Revisiting the Sheepbed Mudstone of Gale Crater, Mars – A comprehensive reappraisal of its depositional, diagenetic, and burial history

Abstract:

The Sheepbed mudstone forms the base of the succession examined by the Curiosity rover and is also the first bona fide mudstone known on another planet. Many initial papers have discussed a lacustrine depositional setting, geochemical features, and implications for habitability. Careful review of all collected images and associated data prompts us to update interpretations of depositional regime, diagenesis, and burial history. The lake basin received sediment pulses associated with localized scouring, lateral thinning of beds, and decelerating flows. Bed cross sections suggest mm-cm scale layering to be the norm, probably recording distal pulses of fluvial sediment injections (fine grained hyperpycnites), fall-out from river plumes, and some eolian supply. Prior diagenesis studies focused on synaeresis cracks and mm-scale nodules but contrary to other genetic models we consider centrifugal diffusion could have formed hollow nodule shells, some continuing until filled. There is also an element of stratiform cementation and formation of thin, laterally extensive concretions that establish outcrop appearance of the uppermost meter of the Sheepbed mudstone.

Typical surface mud with 80 vol% water has a water/rock ratio of about 1.4 (assuming basaltic debris), and this ratio declines with further compaction. Given that clay formation is thought to require rather higher w/r, the observation of around 20% clays in these rocks seems to argue against in situ clay generation, and instead suggests that they were derived from the crater’s periphery and carried into the basin by fluvial discharge. This view is also supported by the presence of significant amounts (>30%) of likely highly reactive amorphous component(s) and Fe-forsterite in John Klein, and the need for an initial presence of clay minerals to enable synaeresis. The virtual lack of Fe-forsterite in Cumberland is more supportive of in situ alteration, although significant amorphous material exists in Cumberland.