Mass balance of Martian sedimentary fans and valleys

Dozens of sedimentary fans have been identified on Mars and have been interpreted as alluvial fans or deltas. However, the extent to which these deposits represent the complete eroded mass of the valleys that drain into them, the extent to which erosion has removed fan material, and the extent to which sediment bypass occurred during fan deposition into a distal water body remain unknown. This study investigates the role of emplacement versus modification following deposition in a catalog of martian fans to determine the extent to which such deposits have been preserved. A mass balance approach was taken; by calculating the present volumes of fans and the valleys feeding them, the percentage of eroded valley sediment that has accumulated and remained within the fan deposits can be determined. Based on measurements of 32 valley and fan volumes calculated using CTX stereo DEMs, we find two major classes of landforms: isolated inlets with lower stream orders and fans of approximately equal volume to the source valleys, and regionally-integrated valley networks with higher stream orders and much smaller fan volumes than valley volumes. If stream order correlates with valley age on Mars, then these results imply change in martian erosion and deposition patterns over time. We infer that hydrodynamic sorting in older, wetter systems resulted in preferential deposition of fines in fans formed by higher stream order valleys, and, ultimately more erosion in these fine-grained deposits, while younger, drier systems created deposits with intermixed sediment sizes that remain preserved today due to the greater protection of the fines from post-depositional erosion. These observations are consistent with a waning hydrologic cycle throughout martian history.Geological Science