ANCIENT PLAYA IN GLENELG REGION, GALE CRATER, MARS: EVIDENCE DERIVED FROM THE POLYGONAL FLAGSTONE NETWORK IN MUDSTONE IMAGED BY CURIOSITY

An extensive bedding plane of mudstone forms the exposed stratigraphic base of the Glenelg region of Gale Crater. It was studied extensively with the Curiosity Rover in the first 200 sols of the MSL mission. This bedding plane is broken up into slabs, forming a distinct flagstone-like network at a smaller scale than has been discernible in satellite data. High quality images from Mastcam, Mahli, and Navcam cameras of the Curiosity Rover reveal extensive flagstone-type fracture pattern in the Glenelg Region of Gale Crater, Mars. The flagstones are approximately 1 m in width and are uniformly spaced and occur on bedding planes of mudstone lithologies. Such characteristics indicate that the observed polygonal fracture pattern of the Glenelg region has an origin different from polygons of Martian high latitude interpreted as thermal contraction of ice-cemented permafrost. The flagstone-type fracture pattern, mudstone lithology, presence of swelling clay, and occurrence in a dominantly sedimentary sequence suggest that the polygonal fracture pattern of the Glenelg Regionare formed by desiccation in a drying playa. These findings complement diverse, recent studies suggesting that Glenelg is a former lakebed at the base of an alluvial fan. This would imply that Glenelg polygonal flagstones formed when conditions were suitable for formation of standing body of liquid water which allowed deposition of mudstone lithology. A possible change in climate resulted in drying of the lake, exposure of mudstones, and formation of desiccation cracks.