DEPOSITIONAL ENVIRONMENTS OF THE MURRAY FORMATION AT THE PAHRUMP HILLS LOCALITY, GALE CRATER, MARS: SEDIMENTATION ON A LAKE-FLOOR FAN DRIVEN BY CLIMATIC-RELATED LAKE-LEVEL FLUCTUATIONS

This study presents characteristics of the basal 20 meters of the Murray formation at the Pahrump Hills locality in Gale crater, Mars. The following four lithofacies are recognized based on observations made on MAHLI and Mastcam images. (1) The laminated mudstone lithofacies is the dominant rock type at Pahrump Hills. Its lamina ranges in thickness from 0.1 mm to 0.3 mm. Laminations are commonly truncated by scour surfaces. (2) The muddy sandstone lithofacies occurs in 1 m to 5 m-thick layers and grades into the laminated mudstone laterally and vertically. Layers display massive, laminated, and cross-bedding sedimentary structures. (3) The coarse-grained sandstone lithofacies forms lenses that are encased in laminated mudstone. It shows excellent cross-bedding. (4) The speckled bed lithofacies is sparsely occurring thin layers of mudstone with scattered grains of medium to coarse sand.

Mud cracks, wave ripples, strandline deposits such as beach rocks, exposure surfaces, and features specific to aeolian processes were not observed at Pahrump Hills. The absence of these shallow-water indicators suggests that the basal 20 m of the Murray formation was deposited in deep waters, below the influence of waves. Discontinuous laminations, abundant scour surfaces, and the presence of cross-beds indicate that deposition of the laminated mudstone and muddy sandstones lithofacies was dominated by bedload traction associated with bottom currents most likely of gravity flow origin such as tubidity currents. The geometry and sedimentary structures of the coarse-grained sandstones are consistent with deposition in subaqueous channels (chutes). Speckled beds are typical of fine-grained debris flows that occur such environments. Combined, these features are indicate a lake-floor fan depositional system in a deep lake.

In the absence of tectonics, sedimentation was controlled by runoff and sediment supply, both a function of climate. Sandstone and mudstone layers were deposited during lake-level rises or highstands, likely corresponding to warm periods when runoff and therefore sediment supply were at their maximum. Sedimentation may have slowed or stopped during lake-level lowstands of cold and dry intervals when the lake became saline leading to the growth of sulfate nodules in sediments.