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Basement Extension and Salt Mobility, Southern Grand Banks, Newfoundland

Hugh R. Balkwill, Franklyn Legall

The Grand Banks, an especially wide cratonic segment of the North American Atlantic continental shelf, extended vigorously during Late Triassic-Aptian rift-phase episodes, accompanied by syntectonic basin filling and large-scale structural disruption. Thereafter, the rift-disrupted domain underwent drift-phase subsidence and was buried by a seaward-prograding continental terrace wedge, in which progressively feeble extension is evident.

Rift-faulted cratonic basement is perceptible on industry-acquired reflection seismic profiles from the southern Grand Banks. The profiles also show that Carboniferous and Lower Jurassic salt were the main levels of supracrustal detachment during Jurassic and Early Cretaceous extension. Large salt-mobilized structures within the rift-phase succession include fault-zone sheaths, elongate pillows, aligned piercement spires, and immense walls. These elements parallel large extension faults in basement and, in many places, are superposed on the faults. In striking contrast, upper Aptian and younger drift-phase strata are regionally sub-horizontal, and are broken to middle and late Tertiary stratigraphic levels by only a few small extension faults and aligned diapirs.

We interpret the structural/stratigraphic relationships in the southern Grand Banks to indicate that episodic Mesozoic and Cenozoic basement extension was the principal dynamic agent in determining the timing of salt structures, their orientations, and styles of disruption on enclosing strata. This genetic association may be applicable to other parts of the Grand Banks tectonic province, and possibly to other extensional cratonic margin basins.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.