ABSTRACT

Significant differences in external geometry, stratification and grain size exist between sandstone bodies in the Terra Cotta Member and those intercalated within claystone of the Janssen Member of the Cretaceous Dakota Formation (Kansas Geological Survey nomenclature) in southeastern Nebraska. Comparison of these criteria with modern environments suggests that Dakota streams became progressively more sinuous during the depositional history of the formation.

Relatively low sinuosity bedload streams, flowing towards the southwest, deposited medium-grained, dominantly tabular cross-stratified sandstone bodies near the base of the formation. As the Cretaceous sea transgressed eastward, these streams became less competent and began to meander, depositing fine-grained, trough and epsilon cross-stratified, lens-shaped sandstone bodies in the Janssen Member. Continued transgression drowned estuaries and floodplains of these streams. Marine shales and limestones of the overlying Graneros Formation reflect normal marine conditions which completed the depositional history of the Dakota Formation.