Evidence for Mobile Element Recharge in the Murray Formation near the Siccar Point Unconformity (Glen Torridon, Gale Crater, Mars)

Abstract:

In late 2019/early 2020, in the Glen Torridon (GT) region of Gale crater, the Curiosity rover explored a section of the Murray formation, referred to as the “fractured Intermediate Unit” (fIU), located immediately below the basal Siccar Point unconformity. This erosional unconformity separates the mudstone-dominated Murray formation from the sandstone-dominated Stimson formation (which at this location forms the surface of the Greenheugh pediment). In orbital imagery, the Murray rocks located within ~15 m of horizontal distance of the contact are distinctively light-toned compared to the rest of the fIU, suggesting possible compositional differences. Here, we summarize the geochemical trends observed by the ChemCam instrument in these terrains.

Away from the light-toned layers, the fIU bedrock has a composition broadly in line with the rest of GT. However, as the rover climbed up toward the contact, ChemCam measured significant changes, most notably a decrease in SiO₂ (from ~56 to ~49 wt%) and an increase in CaO (from ~2.4 to ~4.4 wt%). In addition, the ChemCam sum of oxides decreased from ~98 to ~94 wt%, and the Chemical Index of Alteration (CIA) dropped from ~54 to ~45. For comparison, the Stimson rocks above have a CIA of ~39.

These trends confirm that the light-toned appearance of the Murray rocks near the contact is indeed associated with geochemical variations. The abundance of diagenetic features in these rocks compared to the rest of GT suggests that these variations are not simply due to a change in the sediment source (i.e., it suggests that the proximity of the unconformity is not a coincidence). On the other hand, the decrease in CIA near the contact is not consistent with the development of a weathering profile that could have occurred after the formation of the erosional surface, because the top horizons of such profiles are typically depleted in mobile cations, resulting in increased CIA. Instead, it appears that the light-toned Murray rocks, if initially similar to the rest of the fIU, have been recharged in some mobile cations (including Ca), along with other elements such as H, F, S or Cl in order to explain the lower sum of oxides. Altogether, these observations add to growing evidence for the persistence of aqueous processes late in the history of Gale crater, possibly after the emplacement of the Stimson formation.