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Sequential Evolution and Associated Sedimentation Styles in a Fluvial Channel Belt: Three-Dimensional Outcrop Study of the Cedar Mountain Formation, Utah


The Cretaceous fluvial Cedar Mountain Formation crops out southwest of Green River, Utah across a 62 square kilometer area. Outcrops consist of multiple fluvial channels belts that are exposed in three-dimensions as sinuous ridges across the field area. The exposed segments of the channel belts range in length from 0.75 km to 6 km long. This study focuses on one portion of one channel belt that is exceptionally well exposed over a down-current distance of 0.75 km in length. Strata in this segment are dominated by down-stream accreting bars, with a minor amount of lateral accreting bars. The channel belt of interest has a width (w) of 80 meters, average thickness (t) of 6.4 meters, sinuosity of 1.2, radius of curvature of 153.55 meters, and aspect ratio (w/t) of 12.5. The exceptional exposure of this channel-belt segment provides a unique opportunity to document the sequential evolution and lithofacies distributions. Data collected in this study include detailed stratigraphic columns, paleocurrents, and interpreted photopanels, which were in turn used to create longitudinal and lateral cross-sections of the channel belt. Results are the following. First, the channel evolved in a three-stage process: (1) incision, (2) lateral migration, and (3) filling. Features associated with the incisional phase are large-scale gutters filled by conglomerate composed of extraformatinal clasts and carbonate nodules from the adjacent floodplain. Strata associated with the lateral migration stage are composed of laterally accreting point bars located at the apexes of the channel bends and are composed predominantly of planar to ripple laminated sandstones. Strata associated with the final filling of the channel belt are composed of downstream accreting bars containing cross-stratified sandstones. Measurements and concepts resulting from this study shed light on the long-term evolution of sinuous, fluvial-channel belts and can be use to constrain reservoir models of similar fluvial channels.