ABSTRACT
The Tioga bentonites were first noted by Fettke in 1931 from well cuttings
in the Tioga gas fields of Tioga County, Pennsylvania. They have been used
by many investigators (Oliver, 1954, 1956; Dennison, 1963; Dennison and
Textoris, 1967, 1978; Epstein, 1986; Smith and Way, 1983; Way et al., 1986)
as a time-plane to correlate stratigraphic units from eastern Pennsylvania
to Illinois and from southern Ontario to Virginia. The Tioga bentonites
are recognized as seven major volcanic ash layers labeled Tioga-A - Tioga-G
as well as several minor layers labeled Tioga-A1 - Tioga-A3 (Way et al.,
1986). They occur within the Onondaga (Middle Devonian) limestone and the
Marcellus (Middle Devonian) shale, and by definition mark the top of the
Onesquethaw stage (Dennison 1960). The Tioga bentonites are Eifelian (Middle
Devonian) and the Tioga-B layer has been recently dated as 390.0 Ma ±0.5
Ma (Roden et al., 1990) using U-Pb analysis of monazite phenocrysts.
The Tioga bentonites have been inferred to cover 264,180 km2
of the Appalachian basin of the northeast United States (Dennison and Textoris,
1987) and occur in two dissimilar stratigraphic units; hence the need for
precise correlation is paramount. High precision electron microprobe analysis
of the rhyolitic melt (i.e., glass) inclusions found in the quartz phenocrysts
allows for the geochemical fingerprinting of individual bentonite layers.
Using this technique, only four out of fourteen localities previously
reported as the Tioga bentonites geochemically correlated to bentonite
layers present at Frankstown, PA. The absence of bentonite layers containing
quartz phenocrysts with melt inclusions at the ten other sample locations
indicate that bentonite layers equivalent to FP-12 and FP-9 may be absent.
This implies that the Tioga group of bentonites may consist of more than
the seven major bentonite layers as previously reported. The geochemical
analyses of the rhyolitic melt inclusions of this study indicate that the
eruptions responsible for the Tioga bentonites (as restricted here) are
from a heterogeneous (i.e., zoned) magma chamber.
Waechter, J.W., 1993. Correlation of the Tioga Bentonites using rhyolitic
melt inclusions found in quartz phenocrysts as geochemical fingerprints.
Unpublished MSc. thesis, State University of New York at Albany. 161
pp., +xvi
University at Albany Science Library call number: SCIENCE Oversize
(*) QE 40 Z899 1993 W34
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