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Unpublished MSc.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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