SOFT-SEDIMENT DEFORMATION AND DUNE DYNAMICS IN THE JURASSIC NAVAJO ERG

The aeolian architecture of the Navajo Sandstone, in the Glen Canyon region of the Colorado Plateau, includes abundant soft-sediment deformation features that sometimes extend for hundreds of meters, laterally, and tens of meters, vertically. Many of these features are truncated by erosional surfaces, indicating that deformation often affected near-surface deposits. In rare cases, preserved architectural evidence demonstrates that the contemporary topography was altered in the deformation event, potentially affecting dune dynamics in the Jurassic erg.

An extensive (approximately 250 m x 500 m) outcrop of Navajo Sandstone, at the Glen Canyon Dam, in Arizona, provides an unusually complete exposure of architectural features associated with a series of deformation events. Cross-cutting relationships indicate that these events occurred as accumulation proceeded and aeolian deposition continued in the ancient erg. Architectural geometries also provide evidence of surface disruptions, during one event, including the collapse of a large, active bedform and subsequent changes in the pattern of dune sedimentation. The present report details the architecture of this outcrop and interprets the processes involved in its genesis. Attention is focused, particularly, on how deformation affected the local organization of bedforms.

By revealing the dynamic response of migrating bedforms to a specific perturbation, the preserved architecture at the study site provides unique insight into the organizational behavior of the ancient aeolian system. Subsequent to disruption of the system, it appears that sedimentation proceeded along a pathway that led directly to reestablishment of the preexisting pattern. This suggests that the disruption was not accompanied by a change in the primary controls on deposition in this part of the erg; however, it leaves open the possibility that deformation events, themselves, may have influenced surface processes sufficiently to produce a distinctive architectural style. Recognizing this style in terms of local peculiarities in set variability, as well as by association with deformation features, would facilitate the architectural interpretation of more limited exposures and the recognition of other types of perturbation in ancient aeolian systems.