LARGE BASEMENT-INVOLVED INVERSION STRUCTURES AT THE APPALACHIAN DEFORMATION FRONT, WESTERN NEWFOUNDLAND: INTERPRETATIONS OF NEW INDUSTRY SEISMIC REFLECTION DATA AND IMPLICATIONS FOR PETROLEUM PROSPECTIVITY

The Appalachian deformation front in Western Newfoundland has been the target of a cycle of petroleum exploration that began in 1989. This has resulted in six new wells in the Port au Port Peninsula area. The first, drilled in 1994/95, was Port au Port No.1. It penetrated the Cambro-Ordovician platform and underlying Grenville basement in the hanging wall of the SE-dipping Round Head Thrust (RHT), terminated in a broadly folded slice of the platform succession in the footwall of this inverted basement-involved structure and discovered the Garden Hill pool. Subsequent wells confirmed the existence of a thin-skinned triangle zone encasing the Humber Arm Allochthon, and the thick-skinned and inverted nature of the RHT. The most recent well, Shoal Point K-39, was drilled in 1999 to test a structural and stratigraphic model in which the RHT loses reverse displacement to the NE, eventually becoming a normal fault. This model hinged on interpretation of a seismic reflection survey in Port au Port Bay, now in the public domain. The stratigraphic model invoked platform exposure and karsting in the uplifted footwalls of the thick-skinned faults, that were initiated as normal faults in the Taconian foreland, and were later inverted during Acadian orogenesis.

We present a new interpretation of the offshore seismic data in Port au Port Bay, in which the RHT dies or is terminated against another basement-involved and inverted feature, the Piccadilly Head Fault (PHF). This fault, interpreted also as reactivated during Acadian deformation, strikes NE but dips NW where it crosses Port au Port Peninsula. The present reverse offset on the PHF was previously interpreted as the SE-dipping RHT with a normal motion sense. Our interpretation is consistent with mapping on Port au Port Peninsula and north of Stephenville, where all basement-involved faults are inverted and display a reverse sense of motion. It also explains enigmatic seismic reflections overlying faulted platform reflections as the coarsely conglomeratic Cape Cormorant Formation, a unit known to be locally associated with inverted thick-skinned faults. The K-39 well, which targeted the footwall of the RHT, actually penetrated the hanging wall of the PHF. The apparent magnitude of structural inversion across the PHF suggests other possible plays to the east of K-39.