Abstract: Cenomanian-Turonian Paleoceanographic Models for Western Interior Sea: Constraints from Paleoecology and Geochemistry

B.B. Sageman, M. A. Arthur, W. E. Dean

Marine strata of the Greenhorn Cyclothem in the Western Interior basin record the history of epicontinental flooding associated with Cenomanian-Turonian eustatic rise. Gross lithostratigraphic and geochemical trends suggest that there were distinct stages in the oceanographic evolution of the basin. Geochemical data from core samples obtained through the Cretaceous Western Interior Seaway Drilling Project combined with paleoecologic data from outcrop studies provide constraints on the interpretation of these stages. Paleoecologic data include species diversity and abundance, trophic dominance, and life habit, and largely reflect changes in benthic oxygen levels. Geochemical data include CaCO₃, C_org, Hydrogen Index (HI), and ^dgr¹³C of carbonate and org nic matter, as well as Total Sulfur, DOP, ^dgr³⁴S, and a variety of elemental ratios (e.g., Na/K, Mn/Fe, Ba/Al, V/V+Ni). These data can be employed as indicators of trends in primary productivity, changes in sedimentation rates, redox conditions above and within the substrate, and early diagenetic events. Analysis of paleoecologic and geochemical data suggest that each stage in the oceanographic evolution of the Western Interior basin reflects dominance of one of the following possible mixing conditions: (1) stable stratification (strong density contrast, lower water mass dysoxic to anoxic); (2) dynamic stratification (intermittent density contrast, lower water mass mainly dysoxic); and (3) dynamic mixing (little or no density contrast, lower water mass dysoxic to oxic). Mixin of the water column was ultimately controlled by size and depth of the basin, temperature and salinity of northern and southern water masses, and evaporation/precipitation balance.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994