VARIATIONS IN ALLUVIAL FAN MORPHOLOGY IN SOUTHWESTERN TERRA SABEA, MARS, SUGGEST MULTIPLE, DISCRETE DEPOSITIONAL PHASES

Large alluvial fans in martian highland craters have been interpreted as evidence of a late-stage period of fluvial activity between Early Hesperian and Early Amazonian (3.7 to 3.3 Gyr) [1, 2]. One cluster of large alluvial fans is located in craters north of Hellas, including within Harris, Saheki, and two unnamed craters (‘L’ and ‘7’). We document differences in alluvial fan morphology at these sites, and postulate that these differences reflect discrete phases of aqueous activity.

We classified intracrater alluvial fans in southwestern Terra Sabea into three distinct morphological types. Some fans exhibit wide (250-450 m) channels or “chutes” that maintain constant width downslope, have a linear path, and rarely bifurcate. Other fans have narrow (10-200 m), low-relief ridges with low sinuosity that often branch at their distal ends, suggestive of distributaries associated with channelized mud-rich flows [3]. A third category of fans is rugged at the meter-scale with few clear channels or sinuous ridges on the conical form. Fan morphology is spatially concentrated, even in craters that have alluvial fans at all azimuths (e.g., southern portion of crater ‘L’ has ridge-covered fans, while northern fans have chutes).

Variations in fan morphology within the same crater may represent different periods of fan formation: 1) the oldest fans were subjected to extensive erosion and relief inversion, 2) younger fans exhibiting a degraded but not fully inverted surface, and 3) the youngest fans retaining negative relief channels. In several cases, fans with channels transition to inverted relief at their distal ends, a trend previously interpreted as preferential erosion exploiting downfan fining of deposits and leaving the upper fan protected by coarser deposits [3]. Alternatively, composite fan morphology could result from burial of an older, denuded fan surface (inverted channels) by a later stage of alluvial fan activity (chutes). Work is ongoing in the region to characterize fan morphology and to assess their stratigraphic relationships to test the late-stage fan deposition hypothesis and evaluate potential implications on time-varying, localized precipitation patterns.

[1] Moore and Howard, 2005, JGR. [2] Mangold et al., 2012, JGR. [3] Morgan et al., 2014, Icarus.