Abstract: 

Patterns and distribution of fluvial architectural elements in the John Henry Member (Upper Cretaceous, Straight Cliffs Formation) are documented in the southwestern Kaiparowits Plateau, Utah through detailed, outcrop-based facies characterization and mapping across ∼ 20 km down depositional dip. Observed facies associations and fluvial architectures define seven distinct depositional units (DUs), from stratigraphic base to top: DU-0, braided-river channel belts and tide-influenced channels; DU-1, highly amalgamated meandering-river channel belts with local braided and tide-influenced interdistributary channels; DU-2, laterally extensive, highly sinuous meandering-river channel belts; DU-3, straight channels in thick floodplain deposits; DU-4, laterally restricted, moderately amalgamated meandering-river channel belts; DU-5, laterally extensive, moderately amalgamated braided-river channel belts; and DU-6, highly amalgamated braided-river channel belts. The John Henry Member is characterized by two trends: from DU-1 to DU-3, an upward decrease in grain size, channel-belt frequency and lateral extent, and a shift from braided to meandering channel geometries with episodic tidal influence; and from DU-4 to DU-6, an upward increase in grain size, channel-belt frequency and lateral extent, and a shift from meandering to braided channel geometries. Overall, these trends represent a hybrid of previous models for the formation of nonmarine depositional sequences. To the west, the lower trend was bounded at the base by an erosional surface that may represent a local valley-fill succession. Incision is potentially linked to a tectonically induced relative base-level fall, although other factors such as eustasy and autogenic processes cannot be discounted. A subsequent period of high accommodation is reflected in DUs 2, 3, and 4, and correlates to transgressive marginal marine strata located ∼ 60 km down depositional dip. The upper trend in the John Henry Member fluvial strata can be explained by up-dip, mainly tectonic controls on subsidence and sediment supply, and autogenic progradation of a large, distributive fluvial fan system in DUs 4, 5, and 6.

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