HR: 16:00h
AN: P24A-01    [Abstracts]
TI: Stratification, Sediment Transport, and the Early Wet Surface of Meridiani Planum
AU: * Grotzinger, J P
EM: grotz@mit.edu
AF: Massachusetts Institute of Technology, Room 54-616, Cambridge, MA 02139 United States
AU: Athena Science Team, .
AF: Massachusetts Institute of Technology, Room 54-616, Cambridge, MA 02139 United States
AB: Several stratification styles are present in the outcrops investigated by the Opportunity rover at Meridiani Planum. These include planar lamination, low-angle cross-stratification, cross-bedding, ripple cross-lamination, and crinkly and undulatory lamination. Planar lamination and low angle stratification are well-developed in several locations, particularly at Slickrock and Shoemakers Patio in Eagle crater and at Karatepe in Endurance crater. MI images at Slickrock show "pinstripe" lamination where individual laminae can be single-grain thick layers, suggestive of eolian sedimentation. At Shoemaker's Patio, a single cross-bed set with thickness of 5-7 cm is preserved. At Burn's Cliff, in Endurance crater, a single cross-bed set of up to several meters thickness is preserved. In contrast, ripple cross-lamination with festoon geometry is present at several locations within Eagle crater including Last Chance, the Dells, and in an isolated rock (Scoop) on the southwest rim of the crater. Ripple cross-laminae sets are 0.8 to 1.7 cm thick. In the case of Scoop, the sets are possibly climbing. Grain size appears to range from 0.1 to 0.8 mm in diameter. The cross-bedding preserved at Eagle crater suggests both eolian and subaqeous environments. The set at Shoemaker's patio represents deposition from either subaerial or subaqeous dunes. The thicker, meter-scale set at Burns Cliff is most consistent with transport by eolian dunes. In contrast, the small-scale festoon cross-laminae at Eagle crater are indicative of sediment transport in subaqueous ripples. The reconstructed size of former bedforms is inferred to be only a few cm, and therefore inconsistent with eolian dunes which commonly have significantly larger minimum amplitudes. Yet the cross-lamination is significantly larger and of differing geometry from the climbing translatent cross-strata produced by wind ripples. Thus, the size of bedforms that produced the cm-scale cross-laminae preserved at Eagle crater occupy the spectral gap in the size distribution of eolian dunes and ripples. A further clue to the likely subaqueous origin for Eagle cross-laminae is provided by their festoon geometry which requires that the reconstructed ripples have three-dimensional geometry defined by highly sinuous crestlines. In terrestrial settings such bedforms are known only to develop in subaqueous, subcritical flows, with velocities of less than one meter per second. On Mars, the initiation of subaqueous sediment movement is expected to occur at velocities lower than on earth, by a factor of 1.34. With respect to length scales, such as bedform dimensions, the scaling factor turns out to have the opposite sense so that, given dynamically similar flows, ripples on Mars might be larger by a factor of 1.34. Neither of these rescaling effects influences the interpretation that cm-scale, festoon cross-lamination at Eagle crater most probably requires sediment transport in subaqueous, low-velocity flows across the Meridiani plains. The assemblage of primary facies recorded at Eagle crater is most consistent with an environment characterized by episodic inundation by surface water to shallow depths, followed by evaporation, and exposure and desiccation. Terrestrial analogs for such a suite of facies and surface processes include small interdune depressions, playa lakes, and sabkhas adjacent to marginal seaways. All rover observations indicate that the evaporite-bearing cross-bedded unit extends at least on the order of 1 km though MOC and THEMIS data suggest it extends much further.
DE: 5470 Surface materials and properties
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting