NATURE ABHORS AN ANGLE: USING CLAST MORPHOMETRY TO RECONSTRUCT SEDIMENT TRANSPORT HISTORY ON MARS

Rounded rock clasts observed by Mars rovers are strong evidence of past fluvial transport Investigations into their transport history is hindered as existing models of fluvial abrasion generally do not account for basaltic lithologies that are common on Mars but rare on Earth. The history of sedimentary rocks can be interpreted through measurements of clast morphometry, specifically circularity and roundness. Providing a better understanding of basaltic transport relations may allow better determinations of the provenance of martian clasts and how local environmental conditions may have evolved over time. We have identified field sites in three basaltic-dominated environments in contrasting climatic regimes: (1) an alluvial fan along the base of the Hilina Pali escarpment in Hawai’i (temperate environment); (2) a felsic fan (Globe Fan) and a combined felsic and mafic provenance alluvial fan (Lucy Gray) in the Mojave Desert (arid environment); (3) a channel system and associated gravel bar outwash plain downstream of the Þórisjökull glacier in Iceland (cold environment). At 10-20 stations per site, we randomly sample ~100 clasts and measure size and take photographs for shape analysis. To further refine our interpretations, we have also conducted two experiments in a large rotating drum to isolate abrasion-based shape change in basalt and granite. These results are used as comparison to field-based data to infer the relative lithologic impact of mechanical abrasion against other processes found in nature such as selective transport and chemical weathering. Results indicate that basaltic roundness increases faster than typical terrestrial lithologies (e.g. granite). We also find that basalt in nature rounds much faster than the experimentally determined rate. This may be due to the greater influence of chemical weathering on basalt compared to granites in natural environments. Quantifying this relationship will allow for improved interpretations of transport history and paleohydrology, and thus the inferred climatic environment, of rocks observed by present and future Mars rovers.