GEOLOGIC MAPPING AND STRATIGRAPHIC ANALYSIS OF ALLUVIAL FANS IN RODDY CRATER ON MARS

Alluvial fans are broad, gently sloping, semi-conical landforms that are created when sediment is fluvially transported from mountainous terrain onto adjacent lower-gradient surfaces. Fan lobes build up over time during high magnitude, short duration releases of water and sediment. The growing inventory of post-Noachian alluvial fans on Mars has played a significant role in challenging the paradigm that the Hesperian and Amazonian epochs were sub-optimal for widespread precipitation-led fluvial erosion. Detailed mapping and characterization of host craters and their alluvial fans - including age, sedimentology, sequence of fan development, and relation to other landforms - is essential to our understanding of climate and potential habitability on Mars.

Roddy is an ~86 km-diameter, fan-bearing crater on the southwestern margin of Vinogradov crater in southern Margaritifer Terra (~21.6°S, 320.6°E). Roddy formed into at least one pre-existing crater, leading to an asymmetric rim. Later northward erosion of the rim sourced large, coalesced alluvial fans. The rimcrest-to-floor depth of Roddy ranges from ~1.8-3.1 km, with associated depth-to-diameter ratios of 0.02-0.04. A possible central peak remnant rises ~200-300 m above the fan deposits and is surrounded by a darker-toned indurated bench. The estimated original depth and central peak height for an 86 km-diameter crater on Mars ranges from ~2.8-3.6 km and ~385-2065 m, respectively. Comparison of the present-day versus modeled measurements suggests Roddy is infilled with hundreds to ~1800 m of material. The thickness and volume of the fans is not yet known, but interior walls of a 1.3 km-diameter crater on the eastern fan exposes thick sequences of sedimentary layers.

Fan surfaces are deflated, leaving distributary channels, composed of coarser and (or) more resistant material, standing in positive relief. This inversion of fan topography is consistent with a mostly fine-grained component for the bulk of the fans in Roddy. The relationship between the large, well-exposed alluvial fans and other landforms in Roddy including fluidized ejecta on Roddy’s eastern floor, hummocky topography, and aeolian deposits, provides insight into the processes affecting fan development, making this crater a prime location for detailed mapping and analysis.