SPATIAL AND TEMPORAL VARIATION OF FLUVIAL ARCHITECTURE IN A PROGRADING CLASTIC WEDGE OF THE LATE CRETACEOUS WESTERN INTERIOR BASIN (KAIPAROWITS PLATEAU), USA

摘要

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 similar to 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 similar to 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.