Eastern North American Margin Research Articles

Mesozoic carbonate platforms and banks of the eastern North American margin

The Jurassic—Lower Cretaceous carbonate platforms and banks form a discontinuous belt extending over distance of 6000 km from the Grand Banks up to the Bahamas. Six types of carbonate buildups are recognized and document the variability of depositional, paleo-oceanographic and tectonic processes on the eastern North American margin. The composition of the carbonates closely resembles the Recent deposits of the western Great Bahama Bank since oolitic shoals were present near to the shelf edge, and skeletal, peloid wackestones and biomicrites were deposited in the inner part of the platform. Coralstromatoporoid and sponge bioherms were only rare constituents of the carbonate banks. The thickness of carbonate buildups progressively increases southward along the margin, attaining a thickness of more than 5 km on the Bahamas. The carbonate platforms also become younger southwards, which is thought to reflect the northward movement of the North American plate of less than 1.5 cm/yr. The carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics. Building of carbonate ramps which characterized the Early Jurassic, has began during the transitional period between continental rifting and early drift. The second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates.

Mesozoic Carbonate Platforms and Banks of Eastern North American Margin: ABSTRACT

The Jurassic-Lower Cretaceous carbonate platforms and banks form a discontinuous belt extending from the Grand Banks to the Bahamas, a distance of over 6,000 km. Six types of carbonate buildups are recognized that document the variability of depositional, paleo-oceanographic, and tectonic processes on the eastern North American margin. The texture of the carbonates closely resembles recent deposits of the western Great Bahama Bank in that oolitic grainstones were present near the shelf edge, and skeletal, peloid wackestones and mudstones (biomicrites) were deposited in the inner part of the platform. Coral-stromatoporoid and sponge bioherms were rare constituents of the carbonate banks. The thickness of carbonate buildups progressively increases southward along the margin attaining a thickness of more than 5 km in the Bahamas. The platforms also become younger southward, which is thought to reflect the northward movement of less than 1.5 cm/year of the North American plate. The carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics. The building of carbonate ramps characteristic of the Early Jurassic began during a transitional period between continental rifting and early drift of the continental plates in the North Atlantic. The second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates. End_of_Article - Last_Page 941------------

Earthquakes along the passive margin of eastern Canada

An active seismic zone extends along the passive margin of eastern North America from Baffin Island to the Grand Banks of Newfoundland. We have determined the focal mechanisms of several of the earthquakes, including the 1933 Ms 7.3 Baffin Bay earthquake, the largest event ever recorded along the eastern North American margin. The mechanisms show thrust faulting for earthquakes seaward of the 1000 m contour, and primarily normal faulting for earthquakes landward. We propose that these earthquakes are induced by the removal of the Pleistocene glacial loads which extended onto the continental shelf. The deglaciation reactivated basement faults remaining from rifting associated with the opening of the Labrador Sea. Baffin Bay and the Atlantic. A simple flexure calculation yields horizontal extension (normal faulting) in the previously glaciated region, and horizontal compression (thrust faulting) farther seaward, in good general agreement with the observed earthquake mechanisms. The magnitude of the stresses, 100 to 150 bars, is sufficient to reactivate preexisting basement faults. Large passive margin earthquakes may occur as far south along the coast as glaciation extended.