Abstract

Concept-driven sequence stratigraphic models of alluvial-to-coastal-plain successions suggest that fluvial architecture and style should transition from isolated, single-story, channelized sandbodies deposited by single-thread (typically meandering) rivers in mudstone-prone, high-accommodation intervals into densely stacked, amalgamated sandbodies deposited by multiple-thread (typically braided) rivers in low-accommodation intervals. Model predictions of changing fluvial style are tested by comparing the facies character, internal architecture, dimensions, and formative paleohydraulic conditions of representative, major fluvial sandbodies developed at different stratigraphic levels of an alluvial-to-coastal-plain succession developed under a progressively decreasing rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, USA).

The major fluvial sandbodies have a similar facies composition, and consist mainly of cross-bedded, medium-grained sandstone with subordinate mudclast conglomerate, folded sandstone, and inclined sandstones and intercalated siltstones. Facies are arranged within a hierarchy of architectural components. Channel stories (mean width and depth of 90 m and 7.2 m, respectively) represent the migration of a paleochannel segment and adjacent bar, and they are amalgamated laterally into channel belts (mean width and depth of 400 m and 9.2 m, respectively) that in turn are stacked vertically into channel-belt complexes (mean width and depth of 400 m and 20 m, respectively). Channel stories and belts were deposited by a combination of three paleochannel types that occur together in most major sandbodies: (1) single-thread “cut-and-fill” channels; (2) single-thread, laterally accreting channels of low-to-moderate sinuosity; and (3) multiple-thread, wandering-to-braided channels. Estimated paleochannel slopes are uniformly low (< 0.04°), most sandbodies contain sparse, potential marine indicators (e.g., Teredolites-bored logs and tidally modulated? carbonaceous drapes along cross-bed foresets), and paleodischarge estimates imply that multiple-thread channels may have narrowed and branched downstream to form distributary networks. All of these features are consistent with a delta-plain setting.

Channel story and belt stacking patterns within each major sandbody (channel-belt complex) are highly non-uniform, such that (1) there are no systematic trends shared by the sandbodies, (2) sandbodies do not result from systematic, short-term changes in accommodation, such as those associated with the incision and fill of coastal incised valleys, and (3) variability within each sandbody is more pronounced than variability between sandbodies. These results suggest that local variations in sediment flux and transport capacity, combined with local avulsion history, were the principal controls on the architecture and dimensions of the major sandbodies. The similarities in architecture between major fluvial sandbodies imply that these controls were not predominantly governed by proximity to the coeval shoreline (c. 40–100 km) or by long-term tectonic subsidence rate (c. 80–700 m/Myr), which controlled creation of accommodation.