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