CHANNEL DEVELOPMENT ON MARTIAN TERRACED FANS: A NEW CLUE TO FORMATIVE PROCESSES?

Terraced fans on Mars are unique geomorphic features that record sedimentation through time. Unlike alluvial fans or Gilbert-type deltas, terraced fans have stair-step topographies characterized by abrupt changes in fan slope down-dip. While terracing is not a particularly unusual geomorphic phenomenon, the combination of terracing along outer fan edges coupled with up-dip channelization suggests an intriguing history of erosion and mass movements unlike any terrestrial analog. As a result, terraced fans, or stepped deltas, have been variously attributed to mass wasting processes, fluvio-deltaic processes, resedimentation in standing water, and/or preferential erosion of weakly cemented layers.

High resolution imagery (e.g. HiRISE, CTX) and topographic data (e.g. MOLA) can be combined to investigate the formative mechanisms proposed for terraced fan deposition. Here, we use these datasets to create digital terrain models and conduct detailed geologic mapping of an idealized fan system located at Subur Vallis in the Xanthe Terra region of Mars (11.73°N, 307.05°E). The Subur Vallis fan is a wedge of sediment measuring approximately 4.3 km in radius at the widest point and greater than 350 meters thick. The uppermost terraces are characterized by sinuous channels originating in the Subur Vallis canyon to the southwest. The channels are narrower than the feeder canyon, measuring 0.25-0.5 km wide and 1.5 km wide, respectively. Calibration with topographic data suggests that channels are 5-10 meters deep; however, modern aeolian bedforms are common within channels so depth estimates are interpreted as minimum values.

A record of channelization on younger terraces would suggest that distributary fluid-flow processes were active in the last stages of fan deposition and/or helped shape the uppermost fan surface. This conclusion would, in the case of the Subur Vallis fan, confirm the availability of fluids for transport processes in a terraced fan system.