Proglacial sedimentation of late Wisconsin age in Miers Valley, Antarctica

Miers Valley is an ice free valley in the Koettlitz Region of Victoria Land, Antarctica. The eastern basin of this valley has extensive coverage of gypsum and laminated calcite overlying fine silts and sands, while Lake Miers occupies the western basin. Mapping of the surficial sediments has shown that a proglacial lake at least 80m deep extended up the valley (Glacial Lake Trowbridge), dammed to the east by a glacial tongue extending from the coast, C-14 and U/Th dating of the lacustrine carbonates has shown that this lake occupied Miers Valley between 10,000 and 20,000 yrs BP and was at its maximum between 18,000 and 19,000 yrs BP. Dating of individual lamina has shown that calcite deposition was not continuous, but proceeded in at least three pulses. This is supported by stable isotope analyses which show an enrichment in ¹⁸O with time, suggesting the possible concentration through evaporation of ¹⁸O during the lake's history. Impressions within the calcite plates indicate that a crystalline material, possibly gypsum, was crystallizing contemporaneously with the calcite. This salt was subsequently flushed from the system on the draining of the eastern basin. At the final stages of occupation, large quantities of gypsum were deposited into the eastern basin from the glacier terminus and draining of the lake resulted in a surface coverage of gypsum throughout the eastern basin. Shallow mounds of coarse basalt rich drift overlie the carbonate, gypsum and silt beds in places throughout the eastern basin. On the surface ice cover of Lake Miers, similar mounds occur. The ice beneath these mounds is virtually sediment free and the sediment on the surface differs from that on the lake bed. It is suggested that the mounds on the ice raft and the mounds in the eastern basin share a common origin. Supraglacial and englacial sediments carried into the valley with the Ross Ice Tongue were deposited onto the floating ice cover of Glacial Lake Trowbridge. This material was then rafted across the valley as the lake ice was pushed forward by ice fed from the glacial tongue. The sediments remained supported on the lake until they reached the annual melt out moat or the lake drained. They were then deposited on top of the lacustrine sediments, thereby preserving them (as occurred in the eastern basin), or in the case of the western basin a lake remained so the mounds were preserved on the ice cover which had then become immobile. Once the proglacial lake drained, most of the lacustrine sediments were then exposed to the environment. Scanning Electron Microscope studies have shown that the exposed lacustrine carbonates have undergone chemical weathering. These carbonates are pitted and weathered compared to much older and less exposed carbonates in neighbouring Marshall Valley