Stratigraphic correlations using trace elements in apatite from Late Ordovician (Sandbian-Katian) K-bentonites of eastern North America

Abstract

The early Late Ordovician sedimentary rocks of eastern North America contain a relatively large number (>100) of widespread heavily altered tephra layers (K-bentonites). These beds represent an intense period of subaerial volcanism that occurred from ca. 455 to 449 Ma. The sedimentary rocks that contain these K-bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies. Accurate correlation of these ancient ash-fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono-sedimentary provinces. Despite the relatively thorough study of a few of these K-bentonites over the past several decades, the full stratigraphic potential of these beds has yet to be realized. To test the utility of the apatite trace-element K-bentonite correlation method on a larger scale, we studied over 200 K-bentonite samples from the Mohawkian Stage of eastern North America and statistically compared our results with previous studies on the same suites of K-bentonites. Electron microprobe (EPMA) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) results show that apatite trace-element data provide unique bed discriminators. Each of the K-bentonite layers exhibits unique and reproducible trends in Mg, Cl, Mn, Fe, Ce, Y, and other trace-element concentrations in apatite. Statistical evaluation of results from our apatite analyses suggests correlations for 12 K-bentonite beds, providing a significant improvement in stratigraphic resolution. The stratigraphic relations indicated by these new K-bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic, chemostratigraphic, and sequence stratigraphic studies in eastern North America.