INVESTIGATING THE ROLE OF SALT WEATHERING AT GRAND FALLS, AZ

Grand Falls in Leupp, AZ, was incised after a basaltic flow dammed the Little Colorado River, creating a waterfall system that is still in hydrologic disequilibrium today. The episodic nature of overland water flow makes Grand Falls an apt terrestrial analog of Martian channels, which are also shown to be geologically immature. However, erosion at the falls might be continuous. Water might remain during “dry” periods, seeping through the rock and transporting salts. Here, we will discuss how salts have potentially driven landscape change at Grand Falls to better understand fluvial erosion on Mars.

The stratigraphy consists of the relatively resistant Kaibab Limestone overlying the weaker Coconino Sandstone. Undercutting of the limestone is prominent. On the walls of the resulting overhangs are salt streaks as well as fractures and pockets of salts. The surrounding rock flakes off easily, thus we suspect that salts are predisposing rock to erosion during flow events. Most likely, water seeps through the rock at the Kaibab/Coconino contact, conveying salts which crystallize near the surface. Past research has established that salts are capable of causing erosion via crystallization pressure. XRD analysis demonstrates the composition of salts on the overhangs have high concentrations of halite and thenardite. Thenardite has a unique ability to promote rock disintegration because it undergoes large changes in volume during cycles of hydration and rehydration.

Salt-affected rock on the main cliff wall and along the channel wall have an average tensile strength that is 27% and 41% lower than the average tensile strength of salt-unaffected Kaibab, respectively. Both of these areas exhibit undercutting and flaking of surficial rock. Physical weakening of the limestone could be caused by salt crystallization and cycles of wetting and drying. As salt weathering is believed to occur on Mars, continued study of how salts promote geomorphic change at Grand Falls will add to knowledge about weathering processes on Mars.

Fieldwork on the Navajo Nation was conducted under a permit from the Navajo Nation Minerals Department. Any persons wishing to conduct geologic investigations on the Navajo Nation must apply for and receive a permit from the Navajo Nation Minerals Department, PO Box 1910, Window Rock, AZ 86515, (928) 871-6587.