A SEDIMENTARY PLATFORM IN MAGARITIFER SINUS, MERIDIANI TERRA, AND ARABIA A. D. Howard1, J. M. Moore2, R. P. Irwin, III3, and R. A. Craddock3 1Department of Environmental Sciences, P.O. Box 400123, University of Virginia, Charlottesville, VA 22904-4123, [email protected], 2NASA Ames Research Center, MS 245-3, Moffett Field, CA, 94035-100, 3Center for Earth and Planetary Sciences, National Air and Space Museum, Smithsonian Institution, Washington, D.C., 20013.

Introduction: The Margaritifer-Meridiani-Arabia channel to have formed by sub-ice flow, possibly from highlands-lowlands (H-L) transition has long been the site of an impact into the ice. Mawrth Valles and recognized as the most fluvially dissected region of an unnamed shallow outflow channel near 37N and Mars [1-4]. However, the geomorphic evolution of 10E may also have originated by sub-ice flow. this region remains enigmatic, particularly the origin of the layered deposits of Meridiani Terra and Arabia. We suggest that a portion of this regional slope served as a fluvial depositional platform during the late Noachian. A fluvial wedge (the Meridiani Bench (MB), Figure 1) is postulated to have been deposited at elevations of -1200 to -1700 m. in the Margaritifer- Meridiani-Arabia region during the late Noachian/Early Hesperian. The MB drops about 300 m rather abruptly at its northwestward boundary into the Valles Marineris depression, which was a depression prior to erosion from Uzboi flows and chaotic terrain development. This bench roughly corresponds to the smooth unit (Ns) of [1]. The bench also corresponds to a prominent peak in the elevation histogram centered at -1300 m, with valleys incised 100-300m below the benchThe Meridiani Planum layered deposits, including the hematite-rich lacustrine sediments were deposited near the close of deposition on part of this bench. The cratered terrain in Conclusions: In summary, we suggest that a Arabia Terra below about -1800 m may have been fluvial depositional bench (MB) was deposited during occupied by an ice-covered ocean during this period. the late Noachian. The bench was subsequently Previously the low relief of the MB was interpreted to fluvially incised, but this was followed by local have resulted from intensive fluvial erosion rather than deposition of the layered deposits of Meridiani as resulting from fluvial mantling. Evidence for the Planum, including the hematite-rich lacustrine present interpretation includes: 1) A hypsometric sediments at the Opportunity MER rover site. The transect across this region shows a prominent peak at lateral confinement for deposition of the fluvial bench the elevation of the hypothesized MB; 2) A sequence and the later Meridiani layered deposits may have been of paleochannels of Loire Valles indicate initial flow an ice-covered lake/ocean (or possibly a piedmont at the level of the MB surface followed by progressive glacial complex) occupying the low-lying cratered incision and stream captures; 3) With the exception of terrain of Arabia Terra. a few unusual valleys lacking tributary development, References: [1] Hynek, B. M., Phillips, R. J. the cratered terrain below -1800m lacks the valley (2001) Geology, 29, 407-10; [2] Grant, J. A., The networks that are common above this level; 4) geomorphic evolution of Eastern Margaritifer Sinus, Quantification of the topography and crater Mars, Adv. Planetary Geol. (1987), pp. 268; [3] Grant, frequency/degradation of the MB, the superjacent J. A., Parker, T. J. (2002) JGR., 107, 5066, cratered highlands, and the subjacent cratered lowlands doi:10.1029/2001JE001678; [4] Goldspiel, J. M., reveals that the three regions have statistically distinct Squyres, S. W. (1991) Icarus, 89, 392-410. morphometric signatures and cratering statistics; 5) A 600 km valley on the cratered lowlands below the MB lacks tributaries and has a profile that climbs over three broad ridges up to 300m high. We interpret this