Reconstructing Martian environmental change across a major unconformity at Gale crater: sedimentology of the Stimson formation at the Greenheugh pediment

International audienceThe basal Siccar Point group unconformity observed at the Greenheugh pediment is a major regional break in sedimentation within Gale crater. Below the unconformity, mudstones and sandstones of the Murray formation records a predominantly lacustrine palaeoenvironment that was likely habitable. Overlying the unconformity is a distinctive fan-shaped, erosion-resistant, tabular stratal unit (~2-6 m thick) informally called the Greenheugh pediment capping unit. This unit has been identified as part of the Stimson formation, which encodes the signature of surfaces processes and the climate in Gale crater postdating the formation of the unconformity.Between Sols 2633 and 2781, the Curiosity rover was used to investigate the pediment capping unit, where detailed observations of sedimentary architecture, facies and sediment texture were obtained. Textural and grain-size analyses of the sandstone show that it comprises a well-sorted, medium-grained, well-rounded sandstone with an average grainsize of ~425 μm, which suggests aeolian transport processes The dominant sedimentary structure are cross-bedded sandstones composed of 2-4 mm thick, uniform-thickness cross-laminations, that are typical of wind ripple stratification. Architectural observations reveal that the unit is composed of compound cross-sets, with sets that are 0.2-0.7 m thick, that form cosets up to 2.5 m thick. These observations lead us to conclude that the pediment capping unit accumulated as the result of aeolian processes, where wind-blown dunes migrated across the unconformity, leaving preserved aeolian cross strata.Further to this, our observations of sedimentary texture, facies and architecture are consistent with Stimson formation sandstones observed in the Emerson plateau, Naukluft plateau and Murray buttes areas. We conclude that the pediment-capping unit is an up-slope equivalent of the Stimson formation, and that the Stimson formation drapes the unconformity over a ~500 m elevation range. The Stimson formation is the preserved expression of a dry aeolian dune field, that accumulated in a climate of extreme aridity, when Gale crater would likely have been devoid of surface water. The juxtaposition of desert deposits overlying lake deposits indicates a major change in palaeoclimate in Gale crater