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

The complex stratigraphy of layered deposits suggests a diversity of o rigins, ages, and post-depositional modification histories. The comple xities within some layered deposits indicate changes in the dominant s ource materials in space and time. The stratigraphy of layered deposit s in the isolated Martian chasmata Hebes, Juventae and Gangis is not w ell correlated. This indicates that at least these chasmata had isolat ed depositional environments resulting in different stratigraphic sequ ences. Separated layered deposits in Ophir-Candor and Melas Chasmata m ight have been a single continuous deposit in each chasma. Chaotic ter rains are found in conjunction with layered deposits in Juventae, Gang is ard Capri-Eos Chasmata. In these chasmata, layered deposits unconfo rmably overlie chaotic terrains. Chaotic terrain formation may have pr ovided water to form paleolakes, and lacustrine deposition of thick la yered deposits may have occurred if the canyons were closed. A very th ick sequence of the layered deposits has been exposed by erosion. A co mbination of gradual processes such as evaporation of ice and eolian a nd fluvial transport in addition to structural processes may be respon sible 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 c onfined to the depressions or reentrants in western Candor Chasma. The difference in color between these units and the surrounding terrain i s 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 f rom the primary and dominant formation of the layered deposits.