UNPACKING DIAGENETIC OVERPRINTS ASSOCIATED WITH THE CLAY-SULFATE TRANSITION IN GALE CRATER, MARS

The relatively flat-lying sedimentary strata that compose the lower slopes of Mount Sharp allow the Mars Science Laboratory Curiosity rover to explore sequentially younger deposits while traversing upslope, and therefore interrogate the early environmental history of Mars. In Curiosity’s current vicinity, orbital signatures from the Compact Reconnaissance Imaging Spectrometer for Mars instrument reflect a transition region from clay-bearing sediments to Mg sulfate-bearing sediments. Because of their very high solubility, these younger Mg sulfate salts may indicate a drying event, shifting away from an earlier, wetter lacustrine depositional setting in which salt deposits were rare to absent as sedimentary materials.

In this study, we analyze rover observations using the full instrument suite from Martian day (sol) ~2900 onwards, which represents ~90 m of strata, to characterize the textures and chemistry of the abundant and variably expressed diagenetic features that are pervasive in the rocks in this locality. These features include early irregular, clustered mm-scale nodules, rounded ~cm-scale spaced nodules, color variations, and pits, and late-stage Ca-sulfate veins and fracture fills. At elevation ~-4070m, Curiosity drove around an ~8m tall cliff (Mont Mercou) which marked a transition into a ~30m section (to date) of outcrop with scarce occurrences of visible bedding, likely due to diagenetic overprinting of primary stratification. As Curiosity has traveled farther upsection, the size and organization of the nodular features has increased, as has chemical detections of magnesium sulfate enrichment.

By systematically documenting the changing morphology of the diagenetic features and their paragenetic sequence, and correlating them with bulk chemistry as measured by the Alpha Particle X-Ray Spectrometer instrument, we gain important context to identify the transition on the ground into the orbitally identified sulfate-bearing rocks, as well as understand the nature of the contact as it relates to a younger depositional and diagenetic environment that precipitated highly soluble salts consistent with long-term drying.