SEQUENCE STRATIGRAPHY OF LOWER CENOMANIAN - UPPER ALBIAN STRATA IN A FLUVIAL-ESTUARINE SETTING, DAKOTA FORMATION IN S.E. NEBRASKA-N.C. KANSAS

The marginal marine middle part of the Dakota Formation in Jefferson County, NE and Washington County, KS correlates with nonmarine-estuarine facies of the Dakota Formation in western Iowa and northeastern Nebraska, and mixed marine facies of the Dakota Formation in Kansas. This interval is bounded by the “D2” erosional surface that separates Cenomanian from Albian strata and forms an important regional stratigraphic marker.

In Jefferson County, NE, mudstone strata are unconformably overlain by cross-bedded sets of sandstones, and mudstones that contain Cenomanian palynomorphs (e.g. Foveogleicheniidites confossus and Cicatricosisporites crassiterminatus); while paleosols in close lateral proximity contain Albian palynomorphs (e.g. Quadricolpites reticulatus and Impardecispora marylandensis). A major sea-level fall occurred towards the end of the Albian and into the beginning of the Cenomanian creating incised valleys into the previously deposited Upper Albian strata. This erosional surface is the manifestation of the “D2” surface. Early Cenomanian marine transgression created at least 25 m of accommodation space as the sea flooded the incised “D2” surface. The filling of these incised channels resulting in deposition of a Type-1 stratigraphic sequence that consisted of sand and mud bodies. The lithologic characteristics, distributions and geometries are consistent with bay-head delta complexes, wetland/floodplain deposits, and low-gradient fluvial channels. Adjacent to these valley-fill deposits, amalgamated paleosols containing Late Albian palynomorphs represent the interfluve remnants of the “D2” surface. Flooding surfaces within this sequence may represent times when minor sea-level rises caused the slowing of sediment accumulation rates. Similar incisions and fillings have been documented in Cass and Sarpy counties, NE. Detailed sedimentologic and stratigraphic study of these sandstone units allowed the definition of systems tracts and the increases the resolution of a sequence stratigraphy scheme that relates nonmarine portions of the “mid”-Cretaceous along the eastern margin to marine portions of the Western Interior Seaway.