EVAPORITIC-LACUSTRINE MUDSTONE LAMINITES AND PRODELTA MUDLOBES - CONTINUITY AND CHANGE IN THE HARTMANN’S VALLEY INTERVAL, MURRAY FORMATION, GALE CRATER, MARS

The sedimentary fill of Gale crater on Mars is dominated by lacustrine strata that are the focus of investigations into surface processes, climate change, and habitability early in the planets history. We focus on the lower part of the explored portion of the Murray formation on Aeolis Mons (Mt Sharp) in the Pahrump Hills and Hartmann’s Valley field areas. At Pahrump Hills, sedimentary features and geochemical attributes of exposed mudstones suggest accumulation in an underfilled lake basin in which lake levels, shorelines, and salinities fluctuated greatly at various temporal scales. The immediately superjacent stratigraphic interval, designated by the rover team as the Hartmann’s Valley member, has previously been interpreted as an eolian-deposited sandstone interval. Our fresh examination of all available data, however, suggests that the Hartmann’s Valley member is strongly dominated by the same mudstone facies that compose most of the underlying Pahrump Hills member. The one novel facies element encountered in Hartmann’s Valley are undulose bedded to convex bodies of mudstone that are oriented roughly north-south, suggestive of sediment supply from a northern fluvial source. Mudstones analogous in scale and character to these undulose mudstones occur on Earth in multiple modern and ancient lacustrine successions and are related to deposition of prodelta mud lobes. The mudstones of the Hartmann’s Valley interval record a continuation of the underfilled evaporative lake system proposed previously for the underlying Pahrump Hills member but with somewhat more variable environmental conditions. Hartmann’s Valley mudstones contain more abundant matrix CaSO4, suggesting more severe evaporitic conditions, whereas the interbedded undulose bedded mudstones signify intermittent freshening and buildout of mud-dominated deltas into the lake. The textural similarities between Pahrump Hills and Hartmann’s Valley strata suggest that the Gale lacustrine system operated at a stable depositional mode with similar environmental parameters during both stratigraphic intervals.