MAADIM-VALLIS REVISITED THROUGH NEW TOPOGRAPHIC DATA - EVIDENCE FOR AN ANCIENT INTRAVALLEY LAKE

Based on a comprehensive study of the recent topographic map USGS MC-2 3 SE I-2119 (USGS, 1991, Topographical Map of Aeolis) plus geomorphic and digital elevation, we have constructed a geometric model of Ma'adi m Vallis, The sedimentary history of the valley was determined by the damming of the valley by two impact craters, one by the southern crate r over Gusev rim and the second located 120 km upstream. The presence of an intravalley lake system is inferred from independent sedimentary analysis and from geomorphic evidence for valley benches. Peripheral and parallel topographic contour intervals of the putative intravalley lake shores suggest that the lake was covered by a thick ice sheet. T he longitudinal profile of the valley floor shows a peculiar reverse s lope located 300 km upstream from the southern Gusev crater rim. We co nclude that this is an accumulation of sedimentary deposits adjacent t o the crater rampart which defined the northern boundary of the lake. When the water level reached the lowest point of the west bank, which is 0.4 km below the top of the rim of Gusev crater, it began to floor the surrounding plateau, As the intravalley lake level raised, its ice cover formed a wide lateral bench. Later, this ponded water was funne led into Gusev crater through the breaching of the southern Gusev ramp art by an impact crater. The paleolacustrine sediments transported by the lake water that channeled through the rampart debris slope mantled the floor of Gusev crater. The sublacustrine sediments are promising targets for future missions and of high priority in the search for pas t life. (C) 1996 Academic Press,Inc.