EVOLVING MODELS FOR FACIES AND COMPOSITIONAL VARIATIONS IN THE MURRAY FORMATION MUDSTONE, GALE CRATER, MARS (Invited Presentation)

The Murray formation, Gale crater, Mars contains one major depositional facies – laminated mudstone – deposited in a lake; and minor additional facies – ripple cross laminated and trough cross bedded sandstones – representing subaqueous delta foreslope, fluvial, or eolian environments. The persistence of fine lamination, locally with scour and drape truncation surfaces, and absence of desiccation cracks, prism cracks, intraclasts, displacive evaporite crystals and nodules all suggest a perennial lake formed at depths great enough to avoid seasonal desiccation. Possible eolian and fluvial deposits would indicate base level lowering during longer-term lake level oscillation. Two compositional facies are observed based on elemental and mineralogic data, a hematite-sulfate (HS) facies and magnetite-silica (MS) facies. The HS facies comprises the lower few meters of stratigraphy exposed in the foothills of Mt Sharp, and transitions upward without any major change in the physical facies into the MS facies which persists through ~10 meters up to higher levels in the Murray formation. Thinning of lamination from ~2 mm to ~0.5 mm coincides with the compositional facies transition. At the highest stratigraphic levels studied by Curiosity hematitic rocks are encountered again but lack significant sulfates that are not associated with later diagenesis; physical facies are a mixture of finely laminated mudstones and trough crossbedded fine sandstones. Two models best fit the physical and compositional data. In the first case, variations in the composition of fine clastic detritus delivered to the lake via marginal sediment plumes, coupled with redox oscillations in the composition of authigenic minerals precipitated from the lake. In the second model, originally reduced sediments of either detrital or authigenic origin are oxidized during burial diagenesis related to circulation of oxidizing groundwaters, converting reduced species, e.g. magnetite and sulfides, to hematite and Fe-sulfates. In either case, at least some fraction of the high silica material in the HM facies is likely be detrital (tridymite), and it seems most likely that Mg-sulfates which form concretions are related to burial diagenesis and not the primary environment.