Alteration at the Base of the Siccar Point Unconformity and Further Evidence for an Alkaline Provenance at Gale Crater: Exploration of the Mount Sharp Group, Greenheugh Pediment Cap Rock Contact With APXS

• Published in: Journal of geophysical research. Planets 2022-11, Vol.127 (11), p.n/a
• Main Authors: Thompson, L. M., Spray, J. G., O’Connell‐Cooper, C., Berger, J. A., Yen, A., Gellert, R., Boyd, N., McCraig, M. A., VanBommel, S. J.
• Format: Article
• Language: English
• Subjects: alkali source rocks at Gale crater; Alpha particles; Alpha rays; Composition; Data acquisition; enhanced fluid flow and alteration along the Basal Siccar Point group unconformity; evidence for the long‐lived role of water and aqueous alteration and hence; Fluid dynamics; Fluid flow; habitable environments; Mars craters; Mars surface; Outcrops; Potassium; Rocks; Sandstone; Sodium; Unconformity; Wind erosion
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2021JE007178

Chemical data acquired by Curiosity's Alpha Particle X‐ray Spectrometer during examination of the contact between the upper Mount Sharp group and overlying Stimson formation sandstones at the Greenheugh pediment reveal compositional similarities to rocks encountered earlier in the mission. Mount Sharp group strata encountered below the Basal Siccar Point group unconformity at the base and top of the section, separated by >300 m in elevation, have distinct and related compositions. This indicates enhanced post‐depositional fluid flow and alteration focused along this contact. Sandstone targets exposed immediately above the unconformity have basaltic compositions consistent with previously encountered eolian Stimson formation sandstones, except at the contact, where they show the addition of S. Resistant sandstone outcrops above the contact have higher K, Mn, and Na and lower Ni concentrations that primarily reflect changes in provenance. They are compositionally related to cap rock float blocks encountered as Curiosity climbed through the Mount Sharp group, and Bradbury group sandstone outcrops. The higher K, pediment sandstones are interpreted to have a similar provenance to some Bradbury group sandstones, further evidence for widespread, alkaline source rock within and/or in the vicinity of Gale crater. The Bradbury and Siccar Point groups may both be younger than the Mount Sharp group. Alternatively, an alkaline source area in and around Gale crater has been eroded by both water and wind at different times (both before and after deposition of the Mount Sharp group), during the evolution of the crater and its infill. Plain Language Summary Chemical data acquired by Curiosity's Alpha Particle X‐ray Spectrometer during examination of an important regional contact between two rock units in Gale crater, Mars, reveal relationships with other rocks encountered much earlier in the mission. These relationships provide evidence for erosion by wind and water of various composition source rocks. These include more typical Martian basaltic and more alkaline composition rocks (e.g., higher potassium and sodium). Distinct chemistry within rocks immediately underlying the contact, separated by more than 300 m in elevation, indicate enhanced fluid flow and alteration along the contact. These relationships provide important insights into regional and crater‐scale processes and evidence for the long‐lived role of water and aqueous alteration and hence, habitable environmen