--> Deep-Water Fan Sedimentation on Mars: An Example From the Murray Formation at Pahrump Hills Locality, Gale Crater

AAPG ACE 2018

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Deep-Water Fan Sedimentation on Mars: An Example From the Murray Formation at Pahrump Hills Locality, Gale Crater

Abstract

The Murray formation is a 260-meter thick mudstone that is exposed at the foothills of Mt. Sharp in Gale crater, Mars. This study presents characteristics of its basal 20 meters at the Pahrump Hills locality that were investigated in detail by the Curiosity Rover. The following three lithofacies are recognized: (1) the laminated mudstone, (2) the muddy sandstone, and (3) the coarse-grained sandstone.

The laminated mudstone lithofacies is the dominant lithology. Each lamina ranges in thickness from 0.2 mm to 2 mm. Laminations display a rhythmic pattern consisting of alternating resistance and non-resistant lamina. Most importantly, laminations extend only for few meters before being truncated by other laminations. The muddy sandstone lithofacies occurs as elongated lenses interbedded with the laminated mudstone. One investigated in detail is about one meter thick and has three intervals. Its basal part is massive and transitions upward into a laminated interval which grades upward to a cross bedded layer. The coarse-grained sandstone lithofacies forms lenses within the laminated mudstone. It shows excellent cross-bedding.

Features indicative of shallow waters such as mud cracks, wave ripples, strandline deposits, exposure surfaces, and aeolian characteristics were not observed. This suggests that the basal 20 m of the Murray formation was deposited in deep waters. Discontinuous laminations, abundant scour surfaces, and the presence of cross-beds indicate that deposition was dominated by traction sedimentation. A mechanism capable of producing these features in deep waters is gravity flow processes in a deep lake or a marginal sea that had developed in Gale crater. As such, the muddy sandstone and the overlying mudstone formed by waning bottom currents such as turbidity flows. The geometry and sedimentary structures of the coarse-grain, cross-bedded sandstones are consistent with deposition in subaqueous channels (chutes) or channel-levee complexes. The most likely environment that encompasses these features is a fan depositional system at the bottom of a deep lake or a marginal sea.