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Glacial Geology Glacial geology Late Cenozoic glacial history the maximum former expansion of these outlet glaciers. The upper limit marks the maximum former extent of mesa ice caps of the Terra Nova Bay region, when they spilled down valley walls between exposed ridges. northern Victoria Land, Antarctica A well-developed trimline on nunataks represent maximum former thickening of these mesa ice caps. At the head of Campbell Glacier in the southern Mesa Range, GIUSEPPE OROMBELLI the lower trimline matches that previously mapped in the Pa- cific Ocean drainage of the Outback Nunataks and Rennick Dipartimento di Scienze della Terra Glacier (Denton et al. 1986). Together, this overall trimline limit Llniversità di Milano marks the maximum recognized level reached by outlet glaciers and in northern Victoria Land. Centro di Studio per la Stratigrafico In the absence of isotopic dates, Denton et al. (1986) pos- e la Petrografia della Alpi Centrali tulated two widely differing age models (late Wisconsin and Milan, Italy pre-late Quaternary) for the trimline. Some insights in this regard come from the Terra Nova Bay region. Here, Terra Nova drift ("Younger" drift of Stuiver et al. 1981) is prominent near GEORGE H. DENTON the coast. This drift extends to 300-400-meter elevation in the Northern Foothills (Baroni 1989), and it covers both Inexpres- Department of Geological Sciences sible Island and Tarn Flat. Terra Nova drift is commonly con- and tinuous but in places consists of scattered erratics resting on Institute for Quaternary Studies bedrock. Exposed bedrock below the Terra Nova drift limit is University of Maine Orono, Maine 04469 rarely striated in the coastal area. Terra Nova drift is little weathered, shows common perched boulders, and in places exhibits an ice core. In the Northern Foothills ice cores from In January and February of 1989 and of 1990, we carried out 240-meter elevation exhibit a significant marine component. a joint investigation of the late Cenozoic glacial history of the On Tarn Flat, Terra Nova drift includes numerous eskers. In Terra Nova Bay region on the west coast of the Ross Sea in Inexpressible Island and the Northern Foothills raised marine northern Victoria Land. Our purpose was to examine past beaches now up to 30 meters in elevation are cut into Terra longitudinal profiles of outlet glaciers that drained through the Nova drift (Baroni and Orombelli in press). Numerous radi- Transantarctic Mountains into Terra Nova Bay, some from the ocarbon dates from these beaches define an emergence curve inland ice sheet and some from local mountain accumulation that shows declining rates of Holocene uplift that began shortly areas. By tying our new results to those previously obtained before 7,505±230 (GX-14063) years ago (uncorrected for the farther north in the Rennick Glacier drainage system that flows reservoir effect). Shells from marine muds included in Terra into the Pacific Ocean (Denton et al. 1986), we hoped to de- Nova drift at about 200-meter elevation in the Northern Foot- cipher ice-sheet behavior in northern Victoria Land during late hills date to more than 33,000 (GX-13605) (Baroni and Orom- Cenozoic glacial cycles. Our project was jointly funded and belli 1989). In all these characteristics, Terra Nova drift is similar supported by Programma Nazionale di Ricerche in Antartide to the well-dated Ross Sea drift in the McMurdo Sound area (Italy) and by the Division of Polar Programs of the National (Stuiver et al. 1981; Denton et al. 1986). We think it is highly Science Foundation (United States). likely, therefore, that Terra Nova drift is late Wisconsin and The geography of Terra Nova Bay is dominated by long earliest Holocene in age. outlet glaciers that flow through the high Transantarctic Moun- The upper limit of Terra Nova drift rises inland alongside tains and terminate either in open waters of Terra Nova Bay outlet glaciers. The figure shows the Terra Nova profile for or in the Nansen Ice Sheet, which floats on inner Terra Nova Reeves Glacier derived from ice-free areas on the north valley Bay. Campbell Glacier flows only from local mountain accu- wall. Here, a sharp break in surface weathering characteristics mulation areas, whereas Reeves and Priestley Glaciers drain separates Terra Nova from older drift. In two localities, a sharp- both local accumulation basins and the peripheral Tabs Dome crested moraine ridge occurs at this break. Below this break, of the east antarctic ice sheet. High plateau mesas between delicately perched boulders and polished and striated granite outlet glacier troughs support ice caps that commonly spill bedrock are common; above the break both are absent. These over plateau edges into these outlet glaciers. characteristics, including striated and polished bedrock below A well-defined erosional trimline with distinct upper and the drift boundary, also occur at Skinner Ridge, where the lower limits is etched into alpine ridges and spurs on valley Terra Nova profile is at or close to the trimline. walls alongside Reeves, Priestley, and Campbell Glaciers. The The figure illustrates that Reeves Glacier was thicker along lower limit, which rises smoothly inland (figure), represents its entire length when it stood at the Terra Nova drift limit. 1990 REVIEW 51 I • I I I I I I I E > \A11 2500 0 CD 0 C) E • D) V 0 C CD E U) C 0 C) -C (I) - 0 U) . U) 0 C) Cl) U- V C CL 1500 X C)- C) .-.--- C 0 Z - . 1000 M 500 Trimline ice surface Terra Nova ice surface Terra I Nova (\ n Present ice surface t f /1 / Nansen Ice Sheet Bay 120 100 80 60 40 20 0 Distance Inland From 164 0 East Latitude (km) Present and former longitudinal profiles of Reeves Glacier. Thickening was greatest, however, in coastal regions, where mapping and exposure-age dating of high-elevation mesa grounded ice filled Terra Nova Bay. The figure also illustrates striated surfaces and perched erratics resting on them. that the trimline is far above Terra Nova drift in the lower This work was support by National Science Foundation grant reaches of Reeves Glacier, but that the trimline is close to or DPP 86-13842. at the Terra Nova limit in the upper reaches. This suggests that the trimline may be metachronous, that is, old near the coast and late Wisconsin and even early Holocene in the in- terior. The trimline profile also suggests that interior ice thick- ened only marginally when coastal ice levels rose to the trimline References level. To clarify and check our reconstruct profiles, we are undertaking exposure-age dating of Terra Nova drift and of Baroni, C. 1989. Geomorphological map of the Northern Foothills near the trimline. the Italian Station (Terra Nova Bay, Antarctica). Meinorie della Società A major remaining problem concerns interpretation of striated Geologica Italiana, 33, 195-211. surfaces and trimlines near thin mesa ice caps. Exposed bed- Baroni, C., and C. Orombelli. 1989. Glacial geology and geomor- rock surfaces adjacent to these caps in the Eisenhower Range phology of Terra Nova Bay (Victoria Land, Antarctica). Memorie della Società Geologica Italiana, 33, 171-193. commonly exhibit polished and striated surfaces, and many Baroni C., and G. Orombelli. In press. Holocene raised beaches at have angular and perched surface clasts. In all respects, these Terra Nova Bay (Victoria Land, Antarctica). Quaternary Research. striated surfaces and perched clasts appear as fresh as those Delisle, G., L. Schultz, H. Hofle, and R. Thierback. 1989. Meteorite below the Terra Nova drift limit beside Reeves Glacier. Similar finds near the Frontier Mountain Range in North Victoria Land. high-elevation striated surfaces occur on granite mesas in the Geologische Jahresbuch, 38, 483-513. Outback Nunataks farther north. Here Denton et al. (1986) and Denton, G.H., J.G. Bockheim, S.C. Wilson, and C. Schluëhter. 1986. Hofle (1989) interpreted mesa striated surfaces in term of ice- Late Cenozoic history of Rennick Glacier and Tabs Dome, northern sheet overriding of the mesas. Hofle (1989) and Delisle et al. Victoria Land, Antarctica. In E. Stump (Ed.), Geological investigations (1989) placed such overriding at more than 3.2 million years in northern Victoria Land. (Antarctic Research Series, vol. 46.) Wash- ago on the basis of an exposure age of bedrock near the present ington, D.C.: American Geophysical Union. Höfle, H.-C. 1989. The glacial history of the Outback Nunataks area ice surface of upper Rennick Glacier. Our new results suggest in western North Victoria Land. Geologische Jahresbuch, 38, 335-355. caution, however, in inferring late Tertiary ice-sheet overrid- Stuiver, M., G. H. Denton, T.J. Hughes, and J.L. Fastook. 1981. History ing, for in the Eisenhower Range, it is possible that mesa of the marine ice sheet in West Antarctica during the last glaciation: striated surfaces reflect expansion of mesa ice caps during Terra A working hypothesis. In G.H. Denton and T.J. Hughes (Eds.), Last Nova glaciation. Solution of this problem awaits extensive great ice sheets. New York: Wiley-Interscience. 52 ANTARCTIC JOURNAL.
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