SYNCHRONOUS FACIES CHANGES AND QUANTIFIED HIATUSES AT THE ONSET OF MID-CRETACEOUS OCEANIC ANOXIC EVENT 2 IN THE WESTERN INTERIOR BASIN (N. AMERICA)
Our recent work correlates high-resolution δ13C and Osi chemostratigraphies from key Western Interior Basin (WIB) localities preserving OAE2 along a distal to proximal and depth transect. Although lithofacies from the different localities are distinct, each site records prominent excursions to unradiogenic Osi compositions signaling enhanced submarine volcanism, which are followed by increased global organic carbon burial (as indicated by positive δ13C shift). Using a published astrochronologic time scale, the time lag between the onset of LIP volcanism before OAE2 and the carbon cycle response is quantified at 2-2.5 precession cycles (~40-50 kyr). The presence of this lag between Osi and δ13C excursions in the Portland Core indicates that the hiatus in the uppermost Hartland Shale near the Cenomanian-Turonian Stage GSSP is relatively brief (<40 kyr) compared to the duration of OAE2.
Within the refined chronostratigraphic framework of bentonite stratigraphy, Osi and δ13C chemostratigraphies, and biostratigraphy, changes in the depositional environment of the WIB can be evaluated at the highest temporal resolution to-date. Notably, carbonate content decreased synchronously in two intervals during the lag between Osi and δ13C excursions in the WIB, and near Bentonite “A” where Osi decreases subtly again. The compilation of datasets from a range of depositional settings demonstrates that carbonate sedimentation was suppressed in the Western Interior Seaway during these two intervals of OAE2. This suggests that the injection of large quantities of CO2 to the ocean-atmosphere system from LIP volcanism altered marine carbonate chemistry or other factors governing carbonate production in the WIB.