Date of Award

5-2002

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

Advisor

George H. Denton

Second Committee Member

Daniel F. Belknap

Third Committee Member

Kirk Maasch

Abstract

Beacon Valley is located in the western Dry Valleys, Antarctica, adjacent to the East Antarctic Ice Sheet (EAIS). The surficial material on the floor of Beacon Valley is segmented into large polygonal landforms separated by trenches. Buried beneath the polygons and surficial material is massive ground ice. One hypothesis is that the buried ice in upper Beacon Valley is glacier ice originating from local debris-covered glaciers. The networks of polygons and trenches form as the buried ice undergoes thermal contraction and sublimation. Contraction cracks that penetrate the surficial material and buried ice in Beacon Valley contain Late Miocene age volcanic ashes. The ashes postdate the buried ice. The preservation of such old ice implies a continuous extreme polar condition in Beacon Valley since late Miocene time. An alternative explanation is that the buried ice in Beacon Valley is modem ground ice that formed from percolation of melted, wind-blown snow that subsequently froze within the sediment mantle. Polygonal landforms would result from the seasonal freeze-thaw of the modem ground ice and surficial material. Continual freeze-thaw action, or cryoturbation, would create a mass of coalesced, modern ice lenses covered with older sediment. The buried ice in this case could be young, and hence could not be used to imply stable climatic conditions in Beacon Valley since the late Miocene. Polygons cover the surface of a debris-covered glacier that fills part of upper Beacon Valley and Mullins Valley. A survey of the debris-covered glacier surface indicates that polygons mature with distance from the equilibrium line. The polygon morphology highlights the transport path of the buried ice in upper Beacon Valley, which can be sourced to the cirque (accumulation zone) at the head of Mullins Valley. The buried ice in upper Beacon Valley is part of a coherent, massive ice body of glacial origin. A gray diamicton is draped over the buried ice. It has textural and weathering characteristics akin to englacial, buried ice sediment. This diamicton is classified as a till that formed from sublimation of buried ice. The sublimation till (28% sand, 69% gravel, and 3% mud) is sorted by narrow contraction cracks in the buried ice that results in sand wedge deposits (83% sand, 11% gravel, and 6% mud). The grain-sizes that comprise sublimation till and sand wedges indicate that sediment is initially derived from sublimation of the buried ice. Deep polygon trenches develop over thermal contraction cracks in the buried ice, and create traps for wind-blown sediment (reworked sublimation till, sand wedge sediment and volcanic ash.) The tops of some contraction cracks were void of sediment, indicative of a sediment starvation. In this case, any primary volcanic ashfall could descend directly into active sand wedges. As sublimation occurs, sand wedges containing volcanic ash can slump over the sublimation till and buried ice. The stratigraphy of massive weathered sand, with stringers of volcanic ash, resting on sublimation till and buried ice is widespread in upper Beacon Valley. Because the contraction cracks and sand wedges are secondary to the buried ice, the ashes contained in them can afford a minimum age for the buried ice. This study supports the concept of the ash chronology previously used (Sugden et al., 1995) to date the buried ice at late Miocene age, and argues for persistent polar conditions in Beacon Valley since that time.

Share