CORRELATION OF CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY WITH SR ISOTOPE DATA FROM UPPER CRETACEOUS SEDIMENTS OF THE NORTH CAROLINA COASTAL PLAIN

The ages of the Late Cretaceous stage boundaries, as illustrated by the geologic timescale, are well constrained by ⁴⁰Ar/³⁹Ar dating through calibration of the Western Interior North American ammonite zonation. Since Late Cretaceous stage boundaries are well constrained through radiometric dating rather than cyclostratigraphy, it is possible to assign provisional ages to microfossil datums and marker species from sample material that contains both abundant calcareous nannofossils and well-preserved megafossils. Thus, correlation of the Late Cretaceous calcareous nannofossil biozonation with the ⁸⁷Sr/⁸⁶Sr calibration curve is possible.

Cenomanian through Maastrachtian age sediments from five cores (Kure Beach, Dixon, Elizabethtown, Smith ES, and Hope Plantation) in the North Carolina Coastal Plain have been examined for calcareous nannofossil content and provisional dates have been assigned using strontium isotope ratios. Estimates obtained from the ⁸⁷Sr/⁸⁶Sr calibration curve are correlated and compared to the Late Createceous calcareous nannofossil biozonation scheme for each core. Confidence between the two techniques is generally high and estimates agree with nannofossil zones, except where recrystallization “reset” the strontium isotope data or where undetected reworking has occurred. A few notable exceptions are ages obtained from Cenomanian sediments of the downdip Kure Beach core. These ages are invariably too young (87.1-88.4 Ma), although the biostratigraphy and approximately half of the Sr estimates from similar sediments in the updip Smith ES and Hope Plantation cores match (Zone CC10 and 95.0-97.0 Ma, respectively). Sediments from the Campanian Bladen and Donoho Creek Formations can be placed in Zones CC21 and CC22, but strontium correlations consistently record two distinct clusters: 76.0-78.0 Ma, which agrees with the biozonation, and 70.0-73.0 Ma, which is too young. This bimodality appears to be regional and has been reported from the northeast Atlantic seaboard as well. Estimates obtained from sediments in the upper Maastrichtian Peedee Formation are too old (76.0-77.5 Ma) from the Dixon core, but correlate well with biostratigraphy in the Kure Beach core (Zones CC25b and CC26).