MARS IS A MIRROR, 1 – UNDERSTANDING THE PAHRUMP HILLS MUDSTONES FROM A PERSPECTIVE OF EARTH ANALOGS THROUGH AN INTEGRATIVE PHYSICAL-CHEMICAL ANALYTICAL-EXPERIMENTAL-MODELING STUDY

The Pahrump Hills area of Gale Crater has the most extensive dataset of physical and geochemical observations yet collected on Mars. We moved towards a holistic interpretation that differs substantially from prior work by integrating sedimentology, stratigraphy, mineral and elemental analyses, geochemical modeling, laboratory experiments, and Earth analogs into a sequence-stratigraphic and paleogeographic framework. We avoided bias during interpretations of spatial trends by explicitly considering tool spatial resolution, target access and facies, and outcrop character.

The 15-m-thick study interval is about 3.5 Gy old and contains five facies that range from fine to coarse detrital mudstone with abundant sediment-incorporative evaporite pseudomorphs to medium sandstone. All sediments are dominated by first-cycle grains of minimally weathered primary igneous minerals but with four distinct provenances. The mudstones show planar-parallel beds, current ripples, and wave-induced structures, with common and widespread truncation. The absence of desiccation and synaeresis cracks is probably due to minimal clay-mineral content, as supported by our lab experiments. Evaporite minerals probably formed on and within detrital muds shortly after accumulation by evapoconcentration and cooling.

The Pahrump succession and its lateral equivalents contain all the sequence-stratigraphic elements known from terrestrial strata, with 16 parasequences in five depositional sequences. Two of the sequence boundaries are unconformities that record significant shifts in the behavior and paleogeographic configuration of the fluvio-lacustrine system. This contrasts with the previous view that all facies are genetically related. Changes in provenance that integrate changing drainage basin configurations, type of exposed bedrock, and changes in weathering regime explain most of the stratigraphic variability in rock composition.

We interpret these strata as an Evaporative lake facies association that accumulated in a through-flow Underfilled lake basin. Lake waters were saline to hypersaline; pore waters ranged from acid to mildly alkaline. Salinity, pH, lake level, and shoreline position fluctuated widely at various temporal scales. Saline lake waters would have been rather hospitable to microbes.