LITHIFICATION OF SEDIMENTARY ROCKS ON MARS – A VIEW FROM CURIOSITY

Over the first 655 sols of its mission, Curiosity has revealed a wealth of information regarding the geologic history of Gale crater, Mars. Perhaps most striking is the overwhelming evidence for water-lain sedimentary rocks: well-indurated conglomerates (with potential clast imbrication), cross-bedded sandstone deposits, and massive sheet sands (some with entrained clasts) that reflect deposition from high-energy flows; fine-grained sands with rippled surfaces that suggest waning flows; and fine-grained clay-rich deposits that reflect deposition in quiet, ponded environments.

Yet Curiosity is setting her sights on more than just evidence for water. Curiosity is using clues gained from the texture of surface materials to predict how persistently environments may have interacted with water, at the surface, within the substrate, or at depth, and to determine to what extent fluid interaction may be recorded in the geochemical and mineral signatures retrieved from these rocks.

Here we present evidence for lithification processes in a variety of sedimentary rock types as observed through Curiosity’s Mast Cameras and the Mars Hand Lens Imager (MAHLI), which has provided high-resolution (16-30 microns per pixel) images of rock surfaces. Although images provide sufficient detail for basic textural analysis, examples of candidate cements, which are often near the limits of resolution are not easily identified. We have identified a variety of potential cement phases including both dark and light-toned cements, pore filling cements that indicate water-saturated conditions, and the potential for multi-generational cement fill. In addition to pore filling features, we also observe apparent pits, which may reflect secondary porosity associated with cement dissolution. Cement composition is not yet well constrained by geochemical data. However, molar plots of Si vs. Fe from ChemCam data often show residual Fe that must occupy a non-silicate phase, likely an Fe-rich cement. A paucity of common near-surface cementing agents (e.g. Ca, Na, K-salts), in addition to relationships between lithified sands and secondary veins, suggests the potential for burial and cementation at depth. Together, these observations provide a growing body of evidence for multiple diagenetic fluid interactions at Gale crater.