Proposed Origin of the Burns Formation (Meridiani Planum, Mars) by Erosion, Reworking, and Diagenetic Alteration of a Grasberg‐Like Precursor

Abstract

AbstractThe sulfate‐rich sandstones of the Burns formation investigated by the Opportunity rover on Meridiani Planum, Mars, are directly underlain by the fine‐grained sedimentary rocks of the Grasberg formation. It was recently shown that, except for differing amounts of MgO and SO3, the Burns and Grasberg rocks have very nearly the same chemical composition, suggesting that both units may be genetically related. Here, quantitative models demonstrate that the chemical composition of the Burns rocks is closely reproduced by the addition of MgO and SO3 to a Grasberg‐like precursor. Comparison with previously proposed chemical models indicates that this new model reproduces the measured Burns rock compositions far better than evaporite models and somewhat better than the addition of sulfur alone to pristine basalt. Based on this result, we propose that the precursor materials for both units were derived from related sources and initially had very similar (or identical) chemical compositions, but magnesium sulfate salts were added to the Burns formation during diagenesis. We further propose an alternative origin of the Burns sandstones that incorporates this new chemical model, which involves (a) deposition of fine‐grained airfall deposits with a composition similar to that of Grasberg, (b) induration and erosion into sand‐sized particles, (c) reworking and deposition of the sand grains by eolian and fluvial processes, and (d) diagenetic alteration of the resulting sediments that includes enrichment in MgO and SO3 through evaporation or freezing of infiltrating groundwater. This new scenario would have substantially different implications for sediment sources, depositional history, and diagenetic processes than other proposed scenarios.