TECTONIC CONTROL ON THE ALTERNATING FLUVIAL STYLE IN THE LATE CRETACEOUS UPPER MEMBER OF THE WAHWEAP FORMATION, SOUTHERN UTAH

The Wahweap Formation in Grand Staircase–Escalante National Monument, Utah consists of lower and middle members that contain isolated fluvial sandstone units encased in floodplain mudstone, an upper member of amalgamated sandstone units with little mudstone, and the capping sandstone member, which contains sandstone and conglomerates with very little mudstone. The sequence reflects a transition from deposition by mixed-load meandering streams to that of bedload-dominated braided streams; however the transition is not simple. Paleocurrents and lithology of the capping sandstone member indicate a change in source area from the south (upper member) to west. Furthermore, detailed analysis of the 115 m thick upper member near the Gut, revealed multistoried, inclined heterolithic units alternating with three thick sandstone units, which are interpreted as meandering and braided stream deposits, respectively.

Changes in fluvial style, and thus the form and architecture of channel deposits, are either a response to external controls, tectonic, base level and climate changes, or they can be related to the inherent geomorphic variability of the river system (e.g., avulsions). In the upper member, deep (several meters) incision of thick sandstone units into heterolithic units suggests external controls are responsible for changes in fluvial style. In addition, the study area is located close to a series of normal faults that were apparently active during deposition of the upper member. Highly distorted sediments (seismites) and preserved sand volcanoes and dikes indicate intense high-magnitude seismic activity, and suggest that the primary control on deposition was related to fault activity.

Cyclic alternations of thick sandstone units and multistoried heterolithic bodies represent pulses of sedimentation, a result of tectonic movements of nearby faults. Individual cycle reflect the development of a high-gradient braided alluvial-fan system, which ultimately changed to a low-gradient alluvial plain before fault reactivation led to the next cycle.