Effect of Standing Water on Formation of Fan-Shaped Sedimentary Deposits at Hypanis Valles, Mars

Fan-shaped sedimentary deposits are key indicators of paleoclimate on Mars. The largest example, a multi-lobed deposit (>1,000 km²) at the outlet of Hypanis Valles, may have formed either subaerially or at the edge of a water body. We used a numerical model to test whether these contrasting basin boundary conditions would cause deposit geometries that are diagnostic and distinguishable from orbit. We compared depositional scenarios with either (a) fully subaerial conditions, (b) a constant water level, or (c) a falling water level. In the model, standing water generates deposits that are thicker, less extensive, and increase in dip angle from proximal to distal locations. Nonetheless, deposit geometries for all scenarios are consistent with orbital observations, which are inconclusive regarding a topset-foreset break that would indicate standing water. Thus, it is possible that the Hypanis deposits formed subaerially, without a water body onlapping the Mars dichotomy boundary.