tabular clnies. The subspherical frms are readily identifiable when their diameters are eight inches r less. If they are larger the lw curvature f the laminae and the small size f the drill chips prevent separatin f the tw frms. The subspherical strmatprids in the Masn City Member and in the Cedar Valley Limestne rarely are greater than eight inches in diameter and they average fur inches. In additin, the distinctly tabular strmatprids never

28 24 STRATICRAPHY OF THE UPPER DEVONIAN have been bserved in expsures f the Cedar Valley Limestne. The subspherical frms are, hwever, fund in the Cedar Valley Limestne, either frming a distinct bistrme r scattered in a limestne matrix. Fr these reasns, and because f the charac teristics f the cntaining limestne, the interval f strmatprid-bearing limestne beginning at 120 feet in the abvementined well is crrelated with the Cedar Valley Limestne. Cuttings frm ther nearby wells als shw lithgraphic lime stne beds f the Cedar Valley (with traces f subspherical strmatprids) immediately belw the Juniper Hill Shale. An utlier seven miles suth f Charles City is the eastern mst expsure f the Shell Rck Frmatin. At this lcatin a maximum f 5y2 feet f the Masn City Member is present n tp f the Cedar Valley Limestne (appendix I, sectin 12). The shale bed at the base f the Masn City Member is the same lithlgy recrded in the vicinity f Marble Rck. The area suth f Greene is the final area t be discussed n this first traverse, and the trend previusly nted between this regin and Nra Springs is maintained. Maximum thickness f the Masn City Member at an expsure ll/2 miles suth f Greene is 4 y2 feet (appendix I, sectin 13). Pleistcene ersin has remved all but three feet f the Rck Grve Member. Sub spherical strmatprids, Amphipra and calcisphere structures are characteristic f the Cedar Valley Limestne at this lcatin. The typical calcisphere is a spherical chamber y&mm in diameter with a msaic f calcite within the chamber and a thin wall f anhedral calcite. This expsure, tgether with thers within a distance f half a mile where less than five feet f the Masn City Member are present, is the mst sutherly ccurrence f the Shell Rck Frmatin. The bedrck farther suth is the Cedar Valley Limestne. Secnd Traverse Stratigraphy Nrthwest f Nra Springs The area included n this traverse is shwn in figure 10. Changes in the Shell Rck Frmatin nrthwest f Nra Springs ccur ver a shrter distance than thse bserved t the suth east. Lithlgic changes, faunal variatins, and an abrupt de crease in thickness f the frmatin are cnspicuus. As previ usly nted, the Shell Rck Frmatin thins prgressively suth east f the type area because f a decrease in thickness f the

29 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 25 Kensett n W-I806Z Nr Springe Mp Scale 2= miles 12 (0 Pleistcene Nr Cedar Valley Limestne Masn City Rck Grve Figure 10. Rute f traverse 2; numbers refer t sectin descrip tins in appendix I; W refers t the well lg descriptin in appendix II.

30 20 STRATIGRAPHY OF THE UPPER DEVONIAN Figure 11. Subspherical strmatprids and Amphipra in lwer bi strme f the Nra Member; nte hand level fr scale. upper beds and the absence f lwer beds as they verlap upn the Cedar Valley Limestne. Still, a prtin f at least tw f the three members can be recgnized in expsures and in the subsurface sectin. In sharp cntrast t the sutheast area, the Nra Member is the nly interval f Shell Rck strata present abve the Cedar Valley Limestne nrth f a pint abut three miles nrthwest f Nra Springs. This relatinship is bserved at all lcatins farther nrthwest and t the west near Masn City. Belanski (p. 367) crrelated the carbnate beds which underlie the Nra Member in this area with the Rck Grve and Masn City Members f the Shell Rck Frmatin. Hwever, it is demnstrated here that these carbnate beds are crrelative with the Cralville Limestne member f the Cedar Valley Lime stne. The cmpsitin f the upper strmatprid bistrme f the Nra Member is cnstant wherever this unit ccurs. This massive bistrme, cmpsed almst whlly f clnies f tabular strmatprids, is expsed nrthward t the twn f Plymuth in nrtheastern Cerr Grd Cunty. Farther nrth, Pleistcene and Recent ersin have cut either int a lwer bed f the Nra Member r int the beds f the Cedar Valley Lime stne. Shaly beds ccur lcally in the middle argillaceus unit

31 SHELL HOCK FORMATION OF NORTH-CENTRAL IOWA 27 Figure 12. Large subspherical strmatprid; right prtin was brken ff and carbnate sediment subsequently filled the vid; arrws mark brken edge. f the Nra Member, but argillaceus dlmite is the dminant lithlgy. A change is nted in the lwer strmatprid bi strme f the Nra Member t the nrthwest. Clnies f the tabular strmatprids, s cnspicuus in the type area gradu ally give way t Amphipra. subspherical strmatprids, and a few tabulate and rugse crals. Still farther nrthwest, this interval is strngly dlmitized and fewer strmatprids are present. The first change within the Nra Member is evidenced in an expsure alng the eastern bank f the Shell Rck River near the Flyd-Cerr Grd Cunty line (appendix I, sectin 14). A thin bed f argillaceus dlmite is present within the shale unit that separates the upper and lwer strmatprid bistrmes. This carbnate bed is nt present in the type area, but a similar ccurrence f a carbnate bed in this stratigraphic psitin was nted in the Rckfrd area (appendix I, sectin 9). A few Amphipra ccur thrughut the lwer tabular strmatprid bistrme, but this slender, stalked strmatprid is the dmi nant frm near the tp f the bistrme. Less than a ft f the Rck Grve Member is expsed at this lcatin.

32 28 STRATIGRAPHY OF THE UPPER DEVONIAN Changes within the Nra Member are exemplified further in an expsure apprximately three-furths f a mile nrthwest f the last-described sectin. At this expsure (appendix I, sectin 15) the middle argillaceus unit is dminantly a carbnate inter val. The ccurrence f subspherical strmatprids fr the first time with the tabular strmatprids in the lwer bi strme is striking. Less than tw feet f the Rck Grve Mem ber are expsed abve river level at this lcatin. Cntinuing nrthwesterly, the matrix f the lwer strma tprid bistrme is mre dlmitic (appendix I, sectin 16). Clnies f subspherical strmatprids ver ne ft in diame ter ccur here, but the majrity are much smaller. These sub spherical frms almst cmpletely have replaced the massive, tabular frms which are present farther suth. Amphipra cm prises a high percentage f the fauna thrughut the bistrme. Mst f the strmatprids were subjected t sme degree f abrasin and fragmentatin (figs. 11, 12). Weathering f dl mite in the underlying Rck Grve Member has effected a clear utline f crss-laminatins. Only 6.9 feet f the Rck Grve Member are expsed here. This is the mst nrthwesterly ex psure f the Rck Grve Member. A mre cmplete sectin f the Nra Member is expsed in a small ravine abut ne-furth f a mile nrtheast f the lastmentined sectin (appendix I, sectin 17). A remnant f the Juniper Hill Shale is present abve the Nra Member near the head f the ravine, but at all pints nrth f this lcatin the Juniper Hill Shale has been remved by Pleistcene ersin. Again, the mre dlmitic character f the middle argillaceus unit f the Nra is evident in this expsure. The area apprximately three miles nrthwest f Nra Springs is the lcus fr determining the relatinships f Shell Rck- Cedar Valley strata and fr identifying the Shell Rck-Cedar Valley cntact at all lcatins farther nrth and west. T demnstrate the stratigraphic relatinships, it is necessary t examine a discrepancy in crrelatins made by Belanski. Belanski (p ) crrelated the lithgraphic limestne beds which ccur in the vicinity f Rck Falls as a facies equivalent f the lwer strmatprid bistrme f the Nra Member. These lithgraphic units are the same beds that, nrth f Rck Falls and thrughut the regin f Masn City, Belanski crrelated as a facies equivalent f the Rck Grve Member. The lithlgy f

33 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 29 Figure 13. Cntact l Nra Member witli underlying Cedar Valley Limestne alng Keidle's Ulntf (arrw); the successively undercut dlmite beds belw the Nra Member were crrelated with the Rck Grve Member by Belanski, and the basal unit slping utward tward the river was crrelated with the Masn City Member. these beds is mre cmparable t beds that are unquestined crrelatives f the Cralville Member f the Cedar Valley Lime stne in Flyd and Butler Cunties. The relief n the upper surface f this limestne, which is greatest nrth f Rck Falls, is evidence f an uncnfrmity at the tp f the Cedar Valley Limestne, nt an uncnfrmity within the Shell Rck Frma tin as prpsed by Belanski. It fllws then that the dlmite beds beneath the lithgraphic limestne unit are als a part f the Cedar Valley Limestne and are nt equivalent t the Masn City Member f the Shell Rck Frmatin as prpsed by Belan ski. In summary, equivalent beds that were crrelated with the lwer part f the Nra Member at ne lcatin and with the Rck Grve Member at anther lcatin actually are crrelative with the Cedar Valley Limestne. The next expsure t be examined n this traverse is alng a lw bluff (Keidle's Bluff) lcated 3<4 miles nrthwest f Nra Springs (fig. 13; appendix I, sectin 18). At this lcatin, and at less accessible expsures between here and the last-described sectin, the uncnfrmable cntact f the Nra Member with the Cedar Valley Limestne is expsed. The Nra is the nly mem-

34 30 STRATIGRAPHY OF THE UPPER DEVONIAN ber f the Shell Rck Frmatin that is present in this area and thrughut the mre nrthern and western area f the frmatin. The middle argillaceus unit and the lwer strmatprid bi strme f the Nra Member have been altered by dlmitizatin t a greater degree in these nrthern expsures. In additin, there is mre carbnate matrix in the lwer bistrme cmpared with expsures t the suth. The upper tabular strmatprid bi strme is nt expsed alng Keidle's Bluff, althugh it may be present higher n the sd-cvered slpe. The Cedar Valley Limestne is cmpsed f thick- t massive-bedded dlmite in the upper part (Rck Grve Member accrding t Belanski), and a massive calcitic dlmite in the lwer part (Masn City Mem ber accrding t Belanski). Dlmitized subspherical strmatp rids that average abut tw inches in diameter are cncen trated n a bedding plane within the upper unit f the Cedar Valley. Lithlgic changes in bth the Shell Rck Frmatin and the Cedar Valley Limestne are nted at an expsure n the suth bank f the Shell Rck River half a mile west f the nrthern end f Keidle's Bluff. Here, at Baker's Bluff, the upper tabular strmatprid bistrme f the Nra Member is expsed abve the break-in-slpe and a basal dlmite bed is present at the tp f the bluff (appendix I, sectin 19). The lwer bistrme f f the Nra Member is missing. Bed 2 at Keidle's Bluff is cmpsed f dlmite but the equivalent bed at Baker's Bluff is cmpsed f sublithgraphic limestne. Subspherical strma tprids frm ne t three inches in diameter, and Amphipra are cncentrated n a bedding plane in the lwer part f this limestne. A similar ccurrence f strmatprids was nted at Keidle's Bluff. Farther nrth the surface f the Cedar Valley Limestne has greater relief and the dlmitized Nra Member is crrespnd ingly thicker where it fills shallw depressins. The equivalent f the bedding plane with the single layer f strmatprids bserved at Keidle's Bluff and at Baker's Bluff is here a distinct bed frm 0.5 ft t ne ft in thickness. In the regin f Masn City the equivalent bed is a prminent bistrme up t 5.5 feet in thickness (see third traverse). These cnditins are bserved in a quarry sectin half a mile nrthwest f Rck Falls (appendix I, sectin 20). The largest amunt f expsed relief n the tp f the Cedar Valley Limestne is bserved in a quarry alng the west bank f

35 Qurry Expsure, NW, SW, NW Sectin 29, T.98N., R.I9W. Digrmmatic Crss-Sectin f West Face Fl r r ss» Quarry flr. LEGEND 2 O 5? z=z US I. I.' ' I I 1 Dlmite brecciatcd argillaceus Limaalane Calcite filled ca»ili -^ Stalked ttrmatparidt (Amhipra) Scle(feet) (Vertical 8. Hriintl) O PI H t tlcslm Talus Nra-Cralville cntact < > Figure 14. Relief n upper surface f the Cedar Valley Limestne; nte brecciatcd zne at base f Nra Member.

36 32 STRATIGRAPHY OF THE UPPER DEVONIAN the Shell Rck River tw miles nrth f Plymuth (appendix I, sectin 21). Here the relief is seven feet; cnsequently, the dlmitized Nra is highly variable in thickness. Details f variatins in thicknesses f beds are depicted in figure 14. Significant expsures f Shell Rck-Cedar Valley strata are limited nrth f the abve-described sectin. Drill cuttings frm sutheastern Wrth Cunty shw that the Cedar Valley Lime stne is the bedrck and it is cmpsed f either lithgraphic limestne r the underlying dlmite. An example f the frmer cnditin is given in appendix II, W Farther nrth, alng the Shell Rck River east f Kensett and in the vicinity f Nrthwd, utliers f the lwer bistrme f the Nra Member frm the nly remaining prtin f the Shell Rck Frmatin. In additin t subspherical strmatprids numerus specimens f Trigntreta shellrckensis, high- and lw-spired gastrpds, and charphyte gnia (Trchiliscus sp.) ccur in the bistrme at these lcatins. The subjacent beds are thse f the Cedar Valley Limestne, dlmite at a few lcatins and sublithgraphic limestne at ther lcatins. Third Traverse Stratigraphy in the Regin f Prtland and t the Nrthwest Alng the Winnebag River There are n expsures f Shell Rck-Cedar Valley strata between Nra Springs and Prtland. Only prtins f the Lime Creek Frmatin are expsed n either side f the divide that separates the drainage basin f the Shell Rck River frm the Winnebag River (fig. 15). The reader will recall that beds f the Nra Member lie uncnfrmably upn the Cedar Valley Lime stne in the vicinity f Rck Falls. This same stratigraphic relatinship is bserved at Prtland, and, as is the case nrth f Nra Springs, the exact gegraphic terminatin f the Rck Grve and Masn City Members is unknwn (pi. I). A cre (appendix III, cre 2) was drilled ne mile nrth and 1% miles east f Prtland in an attempt t mre accurately define the terminatin f these members and t btain anther cntrl pint where the cntact f the Juniper Hill Shale with the Shell Rck Frmatin culd be identified. Examinatin f the cre revealed that Pleistcene depsits lie uncnfrmably upn the Nra Mem ber and that, in turn, the Nra Member is uncnfrmably in

37 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 33 Figure 15. Rute f traverse 3; numbers refer t sectin descrip tins in appendix I; W-8079, W-13981, etc. refer t well lg descriptins in appendix II; C-l, C-2, etc. refer t cre descriptins in appendix 111. cntact with Cedar Valley strata. Presumably, Pleistcene ersin has remved any beds f the Lime Creek Frmatin which may have been depsited here. The upper tabular strmatprid bistrme f the Nra is cnspicuus in the cre. The lwer beds f the Nra ccur as dlmitized beds similar t thse in expsures nrthwest f Nra Springs. The underlying Cedar Valley beds als are dlmitized. The cntact f the Nra Member with the Cedar Valley Lime stne is expsed in the McEachran quarry half a mile suth f Prtland (appendix I, sectin 22). As in the cre mentined abve, the upper tabular strmatprid bistrme f the Nra Member is cnspicuus. The underlying dark-clred dlmite is the equivalent f the middle argillaceus unit r the lwer strmatprid bistrme f the Nra. Relief n the tp f the Cedar Valley Limestne is abut fur feet, and at a few lcatins in the quarry the beds are sharply truncated. This limestne is the interval thdt, in the river expsures near the quarry, Belan-

38 Mc Eachran Quarry, Prtland, lw Diagrammatic Crss-Sectin f East Face H? H O i**c-tgcwxari -v->--" es "t^t LEGEND Weathered, brken limestne with abundant tabular strmtprids Bistrme cmpsed f labulr strmtprids Selesf Vertical (feetn Hrizntal Dlmite a pc1: i} w',n clnial grwth interrupted by ^n ^5 Ti ItTTJtti chemical and/r clstie depsitin P*^:r\W' Talus Limestne rgillaceus IB Subsphericl strmatprids J^nt- Stalked strmtprids *-» (Amphiper) ^^&Calcite filled cavities Jf*'r\. Wr-Crclville cntct S < H a n c» D Pi < O z > Z Figure 10. Irregular cntact f Nra Member with the Cedar Valley Limestne.

39 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 35 Figure 1"- Caverns frmed within the Nra Member and Cedar Valley Limestne alng the Winnebag River; in this phtgraph the cavern ceil ings are at the base f the tabular strmatprid bistrme f the Nra Member. The bedding plane ccurrence f small subspherical strmatp rids and Amphipra within the Cedar Valley Limestne is marked by arrw. ski crrelated as the equivalent f the Rck Grve Member, and he crrelated the underlying dlmite with the Masn City Mem ber. Once again, a bedding plane ccurrence f small sub spherical strmatprids similar t that in the equivalent stratigraphic psitin in the Rck Falls area (secnd traverse) is present in this quarry. The variatin in bed thicknesses and the irregular Shell Rck-Cedar Valley cntact is illustrated in figure 16. East f the McEachran quarry, n the east bank f the Winne bag River, a series f shallw caverns has been erded within the Cedar Valley Limestne (fig. 17). The ceiling f each cavern ccurs at the base f the upper tabular strmatprid bistrme r within the lwer part f the Nra Member; the flrs f the caverns are near the base f the lithgraphic lime stne bed f the Cedar Valley. A cntinuus weak re-entrant within the limestne marks the psitin f the bedding plane ccurrence f small subspherical strmatprids and Amphipra, again, the same bedding plane feature that is present thrughut the Rck Falls regin.

40 36 STRATICRAPHY OF THE UPPER DEVONIAN The uncnfrmable cntact f the Juniper Hill Shale with the Nra Member is expsed in a smaller quarry pening abut nefurth mile west f the McEachran quarry. The cntact is marked by a cncentratin f fish teeth (Ptyctdus calcelus) within the base f the Juniper Hill Shale, and pyrite is cncen trated within the upper part f the Nra Member. This fish tth zne has been expsed n the nrth side f U. S. Highway 18 n the west side f the Winnebag River in Masn City, and is present in cres drilled near the nrthwestern and suth western crprate limits f Masn City (appendix III, cres 3, 4). A sectin similar t that in the McEachran quarry is expsed n the east bank f the Winnebag River half a mile nrthwest f Prtland. The cntinuity f the bedding plane ccurrence f subspherical strmatprids and Amphipra is bservable at this lcatin als (appendix I, sectin 23). Shales and carbnates f the Lime Creek Frmatin frm the bedrck in the suthern and western parts f Masn City. Cnsequently, cres and drill cuttings prvide the nly data n stratigraphically lwer beds in this area. These data shw significant changes bth in the Nra Member and in the Cedar Valley Limestne as cmpared with the sequence f rcks in the Prtland sectins described abve. The Nra is mre dl mitic in suthwestern Masn City, but the tabular strmatp rids f the upper bistrme still are recgnizable (appendix II, W-8079; appendix III, cre 4). The bedding plane cncentratin f subspherical strmatprids and Amphipra in the limestne f the Cedar Valley, as bserved at Rck Falls and Prtland, is here a prminent bistrme and is recgnizable thrughut the regin f Masn City. This is the bistrme that Belanski (p. 369) crrelated as the equivalent f the lwer strmatprid bistrme f the Nra Member. The limestne interval f the Cedar Valley generally is thicker in the Masn City regin than it is near Prtland (appendix II, W-8079, W-2971). Natural expsures f the upper tabular strmatprid bi strme f the Nra Member are rare in the Masn City area. Usually it is the bistrme in the Cedar Valley Limestne, cm psed f subspherical strmatprids and Amphipra, that frms the uppermst expsed bed alng the streams. Beds f the Nra, and the underlying Cedar Valley Limestne, are ex psed in an artificial drainage cut in the nrthern part f Masn City (fig. 18; appendix I, sectin 24). The upper strmatprid bistrme f the Nra is well develped and is underlain by a

41 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA -, i Figure 18. Cntact f Nra Member with the Cedar Valley Limestne (arrw), nrthern part f Masn City. (See graphic clumn, appendix I, sectin 22). dlmite bed which is the equivalent f the middle argillaceus unit f the Nra Member. A bistrme that is cmpsed dminantly f subspherical strmatprids frms the tp f the Cedar Valley Limestne at this lcatin. Thrugh cmmn usage, the underlying lithgraphic limestne beds lcally are referred t as the "cement ledge". These are the surce beds fr the manufacture f prtland cement in Masn City. The underlying dlmite beds (nt expsed) lcally are referred t as the "Masn City beds". A similar sectin ccurs in a quarry ne mile east f the drainage cut where the dlmite beneath the Cedar Valley Limestne is expsed (appendix I, sectin 25). Subspherical strmatprids are scattered thrughut this dl mite and have been calcitized and/r dlmitized; the mrph lgic structures f many specimens are recgnizable even where they have been dlmitized. "Vermicular" prsity in this dl mite is caused by calcitizatin and slutin f fragmented Amphipra stalks. A stratigraphically higher bed f the Cedar Valley Limestne is expsed in a quarry tw miles nrthwest f the last-mentined sectin (appendix I, sectin 26). This bed lies abve the bistrme f subspherical strmatprids and Amphipra. An interval equivalent t this bed, but f greater thickness, ccurs nrthwest f Masn City and will be discussed

42 38 STRATIGRAPHY OF THE UPPER DEVONIAN T %***# -1 ti^ta Figure 10. Bistrme l subspherical strmatprids within the Cedar Valley Limestne. nd Amphipra later. The strmatprid bistrme in the Cedar Valley Lime stne at this lcatin is shwn in figure 19. The cnspicuus "vermicular" prsity in the lwer dlmite beds f the Cedar Valley Limestne is the result f calcitizatin and slutin f fragmented Amphipra stalks. Beds f the Lime Creek Frmatin that may have been de psited thrughut the nrtheastern prtin f Masn City have been remved by Pleistcene ersin. Drill cuttings three miles east f the last-mentined quarry indicate a reginal cntinuity f Shell Rck Frmatin-Cedar Valley Limestne lithlgies between the Masn City and Rck Falls areas. The cuttings (appendix II, W-13981) shw Juniper Hill Shale lying n dl mitized Nra(?) which is underlain in turn by beds f the "cement ledge" (Cedar Valley f this reprt, Rck Grve f Belanski) and the "Masn City beds" (Cedar Valley f this reprt, Masn City f Belanski). Only a few cntrl pints are available fr examining Shell Rck-Cedar Valley relatinships nrthwest f Masn City. The mst cmplete sectins are expsed in quarries perated by the Prtland cement industries f Masn City. A facies change in the Nra Member and znes f incmplete dlmitizatin within

43 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 39 the Cedar Valley Limestne are apparent in the Lehigh Prtland Cement Cmpany quarry (appendix I, sectin 27). The Nra Member is cmpsed f finely crystalline limestne with a fauna that includes the fllwing: Cyrtspirifer whitneyi, Aulpra, high- and lw-spired gastrpds, charphyte gnia (Trchiliscus sp.), Atrypa, small subspherical strmatprids, and rugse crals. Crinid fragments are abundant, and ne rthceracnic cephalpd was fund. C. whitneyi always has been recgnized as an index f Upper Devnian (G.A. Cper, written cmmuni catin, March 25, 1966). This fauna is very similar t that fund in the Nra Member near Kensett (secnd traverse), except that subspherical strmatprids are mre abundant near Kensett and Trigntreta shellrckensis is the brachipd fund near Kensett rather than C. whitneyi. The prtin f the Nra Mem ber present in this quarry (preserved nly alng the suth face) is crrelated as a facies equivalent f the lwer strmatprid bistrme. The middle argillaceus beds and the upper tabular strmatprid bistrme f the Nra Member are absent, pssibly the result f Pleistcene ersin. Hwever, nn-dep sitin r pre-juniper Hill ersin may best explain their absence because irregular large pckets f Juniper Hill Shale ccur as stratigraphic leak in the underlying Cedar Valley. The dlmite beds beneath the Nra Member are crrelated with the Cedar Valley Limestne. These beds cntain lenses f unaltered lith graphic limestne similar t the limestne beds that are present in the lwer part f the quarry. The lwer beds are equivalent t the "cement ledge" in expsures in the Masn City area, but the interval is much thicker at this lcality. There are tw sub spherical strmatprid bistrmes within the "cement ledge". The lwer bistrme is mre persistent and is prbably the equivalent f the bistrme immediately abve the "cement ledge" in the nrtheast part f Masn City. The dlmite in the flr f the quarry is the equivalent f the "Masn City beds" in quarries and natural expsures in the regin f Masn City. The facies f the Nra Member bserved in the Lehigh quarry is expsed at several lcatins alng the Winnebag River farther nrthwest. At the Lincln Mill site, tw miles nrthwest f the Lehigh quarry, Cyrtspirifer whitneyi is fund in abundance alng a bedding plane. Mst f the ther fssils that ccur in the Nra Member at the Lehigh quarry are present at this lcatin.

44 40 STRATICRAPHY OF THE UPPER DEVONIAN A series f five cres drilled half a mile nrth f the Lincln Mill site cntained a sectin cmparable t that f the Lehigh quarry. A lg f the deepest cre is given in appendix III, cre 5. The uppermst bed at this lcatin is a dlmite interval that is nt present at the Lehigh quarry and is prbably equiva lent t the middle argillaceus bed f the Nra Member. The underlying limestne is similar lithlgically t that f the Nra Member in the Lehigh quarry and the same fauna is present. The few expsures in the area f Fertile and Hanlntwn, Iwa (suthwestern Wrth Cunty) may be utliers f Shell Rck strata. These beds have been dlmitized and their true relatinship with beds f the Shell Rck Frmatin r Cedar Valley Limestne has nt been established. A deep quarry was pened \y2 miles east f Fertile, but the authr was able t bserve nly the upper few feet f sectin which may be dl mitized beds f the Nra Member. Like mst quarries in nrthern Iwa, this quarry is filled with water when nt per ated, and the main quarry face never was expsed during the study reprted here. Subsurface data suth f Fertile demnstrate that the Juniper Hill Shale lies uncnfrmably abve the Cedar Valley Limestne with n intervening Shell Rck strata. Cuttings frm a well three miles suthwest f the Lincln Mill site indicate that the limestne f the "cement ledge" lies immediately beneath the Juniper Hill Shale (appendix II, W-9308). Either the limestne f the "cement ledge" r the underlying dlmite ccur beneath the Juniper Hill Shale at all ther subsurface cntrl pints farther suth and t the sutheast as far as the twn f Rck well. Thus, the maximum westerly extent f the Shell Rck Frmatin lies alng a nrthwest-sutheast trending line frm just west f Fertile t the twn f Rckwell in suth-central Cerr Grd Cunty, and t the suth f Greene in nrthern Butler Cunty. (See figs. 4, 5, 6). DEPOSITIONAL ENVIRONMENT OF SHELL ROCK SEDIMENTS Beds f the Shell Rck Frmatin were depsited in a rela tively small, shallw basin n the erded surface f the Cedar Valley Limestne. This basin trended nrthwest-sutheast and extended ver an area at least 65 miles in length and 25 miles

45 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 41 in width. Depsitin within the basin during Shell Rck time prbably extended frm near the Iwa-Minnesta state line in nrth-central Wrth Cunty, sutheasterly t nrtheastern But ler Cunty. Presumably, the Shell Rck sea encrached upn the Cedar Valley frm the west-nrthwest. (See discussin f faunal relatinships, p. 4). Subaerial ersin ver mst f the Cedar Valley surface prbably was cntinuus in eastern and sutheastern Iwa dur ing much f Shell Rck time. The State Quarry Limestne in nrthern Jhnsn Cunty (Upper Devnian beds which ccur abve the Middle Devnian Cedar Valley Limestne) may repre sent anther lcalized interval f depsitin cntempraneus with at least part f the Shell Rck Frmatin. Furnish and thers (1969, p. 12) state that "the cndnt fauna f the State Quarry is yunger than that f the bulk f the Cedar Valley and lder than that f the Rck Grve Member f the Shell Rck Frmatin f nrth-central Iwa". Masn City Member Initial depsits f the Masn City Member were thin clay seams and lime muds. The latter subsequently were dl mitized. The areas f greatest accumulatin f these depsits were sufficiently shallw t prvide an envirnment favrable fr the develpment f a bistrme f subspherical strmatp rids and crals. Many f the crals and strmatprids shw evidence f slight fragmentatin and abrasin indicative f grwth in shallw water subjected t wave actin. Several authrs (Lecmpte, 1954; Andrichuk, 1958; Klvan, 1964; and Muntjy, 1965) already have interpreted a highly agitated en virnment fr these massive, subspherical r glbular strma tprids. A prminent discntinuity surface was develped cntemp raneusly with r immediately after the grwth f the bistrme. This surface prbably was frmed within the intertidal t slightly abve tide zne after a slight regressin f shallw water frm a nearly level plain f carbnate sediments (Kch and Strimple, 1968, p. 10). This hrizn marks a discntinuity surface fr which n majr time break is invlved but a definite break in sedimentatin is evident. Slight submergence f this surface prvided an envirnment favrable fr the grwth f

46 42 STRATIGRAPHY OF THE UPPER DEVONIAN cystids, edriasterids and stlnal-branching crals and bryzans. A sudden influx f mud mixed with fine carbnate detritus ended this perid f rganic grwth. Gastrpds, subspherical strmatprids, crals, crinids, and brachipds were abundant during the next perid f carbn ate depsitin. Strng wave and current actin generally re sulted in fragmentatin and abrasin f the hard parts f these rganisms. Limestnes rich in crinid ssicles, and which cn tain a few fragmented subspherical strmatprids characterize mst f the remaining Masn City Member. Lcalized rapid depsitin f lime mud is evidenced by the preservatin f crinid crwns with lng stem segments (Strimple and Levrsn, 1969). Charphyte gnia, prbably washed utward frm the intertidal zne, als are present. Near the end f Masn City time cnditins became favrable again fr the establishment f a fauna rich in crals and subspherical strmatprids, but nt t the extent f develping a bistrmal phase. Rck Grve Member The Rck Grve Member is characterized by dminantly argillaceus beds. A perid f smewhat unstable depsitinal cnditins in slightly deeper water is indicated by depsits f thin-bedded, relatively nn-fssiliferus argillaceus limestne separated by shale seams and partings. Bttm dwelling rgan isms that include stracdes and pelecypds, and a few fish plates are the nly fssils in these beds. Subsequent depsitin in a mre unifrm envirnment is evidenced by beds f massive, argillaceus, mderately fssiliferus limestne in the middle f this member. A sublittral envirnment f intermediate depth is suggested by the abundance f brachipds. An increase in the supply f terrigenus mud mixed with fine carbnate detritus marked the depsitin f beds f calcareus clay and lime mud thrughut the area f the Rck Grve Member. The resultant calcareus shales and argillaceus limestnes are barren f fssils except fr the ccurrence f a few cndnts (Andersn, 1964, 1969). Shallwer depsitinal envirnments fr the upper beds f the Rck Grve Member fllwed the gradual reductin in basin relief by the cntinued influx f terrigenus mud. Lime sands were depsited in agitated water and were spread ver the

47 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 43 shallw shelf. Crss-bedding is a cmmn feature in these beds. A decrease in wave activity brught cnditins in which brachipds flurished (principally Platyrachella and Schizphria). Lcalized areas f echinderm grwth are evidenced by crinid ssicles and a few cystid calices in the east-central part f the basin. These carbnate beds subsequently were dlmitized. By the end f Rck Grve depsitin the basin was reduced t a brad, relatively shallw shelf. Nra Member Gallway and St. Jean (1957, p. 69) suggested that clear, warm water, and abundant fd and carbnate material were ptimum cnditins fr the grwth f strmatprids. Intervals f widely spaced laminae indicate grwth under ptimum cnditins whereas intervals f clsely spaced laminae indicate adverse cnditins. Certainly, ptimum cnditins fr strmatprid grwth prevailed during the first stage f Nra depsitin. Cl nies f tabular strmatprids prbably cnverged frm many centers f grwth and develped int a massive bistrme. Depsits f the Nra Member cvered a larger area than did depsits f the lwer members. The larger depsitinal area f the Nra Member is interpreted as a respnse t a transgressive phase f the Shell Rck sea. An eclgic change tward the nrthern area f depsitin is suggested by a gradual replacement f the tabular strmatprids with subspherical strmatprids and stalks f Amphipra. A highly agitated depsitinal en virnment has already been mentined fr the subspherical strmatprids. As the stalks f Amphipra are brken and are partly runded they must have been subjected t sme ersin and prbably transprtatin befre depsitin, als indicative f a shallw, mre turbulent envirnment. A regressive phase ended the perid f rganic grwth. Terri genus muds were depsited upn the bistrme and were mixed with carbnate detritus in lcalized areas. The resultant shales and argillaceus carbnates separate the lwer and upper strma tprid bistrmes. The upper tabular strmatprid bistrme f the Nra Member was depsited during the final transgressive phase f the Shell Rck sea. Tabular strmatprids spread ver even a larger area than*did thse in the lwer Nra bistrme. Peridic

48 44 STRATIGRAPHY OF THE UPPER DEVONIAN strng wave activity near the western limit f the bistrme is indicated by intervals f slightly brken strmatprids with carbnate depsitin interrupting clnial grwth. The upper tabular strmatprid bistrme f the Nra Member cmpleted the sequence f Shell Rck depsitin. Ex cept fr perhaps a slight amunt f subaerial expsure, this unit was affected very little in the nrthern area befre the advance f the Lime Creek sea. In cntrast, subsurface data suggest cn siderable ersin in the suthern area where the Juniper Hill Shale lies directly upn the lwer strmatprid bistrme. PETROGRAPHIC NOTES General Statement Brief descriptins f sme petrgraphic features f Shell Rck and Cedar Valley strata are presented here. A cmprehensive petrlgic study f these units has nt been undertaken. The variatins in lithlgy and the diagenetic prblems within these beds cnstitute a majr research subject in themselves. Phtmicrgraphs shwn in plates II-III illustrate selected petrlgic features f the Shell Rck Frmatin and f the "cement ledge" and "Masn City beds", bth f which frmerly were crrelated with the Shell Rck Frmatin and are herein crrelated with the Cralville Member f the Cedar Valley Lime stne. "Masn City beds" The thin sectin examined frm this interval was prepared frm a sample taken near the middle f the "Masn City beds" in Weaver's quarry (appendix I, sectin 26). The rck is dl mite and the dlmite crystals are subhedral t euhedral. Indi vidual crystals range frm 125 t 325 micrns in size and average abut 215 micrns. The crystals are nn-unifrmly argillaceus and many exhibit clear znal rims (pi. II A). The argillaceus centers suggest that the dlmite was derived by replacement f argillaceus micrite. "Vermicular" prsity in the rck is believed t be caused by slutin f brken Amphi pra stalks. The pres ccur as tubular penings which average 1.5mm in diameter and range frm ne-half inch t three inches in length. They are riented parallel t the bedding.

49 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 15 "Cement ledge" The limestne in beds f the "cement ledge" is cmpsed f slightly argillaceus micrite. Hand specimens generally shw the limestne t be variably pelletiferus, and specimens that appear t be hmgeneus micrite cntain ghst-utlines f pellets in thin sectin (pi. II B). Vid-filling sparry calcite is cmmn. Fractures with n apparent preferred rientatin are sealed with sparry calcite. Fssil allchems generally are limited t fragments f Amphi pra, stracde valves, and a few brachipd fragments. The discntinuus, irregular chambers f Amphipra are filled with sparry calcite r argillaceus micrite (pi. II C). Whle and fragmented stracde valves are cmmn in the Rck Falls and eastern Masn City areas. Nrthwest f Masn City stracde valves are mre abundant and less fragmented. Calcispheres are very cnspicuus in these limestne beds. The typical calcisphere is a spherical chamber frm 110 t 175 micrns in diameter with a carse msaic f drusy calcite within the chamber and a very thin, dark micrcrystalline wall that prbably resulted frm recrystallizatin f an riginal hyaline wall. Rarely, a calcisphere is bserved which pssesses a thick wall cmpsed f radiating calcite spines r prisms (pi. II B, D). The calcisphere structures prbably are the remains f sme frm f plant spre r reprductive bdy (Stantn, 1963). Authigenic dlmite crystals generally are present thrugh ut the micrite but they are mre abundant in the lwer beds. The crystals range in size frm 105 t 575 micrns and average abut 260 micrns. They exhibit unit extinctin, are nnunifrmly argillaceus, and represent replacement f micrite (pi. II E). The dlmite crystals encrach upn Amphipra in a few cases and fill prtins f their uter chambers. Calcitizatin f frmer dlmite crystals in the limestne has ccurred at a number f lcatins. An utstanding ccurrence f this pseudmrphic phenmenn is bserved in the limestne frm the Fster's Mill quarry sectin (fig. 14; appendix I, sectin 21). Crystal faces f the frmer dlmite crystals are retained, but the mass f each is nw cmpsed f randmly riented anhedral calcite (pi. II F).

50 46 STRATIGRAPHY OF THE UPPER DEVONIAN Beds f the "cement ledge" have been mre cmpletely dlmitized at lcatins in and suth f Rck Falls. Only a few lenses and small, irregular patches f limestne remain unreplaced by the dlmite (pi. Ill A). Specimens f Amphipra that ccur at a dlmite-limestne cntact shw very little t n dlmitizatin. Specimens that ccur whlly within the dlmite exhibit scattered dlmite crystals r have been cmpletely dlmitized. Many specimens have been calcitized and dis slved, and remain nly as external mlds f Amphipra stalks. Styllite sutures are very abundant. Organic-rich clay is dis seminated thrughut the limestne and is cnspicuus arund dlmite grains. The rganic cntent suggests depsitin under smewhat restricted water cnditins. Masn City Member Thin sectins were prepared nly frm the limestne upn which the discntinuity surface is develped and frm the ver lying dlmite. The limestne is a bimicrite with fssil allchems that include brachipds, stracdes, crinid stem seg ments and gastrpds embedded in micrite with a minr amunt f micrspar. Slenpra has been identified by J. L. Wray (Marathn Oil C., written cmmunicatin, Dec. 5, 1967). Dis cntinuus micr-fractures are healed with calcite. Euhedral dlmite fills shallw brings in the limestne and deeper brings cntain subhedral t euhedral dlmite in the upper part and sparry calcite in the lwer part (pi. Ill B). Dlmite abve the limestne generally is finer grained than that which fills the brings and is euhedral t subhedral with an argillaceus matrix and minr sparry calcite. Rck Grve Member The thin sectin examined frm this interval was prepared frm a sample f the thin-bedded limestne unit at the base f the member in the Baumgardner Mill sectin (appendix I sectin 8). Fssil allchems are limited t a few stracde valves. Questinable calcisphere structures in the limestne are similar t thse in the "cement ledge" but they generally lack the cnspicuus uter dark wall. These structures are nt apparent in hand specimens. Authigenic dlmite euhedra are cmmn and range in size frm 80 t 125 micrns and average abut 90 micrns (pi. Ill C). Discntinuus fractures are sealed with sparry calcite.

51 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 47 Nra Member Thin sectins were nt prepared frm strmatprids in the bistrmes f the Nra. Only the dlmitic prtin f the Nra in the nrth-central prtin f the basin and the nrthern lime stne facies were petrgraphically studied. The basal Nra in the Fster Mill quarry sectin (appendix I, sectin 21) is a breccia. Breccia fragments vary frm 14mm t 20mm in size and are cmpsed f subhedral, calcitized dlmite crystals frm 65 t 110 micrns in size. The matrix cnsists f euhedral, argillaceus, variably calcitized dlmite crystals frm 125 t 550 micrns in size and average abut 340 micrns. They exhibit wide, clear znal rims, sme f which have been partially calcitized (pi. Ill D). Styllite sutures transect bth the breccia fragments and the matrix. The nrthern limestne facies f the Nra is cmpsed f slightly argillaceus micrite. Fssil allchems are abundant (see descriptin f fssils, p. 39) and shw little r n alteratin. Fssils are mre fragmented in the mst nrthwesterly area f expsure, suggestive f depsitin in an envirnment f strnger wave r current actin (pi. Ill E,F).

52 48 STRATIGRAPHY OF THE UPPER DEVONIAN

53 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 49 PLATES II - III PHOTOMICROGRAPHS

54 50 STRATIGRAPHY OF THE UPPER DEVONIAN EXPLANATION OF PLATE II A. "Masn City beds". Dlmite crystals(d) that retain argil laceus cntent f riginal micrite; many crystals shw clear znal rims(r). Appendix I, sectin 26, unit 1. Plane plar ized light, 30x. B. "Cement ledge". Pellets(P) in micrite, and calcispheres(c) with dark, micrcrystalline walls. Appendix I, sectin 27, unit 2. Plane plarized light, 75x. C. "Cement ledge". Crss sectin f Amphipra; central canal and vesicles in lwer ne-half f specimen are filled with micrite, upper ne-half with calcite. Creek expsure, SE cr. SW14, NEi/J, Sec. 21, T. 97 N., R. 19 W., Cerr Grd Cunty, Iwa. Plane plarized light, 40x. D. "Cement ledge". Calcispheres(C) in pelletal micrite; speci men at lwer left shws greater recrystallizatin. Lcatin same as phtgraph B, unit 6. Plane plarized light, 75x. E. "Cement ledge". Authigenic dlmite(d) in micrite. L catin same as phtgraph A, unit 2. Plane plarized light, 30x. F. "Cement ledge". Calcitizatin f dlmite(c); clear areas at crystal margins are unaltered dlmite. Appendix I, sec tin 21, unit 2. Plane plarized light, 30x.

55 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 51 PLATE n 0.50 mm 0 50mm

56 52 STRATIGRAPHY OF THE UPPER DEVONIAN EXPLANATION OF PLATE III A. "Cement ledge". Unaltered lense f pelletal micrite (dark) cntaining calcispheres(c) abve dlmite(d). Creek ex psure, west side f railrad trestle, NWy4, SEy4 Sec. 27, T. 97 N., R. 19 W., Cerr Grd Cunty, Iwa. Plane plar ized light, 30x. B. Masn City Member. Micrite(dark) f discntinuity surface in Williams quarry; dark clr is caused by finely dissemi nated pyrite; surface is cvered with dlmite(d) and brings int surface filled with dlmite(d). Appendix I, sectin 7, cntact f units 3 and 4. Plane plarized light, 40x. C. Rck Grve Member. Questinable calcispheres(c), and dlmite euhedra(d) in micrite. Appendix I, sectin 8, unit 11. Plane plarized light, 130x. D. Nra Member. Calcitizatin f dlmite; breccia fragment (B) n right side f phtgraph has been calcitized except fr serrate-appearing, clear znal rims n left side f frag ment; nuclei f matrix crystals are almst whlly calcitized(c), but the clear znal rims are dlmite(d); V, vidfilling calcite. Appendix I, sectin 21, unit 3. Plane plar ized light, 30x. E. Nra Member. Fssiliferus micrite cntaining gastrpds (G), crinid ssicles(c) and shell fragments (left center). Appendix I, sectin 27, unit 11. Crssed nicls, 40x. F. Nra Member. Fssiliferus micrite cntaining shell frag ments^) and charphyte gnia(o). Rad ditch expsure, SE cr. Sec. 15, T. 99 N., R. 20 W., Wrth Cunty, Iwa. Plane plarized light, 30x.

57 SHELL ROCK' FORMATION OF NORTH-CENTRAL TOWA 53 PLATE HE 0.125mm mri 0.50mm 0.50mm

58 54 STRATIGRAPHY OF THE UPPER DEVONIAN REFERENCES Andersn, W. I., 1969, Upper Devnian cndnts and the Devnian-Mississippian bundary f nrth-central Iwa: Jur. Palentlgy, v. 40, n. 2, p , 5 pis., 1964, Upper Devnian and Lwer Mississippian cndnts frm nrth-central Iwa: unpublished Ph.D. dis sertatin, State University f Iwa. Andrichuk, M., 1958, Stratigraphy and facies analysis f Upper Devnian reefs in Leduc, Stettler, and Redwater areas, Al berta: Am. Assc. Petrleum Gelgists Bull., v. 42, p , & Wnfr, J. S., 1954, Late Devnian gelgic histry in Stettler area, Alberta, Canada: Am. Assc. Pe trleum Gelgists Bull., v. 38, p American Cmmissin n Stratigraphic Nmenclature, 1961, Cde f stratigraphic nmenclature: Am. Assc. Petrleum Ge lgists Bull., v. 45, n. 5, p Arey, M. F., 1909, Gelgy f Butler Cunty: Iwa Gel. Survey, v. 20, p Belanski, C. H., 1927, The Shellrck Stage f the Devnian f Iwa: American Midland Naturalist, v. 10, n. 10, p , 1928a, The Shellrck Stage f the Devnian f Iwa; stratigraphic and faunal relatinships: American Mid land Naturalist, v. 11, n. 5, p , 1928b, Descriptins f sme typical fssils f the Shellrck Stage: American Midland Naturalist, v. 11, n. 5, p , 6 pis. Calvin, Samuel, 1896, Gelgy f Cerr Grd Cunty: Iwa Gel. Survey, v. 7, p , 1902, Gelgy f Mitchell Cunty: Iwa Gel. Survey, v. 13, p

59 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 55 Carzzi, A. V., 1960, Micrscpic sedimentary petrgraphy: Jhn Wiley and Sns, Inc., New Yrk, Lndn, 485 p. Cllinsn, Charles, etal., 1967, Devnian f the Nrth-Central Regin, United States: in Internatinal Sympsium n the Devnian System, Calgary, 1967, v. 1, p Cper, C. A., etal., 1942, Crrelatin f the Devnian sedi mentary frmatins f Nrth America: Gel. Sc. America Bull., v. 53, p Cummings, E. R., 1932, "Reefs r Biherms?": Gel. Sc. America Bull., v. 43, p Drheim, F. H., & Kch, D. L., 1966, Devnian f nrthern Iwa; Cedar Valley, Shell Rck and Lime Creek: 30th Ann. Tri- State Guidebk, 45 p. Fentn, C. L., 1919, The Hackberry Stage f the Upper Devnian f Iwa: Am. Jur. Sci. 4th, v. 48, p Flk, R. L., 1965, Petrlgy f sedimentary rcks: The University f Texas, Hemphill's, p Furnish, W. M., Glenister, B. F., and Klapper, G., 1969, Nrth American Palentlgical Cnventin, The Middle Palezic palentlgy and bistratigraphy f eastern Iwa: infrmal guidebk, 41 p. Gallway, J. J., and St. Jean, J. Jr., 1957, Middle Devnian Strmatpridea f Indiana, Kentucky, and Ohi: Bull. Am. Palentlgy, v. 37, n. 162, p Kesling, R. V., & Bneham, R. F., 1966, Trchiliscus bellatulus Peck frm the Middle Devnian Dundee Limestne f nrth western Ohi: Cntributins frm the Museum f Palentl gy, The University f Michigan, v. 20, n. 7, p , 3 pis. Klvan, J. E., 1964, Facies analysis f the Redwater Reef Cm plex, Alberta, Canada: Bull. Can. Petrl. Gel., v. 12, p

60 56 STRATIGRAPHY OF THE UPPER DEVONIAN Kch, D. L., 1963, The Lime Creek Frmatin in the area f Garner, Iwa: Iwa Academy f Science Pra, v. 70, p , 1967, The Shell Rck Frmatin f nrth-central Iwa: unpublished Master's thesis, Univ. f Iwa Library, 169 p., and Strimple, 1968, A new Upper Devnian cystid attached t a discntinuity surface: Iwa Gel. Survey, Rpt. Inv. 5, p. 1-49, 9 pis., Wagner, James, & Mettler, S. D., 1963, Upper De vnian in Masn City and Garner areas: unpublished Ge lgical Sciety f Iwa field trip guidebk. Laudn, L. R., 1929, The stratigraphy f the Kinderhk Series f Iwa: Iwa Gel. Survey Ann. Reprt, p Lecmpte, M., 1954, Quelques Dnnees Relatives a la Genese et aux Characteres Eclgiques des "recifs" du Frasnien de l'ardenne: L'Inst. Ry. des Sci. Nat. de Belgique, Victr Van Straelen, vl. Jubilaire 1, p (Translated in Inter. Gel. Rev. 1959, v. 1, n. 7, p ) Lgan, B. W., 1961, Cryptzn and assciated Strmatlites frm the Recent, Shark Bay, Western Australia: Jur. Gel., v. 69, n. 5, p Macbride, T. H., 1902, Gelgy f Kssuth, Hancck and Win nebag Cunties: Iwa Gel. Survey v. 13, p Muntjy, E. W 1965, Stratigraphy f the Devnian Miette reef cmplex and assciated strata, eastern Jasper Park, Alberta: Department f Mines and Technical Surveys, Gelgical Survey f Canada, Bull. 110, 132 p., 12 pis. Nelsn, P. H 1939, Gelgy f Flyd Cunty, Iwa: unpublished Master's thesis, State University f Iwa. Owen, D. L., 1852, Reprt f Gelgy f Wiscnsin and Minne sta, pt. 1, p. 78.

61 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 57 Pettijhn, F. J., 1957, Sedimentary Rcks: Harper and Brthers, New Yrk, 2nd ed., 718 p. Stainbrk, M. A., 1935, Stratigraphy f the Devnian System f the Upper Mississippi Valley: in Guidebk, 9th Ann. Field Cnf., Kansas Gel. Sc, p , 1944, The Devnian System in Iwa: in Dedicatin- Mineral Cnference, Illinis State Gel. Survey, Bull. 68, p Stantn, R. J., Jr., 1963, Upper Devnian calcispheres frm Redwater and Suth Sturgen Lake reefs, Alberta, Canada: Canadian Petrleum Gelgy Bull., v. 11, p , 1 pi. Strimple, H. L., and Levrsn, C. O., 1969, Tw Upper Devnian crinids: Univ. Kansas Palent. Cntrib., Fssil Crinid Studies, Paper 42, Oct. 1, 1969, p , fig. 6, 7. Thmas, A. O., 1913, The relatin f the Lime Creek Shales t the Cedar Valley Limestnes f Flyd Cunty, Iwa: (abs.), Science, n.s., v. 37, p. 459., 1919, Echinderms f the Iwa Devnian: Iwa Gel. Survey, v. 29, p , 12 pis. Thmas, L. A., 1951, Devnian f nrth-central Iwa: 15th Ann. Tri-State Guidebk, 21 p. Whitney, J. D., 1858, Cunty Gelgy central and nrthern cunties f the eastern part f the state: Reprt n the Ge lgical Survey f the State f Iwa, v. 1, part 1, p Williams, I. A., 1899, Gelgy f Wrth Cunty: Iwa Gel. Survey, v. 10, p

62 58 STRATIGRAPHY OF THE UPPER DEVONIAN

63 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 59 APPENDICES Appendix I. Expsure Sectins 1-27 Appendix II. Well Lgs Appendix III. Cre Descriptins Rck clr designatins f expsures, drill cuttings, and cres fllw the "Rck Clr Chart" (Natinal Research Cuncil, 1948) based n the Munsell System. Rck speci mens were cmpared with the clr placques using a flurescent lamp with tw Champin F15T8/D Daylight bulbs.

64 60 STRATIGRAPHY OF THE UPPER DEVONIAN APPENDIX I Natural Expsures and Quarries Sectins 1-27

65 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 61 LEGEND FOR GRAPHIC SECTIONS i i if Limestne m i X Dlmite Shale Tabular strmatprid 0 Clnil cral «& Subspherical strmatprid fy Brachipd Ostracde Stalked strmatprid (Amphipra) Crinid clumnal aaaa Q Styllite Calcite lined r filled cavity "Vermicular" prsity Dlmite euhedrn Calcisphere structure Gastrpd

66 62 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin I Mbr. Fm. Z Ldnd fill, partially»lutnp-ev«red nd vergrwn 01 > s u rr u a: tu x tn e s Cral-slrmtprid bislrme UJ River level S u < UJ u

67 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 63 Stelien 1 TYPE SECTION OF MASON CITY MEMBER Measured alng the east bnk f the Shell Rck River, east line f west>/i. SE'/t. Sec. 7, T. 96 N., R. 18 W., Flyd Cunty, Iwa. Thick- Unit Descriptin ness (feet) IS Devnian System Shall Reck Frmatin Nra Mtmber Limestne, very paleyellwish-brwn, cmpseddminantlyf clniesf tabu lar strmatprids (Actinstrm expansin); irn-stained znes; prly ex psed, partially cvered by fill Rck Crave Mmbtf 14 Dlmite, dark yellwish-range, medium crystalline, subhedral, grading dwn ward t dark yellwish-range, dlmitic shale; thick-bedded; prly expsed Cvered interval 8'5 12 Limestne, yellwish-range, finely crystalline, mderately argillaceus, very dlmitic, slightly silty; "flating" dlmite crystals 1/16 l/32mm; brken beds; thick- t massive-bedded; upper surface prly expsed Limestne, very paleyellwish-brwn, weathering t yellwish-gray; very finely crystalline; cntains a few scattered stracdes; 0.1 t 0.3-ft platy beds sepa rated by shale partings; sharp lwer cntact 2.7 Miltn City Member 10 Limestne, grayish-range t very light live-gray, weathering t mttled pale yellwish-brwn and dark yellwish-range; Finely t very finely crystalline; many subspherical strmatprids frm 3 t 5 inchesin diameter (clred pale ycllwlsh-brwn), scattered crinid clumnals, Athyris sp.and ther brachipd fragments; thick-bedded Limestne, pale yellwish-brwn, very finely crystalline, mttled with light grayish-range, slightly dlmitic mderately argillaceus, finely crystalline limestne; fragmented brachipds, crinid clumnals. and subspherical strma tprids(up t 2 inches in diameter); thick- t massive-bedded, slightly brken; weathers very light live-gray t mderate yellwish-brwn Limestne, grayish-range t pale ycllwlsh-brwn, finely t very finely crys talline; weakly mttled with light grayish-range, finely crystalline, slightly argillaceus, dlmitic limestne; crinid clumnals thrughut, frming encrinital lenses lcally; weathers t mderate yellwish-brwn platy beds; thickbedded Limestne, very pale-range, mttled with pale red, finely t very finely crys talline, sub-cnchidal fracture; Amphipra, small subspherical strmatprids and crals cmmn; rugh, pitted upper surface; distinct upper and lwer cn tacts; massive Limestne, very pale-range, very finely crystalline t sublithgraphic; finely laminated in upper part; slightly dlmitic in lwer part; distinct upper and lwer cntacts; thick- t massive-bedded Limestne, very pale-range t pale yellwish-brwn, finely crystalline; numer us crals, Amphipra, and small subspherical strmatprids; a very distinc tive unit, but merges almst imperceptibly with the underlying bistrme laterally; massive Limestne bistrme, pale-range t pale yellwish-brwn, cmpsed f slightly brken crals, Amphipra, and subspherical strmatprids; massive Limestne, pale yellwish-brwn, sublithgraphic, with scattered lenses and patches f very pale-range, finely crystalline, argillaceus limestne; scattered calcspnr with a few limnite-hcmaute pseudmrphs as inclusins; znes with "flating"!4-%mm dlmite crystals; stracdes rare; thin-bedded Dlmite, pale yellwish-brwn t pale-range, medium crystalline, subhedral t anhedral, calcareus; grades t very pale-range, sublithgraphic limestne laterally; distinct upper and lwer cntacts; thick-bedded 2.6 Cedar Valley Frmatin Cralville Member 1 Limestne, very pale yellwish-brwn, lithgraphic, sharp-edged cnchidal fracture; sme scattered calcspnr; generally mttled with light yellwish-gray, finely crystalline, argillaceus, dlmitic limestne which weathers readily; hair line fractures cmmn, scaled with calcspan small crals and subspherical strmatprids in upper part; brad undulatins n upper surface; expsed t river level 4.5

68 64 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 2 Thlcknetf asm Unit Mbr. Fm. 4.3 Upper Strmlprld bistrme 5.2 Prtllly lump-cvered Z 4.9 Lwer strmatprid bislrme 4.5 -zrz 6.5 r 4.6 )/V Partially > slump-cvered and vergrwn e> r x Z River level

69 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 65 Sectin 2 TYPE SECTION OF ROCK GROVE MEMBER Measured alng the east bank f the Shell Rck River, in the NW'4. SE'4. SE%, NW%, Sec. 17, T. 96 N., R. 18 W., Flyd Cunty, Iwa. Thick- Unit Descriptin ness (feet) Devnian Syttem Shell Rck Frmatin Nra Member Limestne bistrme, very pale yellwish-brwn with znes f light-gray, dminantly cmpsed f clnies f tabular strmatprids (Aclinslrma expansum); galleries f strmatprids filled with calcspar; irregular small pckets and partings f very pale-urange, finely crystalline, slighlly dlmitic limestne between strmatprid lamellae; slightly undulating upper and lwer surfaces; massive 4.3 Shale, dark yellwish-range, slightly dlmitic, silty; laminated 5.2 Limestne bistrme, very pale-range, cmpsed almst entirely f clnics f tabular strmatprids; irregular znes f pale-range, finely crystalline, slightly dlmitic limestne; base mderately undulating; massive Rck Grve Member Dlmite, grayish-range t dark yellwish-range, medium crystalline, sub hedral t anhcdral, slightly calcareus; prus znes; mlds f Platyrachella ulstcrvnsfs, Schizphria /lydensfs, and crinid ssicles abundant in upper part; weak crss-laminatins in lwer part; sharp lwer cntact; thick- t massivebedded Shale, dark yellwish-range t grayish-range, blcky, dlmitic; well indu-, rated, but weathers t sft clay; prly expsed 6.5 Limestne, grayish-range, finely crystalline, mderately t very argillaceus, slightly dlmitic; weathers t sft blcks; massive 4.6 Limestne, grayish-range in upper, mre weathered tw feet, very pale ycllw lsh-brwn belw, very finely crystalline; stracdes cmmn, a few pelecypds; 0.1- t 0.3-ft layers separated by t 0.15-ft partings f grayish-range, slightly dlmitic shale; distinct upper and lwer cntacts 6.9 Min City Member Limestne, grayish-range and very light live-gray t very pale yellwishbrwn, mttled, finely t medium crystalline; abundant crals, brachipds and crinid fragments; upper surface with rugh irn-stained pits; tw subequal units 3.1 Limestne, pale yellwish-brwn, finely crystalline, mderately argillaceus; weathers t a chippy surface, slightly undercut; cncrinital znes; expsed 5.0 Cvered interval t river level 4.0

70 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 4 Mbr. Fm. Upper atrmalprld bistrme Z Lwer strmatprid bislrme O r a> > X & Quarry flr > Creek bed I I a J5

71 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 67 Sectin 3 TYPE SECTION OF THE NORA MEMBER At the "Junctin" in Nra Springs. NE^. NWM. Sec. 18. T. 96 N.. R. 18 W.. Flyd Cunty, Iwa; sectin n lnger expsed; descriptin after Belanski 1927, p Zne- i^.m^«.i««thickness nnle Descriptin (feet) Devnian System Shell Rck Stage Nra Substage Secnd Actlnstrema Zne Psudtcctus Znule Hard, white limestne in ne massive bed, cmpsed f crwded clnics f Srrmatpra, Actinstrma and ther strmatprids Platyrachlla Zne Unnamed Znule Sft, yellw-brwn, mngneslan shale, distinctly stratified 5.0 First Actinstrma Zne Expansum Znule Hard, white limestne in ne massive bed, cmpsed f laminar strmatp rids, particularly Actinstrma expansum. A cmpund idistrmid cmmn with ccasinal tabulate crals. The upper surface mderately undulating; lwer highly s Rck Grve Substage Schiiphrla Zne Ulsterensts Znule Firm, brwnish, impure dlmite in several layers which increase in thickness upward. Upper prtin cntains abundant casts f Plalyrachclla ulsterensis, Schizphria jlydensls and ther fssils. Base cvered by sd 5.0 REFERENCE SECTION FOR Sectin 4 NORA AND ROCK GROVE MEMBERS Abandned quarry, NE'/4, NE'/j, Sec. 17, T. 96 N R. 18 W Flyd Cunty, Iwa. Unit Descriptin ^teetf48 Devnln System Shell Rck Frmatin Nra Member 8 Limestne bistrme, pale ycllwlsh-brwn, cmpsed f clnies f strmat prids (Actinstrma expansum); clnial grwth ften interrupted by lenses f grayish-range, medium t finely crystalline, slightly argillaceus, dlmitic limestne; styllitcs cmmn; a few Atrypa and small hrn crals; massive, slumped int underlying bed Shale, dark yellwish-range and very light live-gray, dlmitic: thin lenses f dark yellwish-range t grayish-range, sft, argillaceus dlmite; a few mlds f Phtyrachella ulsterensis Limestne bistrme, very pale-range t pale yellwish-brwn; cmpsed f clnies f tabular strmatprids (Actinstrma expansum); mderately undulatry base; massive 4.5 Rck Grve Member 5 Dlmite, yellwish-gray with irn-stained znes ranging frm dark yellwishrange t dark reddish-brwn; finely t medium crystalline, subhedral, slightly calcareus, prus; internal and external mlds f Platyrachlla ulsterensis and Schizphria jlydensis (the mre weathered surfaces shw cavities result ing frm disintegratin f internal mlds); weak t prminent crss-laminae in lwer part marked by differential irn-staining; undercut belw; thick- t massive-bedded Dlmite, light live-gray t dark yellwish-range, medium crystalline, cal careus, very argillaceus; intercalated with light live-gray t dark yellwishrange blcfcy, dlmitic shale; thin-bedded Limestne, very pale yellwish-range t light live-gray, finely crystalline, very argillaceus, mderately dlmitic; weathers t a rugh, pitted surface; massive Limestne, very pale yellwish-brwn t light live-gray,very finely crystalline t sublithgraphic; stracdes cmmn, a few pelecypds and fish plates: 0.1- t 0.3-ft layers separated by live-gray shale partings 4.5 Masn City Member 1 Limestne, pale yellwish-brwn, finely crystalline, slightly argillaceus; Strphednta sp., Pachyphyllum sp., and crinid ssicles abundant; shallw, irnstained pits n upper surface; frms flr f creek.

72 68 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 5 COMPOSITE SECTION Abandned quarry, east bank f Fld Creek, nrth f railrad trestle, and natural expsure suth f the trestle. NE'i. SWyA. SE'4. Sec. 13, T. 96 N.. R. 18 W., Flyd Cunty, Iwa. Descriptin Th,lfCJiVf8s I feet I Devnin System Shell Rck Frmatin Nra Member Limestne bistrme. very pale-range t pale yellwish-brwn, cmpsed dminaihly f clnies f tabular strmatprids and Prismatnhyilum sp. with a few brachipd fragments; cntains znes f grayish-range, argilla ceus, dlmitic limestne; ne resistant, massive ledge, but slumped int underlying bed 3.7 Rck Grve Member Dlmite, grayish-range t dark yellwish-range, finely l very finely crys talline, calcareus, argillaceus, shaly znes; incmplete mlds f Ptatyruclielhi ulsterensis and Schizphria flydensis 0.9 Shale, yellwish-range t grayish-range, slightly dlmitic, sft 4.9 Limestne, very light live-gray t grayish-range, finely t very finely crystal line, argillaceus, shaly znes; very fssiliferus in upper part including valves f Schizphria, Atrypa, and Strphednta, small gastrpds, crinid clumn als, and a few small subspherical strmatprids; weathers t a rugh, knbby surface; massive 5.8 Limestne, very pale yellwish-brwn, weathers t light live-gray and bluishgray, very finely crystalline, subcnchidal fracture: ccurs in 0.1-ft t 0.3-fl layers separated by grayish-range shale partings. This bed is prly expsed in the flr f the quarry: it can be bserved suth f the railrad trestle; expsed t creek 2.5

73 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 69 Sectin 6 ROADSIDE EXPOSURE Measured in east bank f rad ditch. NWI4. SW14, SWyt Sec. 16, T. 96 N.. R. 18 W., Flyd Cunty. Iwa. Descriptin ""(fee 53 Cretaceus System Undifferentiated Sandstne, light reddish-brwn t pale yellwish-range, very carse t very fine sand-sized, majr grade.} t '4mm: cmpsed f subrundcd t curvi linear and runded, weakly plished quartz and chert grains; calcareus cement; irn-stained: scattered fine t medium sand-sized hematite grains; ccurs in 0.1-ft t 0.3-ft brken layers; mderately t very friable; expsed t bt tm f ditch 3.2

74 70 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 7 Mbr. Fm. Lwer ( bistrme strmatprid w «> X. O O rr u x Discntinuity surface c VI 2 ^^ ^-r-y Ouarry flr

75 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 71 Sectin 7 WILLIAMS QUARRY Lcated in E',4. NW14, SW14. Sec. 28, T. 96 N R. 18 W Flyd Cunty, Iwa; measured alng cast face. _. "rilieripun ^eetp Devnian System Shell Rck Frmalin Nra Member 12 Limestne bistrme, very pale yellwish-brwn, cmpsed almst entirely f clnics f tabular strmatprids (/tcfin.iirmci expansum): znes f dark ycllwish-arange lmitic limestne between sme strmatprid lamellae; massive Rck Grv Member Dlmite, dark yellwish-range t pale yellwish-brwn, finely crystalline; a few mlds f Schizphria /iydensis and Ptatyrachella ulsterensis; irnstained; friable; weathers t dlmite sand; thick-bedded Shale, dark yellwish-range, laminated, slightly dlmitic, sft Limestne, mderate yellwish-range t light live-gray, finely crystalline, slightly dlmitic, argillaceus; weathers t a ndular surface; a few micrstyllitcs; massive-bedded Limestne, light live-gray, very finely crystalline, suhcnchklal fracture; 0.1- t 0.3-ft cntinuus layers separated by grayish-range shale partings; stra cdes cmmn, pclccypds present, fish plates rare 2.3 Masn Cily Member 7 Limestne, light live-gray and medium light-gray, weakly mttled, finely crys talline, argillaceus znes; scattered crinid fragments, cncrinital lenses. Strphcdnta sp., Camarphria paupera, Spirifer carilinalis, a few Pachyphyllum sp, subspherical strmatprids; upper surface with shallw irn-stained pits; massive-bedded Limestne, pale yellwish-brwn, mttled with grayish-range, very finely crys talline, argillaceus; scattered crinid fragments, an cncrinital zne 1.3-feet belw tp; massive-bedded Limestne, yellwish-brwn, and live-gray, mttled, finely crystalline: scat tered crinid fragments and a few small subspherical strmatprids; tp f bed marked by a 0.3-ft interval f mttled light live-gray and dark-gray, finely crystalline, argillaceus, pyritic, algalt?) limestne with small pckets f crinidal debris; massive-bedded Dlmite, light live-gray, weathering t grayish-range, medium crystalline, euhedral t subhedral, calcareus, argillaceus, shaly znes; planar upper sur face, very irregular lwer surface filling in and arund knbby limestne f underlying bed Limestne, light brwnish-gray, weathering t pale yellwish-brwn, very finely crystalline; irregular knbby upper surface an intrafrmatinal discntinuity surface with 0.2- t 0.7-fl relief: fauna f attached bicense includes Adcctcyslis williamsi, Sirbilcysliles calvini, Agclacriniles hnveri. Aulpra sp., Hernclia sp., and Spirrbis; spnge(?) brings in limestne are filled with dlmite f bed 4; lwer part f bed cntains brachipds, crinid ssicles, sub spherical strmatprids and Amphipra, crals and gastrpds: relief n tp surface decreases t the suth as the verlying bed increases in thickness; maxi mum measured thickness Limestne, pale yellwish-brwn t light live-gray, sublithgraphic t lith graphic; a few scattered strmatprid fragments: intraclasts in lwer part: "flating" dlmite crystals cmmn; argillaceus at base; thick-bedded Quarry flr dlmite, light live-gray, medium-grained, sub-saccharidal, slightly calcareus, argillaceus znes.

76 72 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 8 Mbt Fm. u K U r -i UJ x JStrmtprld-cral bislrme v> S River level 1^1^ 1 u > *

77 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 73 Sectin 6 ABANDONED MILL SITE (BAUMGARDNER MILL) Left bank f Shell Rck River, west line f SE>4. SV/yA. SWyt. Sec. 28, T. 96 N.. R. 18 W., Flyd Cunty, Iwa. Unit Descriptin '"('iet?'" Devnian System Shell Rck Frmatin Rck Grve Member 12 Limestne, yellwish-range, finely crystalline, dlmitic, very brken beds, weathering t a rugh, pitted surface; thick-bedded argillaceus; Limestne, light live-gray t very pale yellwish-brwn, very finely crystalline, sub-cnchidal fracture; 0.1- t 0.3-ft cntinuus layers separated by grayishrange shale partings; bttm 0.1 ft laminated in '/s-inch layers; stracdes cmmn, pclccypds rare 3.8 Masn City Mmber 10 Limestne, very pale yellwish-brwn, finely t very finely crystalline, with ar gillaceus znes; mderately fssiliferus, including Atrypa sp., Camarphria paupera, Slrphednta sp., crinid fragments, cncrinital znes and a few small subspherical strmatprids; upper surface with shallw irn-stained pits; massive-bedded Limestne, very pale yellwish-brwn t grayish-range, finely t very finely crystalline, slightly argillaceus; scattered crinid fragments, cncrinital znes, brachipd fragments; dlmitic in lwer part; thin- t thick-bedded Limestne, pale yellwish-brwn, very finely crystalline, slightly argillaceus; a few scattered crinid fragments; massive-bedded Limestne, very pale yellwish-brwn, finely crystalline, slightly dlmitic; mttled with grayish-range, very finely crystalline, slightly argillaceus, dl mitic limestne; cncrinital znes and a few small subspherical strmatprids; thin-bedded Limestne bistrme, cmpsed f subspherical strmatprids (averaging fur t six inches in diameter) and a few clnialcrals in a matrix f medium-gray, finely crystalline, argillaceus limestne Limestne, pale yellwish-brwn, sublithgraphic t lithgraphic; "flating" dlmite crystals; intraclasts in lwer 0.3-ft; thin-bedded Dlmite, grayish-range, medium crystalline, slightly calcareus; scattered small calcltc-lined vugs near middle; very argillaceus in lwer part; thin- t thick-bedded, Shale, yellwish-gray t yellwish-range, dlmitic; weakly laminated; sft Dlmite, very pale yellwish-brwn t dark yellwish-brwn, medium crystal line, subhedral, calcareus; sclecdnt fragments in middle; thin-bedded; ter raced tward river 2-4 Cedar Valley Frmatin Cralville Mmber 1 Limestne, pale yellwish-brwn with sme dusky-red mttling, lithgraphic; scattered calcspar; a few calcisphere structures averaging i/8mm in diameter; mttled with pale-range, argillaceus dlmite in tp neft which weathers readily, yielding a rugh, pitted surface; thick-bedded; terraced t water

78 74 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 9 Mbr. Fm Upper strmatprid bistrme Lwer strmatprid bistrme (9 Sectin 9 ROCKFORD SECTION 18 w'vt'd C Winnj1*180 River' west 4lde f brid8c- NEW- Swl/f NWV4. Sec. 15, T. 95 N, R. Unit Descriptin Thickness _ tteett Devnian System Shell Rck Frmatin Nra Member 7 Limestne bistrme, light yellwish-gray t very pale-range, cmpsed al mst entirelyf clnies f tabular strmatprids (Actinstrma expansum): lenses f pale yellwish-brwn, medium grained, slightly dlmitic Hrnestne small Atrypa; massive-bedded ;..' Shale, grayish-range t dark yellwish-range, dlmitic, sft; weakly laml- "* Limestne, ple yellwish-brwn t grayish-range, very finely t finely crystalune; znes with "flating" dlmite crystals: Aulpra cmmn, few scat tered crinid fragments; fucidal markings n bedding planes; thin-bedded Shale, grayish-range t drk yellwish-range, slightly dlmitic; firm iilmslne..b'?5,rsmel v?ry P/^MWTinge tvery pale yellwish-brwn, cmpsed almst entirely f clnies f tabular strmatprids (Actinstrma expan sum); lenses f grayish-range, finely crystalline dlmitic limestne; even upper cntact, undulating lwer cntact; massive-bedded ,8 Rck Grve Member 2 5?J?nU?l.dn? yellwish-range and pale ycllwlsh-brwn, medium crystalline: ^i?fiiwl&.?buj)danj ^S" "I**"0"1 clumnals, numerus mlds YPfatyrchelta baltardl and Schizphria rcfe/rdcnste; Atrypa sp. cmmn- venr few Strbilcystitcs(?) mlds; ne bed , 15 1? f?u^'.yeli0s?j3h'ra!nl5e crystalline; slightly argillaceus, t very weakly P?"e calcareus; yeuwlsh-brwn, priii; finely brken t beds medium 7..TV 2.2 Slump-cvered t river level.

79 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 75 Sectin 10 ROADCUT EXPOSURE Beginning 150 feet west f a tributary t Fld Creek and including the west (right) bank f the tributary; SWcr.. SE>4, Sec. 25. T. 95 N.. R. 17 W.. Flyd Cunty. Iwa. ' _..,, Thickness Unit Descriptin (feet) Devnian System Shell Rck Frmatin Rck Grve Mmbr Limestne, very pale yellwish-brwn, very finelycrystallinet sublithgraphic; stracdes cmmn, a few crinid fragments; 0.1-t 0.3-ft layers with a few pale yellwish-brwn shale partings 6.8 Masn City Member Limestne, grayish-range-pink t very pale yellwish-brwn, finely crystalline; brachipd and crinid fragments, smallgastrpds, small subspherical strma tprids, crals, and bryzans; charphyte gnia rare; tp surface with shallw irn-stained pits 0,8 Limestne, pale yellwish-brwn t light grayish-range, veryfinely crystalline: scattered crinid fragments and encrinital lenses, stracdes cmmn, few brachipds and small gastrpds; fucidal markings n bedding planes; thinbedded; lwer part partially talus-cvered 4-3 Cedar Valley Frmatin Cralville Member Limestne, very ple-range t very pale yellwish-brwn, lithgraphic; abun dant calcisphere structures; scattered calcspar; weakly mttled with patches f dark yellwish-range, finely crystalline, slightly dlmitic, slightly argil laceus limestne in upper three feet; lwer fur feet packed with subspherical strmatprids; thick- t massive-bedded; prly expsed t creek bed Sectin II ABANDONED QUARRY Suth bank f Ackley Creek, SW^. SWi/4, NW^, Sec. 20, T. 94 N., R. 17 W., Flyd Cunty, Iwa. " ~ ~ " ~~" ~"T 7T. ~ Thickness Unit Descriptin (ittit Devnian System Shell Rack Frmatin Masn City Member 3 Limestne, yellwish-range t pale yellwish-brwn, very finely crystalline; slightly dlmitic znes; fssils includescattered crinid fragments, stracdes, Dentafium, and a few brachipd fragments; thin- t thick-bedded Shale, very pale yellwish-range, very calcareus, sft; fssiliferus, including abundant crinid fragments, brachipds (Atrypa reticularis, Schizphria sp.) and numerus small gastrpds; distinct uppercntact, uneven lwer cntact CedarValley Frmatin Cralville Member 1 Limestne, very pale-range t light grayish-range, lithgraphic t sublith graphic; scattered calcisphere structures averaging '^mm in diameter; gastr pds and small subspherical strmatprids cmmn; thick- t massive-bedded; t quarry flr 13.8

80 76 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 12 QUARRY SECTION Suth bank f Dry Run Creek, NE>/t. SEVJ. SW/,. Sec. 7, T. 94 N.. R. 15 W.. Flyd Cunty. Iwa. Unit Descriptin feeu** Devnian System Shell Rck Frmatin Masn City Member 5 Limestne, pale yellwish-brwn t light grayish-range, very finely crystalline, mttled with irregular patches f yellwish-range, finely crystalline, argillace us, slightly dlmitic limestne; crinid fragments scattered thrughut, stra cdes cmmn: thin-bedded, brken Shale, dark yellwish-range, blcky, very calcareus; cntains scattered ndular, yellwish-range t pale yellwish-brwn, finely crystalline limestne; lwer 0.4 ft mre indurated and unifrm; slightly undulating base 2.4 Cedar Valley Frmatin Cralville Member 3 Limestne, very pale yellwish-brwn t pale-range, lithgraphic t sublith graphic, pelletiferus znes; scattered calcspar; numerus stracdes; abundant calcisphere structures averaging '/srnrn in diameter; irregular upperand lwer cntact; massive Limestne bistrme, cmpsed f pale yellwish-brwn subspherical strma tprids in a matrix f grayish-range t very pale-range, very finely crystal line, argillaceus limestne; massive; variable thickness; maximum Limestne, pale-range, lithgraphic, scattered calcspar; numerus stracdes; abundant calcisphere structures averaging %mm in diameter; numerus styllite seams; massive- t thin-bedded with clay partings between beds 5.1 Sctin 13 ROADSIDE AND CREEK BANK EXPOSURE West side f Iwa Highway N. 14, nrth bank f Cldwater Creek, NE'4, NE'4, Sec. 14, T N., 17 W., Butler Cunty, Iwa. f Devnian System Shell Rck Frmatin Rck Grv Member Descriptin ""tftett* Limestne, grayish-range, very finely crystalline t sublithgraphic, in 0.1- t 0.2-ft layers: stracdes present; lwer cntact prly expsed 2.9 Masn City Member Limestne, pale yellwish-brwn, very finely crystalline, mttled with irregular patches f grayish-range, very finely crystalline, slightly dlmitic limestne; abundant crinid fragments, Atrypa sp. cmmn, a few gastrpds and rugse crals; lwer cntact prly expsed 4.4 Cedar Vally Frmatin Cralvlll Mmbr Limestne, very pale yellwish-brwn t pale yellwish-range, lithgraphic, mttled with irregular patches f dark yellwish-range, sft, argillaceus, dlmitic limestne cntaining scattered grains f carse sand-sized, runded, plished quartz; a few calcisphere structures averaging '/smm in diameter; abundant subspherical strmatprids up t six inches in diameter, and Amphi pra; weathers with fssils in relief and exhibits a pitted surface where the sfter, dlmitic limestne has been remved; massive; expsed 2.3

81 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 77 Sectin 14 Iwa. Unit Sectin 14 SHELL ROCK RIVER SECTION East Oeft) bnk f river, SEt/«, NE^, NEW. Sec. 12. T. 96 N., R. 19 W.. Cerr Grd Cunty. Descriptin Thickness (feet) Devnian System Shell Rck Frmatin Nra Member Limestne bistrme, ple-range t paleyellwish-brwn, cmpsedalmsten tirely f clnics f tabular strmatprids (Actinstrma expansum): lenses f dark yellwish-range, sft, argillaceus, dlmitic limestne; ne resistant, massive bed 6-8 Shale, grayish-range, sft, calcareus t slightly dlmitic, blcky t weakly laminated; a few prly preservedmlds f Platyrachclla sp.; variablethickness Dlmite, mderate yellwish-brwn, medium crystalline, subhedral t anhedral, very calcareus, argillaceus; small, irregular vugslined with scalenhedral cal cite; this bed is nly 0.5-ft thick at suthern end f expsure; ne bed; maximum ',4 Shale, as in bed 5, gradatinal with bed 4 lcally; mderately undulating base. variable thickness Limestne bistrme, very pale yellwish-brwn, dminantly cmpsed f cl nies f tabular strmatprids; the tp 0.3 t 0.8 ft is packedwith Amphi pra, fragmented tabular strmatprids and scattered crals; a few Amphi pra scattered thrughut the lwer part, subspherical strmatprids rare; undulatingbase; ne massive bed 4.8 Rck Grve Member Dlmite, mderate yellwish-brwn, medium crystalline, subhedral t an hedral,very calcareus, prus; very fssiliferus, Including mlds f Platyrachella ulsterensis, Atrypa sp., and abundant crinid clumnals; expsed 0.5 Cvered t water 3.1

82 78 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 15 Mbr. Fm. Upper bistrme; tbulr strmtprids Lwer bistrme; tabular and subspherlcl strmatprids and cralt X CO a > s t River level y u rr

83 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 79 SHELL Sectin 15 ROCK RIVER SECTION East (left) bank f river, Ctr. NW^, SW'4, SE^, Sec. 1, T. 98 N., R. 19 W., Cerr Grd Cunty, Iwa. Unit Descriptin '"tfeetf" Devnian System Shell Rck Frmatin Nra Member 6 Limestne bistrme, very pale-range, cmpsed almst entirely f cl nies f tabular strmatprids (Actinstrma expansum); irregular laminae f light-brwn, finely crystalline, argillaceus, dlmitic limestne between strma tprid clnies; mderately undulating base; ne resistant, massive bed Shale, mderate yellwish-brwn, dlmitic, weakly laminated; distinct upper and lwer cntact Limestne, pale yellwish-brwn, very finely t finely crystalline, argillaceus; "flating" medium crystalline dlmite crystals; disseminated carbnaceus(?) flecks; mderately fssiliferus, including crinid clumnals, brachipd frag ments, and a few small subspherical strmatprids; grades t mderate yellwish-brwn and light live-gray, medium crystalline, anhcdral t sub hedral, slightly calcareus dlmite in lwer 0.8 ft; thin-bedded Dlmite, light grayish-range, finely crystalline, anhcdral t subhedral, very argillaceus, slightly calcareus; grades t shale laterally; variable thickness; maximum Limestne bistrme, very pale yellwish-brwn t grayish-range, cmpsed f clnies f tabular strmatprids (dminant), subspherical strmatprids, and a few crals and Amphipra; upper 1.8 feet dlmitic; strmatprids are fragmented and Cladpra sp. abundant; ne massive bed 4.4 Rck Grve Member 1 Dlmite, pale yellwish-brwn, medium crystalline, subhedral t anhedral, very slightly calcareus; few mlds f Platyrachclla ulsterensis and crinid clumnals; weak crss-laminatins brught ut by differential weathering and irn-xide staining; thick-bedded; expsed. 1.4 Cvered t water 6.1

84 80 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 16 Mbr. Fm. } Lwer bislrme; subspherlel strmtprids, Amphlr. crals, tabular strmatprids River level Sectin 16 SHELL ROCK RIVER SECTION West (right) bank f river, Ctr. NW'4, SE^, NW14, Sec. 1, T. 96 N., R. 19 W Cerr Grd Cunty, Iwa. Descriptin Thickness tfeet) Devnian System Shell Rck Frmatin Nra Member Shale, yellwish-range,sft, dlmitic;mixed with yellwish-range, calcareus, argillaceus dlmite 1,7 Limestne bistrme, light grayish-range t pale yellwish-brwn, cmpsed f subspherical strmatprids (dminantly 2"-6" in diameter, a few as large as 14" in diameter), Amphipra, crals, a few clnies f tabular strmatp rids, and brachipds (Trigntreta shellrcfcensis); matrix is finely crystalline, slightly argillaceus, very dlmitic limestne; ne massive, resistant bed... Rck Grve Member Dlmite, yellwish-range t mderate yellwish-brwn, medium crystalline, slightly calcareus; very fine crss-laminatins in upper part; thick-bedded.. Shale, yellwish-range t light live-gray, sft, dlmitic; expsed Cvered t water i '

85 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 81 Sectin 17 RAVINE SECTION Alng bth sidesf ravine. NW14, SW(4, NW14, NE'/ Sec. 1.T. 96 N R. 19W., Cerr Grd Cunty, Iwa. "..,.. Thickness Unit ^ Descriptin qeeti Devnian System Lim Creek Frmatin Juniper Hill Member 6 Shale, light bluish-gray, weathers t light grayish-range; weakly laminated, slightly dlmitic; crushed spre carpsabundant; expsednly at upperend f ravine 2.9 Shell Reck Frmatin Nra Member 5 Limestne bistrme, very light live-gray t very pale yellwish-brwn, cm psed almst entirely f clnies f tabular strmatprids: irregular lenses f grayish-range, very finely crystalline, slightly dlmitic limestne between strmatprid clnies; ne resistant, massive bed Shale, dark yellwish-range, sft, slightly dlmitic Limestne, pale yellwish-brwn, finely crystalline, dlmitic; cncrinital znes, Platyrachclla ballardi cmmn; abundant carbnaceus flecks; grades t dl mite in lwer part; thick-bedded Dlmite, light live-gray, finely crystalline, very argillaceus, calcareus; fucida) casts cmmn; shaly znes; thick-bedded Limestne bistrme, pale t dark ycllwlsh-brwn, cmpsed f subspherical strmatprids in a matrix f dark yellwish-range, finely crystalline, slightly argillaceus, dlmitic limestne; expsed 1.3

86 82 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 18 Thickness (feet) Unit Mbr. Fm Lwer bistrme equivalent i r tu X 7.9 Bedding plane layer f ttrmatpridt 8.3 > S rr < UJ River level

87 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 83 Sectin 18 KEIDLE'S BLUFF SECTION East(left) bnk f Shell Rck River. Ctr. f E^, SWtf. Sec.26.T. 97N.. R. 19W Cerr Grd Cunty, Iwa; measured at nrth end f bluff. Thickness unit Descriptin,feet) Devnian System Shell Rck Frmatin Nra Member Limestne, grayish-range, finely crystalline, slightly dlmitic; fssils are frag mented and include Amphipra, brachipds, and a few abraded subspherical strmatprids averaging fur inches in diameter; the limestne is mttled with patches f pale yellwish-brwn, medium crystalline, calcareus dl mite cntaining scattered, small calcltc-lined vugs; upper surface sd-cvered; ne bed -4 Dlmite, pale t dark yellwish-brwn, medium crystalline, subhedral t anhedral, slightly calcareus; variable prsity; dusky yellwish-brwn carbna ceus partings in lwer part; thick-bedded 2.7 Dlmite, dark yellwish-brwn t grayish-range, medium crystalline, slightly calcareus; prus znes; abundant slightly altered Amphipra in upper part, a few Amphipra and abundant small, subspherical strmatprids belw; massive; lwer surface mderately undulating 2.6 CedarValley Frmatin Cralville Member Dlmite, pale yellwish-brwn, medium crystalline, subhedral, slightly cal careus; very tugh; thick- t massive-bedded; bedding plane with altered twinch diameter subspherical strmatprids fur feet belw tp; beds suc cessively undercut; springs are present at the base 7.9 Dlmite, dark yellwish-brwn, medium crystalline, subhedralt anhedral, cal careus; finely disseminated carbnaceus flecks which yield the dark clr; abundant calcite-fillcd vugs and calcite "stalks", prbably the remains f sub spherical strmatprids and crals, and Amphipra respectively; weathers t a rugh, knbby surface slping ut tward river; massive; t water 8.3

88 84 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 19

89 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 85 Sectin 19 BAKER'S BLUFF SECTION Alng nrth line f SE'/J. SE'/i, Sec. 27, T. 97 N., R. 19 W., Cerr Grd Cunty, lwn. n...^nli.. Thickness Descriptin (feet! Devnian System Shell Rck Frmatin Nra Member Limestne bistrme, very pale yellwish-brwn t pale-red, cmpsed almst entirely f clnies f tabular strmatprids; irregular lenses f grayishrange, finely crystalline, argillaceus, dlmitic limestne between strmat prid clnies; upper and lwer surfaces prly expsed (expsed near head f spring-fed drainage abut 40 feet suth f bluff); massive 4.3 Dlmite, pale yellwish-brwn t grayish-range, medium crystalline, subhedral t anhcdral, calcareus; small, calcitc-lined and filled vugs cmmn in middle; trace f strmatprid fragments; thick-bedded; variable thickness Cedar Valley Frmatin Crlville Member Limestne, grayish-range-pink, sublithgraphic: scattered subspherical strma tprids frm tw t five inches in diameter, numerus Amphipra: calcispheres averaging %mm in diameter cmmn, mre abundant in lwer part; numerus styllitc scams; bedding plane cncentratin f small subspherical strmatprids and Amphipra 2.8 feet belw tp; unit grades t dlmite laterally; massive- t thick-bedded; variable thickness Dlmite, pale yellwish-brwn, medium crystalline, subhedral, slightly t very calcareus; scattered calcite "stalks" are altered Amphipra; thick-bedded Dlmite, dark yellwish-brwn t dusky yellwish-brwn, medium crystalline, subhedral, calcareus: calcite "stalks" in upper part (altered Amphipra), cal cite vugs up t six inches acrss in lwer part (altered strmatprids r crals); disseminated carbnaceus flecks thrughut; massive; weathers t a rugh, knbby surface; t water 3.7

90 86 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 20 Thickness (feet) Unit Mbr. Fm. Rd 2.5 Upper strmtprid bistrme r 9.8 t 9.9 >- UJ S u 3 rr 4 O ui 6.5 Quarry flr

91 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 87 Sctln 20 QUARRY SECTION Hlf a mile nrthwest f Rck Falls; NEi^. SE>/J, SE^. SWft. Sec. 16. T. 97 N.. R. 19 W.. Cerr Grd Cunty, Iwa; cmpsite sectin f west face. Unit Descriptin eetf Devnian System Shell Rck Frmatin Nra Member 4 Limestne bistrme, pale-range, cmpsed f clnies f tabular strmat prids; prly expsed, bserved nly n west side f rad west f quarry; expsed 0.4 Cvered Interval; rad bed, sil Dlmite, pale yellwish-brwn t grayish-range, medium crystalline, sub hedral t anhedral. calcareus; abundant calcite-lined and filled vugs (calci tized strmatprids r crals?); highly fractured and brken in upper nehalf; very argillaceus; thick- t massive-bedded; fills shallw depressin in Cedar Valley; present nly in nrthwest crner f quarry: maximum 9.8 Cedar Valley Frmatin Cralville Member 2 Limestne, very pale yellwish-brwn t grayish-range-pink, sublithgraphic, variably pelletiferus; abundant Amphipra and scattered small subspherical strmatprids; a cnspicuus zne frm 0.6 t 1.0 ft in thickness and 3.5 feet belw tp is crwded with Amphipra and subspherical strmatprids; calcisphere structures averaging '/»mm in diameter ccur thrughut this unit but are mre abundant in the lwer part; upper part grades t dlmite later ally; massive-bedded; variable thickness Dlmite, pale t dark ycllwlsh-brwn, medium crystalline, subhedral t an hedral, slightly calcareus; a few calclte-filled vugs up t three inches in diameter; znes f "vermicular" penings which are remains f calcitized and disslved Amphipra; three t five styliite seams; massive; t quarry flr - 6.5

92 88 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 21 QUARRY NEAR ABANDONED MILL SITE (FOSTER'S MILL) West bank f Shell Rck River; NV/y SW"4, NWi/ Sec. 29, T. 98 N., R. 19 W., Wrth Cunty, Iwa. Unit Descriptin cel!s" Devnian System Shell Rck Frmatin Nra Membr 4 Dlmite, pale range t palc-ycllwish-brwn, medium crystalline, calcareus; abundant calcitc-lincd and filled vugs up t five inches in diameter (altered subspherical strmatprids and crals): "vermicular" penings cmmn (cal citized and disslved Amphipra): upper part very irn-stained: massive, brken beds; variable thickness Dlmite breccia, cmpsed f light live-gray, medium crystalline, subhedral, calcareus, angular dlmite fragments up t ne inch in length; fragments are in a matrix f medium-gray t light live-gray, medium crystalline, calcareus, slightly argillaceus dlmite; tp f unit has a discntinuus zne f medium light-gray, medium crystalline, very argillaceus, finely laminated dlmite shwing cntrted bedding Cedar Vally Frmatin Cralville Mmber 2 Limestne, pale-range, with znes f yellwish-gray t light-gray and grayishrange-pink mttling, sublithgraphic t lithgraphic, variably pelletiferus; "flating" dlmitc(?) crystals, mre abundant in lwer part; abundant calci sphere structures averaging '/nmm in diameter; a few Amphipra; highly ir regular upper surface, distinct lwer cntact; massive-bedded; nt everywhere present Dlmite, very pale yellwish-brwn, medium crystalline, slightly calcareus, subhedral; mderate prsity; sme "vermicular" prsity, the walls lined with carsely crystalline dlmite; massive; t flr f quarry 5.7 Sectin 22 McEACHRAN QUARRY One-half mile suth f Prtland, suth (right) bank f Winnebag River, SW/t. SWi/ NW'4, Sec. 20, T. 96 N.. R. 19 W., Cerr Grd Cunty, Iwa: see figure 17. ttait Descriptin (fee?p Dvnlan 5yslm Shell Rck Frmatin Nra Member 4 Limestne bislrme, very pale-range t grayish-range, cmpsed almst entirely f clnies f tabular strmatprids; lwer part exhibits interrupted clnial grwth f strmatprids as evidenced by alternating layers f strmatprids and limestne; small Atrypa and rugse crals cmmn in upper part; lwer surface very irregular with up t 3 feet f relief and cn frming t depressins in underlying unit: massive Dlmite, pale t dusky yellwish-brwn and live-black, medium crystalline, subhedral, very tugh: abundant calcitc-fillcd vugs frm I inch t 1 ft acrss (calcitized subspherical strmatprids); znes with cncentratins f carb naceus flecks, especially near base: irregular upper and lwer surfaces cn frming t surface f underlying unit; in cntact with unit 1 where unit 2 is absent; massive Cedar Valley Frmatin Cralville Member 2 Limestne, medium-gray t light live-gray and very pale yellwish-brwn, sub lithgraphic, variably pelletiferus; cntains many calcisphere structures aver aging '/Kmm in diameter; subspherical strmatprids and Amphipra scattered thrughut; distinct bedding plane cncentratin f smaller subspherical strma tprids and Amphipra apprximately 2 feet belw tp; numerus styllite scams: massive; unit nt everywhere present alng the east face Dlmite, very pale t dusky yellwish-brwn, medium crystalline, subhedral t euhedral; very tugh; scattered subspherical strmatprids (calcitized). and znes f "vermicular" prsity (result f calcitizatin and slutin f Amphipra): massive- t thick-bedded; tp f this unit frms flr f upper lift; t flr f lwer lift 17.0

93 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 89 Sectin 23 Thickness (feet) Unit Mbr. Fm '^jjsis^ V_ w O z /g&) / s 1 /^jq C _l _l UJ X </> """^flli-. Bedding plne layer ^i-'-j * strmatprids ~-$w _^ ' / 7<- a> '5 w O >- UJ -i _i a: < Q Ul O / s\ River level Sctin 23 PORTLAND RAILROAD TRESTLE ABUTMENT East bank f Winnebag River; SEW, SE'4, NWJ4, SEJ4, Sec. 18, T. 96 N., R. 19 W Cerr Grd Cunty, Iwa. Descriptin Thickness (feet) Devnian System Shell Rck Frmatin Nra Member Limestne bistrme. very pale-range,cmpsed f clnics f tabular strma tprids; distinct lwer cntact; massive 1.6 Dlmite, yellwish-range t pale yellwish-brwn, medium crystalline, sub hedral, calcareus; small calcite-lined vugs averaging 2 inches in diameter are cmmn; slightly brken beds; bradly undulating lwer cntact; thick-bedded 3.6 Cedar Valley Frmatin Cralville Member Limestne, very light-gray t pale-range, sublithgraphic, pelletiferus znes; abundant calcisphere structures averaging %mm in diameter: a few subspheri cal strmatprids and Amphipra; bedding plane cncentratin f small sub spherical strmatprids and Amphipra 2.9 feet abve base; numerus styllite scams; bradly undulating upper and lwer cntacts; thick- t massivebedded 6.8 Dlmite, pale t dark yellwish-brwn, medium crystalline, slightly calcareus; a few subspherical strmatprids 1.2 feet belw tp; thick- t massive-bedded; t water 3.9

94 90 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 24 Thleknett (feel) Unit Mbr. Fm. f S-$3ui \ Upper bistrme; csfellss / lbular strmtprids 3.S 3 / S^i z O O rr _i _j UJ X w I >- Ul _l -1 $ *t "'«K 1 u rr < a Ul *?J>sSJk 1 c/rs Flr f cut Sectin 24 DECKER DRAINAGE CUT Abut 500 feet west f Winnebag River, Near Ctr. suth line SW14, Sec. 34, T. 97 N., R. 20 W., Cerr Grd Cunty, Iwa. Unit Descriptin Thickness feet) Devnian System Shell Rck Frmatin Nra Member Limestne bistrme, cmpsed almst entirely f clnics f tabular strma tprids (pale-range) in a matrix f dark yellwish-range t light-brwn, finely crystalline, slightly argillaceus, dlmitic limestne; massive 3.4 Dlmite, grayish-range t dark yellwish-range, medium crystalline, sub hedral, very calcareus, argillaceus; small calcite-lined vugs and irregular veinlets f calcite: slightly brken beds; massive- t thick-bedded 3.5 Cdar Valley Frmatin Cralville Member Limestne bistrme, cmpsed almst entirely f subspherical strmatprids and Amphipra; strmatprids are very pale-range t very pale yellwishbrwn, in a matrix f grayish-range t very pale yellwish-brwn, finely crys talline, slightly dlmitic limestne; prtins f a few strmatprids are cal citized; mderately undulating upper and lwer surfaces; weathers t a rugh knbby surface; massive 8.6 Limestne, very pale-range t light yellwish-gray, sublithgraphic, variably pelletiferus; abundant calcisphere structures averaging %mm in diameter; trace f Amphipra; numerus styllite seams; "flating" medium crystalline dlmite(?) rhmbs in lwer part; thlck-bcddcd; t water 5.6

95 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 91 Sectin 25 IDEAL QUARRY Nrth side f slugh, SW14. SW'/J, Sec. 35, T. 97 N., R. 20 W., Cerr Grd Cunty, Iwa. UnU Descriptin ^tfeet)" Devnian System Shell Rck Frmatin Nra Member Limestne bistrme, cmpsed f tabular strmatprids (very pale-range) in matrix f grayish-range t dark yellwish-range, medium crystalline, cal careus, dlmite; irn-stained; undulating lwer surface; brken, massive bed 5.7 Dlmite, paleyellwish-brwn t grayish-range, medium crystalline,subhedral, very calcareus, variably argillaceus; irn-stained: small calcite-lined vugs cmmn; undulating upper and lwer surfaces; thick-bedded 4.6 Cder Valley Frmatin Cralville Member Limestne bistrme, cmpsed almst entirely f subspherical strmatp rids and Amphipra; strmatprids are very pale-range, in 0 matrix f grayish-range, finely crystalline, slightly dlmitic limestne; mderately un dulating lwer surface; massive 5.5 Limestne, light grayish-range t light yellwish-gray, sublithgraphic, vari ably pelletiferus; abundant calcispherestructures averaging 14mm in diameter; a few small subspherical strmatprids; Amphipra cmmn; numerus styllite seams; "flating" medium crystalline dlmitc(?) rhmbs in lwer part; thick- t massive-bedded 10.5 Dlmite, pale t dark yellwish-brwn, medium crystalline, subhedral, very slightly calcareus; znes with "vermicular" prsity arc remains f calcitized and disslved Amphipra; scattered calcitized and/r dlmitized subspherical strmatprids; massive- t thick-bedded; t quarry flr 9.7

96 92 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 26 Thickness (feet) Unit Mbr. Fm Upper bistrme; Tbulr strmatprids X. rr UJ x t/> IO.I f "Cement ledge" UJ *- G9?1 //~ rr 2 UJ 10.8 / 0/ > "Masn City beds" -> >- ' -

97 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 93 Sectin 26 WEAVER'S QUIMBY QUARRY Nrth f Winnebag River; NW/t. NW/ Sec 27, T. 97 N., R. 20 W., Cerr Grd Cunty, Iwa. Unit Descriptin (feet) Devnian System Shell Rck Frmatin Nra Member 6 Limestne bistrme, cmpsed almst entirely f clnies f tabular strma tprids (pale-range) in a matrix f dark yellwish-range, finely crystal line, dlmitic limestne; present nly near middle f nrth face; massive Dlmite, dark yellwish-range t very pale yellwish-brwn, medium t carsely crystalline, calcareus; small calcite-filled vugs cmmn: weathers t friable dlmite sand; massive- t thick-bedded; present nly n nrth face Cedar Valley Frmatin Cralville Member 4 Dlmite, similar t bed 5 but with znes f very pale-range t grayish-range sublithgraphic, mderately pelletiferus limestne cntaining numerus calci sphere structures averaging %mm in diameter;abundant Amphipra;small sub spherical strmatprids cmmn; massive Limestne bistrme, cmpsed dminantly f subspherical strmatprids (very pale-range), Amphipra. and a few hrn crals; in a matrix f finely crystalline, slightly argillaceus, dlmitic limestne; massive; irregular upper and lwer cntact Limestne, medium light-gray t yellwish-gray and pale grayish-range, sub lithgraphic, variably pelletiferus; Amphipra cmmn; abundant calcisphere structures averaging Vfemm in diameter; "flating" medium crystalline dl mite^) rhmbsin lwer part, increasing twardbase; numerus styllite seams; thick- t massive-bedded Dlmite, dark t pale yellwish-brwn, medium crystalline, slightly calcareus; scattered calcite-filled vugs in upper part (calcitized subspherical strmatp rids); "vermicular" prsity the result f calcitizatin and slutin f Amphi pra; thick- t massive-bedded; t quarry flr 10.8

98 94 STRATIGRAPHY OF THE UPPER DEVONIAN Sectin 27

99 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 95 Sectin 27 LEHIGH PORTLAND CEMENT COMPANY QUARRY Sectin measured near suthwest crner f quarry; SW'/J, SE'4, Sec. 13, T. 97 N., R. 21 W., Cerr Grd Cunty, Iwa. Unit Descriptin gff" Devnian System Shell Rck Frmatin Nra Member II Limestne, very pale-range t pale grayish-range, very finely crystalline; a few discntinuus styllite seams; fssils include Cyrtspiri/er whitneyi, Atrypa sp., Aulpra sp., charphyte gnia (Trchilfseus sp.), high- and lw-spired gastrpds, a few small subspherical strmatpridsand rugse crals,crinid fragments, and rthceracnic cephnlpds (very rare); sharp lwer cntact; thick-bedded; present nly in suthwest part f quarry 3.8 Cedar Valley Frmatin Cralville Member 10 Dlmite, mderate t dusky yellwish-brwn, medium crystalline, subhedral, very slightly calcareus; a few small calcite-filled vugs; clsely spaced styllite seams in middle; znes with finely disseminated pyrite; mderately undulating base; massive Dlmite, pale t dark yellwish-brwn, medium crystalline, subhedral, very tugh; numerus calcite-filled vugs frm 2 t 5 inches acrss; mderately un dulating upper and lwer surfaces; thick-bedded Dlmite, pale yellwish-brwn, with znes f medium dark-gray mttling, medium crystalline, subhedral, very tugh; a few small calcite-filledvugs; abun dant "vermicular" penings averaging 1.5mm in diameter and 2 inches in length (calcitized and disslved Amphipra); sharp upper and lwer cntacts; massive Dlmite, pale t dusky yellwish-brwn, medium crystalline, subhedral, mder ately calcareus; very tugh; subspherical strmatprids cmmn, averaging 6 Inches in diameter (generally calcitized); highly undulating lwer surface; massive Nte: unaltered lenses f yellwish-gray t medium light-gray, sublithgraphic, calcispherc-bearing limestne ccur within the abve dlmite units. 6 Limestne, yellwish-gray t very pale-range nd medium gray, weakly mt tled, sublithgraphic t lithgraphic; calcisphere structures averaging '/fcmm in diameter cmmn; Amphipra and stracdes abundant, very few small sub spherical strmatprids; znes with "flating" medium crystalline dlmitef?) crystals; irregular upper and lwer cntacts; massive Limestne bistrme. yellwish-gray and dark-gray, mttled, cmpsed f slightly brken subspherical strmatprids and Amphipra in a matrix f pale-green clay: strmatprid fragments frm I t 3 inches acrss; ne dis cntinuus bed Limestne, yellwish-gray t medium light-gray, sublithgraphic; znes with mixed grayish-green clay in upper part; a few 2 t 3 inch-diameter subspherical strmatprids; numerus Amphipra in lwer part; thick-bedded Limestne bistrme, as in bed 5; subspherical strmatprids average 5 inches in diameter; variable thickness; ne bed Limestne, very pale-range t light-gray, sublithgraphic; calcisphere struc tures averaging '/,mm in diameter very abundant: numerus styllite scams; "flating" medium crystalline dlmitef?) crystals (znes with heavier cncen tratins); thick- t massive-bedded; t quarry flr Quarry flr: dlmite, pale t dark yellwish-brwn, medium crystalline, mderately calcareus; trace f scattered carbnaceusf?) flecks.

100 98 STRATIGRAPHY OF THE UPPER DEVONIAN

101 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 97 APPENDIX II WELL LOGS Numbers fllwing wner's name, fr example W-9845, are Iwa Gelgical Survey reference numbers.

102 98 STRATIGRAPHY OF THE UPPER DEVONIAN RUDD TOWN WELL, W-9845, Center nrth line, SW^, Sec. 18, T. 96 N., R. 17 W., Flyd Cunty, Iwa. "ggff1 descriptin gg~ Pleistcene 0-25 Glacial drift 25 Devnian System Shell Rck Frmatin Rck Grve Member Limestne, very pale yellwish-brwn t light live-gray, finely crystalline, mderately argil laceus, dlmitic znes; a few brachipd fragments Limestne, very pale yellwish-brwn, finely crystalline, argillaceus, slightly dlmitic; trace f brachipd and crinid fragments Limestne, pale yellwish-'brwn, "sublith graphic, subcnchidal fracture; trace f stra cde valves 2 Masn City Member Limestne, pale yellwish-brwn t live-gray, very finely crystalline, argillaceus znes; abundant crinid fragments, numerus stra cdes, trace f subspherical strmatprids and charphyte gnia (Trchiliscus sp.) Limestne bistrme(?), very pale yellwishbrwn, cmpsed f subspherical strmatp rids and a few crals (galleries f strmat prids filled with calcite) 4 Cedar Valley Frmatin Cralville Member Limestne, pale-range, weakly mttled with light-gray, lithgraphic t sublithgraphic; a few calcisphere structures frm 1/8 t l/16mm in diameter; scattered stracde valves and very few Amphipra 15

103 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 99 FULLERTON ESTATE WELL, W-11541, SE Cr., SV/y4, SEy4, NWV4, Sec. 30, T. 95 N., R. 18 W., Flyd Cunty. Iwa. "fiff Descriptin ggf Pleistcene 0-5 Glacial drift 5 Devnian System Lime Creek Frmatin Cerr Grd Member 5-55 Limestne, light grayish-brwn, finely crystal line, mderately t very argillaceus; interbedded with light-gray, sft, blcky, calcareus shale; very fssiliferus, including brachi pds, crinid fragments, bryzans and crals; charphyte gnia in lwer ten feet; upper 15 feet highly weathered and xidized 50 Juniper Hill Member Shale, light-gray t light live-gray, blcky t weakly laminated, dlmitic in upper part, cal careus in lwer part; spre carps cmmn in lwer ten feet 65 Shell Rck Frmatin Nra Member Limestne bistrme, yellwish-gray t lightgray, cmpsed f clnies f tabular strma tprids; weak t mderate prsity where galleries f strmatprids are nt filled with calcite 5 Rck Grve Member Dlmite, pale yellwish-brwn, medium crys talline, subhedral, very slightly calcareus Dlmite, greenish-gray, medium crystalline, very argillaceus, shaly znes; grading t shale in lwer part 7

104 100 STRATIGRAPHY OF THE UPPER DEVONIAN Fullertn Estate Well (Cntinued) Interval np _rinh- Thickness (feet) Descriptin (feet) Limestne, light yellwish-gray, very finely crystalline; live-gray shale partings; stra cdes cmmn 1 Masn City Member Limestne, light live-gray t pale yellwishbrwn, finely crystalline, mderately t very argillaceus; scattered crinid and brachipd fragments; finely disseminated carbnaceus(?) flecks 10 Cedar Valley Frmatin Cralville Member Limestne, pale-range t pale yellwishbrwn, very finely crystalline, dlmitic znes; few stracdes; traces f calcisphere structures averaging y8mm in diameter Dlmite, pale yellwish-brwn, finely crystal line, anhedral t subhedral, slightly calcareus, slightly silty; scattered carbnaceus flecks..

105 SHELL ROCK FORMATION' OF NORTH-CENTRAL IOWA 101 STAUDT WELL. W-17056, SE>4, SE>4, NE14. NE>/4, Sec, 28, T. 95 N., R. 17 W., Flyd Cunty, Iwa. Interval T-^,...i *;., Descriptin Thickness (feet)* (feet) Pleistcene 0-35 Glacial drift 35 Cretaceus System Dakta Grup Dakta Sandstne? Sandstne, dark yellwish-range t pale yel lwish-range, very fine t medium sand-sized, very calcareus, argillaceus, very silty, cm psed f subangular t subrunded and runded quartz grains, majr grade 1/8 t l/16mm; weak calcite cement, lcalized irn-xide ce mentatin, friable; interbedded r mixed with clayey, calcareus siltstne; scattered mica flakes Sandstne, pale yellwish-range, very carse t very fine sand-sized, cmpsed f sub runded t runded and subangular frsted quartz grains, and a few slightly plished chert grains; dminantly free, little calcareus ce ment, sme lcalized irn-xide cementatin; small amunt f mixed reddish-range shale in lwer part Shale, reddish-range and pale yellwishrange, very weakly laminated, mderately calcareus; slightly sandy; scattered mica flakes 5 Devnian System Shell Rck Frmatin Masn City Member Limestne, pale-range, very finely crystalline, with argillaceus and slightly dlmitic znes; fssils include crinid fragments, brachipds and a few stracdes 6

106 102 STRATIGRAPHY OF THE UPPER DEVONIAN Staudt Well (Cntinued) Interval TWprinrinn Thickness (feet) Descriptin (feet) Dlmite, light grayish-range, finely crystal line, very calcareus; sme pin-pint prsity; free calcite Dlmite, light grayish-range, finely crystal line, slightly calcareus; small vids result f slutin f fssil fragments; abundant un altered crinid fragments 5 Cedar Valley Frmatin Cralville Member Limestne, very pale-range, lithgraphic; a few calcisphere structures Vsmm m diameter; mttled with pale yellwish-range, finely crys talline, subhedral t euhedral, calcareus dl mite which fills brings(?) and irregular cracks and vids 6

107 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 103 STABENOW WELL, W-8386, NW'/4, SW»4, NW«/J, NW14, Sec. 3, T. 93 N., R. 18 W., Butler Cunty, Iwa. Interval nrwrinhnri Thickness (feet) Descriptin (feet) Pleistcene Glacial drift 100 Devnian System Lime Creek Frmatin Juniper Hill Member Shale, light-gray, blcky t weakly laminated, slightly calcareus; trace f pyrite; spre carps in lwer five feet 20 Cedar Valley Frmatin Cralville Member Limestne, very pale-range, sublithgraphic, with very light grayish-range, dlmitic znes; 40 percent f cuttings are subspherical strmatprids which may be scattered thrughut the bed r may be part f a distinct bistrme Limestne, very light yellwish-gray t white, sublithgraphic, "flating", dlmite crystals; a few strmatprid fragments, and calci sphere structures averaging y8mm in diameter Dlmite, very pale yellwish-brwn, medium crystalline, slightly calcareus, slightly prus; calcite lining small vids Limestne, very pale-range, very finely crys talline t sublithgraphic, scattered "flating" dlmite hedra; mixed and/r interbedded with pale yellwish-brwn medium crystalline, cal careus dlmite; a few strmatprid frag ments 11

108 104 STRATIGRAPHY OF THE UPPER DEVONIAN JOHANNS WELL. W SEV4, SE>/4. SWi/,, Sec. 14, T. 98 N., R. 19 W., Wrth Cunty, Iwa. Interval (feet) Descriptin npsr-rinhnn Thickness (feet) Pleistcene 0-15 Glacial drift 15 Devnian System Cedar Valley Frmatin Cralville Member Limestne, light grayish-range, sublith graphic t lithgraphic, weakly pelletiferus; a few scattered Amphipra; abundant calci sphere structures averaging y8mm in diameter Dlmite, grayish-range, medium crystalline, subhedral t anhedral; calcitic; prus znes Dlmite, very pale yellwish-brwn, finely crystalline, subhedral t anhedral, slightly cal careus; trace f calcspar, prbably derived frm calcite-lined vugs Dlmite, light live-gray, medium crystalline, slightly silty, calcareus, and argillaceus; a few scattered carbnaceus r bituminus flecks Dlmite, light greenish-gray, medium crystal line, silty, slightly calcareus, slightly argil laceus; finely disseminated pyrite Dlmite, pale t dark yellwish-brwn, me dium crystalline, slightly calcareus; trace f pyrite and calcspar Limestne, light yellwish-gray t light-gray very finely crystalline t sublithgraphic; znes with weak utlines f y4 t y6mm dlmite(?) crystals; trace f Amphipra(?) 5

109 SHELL ROCK FORMA'ITON OF NORTH-CENTRAL IOWA 105 MASON CITY, City well 14, W- 8079, NE Crner Sec. 15, T. 96 N., R. 20 W., Cerr Grd Cunty, Iwa. Interval ^ZTZiZTiHi Thickness (feet) Descriptin (feet) Pleistcene 0-10 Glacial drift 10 Devnian System Lime Creek Frmatin Juniper Hill Member Shale, light grayish-range and very light-gray, mixed, lumpy, slightly calcareus; trace f spre carps 9 Shell Rck Frmatin Nra Member Limestne bistrme, very pale-range t very pale yellwish-brwn, cmpsed dminantly f clnies f tabular strmatprids; scattered dlmite crystals in galleries f strmatp rid; highly dlmitic znes between sme strmatprid lamellae Dlmite, pale yellwish-brwn, medium crys talline, slightly argillaceus, calcareus Cedar Valley Frmatin Cralville Member Limestne bistrme, very pale-range t me dium light-gray, cmpsed almst entirely f subspherical strmatprids and Amphipra; subspherical strmatprids are clred very pale-range, Amphipra are generally lightgray; matrix is light-gray, finely crystalline, dlmitic limestne t medium light-gray, me dium crystalline, subhedral, calcareus dl mite

110 106 STRATIGRAPHY OF THE UPPER DEVONIAN Masn City, City Well 14 (Cntinued) Interval np«a»rintinn Thickness (feet) Descriptin (feet) Limestne, yellwish-gray t very pale range, sublithgraphic, variably pelletiferus; calci sphere structures averaging y6mm in diameter very abundant; Amphipra cmmn, trace f subspherical strmatprids; few small vids filled with calcite and minr pyrite; medium crystalline "flating" dlmite(?) crystals a- bundant in lwer 4 feet Dlmite, pale yellwish-brwn t light brwn ish-gray, medium crystalline, subhedral, slightly calcareus; trace f "vermicular" prsity (cal citized and disslved Amphipra) 11

111 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 107 MASON CITY, City Well 12, W-2971, NEJ4, SE^, NE'/J, SE14, Sec. 16, T. 96 N., R. 20 W., Cerr Grd Cunty, Iwa. Interval n-..r _H_n Thickness (feet) Descriptin (feet) Pleistcene 0-7 Glacial drift 7 Devnian System Lime Creek Frmatin Cerr Grd Member 7-13 Dlmite, grayish-range t pale yellwishrange, finely t medium crystalline, anhedral t subhedral, very calcareus, very argillaceus, shaly znes Dlmite, light live-gray t brwnish-gray, finely crystalline, mderately argillaceus, cal careus; finely disseminated pyrite 7 Juniper Hill Member Shale, very light live-gray, blcky, slightly calcareus t weakly dlmitic Shale, as abve, interbedded with medium-gray t light live-gray, finely crystalline, argilla ceus, calcareus, silty dlmite Shale, very light live-gray, sft, lumpy, slighty dlmitic; spre carps in lwer 10 feet 28 Shell Rck Frmatin Nra Member Tabular strmatprid bistrme, grayishbrwn t very pale yellwish-brwn (strma tprids lighter-clred), altered t medium crystalline, slightly calcareus dlmite (str matprid structures still recgnizable); trace f pyrite 7

112 108 STRATIGRAPHY OF THE UPPER DEVONIAN Masn City, City Well 12 (Cntinued) Interval r>a.nrlnnnn Thickness (feet? Descriptin (feet) Cedar Valley Frmatin Cralville Member Subspherical strmatprid bistrme, me dium-gray and pale-range (strmatprids lighter-clred), altered t medium crystalline, subhedral, slightly calcareus dlmite (str matprids still recgnizable); calcite in vugs Limestne, yellwish-gray t very light-gray, sublithgraphic, weakly pelletiferus znes; cal cisphere structures averaging %mm in diame ter very abundant; trace f Amphipra and sub spherical strmatprids; styllite seams; "flating" medium crystalline dlmite(?) crys tals in lwer part Dlmite, pale t dusky yellwish-brwn, me dium crystalline, subhedral; carbnaceus(?) flecks in darker-clred znes; free calcspar Dlmite, light live-gray, finely crystalline, anhedral, argillaceus, slightly calcareus... 4

113 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 109 HEBEL WELL, W-13981, NE14, NEy4, SEy4, NE14, Sec. 25, T. 97 N., R. 20 W., Cerr Grd Cunty, Iwa. Interval -._.,_,.,- Thickness (feet) Descriptin (feet) Pleistcene 0-45 Glacial drift 45 Devnian System Lime Creek Frmatin Cerr Grd Member Limestne, light yellwish-gray, very finely crystalline, argillaceus 1 Juniper Hill Member Shale, very light live-gray t light-gray, blcky, slightly dlmitic; crushed spre carps cmmn in lwer 15 feet 39 Shell Rck Frmatin Nra Member(?) N samples recvered; this interval may be the upper tabular strmatprid bistrme f the Nra Dlmite, dark yellwish-brwn, medium crys talline; trace f subspherical strmatprids; free calcite; this interval is prbably the equivalent f the middle argillaceus beds f the Nra Cedar Valley Frmatin Cralville Member N samples recvered; this interval may be the subspherical strmatprid bistrme f the Cralville

114 110 STRATIGRAPHY OF THE UPPER DEVONIAN Hebel Well (Cntinued) Interval n rrintinn Thickness (feet) Descriptin (feet) Limestne, yellwish-gray, sublithgraphic; packed with calcisphere structures averaging y6mm in diameter; Amphipra rare; styllite seams cmmn; medium crystalline dlmite (?) crystals in lwer part; this is the "cement ledge" lithlgy f the Masn City area Dlmite, very pale yellwish-brwn, medium crystalline, subhedral t anhedral, calcareus; this is the interval f the "Masn City beds" f the Masn City area.

115 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 111 MATSON WELL, W-9308, SEV4, SE^, NEtf, Sec. 19, T. 97 N., R. 21 W., Cerr Grd Cunty, Iwa. interval Descrintin "" Thickness (feet) Descriptin (feet) Pleistcene 0-70 Glacial drift 70 Devnian System Lime Creek Frmatin Cerr Grd Member Dlmite, pale yellwish-brwn, medium crys talline, anhedral t subhedral, slightly calcar eus 5 Juniper Hill Member Shale, very light-gray, weakly laminated t blcky, slightly dlmitic; spre carps cmmn in lwer 10 feet 45 Cedar Valley Frmatin Cralville Member Limestne, light yellwish-gray, sublith graphic; a few stracde fragments and traces f subspherical strmatprids; calcisphere structures averaging y8mm in diameter are cmmn; styllite seams present; this is the "cement ledge" lithlgy f the Masn City area Dlmite, pale yellwish-brwn, and medium light-gray; medium crystalline, subhedral t anhedral, slightly calcareus; scattered car bnaceus^) flecks in upper 5 feet; free calc spar and calcite-lined vugs; this is the interval f the "Masn City beds" f the Masn City area 15

116 112 STRATIGRAPHY OF THE UPPER DEVONIAN

117 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 113 APPENDIX III CORE DESCRIPTIONS

118 Ill STRATIGRAPHY OF THE UPPER DEVONIAN Cre 1 ANDREWS PROPERTY CORE West f Winnebag River, SW>4, NE14, SE14, Sec. 36, T. 96 N., R. 19 W., Cerr Grd Cunty, Iwa; descriptin f beds and crrelatins as recrded by Iwa State Highway Cmmissin, with added ntatins by authr. Unit Descriptin ^^flst* Devnian System Shell Rck Frmatin Nra Member 1 Limestne, light-gray, fine-grained t sublith graphic; hard and sft znes; a mass f laminar strmatprids Shale, buff t greenish-blue, magnesian Limestne, brwnish-gray t gray, mediumgrained t sublithgraphic; earthy texture; numerus hard, black, wavy shale partings; gradatinal base Shale, blue-gray, magnesian Limestne, cream with brwn and black mt tling; a mass f laminar strmatprids; very hard; sme clay arund the strmatprids Rck Grve Member 6 Dlmite and shale: 6A) Dlmite, brwn t dark-gray, finely crystalline; hard; numerus green siltstne inclusins 2.6 6B) Shale, bluish-gray, magnesian; hard, silty; gradatinal cntact at base Dlmite, brwn, with numerus incipient black shale partings; sme lighter clred cal-

119 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 115 Cre 1 (Cntinued) _... Thickness Unit Descriptin (feet) citic znes cmpsed f a fssil hash; fine grained t very finely crystalline; hard; tw massive beds Limestne, light-gray, fine-grained, argillace us; hard; 0.2-ft beds interbedded with darkbrwn, calcareus shale f the same thickness; irregular cntact at base 3.2 Masn City Member 9 Limestne, dark grayish-brwn with white fs sil markings, fine-grained, dlmitic; hard; numerus incipient black shale partings; beds average 2.0 feet thick; numerus brachipds, tetracrals, crinidal znes and a few strma tprids Dlmite, dark-gray, fine-grained, argillaceus; sft; very sharp cntact at base Limestne, cream t light-brwn; almst cm pletely cmpsed f hard massive and laminar strmatprids surrunded by incipient black shale partings 7.1 Nte: The authr believes the strmatprids in this bed are subspherical frms rather than laminar frms. Cedar Valley Frmatin* Cralville Member 12 Limestne, cream, lithgraphic; blue-green clay fills and shale partings giving bed a brecciated appearance; sharp cntact at base Dlmite, dark greenish-gray at tp t lightgray with black clr laminatins at base; fine-

120 116 STRATIGRAPHY OF THE UPPER DEVONIAN Cre 1 (Cntinued) Unit Descriptin c ness grained t very finely crystalline; hard; calcitic; grades int underlying bed Dlmite, blue-gray with lighter clr mttling which gives a brecciated appearance; earthy texture, argillaceus; gradatinal cntact at base Dlmite, dark-brwn with blue-green mt tling; very finely crystalline; very hard; minutely vesicular; lwer 1.0-ft clr lami nated; massive sharp black styllitic seam at base Dlmite, brwnish-gray with varius shades f gray, brwn and cream clr mttling giving a brecciated appearance; medium t fine grained t finely crystalline; hard; numerus fssil vugs; beds average 0.8-ft thick with black styllites between; lwer 3.0 feet with in cluded green clay and finely clastic znes * The authr places the Shell Rck-Cedar Valley cntact at the tp f bed 12; in this descriptin by the Iwa State Highway Cmmissin the Shell Rck-Cedar Valley cntact was placed at the tp f bed 16.

121 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 117 Cre 2 SHANKS PROPERTY CORE Tw miles nrtheast f Prtland, SE14, SE>4, SWi/J, SEy4, Sec. 9, T. 96 N., R. 19 W., Cerr Grd Cunty, Iwa. tt!* n i tian Thickness Unit Descriptin (feet) Pleistcene 1 Glacial drift 36.7 Devnian System Shell Rck Frmatin Nra Member Limestne bistrme, very pale-range, cm psed dminantly f tabular strmatprids; tp 0.2 ft with irregular patches f dusky yellwish-green argillaceus dlmite; massive 7.3 Dlmite, light live-gray, finely crystalline, anhedral, very argillaceus 0.8 Dlmite, dark yellwish-brwn, medium t finely crystalline, subhedral; slightly prus; calcite-filled vug near bttm; thick-bedded Dlmite, dark yellwish-brwn t dark livegray, finely crystalline, argillaceus; calcitelined vugs 2.8 Cedar Valley Limestne Cralville Member Dlmite, pale yellwish-brwn t dark yellw ish-brwn, finely t medium crystalline, sub hedral; scattered subspherical strmatprids and Amphipra; few calcite-filled vugs; numer us styllite seams 15.3

122 118 STRATIGRAPHY OF THE UPPER DEVONIAN BURWELL Cre 3 PROPERTY CORE West bank f tributary t Calmus Creek, NE14, NEy4, NE%, SEV4, Sec. 36, T. 97 N., R. 21 W., Cerr Grd Cunty, Iwa. Unit Descriptin ^ftstt" Pleistcene 1 Glacial drift 3.4 Devnian System Lime Creek Frmatin Cerr Grd Member 2 Dlmite, yellwish-range, medium crystal line, slightly calcareus, argillaceus; brachi pd mlds; sft, brken beds Dlmite, grayish-range, finely crystalline, slightly calcareus, argillaceus; interbedded with bluish-gray shale 6.5 Juniper Hill Member 4 Shale, medium gray t medium bluish-gray, slightly dlmitic; lwer 0.4 ft is medium dark-gray, finely crystalline, argillaceus dl mite with embedded carse- t medium-grained quartz sand and abraded fish teeth (Ptyctdus calcelus 33.3 Shell Rck Frmatin Nra Member 5 Dlmite, medium live-gray t dark yellwishbrwn, medium t finely crystalline; a few brachipds and hrn crals; carbnaceus clay partings; slightly vuggy; massive-bedded Dlmite, medium-gray t light greenish-gray, finely crystalline; abundant Cladpra(J), Am phipra and small subspherical strmatprids

123 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 119 Cre 3 (Cntinued) *r.. r> _i«n «Thickness Unit Descriptin (feet) belw tp 1.2 feet (fssils calcitized except fr uter rim) 4.3 Cedar Valley Limestne Cralville Member 7 Dlmite, very pale yellwish-brwn, medium crystalline; prminent "vermicular" prsity (disslved Amphipra) Dlmite, pale yellwish-brwn, medium crys talline; abundant subspherical strmatprids (calcitized centers, dlmitized rims); styllite seam near tp and base Dlmite, dusky yellwish-brwn t pale yel lwish-brwn, medium crystalline; mttled with burrw(?) markings Dlmite, medium-gray, finely crystalline, very tugh; mderate prsity 0.8

124 120 STRATIGRAPHY OF THE UPPER DEVONIAN Cre 4 MASON CITY BRICK AND TILE CLAY PIT Western edge f Masn City, suth f U. S. 18, SW14, SE^, NW14, SW14, Sec. 8, T. 96 N., R. 20 W., Cerr Grd Cunty, Iwa; cre drilled in flr f pit ne ft abve base f Juniper Hill Shale. Unit Descriptin T^feet?SS Devnian System Lime Creek Frmatin Juniper Hill Shale 1 Shale, medium-gray t very light live-gray, slightly dlmitic; a few crushed spre carps Dlmite, light live-gray, finely crystalline, calcareus, argillaceus; embedded carse t very fine sand-sized quartz grains; abraded fish teeth abundant; sft, elngated phsphatic (?) grains 2mm in length 0.4 Shell Rck Frmatin Nra Member 3 Dlmite, very dark yellwish-brwn and pale yellwish-brwn, mttled, medium crystalline, subhedral, slightly calcareus; packed with dlmitized Amphipra and Cladpra(?); scat tered fragments f tabular(?) strmatprids; massive- t thick-bedded Dlmite, very pale yellwish-brwn and me dium dark-gray, mttled, medium crystalline, subhedral, slightly calcareus; scattered tabu lar strmatprid laminae frm ys inch t y4 inch thick; massive 2.7 Cedar Valley Limestne Cralville Member 5 Dlmite, dark yellwish-brwn in upper part, dark live-gray in lwer part, finely crystalline;

125 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 121 Cre 4 (Cntinued) Unit Descriptin ^SK) scattered carbnaceus flecks; lwer 2 feet fractured and brken; massive- t thickbedded 5.7 Limestne, very light live-gray, carsely crys talline, with abundant embedded quartz sand and silt grains; znes with crss-laminatins; 0.4-ft interval f very light live-gray, sandy shale at tp; irregular upper and lwer cn tacts 4.8 Dlmite, medium-gray and yellwish-gray, mttled, medium crystalline, very calcareus; vuggy; thick-bedded 2.6 Limestne, medium-gray t very light livegray, very finely crystalline, pelletiferus; abun dant calcisphere structures averaging '/8mrn in diameter; styllite seams cmmn; partially dlmitized in lwer 2 feet 7.0 Dlmite, pale yellwish-brwn, finely t me dium crystalline, calcareus; "vermicular" prsity (Amphipra mlds); massive- t thickbedded 5.7

126 122 STRATIGRAPHY OF THE UPPER DEVONIAN STROTHER'S Cre 5 PROPERTY CORE One-half mile nrthwest f Lincln Mill site, NW14, NW«4, SW%, SE14, Sec. 10. T. 97 N.. R. 21 W., Cerr Grd Cunty, Iwa. Unit Descriptin feet?ss Devnian System Shell Rck Frmatin Nra Member 1 Dlmite, pale yellwish-brwn t light-gray, medium crystalline, subhedral 1.1 Limestne, yellwish-gray, sublithgraphic; fssils include Cyrtspirifer whitneyi, Aul pra, Atrypa, gastrpds, crinid ssicles, small subspherical strmatprids, and charphyte gnia (Trchiliscus sp.); a few "flating" medium crystalline dlmite(?) crystals; nu merus styllite seams 5.2 Cedar Valley Frmatin Cralville Member Dlmite, dusky yellwish-brwn t dark yel lwish-brwn, medium crystalline, subhedral, slightly calcareus; alternating laminatins f yellwish-brwn and dark-gray dlmite with carbnaceus(?) flecks in upper part; interca lated greenish-gray clay in lwer 0.6 ft; sharp upper and lwer cntacts 1.7 Dlmite, pale t dark yellwish-brwn, me dium crystalline, subhedral; very tugh; znes f "vermicular" prsity (calcitized and dis slved Amphipra); recgnizable Amphipra in lwer part; styllite seams cmmn; 0.3-ft light live-gray clay seam at base; thick- t massive-bedded 10.3

127 SHELL ROCK FORMATION OF NORTH-CENTRAL IOWA 123 Cre 5 (Cntinued) tt i* r-. ^.i,i^«thickness Unit Descriptin (feet) 5 Dlmite, medium light-gray and light livegray, weakly mttled, medium crystalline, cal careus; massive 3.8 Dlmite, pale yellwish-brwn, medium crys talline, subhedral, prus; massive 2.7 Dlmite, light live-gray and medium darkgray, mttled, medium crystalline, calcareus; clsely spaced styllite seams 2.8 Limestne, medium light-gray and yellwishgray, weakly mttled, sublithgraphic; calci sphere structures averaging ysmm in diameter cmmn in upper part, very abundant in lwer part; scattered small subspherical strmatp rids and Amphipra, a few stracdes; styl lite seams in upper part; thick- t massivebedded 9.1 Limestne, light-gray t yellwish-gray, sub lithgraphic; variably dlmitic, and with znes f pale yellwish-brwn, medium crystalline, subhedral, calcareus dlmite; styllite seams cmmn in upper part; thick- t massivebedded 14.2

128