60-55 The Influence of Bedrock Fractures on Stream Flow Direction in the Northeastern Allegheny Plateau

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Authors:

Abstract:

Numerous bedrock gorges have cut down through glacial drift to etch into Paleozoic sedimentary rocks in the northeastern Allegheny Plateau in New York. Bedrock floored channels reveal a plethora of near vertical fractures which transect multiple sedimentary layers. A prime example of a fracture controlled channel occurs near Milford New York, where vertical fractures form 5-10 m high valley side walls. Vertical fractures rupture and parallel the channel floor, facilitating stream erosion via plucking. The lower end of the gorge makes an abrupt turn when it first encounters the vertical fractures. Such fractures appear to control the channel direction. Further investigation shows the Milford case to be an anomaly. 

We mapped over 900 potential bedrock gorges and fracture sites in GIS, visited over 30 sites with bedrock floored streams, and measured strike and dip of over 2000 vertical fractures. Stream bearing measurements were extracted from the USGS 3D Viewer. We find that the region contains a spectrum of near vertical fracture orientations with three modes. The strongest cluster is 050/230 (northeast/southwest), the next most common is 015/195 (north-northeast/south-southwest), with a weaker broad cluster at 115/295 (east-southeast/west-northwest). Few vertical walled channels or valley side walls of comparable scale to Milford gorge were encountered. Rather, a spectrum of fractures criss-crossed channels. Bedrock valley side walls were typically irregular surfaces. We find the likelihood of a stream aligning with a vertical fracture to be modest. Roughly 25% of bedrock floored streams do not align with a vertical fracture (within a tolerance of 20 degrees). At Milford gorge, the type locality, stream bearing coincides with vertical fractures 70% of the time whereas most channels see multiple fracture directions resulting in a modest to poor correspondence that challenges the idea that vertical fractures control stream direction.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9173

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