Chapter 14 The Atlantic Margin Basins of North America

Abstract

Abstract The Atlantic margin of North America represents the classic “Atlantic-type” continental margin, notably the margin off the east coast of the United States, which was the site of five deep offshore stratigraphic test holes wells (the Continental Offshore Stratigraphic Test, or COST series) drilled in 1976–1979 on the Georges Bank, the Baltimore Canyon Trough, and the Southeast Georgia Embayment. Data from these holes were used in the development of what have become standard backstripping methods and subsidence models for extensional continental margins. Development of the margin began with the initial rifting of Pangea in the Triassic. Sea-floor spreading began in the central Atlantic Ocean during the early Middle Jurassic, and extended northward past Newfoundland beginning in the Late Jurassic. Active sea-floor spreading generated the Labrador Sea and Baffin Bay between the Cenomanian and the end of the Oligocene. Structural styles vary along the Atlantic margin. Off the southern U.S. margin, the edge of the continental margin was affected by magmatic underplating and extensive volcanism during the Jurassic. The Newfoundland continental margin developed by the processes of crustal thinning and crustal detachment. The Grand Banks area was affected by two distinct phases of rifting and flexural subsidence as extension occurred in the central Atlantic, to the south, from Late Triassic to Early Jurassic, and in the North Atlantic, to the northeast of the bank, from Late Jurassic to Mid-Cretaceous. The thickness of Jurassic-Recent sedimentary deposits on the continental margin locally reaches 25 km. Transects across the margin show a series of largely non-marine rift basins, capped by a breakup unconformity, above which is a seaward-thickening wedge of prograding shallow- to deep-water marine deposits. Evaporites are widespread at the base of this section from the Grand Banks to the Bahamas. Carbonates dominate the remaining deposits in the south, notably in the Bahamas area, but as the North American continent drifted northwestward through the Mesozoic, carbonate sedimentation gradually became less important in more northerly parts of the continental margin. On Georges Bank, carbonate sedimentation ended in the Mid-Cretaceous, whereas on the Grand Banks it had essentially come to an end by the close of the Jurassic. Shallow-marine and deltaic clastics comprise much the remaining succession throughout the length of the Atlantic margin. The discovery of major petroleum resources beneath the Grand Banks in 1979 led to extensive seismic and offshore exploration work there, and additional oil and gas resources have been discovered and developed. Gas reserves have been developed off Nova Scotia, and undeveloped gas reserves are located on the Labrador shelf, but no commercial discoveries have been made in the U.S. portion of the Atlantic margin.