Abstract

The internal architecture and genetic-stratigraphic analysis of coastal-plain fluvial successions has applications to both academic understanding of continental-margin stratigraphy and to the economic evaluation of subsurface hydrocarbon and water reservoirs. Despite the abundance of exposed ancient examples of such coastal-plain to shallow marine transitions, however, the lateral facies changes and stratigraphic stacking patterns of such transitions remain largely unresolved in detail. This study presents a sedimentological and stratigraphic analysis of the Campanian Masuk Formation in the Henry Mountains syncline of Utah, western USA, and proposes a sequence-stratigraphic model to explain the geometries of fluvial-channel lithosomes found in the unit. At its type section, the Masuk Formation is an ~ 220-m-thick succession of interbedded sandstone, mudstone, and coal with minor intraformational breccias. Six facies are defined: Facies 1, coal and carbonaceous shale, records accumulation of organic sediments in coastal-plain mires; Facies 2, mudrock, records coastal-plain floodbasin settings; Facies 3, thinly interbedded sandstone and siltstone, represents estuarine-basin deposits with one interval of marine flooding; Facies 4, admixed siltstone-sandstone with intraformational breccia, represents chaotically filled coastal plain channels; Facies 5, interbedded sandstone-siltstone with inclined heterolithic stratification (IHS), records deposits of coastal-plain channels with variable sediment supply; and Facies 6, trough-cross-bedded sandstone, records sand-dominated coastal-plain channel deposits. Facies 4, 5, and 6 all show evidence of coastal and tidal influence on sediment accumulation (IHS, low-diversity trace-fossil assemblages, bimodal to bipolar paleocurrent distributions, mud drapes on small-scale sedimentary structures) to varying degrees. Facies analysis and tracing of key surfaces such as regionally extensive erosion surfaces (sequence boundaries) and flooding surfaces suggest that the entire Masuk Formation (and the uppermost part of the underlying Muley Canyon Sandstone) is a single depositional sequence. The typically multilateral channel bodies are traceable in a depositional strike direction over distances much greater than empirically calculated estimates of channel-belt width. This pattern suggests an allogenic control on stacking patterns. Multistory and multilateral channel bodies typically have a Facies 4 unit at the base, overlain by one or more stories of Facies 5, and in turn capped by Facies 6. This stacking pattern could reflect cycles of incision, initial lowstand channel filling, and subsequent backfilling of estuarine channels during transgressions. This pattern further hints at the possibility of preservation of high-frequency sequences nested within the longer-term sequence. This study provides new insights into the stacking patterns of coastal-plain fluvial successions and their interpretation.