Geochemistry- and Petrophysics-Based Detection of Breaks in the Stratigraphic Record of Cenomanian–Turonian Deposits, Eastern Cretaceous Western Interior Seaway: How Complete is the Stratigraphic Record and Why Does it Matter?

The stratigraphy and paleoenvironmental history of Cenomanian–Turonian (including OAE2) deposits along the eastern margin of the Cretaceous Western Interior Seaway (KWIS) are much less studied than time-equivalent rocks of the central-western portions (e.g., Graneros, Bridge Creek, Fairport members of the Mancos Shale) or along the western Gulf Coast (e.g., Eagle Ford Group). In stable cratonic areas such as present-day Manitoba and Saskatchewan, eustatic fluctuations of any magnitude, changes in oceanic circulation, or other phenomena could lead to non-deposition and erosion. Detecting gaps in the stratigraphic record is essential for reconstructing paleogeography, paleoceanography, and paleobiology but this topic has not previously been addressed in the paleontological or geochemical literature.

We integrate elemental and organic geochemistry (hand-held XRF and Rock-Eval), stable isotope geochemistry (δ13CORG), and wireline logs to illustrate the presence of significant stratigraphic gaps in Cenomanian-Turonian deposits of our study area. We show how this new perspective challenges some existing interpretations that are still used to reconstruct oceanic circulation patterns in the KWIS at this time. For example, previous studies have noted abrupt increases in the abundance and diversity of foraminiferal, molluscan and ammonite faunal assemblages at this time, and linked them to sudden breaches of a sill located at the southern margin of the KWIS. Our results suggest that, at least in some cases, the apparent abruptness of these faunal changes could be related to gaps in the stratigraphic record.

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