THE SEDIMENTARY BEDROCK OF THE MARTIAN HEAVILY CRATERED TERRAIN

Mars has a sedimentary (largely mafic, clastic) rock record of depositional and diagenetic environments chronocorrelative with Earth's Hadean and early Archean Eons. Like early Earth, the original surface of early Mars was heavily damaged by impactors and weathered and eroded in the presence of gravity and gaseous (wind), liquid (water), and, perhaps, solid agents (ices). For the past 15 years, the discussed and examined examples of Martian sedimentary rock were found largely in four settings: in craters, in the Valles Marineris chasms, the "Medusae Fossae Formation," and the plains-forming bedrock of Meridiani Planum and some of the plains cut by the Valles Marineris. The works that describe these are bracketed by a seminal paper published in 2000 (doi:10.1126/science.290.5498.1927) and a key overview and synthesis published in 2012 (doi:10.2110/pec.12.102). Owing to the recent achievement of > 95% coverage of Mars at a uniform 6 m/pixel, coupled with new geomorphic and sedimentologic observations made at the Curiosity rover site in Gale crater, it is now clear that the previously discussed examples are a sub-set of a greater whole. Sedimentary rock occurrences, mainly of a clastic nature, are widespread across Mars. In particular, they compose the bulk of the bedrock of the heavily cratered terrain, including crater interiors, intracrater areas, and the rock in which the craters formed; very little consists of igneous rock bodies. Bedrock is at or very near the surface almost everywhere in the heavily cratered terrain. It is a patchwork of differentially wind-eroded bodies of rock of differing chronostratigraphic position protected temporarily, on geologic time scales, by cap rocks, lags, and recent sediment deposits. In some places the patchwork has been punctured and overprinted by the results of more recent impact events (damaged bedrock, impact-generated debris, impact-induced geomorphic change such as water release) and/or polar/periglacial materials. Some sedimentary rock occurrences are readily recognized throughout the heavily cratered terrains because the erosional form of the rock body resembles the original depositional sediment body form (e.g., lithified eolian dunes, fluvial deltas, alluvial fans, stream channel sediments).