SOFT-SEDIMENT DEFORMATION AND DUNE COLLAPSE IN THE NAVAJO SANDSTONE, UTAH

In 1982, Daniel Horowitz suggested a model for the generation of large-scale soft-sediment deformation in eolian formations. This was among the first models suggested for such features, and Horowitz intended it as a general explanation. More recent research by Bryant and others, however, has revealed features which do not fit a dune collapse model, such as deformation features which attenuate upward, and others which likely took place during an aggradational phase of erg development. While not all outcrops can be attributed to the dune collapse model suggested by Horowitz, it is well developed and makes several testable predictions. This report will apply these predictions to the outcrop on the Canyon Overlook Trail in Zion National Park in Southern Utah, which displays more distinct physical features and can therefore be used to suggest more robust criteria for the identification of the dune collapse process.

One of the main predictions that Horowitz makes is that material will be shifted in a downwind direction, as the steeper lee face of a large dune founders into the liquefied interdune deposit. He based his model on contorted fabrics in outcrops in Red Rock Canyon, Nevada, and off Highway 89 West of Kanab, Utah. Our outcrop in Zion, however, includes interdune carbonate muds, which added some support to the original deposit while lubricating shear planes, leading to distinct thrust faulted structures. These thrust faults have their strike perpendicular to the local paleo-wind direction, and they dip upwind, as predicted by the dune collapse model. Furthermore, deformation is clearly constrained to a single large cross-bed set, and the upper contact is truncational. Interdune carbonate mud layers were pushed to the surface in several locations, and were more resistant to erosion, leading to an irregular topography which is clearly preserved in the outcrop.

The outcrop in Zion is dissected by modern erosion in a way which reveals a great deal of the three-dimensional architecture. Therefore, by incorporating 3D modeling into traditional mapping techniques, this report will suggest more robust physical criteria for recognizing dune collapse.