Statistical analysis of APXS-derived chemistry of the clay-bearing Glen
Torridon region and Mount Sharp group, Gale crater, Mars
Abstract
The Glen Torridon stratigraphic sequence marks the transition from the
low energy lacustrine-dominated Murray formation (Mf) (Jura member: Jm)
to the more diverse Carolyn Shoemaker formation (CSf) (Knockfarril Hill
member: KHm; Glasgow member: Gm), indicating a change in overall
depositional setting. Alpha Particle X-ray Spectrometer (APXS) results
and statistical analysis reveals that the bulk primary geochemistry of
Mf targets are broadly in family with CSf targets, but with subtle
compositional and diagenetic trends with increasing elevation. APXS
results reveal significant compositional differences between Jm_GT and
the stratigraphically equivalent Jura on Vera Rubin ridge (Jm_VRR).
APXS data defines two geochemical facies (high-K or high-Mg) with a
strong bimodal grain distribution in Jm_GT and KHm. The contact between
KHm to Gm is marked by abrupt sedimentological changes but a similar
composition for both. Away from the contact, the KHm and Gm plot
discretely, suggesting a zone of common alteration at the transition
and/or a gradual transition in provenance with increasing elevation in
the Gm. APXS results point to a complex history of diagenesis within
Glen Torridon, with increasing diagenesis close to the Basal Siccar
Point unconformity on the Greenheugh pediment, and with proximity to the
beginning of the clay sulfate transition. Elemental mobility is evident
in localized enrichments or depletions of Ca, S, Mn, P, Zn, Ni. The
highly altered Hutton interval, in contact with the unconformity on
Tower butte, is also identified on Western Butte, indicating that the
“interval” was once laterally extensive.