Diagenesis of the Clay-Sulfate Stratigraphic Transition, Mount Sharp Group, Gale Crater, Mars

Abstract

The diversity and abundance of diagenetic textures observed in sedimentary rocks of the clay-sulfate transition recorded in the stratigraphic record of Gale crater are distinctive within the rover's traverse. This study catalogs all textures observed by the MAHLI instrument, including their abundances, morphologies, and cross-cutting relationships in order to suggest a paragenetic sequence in which multiple episodes of diagenetic fluid flow were required to form co-occurring color variations, pits, and nodules; secondary nodule populations; and two generations of Ca sulfate fracture-filling vein precipitation. Spatial heterogeneities in the abundance and diversity of these textures throughout the studied stratigraphic section loosely correlate with stratigraphic unit, suggesting that grain size and compaction controls on fluid pathways influenced their formation; these patterns are especially prevalent in the Pontours member, where primary stratigraphy is entirely overprinted by a nodular fabric, and the base of the stratigraphic section, where increased textural diversity may be influenced by the underlying less permeable clay-bearing rocks of the Glen Torridon region. Correlations between quantitative nodule abundance and subtle variations in measured bulk rock chemistry (especially MgO and SO₃ enrichment) by the Alpha Particle X-Ray Spectrometer instrument suggest that an increase in Mg sulfate upsection is linked to precipitation of pore-filling diagenetic cement. Due to a lack of sedimentological evidence for widespread evaporite or near-surface crust formation of these Mg sulfates, we propose three alternative hypotheses for subsurface groundwater-related remobilization of pre-existing sulfates and reprecipitation at depth in pore spaces.