P069-0001
APXS Compositional Investigation of the Greenheugh Pediment Capping Unit and Underlying Murray Formation: Déjà Vu

Tuesday, 15 December 2020
Poster
Lucy M Thompson1, Jeffrey A Berger2, Catherine O'Connell-Cooper1, Albert Yen3, Ralf Gellert4, John G Spray1, Michael McCraig4 and Scott VanBommel5, (1)University of New Brunswick, Fredericton, NB, Canada, (2)NASA Johnson Space Center, Houston, TX, United States, (3)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (4)University of Guelph, Guelph, ON, Canada, (5)Washington University in St. Louis, St. Louis, United States
Abstract:
Curiosity recently examined the contact between the predominantly fine grained, lacustrine upper Murray formation and overlying, resistant sandstones that cap the erosional Greenheugh pediment. Chemical data acquired by APXS reveal compositional similarities between these rocks and those encountered earlier in the mission.

Murray formation targets analyzed within 3 m of the contact exhibit elevated Na, excess Ca (not as CaSO4) and similar elemental patterns to Murray bedrock from the base of the section, within 5 m of overlying, capping sandstones. Post-depositional concretions and veins within the Murray targets closest to the pediment contact fall into two categories; those with elevated Mn and Fe, and those with elevated Mg (+/- K). The distinct composition of Murray strata close to the contact with overlying, capping sandstones at both the base and top of the section may indicate enhanced post-depositional fluid flow and alteration associated with this contact.

Sandstone targets exposed immediately above the pediment have basaltic compositions consistent with those of Stimson formation sandstones, some with elevated S. More massive and resistant sandstone outcrops, ~4 m above the contact have higher K, Mn and Na and lower Ni concentrations than underlying and typical Stimson sandstone. This may reflect a change in provenance from the typical basaltic composition Stimson, or changes in transport/sorting or diagenesis. The higher K sandstones are compositionally similar to several potential cap-rock, float blocks analyzed by APXS as Curiosity climbed through the Murray formation as well as Bradbury group sandstone outcrops encountered at the very beginning of the mission. This indicates a possible relationship between the Bradbury group and Stimson formation. The more massive, higher K, pediment sandstones may have a similar provenance and/or depositional setting as at least some of the Bradbury group sandstones.

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