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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/11/1275/3419206/i0016-7606-94-11-1275.pdf by guest on 30 September 2021 i ¿oO FAURE AND OTHERS 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/11/1275/3419206/i0016-7606-94-11-1275.pdf by guest on 30 September 2021 Rb-Sr DATES, GLACIAL DEPOSITS, WISCONSIN RANGE 1277 Wisconsin Plateau receded.