Valles Marineris dune sediment provenance and pathways

Although low-albedo sand is a prevalent component of the martian surface, sources and pathways of the sands are uncertain. As one of the principal present-day martian sediment sinks, the Valles Marineris (VM) rift system hosts a diversity of dune field populations associated with a variety of landforms that serve as potential sediment sources, including spur-and-gully walls, interior layered deposits (ILDs), and landslides. Here, we test the hypothesis that VM dune fields are largely derived from a variety of local and regional (intra-rift) sediment sources. Results show several dune fields are superposed on ancient wall massifs and ILDs that are topographically isolated from extra-rift sand sources. Spectral analysis of dune sand reveals compositional heterogeneity at the basinal-, dune field-, and dune-scales, arguing for discrete, relatively unmixed sediment sources. In Coprates and Melas chasmata, mapping is consistent with the principle sand source for dunes being Noachian-aged upper and lower wall materials composed of primary (igneous) minerals and glasses, some of which show evidence for alteration. In contrast, dune fields in Capri, Juventae, and Ganges chasmata show evidence for partial sediment derivation from adjacent Early Hesperian-aged hydrated sulfate-bearing ILD units. This finding indicates that these ILDs act as secondary sand sources. Dunes containing "soft" secondary minerals (e.g., monohydrated sulfate) are unlikely to have been derived from distant sources due to the physical weathering of sand grains during transport. Isolated extra-rift dune fields, sand sheets, and sand patches are located on the plateaus surrounding VM and the adjoining areas, but do not form interconnected networks of sand pathways into the rift. If past wind regimes (with respect to directionality and seasonality) were consistent with more recent regimes inferred from morphological analysis (i.e., dune slip faces, wind streaks), and were sufficient in strength and duration, small dune populations within Aurorae Chaos and north of eastern VM might have resulted from extended sand pathways into VM. However, we favor local and regional derivation of dune sand from a variety of intra-rift lithologic sources for most cases. Dune sand sources and the mechanism by which the sand is liberated are discussed in the context of findings described herein, but are broadly applicable to analysis of sediment production elsewhere on Mars.