POTHOLE SEDIMENTS: AN INTERPLAY OF WIND, RAIN, AND SANDSTONE PERMEABILITY

Dozens of pits (dubbed “potholes”) shaped like bowls, cylinders, and bathtubs, have been formed in the upper surface of the Lower Jurassic Navajo Sandstone of Rock Window Mesa in NE Arizona. They range from quite small to over 200 ft across, and can have walls more than 100 ft high. The walls range from vertical to gently sloping. Sediment levels in the potholes can be highly variable. Though sand is regularly blown across the top of the mesa and collects in many areas on the mesa, many of the potholes contain very little sediment—the processes controlling sediment levels in these potholes have not been adequately discussed in the literature.

We set up multiple anemometers to simultaneously measure the wind in and above the potholes—the velocities of the winds at the bottoms of the potholes tend to be about 50-80% those of the winds across the top of the mesa. Even with this reduction in wind power at the bottoms of the potholes, however, we observed wind regularly lifting sand up and out of the potholes when the wind velocities over the top of the mesa climbed to over 30 mph. When mud cracks and mud curls form, the strong winds also lift the mud curls up and out of the potholes. We also observed the deposition of sediment in the potholes by the wind. The sand grains and small pebbles of sandstone tend to enter the potholes in a slow trickle (though stronger winds bring a faster trickle), whereas the strong gusts tend to lift small clouds of sand out of the potholes all at once.

Rain comes to this region sporadically, and some potholes drain much slower and stay wetter than others. When water collects in the potholes, it tends to trap the trickle of sediment that comes in, as even damp sediments are not readily removed by strong winds. We have observed that potholes that retain water most of the year have much deeper sediment than those that are dry most of the year. While the catchment area for each pothole is certainly an important factor affecting the amount of water it receives, the amount of water it retains over the course of the year appears to be more strongly affected by the permeability of the sandstone walls of the pothole.

We conclude that wind, the amount of water each pothole collects, and the permeability of the sandstone that forms the potholes' surfaces are the most important factors affecting sediment levels in the potholes.