Alpha Particle X-ray Spectrometer (APXS) (MSL) Analysis of the Glen Torridon Clay Rich Units, below the Greenheugh Pediment, Gale Crater, Mars

Abstract:

Ongoing in situ investigation of the phyllosilicate trough “Glen Torridon” [GT] by the Curiosity (MSL) rover has allowed the correlation of orbitally defined units with sedimentologically defined members, and with bulk geochemical trends, as identified by the APXS instrument. The Jura mbr (herein Jura_Gt), comprising mudstones (abundant pebbles, rare bedrock) and (less commonly) sandstones, roughly correlates with the smooth clay-bearing unit (sCBU), identified from orbit. Up elevation, the Knockfarril Hill mbr (KH), a mixture of sandstones and (less commonly) mudstone, roughly correlates with the fractured clay bearing unit (fCBU). A third unit, the fractured intermediate unit (fIU), (predominantly sandstone) lies between the clay-rich units and the overlying sulfate units, which represent a significant change in environmental conditions from the fluvio-lacustrine Murray formation units.

Changing depositional environments within GT are indicated by the transition from mudstone-dominated (Jura_Gt) to sandstone-dominated (KH & fIU) lithologies. Bulk chemistry is broadly similar for Jura_Gt and KH, suggesting a common source; the fIU shows a broad range for elements such as Na, Cl, SO. Locally, Mg, K, Zn, Mn & Br are amongst the highest identified in > 400 m of Murray fm to date. K concentrations and associated elemental relationships (K is negatively correlated with Mn, Mg & Zn, but positively with Si & Ni) are strongly correlated with grainsize (typically: fine grained – high K/low Mg; coarse grained – low K/high Mg).

Whilst the Jura_Gt is stratigraphically equivalent to, and likely syn-depositional with, the Jura mbr on the Vera Rubin ridge (VRR), a change in bulk chemistry is identified from the ridge (VRR) to the trough (GT). GT targets trend (with some variations with respect to members) to lower Si, Al, Na, Ni, Mn and higher Ti, Fe, Mg, K, P, Cr, Zn, Br. This geochemical change is also reflected mineralogically (in drill samples), from the hematite-rich (up to 15 wt. %), clay-poor VRR to the clay-rich (≥ 30% wt.), hematite-poor GT. The correlation of strong geochemical trends (K-Mg-Zn-Mn) with grainsize in the GT targets are indicative of sorting processes; differences between the VRR and GT are attributed to later post-depositional alteration, related to the ridge and the Greenheugh pediment.