HR: 14:25h
AN: P23D-04    [Abstracts]
TI: MRO Observations of Light-toned Layered Deposits along the Plains Adjacent to Valles Marineris
AU: * Weitz, C M
EM: weitz@psi.edu
AF: Planetary Science Institute, 1700 E Fort Lowell Suite 106, Tucson, AZ 85719, United States
AU: McEwen, A S
AF: University of Arizona, Lunar and Planetary Lab, 1629 East University Blvd., Tucson, AZ 85721, United States
AU: Milliken, R E
AF: Jet Propulsion Laboratory, 4800 Oak Grove Dr. MS 183-301, Pasadena, CA 91109, United States
AU: Grant, J A
AF: Smithsonian Institution, Center for Earth and Planetary Studies, National Air and Space Museum, Washington, DC 20560, United States
AU: Team, H S
AB: We report on light-toned layered deposits (LTLD) that are located in several locations along the Hesperian-aged plains adjacent to Valles Marineris, including (1) southwest of Melas Chasma, (2) south of Ius Chasma, (3) south of West Candor Chasma, (4) west of Ganges Chasma, and (5) west of Juventae Chasma. The LTLD on the plains are only seen in locations where there has been erosion of an overlying meters-thick darker-toned unit or along crater and trough walls so they are likely more extensive in area than seen by outcrop exposures. Based upon exposures along crater and trough walls, thicknesses for the deposits can be up to 200 meters. The LTLD along the plains that we have analyzed with HiRISE data show complex stratigraphic sequences where beds can be divided based upon variations in their color, albedo, thickness, and erosional style. Polygonal fractures seen along some bedding surfaces can vary in size and shape. Color differences seen in HiRISE images appear to reflect compositional variations rather than surficial contamination (i.e., dust or basalt sand). Pitted and rounded features are common to some layers along the plains but not seen in LTLD within troughs. We have noticed that similar beds can repeat throughout an exposed sequence, suggesting a cyclic process for deposition. The layered beds appear alike in morphology for all five locations, although the deposit south of West Candor has less evidence for layering compared to the other four locations. The morphology and color variations seen in the LTLD along the plains appear distinct from those seen for LTLD inside the troughs, suggesting different depositional processes. Proposed origins for the LTLD along the plains include eolian, fluvio-lacustrine, and pyroclastic volcanism. A pyroclastic origin is likely based upon their association with the Hesperian-aged lava plains, their cyclic nature and lithologies that are consistent with pyroclastic deposits on Earth, and the identification of hydrated silica-rich phases in CRISM data. However, at Ganges and Juventae Chasmata the LTLD are adjacent to a valley (Ganges) or postulated inverted channels (Juventae and Ganges) that could support a fluvial origin at these two sites. There is no evidence for fluvial activity at the other three locations, though. If some of the deposits are pyroclastic in origin, their limited distribution implies eruptions were localized and probably associated with emplacement of the lava plains that surround the canyons, rather than in association with Tharsis volcanism. In the case of a fluvio-lacustrine origin for the deposits at Ganges and Juventae Chasmata, water activity could have resulted in the formation of depositional fans composed of the light-toned beds.
DE: 5464 Remote sensing
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting