MORPHOLOGICAL DIFFERENCES IN MARTIAN STEPPED FANS (Invited Presentation)

Stepped fans—conical forms with stair-step topography—are relatively rare on both Earth and Mars, and their formation mechanisms are poorly understood. Multiple formation scenarios for stepped fans have been proposed, including lava flows, erosive wave action, subaerial alluvial fan deposits, or subaqueous delta deposits. We examined the three-dimensional form of fourteen martian stepped fans in high resolution digital elevation models derived from the HRSC, CTX and rarely HiRISE instruments.

Irrespective of fan size, there were distinct groupings of fan attributes that we interpret are due to differences in formation process. Discrimination of morphologic classes is based largely on differences in the fan radial profile, the mass balance ratio (fan volume to valley volume), and the planimetric configuration of the feeder valley. Class A are steep (>4°), multi-step fans typically with a single, short feeder valley. Volume ratios for Class A are close to unity, indicating minimal sediment loss. Consistent with prior study’s findings, we favor gravity-driven flows with low to no water content, such as mass movement or debris flows, in generating Class A stepped fans.

Class B stepped fans are located in closed basins, fed by a branching valley network, and characterized by shallow average fan slope (<3.5°) that includes subhorizontal planar sections ~1 km in length. The low volume ratios (<0.5) for Class B stepped fans are consistent with significant sediment loss from the terminal deposit. Although all martian fans have experienced post-deposition deflation by aeolian erosion, we attribute the low volume ratio of Class B fans also included dispersal of the fine sediment load in a standing body of water. This interpretation is further supported by the presence of platforms that we infer are associated with water level fluctuations. Numerical simulations by our group and laboratory experiments in prior work have replicated stepped topographic form in rising and falling water conditions. Our on-going work is focused on evaluating if transgressive and regressive scenarios can be distinguished in the available remotely sensed data for Class B fan-deltas, and constraining formation timescales.