ANALYSIS OF JOINTS ACROSS THE UPPER DEVONIAN SONYEA GROUP AND WEST FALLS GROUP BOUNDARY, CONNECTICUT HILL STATE FOREST, SCHUYLER COUNTY, NEW YORK

The southern tier region of New York State is underlain by Upper Devonian strata that includes numerous shale formations. Joints were mapped within the Sonyea Gp and West Falls Group shales within the Connecticut Hill State Forest, located immediately south of Seneca and Cayuga Lakes. The local relief with deeply incised gorges within the state forest provided excellent locations to document the Upper Devonian strata in detail, and to evaluate any relationships between joint orientations, intensity, and lithologic variation. This field work was carried out during the Summer of 2017. The local Sonyea Gp contains interbedded laminated gray mudstone and siltstone at the dm-scale. Within these rocks, there are two prominent subvertical joint sets. The most abundant has a strike that ranges from 320-340, with the maximum at 335, and spacing that ranges from 20-30 cm. A second joint has a strike range of 050-080 with the highest population striking 065, and spacing range of 30-50 cm. The overlying West Fall Group contains gray to black shale with only minor siltstone beds. The joints are similar in orientation to the underlying Sonyea Gp with the most prominent joints striking 320-340 and spacing that ranges from 20-30 cm. The less abundant joints strike 070-090 with the highest population striking 085. The joint spacing ranges from 50-60 cm. All joints described herein have well developed plumose features and therefore developed by Mode I failure. There is a clear correlation between the joints observed and J1 and J2 joints of the Appalachian Basin. The J1 joints have been described by earlier researchers as cross-fold joints associated with the development of the regional folds in the Appalachian Plateau. The NW striking joints observed during this study are consistent in orientation across the lithologic contact and consistent in orientation with J1. Although the bedding in the region is nearly horizontal, subtle systematic variation in dip (4-9 degrees) within the field area clearly defines the crest of a broad open anticline that trends perpendicular to the dominant joints, and plunges less than 2 degrees northeast. The other observed joint set is consistent in overall orientation with J2, but the local variation of 20 degrees in strike appears to be linked to the differences in lithology of the Sonyea and West Falls Goups.