Dynamic stratigraphy and sediment partitioning of high-supply fluvial succession in Maastrichtian source-to-sink system

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Ned, Allison Marie

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The sediment budget and paleogeography was reconstructed for the Maastrichtian fluvial to coastal plain Lance Formation (>200m thick) that developed coevally with the shoreline/shelf Fox Hills Sandstone (>200m thick) and deep-water Lewis Shale (>750m thick) in a complete source-to-sink system in the Washakie and Great Divide Basins of south central Wyoming. The system initiated during the final Western Interior Seaway (WIS) transgression and the onset of the Laramide Orogeny rapid subsidence (>2km in 1.9 My) that largely outpaced sediment flux into the basin so the system became and remained a deep-water (>500m water depth) basin beyond the Lance-Fox Hills shelf prism. The active tectonic setting and rapid subsidence caused the Lance fluvial and coastal plain deposits to aggrade and accumulate behind the generally rising shoreline trajectory of the Fox Hills Sandstone. The depositional succession is subdivided into 15 clinothem units and the Lance Formation is best exposed in outcrops in clinoforms 10, 11, and 12. Subsurface analysis correlates key stratigraphic surfaces across the basin to define the sediment budget and clinoform architecture. Field analysis along clinoform 12 on the east side of the basin details facies and paleohydraulic dimensions. Sediment partitioning shows the regressive and transgressive systems tracts (RST and TST) form complementary wedges such that the RST thickens basinward and the TST thins basinward, reflecting the preferential storage of sediment. Channels measured in the field and subsurface datasets are similar in thickness (2m-16m) and suggest braided channel morphology with channel belts from 6.2-8.4km. N/NE paleocurrent trends departing from the subsurface dataset and previous studies of the system provide evidence of possible tidal influences in a developed shoreline embayment or an east to west supply from the basement-cored Rawlins and Sierra Madre Uplifts in the east. The fluvial Lance Formation paleogeography associated with the RST and TST is primarily driven both by modest, Late Cretaceous relative sea level changes and sediment supply linked to the tectonic setting and climate.



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