SELECT OBSERVATIONS OF THE COARSE SEDIMENT RECORD AT GALE CRATER FROM MARS SCIENCE LABORATORY CAMERAS

A fundamental objective of the Mars Science Laboratory (MSL) science team is to assess potential links between field observations of outcrops and the large-scale geomorphology, especially those landforms associated with Gale crater’s water-rich past. Part of the aqueous activity record are in coarse sediments which have been observed occassionally along Curiosity’s ~19.5 km traverse in patchy outcrops of conglomerates and pebbly sandstones.

We highlight the diversity of sedimentologic characteristics observed in the coarser facies at Gale crater. Early in the mission, weakly stratified, moderately sorted, clast-supported conglomerates with rounded to subrounded gravels were interpreted as fluvial deposits. The Darwin outcrop has two coarse-grained facies: isolated pebbles within a sandy matrix, and a poorly sorted, clast-supported conglomerate with pock-marked pebbles as evidence of vigorous fluvial flow. Conglomerate float blocks observed on upper Vera Rubin Ridge are noteworthy for the presence of larger subangular clasts (very coarse gravel) than previously seen.

Very few observations have yielded insight into paleoflow direction. Pebble-rich, cross-stratified sandstones are present at Panamint Butte and the base of one Murray Butte, but no preferential clast orientation is apparent. However, two examples of candidate imbricated clasts are located at Baker River and Piute. These stacked clasts indicate northward sediment transport, raising the possibility that Curiosity may encounter coarse sedimentary rocks while driving up Gediz Valles.

The diversity of coarse sedimentary rocks indicate a variety of aqueous processes, but challenges remain in definitively reconstructing the geographic extent and temporal sequence of flows. The discontinuous outcrop distribution, poor exposure of facies contacts, and presence of unconformities are all factors that complicate determination of stratigraphic interpretations. On-going work seeks to refine and test various scenarios such as potential association with the Peace Vallis fan that incised the northwestern crater wall, the canyons carved into Aeolis Palus, an early (pre-Aeolis Palus) southward prograding fluvio-deltaic system, as well as potential localized aqueous environments with no preserved geomorphological expression.