Abstracts Northeastern Geology and Environmental Sciences, v. 22, no. 3

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HIGH-RESOLUTION STRATIGRAPHY OF THE OHIO SHALE (UPPER DEVONIAN), CENTRAL OHIO

Laura Walker Hellstrom and Loren E. Babcock
Department of Geological Sciences, The Ohio State University, Columbus, OH 43210

ABSTRACT: The Ohio Shale (Upper Devonian: Famennian) is a black shale unit that was deposited widely through much of the western Appalachian Basin. Although the formation has received much study in certain areas, there has been inconsistency in the application of lithostratigraphic correlation techniques and nomenclature through some important parts of the depositional area. Longstanding correlation and nomenclatural problems have hindered work particularly in central Ohio. New work, combining lithologic studies and gamma-ray profiles, now allows clarification of the lithostratigraphy of the Ohio Shale through the entire outcrop and subcrop belt of Ohio. Subdivisions of the formation recognizable both in northern Ohio, and in southern Ohio and Kentucky, are now traced into central Ohio. Gamma-ray profiles from outcrops in central Ohio, together with subsurface gamma-ray logs, indicate the presence of the Chagrin Shale Member in central Ohio. The Three Lick Bed, interpreted as the upper part of the Chagrin Shale Member, also can be correlated through central and northeastern Ohio. Recognition of the Chagrin Shale Member in central Ohio furthermore provides stratigraphic resolution of the underlying Huron Shale Member and the overlying Cleveland Shale Member. Decameter-scale intervals containing carbonate concretions in the Huron Shale Member are traceable over long geographic distances; however, individual concretionary beds tend to be of a more limited geographic extent.

Long distance, high-resolution correlation of the Ohio Shale across Ohio suggests that lithologic differentiation within the formation is related to forcing mechanisms of at least regional extent. Such mechanisms may have included fluctuation of a pycnocline (oxycline), eustatic sea-level change, or intervals of time during which turbidite influxes caused repeated, short-term oxygenation of the sea floor.

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ANCESTRAL HORSESHOE CRAB (SUBORDER SYNZIPHOSURINA) FROM THE BLOOMSBURG RED BEDS (UPPER SILURIAN) OF NORTHWESTERN NEW JERSEY

Robert Metz
Department of Geology and Meteorology, Kean University, Union, New Jersey 07083

ABSTRACT: The first occurrence in New Jersey of an ancestral horseshoe crab (suborder Synziphosurina), has been identified in overbank flood and crevasse splay deposits of the Upper Silurian Bloomsburg Red Beds from the Delaware Water Gap National Recreation Area. The monospecific presence of the ichnogenus Skolithos strongly suggests a brackish water origin for the synziphosauran-bearing strata.

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RE-EVALUATING THE CANADAWAY GROUP: A REVISED STRATIGRAPHIC CORRELATION CHART FOR THE UPPER DEVONIAN OF SOUTHWESTERN NEW YORK STATE

Gerald J. Smith and Robert D. Jacobi
Department of Geology, 876 Natural Science Complex, SUNY at Buffalo, Amherst, NY 14260; [email protected]

ABSTRACT: A combination of previous studies and new detailed work suggests that Rickard's (1975) correlation chart for Devonian rocks in New York State should be revised for the Upper Devonian sedimentary rocks of southwestern New York State. The area of concern involves the correlation of the stratigraphic section from Lake Erie to immediately east of the Genesee River. A review of investigations by earlier works (Chadwick 1923, 1935a, 1936; Woodruff 1942; Pepper and deWitt 1950,1951; Tesmer 1955; Van Tyne 1982,1983) documents a consistent sequence of units that generally agrees with our own field studies. By integrating stratigraphic sections of Pepper and deWitt (1950, 1951) with outcrop-based, stratigraphic sections we developed in Allegany and Cattaraugus Counties (Jacobi and Fountain 1996; Smith and Jacobi 1998), we propose the following regional correlations, which follow similar correlations proposed by Chadwick (1936), Pepper and deWitt (1950, 1951) and Van Tyne (1982). The Hume Formation is not equivalent to the Dunkirk Formation, but occurs stratigraphically upsection, separated from the Dunkirk Formation by the South Wales and Canaseraga formations. The Forty Bridge Formation should be abandoned and the older term "Gowanda" used since the Gowanda Formation has the same type section as the Forty Bridge. The Wellsville Formation and Whitesville Formation are equivalent to neither the Gowanda nor the Caneadea formations, but overlie the Cuba Formation. The Whitesville Formation occurs stratigraphically upsection from the Wellsville Formation and is separated from the Wellsville Formation by the Hinsdale Formation.

New interpretations in the depositional environments of the upper West Falls and Canadaway groups (Jacobi and Fountain 1996; Smith and Jacobi 1998) has enabled us to further revise the Rickard (1975) correlation chart to display a shallower depositional environment (nearshore to shoreface deposits, instead of basinal to slope deposits), as well as a more varying depositional environment. Furthermore, we propose the formal revision in stratigraphic rank for the major lithostratigraphic units that comprise the Canadaway Group to formation status. Finally, we suggest the stratigraphic column of Woodruff (1942) be used as a guide for the stratigraphic units of the Conneaut Group.

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HYDROFRACTURE EMPLACEMENT OF A MESOZOIC LAMPROPHYRE INTRUDED INTO THE LATE PROTEROZOIC-CAMBRIAN HAZENS NOTCH AND FAYSTON FORMATIONS, JOHNSON, VERMONT

¹Stephen Van Horn, Adam Jones, and William Stansfield
Department of Environmental and Health Sciences, Johnson State College, Johnson, VT 05656
¹now at: Department of Geology, Muskingum College, New Concord, OH, 43762

ABSTRACT: Mesozoic intrusions in the northern Appalachians were emplaced in complexly deformed and jointed Late Proterozoic-Paleozoic regional metamorphic and sedimentary rocks. Typically these dikes occur as en echelon dike segments or as dikes with offset contacts. Magma can create joints by a hydrofracture mechanism as it rises through the crust or it can use older joint sets or other structural features such as foliation or cleavage as pathways. En echelon dikes or offset dikes can form by either of the above mechanisms. Dikes with offset contacts may also form by post-emplacement faulting.

Dikes from the northern Appalachians have been used to determine the orientation of paleostress in the crust. The emplacement mechanism of a dike has important implications for its use in the determination of paleostress. Only if the dike was emplaced by a hydrofracture mechanism does the orientation of the dike reflect the stress conditions during emplacement.

Near the town of Johnson, Vermont, a lamprophyre dike crops out discontinuously along a 1500 m section of a north-flowing stream that exposes the Late Proterozoic-Cambrian Hazens Notch and Fayston formations in the Green Mountain Anticlinorium. The Hazens Notch and Fayston formations consist of silvery gray-green schist with abundant albite porphyroblasts. Both formations have quartz-filled joint sets that strike N15°W and E-W and an unfilled joint set that strikes N30°E to N50°E. The orientation of the dominant foliation (S2 averages N75°E, 33°SE) is subparallel to the limbs of isoclinal folds (F2). The Johnson dike crops out as seven discrete en echelon segments, striking N17°E to N42°E. Individual dike segments are continuous but contain offsets of the dike contact perpendicular to strike. Horns, bridges, and country rock xenoliths are present at several of the contact offsets. Dike-parallel joints are present in the country rock near most of the dike segments.

In an attempt to determine if pre-existing structures influenced the emplacement direction of the dike we measured joint and foliation orientations in the local area. Foliation appears not to have influenced emplacement because there is no relationship between foliation and dike segment orientations. Evidence against emplacement along pre-existing joints includes: i) lack of a splay geometry at the termination of individual dike segments, ii) the presence of numerous dike-parallel joints associated with the dike segments, and iii) no evidence for shear motion across the dike. The dike emplacement model that best fits the Johnson dike is a hydrofracture model which results in the creation of an en echelon fracture array during emplacement. The presence of horns, bridges, country rock xenoliths and dike-parallel joints support this interpretation. Careful determination of the dike emplacement mechanism should allow accurate paleostress determinations to be made even in structurally complex areas.

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