DINOSAUR-INFLUENCED AVULSIONS IN THE UPPER JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH

The Upper Jurassic Salt Wash Member of the Morrison Formation in east-central Utah contains low sinuosity, ribbon-shaped fluvial channel sandstones enclosed by variegated mudstones and siltstones. Channel sandstones formed when avulsion (the relatively abrupt shift of a river to a new channel) relocated a channel, after which extensive in-channel and minor near-channel sand deposition occurred. Interpretation of sedimentological, paleontological, and paleoclimatic data, and comparison with a possible modern analog, suggest that channel avulsions and the subsequent deposition of ribbon sandstones may have occurred when large Jurassic dinosaurs, such as sauropods, partially or completely blocked active channels at death, thereby forcing discharge out of channels and into overbank areas. Dinosaurs also trampled deep pathways into the damp, soft floodplain substrate, creating channel-like conduits that focused overbank flow during flooding. In places where overbank flow concentrated in these deep, channel-shaped trails, a new channel course was scoured, and an avulsion was completed. As time passed and the system aggraded, this process of dinosaur-influenced avulsion ("dinovulsion") recurred, and the present architecture of isolated, low-sinuosity sandstone ribbons resulted.