Pothole formation on Rock Window Mesa, Chinle valley, AZ

Abstract

In areas of the Colorado Plateau, rock basins, or potholes, occur in abundance. Because of their impressive size and often enigmatic location, their presence begs an explanation. Our goal in undertaking this research is to suggest a model for the origin of Rock Window Mesa's potholes, by which we might better interpret the geomorphological history of the mesa and Chinle Valley. Data collection consisted of both field work and analysis of a high-resolution 3D point cloud of the mesa. The southern and western edges of the mesa, closest to the Chinle Wash, and close to where knickpoints may have been, are the areas of highest pothole density. The pothole-forming processes disrupted drainage systems, and pothole sizes do not correlate with the catchment areas of the potholes. There are abundant, fluvially deposited erratic pebbles and cobbles on the mesa, and the joints and cracks that are present in the bedrock exert little to no control over pothole morphology. These data suggest that the potholes were formed subaqueously. A proposal that may point us to a possible mechanism is that similar potholes, in the Navajo Sandstone exposed in the Henry Mountains, appear to have been formed via fluvial processes in over-steepened tributary junctions. At our research location, the water that carved out the potholes appears to have come from ENE, and the point at which this water met the Chinle Wash may have been over-steepened as a result of rapid incision of the Chinle Wash through the poorly lithified Navajo Sandstone.