MARS IS A MIRROR, 2: THE PAHRUMP HILLS MEMBER OF THE MURRAY FORMATION, GALE CRATER, MARS—AN EVAPORATIVE LAKE FACIES ASSOCIATION FORMED IN AN UNDERFILLED THROUGH-FLOW LAKE BASIN TYPE

Our holistic study of the Pahrump Hills section of the Murray Formation integrated sedimentology, stratigraphy, mineral and elemental analyses, geochemical modeling, lab experiments, and Earth analogs in a sequence-stratigraphic and paleogeographic framework. It shows these strata accumulated as an evaporative lake facies association in an underfilled lake basin with closed surface hydrography but through-flowing hydrology. Lake waters were saline to hypersaline; lake levels, shorelines, and salinities fluctuated greatly at various temporal scales. Previous work envisioned a freshwater lake with stable lake levels.

These ca. 3.5 Gy rocks range from fine detrital mudstone to medium sandstone. Parasequence stacking mainly records vertical aggradation of the products of desiccation cycles. Mudstone bedding comprises planar-parallel and graded beds, current ripples, wave-formed structures, and common and widespread truncation. Sandstone beds contain current ripples, planar-tabular and trough cross-beds, compound cross-beds, foreset bundles, reactivation surfaces, mud drapes, and mud clasts.

The most common mudstone facies has alternating mm-scale layers: softer layers consist of poorly sorted mudstone, with coarse silt-size particles dispersed in a matrix of medium to fine mud; harder layers have a similar matrix but contain sand-sized sediment-incorporative crystal pseudomorphs with rhombic, pyramidal-triangular, lath-blade, and lozenge shapes. These shapes imply that multiple evaporite minerals were present initially (supported by XRD, compositional data, and geochemical modelling). Gypsum was most common; epsomite, thenardite, gaylussite/pirssonite, and kerolite occurred. CheMin data suggest the original minerals were altered during burial and exhumation.

A lack of bedded evaporites at Pahrump can be explained by the position of these strata in a marginal subbasin with through flowing groundwater. Bedded evaporites would have formed in remnant brine pools in the overall basin center where groundwater discharged.

Placing these strata into the context of lake-basin systems on Earth enables detailed prediction away from sample control, selection of appropriate analogs, and suggests that the elevated salinity lake waters were potentially hospitable to microorganisms.