Textural characteristics of some glacial sediments in Taylor Valley, Antarctica

Abstract

Taylor Valley is an ice-free valley on the McMurdo Sound coast of southern Victoria Land, Antarctica. Lower Taylor Valley is filled with glacial drift at least 5.5 My old, which has been cored by the Dry Valley Drilling Project to a depth of 320 m. The glacial drift from DVDP holes 8 to 12 in lower Taylor Valley is grouped into six broad sediment types on the basis of full grain size analyses. The standard sediment textural statistics were found to be ineffective at grouping samples for the purpose of interpreting conditions during sediment deposition, and features of histograms and cumulative frequency curves have been used instead. The textural types are: Type I: poorly to very poorly sorted sandy gravel; traction current lag deposits and high intensity traction current deposits, Type II: poorly to very poorly sorted muddy sandy gravel or gravelly sandy mud; low density gravity flow deposits, high density traction current deposits or winnowed diamicton, Type III: well to poorly sorted sand or gravelly sand; moderate intensity traction current deposits, Type IV: poorly to very poorly sorted silty clayey sand to very poorly sorted gravelly sandy mud; low intensity traction current deposits, Type V: poorly to very poorly sorted silt and clay with sand and gravel-size debris; suspension deposits with ice-rafted debris, and Type VI: very poorly sorted gravelly muddy sand and sandy mud; diamicton, either till or gravity flow deposits. From the textural analysis it is inferred that the traction deposited sediment is derived from a diamicton source. Winnowing of very poorly sorted sediment by water is recorded in different stages by different sediment types. Starting with the diamicton (type VI), very minor winnowing only slightly modifies the original grain size distribution (type II). As the length of time the water acts increases, more material is transported and a lag gravel is left (type I). Once in the traction system, sediments fall into a naturally occurring preferential sorting trend (type III is better sorted than types I and IV). The traction transported sediment tends to become better sorted as the mean grain size tends to 2.5 Φ (0.18 mm). In preparing for the interpretation of the DVDP core sediment, samples were collected for grain size analysis from the glacial drift deposits in different Taylor Valley environments. Debris layers within the Taylor Glacier have a type VI texture and are probably a comminution product the most likely parent being basement granite. Ice-cored drift (meltout till) at the Taylor Glacier snout has a similar texture (type VI) to the englacial layers. Glacial debris has flowed by gravity (winnowed debris flow, type II) from the valley walls on to the surface of the Tedrow Glacier. Ice marginal, distant proglacial and supraglacial glaciofluvial deposits have grain size distributions typical of non-glacial stream deposits (type III). New Harbour beach at the mouth of Taylor Valley has more poorly sorted sediment when compared with beaches in more temperate climates. The grain size distribution of sand (type III) is like the adjacent stream sand. It is suggested that the sea ice, which covers the bay for 90 percent of the time, damps the waves and hence lessens the sorting effect the waves would otherwise have had on the sediment. The deposition of drift sequences in lower Taylor Valley is thought to have been within or near to the glaciomarine environment throughout Late Cenozoic time and is interpreted as recording the fluctuations in the ice cover and the thermal conditions at the base of the ice in the lower Taylor Valley. The tentative sequence of glacial events compares favourably with the previously established chronology of southern Victoria Land and climatic events in the Southern Hemisphere.