Transportation of detrital materials on the lunar surface: evidence from Apollo 15

The thickness frequency distribution of stratigraphic layers intersected by the Apollo 15 deep core suggests that the majority of impact events reworking the lunar soil are small and produce ejecta blankets with an average thickness of less than 1·D5 cm. The energy frequency distribution of the meteorites producing the layers may be bimodal. The impacting meteorites produce both normal and reverse graded beds which appear to be the end products of two depositional mechanisms. First, the normally graded beds appear to be produced in base surges as escaping gases fluidize the flowing debris and larger particles move downwards in response to Stokes Law. Second, if the gas loss from the base surge is excessive the fluidization may cease and inertial grain flow dominates. In this situation the beds are reverse graded as larger particles move under dispersive pressure to the region of minimum shear stress at the upper boundary of the base surge. The same processes also produce measurable shape sorting of the particles in the beds.