ABSTRACT: Onshore-Offshore Biofacies Patterns, Greenhorn Eustatic Cycle, Western Interior Cretaceous Epicontinental Seaway, North America

Erle G. Kauffman, Bradley B. Sageman

The Cretaceous Western Interior foreland basin provides an ideal testing ground for models of evolution and dispersion among benthic communities. This basin has yielded high-resolution stratigraphic/paleobiologic data, allowing successive mapping of organism/environment interactions within narrow time intervals. Biological patterns of adaptation, diversity, life habit, population structure, trophic strategies, community composition/structure, and biogeography define biofacies in this epeiric sea. Biofacies distributions are documented for the 6-m.y. Greenhorn eustatic cyclothem (latest Albian-middle Turonian). Two high-resolution cross sections provide a stratigraphic framework: (1) Utah/Arizona to Minnesota, transecting diverse tectonic/sedimentologic provinces within th same warm temperate climatic zone and (2) Alberta to Oklahoma/New Mexico, crossing three biogeographic subprovinces within basinal facies. Plotting biofacies distributions on the cross sections allows the spatial and temporal evolution of Western Interior benthic communities to be studied. Resulting patterns suggest four points. (1) Onshore-offshore and latitudinal trends in shell size, thickness, and ornamentation, community diversity/structure, and the development and dispersion of adaptive characters reflect changes in temperature, benthic oxygen levels, and substrate characteristics. (2) These trends indicate that diverse short-term and long-term oxygen depletion events occurred during Greenhorn deposition, modifying community structure and reflecting dynamic paleo-oceanographic cha ges in the seaway. (3) Biofacies/lithofacies data in conjunction with paleogeographic reconstructions allow modeling of community dispersion in response to trends in chemical and physical sedimentologic events. (4) Integration of community data and paleo-oceanographic models with latest Albian through middle Turonian eustatic history furthers our understanding of the relationship between sea level changes and community evolution in epeiric seas.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990