FACIES ANALYSIS AND STRATIGRAPHIC CONTEXT OF THE PAHRUMP HILLS OUTCROP, TYPE LOCALITY OF THE BASAL MURRAY FORMATION, GALE CRATER, MARS

The Mars Science Laboratory Curiosity rover’s arrival at the Pahrump Hills outcrop marked the start of the mission’s exploration of Aeolis Mons. At Pahrump Hills, the rover team examined a 14 meter-thick section of the Murray formation, the basal stratigraphic unit of Aeolis Mons, using Curiosity’s instruments and tools to document compositional variations and sedimentary structures and textures. This study presents an analysis of the stratigraphic context and sedimentary facies observed at Pahrump Hills, providing a descriptive baseline and depositional framework with which to interpret the team’s continued observations of the Murray formation as Curiosity ascends Aeolis Mons.

Four main sedimentary facies were observed at Pahrump Hills. A conglomerate facies composed predominantly of gravel-sized clasts crops out intermittently near the base of the section, but the age relationship of this facies relative to the rest of the section is uncertain due to a lack of internal bedding and clear contacts with nearby outcrop. Approximately 12 of the 14m of section exposed at Pahrump Hills consist of thinly laminated mudstone characterized by uniform, very fine-grain size and abundant mm-scale even and parallel laminations. The presence of low-angle truncation surfaces throughout the section interpreted as scour-and-drape structures suggest that this facies is best interpreted as a stacked sequence of river-generated hyperpycnal flow deposits in a lacustrine setting. Interbedded within the mudstone is a cross-laminated, very fine to coarse sandstone facies that occurs as discrete lenses bounded by basal erosional scours. Stacks of climbing ripples indicate unidirectional current flow in subaqueous channels proximal to the source of the plunging river plume. Pahrump Hills is capped by the thickly laminated sandstone facies, which contains well-rounded medium to coarse sand grains. This facies is conformable with underlying thinly laminated mudstones, and is consistent with a coarsening upward trend. The transition from predominantly mudstones at the base of Pahrump Hills to interbedded sandstones and mudstones in the upper half of the section records progradation and a transition to a more proximal lacustrine depositional setting up-section.