Abstract

Drilling during the current second phase of exploration and production in the Appalachian Basin has provided structural information which must be considered in the third phase of deep exploration which has recently been initiated. Classical textbook descriptions and generalizations are misleading unless qualified and modified in light of existing but poorly publicized and relatively new information.

There is a Siluro-Devonian mobile zone beneath the Appalachian Plateaus which approaches the structural pattern and complexity of the surface structures of the Valley and Ridge province to the southeast. The less-couples post-Devonian surface anticlines of the Plateaus therefore may not be reliable guides to the position of the disharmonic Siluro-Devonian structures. There is even the possibility that productive subsurface highs will be found beneath surface lows to mark the terminal discovery period of second phase production.

Rasp-type gravens are along the axes of the Siluro-Devonian anticlines in some areas, whereas southeast-dipping thrust faults are characteristic of others. Experimental analogies exist for explaining the origin of these patterns from either lateral or vertical stresses.

There appears to be no sudden termination of "southern type" thrust faulting northeastward along the trend of the Appalachians in southeastern West Virginia or neighboring Virginia.

Conformity of Siluro-Devonian structures to the Cambro-Ordovician structures which are the main targets of the third phase exploration has not been established. With known vertical and lateral disharmonies, it appears that intelligent completion of prospecting of the Siluro-Devonian structures, and location of third phase prospects in the Cambro-Ordovician must depend on successful adaptation of geophysical prospecting to Appalachian conditions.