RAPID INUNDATION OF GALE CRATER, MARS (Invited Presentation)

Gale crater, the field site of the Curiosity Rover, is located near the equator of the planet Mars along the boundary that separates the Southern Highlands region from the Northern Lowlands area. It formed by an impact during the Late Noachian time, about 3.75 – 3.6 Byr ago. Nine distinct lithologies are identified; six have been investigated by the rover in detailed during past 7 years. This study evaluates the relationship between the two oldest exposed rock units that recorded the rapid inundation of Gale crater.

The Hummocky Plains Unit is the oldest lithologic unit because it underlies all other strata so far examined by the rover. It consists of an unsorted conglomerate with rounded grains as large as 20 cm in size. Its basal contact is not exposed but its top surface slopes northward (rises in elevation toward south). In areas where it was not subjected to extensive erosion, it forms asymmetric ridges which are up to 5 m high and cross-bedded indicative of deposition by huge floods.

The Murray formation is 310 m thick and overlies the Hummocky Plains Unit with a thin gradational contact. Its dominant lithology is a laminated mudstone but it also has thin interbeds of siltstone and sandstone. Laminations extend for few meters before being truncated. The basal lithologies of the Murray formation lack shallow-water features such as ripple marks and mud cracks indicating that it was deposited in deep-waters below the influence of wave actions.

Deposition of laminated, deep water mudstones directly over the fluvial conglomerate of the Hummocky Plains Unit suggests a rapid and geologically instantaneous inundation of Gale crater. One possible cause is the drowning of the crater by a large body of water from the Northern Lowlands: possibly the Northern Ocean. Earth’s analogy is Jurassic strata of the Northern U.S. Gulf Coast where laminated deep-water strata of the Smackover Formation deposited on alluvial fan conglomerate of the Norphlet Formation during the expansion of Gulf of Mexico. However, sedimentological and stratigraphic characteristics indicate that the more plausible interpretation is the rapid filling of the crater by huge floods produced by melting of ice and snow on the Southern Highlands of Mars. The Earth’s analogy for this scenario is formation of deep lakes along flood paths during Earth’s Pleistocene Epoch.