High-Resolution Sedimentological and Chronological Correlation of the Bridge Creek Limestone Member of the Greenhorn Formation over 200 km

The Cenomanian-Turonian Ocean Anoxic Event 2 interval in the central North American Western Interior Seaway is characterized by distinct limestone-marl bedding couplets that represent one of the best geochemically and cyclostratigraphically constrained sedimentary records preserving orbital cyclicity. This creates a prime opportunity to examine sedimentary facies variability at sub-bed resolution and to evaluate lateral facies continuity across the basin. In this study, the Bridge Creek Limestone Member (BCL) of the Greenhorn Formation was measured at millimeter- to centimeter-scale in the USGS # 1 Portland and Aristocrat-Angus-12-8 (“Angus”) cores. Detailed facies analysis reveals changes in depositional conditions at a much finer temporal resolution because individual limestone and marl beds often consist of a stack of different facies. By integrating carbonate content and iron and manganese elemental data, the BCL can be correlated at a facies-to-facies level. The correlative, strikingly similar facies successions between the two cores approx. 200 km apart imply that vertical facies changes reflect contemporaneous environmental changes pervasive across the basin. Using facies boundaries as timelines, the BCL can be sub-divided into at least 25 chronological units. Most chronological units are thicker in the Angus core, indicating higher detrital sediment input, likely derived from intrabasinal erosion when considering the regional paleogeography. However, many chronological units in the lower and middle parts of the BCL are thicker in the Portland core, which can be evidently linked to greater volcanic ash input. Lateral changes in the thickness of chronological units have important implications for sediment provenance and transport mechanisms, and the facies-to-facies relationship offers process-based insights that complement existing models focused primarily on lithological alternations. The study provides a rare example of how the integration of detailed sedimentological and geochemical data allows high-resolution chronological correlations. The succession of sedimentary facies thus represents a powerful tool for strengthening, refining, or compositing correlations beyond traditional lithologic or proxy-based approaches.