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

The Red Bird section of the Pierre Shale in eastern Wyoming contains a rela tively complete sequence of fine-grained marine clastics that were deposite d 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-stu died, high-resolution ammonite biostratigraphic sequence, by which the far- hung exposures of the seaway sediments are correlated across this region, b ut they are also isotopically well-dated due to the presence of numerous sa nidine-bearing volcanic ash layers. The magnetostratigraphy of the Red Bird section consists of three geomagnetic reversals which can be independently calibrated by seven Ar-40/Ar-39 isotopic ages in an interval that spans 12 million years of the Campanian and Maastrichtian stages. The magnetostrati graphic section can be confidently correlated to that part of the geomagnet ic polarity rime 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 +/- 0.7 Ma; C31r/C31n, 69.01 +/- 0.5 Ma. The C33n/C32r reversa l boundary cannot be identified with complete confidence but it is certainl y younger than the 74.62 +/- 1.2 Ma 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 previousl y 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-equivale nt European pelagic microfossil zonation based on the magnetostratigraphic reference section at Gubbio in north-central Italy. (C) 1999 Academic Press .