Thermophysical Signatures of Sedimentary Processes on Alluvial Fans

Alluvial fans are common features in arid terrestrial environments where subaerial runoff surfaces transition from relatively steep slopes with confined flow to shallow slopes with unconfined flow. Visible-wavelength orbital images from Mars have been used to suggest that alluvial fans also exist there. Terrestrial alluvial fans display a variety of different surface features that are indicative of the mode of flow (which is related to ratio of water to rock) and sediment lithology, such as gravel bars, sheetflood deposits, debris flows, and incised and/or braided distributary channels. Many of the processes that form these features act to sort or distribute particles by size to different degrees. Fan surfaces may also become indurated as the result of mud in the flow or secondary modification processes.

Thermal infrared imaging provides a way of mapping the processes that have operated on the uppermost surfaces of alluvial fans. Surfaces that display relatively large diurnal temperature variation consist of smaller grain sizes, whereas those with smaller thermal variation are either made up of larger grain sizes or are more indurated. Spatial patterns in relative grain size distributions provide an indication of processes that sort. We have thermally imaged a diverse set of alluvial fans in Owens Valley and Death Valley, CA, to assess the utility of this method. Images were collected from the ground several times per hour over the course of a diurnal cycle, and at multiple times of day from above using a light aircraft. We have also used lower-resolution ASTER orbital thermal images of the fans as an analog for THEMIS thermal images available from Mars. Initial evaluation of the data indicates that many of the sedimentary processes that operate on the surfaces of terrestrial alluvial fans leave distinct spatial/thermophysical patterns, beyond what may be observed in more conventional visible wavelength images.