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Unpublished MSc.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
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