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Sedimentary facies associated with Antarctica’s floating ice masses

By
John B. Anderson
John B. Anderson
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Douglas S. Kennedy
Douglas S. Kennedy
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Michael J. Smith
Michael J. Smith
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Eugene W. Domack
Eugene W. Domack
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Published:
January 01, 1991

Large floating glaciers are presently confined to the polar regions of Antarctica. The sedimentary facies associated with these ice masses therefore appear to be unique to the polar setting. However, there is a wide range of glacial maritime settings in Antarctica, and no single model is adequate to illustrate the variety of lithofacies to be found in these different settings.

The largest floating ice masses of Antarctica are the Ross, Ronne-Filchner, and Amery Ice Shelves, which are the floating extensions of the ice sheet and occur at the confluence of large ice streams. Theoretical studies of the dynamics of large ice shelves suggest that the basal debris they transport is associated within a fairly narrow (tens of kilometers wide) transition zone where the ice is intermittently coupled to the sea floor. The zone is characterized mainly by subglacial deposits (basal tills) and glacial marine diamictons (transitional glacial marine sediments). These two sediment types are virtually identical.

In the Ross Sea, surface sediments consist of diatomaceous glacial marine sediments. These sediments are relatively thin over most of the continental shelf, where they rest in sharp contact on basal tills. Transitional glacial marine sediments are rare, and basal tills grade offshore into bioclastic carbonates. These stratigraphic relations imply that the ice sheet was fully grounded on the shelf during the last glacial maximum and that seaward of this expanded ice sheet the sea floor was starved of terrigenous sediment. The retreat of the ice sheet from the continental shelf was so rapid that sub–ice shelf sediments were not deposited. Rapid retreat is expected within a foredeepened shelf setting.

Fringing ice shelves represent the second most extensive bodies of floating glacial ice in Antarctica. They may contain a significant portion of ice that is accreted onto the ice shelf directly, either by surface accumulation or basal freezing of seawater. They tend to be thinner than large ice shelves, which implies conditions more suitable for basal freezing. Fringing ice shelves may contain significant englacial and supra-glacial debris that is entrained by the ice shelf from nearby tributary glaciers originating near mountains and nunataks. This implies that fringing ice shelves are more efficient at delivering sediment seaward of the grounding line than are large ice shelves. The Larsen Ice Shelf, as an example, has associated with it an extensive diamicton facies. These diamictons are overlain by predominantly terrigenous, glacial marine sediments whose grain size shows clear influence of marine currents on sedimentation.

The third type of ice shelf setting studied (the George VI Ice Shelf) is one that is confined by valley walls. It is the only one of the three examples studied in which an ice shelf recessional lithofacies is clearly recognized. This lithofacies is composed of transitional glacial marine sediments that grade upward into terrigenous muds, which are inferred to be derived from subglacial meltwater outflow. The sub–ice shelf facies is overlain by diatomaceous muds and oozes, which indicates open-marine conditions.

Ice tongues, while small in size, represent a significant mode of ice drainage from the Antarctic continent. The two examples studied include the shelf areas adjacent to the Drygalski Ice Tongue and the Mertz and Ninnis Ice Tongues. There is an apparent absence of a transitional glacial marine facies in these ice tongue settings. This implies more widespread dispersal of debris by currents and the icebergs that calve from the tongues. Marine facies associated with ice tongues contain a large biogenic component, which is primarily siliceous.

The floating portions of tidewater glaciers represent the smallest element of floating ice masses in Antarctica, and the only ones that exist in subpolar regions of the northern Antarctic Peninsula. Recent observations have led to the discovery of tunnel valleys beneath tidewater glaciers of the northern Antarctic Peninsula. These tunnel valleys are believed to act as conduits for cooling of seawater and limited subglacial meltwater outflow to the bays and fjords. The sedimentary facies associated with floating tidewater glaciers consist of massive and laminated muds within thin sand layers. Associated facies include diamicton and siliceous mud.

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GSA Special Papers

Glacial marine sedimentation; Paleoclimatic significance

John B. Anderson
John B. Anderson
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Gail M. Ashley
Gail M. Ashley
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Geological Society of America
Volume
261
ISBN print:
9780813722610
Publication date:
January 01, 1991

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