New Constraints on Grain Size of Eolian Sediments in the Stimson Sandstone, Gale Crater, Mars and Implications for Paleoclimate

Grain size distributions in sedimentary rocks encode valuable information about the conditions in which the rocks formed. Because different grain-size fractions move through different transport modes for a given wind strength, grain-size distributions in eolian sediments can provide information about atmospheric density and wind speeds. Eolian deposits are of particular interest on Mars because we can compare modern and ancient conditions. Previous work has identified a difference between the grain size distributions of the ancient eolian Stimson formation in Gale crater and current Martian dunes (Banham et al., 2018, JGR). Unfortunately, grain size is not easily constrained on Mars due to dust, surface textures, and limitations of science cameras, so creative approaches are needed. In this work, we utilize a novel digital grain size card to qualitatively measure grain size and group targets in the Stimson sandstone by texture based on images taken by the Mars Science Laboratory Curiosity rovers Mars Hand Lens Imager (MAHLI) and ChemCam Remote Micro-Imager (RMI). We then use ImageJ to measure grain size distributions for selected MAHLI images from each texture group. We find that the most common textures contain bimodal grain size distributions, characterized by sparse coarse sand and dominant very fine to medium sand grains. Eolian transport often leads to bimodal grain-size distributions at the scale of individual bedforms, consistent with our measurements and previous interpretations. While we are limited by the non-random sampling of images obtained by the MAHLI and RMI cameras, our findings indicate that the modal grain size of grains apparently transported by saltation in the Stimson is substantially coarser (~300 m) than the modal grain size of active dunes (~120 m). The modal diameter of the coarse grains is ~900 µm. The disparity in grain size between ancient and current dune fields suggests that either sediment supply or atmospheric conditions that enabled the transport and deposition of the Stimson sandstone may have been different than current conditions. We will present the implications for the sediment source, paleoclimate, and atmospheric conditions at the time of Stimson deposition.