55-4 High-Resolution Sedimentological and Chronological Correlation of the Bridge Creek Limestone Member of the Greenhorn Formation between the USGS # 1 Portland Core and the Aristocrat-Angus-12-8 Core

Session: Sequence Stratigraphic, Geochemical, and Geochronologic Correlation of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) in the Cretaceous Western Interior Seaway (KWIS) and the Gulf Coast

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Abstract:

The Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) interval in the central North American Western Interior Seaway is characterized by distinct limestone-marl bedding couplets. Decades of research on these couplets has made them one of the best geochemically and cyclostratigraphically constrained sedimentary records preserving orbital cyclicity. This creates a prime opportunity for a detailed analysis of microfacies in different cores that recovered equivalent cycles. In this study, the Bridge Creek Limestone Member (BCL) of the Greenhorn Formation was measured at mm-cm scale in both the USGS # 1 Portland and Aristocrat-Angus-12-8 (“Angus”) cores. Six sedimentary facies types, along with bentonite beds, were identified in both cores based on lithology and sedimentary and bioturbation characteristics. Detailed sedimentologic analysis reveals changes in depositional conditions at a finer temporal resolution because, notably, single limestone or marl beds often consist of a stack of different facies. By integrating carbonate content and elemental data of Fe and Mn, most of the BCL can be correlated based on both lithological beds and facies-to-facies, confirming the preservation of orbital signals in both cores. The correlative, strikingly similar facies succession between the two cores ~ 200 km apart implies that vertical changes in facies type can be linked to contemporaneous environmental changes across the basin. Using facies boundaries as timelines, the BCL can be divided into at least 25 chronological units. Most chronological units are thicker in the slightly more offshore Angus core, indicating higher sediment supply, likely derived from intrabasinal erosion when considering the regional paleogeography. The higher detrital input in the Angus core is also supported by the correlative facies change from the Angus core to the Portland core (e.g., from laminated marl to laminated limestone, respectively). However, the majority of chronological units before the early Turonian peak transgression (i.e., Bed 97 in the BCL sensu Cobban and Scott 1972) are thicker in the Portland core, which can be evidently linked to higher volcanic ash input. The study provides an example of how the integration of detailed sedimentologic and geochemical data allows high-resolution chronological correlations. Lateral changes in the thickness of chronological units have implications for sediment provenance and transport mechanisms, and the facies-to-facies relationship offers process-based insights that complement existing models focusing more on explaining lithological alternations (i.e., limestone vs. marl).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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