DEFORMATION EVENT ANALYSIS IN ANCIENT EOLIANITES: PSEUDO-BEDDING AND OTHER ARTIFACTS OF SOFT-SEDIMENT DEFORMATION IN THE NAVAJO SANDSTONE (Invited Presentation)

In many places, the Navajo Sandstone and other ancient eolianites of the Colorado Plateau were affected by penecontemporaneous deformation events. Deformation obscured primary depositional relationships, introducing severe challenges to facies interpretation, but providing new lines of evidence for event analysis.

Unlike the regular successions of crossbeds that are typical of the Navajo Ss, sedimentary architectures in zones of deformation often contain large volumes of apparently massive sandstone, bounded by dramatically stretched, bent, and convoluted primary structures. In many instances, the upper extents of these zones of deformation terminate abruptly at a horizontal bounding surface which, elsewhere, separates successive crossbed sets. At some locations, a stacked succession of distinct deformation zones appears, apparently produced by multiple events. Such interpretations can be problematic, however, where outcrop exposures are limited.

In southern Utah, a complete 600-m cross-section of a deformation feature in the Navajo Sandstone is exposed in the walls of Kanab Canyon. There, deformation morphologies occur within a well-defined context of intact depositional structures. Gradational transitions into undisturbed primary structures appear at the base of the complex and at both lateral extremities. An upward gradation from greater to lesser degrees of deformation abruptly terminates against a horizontal bounding surface, where the truncation of synsedimentary faults enables reliable placement of the deformation event within the depositional history. Correlative features, exposed on the opposite canyon wall, provide three-dimensional control on the morphology of the deformation complex.

The fortuitous exposure of deformation features in the Kanab Canyon outcrops enables an unusually well-constrained interpretation of architectural changes due to deformation dynamics. These changes include the production of a succession of parallel strata along the primary shear zone within the deformation complex. This distinct example of pseudo-bedding provides an important key to understanding similar features in less completely exposed deformation complexes at other locations, such as White Pocket, Arizona.