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Minor Expansion of Alpine Glaciers in Wright Valley, Antarctica

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Minor Expansion of Alpine Glaciers in Wright Valley, Antarctica basis of areal extent and distribution of erratic lithologies, we Minor expansion of alpine glaciers in conclude that silty tills represent expansion of east antarctic ice Wright Valley, Antarctica seaward through Wright Valley. Alpine glacier deposits. Alpine I drift, located less than 5 meters from the sides of present-day glaciers, is a gravelly diamicton containing ventif acts and lacking striated clasts. Alpine I drift is BRENDA L. HALL AND GEORGE H. DENTON virtually unweathered and forms discontinuous, ice-cored mo- raines. Alpine I drift overlies, and thus is younger than, Alpine Department of Geological Sciences II drift. and Inst it utefor Quaternary Studies Alpine II drift is located 15 to 150 meters from the sides and one University of Maine kilometer in front of present-day glaciers. The drift is a coarse- Orono, Maine 04469 sand diamicton containing ventifacts and lacking striated clasts. Surface boulders display cavernous weathering. Alpine II drift Here we present information on Plio-Pleistocene alpine gla- beside Bartley and Meserve Glaciers contains basalt clasts. Al- cier activity and paleoclimate in east-central Wright Valley that pine II drift forms small (0.2 to 1 meter high) discontinuous bears on Pliocene paleoclimate and east antarctic ice sheet dy- moraines. These moraines extend onekilometer in front of present- namics. day glacier termini. Alpine II drift overlies and is younger than We collected over 400 samples from the Wright Valley drifts Alpine III and IV drifts. for sedimentologic and lithologic analysis. We separated alpine Alpine III drift, located 20 to 150 meters from the sides of difts, numbered from youngest to oldest, into four map units, current glaciers, is a highly weathered, sandy diamicton with bsed on areal extent, geometric relationships, morphology, numerous ventifacts and no striated stones. Cavernously weath- sirficial and internal weathering, sedimentology, and lithology. ered and shattered boulders mantle the drift surface. Over 95 411 silty tills are grouped into one map unit. Table 1 presents the percent of the clasts in the left lateral Alpine III at Meserve gneral stratigraphy and distinguishing characteristics of each Glacier are composed of basalt although most Alpine III drifts are tap unit. The figure shows the distribution of alpine drift and comprised of granite, gneiss, and dolerite clasts. Alpine III drift silty till near Bartley Glacier. forms large (more than 10 meters high), bulky moraines. These Silty tills. Silty tills, containing sandstone erratics, striated moraines overlie and are younger than Alpine N drift. ones, and ventifacts, occur discontinuously from the valley Alpine IV drift, located 75 to 200 meters from the sides of oor to 1,150 meters elevation. Silty tills lack basalt clasts. All present-day glaciers, is a highly weathered, sandy diamicton that IsIty tills underlie, and are older than, the alpine drifts. On the includes abundant ventifacts and lacks striated stones. Alpine N Table 1. List of east-central Wright Valley alpine drifts and silty tills, from youngest to oldest Deposit Age Distinguishing characteristics Interpretation (millions of years) Alpine I drift <<<3.5 Small, 1-rn-high, unweathered, ice-cored moraines Slight expansion of cold-based No striated clasts but numerous ventifacts alpine glaciers Alpine II drift <<3.5 Small, 1-rn-high, ice-free moraines. Cavernously Expansion of cold-based alpine weathered surface boulders, ventifacts, basalt clasts, glaciers in Wright Valley but no striated clasts Alpine Ill drift <3.5 Large, 10-rn-high, broad moraines. Cavernously Expansion of cold-based weathered and shattered surface boulders, numerous alpine glaciers incorporated ventif acts, but no striated clasts. One moraine is composed of 95% basalt clasts Alpine IV drift 3.5-3.8 Large, 10-rn high, broad moraines. No upstanding Expansion of cold-based surface boulders. Strongly oxidized to >100 cm depth. alpine glaciers Numerous incorporated bentif acts, but no striated clasts. Most Alpine IV moraines lack basalt clasts Peleus till and >3.5-3.8 Silty diarnictons lacking moraine ridges, which occur Expansion of partially warm-based other silty tills discontinuously from the valley floor to 1150 rn elevation, east antarctic ice seaward through Contain striated clasts, ventif acts, sandstone erratics, Wright Valley but lack basalt clasts 992 REVIEW 55 drift flanking the west side of Bartley Glacier contains reworked ages. In one case involving the Meserve Glacier, basalt clasts al basalt cobbles and gravel, whereas Alpine IV drift at other gla- provide minimum ages for underlying deposits. The higF1y ciers lacks basalt clasts. Alpine N drift is strongly oxidized to basaltic, left lateral Alpine III drift at the Meserve Glacier is lass depths exceeding 100 centimeters. There are no upstanding than 3.5 million years ago, based on multiple age determinations boulders on the drift surface. Alpine IV drift also forms large, from incorporated basalt clasts. The underlying Alpine IV drift bulky moraines. Alpine IV drift, which is the oldest alpine drift almost surely dates to more than 3.5 million years ago, because it in Wright Valley, overlies Peleus till (Prentice 1982; Prentice et al. lacks basalt clasts. The assumption is that, had the source volcano 1987). in the upper catchment area erupted before emplacement of Cold-based glaciers deposited all alpine drifts in Wright Val- Alpine N drift, then this drift would contain basalt clasts. The ley. We infer this basal thermal regime from the coarse texture of similar extent of Alpine III and N advances and the large percent the drifts, the lack of incorporated striated stones, the presence of (greater than 95 percent) of basalt clasts within Alpine III drift abundant ventifacts, and the preservation of preexisting surface support the validity of this assumption. features and morphology. In addition, outwash plains, kame We conclude that the last ice flow from East Antarctica sea- terraces, and ice-contact heads, which record temperate glacier ward through Wright Valley occurred before 3.5 to 3.8 million activity in nearly all alpine valleys north of the Antarctic Conver- years ago. Silty tills, indicative of through-flowing east antarctic gence, are not associated with alpine drifts or silty tills in Wright ice, all underlie large, bulky Alpine IV moraines dated to 3.5 to 3.8 Valley. This suggests that temperate surface ablation zones, million years ago. In addition, any ice-sheet overriding of the which are common north of the Antarctic Convergence, have not Transantarctic Mountains, which necessitates through-flowing existed in Wright Valley since before deposition of Alpine IV drift ice in Wright Valley, must have occurred before this time. Lak and older silty tills. of large ice-sheet expansion since 3.5 to 3.8 million years ago s Chronology. Fifteen argon-40/argon-39 age determinations on consistent with only a 1-kilometer advance of alpine glaciers, whole-rock basalt associated with alpine drifts provide an abso- located less than 50 kilometers from the ice sheet. lute chronology. We graded argon-40/argon-39 age determina- Cold-desert conditions have persisted since at least 3.5 to 3.8 tions A, B, or C, based on the quality of our data (Wilch 1991; Hall million years ago. We infer this paleoclimate from evidence for 1992). Whenever possible, we used only grade A data to deter- cold-based glaciation, abundant ventifacts incorporated within mine the limiting ages for a deposit. Table 2 presents all grade A alpine glacier drifts, and, most important, the lack of temperate and B age determinations. Table 1 shows the resulting limiting ice landforms, such as outwash plains. Our evidence sugges s ages on alpine drifts. that, at least in the Wright Valley sector of the East antarctic i e Argon-40/argon-39 dates of basalt clasts reworked into gla- sheet, air temperatures in the last 3.5 to 3.8 million years have cial drifts afford maximum ages, which provide critical upper never warmed enough to allow superposition of the temperate limits on the ages of the drifts, placing the drifts in the mid-late ablation zones that we feel are necessary for ice-sheet collapseS Pliocene or younger and precluding early Pliocene or Miocene East Antarctica. Table 2. Argon-40/argon-39 age determinations for basalt clasts incorporated within alpine glacier drifts Deposit and age interpretation Age results (Ma)(million years) West Meserve Glacier: Dated basalt clasts incorporated within Alpine II drift; 3.6 ± 0.2 inferred maximum age for Alpine II drift 3.7 ± 0.3 3.7 ± 0.1 3.15 ± 0.06 West Meserve Glacier: Dated basalt clasts incorporated within Alpine III drift; 3.4± 0.1 inferred maximum age for Alpine II drift and inferred minimum age for underlying, 3.5 ± 0.1 basalt-free, Alpine IV drift East Bartley Glacier: Dated basalt clasts incorporated within Alpine II drift; 3.6 ± 0.1 inferred maximum age for Alpine II drift 3.7 ± 0.1 3.7 ± 0.2 3.7 ± 0.1 West Bartley Glacier: Dated basalt clasts incorporated within Alpine IV drift; 3.6 ± 0.2 inferred maximum age for Alpine IV drift 3.7 ± 0.3 3.8 ± 0.2 3.9 ± 0.3 4.3 ± 0.3 56 ANTARCTIC JOURN This research was supported by National Science Foundation of Sessrumir Valley, western Asgard Range, Antarctica: Implications rant DPP 89-18942. Dick Kelly drafted the figure. for late Tertiary ice-sheet overriding. Antarctic Journal of the U.S., 25(5),53-55. Pi entice, M. L. 1982. Surficial geology and stratigraphy of central Wright Valley, Antarctica: Implications for Antarctic Tertiary glacial history. References Master of Science thesis, University of Maine, Orono, Maine. Prentice, M. L., C. H. Denton, L. H. Burckle, and D. A. Hodell. 1987. /ckert, R. P., Jr. 1990. Surficial geology and stratigraphy in Njord Valley, Evidence from Wright Valley for the response of the antarctic ice sheet western Asgard Range, Antarctica: Implications for late Tertiary to climate warming.
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