Discontinuous Drainage Systems of NE Hellas Basin, Mars

Abstract:

We mapped several valley and channel systems located on the plains NE of Hellas Basin, NW of Dao, Harmakhis, and Reull Valles, using CTX, THEMIS, HiRISE and HRSC data. The dissected terrain is comprised of early Hesperian lava flows. Drainage systems consist of deep, narrow valleys that we interpret as bedrock reaches and small outflow-like, flat-floored channels, that are approximately 1-2 km wide and contain depositional bedforms. In these systems, approximately 130 m deep narrow reaches alternate with wide, shallow sediment-dominated reaches. This morphologic pattern is probably the result downcutting and erosion of bedrock by stream flow and subsequent deposition of the eroded material in the wider reaches downstream in response to local topographic and lithologic changes. The floors of the wider channel reaches contain several stream-lined smooth-surfaced islands, which we interpret as possible bar deposits. In some of these non-terminal depositional reaches, the channels are completely filled by deposits but further downstream the boundaries of the channel walls become apparent again. We interpret these locations as possible sites where stream flow spreads out and infiltrates into the subsurface and then subsequently outflows again to surface where channel walls become more defined. Some channels seem to be associated with the pitted latitude dependent mantle, whereas elevated islands remain smooth and free of pits. In the upper reaches of one channel system, an assemblage of intra-channel features is repeated: knobs, sinuous ridges and elongate, channel-jamming deposits similar to the morphology resulting from glaciers or rock glaciers. One channel system begins with several theater-shaped heads, suggesting a possible formation by sapping. Channel heads are cut into a high-thermal-inertia unit, possibly basaltic bedrock. We propose that this setting is suggestive of terrestrial discontinuous ephemeral stream channel systems.