Sediment Movement Along the Eastern Margin of the Cretaceous Western Interior Seaway, as Revealed by Cross-stratification in the Dakota Formation, Washington County, Kansas

Abstract

A detailed paleocurrent study of Cretaceous Dakota Formation sandstones in Washington County, Kansas was undertaken to determine directions of sand movement and depositional environments along the eastern margin of the Western Interior Seaway. Azimuth measurements of cross-stratified units were plotted in “rose diagrams” and examined with respect to their geographic and approximate stratigraphic position. A general southwestward direction of sediment transport is indicated (244° mean azimuth). Although mean paleocurrent directions can differ markedly from outcrop to outcrop, most sandstones in the lower part of the Dakota Formation generally have unimodal paleocurrent directions. Some of these sandstones have channel-like geometry and are situated in paleovalleys, with their mean paleocurrent direction oriented subparallel to the channel axis. Fluvial deposition is indicated. Higher in the Dakota Formation unimodal and bipolar paleocurrent azimuths are oriented nearly perpendicular to the axis of the sandbody, suggesting tidally influenced deposition. Unimodal paleocurrents again dominate the highest part of the Dakota Formation, but with overall less directional variability than in the lower part of the unit. Sheetlike sandbody geometry is suspected and more open-marine deposition could be inferred, but fluvial depositional conditions also may be possible.