GEOLOGIC EVOLUTION OF THE TYRAS VALLIS SYSTEM, MARS

High Resolution Stereo Camera (HRSC) data [1], with their stereo capabilities, allow the investigation of Martian sedimentary features at unprecedented details. Using these new data, we reconstructed the hydrological history of an unnamed Noachian [2] complex crater in the Xanthe Terra region. The crater hosted a lacustrine basin fed by a dense and centripetal drainage system, developed along its inner rim, and by the Tyras Vallis channel. Where the Tyras Vallis feeder channel opens into the crater, is visible a prominent fan-shaped distributary feature, characterized by a central terrace and two small longitudinal scarps. The delta-like deposit covers an area of about 83 km², with a 13 km radial length and an average slope gradient of about 3.5 degrees. The apex of the fan lies ~900 m above the crater floor. To investigate the geology of the Tyras Vallis paleolacustrine system, we considered the geomorphologic characteristics of the mapped units and determined their relative ages based on stratigraphic relationships. Evidence of strandlines within the crater provided constraints about the past presence of a standing body of water with its wave-related processes. Topography derived by HRSC stereo data supported the analysis of the Tyras fan deposit and allowed to reconstruct the overall lacustrine depositional history from the parallel observations of the Tyras fan and crater floor deposits. Two major stands of the water level have been inferred at 700 m and 550 m above the crater floor. Our reconstruction reveals a complex sedimentary evolution of the fan, which underwent deltaic and alluvial sedimentation, passing from a river-dominated progradational regime to a wave-dominated erosional regime. Groundwater processes and intrinsic fluctuations of the sapping dynamics likely affected the transport capacity of the Tyras Vallis and the changes of the water level in the paleolake. Moreover, the heating effects of cratering [3] might have affected the area, initiating, rejuvenating or accelerating the recharge of the aquifer and the basin evolution. This eventuality advised against any global paleoclimatic extrapolation.

References: [1] Neukum, G., et al. (2004), ESA SP-1240, 17-35; [2] Scott, D.H., and K. L. Tanaka (1986), USGS Misc. Inv. Ser. Map, I-1802-A; [3] Brakenridge, G. R., et al. (1985), Geology, 13, 859-862.