Provenance and Groundwater Lithification of the Stimson Sandstone, Gale crater, Mars

Abstract:

The Stimson formation consists of eolian sandstones that unconformably overlie the dominantly fluvio-lacustrine sediments of the Mount Sharp group rocks in Gale crater, Mars. The basal Stimson unconformity surface parallels the modern slope of Mt. Sharp, indicating that the Stimson formed significantly later than the Mt. Sharp group. Indeed, Gale crater had filled sufficiently to bury hundreds of meters of fluviolacustrine sediments when the crater was evacuated to form a proto-Mt. Sharp. Sand dunes accumulated, which were buried sufficiently to lithify and fracture the resulting Stimson sandstone prior to the wind evacuating the crater again to form the modern topography (an ongoing process, with more sediment forming the Bagnold sand dunes). Multiple episodes of groundwater must have been present after Stimson deposition because it is well cemented, fractured, and fluids moved through fractures, leaving a record of alteration halos and veins. Here, we focus on the first and most extensive groundwater episode, in which the sand was cemented into sandstone.

The Curiosity rover team investigated several outcrops of the Stimson, including two prominent locations near the base of Mt. Sharp called the Emerson and Naukluft plateaus, and, recently, a portion of the Greenheugh pediment ~300 m higher in elevation. Here, we use compositional results from the Naukluft and Emerson sections, interpreted with MAHLI images and CheMin mineralogy, to investigate the characteristics of the sediment provenance and the cementation processes for these rocks. All ancient and modern sands in Gale have a generally basaltic provenance, however, the provenance for the lithified Stimson is mineralogically distinct from older rock units and modern sands. The sandstone lithification appears to have been dominated by the conversion of any olivine into iron oxides, as previously noted by Yen (2017, EPSL) and Hausrath (2018, GRL), and we also see evidence for localized Mg mobility within the amorphous component and in some concretions. We interpret the majority of compositional differences in the unit to be related to groundwater dissolution of primary materials during lithification. Broadly, this work shows that even compositionally basaltic caprock units on Mars may have had significant groundwater interaction.