MID-CRETACEOUS SHORELINES OF THE CRATONIC MARGIN, WESTERN INTERIOR BASIN: WHERE HAVE ALL THE DELTAS GONE?
G.A. Ludvigson, B.J. Witzke, E.P. Kvale, R.L. Brenner, T.S. White, L.A. Gonzalez, and R.M. Joeckel
The Geological Society of America
32nd Annual North-Central Section Meeting
Ohio State University, Columbus, OH, March 19-20, 1998
1998 Abstracts with Programs, v. 30, no. 2, p. 57
Previously-published depositional models for mid-Cretaceous (Albian-Turonian) shoreline deposits along the eastern margin of the Western Interior Seaway have included sandstone units that accumulated in barrier bar, shoreface, marine bar, and delta distributary settings, although none of these interpretations has proven satisfactory. Large-scale constructional deltas are not present, and marine sandstones are limited to thin, burrowed horizons beneath transgressive lag deposits. Our studies indicate that much of this sedimentary record is retrogradational in nature, and that most of the fluvial sand transported into shoreline settings was trapped at the heads of large-scale paleoestuaries. Sedimentary evidence for widespread estuarine deposition includes: 1) thick sections of carbonaceous mudstones (previously interpreted as fluvial) that lack marine body fossils, but contain marine palynomorphs (dinoflagellates and/or acritarchs); 2) sandstones with tidal bundling recorded by clay drapes; 3) lateral accretion deposits with inclined heterolithic stratification; 4) tidal rhythmites in laminated fine-grained sediments; 5) early carbonate cements in tidally-influenced sandstones with carbon and oxygen isotopic trends recording meteoric-marine fluid mixing; 6) early sphaerosiderite nodules in gleyed wetland paleosols with carbon and oxygen isotopic trends recording meteoric-marine fluid mixing. Factors that contributed to widespread estuarine deposition include: 1) low fluvial gradients for rivers draining cratonic lowlands; 2) further reduction of sediment supply because of extensive armoring of the cratonic landscape by dense vegetative cover, with attendant sediment trapping in fluvial depositional systems; 3) limited accomodation created by successive retrogradational steps, as dictated by low fluvial gradients and rates of subsidence.