STRATIGRAPHY AND EROSIONAL LANDFORMS OF LAYERED DEPOSITS IN VALLES-MARINERIS, MARS

The complex stratigraphy of layered deposits suggests a diversity of origins, ages, and post-depositional modification histories. The complexities within some layered deposits indicate changes in the dominant source materials in space and time. The stratigraphy of layered deposits in the isolated Martian chasmata Hebes, Juventae and Gangis is not well correlated. This indicates that at least these chasmata had isolated depositional environments resulting in different stratigraphic sequences. Separated layered deposits in Ophir-Candor and Melas Chasmata might have been a single continuous deposit in each chasma. Chaotic terrains are found in conjunction with layered deposits in Juventae, Gangis ard Capri-Eos Chasmata. In these chasmata, layered deposits unconformably overlie chaotic terrains. Chaotic terrain formation may have provided water to form paleolakes, and lacustrine deposition of thick layered deposits may have occurred if the canyons were closed. A very thick sequence of the layered deposits has been exposed by erosion. A combination of gradual processes such as evaporation of ice and eolian and fluvial transport in addition to structural processes may be responsible for this erosion. Another alternative is that catastrophic water release under the layered deposits disrupted and initiated erosion of the layered deposits. Newly identified units of anomalous color are confined to the depressions or reentrants in western Candor Chasma. The difference in color between these units and the surrounding terrain is most consistent with a somewhat greater content of bulk crystalline hematite in these anomalous units. The presence of the Candor units is a result of original and/or secondary deposition which is different from the primary and dominant formation of the layered deposits.