Date of Award
Master of Science (MS)
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, John W., "Correlation of the Tioga Bentonites using rhyolitic melt inclusions found in quartz phenocrysts as geochemical fingerprints" (1993). Geology Theses and Dissertations. 95.