APPALACHIAN FORELAND BASINS BENEATH THE GULF OF ST. LAWRENCE

The northernmost Appalachian foreland basins are largely concealed beneath the Gulf of St. Lawrence. Seismic reflection data, gathered off the west coast of Newfoundland, give subsurface information required for interpretation of structures. Ties to onshore stratigraphy and wells are scarce, and corresponding on-land successions are highly deformed in the Humber Zone of Newfoundland. Reflections can be tied to prominent bathymetric features and magnetic anomalies within the Long Point Group. This allows identification of several significant geologic boundaries with reasonable confidence.

Strata of the foreland basin were deposited during time intervals correlative with events in Appalachian development. The St. George Unconformity marks the base of the foreland basin and onset of Taconian deformation. In seismic data this boundary is interpreted as a high amplitude peak that demonstrates clear angular discordance with underlying reflectors. The overlying Middle Ordovician deep-water siliciclastic Goose Tickle Group is associated with tectonic loading during Taconian orogenesis and, offshore of Port au Port, is relatively thin (~200 m).

The Late Ordovician Lourdes Limestone, a prominent reflector, locally truncates this basin fill and regionally marks the base of the Long Point Group. The varying position of this reflector, relative to the Tea Cove Thrust at the thrust front of the orogen, may be due to ramping of the thrust from south to north, or due to stratigraphic onlap of the Long Point Group onto underlying units. A relatively thicker clastic succession (Winterhouse and Misty Point Formations) overlies the Lourdes Limestone, and standard tectonic scenarios do not offer a straightforward explanation for this second phase of Ordovician subsidence. This package shows pronounced southward thickening, suggesting that subsidence may be a result of tectonic loading in the Quebec segment of the orogen, along strike to the south.

A Salinian unconformity is recognized due to the gap in the sedimentary record on land. Younger strata rest with only subtle discordance on the underlying Long Point Group suggesting Salinian deformation of the former Laurentian margin was not intense. However, a thick late Silurian - Devonian succession records the development of a substantial Acadian foreland basin.