MODELS FOR COMPOSITIONAL VARIATIONS IN THE MURRAY FORMATION MUDSTONE, GALE CRATER, MARS

The Murray formation, Gale crater, Mars is distinctive in containing elevated silica compositions relative to other Gale rocks exposed in outcrop. These high silica rocks are also marked by their fine grain size of < 60-70 microns and mm-scale lamina. Lamination is characteristically parallel, with horizontal to gently inclined attitude, and absence of cross-stratification. Sediment-filled mudcracks (evidence of desiccation) are absent, as are outsized clasts, such as pyroclasts, impact ejecta blocks, or glacial dropstones. Early to late diagenetic textures are present including dendritic concretions, prismatic crystal pseudomorphs, and calcium sulfate-filled fractures. High silica rocks show repeating characteristic lamina thicknesses reminiscent of terrestrial lake sediments. Deposition is likely to have occurred in a pro-delta environment where clastic sediment was delivered as river plumes, and perhaps also as authigenic phases from lake waters. Chemical and mineralogical analyses indicate that the majority of Murray formation mudstones are composed of roughly equal proportions of clastic igneous minerals, and secondary phases (e.g., phyllosilicates), including amorphous mineraloids of probable secondary origin. The mudstones are sourced by incipiently weathered basalt, and are possibly cemented by Ca-Mg-sulfates and silica, the latter in the form of cristobalite or opal. Redox cycling is indicated by variable amounts of hematite, magnetite, and jarosite, and likely reflects dynamic primary lake water and/or diagenetic redox conditions. For a smaller number of samples silica contents reach values as high as ~70-85 wt %. The fine-scale lamination is not disrupted by post-depositional weathering or alteration processes. Hydrothermal and acid-sulfate weathering mechanisms are frequently invoked to explain the occurrence of siliceous rocks on Mars, and it is also possible that silica enrichment represent distal silicic volcanic ash or eroded silica-rich detritus. However, available sedimentological, geochemical, and mineralogic data may best support a fourth hypothesis: that silica enrichment occurred via chemical precipitation as sediment within a subaqueous lake setting, perhaps similar to terrestrial banded iron formation.