A LARGE INCISED FLUVIAL CHANNEL WITHIN THE MID-CRETACEOUS DAKOTA FORMATION, WASHINGTON COUNTY, KS: EVIDENCE OF CRETACEOUS GLACIATION?

The Dakota Formation (mid-Cretaceous) of Washington Co., KS was deposited in a marginal marine setting. Estuarine sediments that filled incised valleys formed the lithologies that dominate the Upper Albian/Lower Cenomanian strata along this non-tectonically active eastern margin of the Cretaceous Western Interior Seaway (KWIS).

An excellent outcrop of an incised channel fill within the Dakota Formation is exposed along U.S. 36 in central Washington Co., KS (M-276 site). The channel has at least 13 m of vertical relief with the erosion surface visible at the top of the outcrop, suggesting that the observed erosion is only a minimal measurement. The collecting and identification of Upper Albian palynomorphs (Impardecispora marylandensis and Quadricolpites reticulatus) demonstrates that the valley was incised during or after the Late Albian and is not part of the sub-Cretaceous erosional surface. This valley incision is the manifestation of a regional erosion surface that separates Upper Albian from Lower Cenomanian strata. Evidence from Jefferson Co., NE suggests that over 25 m of incision may have occurred during the Cenomanian. Traditional studies did not include glacioeustasy to explain over 13 m of erosion within the Dakota Formation, since it was deposited under global “greenhouse” conditions.

Recent research based on geochemical and stratigraphic observations suggest a possible glacial component to the observed sea-level changes during the mid-Cretaceous. Possible eustatic mechanisms that have been investigated include: foreland bulge tectonics, regional to plate-scale tectonics, thermal expansion/contraction of sea water, and glacioeustasy. If comparisons with recent research are combined with careful analyses of field data and eustatic sea-level change mechanisms, glacioeustasy as a possible partial cause for the M-276 channel incision can't be ruled out. Lack of direct evidence for large Cretaceous continental ice-sheets precludes them from our interpretation. Rather, extensive and global alpine glaciers fed by warm and wet climate cycles are hypothesized to account for sea-level fluctuations that resulted in valley incision and subsequent filling.