Soil Relative Dating of Moraine and Outwash-terrace Sequences in the Northern Part of the Upper Arkansas Valley, Central Colorado, U.S.A.

Transcription

1 Arctic and Alpine Research, Vl. 3, N.4, 1998, pp Sil Relative Dating f Mraine and Outwash-terrace Sequences in the Nrthern Part f the Upper Arkansas Valley, Central Clrad, U.S.A. Alan R. Nelsn U.S. Gelgical Survey, MS 966, PO Bx 2546, Denver, Clrad 8225, U.S.A. anelsn@usgs.gv Ralph R. Shrba U.S. Gelgical Survey, MS 913, PO Bx 2546, Denver Federal Center, Denver, Clrad 8225, U.S.A. shrba@usgs.gv Abstract Prfile develpment indices fr sils develped in mraines and utwash near Twin Lakes and in utwash near Leadville supprt the crrelatin f mraines with subdued mrphlgy and tw high utwash terraces with the Bull Lake glaciatin (ca ka) and the crrelatin f hummcky mraines and tw lw utwash terraces with the Pinedale glaciatin (ca ka). Elsewhere in the nrthern part f the upper Arkansas Valley, glacial sequences are crrelated by mapping utwash terraces near the muths f majr tributaries f the Arkansas River. Near Twin Lakes, indices fr sils n lw, uter lateral mraines suggest that the lder Pinedale glaciers extended beynd the margin f high, yunger Pinedale lateral mraines with hummcky tpgraphy. A few subdued mraines near Twin Lakes and Leadville prbably recrd ne r mre glaciatins significantly lder than the Bull Lake. The dwnvalley extent f Pinedale glaciers in the Msquit Range n the east side f the Arkansas Valley is uncertain: mst likely, Pinedale glaciers were almst as extensive as Bull Lake glaciers but built n prminent terminal mraines at their maximum psitins. Intrductin Crrelatin Methds Sme f the best-preserved sequences f mraines and utwash terraces in the suthern Rcky Muntains were depsited in the nrthern part f the upper Arkansas Valley between Leadville and Clear Creek (Fig. I). The ages f these glacial sequences have been debated since 1869, when the nted gemrphlgist, William Mrris Davis, first bserved them as a student (Davis, 195). The stratigraphic nmenclature fr, and the crrelatin f, the sequences have undergne numerus revisins (Shrba, 1977; Nelsn et ai., 1984; Richmnd, 1986a). Our ideas abut the numerical ages and genesis f these depsits are mre detailed than thse f the earliest wrkers in the area, but ur map-unit bundaries differ little frm thse f Westgate (195) and Capps (199). Our wrk in the nrthern part f the upper Arkansas Valley fcused n relative dating f a sequence f mraines (Shrba, 1977) and utwash terraces (Ostenaa et ai., 1981) in the lwer Lake Creek valley near the frmer Twin Lakes, nw Twin Lakes Reservir (referred t belw as "Twin Lakes"; Fig. 2) and n crrelating them with similar landfrms and depsits f the Bull Lake and Pinedale glaciatins elsewhere in the Rcky Muntains (e.g., Richmnd, 1986a; Hall and Shrba, 1993). The mre accurate ages fr depsits that such crrelatins prvide imprve recnstructins f frmer valley glaciers, which are an imprtant aspect f understanding Pleistcene climate change in the Rcky Muntains (Lenard, 1989; Brugger, 1994). The ages als help define the earthquake hazard psed by late Pleistcene faults alng the glaciated eastern flank f the Sawatch Range (Fig. IA) by cnstraining the times when glacial depsits were faulted (Ostenaa et ai., 1981; McCalpin, 1982; Clman et ai., 1985). A preliminary versin f this paper (Nelsn and Shrba, 1984), including much f the supprting data, appeared in an unpublished field guide (Nelsn et ai., 1984). Sil prperties, particularly thse f the B hrizn, are the mst useful criteria fr distinguishing till and utwash in mraines and terraces f different relative age in the upper Arkansas Valley and fr crrelating these depsits and landfrms with thse f the Bull Lake and Pinedale glaciatins elsewhere in the Rcky Muntains (Shrba and Birkeland, 1983; Richmnd, 1986a; Hall and Shrba, 1993). We applied standard methds widely used in sil chrnsequence studies (e.g., Birkeland et ai., 1991) in describing and analyzing sils n the mraines and terraces near Twin Lakes (Fig. 2) and at five ther sites near Leadville and Granite (Fig. IB). We summarized prperties f sil develpment thught t crrespnd mst strngly with landfrm and depsit age (Table 1) using the prfile develpment indices (PDIs) f Harden and Taylr (1983; calculatins dne with an updated versin f the spreadsheet designed by Nelsn and Taylr, 1985) and several ther measures, such as accumulatin indices (AIs; Birkeland and Burke, 1988; Harden et ai., 1991). In rder t make ur indices cmparable t thse fr sils described n the type depsits f the Pinedale and Bull Lake glaciatins in the Wind River Range f Wyming, we used the same prfile thickness (26 cm) and the same prfile prperties (except ph) fr calculatin f indices as did Hall and Shrba (1993; 1995) in their studies f sils n the type depsits; this prcedure added basal hrizn thickness t all but ne f ur sils (Table 1). As in mst ther sil chrnsequence studies, we assume that similarities in sil parent materials, present vegetatin, and present climates (mean annual temperature, abut 2 C; mean annual precipitatin, cm) between the type area f the Pinedale glaciatin n the west flank f the Wind River Range and Twin Lakes imply cmparable lng-term rates f sil develpment (Shrba, 1977). A secndary means f distinguishing mraines and terraces f differing relative age invlved mapping remnants f as many as five distinct utwash terraces (terraces I thrugh 5 n Figs. A. R. NELSON AND R. R. SIIROI3A / 349

2 B A Majr peak i km 1 c '" c: =g "'''' ~.~ c:: INFORMAL REGIONAL ALLOSTRATIGRAPHIC UNITS,--,--, r I r--- 1 } } Depsits f the Pinedale glaciatin Depsits f the Bull Lake glaciatin Depsits f pre-bull Lake age TYPE OF DEPOSIT N Till and related depsits f the Pinedale glaciatin Till and related depsits that pre-date the Pinedale glaciatin ::::::':: Outwash depsits ::.:..:.. f the Pinedale and pre-pinedale glaciatins 2 3 I! km Sil prfile lcatin Recnnaisance prfile (Lcke) FIGURE I. A: Lcatin f the upper Arkansas Valley (lightly shaded area between the Sawatch and Msquit ranges) in central Clrad. Dashed line shws ur infrmal bundary between the nrthern and suthern parts f the upper Arkansas Valley. Darkly shaded area lcates map A within the state f Clrad (inset in A). B: Generalized map shwing Pinedale-age glacial depsits (mstly till), utwash depsits, and pre-pinedale-age glacial depsits, and the relative ages f these depsits in the nrthern part f the upper Arkansas Valley. Additinal sil prfile lcatins (slid dts) near Twin Lakes Reservir 35 / ARCTIC AND ALPINE RESEARCH

3 I and 3) thrughut the nrthern part f the valley (Ostenaa et ai., 1981). By cmparing the relative heights and psitins f mraines and terraces at the muths f the majr tributary drainages t the heights and psitins f the mapped terraces extending suth frm Leadville we crrelated these tributary glacial sequences with the sequence at Twin Lakes (Fig. 2). The crrelatins relied heavily n the interpretatin f aerial phtgraphs, limited bservatins f sils and ther weathering features, and unpublished wrk by W. W. Lcke III, n the sils and glacial depsits near the muth f Half Mn Creek (Fig. IB). Our mapping f the extent f Pinedale glaciers n the west flank f the Msquit Range suth f Leadville is based entirely n interpretatin f aerial phtgraphs. At a few sites near Twin Lakes, Half Mn Creek, and Leadville, we used the degree f subsurface stne weathering and preservatin f mraine mrphlgy t identify bundaries between tills f different ages (Figs. I and 2; as fund by Burke and Birkeland, 1979, Pinedale stnes are little weathered and Bull Lake stnes are cmmnly weathered t grus). In the fllwing discussin we use ur mst detailed evidence-the sils data frm mraines at Twin Lakes and utwash terraces near Leadville-t supprt crrelatins f these depsits and landfrms with similar depsits and landfrms f the Pinedale and Bull Lake glaciatins elsewhere in the Rcky Muntains. We then rely n ur mapping f utwash terraces and mre limited sil and mrphlgy data t crrelate ther mraines and terraces in the nrthern part f the upper Arkansas Valley with thse near Twin Lakes and Leadville (Fig. IB). Stratigraphic Nmenclature Tw different sets f stratigraphic (relative-age) terms are applied t the glacial depsits in the nrthern part f the upper Arkansas Valley. We use the terms "Pinedale," "Bull Lake," and "pre-bull Lake" as relative-age terms fr the glacial depsits cmpsing the mraines at Twin Lakes (Fig. 2; Shrba, 1977) and the utwash terraces and mraines near the muths f ther majr tributary valleys between Leadville and Granite (Figs. 1, 3). These depsits are infrmal allstratigraphic units f the Nrth American Stratigraphic Cde (1983), whereas the terms "Pinedale" and "Bull Lake" als crrespnd with "glaciatins" in the event classificatin scheme prpsed by Jhnsn et ai. (1997). All three terms have been used in the same sense in mst previus studies f glacial depsits in the Rcky Muntain regin (e.g., Meierding and Birkeland, 198; Richmnd, 1986a; Chadwick et ai., 1997). Hwever, we als use numbers that crrespnd with the terraces near Leadville (sequentially numbered, frm ldest t yungest) t indicate a range f relative ages fr depsits whse ages are uncertain. Fr example, utwash terraces 2 and 3 generally crrelate with Bull Lake mraines and utwash terraces 4 and 5 generally crrelate with Pinedale mraines (Figs. IB, lc). Hwever, n the west flank f the Msquit Range, we designate sme mraines that we studied nly n aerial phtgraphs as ranging in relative age frm 2 t 4 (these map units are unshaded n Figure IB because they may have been depsited during either the Pinedale r Bull Lake glaciatins). We have n numerical ages fr glacial depsits in the nrthern part f the upper Arkansas Valley, but n the basis f ur relative-age data we suggest that the majr depsits are bradly crrelative with depsits f the Pinedale and Bull Lake glaciatins as summarized by Meierding and Birkeland (198), Prter et ai. (1983), Richmnd (1986a), Hall and Shrba (1993), and Chadwick et ai. (1997). Studies f the past tw decades using radicarbn, bsidian hydratin, and Uranium-series dating indicate ages f abut 14 t 47 ka fr depsits f the Pinedale glaciatin, 13 t 16 ka fr depsits f the Bull Lake glaciatin, and 3 t 7 ka fr depsits f ne r mre pre-bull Lake glaciatins (Pierce et ai., 1976; Sturchi et ai., 1994; Gillespie and Mlnar, 1995). Recent csmgenic-istpe dating f the surfaces f bulders n mraines in the Wind River Range f Wyming yields ages f ka fr Pinedale mraines (Gsse et ai., 1995), ka fr tw grups f Bull Lake mraines f differing relative age, and> 13 ka fr mraines frm tw lder glaciatins that were previusly mapped as Bull Lake (Chadwick et ai., 1997; Phillips et ai., 1997). Thus, althugh we use the singular "glaciatin" with the terms "Bull Lake" and "pre-bull Lake," each f these glaciatins includes multiple glacier advances f differing relative age. Crrelatin with the early Wiscnsin glaciatin f central Nrth America (abut 6-8 ka) has been suggested fr a few glacial depsits in the Rcky Muntains with weathering r sil characteristics intermediate between thse f adjacent Bull Lake and Pinedale depsits (Prter et ai., 1983; Clman and Pierce, 1986; Richmnd, 1986b; Hall and Shrba, 1995). Althugh a glaciatin f intermediate age cannt be entirely precluded, Phillips et ai. (1997) date the yungest Bull Lake mraines in the Wind River Range as n yunger than 95 ka (Chadwick et ai., 1997). Mraines and Outwash Terraces near Twin Lakes PREVIOUS WORK The early descriptins f the glacial features in the Twin Lakes area (e.g., Hayden, 1874; Capps and Leffingwell, 194; Davis, 195), fllwed by the mre detailed wrk f Westgate (195) and Capps (199), established this area as a classic example f alpine glaciatin in the Rcky Muntains. Later, less detailed studies cncerned primarily with the mraines at Twin Lakes (Ray, 194; Richmnd and Twet, 1965) resulted in further subdivisins. Twet and Case (1972: C 1) identified nine tills at Twin Lakes based n the relative psitins f mraines are shwn n Figure 2. Triangles near Half Mn Creek mark lcatins f recnnaissance prfile descriptins by W. W. Lcke /II (written cmmunicatin, 1975). Map shws utline f Twin Lakes Reservir prir t cnstructin f a new dam; the new reservir submerges lw land between the lakes and arund their shres. C: Explanatin and crrelatin f depsits shwn in B, which are infrmal allstratigraphic units f the Nrth American Stratigraphic Cde (1983). The tenns "Pinedale" and "Bull Lake" glaciatins are used in the sense f diachrnic events, as prpsed by Jhnsn et al. (1997). Outwash depsits f the yungest phases f the Pinedale glaciatin are nt shwn. Numbers shw relative-age grups based n crrelatins with the sequence f terraces nrthwest f Leadville. Depsits with relative-age numbers 2-4 are nt shaded due t uncertainty as t the maximum extent f Pinedale glaciers, particularly in the Msquit Range. A. R. NELSON AND R. R. SHROBA / 351

4 Glacial Depsits Till D D t,;-~j Outwash 1::; :.. :1 Pinedale <.--: ':: glaciatin 1:: :: ::.1 Bull Lake C.::-. glaciatin pre-bull Lake glaciatin Sil prfile lcatin Bulder weathering site Reservir I N \.5 I km 39'5' N FIGURE 2. Map f glacial depsits (primarily till and utwash; Hlcene landslide and alluvial depsits nt shwn) f the Pinedale and Bull Lake glaciatins and depsits f a pre-bull Lake glaciatin in the lwer Lake Creek valley near Twin Lakes Reservir (mdified frm Shrba, 1977). Unshaded areas are reservirs r unmapped, nn/glacial depsits. Data frm weathering sites are discussed by Nelsn et al. (1984, Part I). In the nrthwest cmer f the map near prfiles 1 and 11, a subdued mraine (primarily till?) f prbable pre-bull Lake age (see text) lies between utwash and till f the Bull Lake glaciatin. The psitin f this mraine suggests that the adjacent utwash was depsited frm the Half Mn Creek drainage (Fig. 1B); the utwash may include sme unmapped pre-bull Lake utwash. The map shws the utline f Twin Lakes Reservir prir t cnstructin f a new dam; the new reservir submerges lw land between the lakes and arund their shres. and their interpretatin f u.s. Bureau f Reclamatin drill-hle data. They nted in sme drill hles that "weathered znes" lcally verlain by "black sils" separated fur f the tills. Recnnaissance mapping by Twet and Reed (1973) and Twet et a!. (1978) als differed significantly frm that f earlier studies. Richmnd (1986a, his table 2) crrelates Till N.3 f Twet and Case (1972) with lder Bull Lake mraines, Till N.4 with yunger Bull Lake mraines and the till f the maximum advance f Pinedale glaciers, and Till Ns. 5-7 with Pinedale recessinal mraines. MORAINE MORPHOLOGY AND SURFACE-WEATHERING FEATURES Mraines with tw distinct mrphlgies enclse the basin f Twin Lakes (Figs. 1B, 2). Nrth f the reservir the utermst mraines have wider crests, gentler slpes, fewer surface bulders, and a greater degree f surface-bulder weathering than d the inner, mre prminent mraines (Nelsn et a!., 1984, Part I, table 1). The uter mraines are als mre dissected and cntain far fewer small, clsed depressins than the higher (by 3 m) inner mraines. All previus wrkers have mapped the bundary between these tw grups f mraines in abut the same place (Richmnd, 1986a). Based n mrphlgy and weathering characteristics we assign these mraines t the Bull Lake and Pinedale glaciatins. Suth f Twin Lakes, Shrba's (1977) bundary between tills f the Bull Lake and Pinedale glaciatins differs frm earlier studies except fr that f Westgate (195). Based primarily n the degree f sil develpment (discussed belw), Shrba (1977) placed this bundary suth f the reservir, between the tw lw, subdued, utermst lateral mraine ridges (between sil prfiles 6 and 8, Fig. 2) rather than in the small valley between thse ridges and the high, steep lateral mraines 2 m t the nrth (nrth f prfile 6, Fig. 2). The mrphlgies f the tw subdued ridges are smewhat similar, but the inner ridge has a number f small, undrained depressins that indicate an age similar t that f the high (Pinedale) lateral mraines. We infer frm recnnaissance surface-weathering data cllected n the high and subdued mraines (Nelsn et a!., 1984, Part I, table 1) that the innermst subdued lateral mraine may be nly slightly lder than the high (Pinedale) lateral mraine. The difference in the degree f surface weathering between the innermst subdued mraine and the high mraines is much less prnunced than that between Bull Lake and Pinedale mraines in many ther valleys in the Rcky Muntains (e.g., Meierding and Birkeland, 198; Richmnd, 1986a). AGE-RELATED PROPERTlES OF SOILS ON MORA1NES Surface sils frmed in mrainal depsits (primarily till) f different ages near Twin Lakes (Fig. 2) differ in their degree f develpment f B hrizn prperties (Shrba, 1977). Prperties that are mst strngly crrelated with age are the thickness f the B hrizn, rubificatin index, fur-prperty PDI (rubificatin, melanizatin, texture, and structure), maximum percent clay, prfile-weighted mean f percent clay, and the clay accumulatin index (Table 1; Fig. 4). Sils frmed primarily in till f the Pinedale glaciatin differ smewhat in their degree f develpment (Table 1, Fig. 4). Mst sils have cambic B hrizns r weak argillic B hrizns; ther sils develped n very sandy tiii r that may have been erded (prfiles 4 and 5; e.g., Dahms, 1994) lack B hrizns. B hrizns are best develped at stable sites, where thin less caps till (prfile 6) and at sites where the tiii is silty (prfile 2). The cambic hrizns f prfiles 1 and 2 are abut 2 cm thick and have less than a 2% increase in clay, relative t the C hrizn. These sils have very lw structure and texture indices f I ARCTIC AND ALPINE RESEARCH

5 TABLE J Measures f prfile develpment fr sils n till and utwash in the nrthern part f the upper Arkansas Valley, Clrad" Described PrfIleb depth (cm) B-hrizn thickness (cm)c Prfile Develpment Indices c Fur- Rubifi- Melani- Dry cn- Clay prperty catin Texture Structure zatin sistence ph films PDld Sils n till f the Pinedale glaciatin Clay cntent (%)e PrfIlewt. mean Max AI Max J CaC 3 cntent (%)e Prfilewt. mean AI Sil n utwash f the Pinedale glaciatin >8 f Sils n till f the Bull Lake glaciatin >13(3) >21(69) >51(144) >7(15) >18(66) >63(222) >23(63) _II Sil n till f a pre-bull Lake glaciatin ?> ~ Z m r C/l z > z ~ ~ VJ :I: ;<: c:l >-... w VI w g >75 Sils n utwash f the Bull Lake glaciatin >47(8) >36(64) >65(128) >6(7) >3(61) >12(217) >39(7) 18 Field and labratry methds as described by Birkeland et al. (1991). Dash indicates n data II >8.(16.2) >895(1895) >.5(1.6) >13(427) b Prfiles I thrugh and 1 were riginally designated TL-7, TL-9. TL-IO. TL-IIA. TL-II. TL-12. TL-13. and TL-14. respectively by Shrba (1977) and Nelsn and Shruba (1984). Prufiles and II thrugh 18 were riginally designated , respectively by Nelsn and Shrba (1984)., Cumulative thickness f Bw. Bt. and Btk hrizns. J Prfile develpment index f Harden and Taylr (19X3). Fur-prperty POI is calculated with rubificatin. texture. structure. and melanizatin indices t make the POI cmparable t thse f Hall and Shruba (1995). Fr prfiles that did nt include the C hrizn. we assumed that the B hrizn was < 1 em thick and estimated prperties fr a Cx hrizn assumed t extend t a depth f 26 em (the rest f the added hrizn thickness). Values in parenthe!-.es are thse btained if the B hrizn is assumed t extend t 26 em. e Maximum percentage f clay r carbnate in the m~t clay-rich r carbnate-rich hrizn. prfile weighted means f clay r carbnate percentages in all hrij:ns. and clay r carbnate accumulatin indices (AI; methds described in Harden et al ; Birkeland et al ; Hall and Shrba. 1995). Rman numeral superscripts next t carbnate percentages indicate hrizns with carbnate stage I r II mrphlgy (Birkeland et al ). The minr amunt f secndary carbnate in prfile 1 is prbably due t the frest vegetatin and the likelihd that secndary carbnate was leached frm sils in the upper Arkansas Valley during glaciatins (Machette. 1985)., The ttal thickness f the Bt and Btk hrizns in this sil is prbably nt much mre than 1 cm. because the thickest Bt hrizn that we knw f that is frmed in till f similar age and cmpsitin is 95 em thick (Madle and Shrba. 1979). 'The unusually thin Bt hrizn f prfile II. cmpared t its mrphlgy. suggests that the upper part f the 13 hrizn has been erded. perhaps during the Pinedale glaciatin.

6 I z fi > 34 ~--r=====================~ ~n=ei EXPLANATION LU -' 27 LU 26 S 3 Terrace N. r letter designatin K~ Terrace surface,-\ Outline f mraine Sil prfile lcatin A Vlcanic ash lcality (Sctt, 1975) N lsw EAST FORK MORAINES / TURQUOISE LAKE / / /' MORAINES I ~J 4 (\ 2.A'113 t I \~4~ 3-1 \~ t TWIN LAKES MORAINES 1-2 / 5- LEADVILLE CLEAR CREEK,'I d/ MORAINES t,\. ~ ~ '\ 1\ I \.,: 2.::Ie-... -:=--=-,-- ~ I \ I. r, II 2-3..,.I»>' - -- ~ ~,\ I \ II' -::. Ji" I \ I \ 2-3 I.e- 1\, \~4 \ \ I -- I' I HalfMn Creek Cntinenta~ Divide 5 N I _I Carlyrl Br wrls Creek Cttnwd Creek (based n mapping f Sctt et ai., 1975) (based n mapping f Sctt, 1975) I I FIGURE 3. Lngitudinal prfiles (slid line segments) f alluvial terraces alng the axis f the upper Arkansas Valley. The utline f terminal mraines (dashed lines, pattern) alng the central part f the valley, lcatins f sme f the sil prfiles shwn n Figures 1B and 2, and vlcanic ash lcalities are als shwn. Distance is frm Climax, 16 km nrtheast f Leadville (Fig. 1A). The dashed arrws at the end f the upper prfile and the beginning f the lwer prfile shw where the prfiles wuld jin if they were cnnected. Terraces in the suthern half f the valley were mapped by Sctt (1975) and Sctt et at. (1975). The numbers are infrmal relative-age grups based n dwn-valley crrelatin f the terrace sequence at Leadville (Fig. 1 C). The mrphlgy and surface weathering features f adjacent mraines, and sils n the terraces, suggest the fllwing relative-age crrelatins: 1, pre-bull Lake; 2 and 3, Bull Lake; 4 and 5, Pinedale; N, K, and I refer t the "Nebraskan," "Kansan," and "Illinian" terraces, respectively, f Sctt (1975), which are f middle and early Pleistcene age (Sctt, 1984; Clman et al., 1985; Richmnd, 1986a). Terraces 2-3 near the Twin Lakes and Clear Creek mraines are much higher abve the river than the equivalent terraces t the nrth and suth, prbably because utwash frm Bull Lake glaciers aggraded rapidly whenever glaciers in the valley f Clear Creek, r in a valley 3 km suth f the creek, blcked utwash drainage thrugh the narrw canyn belw Granite (Sctt, 1984; Fig. 1A). and POls f 4-5; prfile 2 cntains the mst clay (AI f 15) and significant calcium carbnate (AI f 256). Argillic B hrizns f prfiles 3 and 6 are f similar thickness, but they have a 3.5% increase in clay relative t Cx hrizns and have similar r higher sil and clay-accumulatin indices than prfiles 1 and 2. Bth types f B hrizns have redder hues and higher chrmas than the underlying unxidized till and have rubificatin indices f By cmparisn, sils in till f the Pinedale mraines at Fremnt Lake, Wyming, cmmnly have smewhat higher values fr mst f these indices (Fig. 4; Hall and Shrba, 1993, 1995). Exceptins are the similar rubificatin indices fr sils at Twin Lakes and Fremnt Lake (means, 13.5 ± 7.6 versus 12.3 ± 4.3) and the much higher carbnate accumulatin indices fr prfiles 2 and 3 at Twin Lakes than thse fr sils at Fremnt Lake. Indices fr sils frmed in till f the mraines with Bull Lake mrphlgy at Twin Lakes (prfiles 8 and 9) indicate strnger sil develpment than fr sils in till f the mraines f the Pinedale glaciatin (Table I, Fig. 4). Mrever, the degree f develpment f the Bull Lake sils is similar t that f sils develped in till assigned t the Bull Lake glaciatin in ther areas f the Rcky Muntains (Shrba and Birkeland, 1983). The Bull Lake sils near Twin Lakes have thick (>45 cm) argillic B hrizns with abslute increases in clay f 11-18%. Indices fr sme prfile prperties, particularly fr prfile 8, are similar t the highest values fr Pinedale sils at Twin Lakes, but structure and ther indices that reflect clay cntent are higher 354 / ARCTIC AND ALPINE RESEARCH

7 X w ~ I- z W ::::.. -.J UJ > w w -.J u::: cc ,, 15 6,, pre-bull , <6\~ Lake glaciatin 1 5, , 6. ", 3 Bull Lake glaciatin 2, 1 9 "5 - \" '?', 7 ~ \ 1 2.';4 \ _ Pinedale glaciatin -r--.---,---,---~-,---, ,, CLAY ACCUMULATION INDEX Arkansas Valley Fremnt Lake till and 6. UDNash FIGURE 4. Plt f the fur-prperty prfile develpment index (rubijicatin, texture, structure, and melanizatin) versus clay accumulatin index (Table 1) fr sils n till (slid circle) and utwash (slid square) in the nrthern part f the upper Arkansas Valley and near Fremnt Lake, Wyming. Numbers crrespnd with thse f sil prfiles in Table I, lcated in Figures I Band 2. Heavy dashed lines indicate pssible bundaries between indices fr sils n depsits f the Pinedale, Bull Lake, and pre-bull Lake glaciatins. Values fr prfiles 9 and 18 are maximums (arrws n prfiles 9 and 18 shw hw true values may be lwer) as explained in Table I. N labratry data are available fr prfile 9, ther prfile characteristics suggest that if data were available, the sil wuld plt in the range fr the Bull Lake glaciatin. Open circles shw values fr the same indices fr sils n Bull Lake and Pinedale mraines at Fremnt Lake, the type area f the Pinedale glaciatin in the Wind River Range (Hall and Shrba, 1995, their tables 3 and 4). High clay accumulatin indices fr sils frmed in till f the Bull Lake glaciatin are prbably due chiefly t the high clay cntent f the till. Open triangles shw indices fr sils n mraines at Fremnt Lake suggested by Hall and Shrba (1995) t date frm an intermediate glaciatin between the Pinedale and Bull Lake; recent cm genic ages fr these mraines suggest an age f ka (Phillips et al., 1997). than thse fr the Pinedale sils. Only minimum indices can be estimated fr prfile 9, because the prfile did nt include the Cx hrizn (Table I). The thick argillic B hrizns, high clay cntents, and the stage II carbnate mrphlgy (Birkeland et ai., 1991) f prfiles 8 and 9 indicate that these sils are much lder than the sils in till f the Pinedale glaciatin at Twin Lakes. Data fr ne prfile suggest that a subdued mraine nrth f Twin Lakes may greatly predate the Bull Lake glaciatin. The clay cntent (28%) and mst sil indices fr prfile 1 (Fig. 2) are twice as high as cntents and indices fr the tw sils n mraines with Bull Lake mrphlgy at Twin Lakes, suggesting a significantly greater age fr this sil (Table 1; Fig. 4). Hwever, nly indices that reflect the percentage f clay are higher than thse fr sils n nearby less-capped utwash that grades int the Bull Lake mraines (discussed belw). Rubificatin and structure indices are significantly higher than the same indices fr sils frmed in the mre clay-rich (12-19% clay) till f the utermst Bull Lake mraines at Fremnt Lake, Wyming (Fig. 4; Hall, 1989; Shrba, 1989; Hall and Shrba, 1995), which may well be lder than many mraines with Bull Lake mrphlgy and relative weathering characteristics in the Rcky Muntains (Chadwick et ai., 1997). All clasts in the B hrizn f prfile 1 except quartzites are cmpletely grussified-additinal evidence f parent material f pre-bull Lake age. Althugh weathering-rind thicknesses f clasts n the subdued mraine are abut the same thickness as thse n Bull Lake mraines, rind thicknesses usually increase very slwly with time n pre-bull Lake depsits (Cleman and Dethier, 1986). At present, we crrelate the mraines at prfiles 8 and 9 with the Bull Lake glaciatin (relative-age grup 2-3, Fig. 1) and assign the mraine at prfile 1 t a pre-bull Lake glaciatin n the basis f its high clay cntent (Table I; Fig. 4). Indices fr prfiles 8 and 9 are sufficiently lw t suggest they may crrelate with the Mraine V sils f Hall and Shrba (1995) n the yungest Bull Lake mraine at Fremnt Lake, Wyming, as riginally mapped by Richmnd (1987). Hall and Shrba (1995) crrelate this yungest Bull Lake mraine with the early Wiscnsin glaciatin (dating frm abut 6-8 ka), but recent csmgenic-istpe dating f Mraine V and crrelative mraines at Bull Lake n the east flank f the Wind River Range yields ages f abut ka (Phillips et ai., 1997). Thus, all subdued mraines at Twin Lakes may als date frm at least 95 ka. Because differences in the degree f sil develpment n lder and yunger Bull Lake mraines at Fremnt Lake imply ice advances f differing relative age (Hall and Shrba, 1995), sme utermst mraines with Bull Lake mrphlgy at many ther Rcky Muntain lcalities may be cnsiderably lder than 13 ka (Chadwick et ai., 1997). Nevertheless, mre detailed mapping f mraines in the upper Arkansas Valley incrprating csmgenic-istpe and ther numerical ages are needed t cnfirm these crrelatins. AGE-RELATED PROPERTIES OF SOILS ON OUTWASH TERRACES Remnants f tw gravelly terraces, which based n their height abve the Arkansas River predate the Hlcene (Terrace Ns. 3 and 4 f Behre, 1933; terraces 4 and 5 f this study), extend discntinuusly alng the river abve and belw the Twin Lakes mraines (Figs. lb, 3). At Balltwn (Figs. IB, 2) terrace 4 (15-25 m abve the river) is largely utwash because it grades int meltwater channels which head in Pinedale mraines t the west. A few prbably crrelative strath terraces at similar elevatins have been cut n bedrck n the east side f the river. The few remnants f terrace 5 in this area (4-1 m abve the river) als cnsist f carse gravel suggesting an utwash rigin during the later part f the Pinedale glaciatin. The sil n Pinedale utwash f terrace 4 (prfile 7, Fig. 2) is much less well develped than the sils n higher utwash terraces. The weak develpment is partly due t the absence f a mantle f less r slpewash, as is present n higher terraces. Prfile 7's indices are similar t thse f the mre strngly develped sils n mraines f the Pinedale glaciatin (prfiles 3 and 5) and the mre weakly develped Pinedale sils n mraines at Fremnt Lake, Wyming (Fig. 4; Hall and Shrba, 1995). N sils were described n terrace 5. The high utwash terraces near Twin Lakes (Terrace N.2 f Behre, 1933; terrace grup 2-3 f Fig. 1) are preserved n the west side f the river, frm Granite nrth t Half Mn Creek (Fig. 3), at an average elevatin f 11 m abve the Arkansas River. The main terrace surface grades int the subdued uter mraines with Bull Lake mrphlgy that enclse Twin Lakes (Fig. 2), a gemrphic relatin that suggests that the terraces were frmed during the depsitin f the mraines. A. R. NELSON AND R. R. SHROBA / 355

8 The sils n the high utwash terraces are generally similar t sils n the Bull Lake mraines (Table 1, Fig. 2). Hwever, mst f the high-terrace sils are develped in a mantle (thicker than the surface A hrizn) f less and underlying less-derived slpewash and/r a mixture f frst-stirred less and utwash, as well as in the underlying utwash. In sme sils the texture f this less mantle is fine-grained and unifrm, but in thers (prfiles 14 and 16) the presence f clasts in the slpewash r frst-mixed sediment requires designatin f a parent material separate frm that f the verlying less and underlying utwash. The grain-size distributin f the matrix f the utwash is similar t that f sandy till. The strng develpment f the cambic B hrizns develped in the less and less-derived sediment f sme sils (prfiles 13, 14 and 16), especially thse that grade with depth int argillic B hrizns r int mre strngly develped cambic hrizns in the underlying utwash parent material, suggests that the lwer layers f less and lessderived sediment d nt significantly pstdate the Pinedale glaciatin. The ldest less n the high utwash terraces may be similar in age t the lder f tw less sheets identified n Bull Lake terraces alng the Clrad River, 1 km t the nrthwest (Shrba, 1994). A much yunger age is indicated, hwever, fr mantles f less-derived sediment at sme sites. A single radicarbn age f 3995 ± 1 I C yr BP (1-11,148) frm a lens f charcal at the base f the mantle f prfile 16 reveals that (1) sme f the less-derived sediment verlying the utwash is mid-hlcene is age, and (2) cambic hrizns can develp in lamy parent material in a few thusand years. The 5-cm-thick cambic hrizn frmed in the less-derived sediment has mderate structure, 7.5YR-hue clr, and 2% mre clay than the underlying C hrizn. Indices fr sils n the high utwash terraces (prfiles 13, 14, 15, and 16; Table 1) are similar t r higher than thse fr the sils n the Bull Lake mraines (Fig. 4). This is prbably due t the higher clay cntent and higher misture-hlding capacity f their less and less-derived mantles. PDIs range frm 34 t 61, similar t the range fr Bull Lake sils at Fremnt Lake, Wyming (Hall and Shrba, 1995). Indices fr prfile 11 are much lwer; its argillic B hrizn is s thin (8 cm thick) that we infer that it was erded during the Pinedale glaciatin. B hrizns n the ther less-and-utwash sils are cm thick, have clay cntents f 1-2%, and clay accumulatin indices f ; clay films in pres and cating clasts indicate clay illuviatin (Table 1). Carbnate mrphlgy appraches stage III in prfiles 14 and 16, and carbnate accumulatin indices range frm 216 t 2517, encmpassing the range fr carbnate AIs at Fremnt Lake. Mrphlgic features bserved in thin sectins indicate engulfment f the lwer part f the argillic B hrizns by secndary carbnate (Fig. 5); similar features have been attributed t a change frm mist climate during glaciatins t significantly drier misture regimes during interglaciatins (Reheis, 1987). Grussificatin f all clasts except quartzites in a sil n a strath terrace well belw the high utwash terraces (prfile 12, Fig. 2, abut 45 m abve the river) suggests that the sil parent material is at least middle Pleistcene in age (pre-bull Lake). Sil indices, hwever, indicate that the sil may be abut the same age as the sils n the high utwash terraces (prfiles 13, 14, 15, and 16; Table 1); perhaps the sil develped n a strath cut by the river int pre-bull Lake alluvium near the end f the Bull Lake glaciatin after the river had incised t a level belw that f the high terraces. FIGURE 5. Thin sectin (lox, plarized light) f the 3Bt3bk hrizn f prfile 15 n Bull Lake utwash suth f the Twin Lakes mraines (Fig. 1B) shwing clay films (labeled "f") cating sand grains and yunger carbnate (labeled "c") cating the clay films. Mraines and Terraces f Bull Lake Age near Leadville PREVIOUS WORK Capps and Leffingwell (194) and Capps (199) recgnized evidence fr tw distinct perids f glaciatin in the Leadville area where they identified and mapped tw ages f glacial depsits with different surface mrphlgy and tw levels f terrace gravels (Fig. IB). Emmns et al. (1927) supplemented the wrk f Capps (199) with additinal descriptin f the surficial depsits near the mines f the Leadville area. Behre (1933) discussed five alluvial terraces in the Leadville area, but did nt publish any detailed mapping. Ray (194) later crrelated mraines in the tributary valleys near Leadville with his glacial sequence in the Clrad Frnt Range based n his limited recnnaissance study in the Leadville area. In remapping the glacial depsits f the Leadville mining district, Behre (1953) generally supprted the earlier wrk f Capps (199) and Emmns et al. (1927), but like Ray (194), assigned mst depsits t the Wiscnsin glaciatin. Twet's (1974) extensive mapping and gelgic studies in the Leadville area resulted in nly a general subdivisin f the Quaternary depsits. AGE-RELATED PROPERTIES OF SOILS ON OUTWASH TERRACES We use the sils develped n tw utwash terraces 4 km nrthwest f Leadville, which can be traced t mraines in the valley f the East Frk f the Arkansas River (terraces 2 and 3, Fig. IB), t crrelate bth the terraces and mraines with the Bull Lake glaciatin. The sils n the terraces are develped in a mantle f less, less-derived slpewash r frst-stirred sediment, and/r underlying, very gravelly utwash. Sil prfile 18 n the highest terrace (terrace 2 f Figs. IB and 3) has indices like thse f the sils n the utwash terraces that grade int the Bull Lake mraines at Twin Lakes (Table 1). The argillic B 356 / ARCTIC AND ALPINE RESEARCH

9 hrizn f the sil is >75 cm thick with an abslute 1% clay increase relative t the A hrizn (the C hrizn was nt expsed) and has mderately thick clay films that line pres and cat clasts. Althugh this sil has acid sil phs, as wuld be expected at its mist, frested lcatin, the lwer 3 cm f its argillic B hrizn has stage I carbnate mrphlgy. Mst f ur sils that cntain calcium carbnate are at dry, sagebrushcvered sites abut 2 m lwer and have alkaline sil phs (Fig. 2; Shrba, 1977). Thus, the stage I carbnate may be evidence f an earlier climate, drier than present. Our terrace 3, which crrespnds t much f Terrace N. 2 f Behre (1933), is 12 m lwer than ur terrace 2. Althugh the argillic B hrizn f prfile 17 n terrace 3 is nly 39 cm thick, it has the highest rubificatin index (117) and POI (7) f any f ur sils, 8% mre clay than the underlying Cx hrizn, and mderately thick clay films that line pres and cat clasts (Table 1). A nearly identical sil was bserved in a 2-m-deep excavatin in an utwash channel graded t terrace 3 west f Leadville. The indices fr the sils n terraces 2 and 3 indicate that the sils are crrelative with thse n Bull Lake mraines and utwash terraces near Twin Lakes and with sils f the Bull Lake glaciatin in the Wind River Range (Fig. 4). Althugh terrace 3 is lwer than and therefre yunger than terrace 2, the similarities in indices between the sils d nt indicate an age difference as large as that suggested by Hall and Shrba (1995) between the Bull Lake and Mraine V (yungest Bull Lake) mraines at Fremnt Lake, Wyming. MORAINE MORPHOLOGY AND RELATIONS OF OU1WASH TERRACES WITH MORAINES Terrace 2 grades int subdued mraines n bth sides f the Arkansas River nrthwest f Leadville (Fig. IB). Neither surface bulder-weathering data nr sils data have been cllected n mraines in the Leadville area; hwever, the gentle slpes, wide crests, and few undrained depressins n the subdued mraines are typical f mraines f Bull Lake age r lder. Twet (1974; 1978, p. 18) suggested that the utermst f these mraines was f pre-bull Lake age, but an expsure near the east end f the utermst mraine shwed sil develpment and clast weathering mre typical f depsits f Pinedale age. Because a mraine f Pinedale age culd nt lie dwn valley frm Bull Lake r lder mraines, the crest f this mraine must have been erded. We assign these uter mraines t the Bull Lake glaciatin n the basis f indices fr prfile 18 n terrace 2 (Table 1). Immediately upvalley frm the mraines crrelated with terrace 2, terrace 3 grades int smaller mraines which have slightly steeper slpes and a few mre undrained depressins than the uter mraines t the west (Fig. IB). Althugh Capps (199) mapped these mraines within his depsits f the last glaciatin, sils data frm the adjacent terraces d nt suggest a substantial age difference between the terrace-3 and the terrace-2 mraines. Unfrtunately, we have neither rck-weathering nr sils data frm these mraines. We are uncertain whether r nt the lateral mraines and subdued glacial depsits that mantle the bedrck abve the Pinedale mraines and utwash nrth and east f Leadville accumulated during the depsitin f terraces 2 r 3 r during an earlier pre-bull Lake glaciatin. Hwever, utwash depsits that grade int the mraines nrth f Leadville als grade int slightly hummcky mraines at the muth f Evans Gulch (labeled 2-4, Fig. IB). Because the utwash surfaces at abut the same elevatin as terraces 2 and 3 grade int the mraines f similar mrphlgy depsited in the valleys f the East Frk and Evans Gulch nrth and east f Leadville, we infer that bth mraines are abut the same age and that they prbably were depsited during the Bull Lake glaciatin. We discuss the pssibility f a pst-bull Lake age fr these mraines belw. Mraines and Terraces in Other Parts f the Valley MORAINES AND TERRACE DEPOSITS OF PRE-BULL LAKE AGE Several high terrace depsits and ne area f subdued mraines n the east side f the Arkansas River, suth f Leadville, cnsiderably predate the Bull Lake glaciatin. We assign mst utwash-terrace surfaces and depsits near Leadville t terrace 2 (Bull Lake age), but we indicate uncertainty in ur ages fr sme f the mre distal surfaces and depsits in the center f the valley by assigning them t terrace grup 1-2 (Fig. IB). Fr example, the very subdued mrphlgy and greater distance f a 1-2 mraine (suthwest f Leadville) frm the head f the Arkansas Valley near Climax relative t that f terrace-2 (Bull Lake) mraines is typical f mraines f pre-bull Lake age. Based largely n a crrelatin with the glbal xygen-istpe recrd, Richmnd (1986a) suggests a middle middle Pleistcene age (42-62 ka) fr this mraine (pssibly till N.2 f Twet, 1961) and an early middle Pleistcene age (62-78 ka) fr an lder till-like depsit identified by Twet (1961; Till N.1) near Leadville. Weathering characteristics and relative height in relatin t terraces that we trace int Bull Lake mraines shw that many f the high terrace depsits in grup 1-2 are f pre-bull Lake age. Sme high terraces, such as thse that rise eastward tward the Msquit Range and grade int pediment depsits, may nt be linked t glaciatins (parts f Terrace N. 1 f Behre, 1933: 8). Twet (1961, 1974) mapped sme f the ldest terrace depsits as Malta Gravel f pre-bull Lake age (crrelated by Richmnd, 1986a, t the middle middle Pleistcene). Such an age is indicated by the cmplete grussificatin f all carsegrained granitic clasts in the upper parts f sme depsits f terrace grup 1-2. Capps (199: 17) reprted xidatin t depths in excess f 25 m in surficial depsits in a mine near Leadville; hwever, this apparent xidatin may have been inherited frm hydrthermally altered bedrck. Anther way t explain clast grussificatin in sme grup 1-2 depsits is thrugh the cutting f strath terraces in highly weathered lder depsits by meltwater during Pleistcene glaciatins. Behre (1933) crrelated several smth, gently slping bedrck surfaces east and nrth f Granite with utwash terraces f grup 1-2 based n their similar relative heights abve the Arkansas River. Althugh sme surfaces cut n bedrck may be dwn-drpped remnants f a late Ecene ersin surface, we cncur with Richmnd (l986a) that mst grup 1-2 depsits are prbably equivalent in age t gravels 3 and 4 f Van Alstine (1969), the middle Pleistcene alluvial depsits f Sctt (1984; Kansan and Illinian (?) Alluviums f Sctt, 1975), and the pre-bull Lake depsits f Clman et al. (1985; als Richmnd, 1986a, his table 2) in the suthern part f the upper Arkansas Valley (Figs. la, 3). MORAINES AND TERRACE DEPOSITS OF BULL LAKE AND PINEDALE AGES As discussed abve fr the Twin Lakes and Leadville areas, many alluvial terraces lwer than thse in grup 1-2 can be traced int mraines in ther tributary valleys f the nrthern A. R. NELSON AND R. R. SHROAA / 357

10 part f the upper Arkansas Valley demnstrating a glacifluvial rigin fr these terraces. The high terraces alng the west side f the Arkansas River were mapped as Terrace N.2 by Behre (1933). as part f the "high terrace gravels" by Capps (199). and as the "earlier gravels" by Westgate (195). Because we are uncertain f crrelatins f terraces 2 and 3 suth f Leadville. these terraces and their crrelative mraines are assigned t terrace grup 2-3. which crrelates with the Bull Lake utwash (lder and yunger utwash nt differentiated) f Sctt (1975) in the suthern part f the upper Arkansas Valley (Figs. la 3). These terraces grade int mraines with subdued mrphlgy characteristic f Bull Lake mraines (e.g. Meierding and Birkeland. 198; Richmnd, 1986a) in the valleys f Half Mn, Clear Creek, and Evans Gulch. Our map f the mraines and utwash terraces near the muth f Half Mn Creek (Fig. IB) is similar t thse f Capps (199), Behre (1933), Twet and Reed (1973), and a recnnaissance study by W. W. Lcke III (written cmmunicatin, 1975). Lcke used quantitative measurements f mraine mrphlgy (slpe angles and crest widths), surface bulder-weathering parameters (frequency, percent split, rind thickness, and pit depths), and sil clr and texture at eight sites t distinguish mraines f three relative ages. Tw f the three grups f mraines have crrespnding utwash terraces. Mraines that we crrelate with the Pinedale glaciatin n the basis f hummcky tpgraphy and Lcke's surface bulder-weathering data (Nelsn et a\., 1984, Part 1) and sil indices, extend eastward ut f the valley f Half Mn Creek and then trend nrtheastward (Fig. IB). The ice that depsited these hummcky mraines appears t have been deflected nrtheastward by higher, mre subdued mraines t the east. Lcke subdivided the subdued mraines int a Bull Lake grup and a pre-bull Lake grup n the basis f differences in their relative relief and slpe angles. Hwever, rubificatin, maximum percent clay. and clay accumulatin indices fr the single sil used by Lcke t suggest a pre-bull Lake age fr the easternmst mraines have similar index values as sme f the sils in Lcke's Bull Lake grup, and these values fall within the ranges f the same indices fr the sils n the Bull Lake mraines at Twin Lakes (Fig. 2). Fr this reasn, and until mre data are available, we crrelate all the lder mraines at Half Mn Creek with the Bull Lake glaciatin. In ther tributary valleys, as well as in the valleys f Twin Lakes, the East Frk f the Arkansas, and Half Mn Creek (discussed abve). ur terraces 4 and 5 grade int mraines with hummcky mrphlgy characteristic f Pinedale mraines (e.g., Meierding and Birkeland, 198; Richmnd, 1986a). These terraces are equivalent t Behre's (1933) terraces N.3 and 4, and they include the "lw terrace gravels" f Capps (199) and the "later gravels" f Westgate (195). Alng the axis f the Arkansas Valley nrth f the Twin Lakes mraines, these terraces are smetimes difficult t distinguish frm yunger, lwer terraces f Hlcene age. Hwever, suth f the canyn belw Granite, terraces 4 and 5 are distinct and present at heights f apprximately 2 t 25 m and 1 t 12 m abve stream level, respectively (Figs. IB, 3). A series f alluvial fans, discntinuus terrace depsits, and small surfaces cut n bedrck suth f Bx Creek (Fig. IB) prbably crrelate with terraces 4 and 5, but nne f the terrace surfaces can be traced cntinuusly thrugh the narrw canyn belw Granite t the terraces mapped by Sctt (1975) nrth f Buena Vista. Based n their relative height abve the Arkansas River (Fig. 3), relatins with mraines with mrphlgies typical f thse f the Pinedale glaciatin, and the relative degree f sil develpment n the terraces (discussed abve), we crrelate terrace 4 with the lder Pinedale utwash f Sctt (1975) and terrace 5 with his yunger Pinedale utwash in the suthern part f the upper Arkansas Valley. Sctt (1984) used the thusands f huge bulders (many >6 m lng) in the utwash f terrace 4 near Buena Vista t infer multiple utburst flds f meltwater dammed by a Pinedale glacier abut 3 km suth f the muth f Clear Creek (Fig. la). UNCERTAIN EXTENT OF PINEDALE AND BULL LAKE GLACIERS ON THE WEST FLANK OF THE MOSQUITO RANGE Ice extent n the west flank f the Msquit Range during the Pinedale and Bull Lake glaciatins is uncertain, particularly suth f Leadville. In Iwa Gulch, Empire Gulch, and alng Unin Creek (Fig. IB), mraines at valley muths are present in the same relative psitin (distance frm the cirques) as the uter mraines in Evans Gulch at Leadville (labeled 2-4, Fig. IB), suggesting that they may als be f Bull Lake age. Hwever, the mraines in the valleys suth f Evans Gulch (als labeled 2-4, Fig. IB) have sharper crests and mre undrained depressins than thse at the muth f Evans Gulch, leaving pen the pssibility that the mraines farther suth significantly pst-date the Bull Lake glaciatin. These mrphlgically yunger mraines might be bradly crrelative with the ldest depsits f the Pinedale glaciatin identified in nrthwestern Wyming (Sturchi et a\., 1994), with early Wiscnsin depsits pstulated t date frm abut 6-8 ka at a few lcalities in the Rcky Muntains (Clman and Pierce, 1986; Richmnd, 1986b), r, pssibly, the mraines were depsited during the Bull Lake glaciatin (13-16 ka) and the mre subdued mraines in Evans Gulch and the East Frk f the Arkansas at Leadville significantly predate them. Terrace 4 alng the East Frk f the Arkansas has hummcky tpgraphy (the utwash must have cntained blcks f stagnant ice) upvalley frm the terrace-3 mraines, and grades int lw, hummcky mraines 4 km farther upvalley (Fig. IB). N sil data were cllected n these mraines, but they are mrphlgically similar t the Pinedale mraines near Twin Lakes and elsewhere in the Rcky Muntains. Althugh they are small, they have steep slpes, narrw crests, and abundant, deep, undrained depressins. If mraine mrphlgy is the nly relative-age criteria, the maximum extent f Pinedale glaciers in the valley f the East Frk f the Arkansas was at the psitin f the hummcky mraines mentined abve (Fig. IB). By the same reasning, Pinedale glaciers extended n farther dwn the valleys f Evans Gulch, Iwa Gulch, and Empire Gulch than the small, narrw mraines 2 t 4 km upvalley frm the utermst, mre subdued mraines in these valleys. This interpretatin implies that, at their greatest extent, Pinedale glaciers n the west flank f the Msquit Range were significantly less extensive than Bull Lake glaciers (Pinedale-t-Bull-Lake-glacier length ratis f ). But Pinedale-t-Bull-Lake ratis are much higher (typically abut.95) n the west flanks f ther nearby muntain ranges-fr example, the Frnt Range t the nrtheast (Meierding and Birkeland, 198; Meierding, 1982), the Sangre de Crist Range t the sutheast (McCalpin, 1982), and the Sawatch range t the west (Brugger, 1994)-and elsewhere in the Rcky Muntains. If cnfirmed, the much less extensive glaciers n the west flank f the Msquit Range during the Pinedale glaciatin wuld have reginal paleclimatic implicatins (e.g., Lenard, 1989). A mre likely interpretatin fr the mrphlgy and psitin f the vahey-muth mraines n the west flank f the Ms- 358 / ARCTIC AND ALPINE RESEARCH

11 quit Range is that the mst extensive Pinedale advance reached the utermst mraines, but did nt linger lng enugh r transprt enugh debris t depsit large, hummcky mraines. Assuming stagnant ice prduced the hummcky tpgraphy f utwash terrace 4 alng the East Frk f the Arkansas, the terrace and the mraines immediately upvalley frm it must be f similar age because an lder utwash surface wuld nt be preserved immediately adjacent t yunger mraines upvalley. This alternative interpretatin requires Pinedale ice in the valley f the East Frk t have advanced fr a shrt time at least t the east edge f the mraines crrelative with terrace 3 and then t have receded and stabilized at the psitin f the upvalley mraines. The small size f the crrelative up valley mraines in the valleys t the suth (labeled 4-5, Fig. lb) suggests that they culd be recessinal mraines rather than the terminal mraines f a Pinedale advance. The 2-4 range in relative ages assigned the larger mraines in these valleys (Fig. IB) reflects ur uncertainty n the maximum extent f Pinedale and Bull Lake glaciers. Sils and ther relative-weathering data may help determine the psitin f the Pinedale-Bull Lake bundary in these valleys. Such techniques cmbined with new methds f csmgenic-istpe dating f bulders n mraines (e.g., Phillips et ai., 1997) might reslve these and ther questins abut the glacial chrnlgy f the nrthern part f the upper Arkansas Valley that were first raised well ver a century ag. Cnclusins Age-related prperties f sils n mraines and utwash terraces prvide a means f crrelating glacial sequences at the muths f majr tributaries f the upper Arkansas River in central Clrad with sequences at tw f the type areas f Rcky Muntain glaciatin in the Wind River Range f Wyming. Mraine mrphlgy, relative psitin f mraines and terraces, and the distributin f utwash terraces within the nrthern part f the upper Arkansas Valley are secndary means f crrelatin f sequences in this area. Prfile develpment indices fr sils develped in mraines and utwash near Twin Lakes and in utwash near Leadville supprt the crrelatin f subdued mraines and tw high utwash terraces with the Bull Lake glaciatin (ca ka) and the crrelatin f hummcky mraines and tw lw utwash terraces with the Pinedale glaciatin (ca ka). Near Twin Lakes, indices fr sils n lw, uter lateral mraines suggest that Pinedale glaciers extended several hundred meters beynd the margin f the high lateral mraines with hummcky tpgraphy. A few subdued mraines near Twin Lakes and Leadville prbably recrd ne r mre glaciatins significantly lder than the Bull Lake. Crrelatins using sils data and new csmgenic-istpe ages f >95 ka fr the yungest Bull Lake mraines in the Wind River Range suggest that all subdued mraines in the nrthern part f the upper Arkansas Valley may be at least this ld. Crrelatins and ice extent during the Pinedale and Bull Lake glaciatins are much less certain fr the smaller, less extensive glacial sequences n the west flank f the Msquit Range than fr thse n the east flank f the Sawatch Range, primarily because sil and ther types f relative-age data are unavailable fr the Msquit Range. Althugh Pinedale glaciers in the Msquit Range may have been much less extensive than Bull Lake glaciers, the paleclimatic anmaly implied by such a glacial histry in cmparisn with ther ranges in the Suthern Rcky Muntains suggest instead that Pinedale glaciers extended t r beynd valley muths but built n prminent terminal mraines. Acknwledgments Nelsn's research in the upper Arkansas Valley was part f a seismic hazard study cnducted while he was emplyed by the U.S. Bureau f Reclamatin (Denver). Shrba's wrk n the Twin Lakes mraines was part f his Ph.D. thesis at the University f Clrad (Bulder). We thank P. W. Birkeland, D. A. Ostenaa, G. R. Sctt, J. T. Sullivan, and O. L. Twet fr advice; Ostenaa, Sullivan, and S. L. Lsh fr help with field wrk; Peter Klein fr preparing sil thin sectins; and especially W. W. Lcke III, fr sharing his unpublished 1975 class reprt, data, and ideas abut Half Mn Creek. P. W. Birkeland, D. R. Muhs, and D. A. Ostenaa prvided valuable cmments n earlier versins f this paper. References Cited Behre, C. H., Jr., 1933: Physigraphic histry f the upper Arkansas and Eagle Rivers, Clrad. Jurnal f Gelgy, 41: Behre, C. H., Jr., 1953: Gelgy and re depsits f the west slpe f the Msquit Range. US. Gelgical Survey Prfessinal Paper, pp. Birkeland, P. W. and Burke, R. M., 1988: Sil catena chrnsequences n eastern Sierra Nevada mraines, Califrnia. Arctic and Alpine Research, 2: Birkeland, P. w., Machette, M. N., and Haller, K. M., 1991: Sils as a tl fr applied Quaternary gelgy. Utah Gelgical and Mineral Survey Miscellaneus Publicatin, pp. Brugger, K.A. 1994: Implicatins f dynamic mdeling f glacial systems fr late Quaternary paleclimate and style f deglaciatin f the suthern Sawatch Range, Clrad. Gelgical Sciety f America Abstracts with Prgrams, 26 (n. 7): A-51O. Burke, R. M. and Birkeland, P. W., 1979: Reevaluatin f multiparameter relative dating techniques and their applicatin t the glacial sequence alng the eastern escarpment f the Sierra Nevada, Califrnia. Quaternary Research, 11: Capps, S. R., 199: Pleistcene gelgy f the Leadville quadrangle, Clrad. US. Gelgical Survey Bulletin, pp. Capps, S. R. and Leffingwell, E. D. K., 194: Pleistcene gelgy f the Sawatch Range near Leadville, Clrad. Jurnal f Gelgy, 12: Chadwick, O. A., Hall, R. D., and Phillips, F. M., 1997: Chrnlgy f Pleistcene glacial advances in the central Rcky Muntains. Gelgical Sciety f America Bulletin, 19: Clman, S. M. and Dethier, D. p., 1986: Rates f Chemical Weathering f Rcks and Minerals. Orland, Flrida: Academic Press. 63 pp. Clman, S. M. and Pierce, K. L., 1986: Glacial sequence near McCall, Idah: Weathering rinds, sil develpment, mrphlgy, and ther relative-age criteria. Quaternary Research, 25: Clman, S. M., McCalpin, J. P., Ostenaa, D. A., and Kirkham, R. M., 1985: Map shwing upper Cenzic rcks and depsits and Quaternary faults, Ri Grande rift, suth-central Clrad. US. Gelgical Survey Miscellaneus Investigatins Map, , scale 1:125,,2 sheets. Dahms, D. E., 1994: Mid-Hlcene ersin f sil catenas n mraines near the type Pinedale Till, Wind River Range, Wyming. Quaternary Research, 42: Davis, W. M., 195: Glaciatin f the Sawatch Range, Clrad. Bulletin f the Museum f Cmparative Zlgy, Cambridge, Mass., Harvard University, 49: Emmns, S. F., Irving, J. D., and Lughlin, G. F., 1927: Gelgy A. R. NELSON AND R. R. SHROBA / 359

12 and re depsits f the Leadville mining district, Clrad. u.s. Gelgical Survey Prfessinal Paper, pp. Gillespie, A and Mlnar, P., 1995: Asynchrnus maximum advances f muntain and cntinental glaciers. Reviews f Gephysics, 33: Gsse, J. c., Evensn, E. B., Klein, J., Lawn, B., and Middletn, R., 1995: Precise csmgenic lobe measurements in western Nrth America-Supprt fr a glbal Yunger Dryas cling event. Gelgy, 23: Hall, RD., 1989: Cntributin t 1989 Friends f the Pleistcene Trip, Wind River Muntains. Indianaplis: Indiana University, Dept. f Gelgy (unpublished reprt). 5 pp. Hall, R. D. and Shrba, R R, 1993: Sils develped in the glacial depsits f the type areas f the Pinedale and Bull Lake glaciatins, Wind River Range, Wyming, U.S.A Arctic and Alpine Research, 25: Hall, R D. and Shrba, R. R, 1995: Sil evidence fr a glaciatin intermediate between the Bull Lake and Pinedale glaciatins at Fremnt Lake, Wind River Range, Wyming, U.S.A. Arctic and Alpine Research, 27: Harden, J. W. and Taylr, E. M., 1983: A quantitative cmparisn f sil develpment in fur climatic regimes. Quaternary Research, 2: Harden, J. W, Taylr, E. M., Hill, c., Mark, R. K., McFadden, L. D., Reheis, M. c., Swers, J. M., and Wells, S. G., 1991: Rates f sil develpment frm fur sil chrnsequences in the suthern Great Basin. Quaternary Research, 35: Hayden, E v., 1874: Annual reprt f the U.S. Gegraphical and Gelgical Survey f the territries, embracing Clrad, Washingtn, D.C.: Gvernment Printing Office. 718 pp. Jhnsn, W H., Hansel, A. K, Bettis, E. A., III, Karrw, P. E, Larsn, G. J., Lwell, T. v., and Schneider, A. E, 1997: Late Quaternary tempral and event classificatin, Great Lakes Regin, Nrth America. Quaternary Research, 47: Lenard, E. M., 1989: Climatic change in the Clrad Rcky Muntains: Estimates based n mdem climate at late Pleistcene equilibrium lines. Arctic and Alpine Research, 21: Machette, M. N., 1985: Calcic sils f the suthwestern United States. In Weide, D. (ed.), Sils and Quaternary Gelgy f the Suthwestern United States. Gelgical Sciety f America Special Paper 23. Bulder, Cl.: Gelgical Sciety f America, Madle, R E and Shrba, R R., 1979: Till sequence and sil develpment in the Nrth St. Vrain drainage basin, east slpe, Frnt Range, Clrad. In Ethridge, E G. (ed.), Field Guide Nrthern Frnt Range and Nrthwest Denver Basin, Clrad. Gelgical Sciety f America, Rcky Muntain Sectin 32nd Annual Meeting, Frt Cllins, Clrad, McCalpin, J. P., 1982: Quaternary gelgy and netectnics f the west flank f the nrthern Sangre de Crist Muntains suth-central Clrad. Clrad Schl f Mines Quarterly, 77 (3). 97 pp. Meierding, T. c., 1982: Late Pleistcene glacial equilibrium-line altitudes in the Clrad Frnt Range: A cmparisn f methds. Quaternary Research, 18: Meierding, T. C. and Birkeland, P. W, 198: Quaternary glaciatin f Clrad. In Kent, H. C. and Prter, K W., (eds.), Clrad Gelgy: Denver, Clrad, Rcky Muntain Assciatin f Gelgists, Nelsn, A. Rand Shrba, R. R., 1984: Part II-Mraine and utwash-terrace sequences and sil develpment in the nrth graben f the upper Arkansas Valley, central Clrad. In Nelsn, A. R, Shrba, R. R., and Sctt, G. R., Quaternary Depsits f the Upper Arkansas River Valley, Clrad. Bulder, Clrad: American Quaternary Assciatin, 8th Biennial Meeting, August 16-17, 1984, unpublished guide fr Field Trip N.7, 25-5 Nelsn, A. R., Shrba, R. R., and Sctt, G. R, 1984: Quaternary Depsits f the Upper Arkansas River Valley, Clrad. Bul- der, Clrad: American Quaternary Assciatin, 8th Biennial Meeting, August 16-17, 1984, unpublished guide fr Field Trip N.7, 57 pp. Nelsn, A R and Taylr, E. M., 1985: Ltus template t calculate sil develpment indices. Denver, Clrad, Cmputer Oriented Gelgical Sciety, Public Dmain Diskettes Gelgical Applicatins fr Yur Micr, 3: (file SOILIND). Nrth American Cmmissin n Stratigraphic Nmenclature, 1983: Nrth American stratigraphic cde. American Assciatin f Petrleum Gelgists Bulletin, 67: Ostenaa, D. A, Lsh, S. L., and Nelsn, A. R, 1981: Evidence fr recurrent late Quaternary faulting, Sawatch fault, upper Arkansas Valley, Clrad. In Junge, W. R. (ed.), Clrad tectnics, seismicity and earthquake hazards. Clrad Gelgical Survey Special Publicatin, 19, Pierce, K L., Obradvich, J. D., and Friedman, 1., 1976: Obsidian hydratin dating and crrelatin f Bull Lake and Pinedale glaciatins near West Yellwstne Mntana. Gelgical Sciety f America Bulletin, 87: Phillips, EM., Zreda, M. G., Gsse, J. c., Klein, J., Evensn, E. B., Hall, R. D., Chadwick, O. A, and Sharma, P., 1997: Csmgenic 36CI and lobe ages f Quaternary glacial and fluvial depsits f the Wind River Range, Wyming. Gelgical Sciety f America Bulletin, 19: Prter, S. C., Pierce, K L. and Hamiltn, T. D., 1983: Late Wiscnsin muntain glaciatin in the western United States. In Wright, H. E., Jr. (ed.), Late-Quaternary Envirnments f the United States, v. 1, The Late Pleistcene, Prter, S. C. (ed.). Minneaplis: University f Minnesta Press, Ray, L. L., 194: Glacial chrnlgy f the suthern Rcky Muntains. Gelgical Sciety f America Bulletin, 51: Reheis, M. c., 1987: Climatic implicatins f alternating clay and carbnate frmatin in semiarid sils f suth-central Mntana. Quaternary Research, 27: Richmnd, G. M., 1986a: Stratigraphy and crrelatin f glacial depsits f the Rcky Muntains, the Clrad Plateau, and the ranges f the Great Basin. In Sibrava, v., Bwen, D. Q., and Richmnd, G. M. (eds.), Quaternary Glaciatins in the Nrthern Hemisphere. Quaternary Science Reviews, 5: Richmnd, G. M., 1986b: Stratigraphy and crrelatin f glaciatins in Yellwstne Natinal Park. In Sibrava, v., Bwen, D. Q., and Richmnd, G. M. (eds.), Quaternary Glaciatins in the Nrthern Hemisphere. Quaternary Science Reviews, 5: Richmnd, G. M., 1987: Type Pinedale Till in the Fremnt Lake area, Wind River Range, Wyming. Gelgical Sciety f America Centennial Field Guide-Rcky Muntain Sectin, Richmnd, G. M. and Twet, O. L., 1965: Glaciatin f the Sawatch Muntains. In Schultz, C. B. and Smith, H. T. U. (eds.), Internatinal Assciatin fr Quaternary Research, 7th Cngress, Guidebk fr Field Cnference E, Nrthern and Middle Rcky Muntains. Lincln, Nebraska: Academy f Sciences, Sctt, G. R, 1975: Recnnaissance gelgic map f the Buena Vista quadrangle, Chaffee and Park Cunties, Clrad. U.S. Gelgical Survey Map MF-657, 1:62,5 scale. Sctt, G. R, 1984: Part III-Pleistcene flds alng the Arkansas River, Chaffee Cunty, Clrad. In Nelsn, A R, Shrba, R. R., and Sctt, G. R., Quaternary Depsits f the Upper Arkansas River Valley, Clrad. Bulder, Clrad: American Quaternary Assciatin, 8th Biennial Meeting, August 16-17, 1984, unpublished guide fr Field Trip N.7, Sctt, G. R., Van Alstine, R E. and Sharp, W N., 1975: Gelgic map f the Pncha Springs quadrangle, Chaffee Cunty, Clrad. U.S. Gelgical Survey Map MF-6587, 1:62,5 scale. Shrba, R R, 1977: Sil develpment in Quaternary tills, rck- 36 / ARCTIC AND ALPINE RESEARCH