REGIONAL CORRELATION OF THE MIDDLE DEVONIAN TIOGA K-BENTONITES USING APATITE TRACE ELEMENT FINGERPRINTING

The Middle Devonian Tioga K-bentonites have long been recognized as a valuable marker and regional correlation tool. Despite nearly fifty years of study, regional correlations of specific Tioga beds remain tenuous, particularly between proximal (Appalachian Basin: New York and Pennsylvania) and distal (Wabash Platform: Ohio and Indiana) areas. However, new techniques of geochemical fingerprinting now allow precise correlations to be made across wide geographic areas.

Samples of the Tioga B and F beds (terminology of Way et al., 1986) were obtained from six Appalachian Basin localities. Coeval (late Eifelian) K-bentonites were also sampled from central Ohio (Slate Run--Columbus Fm/Delaware Fm contact) and northeastern Indiana (Woodburn Quarry--upper part of the Detroit River Group). Apatite phenocrysts were handpicked from heavy mineral separates of each bed. Suites of individual apatite grains were analyzed for rare earth elements (REE) using inductively coupled plasma-mass spectrometry (ICP-MS). The analysis method used was a slight modification of that outlined by Shaw (2003).

Two of the Tioga B samples in this study duplicate localities analyzed by Shaw (2003). The REE signatures in both studies are statistically identical, showing that the ICP-MS data are reproducible. The Tioga B and F beds are readily distinguishable from each other at all of the sampled Appalachian Basin localities. Light- to middle-REEs (La-Sm, Gd and Tb) are the best discriminators. In this range, chondrite-normalized REE plots show a distinct separation between the two beds, with considerable intra- but no interbed overlap of mean values at the 95% confidence level. Moreover, the curve shapes are different, with the Tioga F exhibiting a much higher La/Yb ratio. All samples show a pronounced Eu anomaly. Slate Run apatites show two distinct clusters in a plot of La/Yb vs. Eu/Eu*, possibly indicating a condensation of multiple eruptive events. However, most of the Slate Run phenocrysts show an REE fingerprint identical to that of the Appalachian Basin Tioga F.