INSIGHT INTO PALEOENVIRONMENTS FROM SEDIMENTARY ROCKS ALONG THE MARS SCIENCE LABORATORY (MSL) CURIOSITY’S TRAVERSE

Past depositional processes and aqueous environments in Gale crater are preserved in sedimentary rocks observed by the Curiosity rover. Many of the sedimentary rocks are fluvial-lacustrine deposits apparently derived from the crater rim. Details of the duration and extent of ancient aqueous activity in Gale crater are still under investigation.

The rover landed on plains several kilometers northwest of Mt. Sharp and down slope from the Peace Vallis alluvial fan. Initially the rover drove ~400 m east across Bradbury Rise and descended into Yellowknife Bay for a drilling campaign before beginning the ~8 kilometer drive southwest to Mt. Sharp.

On the Bradbury Rise plains, the rover encountered isolated outcrops of pebble conglomerates representing the first in-situ evidence for sustained fluvial transport over the martian surface. Using the observed grain size distribution, we estimate the threshold flow conditions required to transport the sediment: depth 0.03 - 0.9 m, average velocity 0.20 - 0.75 m/s. Flows likely exceeded these minimum values, resulting in deposits with imbricated clasts, clusters of clast-supported pebbles, and parallel stratification. The geomorphic setting suggests these conglomerates are distal alluvial fan deposits associated with the Peace Vallis system.

The 5-meter thick Yellowknife Bay section is predominantly sandstones of basaltic composition. The 1.2-m thick Shaler outcrop is a heterogeneous assemblage of interstratified platy sandstones separated by recessive presumably finer-grained beds. Pin-stripe laminations, compound cross-bedding, and fining upward sequences are consistent with a complex scenario of fluvial deposition with eolian reworking. The basal Sheepbed member is a clay-bearing mudstone inferred to result from suspension settling in a lacustrine or playa setting. A diverse set of diagenetic features in this mudstone, including nodules, raised ridges and cross-cutting sulfate-filled fractures, indicate multiple phases of aqueous alteration.

Patchy outcrops of varying conglomerate and sandstone facies are on the hummocky and ridged geomorphic units of the Bradbury plains, including at two waypoints, Darwin and Cooperstown. Facies attributes are generally consistent with fluvial deposition, with some evidence suggesting vigorous flows.