Crosscutting vs. Abutting Joints: a Reflection of Joint-Normal Effective Stress

Terry Engelder¹ and Gary Lash^2
1Geosciences, Penn State, University Park, PA
²Geosciences, SUNY Fredonia, Fredonia, NY

Whether joint sets crosscut depends on magnitude of the effective normal stress that acts to close the earlier joint set. Conventional wisdom holds that exhumation-related joints are late-stage cross joints that abut systematic joints because the systematic joints have opened under reduced effective stress (Gross, 1993). The same wisdom holds that deep-seated joints crosscut because high effective normal stress holds the first joint set closed. However, the role of joint normal stress on joint crosscutting has never been demonstrated using tight stratigraphic control where the same joint sets have propagated under relatively deep and relatively shallow conditions. Devonian black shale of the Catskill Delta complex hosts east-northeast (ENE)-trending joints that either crosscut or abut cross-fold (CF) joints. Joints of both sets crosscut exclusively in the more proximal, deeper region of the delta complex. In equivalent black shale in the distal delta, ~25% of the observed joint intersections are crosscutting whereas ≈ 75% of the ENE joints abut CF joints indicating late reactivation of ENE joints. ENE joints in distal deposits were the first to propagate in these rocks but were neither as long nor pervasive as in the more deeply buried proximal delta deposits. Deeper into the distal section (≈130 m), however, ENE joints are much better developed and all intersections are crosscutting. Field observations from across the delta complex suggest that CF joints cut ENE joints when burial-related effective joint-normal stress caused ENE joints to close creating a frictional contact. Reactivated ENE joints abut CF joints that opened as a consequence of the relief of joint-normal effective stress during exhumation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas