NUMERICAL SEDIMENTARY MODELING OF MARTIAN STEPPED FAN-DELTAS

We are investigating the hydrology and sedimentology of martian stepped fan-deltas using a modified (adapted for martian conditions) version of the Sedflux model, whose concepts have been vetted in many different terrestrial settings and also for Mars. Stepped fans are distinguished from other martian conical sedimentary deposits principally by their concentric stair-step topography, as well as their relatively small size (<100 km²). Seven stepped fans with meter-scale topographic data vary in profile shape from steep (4-10° average fan slope), thick (~1.3 km) form, to flatter fans with broad platforms (average slope <1.5° over >1 km radial length), as well as composite cases. Stepped fans make up about one fourth of the known candidate martian fan-deltas.

Our main aim is to test the hypotheses put forth for the origin of these sedimentary bodies. Specifically, the (forward) simulations test several hydrologic scenarios, sediment discharges, grain size, basin bathymetry, timing of emplacement, water level variations, and morphodynamic evolution to characterize the hydrologic and sedimentary settings that match the observational evidence as reconstructed from available topography.

Results from simulations at constant water and sediment discharges while changing water level suggest that the formation of steps occur only when water level increases through time. We tested several water level scenarios from monotonic rise to complex water level variations including the occurrence of several (and of different duration) highstands followed both by continuing level rise and/or intervening temporary falls within the overall level rise. The morphodynamics of the resultant deposits show that rather regularly spaced steps are formed under monotonic level rise (in agreement with results from laboratory tank simulations), while longitudinal intermediate terraces are formed during the highstands. More interestingly, later steps do not bury previously formed steps even while maintaining constant river water and sediment discharges. Therefore, previously hypothesized retrogradational depositional conditions (i.e. water level rise with significant decrease of sediment flux) are not necessarily required to form stepped fans.