EVIDENCE FOR MUDFLOWS IN KASEI VALLES, MARS

Kasei Valles is generally thought to have formed largely by catastrophic floods. High-resolution images (1.5-12 m/pxl) acquired from the Mars Orbiter Camera (MOC) reveal a pervasive surface texture that is a record of the last stage of activity within these channels. This surface texture is characterized by corrugated plates of material that are 1 km in their longest dimension and bounded by ridges or occasionally troughs. Plates can form a tight mosaic or exhibit irregular spacing in a configuration reminiscent of rafted jigsaw pieces. Interplate spaces are rough and can be cracked. Particularly noteworthy is the relationship of this texture to two inner channels (140 km long, 1 km wide, ~200 m deep) located in the headward reaches of the southern, east-west branch (~21.2° N, ~72.5° W). A campaign to acquire contiguous coverage of the inner channels has been undertaken, including the use of ROTO’s (Roll-Only Targeted Observations); ~85% has been imaged to date. These pictures reveal the platy surface texture is present throughout the inner channels (headward, on floor and downlope of channels) indicating the material navigated through and probably carved the narrow channels. The margins of the platy surface texture is rugged (sometimes bouldery), semi-lobate and locally embays topographic obstacles. Platy surface texture is sparsely cratered with ~5 times fewer 200 m diameter craters than the adjacent non-platy surfaces. Although the surface texture is somewhat similar to lava flows, constructional forms expected at the inner channel heads, where fluid lava cascaded over the escarpment, cooled and solidified, are not present. The material properties associated with its expression are most consistent with emplacement by a mudflow. Mudflows are known to transition between viscous Newtonian behavior at low sediment concentrations to properties of a visco-plastic fluid when particle concentration exceeds 60%. Consequently, mudflows are capable of eroding channels, maneuvering through narrow or sinuous paths, and developing lobate terminations and corrugated forms, all attributes observed within the study region. Future work involves detailed mapping of the regional extent and continuity of platy surface texture that has been observed in both northern and southern Kasei Valles channels, as well as Echus Chasma.