SIGNIFICANCE OF FLOOD DEPOSITS IN GALE CRATER, MARS
In areas where the HPU was not subjected to late erosion, it forms 300-500 m-long asymmetric ridges which are about 150 m apart. Ridges are up to 5 m high and cross-bedded indicating flow direction toward north. Supporting this conclusion is the elevations of the top surface of the HPU that decreases northward, suggesting that it formed a north-sloping (down flow) surface at the end of its deposition. These characteristics indicate that asymmetric ridges are giant gravel dunes. They are indicative of deposition by large floods rather than sedimentation by ordinary fluvial systems. Flood waters entered Gale crater through its southwestern rim and flowed northward.
Giant gravel dunes of Gale crater are identical to giant current ripples of Lake Missoula and Altai Mountains flood deposits of the Pleistocene Epoch. This suggests that Martian floods originated by glacial outbursts, as did their Earth’s counterparts. This scenario implies that Mars was cold and dry. But, high obliquity and high eccentricity under a modest atmospheric pressures caused above freezing (warm) temperatures over large areas of the Northern Lowlands and equatorial regions of Mars (including Gale crater) where liquid water accumulated. The Southern Highlands remained cold due to adiabatic cooling and melting of its glaciers produced flood waters that flowed to Gale crater. Alternatively, flood waters could be attributed to sustained heavy precipitation under a warm and wet Mars. Unless occurred very rapidly, this scenario should have also produced extensive chemical weathering at the source area. The presence of abundant unstable minerals such as olivine, pyroxene, and plagioclase in fine-grained sediments of Gale crater favors the cold and dry model.