Encounters with an unearthly mudstone: Understanding the first mudstone found on Mars

  • Juergen Schieber
    Department of Geological Sciences Indiana University 1001 East 10th Street Bloomington IN 47405‐1405 USA
  • David Bish
    Department of Geological Sciences Indiana University 1001 East 10th Street Bloomington IN 47405‐1405 USA
  • Max Coleman
    Jet Propulsion Laboratory California Institute of Technology and NASA Astrobiology Institute 4800 Oak Grove Drive Pasadena CA 91109 USA
  • Mark Reed
    Department of Geological Sciences University of Oregon 1585 East 13th Avenue Eugene OR 97403 USA
  • Elisabeth M. Hausrath
    Department of Geoscience University of Nevada Las Vegas 4505 South Maryland Parkway Las Vegas NV 89154 USA
  • John Cosgrove
    Department of Earth Science and Engineering Imperial College London SW7 2AZ UK
  • Sanjeev Gupta
    Department of Earth Science and Engineering Imperial College London SW7 2AZ UK
  • Michelle E. Minitti
    Planetary Science Institute 1700 East Fort Lowell Road #106 Tucson AZ 85719 USA
  • Kenneth S. Edgett
    Malin Space Science Systems 5880 Pacific Center Boulevard San Diego CA 92121 USA
  • Mike Malin
    Malin Space Science Systems 5880 Pacific Center Boulevard San Diego CA 92121 USA

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<jats:title>Abstract</jats:title><jats:p>The Sheepbed mudstone forms the base of the strata examined by the Curiosity rover in Gale Crater on Mars, and is the first <jats:italic>bona fide</jats:italic> mudstone known on another planet. From images and associated data, this contribution proposes a holistic interpretation of depositional regime, diagenesis and burial history. A lake basin probably received sediment pulses from alluvial fans. Bed cross‐sections show millimetre to centimetre‐scale layering due to distal pulses of fluvial sediment injections (fine‐grained hyperpycnites), fall‐out from river plumes, and some aeolian supply. Diagenetic features include mineralized synaeresis cracks and millimetre‐scale nodules, as well as stratiform cementation. Clay minerals were initially considered due to <jats:italic>in situ</jats:italic> alteration, but bulk rock chemistry and mineralogy suggests that sediments were derived from variably weathered source rocks that probably contained pre‐existing clay minerals. X‐ray diffraction analyses show contrasting clay mineralogy in closely spaced samples, consistent with at least partial detrital supply of clay minerals. A significant (<jats:italic>ca</jats:italic> 30 wt%) amorphous component is consistent with little post‐depositional alteration. Theoretical modelling of diagenetic reactions, as well as kinetic considerations, suggest that the bulk of diagenetic clay mineral formation occurred comparatively late in diagenesis. Diagenetic features (synaeresis cracks and nodules) were previously thought to reflect early diagenetic gas formation, but an alternative scenario of synaeresis crack formation via fabric collapse of flocculated clays appears more likely. The observed diagenetic features, such as solid nodules, hollow nodules, matrix cement and ‘raised ridges’ (synaeresis cracks) can be explained with progressive alteration of olivine/glass in conjunction with centrifugal and counter diffusion of reactive species. Anhydrite‐filled fractures in the Sheepbed mudstone occurred late in diagenesis when fluid pressures built up to exceed lithostatic pressure. Generating fluid overpressure by burial to facilitate hydraulic fracturing suggests a burial depth of at least 1000 m for the underlying strata that supplied these fluids.</jats:p>

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