MAGNETIC SUSCEPTIBILITY AND GEOCHEMISTRY FOR THE CENOMANIAN/TURONIAN BOUNDARY GSSP WITH CORRELATION TO TIME EQUIVALENT CORE

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 analyses and correlation among C/T sections. We have collected samples for bulk (initial) low-field magnetic susceptibility (MS) measurement and high-resolution geochemical analyses from two C/T sequences: (1) the well-defined but somewhat weathered marlstone and limestone outcrops, in the Upper Cretaceous Western Interior Seaway, of the C/T Global boundary Stratotype Section and Point (GSSP) located at Lake Pueblo, Colorado, and (2) the well-studied and non-weathered marlstone and limestone USGS#1 Portland Core, drilled at Portland, Colorado, ~40 km to the west of the C/T GSSP. Comparing the litho- , chemo-, and magneto-stratigraphy (MS) 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 standard MSEC (Magnetosusceptibility Event and Cyclostratigraphy) zonation for the C/T boundary is constructed and 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 analyses 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 argue here that while sediment accumulation rate is relatively high in the upper Cenomanian, a change toward lower accumulation rates in the GSSP at the C/T boundary is the result of changes in productivity due to detrital impact on the marine biota in these sequences. This reduction in accumulation rates at the GSSP is due to reduced productivity following the C/T extinction events resulting from global warming associated with an Oceanic Anoxic Event, OAE2, beginning in the uppermost Cenomanian.