DIAGENETIC HISTORY OF THE EVAPORITE-BEARING BURNS FORMATION, MERIDIANI PLANUM, MARS

Impure evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of approximately equal amounts of altered basaltic mud and chemical constituents, dominated by a distinctive suite of evaporite minerals (jarosite, Mg-, Ca- (±Na-, ±ferrous) sulfates ± chlorides), hematite and possibly amorphous silica. Chemical components formed by evaporation of acidic water derived from interaction with basalts, and subsequent diagenetic alteration. Evaporite minerals and their relative abundances are not an equilibrium assemblage, having been reworked by aeolian and subaqueous transport and having experienced diagenetic mineral growth, transformation and dissolution. An extended diagenetic history includes at least two episodes of cementation, minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features), highly spherical mm-scale hematitic concretions, mm-scale crystal molds, resulting from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and channels, and stratigraphically restricted zones of recrystallization and enhanced secondary porosity. Crystallization of minerals that originally filled molds, early cements and sediment deformation occurred syndepositionally or during very early diagenesis. Other diagenetic features formed during later diagenesis in the phreatic zone or capillary fringe of a groundwater table under near isotropic hydrological conditions. Rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements and pre-date secondary moldic and vug porosity. The second generation of cements likely followed formation of secondary porosity. Other features that are most likely associated with relatively recent meteorite impact, but could also have a very late diagenetic origin or influence, are veins, fracture networks and polygonal structures on rock surfaces that cut across bedding. The inferred paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.