Making sense of martian sediments at the Kimberley, Gale crater
Thursday, 18 December 2014: 10:35 AM
During her crossing of Aeolis Palus, the plains region between the Gale crater rim and Aeolis Mons (Mount Sharp), Curiosity encountered a remarkably rich array of clastic sedimentary rocks primarily of fluvial and lacustrine origin. Our challenge is to reconstruct at fine-scale the ancient sedimentary environment. The Kimberley was chosen as a waypoint for detailed investigation because of a set of distinct 'orbital' stratal units identified in Mars Reconnaissance Orbiter (MRO) HiRISE imagery that showed repetitive stratal relationships. Curiosity's exploration of these outcrops in April and May 2014 provided an excellent 3-D window into complex sedimentary stratigraphy on Mars. Here, we describe in situ observations of sedimentary facies, and their spatial and relative temporal relationships. Sedimentary facies at the Kimberley include conglomerates, cross-bedded sandstones, and fine-grained, cross-laminated sandstones, all of which are overlain by a massive butte-forming unit, that is itself capped by a resistant, dark-toned fine-grained unit. The most distinct and scientifically perplexing facies is characterized by consistent southward-dipping decimetre-thick sandstone beds that can be traced over hundreds of meters. This facies partly equates to a 'orbital' unit characterized by NE-SW oriented lineations (informally termed the 'Striated Unit'). Topography data shows that the unit rises in elevation traced southward toward Mount Sharp. The consistent dip of the south-dipping sandstone facies, its apparent lack of variability in strike orientation, and the presence of prominent south-dipping beds observed in cliff face sections leads us to propose a model in which these represent clinoform geometrical elements. We discuss plausible models to explain the clinoforms (e.g., large-scale bedforms, downstream-accreting or laterally-accreting, small-scale delta mouth bars). In light of these models we consider the larger-scale stratal relationships at the Kimberley.