IMPLICATIONS OF ACTIVE SURFACE PROCESSES FOR THE INTERPRETATION OF THE MARTIAN SEDIMENTARY ROCK RECORD: AEOLIAN SANDS, SEDIMENTS, AND THEIR SOURCES AT GALE CRATER

Sedimentary grains are sorted by their physical properties as they are transported by a fluid and the amount of sorting provides information about sediment transport and fluid processes. Therefore, a quantitative understanding of sorting magnitude on Mars is critical to interpret Martian sedimentary deposits. Here, we focus on transport of sand by wind within the Bagnold Dune Field (BDF) of Gale Crater. We derive CRISM mineral parameter maps and ripple displacements from time correlation of HiRISE images to show that grains are actively being sorted in the BDF. Using CRISM spectral reflectance data, we observe an enrichment in coarse olivine grains in the barchanoid dunes at the NW margin of the field, whereas longitudinal dunes further downwind are dominated by pyroxene signatures. We suggest that enrichment of coarser/denser grains at the upwind edge of the BDF results from the development of an internal boundary layer at the transition from cratered plains to sand due to an abrupt change in surface roughness. An alternative scenario is that the dunes formed over an erodible bed altering the kinetics of saltation through less efficient inelastic collisions. Orbital observations within the dunes that Curiosity will visit in the near future show a positive correlation between olivine enrichment and ripple displacements. To further quantify results, we invert for mineral composition and grain sizes of the sands from visible/near-infrared (VNIR) single scattering albedo spectra of the dunes using a noise-based Bayesian approach. We perform a series of modeling experiments to better identify the chemistry of mafic phases making up the sands, and thus characterize the chemistry of the parent rock(s). Results show spectra can be modeled with a simple mixture of olivine (Fo50-80), plagioclase, and pyroxenes. Finally, we identify possible olivine-bearing source areas for the active sands within Gale Crater. With Curiosity currently investigating its first potentially aeolian sandstones and approaching the active BDF in the coming months, in situ observations will refine, confirm, or refute our hypotheses based on orbital observations and better constrain the source-to-sink history of aeolian sediments and sedimentary rocks at Gale.