Sequence stratigraphy and facies analyses of the Dakota Formation, Jefferson County, Nebraska and Washington County, Kansas

The estuarine to fluvial sediments of the mid-Cretaceous (Late Albian/Early Cenomanian) Dakota Formation of Jefferson Co., Nebraska (NE) and Washington Co., Kansas (KS) were deposited in a marginal marine setting along the eastern margin of the Cretaceous Western Interior Seaway.; Three depositional facies based on various lithic content are recognized in the study area: Facies 1: Fluvial Channel Facies, Facies 2: Paleosol/Interfluve Facies, and Facies 3: Bay Head Delta/Estuarine Facies. The facies interpretation helped confirm that the Dakota Formation was deposited in a marginal marine setting in which low-gradient fluvial systems supplied a wave-dominated, estuary system.; Petrographic analysis of the Fluvial Channel Facies concluded that the sandstones can be classified as quartz-rich lithic arkose. These findings differ slightly from previous studies on Cenomanian Dakota Formation strata in Thurston Co., NE.; Palynostratigraphic, subsurface, and sedimentologic evidence helped to delineate a more accurate sequence stratigraphic framework for the Dakota Formation in the study area. Three large-scale, unconformity-bounded, sequences (D0, D1, and D2) are recognized, within which deposits of the transgressive and falling stage systems tracts are preserved in the Dakota Formation in the study area. While no physical deposits exist for the falling stage and lowstand systems tracts, evidence for their past occurrence can be observed by the erosional nature of the sequence boundaries. Detailed analysis of the systems tracts framework allows delineation of a generalized sea-level curve for the Dakota Formation in the study area.; Analysis of the sequence stratigraphic framework revealed a Late Albian/Early Cenomanian sea-level fall that subsequently created valley incisions of over 25 m into the Late Albian D1 sequence. A careful literature review combined with sequence stratigraphic evidence suggests that a geologically fast-acting eustatic sea-level mechanism lowered worldwide sea-levels by more than 25 m from Late Albian into Early Cenomanian time. A reevaluation of the mid-Cretaceous "greenhouse" world suggests that a glacioeustatic component to the observed sea-level changes may have occurred. A Southern Hemispheric polar ice sheet with limited extent and volume compared to "icehouse" continental ice sheets, along with global alpine glaciers fed by wet climate cycles are hypothesized to account for sea-level fluctuations that resulted in valley incision and subsequent filling in the study area.