PALEOENVIRONMENTS AND STRATIGRAPHIC ARCHITECTURE OF AN ANCIENT LAKE BASIN, GALE CRATER, MARS

Gale crater is located on fluvially dissected, cratered, highlands crust astride the dichotomy boundary on Mars. Sedimentary deposits in Gale were deposited, buried, lithified, exhumed, and eroded before 3.3-3.1 Ga and comprise a large mound (Aeolis Mons). Interfingering with fluvio-deltaic strata of the Bradbury group, is a diverse rock package dominated by mudstones termed the Murray formation. It is ~200 m thick, and has three major facies. Facies 1 consists of finely laminated gray-colored fine-grained rocks that typify the lower part of Murray formation. A variant on this facies resembles the gray laminated rocks, except being more purple in color and containing distinct patches of cm-scale concretions. Persistent fine lamination, coupled with an absence of desiccation cracks, suggests sedimentation in a lake deep enough to avoid regular desiccation. Facies 2 forms an ~25 m thick interval exhibiting meter-scale trough cross-bedding with steep foresets. Cross stratification at this scale is consistent with bedload sediment transport in an aeolian setting, particularly curved-crested dunes, although fluvial systems could produce such bedforms. Facies 3 is a heterolithic mudstone and sandstone. The lithologies include dark red, finely laminated mudstone, cm-scale ripple cross-laminated sandstone, dm-scale cross-stratified sandstone. Supporting evidence for sandstone lenses in the Murray comes from the ChemCam Laser Induced Breakdown Spectroscopy grain size proxy. Concretions locally disrupt primary lamination. Facies 3 also shows distinctive, small-scale polygonal fractures that resemble desiccation cracks. Their presence, with potentially contemporaneous gypsum precipitates, suggests deposition in lake and lake-margin environments dominated by suspension fallout with less common traction deposition. The broad facies arrangement of the sedimentary fill investigated along the >17 km traverse by Curiosity since 2012 is consistent with progradation of terrestrial deposits from the crater margin to a lake that occupied part of the crater interior. Overall, the facies types and architecture are consistent with an overfilled lake basin. In addition to the listed authors, this abstract summarizes the work of the entire Mars Science Laboratory Sedimentology-Stratigraphy Working Group.