Magnetostratigraphy, isotopic age calibration and intercontinental correlation of the Red Bird section of the Pierre Shale, Niobrara County, Wyoming, USA

Abstract

The Red Bird section of the Pierre Shale in eastern Wyoming contains a relatively complete sequence of fine-grained marine clastics that were deposited between 81 and 69 million years ago in the Late Cretaceous epicontinental seaway of the US Western Interior. These units not only contain a well-studied, high-resolution ammonite biostratigraphic sequence, by which the far-flung exposures of the seaway sediments are correlated across this region, but they are also isotopically well-dated due to the presence of numerous sanidine-bearing volcanic ash layers. The magnetostratigraphy of the Red Bird section consists of three geomagnetic reversals which can be independently calibrated by seven40Ar/39Ar isotopic ages in an interval that spans 12 million years of the Campanian and Maastrichtian stages. The magnetostratigraphic section can be confidently correlated to that part of the geomagnetic polarity time scale (GPTS) that ranges from the base of subchron C33n to the base of C31n. Linear interpolation and extrapolation from the isotopic ages gives the following age estimates for these reversal boundaries: C32n/C31r, 70.44±0.7Ma; C31r/C31n, 69.01±0.5Ma. The C33n/C32r reversal boundary cannot be identified with complete confidence but it is certainly younger than the 74.62±1.2Ma age interpolated for the reversal found at the top of C33n. These age estimates make a significant contribution to the calibration of the GPTS for the Cretaceous Period, which has previously relied heavily on interpolation between three or fewer calibration points that are widely spaced in age. In addition, the recognition of the chrons C33 through C31 in this section enables us to correlate the high resolution ammonite zonation of the US Western Interior directly to the time-equivalent European pelagic microfossil zonation based on the magnetostratigraphic reference section at Gubbio in north-central Italy.