--> A Sequence Stratigraphic Perspective on the Murray Formation, a Martian Mudstone Succession in Gale Crater, Mars

AAPG ACE 2018

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A Sequence Stratigraphic Perspective on the Murray Formation, a Martian Mudstone Succession in Gale Crater, Mars

Abstract

The Curiosity rover has examined a thick mudstone succession, the Murray Fm., at Mt. Sharp in Gale Crater. The succession is lacustrine and records a prolonged history of infill under likely lake-level fluctuations. Physical processes of sediment transport and deposition are comparable between Earth and Mars and the defining criteria are geometric. Therefore methodologies for analysis and identification of lake-basin type developed on Earth should also allow for a better understanding of the infill history of Gale crater, recorded in sedimentary features, vertical and lateral facies successions, and stratal geometries.

At Gale crater, compiling a definitive sequence of sedimentary packages that record rising and falling lake levels is challenging due to the variable and discontinuous bedrock exposure. Yet there is evidence, for example, for facies migration in fining-upward successions and distal “sanding” of the succession that may reflect infilling during lake highstands. For stratal surfaces and lake-level changes, multiple examples of erosive downcutting are exposed. Associated sediment bodies are visible in HiRISE images and a few of them were examined by the rover with image mosaics, closeup imaging, and geochemical analysis. The base of these bodies can be coarse sandy, contain mudstone clasts, be variably sulfate enriched, and show cross-bedding and desiccation cracks. Upwards they fine towards the basic mudstone background that dominates the Murray Fm. The incisions are considered erosion into older lake beds during lake-level drop, and coarse basal fills may be (a) lowstand deposits, or may mark (b) the onset of lake-level rise that leads to progressive infilling. In balanced-fill lake-basin successions on Earth, scenario (b) strongly dominates, however, the possibility that scenario (a) was on occasion realized at Gale is suggested by beds with multiple generations of desiccation cracks and strongly Ca-sulfate cemented sandstone beds.

Identification of system tracts in the Murray Fm. seems feasible, but resolving parasequences is hampered by outcrops continuity. Locations where better-cemented thin intervals occur suggest enhanced cementation at flooding surfaces/tops of parasequences. Overall it appears that the sequence-stratigraphic approach can assist in the identification of successive lake basin types and thus provide useful context for constructing detailed interpretations of the record of lake processes on Mars.