Abstract

Stratigraphic profiles of carbon isotopic ratios in organic matter (δ¹³C_org) from Cenomanian and Turanian strata have been established for four locations in the southern part of the Western Interior basin of North America. Each δ¹³C_org profile includes a 2.5-3.5 ‰ positive excursion that spans the Cenomanian-Turonian boundary. Detailed features of the isotopic excursions are consistent and can be correlated on profiles from the western siliciclastic facies, the central pelagic facies, and the eastern carbonate-bank facies. This δ¹³C_org excursion is a valuable stratigraphic marker because it resulted from a geologically brief, global geochemical event (estimated duration, 0.5 m.y.) and because it can be readily distinguished from other isotopic events in the geologic record. The Cenomanian-Turonian excursion and other major shifts in carbon isotopic ratios can provide a means to establish chronostratigraphic boundaries that cross the boundaries of lithofacies and depositional environments.