|The most compelling evidence for the early sealing
behavior of the Dunkirk shale is very pervasive NNW-trending
fractures or joints found only in the upper third of the Hanover
Significantly, very few of these joints extend more than
several decameters into the Dunkirk shale (Figure
Individual joints are more than three meters high, can be
traced laterally for well over 100 meters and exhibit orthogonal
spacings on the order of one to five meters (Figure
The NNW-trending joints are interpreted to be hydraulic
fractures that formed as a consequence of abnormally high pore fluid
pressures at the top of the Hanover shale. Indeed, the locally great
joint height:joint spacing ratio of these joints is most consistent
with their formation as a consequence of elevated pore fluid
pressure (Ladiera and Price, 1981). Abutting relations suggest that
NNW joints are older than NW- and ENE-trending joints, which are
found locally in the Hanover shale.
The fact that NNW joints are roughly coaxial with a very
modest strain produced during Alleghanian compression (Craddock and
van der Pluijm, 1989) suggests that the joints formed as a
consequence of Alleghanian deformation.
«Figure 1: generalized Devonian stratigraphy of western New York. This study focuses on the Dunkirk shale-Hanover shale contact.
|Figure 2: comparative rose plots of joint orientations in the lower and upper Hanover; note abundance of NNW joints in the upper Hanover.|
|Figure 4: field photograph of a very continuous NNW trending joint in the Hanover shale approximately 2-3 m beneath the base of the Dunkirk shale.|
|Figure 6: field photograph of the well-laminated Dunkirk shale, which would have enhanced the strength of the capillary seal.|
Baird, G.C., and Lash, G.G., 1990, Devonian strata and environments: Chautauqua County region: New York State: New York State Geological Association, 62nd Annual Meeting Guidebook, Sat. A1-A46.
contact Gary Lash, Department of Geosciences, SUNY Fredonia. Fredonia, NY 14063