The Cambro-Ordovician carbonate platform was formed in the Iapetus Ocean. Siliciclastic sediments of the early Cambrian Labrador Group underlie the carbonate setting but are considered to be part of the platform succession. Late Cambrian and early Ordovician carbonate sediments of the St. George and Port au Port groups formed a stable platform that seismic data suggests was fairly uniform in thickness across the margin. These groups are denoted in Fig. 3 as red and pink seismic peaks. Regionally, the carbonate platform thins slightly to the north and west.

Early normal faulting of the platform has been related to the opening of the Iapetus Ocean in late Cambrian time. This would involve the Labrador Group, but not the carbonate platform cover. However, a few faults exhibit limited growth at the Port au Port and St. George Groups levels, indicating some reactivation during the deposition of the carbonate platform. This would indicate a stable structural environment for the deposition of the carbonate platform. (Fig. 4).

Fig. 3 - Seismic Line 91-1563

Fig. 4 - Early Ordovician with normal faulting to the southeast.

The St. George unconformity is the result of a large drop in sea level coupled with a peripheral bulge migrating across the margin in response to loading on the outboard margin by thrusted slope sediments (allochthon). Normal faulting occurred in the platform at this time as evidenced in the sedimentation variations of the Table Head Group which is depicted by the yellow horizon in Fig. 6.

Fig. 5 - Cross Section - Initial loading on the outboard margin by thrusted slope sediments.

Fig. 6 - Seismic section Showing Allochthon

Throughout the deposition of the Table Head Group, thrusting of the platform slope facies continued, resulting in the development of a foreland basin setting. The thrusts carried the rich source rock of the Green Point Shale into the foreland basin, and additional source rocks were deposited as flysch sediments were shed from the allochthon (transported slice) to the east. Continued loading of these advancing thrust sheets was likely responsible for continued normal faulting and subsidence of the eastern margin of the platform. (Fig. 5 & 6)

By the mid-Ordovician, a stable shallow sea setting in the foreland basin resulted in the deposition of the Lourdes Limestone (strong seismic peak above blue marker, Fig. 6) over the entire area. This limestone appears from seismic data to be regionally consistent in thickness (~100 m.).

Fig 7 - Cross Section -Present day offshore western Newfoundland.

Seismic data suggests that the platform setting at Anticosti Island and Quebec, to the west, was separated from the western Newfoundland platform by a basement high. This would restrict the oil migration generated in the deep basin, just west of the Newfoundland coastline, to the Newfoundland offshore area since the tight limestone overlying basement would restrict further westward movement.

Throughout the early Devonian, sediments in the foreland basin overlying the Lourdes Limestone were deposited in a regionally consistent “layer-cake” model where these sediments are seen on offshore seismic data.

Fig. 8 - Generalized stratigraphic section of the Anticosti Basin

During the Acadian orogeny (Devonian), compression induced inversion of old normal faults. By the mid to late Carboniferous, this compressional was the dominant regional force and likely responsible for the continuation of fault inversion, pop-up structures in the platform and for imbricate stacking within the thrust sheets of the Humber Arm Allochthon. This ultimately created the triangle zone that is so obvious in the offshore seismic data of western Newfoundland. (Fig. 6 & 7)