MARINE-INFLUENCED SEDIMENTATION IN THE DAKOTA FM, CRETACEOUS (ALBIAN-CENOMANIAN), CENTRAL U. S.; IMPLICATIONS FOR SEQUENCE STRATIGRAPHY AND PALEOGEOGRAPHY IN THE WESTERN INTERIOR

by
B. J. Witzke, G. A. Ludvigson, T. S. White, and R. L. Brenner

The Geological Society of America
1999 Annual Meeting and Exposition
Denver, Colorado, October 25-28, 1999
1999 Abstracts with Programs, p. A-425

The mid-Cretaceous Dakota Fm, eastern-margin area of the Western Interior (WI) in Iowa, Nebraska, and Kansas, is dominated by nonmarine fluvial, floodbasin, and paleosol facies. However, some stratigraphic intervals show evidence of marine-influenced estuarine sedimentation including the presence of marine palynomorphs, tidal rhythmites, inclined heterolithic stratification, burrowed facies (marine ichnotaxa), and pyrite enrichment. The Dakota Fm in this region is subdivided into three stratigraphic sequences, each containing intervals with estuarine facies: 1) lower interval equivalent to the Kiowa-Skull Creek marine cycle; 2) middle interval correlative with the Muddy Fm of the WI (late Albian); 3) upper interval correlative with the lower Greenhorn marine cycle. Estuarine facies extend into Iowa over 400 km east of coeval marine shales of the Kiowa Fm, and correlative estuarine facies are now recognized as far east as the Moose River Basin of Ontario. Estuaries of comparable scale are also identified in overlying Dakota strata of Iowa-Nebraska, which indicate a mid-Cretaceous paleogeography for the WI with a highly embayed eastern coastline. Deposition of the Muddy Fm and its equivalents was marked by a general contraction of the WI seaway, but the extent of the seaway at that time has remained problematic. Estuarine facies in correlative Dakota strata extend into easternmost Nebraska, indicating that the maximum extent of the "Muddy seaway" was significantly greater in the eastern part of the WI than previously recognized. The episodic eastward expansion of estuarine facies during Dakota depositon is interpreted to be a response to parasequence-scale sea-level changes. The stratigraphic resolution of such changes will potentially allow a detailed correlation and sequence-stratigraphic framework between dominantly nonmarine facies to the east and marine-dominated facies to the west. The stratigraphic position of these minor marine incursions will help delineate the complex stratal architecture in the dominantly fluvial/nonmarine facies in the stable eastern cratonal portion of the Western Interior.