THE APPALACHIAN FORELAND BASIN BENEATH THE GULF OF ST. LAWRENCE

The Appalachian thrust front offshore of western Newfoundland is marked by a tectonic wedge or triangle zone inserted into a Middle Ordovician to Lower Devonian foreland basin succession. The basin contains up to 3 km of Paleozoic sedimentary strata and covers over 50000 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 deformed in the Humber Zone of Newfoundland.

The initial foreland basin fill consists of shallow-marine carbonates (Table Point Formation). The transition to overlying clastics (Goose Tickle Group) produces a widely correlated reflection, tied to the Port au Port #1 well. This surface is offset by numerous faults, recording Taconian flexural extension of the Laurentian margin.

A second marker reflector, traced to surface at Long Ledge, is identified as the basal limestone of the Caradocian Long Point Group. Locally, the base of the Long Point Group is unconformable, and the Goose Tickle Group is missing. The bulk of the Long Point Group consists of over 1 km of shallow-marine and fluvial clastic sedimentary rocks, known in incomplete on-land exposure. Within this succession, two paleoplacer concentrations of detrital magnetite produce pronounced magnetic anomalies, correlative with the Odd-twins anomalies offshore, providing two more tie-points in the seismic data.

A bright reflector above the upper magnetic horizon probably represents the base of the Clam Bank Formation (Ludlovian-Pridolian). The lower part of the Silurian is missing at a disconformity corresponding to the Salinian orogeny. Upper parts of the foreland basin succession, well imaged in sea-floor swath bathymetry, represent the Acadian foreland basin. Local unconformable cover of Mississippian age post-dates the Acadian orogeny.

Taconian and Acadian clastic wedges, and overlying unconformities, are clearly identifiable in the foreland basin. However, a pronounced Late Ordovician subsidence of the basin corresponds to no previously described major event within the orogen. The Early Silurian Salinian orogeny is marked by a disconformity without significant evidence for tilting or thrusting, suggesting that it may correspond to an episode of largely vertical movement.