Structural Evolution

While the carbonate platform was likely deposited in a stable environment through the late Cambrian and early Ordovician, there is evidence on seismic data to suggest that limited movement occurred on older faults as the platform developed. With the closing of the Iapetus Ocean during the Taconic orogeny, thrust sheets of the slope and distal facies were emplaced on the platform, creating a foreland basin. Loading of the allochthon along the platform’s eastern margin caused a peripheral bulge to migrate across the platform, causing normal faulting and reactivation of the earlier faults (Fig. 2-1).

Fig. 2-1 - Initial Loading on the Outboard Margin by Thrusted Slope Sediments.

The peripheral bulge also created localized highs where the Aguathuna Formation was preferentially dolomitized. These local highs were also likely locations for karst development within the upper sections of the St. George Group.

By the mid-Ordovician, the foreland basin had been filled by tight limestones, conglomerates and shales of the Table Point Group and thick shales (with some localized sandstones) of the Goose Tickle Group. These sediments were overlain by ~100 m. of the Lourdes Limestone in a stable shallow water environment (Fig 2-2).

The Lourdes Limestone (blue) also directly overlies the leading edge of the allochthon that had moved into the foreland basin. This leading edge appears to have been eroded prior to the deposition of the Lourdes Limestone. This presents an area where the near slope allochthon may have been extensively dolomitized and karstified. By the end of the Devonian, the siliciclastics of the Long Point and Clam Bank Groups had filled the foreland basin.

Fig. 2-2 - Present Day Schematic of Western Newfoundland

Fig. 2-3 - Seismic Line – Major Groups and Structural Features.

The Acadian orogeny of the mid-Devonian started to invert the earlier normal faults of the platform along its eastern margin resulting in the initial stages of “raising” the late Ordovician – Devonian sediments by inverting the “triangle” of allochthon that lay below them. Continued deformation into the late Devonian (or early Carboniferous) generated the imbricate stacking within the allochthon. This stacking combined with fault inversion of the basement blocks created the “Triangle Zone,” as seen in the offshore seismic data, where the Long Point and Clam Bank Groups were raised by the Tea Cove Thrust at the base of the Lourdes Limestone.