CORRELATION OF K-BENTONITES USING APATITE TRACE ELEMENT CONCENTRATIONS

Paramount to developing a clear picture of Earth’s geologic history is stratigraphic correlation of geologically important events. Distinct marker beds or index fossils spanning multiple basins are sometimes lacking, and, even with the increased time-resolution afforded by improved chronostratigraphic methods, correlation among sections with suffiecient resolution to distinguish among alternative explanations for geologic events is often difficult. Precise correlation of marker beds such as the Deicke and Millbrig, two of the most extensive K-bentonite layers in N. America, provide potential isochrononus tiepoints critical to determining the correct stratigraphic position and interregional synchronicity of the GICE (Guttenberg Isotope Carbon Excursion), a positive carbon isotope excursion near the base of the Katian Stage.

Upper Ordovician rocks along the southeastern margin of Laurentia contain numerous widely dispersed, largely facies independent, K-bentonite layers retaining original apatite phenocrysts. Apatites from each K-bentonite layer, when subjected to LA-ICP-MS or EPMA, display reproducable trends in REE and other trace element concentrations that serve as unique bed discriminators. Regional correlations of the Deicke and Millbrig have been successful using this approach. Here we extend this approach throughout the southern Appalachians in an attempt to reduce uncertainty in stratigraphic correlation among sections (and thus, on local changes during the GICE interval), refinements important for improving our understanding of the Ordovician climate system. Preliminary results show that the Fort Payne Deicke and Tidwell Hollow Millbrig display trace element concentrations similar to previous analyses of samples from the Deicke and Millbrig bentonites. However, results for a bentontite at the Fort Payne section identified as the Millbrig, have elemental concentrations that are inconsistent with previous Millbrig correlations, but look similar to the Tidwell Hollow 1.5 sample (carbonate 1.5 m above the Millbrig). When combined with previous results, these data suggest that placement of the Fort Payne Deicke and Tidwell Hollow Millbrig are correct, but show that revision of the stratigraphic placement of the Fort Payne section Millbrig may be necessary.