SPECTACULAR OUTCROPS OF THE CLAY-BEARING UNIT, GALE CRATER, MARS

The Clay-Bearing Unit was originally recognized through orbital investigations of Gale crater, Mars, as layers with abundant phyllosilicate minerals. It occurs in a 0.5 km – 1 km wide valley north of the Vera-Rubin ridge and can be traced for about 15 km in HiRISE images. Investigations so far indicate that its overall characteristics are similar to the Murray formation.

This study includes characteristics of two magnificent outcrops as revealed by Mars Hand Lens Imager (MAHLI) and Mast-Mounted (MastCam) cameras. The first occurs at the transition of the Murray formation to those currently defined as the Clay-Bearing Unit. The outcrop is exposed on the northern flank of the Vera Rubin Ridge and displays rhythmic layers with meter-size, soft sediment deformation features most likely formed by mass flow processes due to slope failure. They indicate a disturbance in Gale crater that was triggered by Marsquakes. Alternatively, they could have also been caused by a change in lake level or a nearby impact.

The second exposure occurs in another equally magnificent outcrop that also shows rhythmic layers. Here, centimeter-thick, resistant mudstone layers alternate with recessive mudstone to silty mudstone layers. Both layer types are internally laminated. Soft sediment deformation and truncations of laminations suggest deposition by mass flow processes. The resistant layers transition into the recessives layers vertically and laterally suggesting two potential causes for the apparent rhythmicity. The first is that resistant layers experienced excessive preferential cementation and became harder than recessive layers. The second is that both layers had equal hardness but recessive layers experienced excessive preferential erosion. The two layers have identical bulk elemental composition as determined by the Alpha Particle X-ray Spectrometer (APXS) instrument except slight increase in Ca and S in recessive layers. This suggests that the second possibility is more likely than the first.