ROCKING AND ROLLING DOWNHILL: A COMPARISON OF BOULDER DISTRIBUTION PATTERNS ON MARTIAN TERRACED FANS, XANTHE TERRA

Terraced fans are a relatively rare subset of fan-shaped features on Mars that display radial, stepped topography and concave-up profiles. This study focuses on three terraced fans located in the Xanthe Terra region: the Camichel Crater fan (2.69˚N, 308.33˚E), the Rypin Crater fan (1.27˚S, 318.98˚E), and an unnamed crater fan (14.09˚N, 335.70˚E). Features common to all three fans are boulders, large rocky detrital fragments visible in high-resolution orbital images. Hypotheses for boulder origins include: (1) impact processes; (2) erosion of steep slopes; (3) preferential removal of finer-grained material through erosion; (4) transport via laminar debris flows; and/or (5) transport via turbulent flood events. The purpose of this project is to conduct high-resolution geologic mapping and test competing hypotheses by comparing boulder distributions on terraced fan surfaces.

Images returned from the High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter (MRO) were used to evaluate small-scale features on the surface of each fan. HiRISE images have a resolution of 0.3 to 0.6 m/pixel; thus, boulders with a diameter of > 0.8 m were mapped in ArcGIS Pro at a consistent scale of 1:3,400. Geologic mapping to identify depositional units was also conducted at a consistent scale of 1:4,000.

Resulting maps show that boulders can be linked to both erosional and depositional processes as well as structural deformation. For example, boulders mapped on the Camichel Crater fan (n > 3,700) and Rypin Crater fan (n > 3,600) are primarily associated with steep topographic scarps adjacent to feeder channel walls or terrace edges, likely a result of over-steepening and erosion. In distal fan units, however, both fans are also characterized by high boulder distributions in light-toned units, which suggest flow competence and boulder transport during deposition. Boulders on the unnamed crater fan (n > 11,700) are distributed in lineaments, possibly linked to regional faulting and deformation. Future work will focus on comparing these patterns to boulder distribution patterns in terrestrial fan settings such as alluvial fans and Gilbert-type deltas.