Chemistry of the Materials Above and Below an Unconformity Between the Murray and Stimson Formations in Gale Crater, Mars

Friday, 18 December 2015: 14:55
2007 (Moscone West)
Horton E Newsom1, Ines Belgacem1, Roger C Wiens2, Jens Frydenvang3, Olivier Gasnault4, Sylvestre Maurice5, Patrick James Gasda3, Samuel M Clegg3, Agnes Cousin5, William Rapin4, Ryan Jackson6, Zoltan Vaci6, Beth Ha6, Diana L Blaney7, Nathan Bridges8, Raymond Francis9, Valérie Payré10, Sanjeev Gupta11, Steven Banham11, Jeff Schroeder7, Fred J Calef III12, Kenneth S Edgett13, Deirdra Fey13, Martin R Fisk14, Ralf Gellert15, Lucy M Thompson16, Glynis M Perrett15, John P Grotzinger17, David M Rubin18, Amy Williams8,19, Linda C Kah20 and Rachel Kronyak20, (1)University of New Mexico Main Campus, Albuquerque, NM, United States, (2)Space Science and Applications, Los Alamos, NM, United States, (3)Los Alamos National Laboratory, Los Alamos, NM, United States, (4)Universite de Toulouse, Toulouse Cedex 4, France, (5)IRAP, Toulouse, France, (6)Univ New Mexico, Albuquerque, NM, United States, (7)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States, (8)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (9)University of Western Ontario, London, ON, Canada, (10)University of Lorraine Nancy, Nancy Cedex, France, (11)Imperial College London, London, United Kingdom, (12)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (13)Malin Space Science Systems, San Diego, CA, United States, (14)Oregon State Univ, Corvallis, OR, United States, (15)University of Guelph, Guelph, ON, Canada, (16)University of New Brunswick, Planetary and Space Science Centre, Fredericton, NB, Canada, (17)California Institute of Technology, Pasadena, CA, United States, (18)University of California-Santa Cruz, Santa Cruz, CA, United States, (19)University of Maryland College Park, College Park, MD, United States, (20)University of Tennessee, Knoxville, TN, United States
MSL began investigating a contact between Murray formation, (fine grained lake deposits) and the younger Stimson formation at Marias Pass in May 2015, on the lower slopes of Mt. Sharp. Images show that the Murray formation, with numerous calcium sulfate veins compared to the Stimson, is truncated at an erosional contact. MAHLI images show a white layer a few mm thick at the contact that might be calcium sulfate. The lowermost beds of the Stimson unit in the Missoula area comprise horizontally laminated or cross-laminated sandstones. The sandstones are poorly sorted with floating granules and very coarse sand grains set in a fine- medium-grained sand ‘matrix’. This material directly above the contact is a resistant, basal ledge-forming layer that also forms numerous blocks of float on top of the eroded Murray. This basal layer contains light toned fragments, possibly calcium sulfate, eroded from the Murray. The poor sorting and presence of sub-angular grains, together with the absence of preferential sorting into size sorted layers would seem to rule out eolian processes for the lowermost beds of the Stimson and suggest fluvial processes were responsible for deposition of these beds. For chemostratigraphy, the distance of each ChemCam or APXS observation above or below the contact was determined from images and the NavCam stereo mesh. The top of the Murray near the Missoula area is variable in composition, and additional analyses are planned to determine if weathering occurred at the eroded surface. Above the contact, the lowest 2 cm of the resistant slab is higher in SiO2, and lower in Al2O3, K2O and Na2O, relative to other Stimson analyses. In a few points with low totals, there is a correlation between Ca and missing components (presumed to be mostly S). These points could be connected to calcium sulfate in the form of cements and/or incorporation of eroded clasts of Murray vein materials.