PREFERENTIAL JOINTING OF UPPER DEVONIAN BLACK SHALE, APPALACHIAN PLATEAU: EVIDENCE FOR AN ANCIENT PRESSURE COMPARTMENT AND SEAL SYSTEM

Heavily fractured black shale units form an integral component of the Upper Devonian sequence of the Catskill Delta Complex, western New York Appalachian Plateau. The 12.5-m-thick Dunkirk shale, the youngest of these thermally mature, carbon-rich deposits, passes upward via a transition zone of interbedded black and gray shale into the organically lean Gowanda gray shale and abruptly overlies the Hanover gray shale. Preferential jointing of the Dunkirk shale is interpreted to reflect the combined effects of early compaction disequilibrium and an extended hydrocarbon generation history. Rapid accumulation of increasingly siltstone-rich deposits that overlie the Gowanda shale during progradation of the Catskill Delta resulted in compaction disequilibrium of deeper pre-Dunkirk shale deposits. Overpressured fluids ascended the permeable Hanover gray shale to the base of the Dunkirk shale, which was acting as a pressure seal at this time. Elevated formation pressures at the top of the Hanover pressure compartment induced propagation of NNW-trending joints early in the Alleghanian Orogeny. Few of these joints propagated more than a meter into the Dunkirk pressure seal. The inferred pressure-depth profile of the Upper Devonian sequence at this time was similar to modern overpressure profiles of the Central Graben of the North Sea where the impermeable, organic-rich Upper Jurassic Kimmeridge Clay Formation maintains abnormal formation pressures in underlying strata. NW joints breached the Dunkirk pressure seal after the Alleghanian stress field had rotated in an anti-clockwise sense. Though omnipresent throughout the Hanover-Dunkirk-Gowanda sequence, NW joints are best developed in the lower, organic-rich half of the Dunkirk black shale and appear to have formed as a consequence of hydrocarbon generation as the Dunkirk neared maximum burial depth. ENE joints, the youngest and best developed systematic joints carried by the black shale, are interpreted to have been generated in a post-Alleghanian remote stress field at or near peak hydrocarbon generation. Throughout this protracted jointing history, overpressured fluids and hydrocarbons migrated up-section from the Dunkirk shale toward the hydrostatically pressured Gowanda shale and overlying strata causing joints to propagate vertically.