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A detailed sedimentologic and stratigraphic analysis facilitated interpretations of depositional environment, sequence stratigraphy, and sandstone body geometry for isolated, top-truncated, shallow marine sandstone bodies of the Upper Cretaceous (Cenomanian-Turonian) Frontier Formation, northeast Bighorn Basin, Wyoming. The Frontier Formation interval is ~160 meters thick and was deposited as a complex clastic wedge that prograded into Cretaceous Western Interior Seaway (KWIS). The vertical interval comprises several incomplete coarsening-upward cycles, composed of basal offshore marine and prodeltaic shales progressively overlain by proximal shallow marine/fluvial facies that are capped by pebble lags. Sedimentary structures, vertical stacking patterns, and lateral variability within these cycles record multiple southward progradational episodes of tide- and wave- influenced, fluvially-dominated deltas. Subsurface correlation of sandstone bodies reveals dip elongate, strike restricted, lensoid, digitate, linear, and lobate geometries, supporting the deltaic interpretation. A sequence stratigraphic analysis of outcrop sections divides the coarsening-upward cycles into multiple parasequences and identifies two types of key stratigraphic surface (transgressive surfaces of erosion, sequence boundaries). Transgressive surfaces of erosion occur as low-relief, laterally extensive pebble lag horizons that top-truncate parasequences, generating mudstone-encased, isolated sandstone bodies. Sequence boundaries are placed at the base of shallow marine sandstones (Peay Member), as well as at the erosional contact between fluvial and shallow marine facies (Torchlight Member). Sequence boundary placement suggests shoreline advancement basinward during relative sea-level lowstand. Relative sea-level transgressive-regressive cycles observed within parasequences were generated from the interplay between allogenic forces (tectonics, climate, eustacy). This study proposes a depositional model explaining the dispersal of southward-deflected, isolated shallow marine deposits 10’s to 100’s of kilometers basinward of contemporaneous shorelines. The interaction between fresh and saline water, pressure gradients, and the Coriolis Effect generated shore-parallel, southward-deflected geostrophic currents that dispersed sediments south, parallel to the KWIS western paleoshoreline. Additionally, this study highlights multiple member-scale lateral pinch-outs and intramember-scale heterogeneities in sandstone bodies. Such findings will assist in future hydrocarbon exploration efforts.
Adviser: Christopher R. Fielding