AGGRADATION OF FLUVIAL GRAVELS IN RESPONSE TO LATE ALBIAN MARINE TRANSGRESSION, EASTERN MARGIN, WESTERN INTERIOR BASIN
R.L. Brenner, G.A. Ludvigson, B.J. Witzke, R.M. Joeckel, A.N. Zawistoski, and E.P. Kvale
The Geological Society of America
1996 GSA Annual Meeting
Denver, Colorado, October 28-31, 1996
1996 Abstracts with Programs, v. 28, no. 7, p. A-122
Long-standing conjectures about possible depositional relationships between the mainly Cenomanian Dakota and the Late Albian Kiowa formations of the eastern margin of the Western Interior Basin are now being clarified by new stratigraphic studies. An onshore-offshore transect through the lower Dakota Formation in western Iowa and eastern Nebraska, shows that gravel-rich fluvial deposits of the basal Nishnabotna Member correlate with transgressive marine shales of the Kiowa Formation. This basal gravel facies (up to 40 m in western Iowa) aggraded in incised valleys during the Late Albian Kiowa-Skull Creek marine transgression. In southeastern Nebraska, basal gravels intertongue with carbonaceous mudrocks that contain diverse assemblages of Late Albian palynomorphs with marine dinoflagellates and acritarchs. Tidal rhythmites within these fine-grained deposits record diurnal, semi-monthly, neap-spring tidal cycles and moderately well-developed semi-annual cycles. By modern analogy, these tidal rhythmites are interpreted to have accumulated at the head of a paleoestuary with at least periodic mesotidal range. They were preserved in accommodation space that was created as incised valleys were flooded by rising sea level. This scenario places the gravel-bearing lower portion of the Nishnabotna Member of the Dakota Formation in the transgressive systems tract within an Upper Albian sequence that was deposited along a tidally-dominated coast, as a result of relatively low rates of sediment supply from eastern cratonic sources, and relatively low wave-energy conditions.