COARSE,WIND BLOWN GRAVEL DEPOSITS, BENNETT PLATFORM, ANTARCTICA: CONSTRAINING WIND VELOCITIES DURING TRANSPORT

Wind blown gravel dunes on the Bennett Platform, Shackleton Glacier area, Antarctica, document transport of very coarse clasts by intense windstorms. The dunes have a wavelength of approximately 10 to 15 meters, a wave height of 10 to 15 centimeters, and consist of gravels that range from 4 to >100 mm in maximum dimension. After measuring clast dimensions, we applied Bagnold’s [1941] empirical formulation for threshold friction velocities* (V) to overcome friction and initiate saltation. We took (length x width x height) 1/3=“D” as an approximation for diameter to apply Bagnold’s equation to constrain V; measurement of dimensions of >1000 clasts yields “D” ranging from 3 to 53 mm. For clasts of this size, calculated threshold friction velocities range from 14.4 m/s for smaller samples up to 110 m/s for the largest. However, it should be noted that impact of saltating grains can move grains exceeding six times the size (volume) of the impactor. Thus, it is possible that our largest clasts do not reflect true friction, or wind, velocities, but rather were transported as a consequence of impacts by wind-induced saltation of smaller particles. At a minimum, assuming that the primary wind-saltated particles were 1/6 the volume of the largest fragments, friction velocities must have reached ~60 m/s. Based on sample measurements, saltation due to impact seems to be the most probable explanation because of the abundance of grains that are 1/6th the volume of the largest sample. Experiments on wind transport of the collected clasts will be conducted to determine an appropriate model of wind transport.

* Threshold friction velocities are the measured at the surface of the gravel and measure the velocity required to cause a sample to undergo saltation. The ambient airflow will be much faster.