98 NORTH AMERICA/Atlantic Margin Figure Three dimensional perspective of the Chesapeake Bay impact crater, which was formed at about 35 Ma on the North American Atlantic Continental Margin The inner and outer crater walls are shown superimposed on a map of the Virginia part of Chesapeake Bay (Reproduced from Poag CW, Koeberl C, and Reimold WU (2004) The Chesapeake Bay Crater: Geology and Geophysics of a Late Eocene Submarine Impact Structure New York: Springer Verlag.) variable amounts, and created numerous synrift basins; drifting, which created hot new oceanic crust initially adjacent to the rifted continent and now far away along the Mid-Atlantic Ridge; and passive subsidence, caused by cooling of the lithosphere and loading of the crust by sediments, which resulted in the development of large offshore sedimentary basins Superimposed on these first-order lithospheric events are various oceanographic, magmatic, fluvial, eustatic, sedimentary, diagenetic, and biological processes that have also affected the final shape of the margin The resulting structure of the margin is an alternating sequence of platforms, which have relatively little sedimentary cover, and basins, which contain enormous thicknesses of sediment (Figure 4) Mesozoic rifting created the exposed basins of the Newark Supergroup (Figure 4) These fault-bounded basins and/or half-grabens form a linear chain along the exposed part of the margin The basins contain deposits up to several kilometres thick Similar synrift basins have been detected seismically and drilled near the coast and beneath the continental shelf The bestknown of these offshore basins is the petroliferous Jeanne d’Arc basin on the Grand Banks, in which the thickness of sediment exceeds 14 km Synrift deposits are inferred to underlie the deepest portions of the margin, but have not yet been sampled Synrift basins are also known on the conjugate north-west African continental margin A prominent margin-wide unconformity separates the synrift basins from the overlying postrift sedimentary units and is called the postrift or the breakup unconformity (Figure 7) Consequently, sedimentary-thickness maps of the margin often include only those units above the postrift unconformity (i.e postrift deposits) The postrift unconformity may become a conformable surface within the deepest sediments in the centres of the postrift basins, but evidence for this is ambiguous, because seismic imaging in these deeper regions is often of poor quality and drilling data are lacking Beneath the Grand Banks, the postrift unconformity is termed the Avalon unconformity Above the postrift unconformity, five major sedimentary basins have been documented From north to south, they are the Scotian basin, Georges Bank basin,