POSSIBLE FLUVIAL ORIGIN OF POTHOLES ON ROCK WINDOW MESA IN CHINLE VALLEY, AZ

Certain sandstones of the Colorado Plateau contain numerous potholes. We chose to study Rock Window Mesa in Chinle Valley, AZ for its accessibility and for the plentiful potholes that mark its top. Our goal in undertaking this project was to better understand the geomorphological history of Rock Window Mesa and Chinle Valley by exploring the origins of the potholes. We spent several months in the field, and also obtained a digital elevation model and orthophoto of the mesa with cm-scale resolution.

Our analyses showed that the pothole-forming processes favored rounder, smaller, shallower potholes, relative to the size and shape distributions of the potholes. They also strongly favored the southern and western edges of the mesa. There are three preferred orientations, and potholes in close proximity to each other are not more closely aligned than those further apart on the mesa.

The pothole-forming processes disrupted drainage systems, and the sizes of the potholes do not correlate with their catchment areas. Also, the mesa is covered with erratic cobbles and pebbles, and pothole morphology does not correlate with surrounding joints and cracks.

The literature presents two possible explanations that are consistent with our data. The first is that during the Upper Miocene/Pliocene there was a general lack of channelized flow, and sediment transport generally took place via flash flooding. Though this would have been a time of aridity, the flash flooding, combined with the relative lack of channelized flow, may have sent high-energy, sediment-laden water over what are now the southern and eastern edges of the mesa.

The other possibility is that the current top of the mesa was, at one point, an over-steepened tributary junction. There are tributaries to the Chinle Wash that bound the mesa, and when the level of the paleodrainage was at the level of the current top of the mesa, possible over-steepening of the junction may have produced energetic, sediment-laden water.

These hypotheses account for the potholes’ general roundness (due to their formation by eddies), the lack of joint control over pothole morphology, the requirement that pothole formation proceed faster than erosion caused by monsoon rains, the lack of correlation between size and catchment area, and the ubiquitous erratic pebbles and cobbles.