BURIED EARLY MESOZOIC RIFT BASINS OF THE US MID-ATLANTIC CONTINENTAL MARGIN

A 1992 map of the exposed and buried rift basins of the US Middle Atlantic continental margin was compiled from existing publications, and, for the onshore and offshore buried basins, interpretations of borehole data, geophysical maps, and seismic reflection profiles. Synrift rocks of the buried basins are assumed to be similar to those of the exposed basins—continental and lacustrine sands and mudrocks with interlayered volcanic rocks. Near the seaward edge of the continental margin, synrift and prerift basement rocks are onlapped by seaward-dipping seismic reflectors representing the wedge of volcanic and sedimentary rocks marking the final breakup of Pangea during the Early Jurassic (~180-200 ma). The post-rift unconformity separates all these rocks from the overlying postrift sedimentary section which accumulated during continental margin subsidence as continental drift proceeded. Published interpretations of ages of the earliest postrift rocks range from Early Jurassic (Sinemurian, 192-197 ma) to Middle Jurassic (Bajocian, 166-174 ma) for areas offshore the Canadian maritimes and New England. In the area seaward of the Baltimore Canyon trough, deep-sea acoustic horizon J3, estimated as Toarcian-Aalenian age (174-184 ma), is present over the upper continental rise and is traced landward to the base of the postrift-rock seismic reflectors.

Results of exploratory drilling on the United States Outer Continental Shelf yielded no commercial hydrocarbon discoveries in the postrift sedimentary section. Synrift rocks are too deeply buried for drilling in far offshore areas, but in nearshore areas drilling could reach these rocks. As in the exposed basins, buried basins probably contain lacustrine hydrocarbon source beds, probably type II or oil-prone as well as type III gas- prone beds perhaps associated with coal deposits. Hydrocarbon-generation models using Lopatin’s method indicate that for the Norfolk rift basin just offshore Virginia buried by about 3000 m of postrift rocks, the upper 800-1000 m of synrift rocks may still be in the oil window. Near Delaware, however, the Fenwick basin is buried by about 6000 m of postrift rocks, and here the model predicts that any source beds be they oil- or gas-prone would be in the dry gas window