TIBERT, NEIL E.¹, R. MARK LECKIE¹, and JEFFREY G. EATON^2
1Department of Geosciences, University of Massachusetts, Amherst, MA
²Department of Geosciences, Weber State University, Ogden, UT

Abstract: Recognition of Relative Sea Level Change in Upper Cretaceous Coal-bearing Strata: A Paleoecological Approach using Agglutinated Foraminifera to Detect Key Stratigraphic Surfaces

Marginal marine agglutinated foraminifera can be used to establish key stratigraphic surfaces in a sedimentary sequence of sea level rise and fall. To demonstrate this, we analyzed samples obtained from transgressive facies of the upper Cenomanian Dakota Formation and regressive facies of the middle Turonian Straight Cliffs Formation from the southwestern United States. Our results indicate that marine flooding surfaces occur on two orders of magnitude. Flooding surfaces that occur within the third-order Greenhorn Cycle are recognizable in transgressive facies usually at major lithostratigraphic boundaries. In the Dakota, the brightly banded C bituminous coals are approximately 1-2 m in thickness, laterally extensive, and low in ash and sulfur. Agglutinated foraminiferal populations that cap the coal zones are characterized by a Trochammina/Verneuilinoides association typical of modern lagoonal/estuarine ecosystems. By tracking the habitat of the estuarine association, we demonstrate a diachronous transgressive event recorded at the contact between the middle (carbonaceous) and upper (brackish) members of the Dakota Formation at Mesa Verde Colorado, Black Mesa Arizona, and Markagunt Plateau Utah. In contrast, high frequency surfaces (fifth-order?) occur within thin (>5m) coal zones that comprise 2 or 3 thin (<0.5 m) lignitic coal seams separated by bayfill mudstone and sandstone. Agglutinated foraminiferal associations that resemble modern marsh populations in their generic composition directly overlie these lignites. Trochammina, Miliammina, and Haplophragmoides occur in decreasing order of abundance. Marsh foraminifera are sensitive indicators for marine influence in ancient coastal sedimentary successions. This research demonstrates that ancient agglutinated foraminifera occupied similar ecological niches as their modern counterparts and provides a testament for their use as paleoenvironmental indicators.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas