A Seismic Refraction Survey

Transcription

1 488 S 14.GS: CIR488 ILLINOIS GEOLOGICAL SURVEY LIBRARY z I J c. 1 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A Seismic Refractin Survey f the Meredsia Channel Area f Nrthwestern Illinis L. D. McGinnis P. C. Heigld ILLINOIS STATE GEOLOGICAL SURVEY Jack A. Simn, Acting Chief Urbana, IL CIRCULAR

2 Digitized by the Internet Archive in 2012 with funding frm University f Illinis Urbana-Champaign

3 A Seismic Refractin Survey f the Meredsia Channel Area f Nrthwestern Illinis L. D. McGinnis 1 and P. C. Heigld ABSTRACT The depth and cnfiguratin f the bedrck surface in the Meredsia Channel area f nrthwestern Illinis were determined by seismic refractin surveying and by interpreting brehle infrmatin. The Meredsia Channel, the upper segment f the Princetn Bedrck Valley system, is a drift-filled segment f the pre- Late Wis cnsinan Mississippi River Valley. The mean elevatin f the bedrck surface in the Meredsia Channel is apprximately 450 feet abve sea level. Hwever, a glacially scured grve mre than 100 feet deep, 3.75 miles lng, and 3, 000 feet wide has been delineated in the channel. The elngate grve, riented east-west alng the suth wall f the channel, is cut t an elevatin f 340 feet abve sea level and tapers upward t mean bedrck surface elevatins f 450 feet n the east end. The grve was cut by a Kansan r pre-kansan ice cap mving eastward. Scattered data in the vicinity suggest that there may be additinal grves in the regin. Althugh grves f nearly this magnitude have been nted elsewhere in glaciated terrane, in the Mackenzie Valley, Canada, fr example, they have nt been described in the drift-cvered bedrck valley systems f the mid- cntinent. The presence f a glacial scur feature f this size in a bedrck valley suggests that what has been called the "deep valley stage" f valley develpment in the mid- cntinent is, at least in part, a prduct f glacial actin. Department f Gelgy, Nrthern Illinis University, De Kalb, IL Illinis State Gelgical Survey, Urbana, IL 618OI 1

4 2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 INTRODUCTION The Meredsia Channel, the upper segment f the Princetn Bedrck Valley system in nrthwestern Illinis, is a drift-filled lwland cnnecting the present-day Mississippi River Valley and the Green River Lwland (fig. 1). The channel narrws t 3.75 miles in width and has surface elevatins ranging frm slightly less than 580 feet t 600 feet abve sea level. Bedrck uplands flanking the channel are mantled with glacial drift and less and rise t elevatins f mre than 700 feet. This reprt presents the results f a study f the bedrck tpgraphy in and arund the Meredsia Channel. The study was made in rder t determine whether the bedrck drainage system west f the present Mississippi River was cnnected t that n the east by a deep, preglacial channel as suggested by Hrberg (1950). The study als prvided a basis fr evaluatin f grund-water resurces in nrthwestern Illinis. Acknwledgments This study was supprted by the Illinis State Gelgical Survey, with the cperatin f Nrthern Illinis University, where the senir authr is a prfessr f gelgy. The authrs were assisted in the field by A. R. Swayan and T. E. Jensen. Previus Studies Hrberg (1950) describes the Meredsia Channel as utlining the curse f the preglacial Mississippi River. His arguments are based n physigraphic evidence and well data. His interpretatin, fllwing that f Leverett (1899), is that "the ancient Mississippi River ccupied the upper Mississippi bedrck valley suth t the present upper rapids in nrthern Rck Island Cunty where it turned sutheastward thrugh buried Princetn Valley t the 'Big Bend' f present Illinis River near Hennepin" (Hrberg, 1950, p ) (see fig. 1). Hrberg (1950) states further (p. 45), "The unusually lw elevatin f bedrck at Dubuque, Iwa, less than 300 feet abve sea-level, is f special interest because it suggests the presence f a deep and narrw bedrck channel alng the valley which elsewhere is nt indicated by the subsurface data nw available. " He shws the thalweg f the valley extending thrugh the Meredsia Channel at elevatins f less than 300 feet; hwever, frm subsurface evidence within the study area, Hrberg (195 0) lists bedrck elevatins as 418 feet at Erie, 434 feet at Prphetstwn (Erie and Prphetstwn shwn in fig. 1), and 425 feet in sec. 31, T.19 N., R. 4 E. Hrberg cncludes (p. 49) that "the preglacial Mississippi Valley lies buried either t the west in Iwa r t the east in Illinis, and that nly the eastward curse is supprted by purely physigraphic evidence. Frye (1938) and Trwbridge (1954) prpsed that the main preglacial valley was in central Iwa and that the present Mississippi Valley in sutheastern Minnesta and nrtheastern Iwa resulted frm diversin by Nebraskan ice and ersin t the deep valley stage during later interglacials. Hrberg (1950) favrs a preglacial age fr the deep valley stage. Frye, Willman, and Black (19 65, p. 48 and 49) state, "It seems prbable that the Nebraskan glacier largely determined the psitin f the Ancient Mississippi River thrughut its curse abve the muth f the Missuri River, and less directly, the psitin f the river suthward t the head f the Embayment regin. "

5 A SEISMIC REFRACTION SURVEY ^ t^m Mi*? V$v^' ~'^&~J&?>' t? OregnJL^^ =^.^s ywh tr'^ / ''^P"}g bam t^r Cattail Channellf?,.!^^.^^! ^V ^---A'' fsftt^zi'* -* v '-Be V Meredsia <y p - ;-./^ ; esc Green River.-'- Lwland ^^%l^& & m ' "iyo-.- ^f^s^i^^ n 9 tn ty Ridged Plain ^Channel^VFC^^V -*= f^j-a^ \ \M" w y : 660 PRh.. I inis R., 3, am c^y HFMNFPIN " Miles Fig. 1 - Physigraphy f the Meredsia Channel area, nrthwestern Illinis. The apprximate axis f the buried Princetn Bedrck Valley is als shwn. E, Erie; P, Prphetstwn. (Adapted frm Bier, 1956.) Andersn (1968) recnstructs the entire glacial histry f the area; f the early histry f the Mississippi, he states, "The fact that the curse f the Mississippi River appears t cincide with the eastern limit f the Nebraskan drift nrth f the Rck Island area suggests that this curse may have been determined by Nebraskan ice" (p. 13).

6 4 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 After the preliminary explratin phase f the present study, Swayan (1969) cncluded, n the basis f finding bedrck elevatins in the channel greater than 400 feet abve sea level, that the preglacial Mississippi rute did nt traverse the Princetn Valley system. He suggested that the Clena Channel in Iwa was the preglacial rute and that diversin tk place during Nebraskan glacial advance. McGinnis, Swayan, and Jensen (19 70) cncurred with this interpretatin; hwever, a deep cut (elevatin less than 360 feet abve sea level) fund in the Meredsia Channel by drilling (data discvered after cmpletin f the study by Swayan) smewhat weakened their argument. Areal Gelgy The bedrck gelgy f the Meredsia Channel area is shwn in figure 2. The Meredsia Channel area is underlain by nearly flat- lying Silurian dlmite. Unmapped utliers f Pennsylvanian strata have been reprted by Andersn (1967) alng the Rck River near the suthern limits f the study area. Scattered utcrps f Pennsylvanian strata have als been mapped as far as 10 miles nrth f Erie. The Galena Dlmite and Maquketa Shale Grups f Champlainian and Cincinnatian (middle and upper Ordvician) age, alng with scattered utliers f Silurian dlmite, frm the bedrck surface in the nrthwest part f the study area. Inferences regarding variatins in bedrck lithlgy can be made frm bservatins f seismic velcities as discussed in the sectin n gephysical measurements. These bservatins suggest that the edge f the Pennsylvanian shales is farther nrth than mapped previusly. Structure Meredsia Channel ccupies an east-west trending structural sag. Fster (1956) gives a structural map f the tp f the Galena Dlmite Grup, shwing that the Galena is dipping due suth, abut 30 feet per mile at a pint abut 6 miles nrth f Erie. The tp f the Galena is at apprximately sea level at that pint; therefre, it wuld be at -180 feet at the twn f Erie. Buschbach (19 65) shws the Galena als rising t nearly sea level abut 25 miles suthwest f Erie and dipping apprximately 20 t 25 feet per mile t the nrth n the nrth side f an anticlinal structure n the Galena SEISMIC REFRACTION MEASUREMENTS The present study utilizes available brehle lgs (fig. 3) and 71 seismic refractin prfiles (fig. 4) t better define the bedrck surface and als t aid in the identificatin f bedrck lithlgies and valley- fill materials. A gravity study was als made with the present study; hwever, since the gravity field prved t be unrelated t bedrck tpgraphy in the Princetn Valley area, it was nt included in this reprt. Because f the widespread distributin f shallw aquifers in the lwlands f the Meredsia Channel regin, wells t bedrck frm which bedrck elevatins can be determined are rare. A seismic refractin study, alng with an incrpratin f all available utcrp and well- cntrl data, was deemed the mst useful way t develp a detailed knwledge f the shape f the bedrck lwlands. A thin drift and less cver in the incised uplands des nt lend itself t gephysical surveying; therefre, this study was cnducted nly in the lwlands

7 A SEISMIC REFRACTION SURVEY PENNSYLVANIAN ORDOVICIAN Maquketa Galena 5 Miles R2E R3 E R4 E R5 E R6 E Fig. 2 - Bedrck gelgy f the Meredsia Channel area (frm Willman and thers, 1967).

8 '. q ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 D-l 00'_ D-2 E3 D-4 D-5 O- O Wd 550 r-:0.- '±10]._- -- ': '. 'q _ - III Bulder (Dlmite) CO;. A. O '. -g- '. '. -'. '. ''.'. 500 '? b.: '.6.'. 0) > a> a> </> Bulder -'.'- \- > ) <u ^ 450 Bulder \ ^.y. 6 'p P. 'py\ '.0': b''.. a' O.O. OP- ^ : 0: Sand O- Gravel S TZZ. Shale and clay Dlmite bedrck 0 e Fig. 3 - Graphic lgs f deep drillhles prvided by the U.S. Army Crps f Engineers. Lcatins f the hles are shwn n figure 6.

9 1 A SEISMIC REFRACTION SURVEY STATION r S Distance (ft) STATION Distance (ft) Fig. 4 - Time- distance curves frm seismic statins ver the deepest prtin f the Meredsia Channel. Lcatins f the statins are shwn in figure 5

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 Of the 71 seismic refractin statins ccupied during the curse f the study, 60 were reversed (sht at each end f the gephne spread); the remainder were sht in ne directin nly. Three cntinuus prfiles were sht acrss prtins f the Meredsia Channel, as illustrated in figure 5. A 24- channel Seismgraph Service Crpratin reflectin-refractin seismgraph wned by Nrthern Illinis University was used during the summer f In later studies we used a GEOSPACE 12- channel refractin seismgraph belnging t the Illinis State Gelgical Survey. During the initial stages f the prject, when depths f the rder f 300 feet t bedrck were expected, a 2400-ft gephne spread was utilized. After several shts in the deeper parts f the Meredsia Channel, as mapped by Hrberg (1950), we fund that ft spreads were sufficient t measure depths t bedrck. Sht hles were drilled t 6-ft depths and laded with 2 punds f Nitramn S primer. The sht was generally placed 100 feet frm the first gephne, and gephnes were placed at 100-ft intervals alng the spread. Seismic velcities within 15 feet f the surface were nt measured. Other studies in Illinis have shwn that velcities in the weathered surficial layers average 1200 feet per secnd. This velcity was used t represent the first seismic layer thrughut the survey. Velcities belw the weathered layer t bedrck ranged frm feet per secnd t 6125 feet per secnd; hwever, at nly ne statin was a velcity greater than 6000 feet per secnd recrded (see appendix). The mean velcity f sediment belw water table in the lwlands is 5130 feet per secnd, which is representative f velcities fr saturated sands and gravels. The prevalence f velcities in the 5000 feet per secnd range reflects the absence f higher velcity glacial till in the subsurface f the lwland areas. Bedrck velcities range frm 9000 feet per secnd t 16, 650 feet per secnd. In general, nly ne bedrck velcity was detected at each site; hwever, at five statins bth a lw bedrck velcity at the bedrck surface and a higher velcity smewhat deeper were measured. Lwer velcities may crrespnd t ld glacial tills, weathered dlmite, r pssibly Pennsylvanian shales r sandstnes, whereas the higher velcities indicate the presence f Silurian dlmites. Taking int accunt all reversed refractin statins, table 1 indicates the velcity distributin in the bedrck. TABLE 1 - VELOCITY DISTRIBUTION IN BEDROCK IN THE MEREDOSIA CHANNEL AREA Velcity interval Number f statins recrding 9,000-10,000 ft/sec 2 10,000-11,000 ft/sec 11 11,000-12,000 ft/sec 6 12,000-13,000 ft/sec 15 13,000-14,000 ft/sec 11 14,000-15,000 ft/sec 7 15,000-16,000 ft/sec 7 16,000-17,000 ft/sec 2 Sme f the lwer bedrck velcities were fund in the lwer Rck River drainage basin in the area sutheast f the Meredsia Channel where Andersn (19 67) nted spradic utcrps f Pennsylvanian shale. Velcities in the 9000

11 1 A SEISMIC REFRACTION SURVEY Y cn \ T3 c t cu u 3 CO 01 cu <+H 0) X! c 4-H a O CO CM c «X CO T3 10 G (0 (0 (U c t^ a C E u CO 1 c <D w D) c JD c O c Tj O CU CO XJ CO jq c c C <D c O -M CO 10 X\ 10 *1 "c ra U. c e c CO -a u u c c 2 a Xi (1) -M X3 a> c CO 1+4 c O S-. c S-. -Q O fo CO 2 c ^3 c c > c (0 _4 c e c c > O C +-> O O Xi 00 O 1 O JD r«*.01 _ UO D> -rh PL, 3A3 D9S 9AOqD J93J 'U0liDA9 3

12 10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 t 12, 000 feet per secnd range prbably represent shale bedrck, and therefre Pennsylvanian strata are suspected t lie beneath the lwer Rck River lwland area. Brehles drilled int bedrck in several lcatins in this area encuntered scattered shale and sandstne f Pennsylvanian age. Since bedrck in the uplands surrunding the lwer Rck River basin has been mapped as Silurian dlmite, the presence f Pennsylvanian sediment in the valley bttms was unexpected and may represent depsitin in a pre- Pennsylvanian ersinal uncnfrmity n the Silurian surface Three nrth- suth cntinuus prfiles acrss, r partially crssing, the Meredsia Channel were cmpleted (fig. 5). All f the statins n these prfiles exhibit bedrck velcities in excess f 12, 000 feet per secnd except fr the suthernmst statin f Prfile B-B' and a statin n the suth end f Prfile C-C. These statins f lw- velcity bedrck n dubt indicate the presence f smething ther than Silurian dlmite DISCUSSION In the initial stages f this study, that is, when the 2400-ft gephne spreads were used and the gravity survey was made, it was assumed that Hrberg's (1950) bedrck tpgraphy map f Illinis was essentially crrect in detail. The map f Hrberg, althugh prven ver and ver again t be crrect in its prtrayal f reginal bedrck tpgraphy, des lack the precisin needed fr studies f a lcal nature. With large amunts f new bedrck data cncentrated in small areas t reslve specific prblems, an ccasinal new interpretatin having reginal significance des result. The authrs believe the present wrk intrduces data f this nature. The bedrck tpgraphic map shwn in figure 6 cntains several ntewrthy features: (fig. 7). 1. Bedrck elevatins in the "deep" channels average 450 feet abve sea level; 2. A clsed depressin in the Meredsia Channel is cut mre than 100 feet belw average depths in the channel; 3. The flat-bttmed, steep-walled Cattail Channel is cut t abut 500- ft elevatin; 4. The present Mississippi Channel, nrth and west f Meredsia, is cut t depths similar t thse f the Meredsia Channel and als cntains a deeply cut grve that may be anther clsed depressin; 5. The brad lwland sutheast f Meredsia Channel des nt fit physigraphically with the Meredsia Channel; 6. Lw-velcity bedrck rests n a high-velcity bedrck surface at elevatins f abut 500 feet in the lwer Rck River Valley suth f Meredsia Channel; 7. The east-west trend f the Meredsia Channel is cntinued in the subsurface nrth f the lwer Rck River Valley. The bedrck tpgraphy map f Hrberg is als shwn fr cmparisn

13 A SEISMIC REFRACTION SURVEY 11 5 Miles A Bedrck utcrp Brehle reaching bedrck Brehle nt reaching bedrck - Seismic refractin line Elevatin f bedrck surface T.20N T.19N R.2E R.3E R4E R.5E R.6E Fig. 6 - Bedrck tpgraphy and seismic refractin lines in the Meredsia Channel area. Lines f gelgic crss-sectins (A-A', B-B', and C-C, fig. 5) are shwn. On the basis f the bservatins listed, it can be stated that a deep channel cut belw 300 feet des nt trend thrugh the Meredsia Channel and that there is n evidence anywhere in the area studied f any bedrck surface elevatins less than abut 340 feet abve sea level. Cnsideratin f crustal flexure due t glaciatin set aside fr the mment, bedrck grade elevatin in the Meredsia Channel (thalweg) is nt less than abut 420 feet. The Rck

14 12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 R2E R3 E R4 E R5 E R6 E Fig. 7 - Bedrck tpgraphy f the Meredsia Channel area, frm Hrberg (1950 [1957]). River Bedrck Valley is erded t an elevatin f abut 500 feet, as is the Cattail Channel. These channels, therefre, never ccupied a psitin as prminent as Meredsia Channel in the histry f Mississippi River drainage. The clsed, 100-ft-deep depressin in the Meredsia Channel is abut 3000 feet wide and extends in an east-west directin nearly 4 miles alng the suth wall f the channel. The depressin was nted after preliminary studies

15 A SEISMIC REFRACTION SURVEY 13 were cncluded during the first summer f seismic explratin in The gephysical study had been cmpleted when the recrds frm U.S. Army Crp f Engineers brehles drilled in in the Meredsia Channel were made available t the authrs. A deep hle (D-5 in fig. 3) appeared t cntradict infrmatin gathered during the seismic study. The brehle was drilled mre than 226 feet (bedrck elevatin less than 360 feet abve sea level) int sands and gravels and did nt penetrate bedrck. Subsequent seismic studies made ver the drill hles cnfirmed the presence f abnrmally lw bedrck (elevatin 340 feet), and therefre the seismic study was expanded t explre in mre detail the cnfiguratin f the lw bedrck surface. Cntinuus seismic prfiles were run bth east and west f the brehle, and it was fund that the brehle was inadvertently but frtuitusly placed ver a clsed depressin. Clsed depressins r very narrw deep channels have lng been suspected by staff members f the Illinis State Gelgical Survey; hwever, this reprt is the first dcumentatin f such a feature. Glacial scur f the dlmite bedrck flr f the channel is the mst likely cause f the depressin. Frye (1963), McGinnis (1968), and Willman and Frye (1969) infer that crustal bending als gives rise t anmalus bedrck elevatins; hwever, the depressin nted in the present paper is prbably nt the prduct f crustal bending inasmuch as it extends ver t shrt a distance. Glacial scur f the Meredsia Channel is cnsistent with the glacial histry f the regin. It is suspected that the Meredsia Channel area was entered at least nce by glaciers during each f the fur majr glaciatins. During Nebraskan and Kansan time the valley may have been entered frm the west, whereas during Illinian and Wiscnsinan time it was entered frm the east (Andersn, 1968; Willman and Frye, 1969). Reprts f gravels f Kansan age resting n the flr f the Meredsia Channel seem t indicate that the elngate depressin culd nt have been cut as late as Illinian r Wiscnsinan time (Willman and Frye, 19 69). Glaciated grves n bedrck have been described by Smith (1948) and Wld and Ostens (1966). Smith describes grves up t 300 feet v/ide, mre than 100 feet deep, and up t 8 miles lng in the Mackenzie Valley f nrthwest Canada. They ccur ver a 50 square mile area and, althugh they ccur within the walls f the majr valley, they are nt cntrlled by lcal tpgraphy but by the directin f ice mvement. Smith believes the entire valley was at ne time cvered by an ice sheet mving essentially parallel with the axis f the majr valley. Wld and Ostens (1966) illustrate bedrck valleys in western Lake Superir that may r may nt be clsed depressins. The travel time f reflectins bserved in the valleys indicates that the relief may be as much as 150 t 200 feet and that the valleys are several miles acrss. The rigin f these depressins remains unclear althugh the Lake Superir basin was certainly scured thrugh glacial prcesses. Flint (195 7, p. 83) suggests that where a main valley is cvered by an extensive ice sheet the ice is thicker ver the valley and less impeded by tpgraphic barriers than is ice ver tributaries. Tributaries are left hanging as the main valley is deepened. The pssibility f deepening a bedrck valley flr belw grade thrugh water actin has been discussed by Matthes (1947). He describes a macrturbulence phenmenn in natural streams underging rising river stages; in this phenmenn a vrtex in the stream prduces upward displacement f the river bed. Lifting, accrding t Matthes, ccurs nt nly in bed-lad materials but

16 14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 in resistant beds as well. The vrtex is assciated with a "bil" n the water surface and ccurs alng the axis f the main current r at ther pints where sufficient energy is develped. Matthes claims this phenmenn is active in the present Mississippi River and is an imprtant agency in bed scur and in pthle frmatin. He refers t the prcess as a "klk," which is a term used by Dutch engineers t explain this vrtex type f bed deepening. In the abnrmal deepening prduced by the "klk" prcess, ne might expect a relatively rugh bedrck flr accrding t illustratins shwn by Matthes If the deepening is a result f glaciatin, ne wuld expect the depressin t be runded and smth. Frm the type f data available, that is, seismic refractin prfiles, it is nt pssible t describe with certainty the micrtexture f the bedrck surface; hwever, at statin 71-6 (figs. 4 and 5B), lcated directly ver the depressin, the seismic data suggest that the bttm is smth and therefre prbably was prduced by glacial scur. Statin 71-7 (figs. 4 and 5B), sht immediately suth f 71-6, displays evidence f bedrck ledges having relief f abut 25 feet. Thus, althugh the valley bttm is hrizntal and planar, the suth walls f the valley rise t the surface in steps as they d in many alluviated valleys in carbnate terrane. The nrth wall f the depressin rises gradually in a manner ne might expect t be prduced by glacial scur. Thus bth glacial and water ersin may be represented in the walls f the depressin. The Meredsia Channel and its depressin may have been initially scured by glaciers mving int a valley frm the west in Nebraskan and Kansan time and later mdified by the plucking actin described by Matthes (1947). Seismic statins and wells t bedrck in the present Mississippi channel nrthwest f the Meredsia Channel indicate bedrck elevatins similar t thse fund in the Meredsia Channel, with the exceptin f several wells in and just t the sutheast f Fultn. Here als lw elevatins (-350 feet) are bserved and anther depressin may be present. Seismic studies were nt cnducted in the vicinity f Fultn t verify the nature f this depressin; therefre, an penended lw was cnstructed n the bedrck tpgraphy map. This lw als is lcated in a narrw cnstrictin similar t, but smaller than, the Meredsia Channel and therefre is prbably similar t the Meredsia depressin. Sutheast f the Meredsia Channel, the Princetn Bedrck Valley flattens ut int a brad lwland. This lwland, in general characteristics, predates Pleistcene glaciatin (Hrberg, 1950). It may have been a physigraphic lw develped n a nrthward extensin f Pennsylvanian shales f the Illinis Basin. It prbably wes its develpment t a cmbinatin f factrs, including structure, stratigraphy, and stream ersin. By means f explratin seismic prfiles, which were lcated in the lwer Rck River Valley t further establish bedrck cnfiguratin, unusually lw bedrck velcities were detected. These velcities ranged frm abut 9000 feet per secnd t 12, 000 feet per secnd, cnsiderably less than the velcities bserved elsewhere. An examinatin f the bedrck recrd frm cres in brehles and a review f Andersn's paper (1967) indicate the presence f Pennsylvanian shale utcrps in the lwer Rck River. Since the elevatin f the bedrck surface in the valley bttm is f the rder f 500 feet and uplands flanking the valley cntain Silurian utcrps, an explanatin must be fund fr the implied structure. Shale bedrck is inferred frm seismic data in the lwer Rck River Valley nrthward t seismic statin 72-9 (fig. 5). At this statin a ledge n the bedrck

17 A SEISMIC REFRACTION SURVEY 15 surface is apparent. The flr f the eastward extensin f Meredsia Channel is assciated with higher velcities f Silurian type. It wuld appear that prir t Pleistcene deepening f the Meredsia Channel, the channel was filled with a tngue f Pennsylvanian sediments. Frm the bservatins discussed, ne can attempt a recnstructin f the early develpment f the bedrck valley in Meredsia Channel and its envirns Beginning with the mst recent glacial events, Andersn (19 68) has shwn that glaciers extended int the Meredsia Channel frm the east in bth Wiscnsinan and Illinian time. If Kansan gravels are cntained in the channel, these glaciers culd nt have mdified the bedrck surface in the valleys. On the basis f data presented in this paper, we cnclude that either Kansan r Nebraskan glacial lbes must have extended int Meredsia Channel. A histry f the valley, based partly n cnjecture and partly upn arguments supprted by bservatins f structure, drainage systems, and glaciers, is as fllws. Glaciers frm the west mved in lbes dwn lwland areas (Hrberg and Andersn, 1956). Drainage nrmally flwing int the lwlands must either have been diverted t rutes alng glacial margins r have backed up as ice marginal lakes and spilled ver divides int tributaries f adjacent systems. Channels, such as the Meredsia Channel, were develped acrss divides by stream ersin. Further ice build-up caused glacial advance int tributary systems deepened by trrential flding and high stages. Glaciers advanced int the deepened tributaries and prduced glacially scured features like thse bserved n the flr f Meredsia Channel. The early Pleistcene histry f the Meredsia Channel prpsed here is in essential agreement with the histry prpsed by Frye in Studies made in the last decade n all drainage rutes f the ancient upper Mississippi River system indicate a yuthfulness f the system and the presence f steep- sided valleys cutting thrugh the uplands with sharp-angle turns and hanging tributary valleys. These characteristics are nt representative f a well-develped preglacial drainage system unaltered by glacial diversin. Frye, Willman, and Black (19 65) suggest that the preglacial drainage f the Mississippi system has been s altered that it is n lnger detectable. On the basis f presumed preglacial gemrphlgy, structure, and stratigraphy and with sme cnjecture, it is pssible t infer a preglacial drainage netwrk as has been dne by Hrberg and Andersn (1956). The cnstraint requiring an explanatin fr abnrmal elevatins in bedrck valleys and ften expressed in previus studies is n lnger a valid cnsideratin if it can be presumed that deepening by glacial scur was a factr in the mdificatin f bedrck valleys. SUMMARY Elevatins in majr bedrck valleys f nrthwestern Illinis average 450 feet abve sea level, mre than 150 feet abve thse pstulated by Hrberg (1950). Althugh Hrberg and Andersn (1956) state that master valleys and principal tributaries have gradients descending tward the Gulf f Mexic, the present study, based n detailed measurements in the Meredsia Channel, illustrates that this cnditin des nt exist here. A clsed and elngate east-west depressin lcated alng the suth wall f the channel descends mre than 100 feet belw average Meredsia Channel elevatins.

18 . 16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 488 Frye (1938) suggests that the "deep valley stage" ccurred during interglacials; hwever, it appears that the "deep valley stage" ccurred abut the same time as glacial maxima since the bedrck valley bttm, at least in the Meredsia Channel, is marked by the effects f glacial deepening. Glacial scur nted in this study prbably ccurred during Kansan glacial maxima. The bedrck valley f the Mississippi River west and nrthwest f the Meredsia Channel has elevatins similar t thse in the Meredsia Channel. A clsed depressin is als suggested near Fultn althugh data are insufficient t dcument clsure. Dcumentatin f large-scale glacial grves n bedrck valley flrs in the mid- cntinent intrduces a new type f glacial feature by which the glacial histry f the regin can be interpreted. Grves as wide as that in the Meredsia Channel have nt been reprted elsewhere in the literature. Large-scale grves nt nly require a reinterpretatin f bedrck tpgraphy in the midcntinent but als change quantitatively the presumed dimensins f glacial drift aquifers Evidence f large-scale glacial scur n the bedrck valley flr in the Meredsia Channel suggests that thick Nebraskan r Kansan glaciers mved eastward acrss the present Mississippi Valley in the study area. REFERENCES Andersn, R. C, 19 67, Sand and gravel resurces alng the Rck River in Illinis: Illinis Gel. Survey Circ. 414, 17 p. Andersn, R. C, 1968, Drainage evlutin in the Rck Island area, western Illinis, and eastern Iwa, in Berqstrm. R. E., ed., The Quaternary f Illinis: Univ. Illinis Cllege f Agriculture Spec. Pub. N. 14, Urbana, IL, p Bier, J. A., 1956, Landfrms f Illinis: Illinis Gel. Survey Map. Buschbach, T. C, 1965, Deep stratigraphic test well near Rck Island, Illinis: Illinis Gel. Survey Circ. 394, 20 p. Flint, R. F., 1957, Glacial and Pleistcene gelgy: New Yrk: Jhn Wiley and Sns, Inc., 553 p. Fster, J. W., 195 6, Grundwater gelgy f Lee and Whiteside Cunties, Illinis: Illinis Gel. Survey Rept. Inv. 194, 67 p. Frye, J. C, 1938, Additinal studies n the histry f Mississippi Valley drainage: Unpubl. Ph.D. thesis, Univ. Iwa, Iwa City, IA, 150 p. Frye, J. C, 1963, Prblems f interpreting the bedrck surface in Illinis: Illinis Acad. Sci. Trans., v. 56, n. 1, p. 3-11; Illinis Gel. Survey Reprint J, 9 p. Frye, J. C H. D. Glass, and H. B. Willman, 1962, Stratigraphy and mineralgy f the Wiscnsinan lesses f Illinis: Illinis Gel. Survey Circ. 334, 55 p. Frye, J. C, H. B. Willman, and R. E. Black, 1965, Outline f glacial gelgy f Illinis and Wiscnsin, in Wright, H. E., Jr., and D. G. Frey, eds., The Quaternary f the United States: Internat. Assc. Quaternary Research, 7th Cng.: Princetn, NJ, Princetn Univ. Press, p ; Illinis Gel. Survey Reprint 1965-N, 19 p.

19 A SEISMIC REFRACTION SURVEY 17 Hrberg, C. L(eland), 1950, Bedrck tpgraphy f Illinis: Illinis Gel. Survey Bull. 73, 111 p. Secnd editin f plate 1, Bedrck surface f Illinis, published in by Illinis Gel. Survey. Hrberg, C. L(eland), and R. C. Andersn, 1956, Bedrck tpgraphy and Pleistcene glacial lbes in central United States: Jur. Gel., v. 64, n. 2, p Leverett, Frank, 1899, The Illinis glacial lbe: U.S. Gel. Survey Mn. 38, 817 p. Matthes, G. H., 1947, Ma crturbule nee in natural stream flw: Am. Gephys Unin Trans., v. 28, n. 2, p McGinnis, L. D., 1968, Glacial crustal bending: Gel. Sc. America Bull. 79, p McGinnis, L. D., A. R. Swayan, and T. E. Jensen, 1970, Gephysical studies f the ancient Mississippi River bedrck valley: Gel. Sc. America Abstracts with Prgrams, Nrth-Central Sect. Furth Annual Mtg., v. 2, n. 6, p Smith, H. T. U., 1948, Giant grves in nrthwest Canada: Am. Jur. Sci., v. 246, p Swayan, A. R., 19 69, Seismic studies f the bedrck valley system in nrthwestern Illinis: Unpubl. M.S. thesis, Nrthern Illinis Univ., De Kalb, IL, 67 p. Trwbridge, A. C, 1954, Mississippi River and Gulf Cast terraces and sediments as related t Pleistcene histry - a prblem: Gel. Sc. America Bull., v. 65, pt. 3, July- Sept., p Willman, H. B., and J. C. Frye, 19 69, High-level glacial utwash in the Driftless Area f nrthwestern Illinis: Illinis Gel. Survey Circ. 440, 21 p. Willman, H. B., and thers, 1967, Gelgic map f Illinis: Illinis Gel. Survey Map Wld, R. J., and N. A. Ostens, 19 66, Aermagnetic, gravity, and sub- bttm prfiling studies in western Lake Superir: in Steinhart, J. S., and T. J. Smith, eds., The earth beneath the cntinents: Am. Gephys. Unin Mngraph 10, p

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22 Illinis State Gelgical Survey Circular p., 7 figs., 1 table, lapp., 2000 cp., 1974 Urbana, Illinis Printed by Authrity f State f Illinis, Ch. 127, IRS, Par

23

24 t v'v/:' CIRCULAR 488 ILLINOIS STATE GEOLOGICAL SURVEY URBANA, IL 61801