GRAVEL DUNES FORMED BY EOLIAN PROCESSES IN A COLD DRY ENVIRONMENT, ALLAN HILLS, ANTARCTICA: A POSSIBLE MARS ANALOGUE

Dunes comprised of gravel sized grains are rare in the rock record except in a few environments. In most cases these features are found in subaqueous environments formed by alluvial discharge on alluvial fans and braided rivers. Gravel dunes also form in other environments during extreme events that include: hurricane force storms, tsunamis, or glacial outburst floods. High velocities are required to transport gravel size particles. In fact, specific requirements are needed for the transportation of gravel sized particles. These requirements include: relatively dense fluids with high flow velocities. Reports of gravel dunes formed by eolian processes are extremely rare as eolian transport requires extreme velocities due to the lower density of air. Recently, eolian gravel dunes have been found in Antarctica including dunes in a gravel sheet in the Allan Hills. The gravel sheet is located on an extensive bedding plane that is exposed to katabatic winds of off the Polar Plateau on the west. The sheet is surrounded on its other three sides by exposures of sandstone, mudrocks, and coal of the Triassic Lashly Formation, dolerite sills of Jurassic age (Ferrar), and abundant scree deposits. Gravel clasts in the sheet consist of highly polished and faceted dolerite ventifacts that range in size from 0.5 to 20 cm (ave 1.9 cm) in diameter. Within the sheet, dune crest are straight to slightly sinuous with an average height of 32 cm and an average dune spacing of 29.6 m. The largest dunes occur at the center of the sheet and decrease in height toward the sheet's edges. Using Bagnold's equation, frictional wind velocities of at least 54.58 m/s (196.5 km/h) are estimated for formation and migration of the dunes. This is consistent with maximum katabatic wind speeds in the nearby Dry Valley region of Antarctica which have been measured at over 320 km/h. Although evidence occurs for eolian transport on Mars, it is unlikely that gravel transport is possible due to the density of its athmosphere (~1/100 of Earth's).