SEDIMENTARY DEPOSITS IN IMPACT CRATERS ON MARS: COMPARISON WITH TERRESTRIAL PLUVIAL LAKE BASINS

Identification of likely sedimentary deposits on Mars has improved with new high-resolution orbital imaging and lander data. Positive-relief deposits with frontal scarps (resembling deltas) are uncommon at the mouths of Martian valley networks, but such deposits have been identified in >20 impact craters to date. These deposits commonly extend several kilometers into the enclosed basins, and some surfaces exhibit distributary channels, but few deposits were entrenched with declining lake level in the crater. This observation contrasts with pluvial lake deltas in the western United States, which were generally dissected as lake level fell. Some craters and intercrater basins on Mars have narrow, pristine outlet valleys that formed late in the process of crater degradation. Small channels with low sinuosity are also locally evident within some contributing valley networks on Mars. These observations suggest a sudden and more substantial decline in erosion rates in the Martian equatorial highlands, relative to the onset of the last interglacial in the U.S. basin and range. Possible shore landforms on Mars remain controversial and difficult to interpret. We photodocumented the lag material and surveyed the microtopography of Pleistocene barrier ridges in two Nevada paleolake basins to determine whether similar landforms might be preserved in the Martian geomorphic environment. The occurrence of these shore landforms would require past wave action on a surface that has since been relatively stable. The reduction of the small crater population on many crater walls and other steep slopes demonstrates a mobile material that could eradicate similar small-scale shore landforms at high levels within a basin.