Rb-Sr provenance dates of feldspar in glacial deposits of the Wisconsin Range, Transantarctic Mountains

q F/XUR.E ' I Department of Geology and Mineralogy and Institute of Polar Studies, The Ohio State University, Columbus,

J H MERCER ' °hi°43210 ABSTRACT than those of feldspar in the plateau till and range only from 0.46 to 0.66. Nevertheless, three feldspar fractions form a straight line on Glacial deposits in the Wisconsin Range (lat. 85° to 86°30'S, the Rb-Sr isochron diagram, the slope of which indicates a date of long. 120° to 130°W) of the Transantarctic Mountains include a 576 ± 21 Ma. The difference in the date derived from the feldspar of deposit of till on the summit plateau at an elevation of 2,500 m the glaciolacustrine sedimeyt may be caused by the presence of a above sea level and glaciolacustrine sediments along the Reedy component of Precambrian feldspar derived from the East Antarc- Glacier. The plateau till and underlying sediments consist of six tic Shield. units that appear to record the replacement of ice-free, periglacial conditions by ice cap glaciation of pre-Pleistocene age. Alterna- INTRODUCTION tively, the plateau till may have been deposited by the East Antarc- tic ice sheet either when it was thicker than at present or when the The glaciation of Antarctica in Cenozoic time was an important Wisconsin Range was lower in elevation. Feldspar size fractions event in the history of the Earth, the effects of which continue to from the plateau till have Rb/Sr ratios that increase with grain size influence climatic conditions and sea level. Comprehensive reviews from 1.4 (67 to 125 Mm) to 4.24 (500 to 1,000 Aim). These size of the accumulated field and laboratory evidence regarding the his- fractions define a straight line on a Rb-Sr isochron diagram and tory of glaciation of Antarctica have been published by Denton and yield a date of 480 ± 21 Ma that is indistinguishable from the age of others (1971), Mercer (1978), and Denton and Hughes (1981). In the granitic basement rocks of the Wisconsin Range dated pre- spite of intense efforts by many scientists, important questions viously. This result therefore supports the hypothesis that the pla- regarding the initial growth of the East and West Antarctic ice teau till was deposited by a local ice cap and suggests that the sheets and their subsequent evolution remain unsettled (Grindley, Wisconsin Range was sufficiently elevated to permit an icecap to 1967; Mercer, 1968, 1972, 1978; Drewry, 1975, 1980; Mayewski, form prior to the growth of the East Antarctic ice sheet. The glacio- 1975; Mayewski and Goldthwait, in press; Stump and others, 1980; lacustrine sediments along Reedy Glacier probably were deposited Barrett and Powell, 1982; Kvasov and Verbitsky, 1981). in an ice-marginal melt-water pond along the margin of a temperate The purpose of this study is to determine the provenance of Reedy Glacier soon after the East Antarctic ice sheet first reached feldspar in glacial deposits of the Wisconsin Range previously de- full size. The Rb/Sr ratios of feldspar in this sediment are lower scribed and interpreted by Mercer (1968, 1978). The provenance

Figure 1. Map of the Wiscon- sin Range, Horlick Mountains, Antarctica. The dark areas repre- sent rock, and the white areas are ice or snow. The locations of the glacial deposits included in this study are shown.

Geological Society of America Bulletin, v. 94, p. 1275-1280, 5 figs., 1 table, November 1983.

1275

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determination is based on Rb-Sr dating of size fractions of feldspar extracted from the deposits. This method was first used by Taylor and Faure (1981) to study late Wisconsinan tills of Ohio and Indi- ana and was subsequently applied by Faure and Taylor (1981) to glacial deposits in the Transantarctic Mountains. The results of this study provide new information about the provenance of feldspar in glacial deposits of the Wisconsin Range and support the interpreta- tion of field evidence by Mercer (1968).

GEOLOGY AND GLACIAL HISTORY

The Wisconsin Range of the Horlick Mountains consists of several dissected plateaus between 120°W and 130°W long, and between 85°S and 86°30'S lat. (Fig. 1). The Reedy Glacier separates the Wisconsin Range on the east from the Queen Maud Mountains to the west (Fig. 1). It is the most easterly of the great outlet glaciers that carry ice from the East Antarctic ice sheet through the Trans- antarctic Mountains to the Ross Ice Shelf. This region includes a basement of igneous and metamorphic rocks of early Paleozoic age (Murtaugh, 1969) overlain by flat-lying sedimentary rocks of the Beacon Supergroup of Permian age (Min- shew, 1966). The latter have been largely removed by erosion and are preserved primarily in a downfaulted block south of the Olen- tangy Glacier (Mirsky, 1969). A typical specimen of granitic base- ment rock collected at Mims Spur along the Olentangy Glacier contains about 30% microcline in grains as much as 4 mm in diame- ter and about 10% plagioclase (andesine) as interstitial grains as much as 1.5 mm in diameter. Much of the plagioclase is present in Grain Size In Micrometers myrmekitic intergrowth with quartz. Age determinations by the whole-rock Rb-Sr method initially indicated a date of 627 ± 22 Ma Figure 2. Grain-size distributions and relative abundances of for the granitic rocks of the Wisconsin Range batholith and a date quartz and feldspar in different size fractions of till from the Wis- of 479 ± 10 Ma for quartz-monzonites, aplites, and pegmatites consin plateau and from Reedy Glacier. For example, about 21% of (Faure and others, 1968). Additional analyses by Faure and others the quartz in the till from the Wisconsin plateau is in the 250 to 500 (1979) later indicated that the foliated granitic rocks can be resolved /¿m fraction. into two suites having different initial 87Sr/86Sr ratios but similar ages of 507 ± 23 Ma and 513 ± 12 Ma. The age of the quartz- monzonites and aplites was revised to 486 ± 9 Ma and that of the The significance of these glacial deposits arises from the fact pegmatites to 473 ± 5 Ma. that they are believed to record the onset of glaciation of this sec- A large deposit of unconsolidated sediment, approximately 40 tion of the Transantarctic Mountains. According to Mercer (1968), m in thickness and consisting of six units, was found and described the climate on the plateau of the Wisconsin Range deteriorated by Mercer (1968) in a shallow depression on the plateau of the from periglacial conditions (unit 1) to permit the formation of small Wisconsin Range at an elevation of about 2,500 m above sea level temperate glaciers (unit 2) that subsequently expanded to form a (Fig. 1). The lowest unit (unit 1, 1 m) consists of fragments of large wet-based ice cap (units 4 and 5). Later, this ice sheet became fine-grained sandstone displaying some imbricate structure that cold and dry-based, partly perhaps because of continuing uplift of Mercer (1968) interpreted as frost-shattered bedrock or the C the Transantarctic Mountains, and began to recede. This sequence horizon of a former soil that originated under periglacial or nongla- of events constitutes the Horlick Glaciation (Mercer, 1968). No cial conditions. Unit 2 (1-2.5 m) is composed entirely of clasts of direct evidence regarding the age of the Horlick Glaciation exists at granitic rocks and may have formed either by downslope movement this time, but, on the assumption that it predates the East Antarctic or by a small local glacier prior to the formation of a more extensive ice sheet, it is more than 15 Ma old (Mercer, 1978). ice cover. The third unit is made up of discontinuous stratified The plateau of the Wisconsin Range combines high elevation lenses of silt and clay that formed in pools of water at the edge of a (2,400 to 3,600 in above sea level) and high southern latitude (about glacier. Unit 4 (30 m) and unit 5 (6 m) are composed of very com- 86° S) and therefore is likely to have supported some of the earliest pact till rich in clay minerals and contain clasts of sandstone, shale, glaciers in Antarctica. For this reason, the existence of a wet-based granite, and metavolcanic rocks derived from the bedrock of this ice cap in the Wisconsin Range in middle Horlick time implies that area. Unit 5 differs from unit 4 only by having a slightly coarser climatic conditions in East Antarctica were still too mild to permit matrix and by being less compact. Mercer (1968) interpreted both an ice sheet to accumulate there on low ground. At this time, the units as lodgment till deposited by an extensive ice cap that covered Reedy Glacier was probably a local valley glacier draining ice from all, or a large part, of the Wisconsin Range plateau. The uppermost the Transantarctic Mountains. As climatic conditions became more layer (unit 6, 1 m) resembles glacial drift of the Reedy I moraine severe in late Horlick time, ice accumulated in the interior of East (Mercer, 1968) and may be ablation till deposited by recession of Antarctica and the Reedy Glacier eventually became an outlet gla- the ice cap on the plateau. cier of the East Antarctic ice sheet, whereas the ice cap on the

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Wisconsin Plateau receded. Later fluctuations in the level of the that the till on the high plateaus of the Wisconsin Range was depos- Reedy Glacier, recorded by the Reedy I to III lateral moraines and ited by the East Antarctic ice sheet rather than by an ice cap cen- drift sheets, may ultimately be related to sea-level changes caused tered over the Wisconsin Range. In that case, the till might contain by fluctuations of the Laurentide ice sheet (Stuiver and others, a mixture of feldspar grains derived both from the basement rocks 1981). in the Wisconsin Range (473 ±5 Ma to 513 ± 12 Ma) and from the An alternative hypothesis is that the glacial deposits on the Wis- Precambrian rocks of the East Antarctic Shield (Grew, 1978; Grew consin Range plateau were formed at the base of the East Antarctic and Manton, 1979). The presence of a Precambrian component in ice sheet. Evidence presented by Mayewski and Goldthwait (in the glacial deposits of the Wisconsin Range is detectable by dating press) and Denton and others (in press) elsewhere in the Trans- size fractions of feldspar by the Rb-Sr method. The reliability of antarctic Mountains indicates that the East Antarctic ice sheet was this method has been demonstrated by a study of glacial deposits thicker and more extensive in the past than at present. Denton and from the Byrd Glacier area (Faure and Taylor, 1981). others (in press) concluded from geomorphic evidence that the The results to be presented in this report are based on the study mountains of southern Victoria Land were twice overridden by ice of two samples of glacial sediment collected by Mercer in the Wis- that flowed in a northeasterly direction. Mercer (1968) found no consin Range during the 1964-1965 field season. No additional evidence for such an event in the Wisconsin Range and still favors samples are available at this time. One of the samples was taken the interpretation presented above. However, Stump and others from unit 4 (middle Horlick Glaciation) on the Wisconsin Plateau (1980) determined that olivine basalts and hyaloclastites at Sheri- (Fig. 1) and is referred to here as the "Plateau Till." The second dan Bluff and on Mount Early at the head of Scott Glacier, about sample was collected from a deposit of stratified drift in an ice-free 225 km southwest of the main plateau of the Wisconsin Range, cirque at the southeastern end of the Quartz Hills adjacent to Reedy were erupted subglacially. Age determinations by the K-Ar and Glacier (Fig. 1). The sample consists of thinly bedded silt and clay 40Ar-39Ar method yielded an average date of 18.32 ± 0.35 Ma for alternating with micaceous sand and contains numerous granules lava flows at Sheridan Bluff and 15.86 ± 0.30 Ma for flows on and pebbles. Mercer (1968) concluded that this sediment was depos- Mount Early. They infer from this that an ice sheet existed in this ited in an ice-marginal lake containing floating ice. The climate area in early Miocene time. Moreover, Stump and others (1980) must have been much warmer when this lake existed than it is today found 900 m of relief on a glacially eroded prevolcanic erosion because small ice-marginal melt-water ponds along the present surface at Sheridan Bluff and concluded that the Transantarctic Reedy Glacier are frozen solid during most of the year and are Mountains had been uplifted and had been subjected to erosion by covered with a few centimetres of water only on exceptionally warm glaciers in early Miocene time. In view of this evidence, it is possible and sunny days during the summer. The age of this sediment is pre-Reedy III, but its relationship to the older Reedy moraines is Mesh Number unknown. Judging from the evidence for warmer climatic condi- 250 120 60 35 18 tions, Mercer (1968) suggested that temperatures at that time were T T 6 to 10 °C warmer than at present. The sample from the stratified drift (64-62 of Mercer) will be referred to as "Reedy Sediment" in this report. • Plateau Till 3.0

O Reedy Sediment GRANULOMETRY AND MINERAL COMPOSITION

The samples were disaggregated in water and separated into 2.5 size fractions by sieving. The abundances of feldspar and quartz tflCD O were determined by X-ray diffraction of aliquots of each size frac- O tion ground to less than 70 /xm. The K-feldspar/plagioclase ratio 0

01 was estimated from the background-corrected intensities at 26 o angles of 27.5° and 28.0° of Cu K-<* X-radiation and is the average of 8 replicate scans obtained by rotating the specimen through 90° o between determinations. Details of the analytical procedure were 91.5 reported by Faure and Taylor (1981). The Plateau Till is unsorted and has a mode (29%) in the 67 to 125 |im fraction (Fig. 2). A much smaller abundance peak (10%) 1.0 occurs in the 250 to 500 jum fraction that is characteristic of grains derived from the sandstones of the Beacon Supergroup, seen also in till from Hatherton Glacier and Table Mountain by Faure and Taylor (1981). The lithologic composition of granules and clasts 0.5 more than 4 mm in diameter consists of 39.3% granitic rocks, 37.5% sandstone, and 23.1% black shale by weight. Some of the black shale clasts may have originated from outcrops of the late Precam- _L _L _1_ brian LaGorce Formation on the Wisconsin plateau (Minshew, 67 125 250 500 1000 1966). Grain Size, Micrometers The grain-size distribution of quartz in the Plateau Till is uni- modal, with a peak in the 250 to 500 /xm fraction that presumably Figure 3. Variation of the K-feldspar/plagioclase ratio with reflects prior sorting of grains in the Beacon sandstones. Feldspar, grain size. determined by planimeter from 4 replicate X-ray diffraction scans,

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has a bimodal distribution with peaks in the 500 to 1,000 and 67 to TABLE I. ANALYTICAL DATA FOR FELDSPAR SIZE FRACTIONS IN GLACIAL DEPOSITS. WISCONSIN RANGE. ANTARCTICA 125 |im fractions. The former is caused by the predominance of

K-feldspar (microcline) in the coarse fractions (500-1,000 nm), 87 Sample Si/e fraction K-spar Rb 87Rb Sr whereas the latter results from the presence of abundance peak of no. plag Sr »"Sr 8bSr plagioclase in the 67 to 125 /urn fraction. The apparent mineralogi- cal separation of K-feldspar from plagioclase was previously Plateau Till reported by Taylor and Faure (1981) for late Wisconsin till in Ohio 1 500 to 1,000 3.01 4.238 12.377 0.79820 2 250 to 500 1.53 3.791 11.064 0.79154 and is probably caused by the preferential grinding of plagioclase 3 125 to 250 0.822 2.431 7.076 0.76442 grains that tend to break along cleavage planes (Pittman, 1969) or 4 63 to 125 0.397 1.471 4.272 0.74202 along microfractures (Haldorsen, 1977). The resulting variation of Reedy Sediment Rb/Sr ratios of feldspar in different grain-size fractions facilitates 1 500 to 1.000 0.559 0.6570 1.905 0.72738 • 2 250 to 500 0.994 0.5095 1.477 0.72403 dating of such fractions by the Rb-Sr method, as shown by Faure 3 125 to 250 1.508 0.4572 1.325 0.72256 4A 63 to 125 0.870 0.4692 1.360 0.72078 and Taylor (1981) in a previous study of glacial deposits in the 4B 63 to 125 0.4807 1.392 0.71945 Transantarctic Mountains. The Reedy Sediment is much better sorted than is the Plateau Till, with a strong concentration of grains (60%) in the < 67 ;um The feldspar in the glacial deposits of the Wisconsin Range fraction (Fig. 2). A small abundance peak in the 250 to 500 pm may be a mixture derived from three possible sources: (1) the gra- fraction (11.6%) may result from the presence of grains derived nitic rocks of the Wisconsin Range of ages ranging from 473 ± 5 Ma from the Beacon sandstone. Granules and clasts more than 4 mm in (pegmatites) to 513 ± 12 Ma (granitic gneiss) based on Rb-Sr whole- diameter are much less abundant (1.6%) than in the Plateau Till and rock isochron dates of Faure and others (1979); (2) the sandstones are composed entirely of igneous rocks of granitic composition. The of the Beacon Supergroup that are mature quartz arenites but con- grain-size distribution of quartz in the Reedy Sediment appears to tain a few percent of feldspar of unknown age in some parts of the be bimodal, with peaks in the 67 to 125 /im and 500 to 1,000 /um section (Minshevv, 1966); (3) the granitic and metamorphic rocks of fractions, in marked contrast to the Plateau Till, which has a well- the Precambrian Shield of East Antarctica of ages ranging beyond developed unimodal quartz distribution pattern. Feldspar also 2.5 Ga (Grew, 1978; Grew and Manton, 1979). appears to be bimodal, but the separation of K-feldspar from plagi- The quartz grains in the Plateau Till are predominantly clear oclase, seen in the Plateau Till, is lacking in this sample. and angular. Rounded grains with frosted surfaces make up less The preferential concentration of K-feldspar in the coarse-sand than 5% of the size fractions. These observations suggest that the fraction of the Plateau Till and the enrichment of the Fine-sand Beacon rocks are not an important source of feldspar in the sample fraction in plagioclase give that till a strong positive correlation of Plateau Till analyzed here. In general, exposures of Beacon rocks between the K-feldspar/plagioclase ratio and grain size (Fig. 3). in the Wisconsin Range are confined to small remnants in a down- The Reedy Sediment is anomalous compared to previously studied faulted block in the valley of the Olentangy Glacier (Fig. 1). No till (Faure and Taylor, 1981) because its K-feldspar/plagioclase ratio is apparently independent of grain size. The difference may be caused by the fact that the Reedy Sediment was deposited in an 0.820 - ice-marginal meltwater pond and. thus may be a mixture of grains derived from different glacial sources.

0,800 - FELDSPAR PROVENANCE DATES

Feldspar plus quartz concentrates in four grain-size fractions 0.780 of the Plateau Till and Reedy Sediment were analyzed for dating by the Rb-Sr method. The analytical procedures and the possible interpretation of such feldspar provenance dates have been pre- 0.760 - sented by Faure and Taylor (1981) and by Taylor and Faure(1981). The analytical results are compiled in Table 1. All four grain-size fractions of the Plateau Till define a straight 0.740 - line on the Rb-Sr isochron diagram (Fig. 4). A least-squares cubic = 0.7144 + 0.0030 regression of these points to a straight line, using the method of York (1969) and the computer program of Faure (1977), yielded a 0.720 - slope of 0.00684 ± 0.00029, an intercept of 0.7144 ± 0.0030, and a linear correlation coefficient of 0.9981. Such a linear array of grain- size fractions of feldspar in till can be generated by two different 0.700 mechanisms: (1) All feldspar grains were derived from sources hav- ing the same age, in which case the line is an isochron the slope of 87 86 which yields the age of the sources, provided the feldspar remained Rb/ Sr closed to Rb and Sr. (2) The data points are mixtures of two feld- spar components derived from sources having different ages and Figure 4. Grain-size fractions of feldspar from a sample of lodg- different Rb/Sr and 87Sr/86Sr ratios. In this case, the linear array is ment till collected on the plateau of the Wisconsin Range. The date a mixing line the slope of which yields a "provenance date" that is calculated from the slope of the straight line is compatible with the intermediate in value between the ages of the two sources (Faure known ages of the granitic basement rocks in the Wisconsin Range. and Taylor, 1981). The numbered points are grain-size fractions identified in Table 1.

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outcrops of sandstone were seen on the Wisconsin Range plateau, although some may exist under the ice and snow that now cover the surface. Moreover, indirect evidence from a till collected from the summit plateau of Table Mountain in southern Victoria Land indi- cates that the feldspar derived from Beacon rocks at that site has a date of about 500 Ma, which suggests that it was derived from highlands underlain by granitic rocks having the same age as the local basement rocks (Faure and Taylor, 1981). For these reasons, the sandstones of the Beacon Supergroup do not seem to have contributed a distinctive component of feldspar to the glacial depos- its on the plateau of the Wisconsin Range. In the absence of a detectable feldspar component derived from the Beacon Supergroup, the significance of the colinearity of the data points in Figure 4 can be evaluated by the following criter- ion: If the date calculated from the slope of the line is significantly older than the known age of the granitic basement rocks in the Wisconsin Range, then a Precambrian component may be present in the till. In that case, one may conclude that the till was deposited by ice originating in East Antarctica because that is the only known source of Precambrian rocks in this part of the Transantarctic Mountains. The date actually derived from the slope of the straight line formed by the feldspar size fractions of the Plateau Till is 480 ± 21 Ma. This date is indistinguishable from the known ages of the granitic basement rocks exposed in the Wisconsin Range. There- fore, there is no evidence to suggest the presence of a component of Precambrian feldspar in this sample. This result thus supports the hypothesis of Mercer (1968), according to which this till was depos- ited by a local ice cap on the Wisconsin Range plateau. A local Figure 5. Grain-size fractions of feldspar from glaciolacustrine provenance of the till also implies that the Wisconsin Range was sediment along Reedy Glacier. The line defined by three of the four sufficiently elevated to permit an ice cap to form before the East fractions yields a date that is older than the ages of the basement Antarctic ice sheet had formed. rocks represented by isochrons A and B from Faure and others The feldspar size fractions of the Reedy Sediment do not fit the (1979). The numbered points are grain-size fractions identified in line in Figure 4 formed by the feldspar of the Plateau Till. Instead, Table 1. fractions 1, 2, and 3 of the Reedy Sediment form a straight line having a slope of 0.00821 ± 0.00030, an intercept of 0.7117 ± 0.0005, and a linear correlation coefficient of 0.9658 (Fig. 5). However, feldspar in the fine-sand fraction of the Reedy Sediment the feldspar in the Reedy Sediment therefore cannot be ruled out. (4A and 4B in Fig. 5) deviates significantly from the line formed by However, the steeper slope of this mixing line compared to isoch- the three coarser fractions. Sample 4A was purified ultrasonically rons A and B requires that the feldspar in the Reedy Sediment was for about 4 to 6 hr similar to the amount of cleaning given fractions derived from two specific sources having 87Sr/86Sr and 87Rb/86Sr 1, 2, and 3. Sample 4B was given more extensive cleaning for about ratios that must lie between isochrons A and B along the observed 12 hr, which decreased its 87Sr/86Sr ratio from 0.72078 (4A) to feldspar mixing line. 0.71945 (4B) and increased its Rb/Sr ratio from 0.4694 to 0.4807. An alternative interpretation can be supported by considering These results suggest that the feldspar grains are coated with altera- the date calculated from the slope of the mixing line. This date is 87 86 tion products having higher Sr/ Sr ratios but lower Rb/Sr ratios 576 ± 21 Ma, which exceeds the known age of the granitic basement than the feldspar. Clauer (1981) came to a similar conclusion in a rocks by a significant margin. The older date may indicate the study of weathered feldspars derived from igneous rocks in the presence of a small amount of feldspar of Precambrian age presum- Republic of Chad. The magnitude of the effect increases with ably derived from East Antarctica. According to the model decreasing grain size because the fine-sand fraction contains a presented by Faure and Taylor (1981, equation 6), the abundance of higher proportion of alteration products per unit weight than do the this hypothetical Precambrian component in the feldspar of Reedy coarser fractions. For example, an alteration layer of 10 yum thick- Sediment is only about 3%, assuming ages of 2.7 Ga and 0.5 Ga for ness constitutes about 28% by weight of the grains in the 63 to the Precambrian and local feldspars, respectively. However, the 125 fraction but only about 4.0% of the 500 to 1,000 /¿m presence of feldspar of Precambrian age in Reedy Sediment is con- fraction. These considerations indicate that the fine-sand fraction sistent with the fact that this glacier drains the East Antarctic ice (63 to 125 (jm) of feldspar in till may be less reliable for dating than sheet. the coarser fractions and therefore should not be used. All of the data points for the Reedy Sediment in Figure 5 lie SUMMARY between isochrons A and B for the local granitic basement rocks. The colinearity of fractions 1, 2, and 3 therefore may be accounted The study of one sample of till deposited by a wet-based glacier for by mixing of grains derived from granitic rocks exposed along on the Wisconsin plateau (elevation 2,500 m above sea level) indi- the walls of the valley occupied by Reedy Glacier. A local origin for cates a Rb-Sr feldspar provenance date of 480 ± 21 Ma. The good

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agreement of this date with the known age of the granitic basement — -- 1980, Late Wisconsin reconstruction for the Ross Sea region, Antarctica: Journal of Glaciology, v. p.231-244. rocks of the Wisconsin Range supports the hypothesis, based on Faure. G,. 197?, Principles of isotope geology; New York. Wiley and Sons, 464 p. Faure, G., and Taylor, K. S.. 1981. Provenance of some glacial deposits in the Transantarctic Mountains has? field evidence, that this deposit was formed by a local ice cap that on Rb-Sr dating of feldspars: Chemical Geology, v. 32, p. 271-290. developed prior to the growth of the East Antarctic ice sheet. No Faure, G., Murtaugh. J. G.,and Montigny. R., 1968. The geology and geothronology of the basemen! corople; of the central Transantarctic Mountains: Canadian Journal of Earth Sciences, v. 5. p. 555-560. evidence was found to suggest that this deposit was formed by the Faure, G., Easain. R.. Ray, P. T., Mclellan, D„ and Shult?. C. H-, 1979, Geochronology of igneous and

metamorphic rocks, centra! Transantarctic Mountains, in Laskar, B.v and Raja Rao. C. S., cds,. Inter- East Antarctic ice sheet. national Gondwana .Symposium, 4th. Proceedings, Volume 2: Delhi, India, Hindustan Publishing Co.. A second sample of glaciolacustrine sediment deposited in a p. B05-K13. Grew. E. S,, 197*1, Precamfcrian basement at Molodezhnaya Station. East Antarctica: Geological Society of melt-water pond along Reedy Glacier has a feldspar provenance America Bwltetin. v. M, p. 801-813. Grew. E. S.. and Mamon. W. I., 1979, Archean rocks in Antarctica: 2.5 billion year uranium-lead ages of date of 576 ±21 Ma. The older date can be attributed to the pres- pegmatites in Enderby Land: Science, v. 206, p, 443-445. ence of a small amount of Precambrian feldspar derived from East Grindley, G, W.. 1967, The geomorphology of the Miller Range. Transa ma rc-tic Mountains, with notes on the glacial history and neoteetonics of Antarctica: New Zealand Journal of Geology and Geophysics, v. 10. Antarctica. Although the presence of such a component in this till is p, 557-598. Haldorsen. S,. 1977, The petrography of till- -a study from Ringsaker, southeastern Norway: Norges Geolo- not certain, it is plausible on geological grounds because the Reedy giske Undcrsokelse (Skrifterj 336, 36 p. Glacier had already become an outlet for the East Antarctic ice Kvasov, D- D,. and Verbitsky, M. Ya., 1981. Causes of Antarctic glaciaiion in the Ceno/oic; Quaternary Research, v. 15. p. 1-1?. sheet when this sediment was deposited. Mayewski. P. A.. 1975, Glacial geology and late Cenazoic history of the Transantarctic Mountains. Antarctica: Ohio State Universitj Institute oi' Polar Studies Report 56, 168 p. Mayewskt. P, A. and Goldthwajj, R. P., in press. Glacial events itt the Transantarctic Mountains: A record of the East Antarctic ice sheet, in Turner, M. D.. arid Splettstoesser. J. F„ eds.. Antarctic Geology: ACKNOWLEDGMENTS Washington. D.C., American Geophysical Union. Mercer, J. H.. 1968, Glacial geology of the Reedy Glacier area. Antarctica: Geological Society of America Bulletin, v. 79. p. 471-486. The petrographic description of a thin section of sample F-64- 1972. Some observations on the glacial geology of the Heard more Glacier area, in Adie, R. J., ed.. Antarctic geology and geophysics: Oslo, Norway. Universuetsforlaget, p. 427-433. !6 was made by T. M. Mensing. Critical comments by P. A. 1978. Glacial development and temperature trends in the Antarctic and in South America, in Van Mayewski, P. E. Calkin, and G. H. Denton are gratefully acknowl- Zinderen ftekker. E. M..ed,, Antarctic glacial history and world pafaeoeimronmem: Rotterdam. A, A. Balkema, p. 73-93. edged. 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