APATITE TEPHROCHRONOLOGY OF LATE ORDOVICIAN K-BENTONITES: CONTENTION, CLUSTERS, AND CORRELATIONS

Extra-basinal K-bentonite correlations in the Paleozoic are contentious despite the success of regional scale correlation efforts utilizing whole rock and single crystal methods. In order to test stratigraphic models that describe facies distributions in continental margins it is critical to correlate beds at high precision across several depositional environments. Toward such correlations, over 200 K-bentonite beds were sampled from the late Sandbian and early Katian stages of eastern North America. Single crystals of volcanogenic apatite from K-bentonites were analyzed using electron microprobe techniques and with a new approach utilizing an ion microprobe (Cameca 3f). Initial results demonstrate that apatite trace element chemistry (F, Mg, Cl, Mn, Sr, Y, Ce, and Nd) of K-bentonite beds can be characterized over long distances and throughout the variety of depositional and diagenetic environments in eastern North America. The Millbrig, Deicke, Elkport, Dickeyville, Brannon, Calmar, and Haldane K-bentonites are proving to be the best for long-range correlation while several other un-named beds correlate between two or more sections. Electron microprobe analyses of Mn in apatite display a bimodal distribution manifest by the relatively high-Mn apatite Utica Cluster K-bentonites and the low-Mn apatite Hagan Cluster K-bentonites. While there may be minor overlap of these two apatite trace element chemistry-based groups, the distribution currently serves as an important correlation heuristic. These new apatite trace element data suggest two sources or an evolving source for the late Sandbian and early Katian K-bentonites of North America. The uniquely complex Millbrig K-bentonite is definitely a composite bed, likely composed of at least two eruptions. On the basis of these findings we present new K-bentonite correlations between the foreland areas of West Virginia (Dolly Ridge Formation), Virginia (Martinsburg Formation), Pennsylvania (Salona to Antes formations), and New York (Utica Shale) and into cratonic deposits in Kentucky (Tyrone and Lexington limestones) and the Upper Mississippi Valley (Decorah and Dunleith formations).