TACONIAN FORELAND BASINS IN THE GULF OF ST. LAWRENCE REGION

The Appalachian foreland basin offshore of western Newfoundland contains up to 5 km of Middle Ordovician to Lower Devonian sedimentary strata and covers over 50,000 km². Numerous seismic profiles allow correlation within the basin, but tie-points to onshore stratigraphy, and to well control, are scarce. Corresponding on-land successions are highly deformed in the Humber Zone of Newfoundland.

Initial Taconian deformation, interpreted to represent collision of an offshore Dashwoods microcontinent with an encroaching arc system, left little record on the margin of Laurentia. Subsequent closure of the intervening Humber Seaway led to margin subsidence and the diachronous arrival of easterly-sourced clastics above the former passive margin. Seismic images show that the margin was cut by numerous normal faults active at this time, interpreted to represent flexural extension of the lithosphere as it entered the Taconian subduction zone. Major normal faults, subsequently inverted as the Round Head and Parsons Pond thrusts) are exposed on Port au Port Peninsula and north of Bonne Bay. Their large throw (several km), unusual orientation, and the presence of thick rift-phase sediments in their hanging walls suggest an origin as reactivated basement structures associated with the St. Lawrence promontory.

Except in localized graben, the overlying clastic fill of the Middle Ordovician foreland basin (Goose Tickle Group) is thin, suggesting that Taconian Allochthons represented a small load on the Laurentian lithosphere, or were still located well to the east of their current position. Traced to the west beneath the Gulf, this basin fill is locally truncated unconformably at the base of the Late Ordovician Lourdes Limestone, a prominent reflector. An overlying clastic succession (Winterhouse and Misty Point formations) is much thicker than the Goose Tickle Group. Standard tectonic scenarios for the evolution of the Newfoundland Appalachians do not offer a straightforward explanation for this second phase of Taconian subsidence. It may have been driven by distributed thickening within the orogen (suggested by Ar-Ar dates on fabrics within the Humber Zone) or possibly by Late Ordovician thrusting on the transform segment of the Laurentian margin now hidden beneath the Gulf of St. Lawrence.