Testing Tehprostratigraphic Techniques with K-Bentonites from the Trenton and Black River Groups of Central Pennsylvania
Correlations of Late Ordovician K-bentonites on the basis of apatite phenocryst chemistry have been successful at a regional scale in the Mohawk Valley of New York and in the Upper Mississippi Valley. Continental scale correlations have recently been attempted using the same techniques, however regionally distinct stratigraphic clusters of K-bentonites must be studied in greater detail to establish the unique character of these long-range tephra deposits. The Salona Formation of central Pennsylvania was chosen in this study because it possesses six well-known K-bentonites within a sequence of 23 K-bentonites, is exposed in several locations spanning more than 100 kilometers, and was previously assigned correlation schemes on the basis of stratigraphic position and whole rock composition. The goal was to sample five locations so as to assess the uniqueness of each of each K-bentonite in stratigraphic succession and between measured sections. The measured sections are in State College, Union Furnace, Ashcomm, Roaring Spring, and Reedsville. For comparison, we present several analyses of K-bentonites from the underlying Nealmont and Curtin Formations. Eight samples anomalously lack apatite phenocrysts. The trace elements Cl, Mg, Mn, Fe, Ce, and Y where analyzed in 20 or more apatite phenocrysts from thirtyfour samples using an electron microprobe. Initial results show that at least three of the beds are very distinct and can be easily correlated between sections. The remaining beds display large variation in phenocryst chemistry requiring a larger number of analyses to adequately characterize the apatite population. Despite this problem, all beds prove to be sufficiently unique within a stratigraphic succession. Our correlations on the basis of apatite phenocryst chemistry are at odds with those on the basis of stratigraphic position and whole rock chemistry. This study shows that long-range correlations of K-bentonites will require sampling at multiple locations within a given region because of varying phenocryst content.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009