High-resolution magnetic susceptibility and geochemistry for the Cenomanian/Turonian boundary GSSP with correlation to time equivalent core
Abstract The Upper Cretaceous Cenomanian/Turonian (C/T) boundary represents the most significant thermal climatic event in the Mesozoic, and it has been well-studied. Here we present a relatively new approach for providing both independent, climate-based cyclostratigraphic analysis and correlation among C/T sections. We have collected samples for bulk (initial) low-field magnetic susceptibility (MS) measurement and high-resolution inorganic geochemical analyses from two C/T sequences: (1) the well-defined, C/T Global boundary Stratotype Section and Point (GSSP) located at Lake Pueblo, Colorado, and deposited in the Upper Cretaceous Western Interior Seaway of North America, and (2) the well-studied marlstone and limestone USGS#1 Portland Core, drilled at Portland, Colorado, ∼ 40 km to the west of the C/T GSSP. Comparing the lithostratigraphy, chemostratigraphy, and magnetostratigraphic susceptibility (developed from magnetic susceptibility (MS) data) indicates that the USGS#1 Portland Core closely compares with the C/T GSSP sequence using any of the three measures, demonstrating that intervals collected and sampled from these two sequences are highly correlated. A preliminary magnetostratigraphic susceptibility zonation for the C/T boundary is presented to which other MS work can be compared. Fourier Transform (FT) analysis of the MS data from the GSSP, when compared with previous FT work using geochemical analysis of Portland core samples shows close similarities to the GSSP, with Milankovitch eccentricity, obliquity and precession bands well-defined in both data sets. Using this result, we assign relative ages to the intervals sampled and evaluate changes in absolute sediment accumulation rates through the C/T boundary interval. We show that while sediment accumulation rate is relatively high in the upper Cenomanian, a change toward lower accumulation rates in the GSSP section at the C/T boundary is due to reduced productivity following the C/T extinction events that resulted from global warming associated with Oceanic Anoxic Event 2 (OAE2) that began in the uppermost Cenomanian.