A ROCK RECORD OF COMPLEX AEOLIAN BEDFORMS IN A HESPERIAN DESERT LANDSCAPE:THE STIMSON FORMATION AS EXPOSED IN THE MURRAY BUTTES, GALE CRATER, MARS

Aeolian strata encode information about ancient planetary surface processes and the climate at the time of deposition. By decoding these strata, insight can be gained into ancient sediment transport processes, bedform kinematics, the landscape and prevailing climate. Deciphering these environmental signatures requires detailed analysis of the sedimentary architecture to reconstruct the bedforms and conditions enabling them to form. Here we show that a distinct sandstone unit exposed on the foothills of Aeolis Mons (informally known as Mount Sharp) in Gale crater (Mars) represents the preserved expression of compound aeolian bedforms which accumulated in the centre of a large dune field. Interrogation of Mastcam image data of the Stimson formation shows that it consists of cross-stratified sandstone beds separated by a hierarchy of erosional bounding surfaces that were formed during dune migration. The presence of two orders of surfaces with distinct geometrical relationships enables us to show that the Stimson landscape comprised large dunes (draas) with smaller superimposed dune bedforms migrating across their lee slopes. Analysis of cross-lamination and second-order surface geometries indicate a complex wind regime that transported sediment to the north, constructing oblique dunes. This reconstructed environment is a direct product of the regional climate, and surface processes active in Gale crater during the Hesperian: the environment is shown to be arid, supporting a large aeolian dune field, which contrasts with earlier humid depositional episodes determined from the lacustrine sediments of the Murray formation. Such fine-scale reconstruction of landscapes on the ancient surface of Mars is important to understanding the past climate and habitability.