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From 1973 to 1978, the U.S. Geological Survey (USGS) procured approximately 20,000 km (12,400 mi) of 24- and 48-channel seismic reflection data along the U.S. Atlantic continental margin in anticipation of pending oil and gas exploration leasing activity (Figure 1). All of these data have been released to the public through the National Geophysical Data Center (NGDC) in Boulder, Colorado.

Interpretive geologic studies based on this regional multichannel seismic reflection grid have identified four major sedimentary basins lying beneath the Outer Continental Shelf, Slope, and Rise off the U.S. Atlantic coast: the Georges Bank Basin, Baltimore Canyon Trough, Carolina Trough, and the Blake Plateau Basin (Figure 2). These basins formed in response to rifting between North America and Africa during the Triassic and Early Jurassic and the initiation of sea-floor spreading during the Early or Middle Jurassic. The maximum thicknesses of postrift sedimentary rocks, (that is sediments deposited after the initiation of sea-floor spreading) are approximately 7, 13, 11, and 12 km (4.3,8, 6.8, and 7.4 mi) respectively from north to south. Prerift or synrift sediments up to 8 km (5 mi) thick can be seen within extensional grabens below a "breakup" or "postrift" unconformity on some profiles.

In this paper, the term "postrift sedimentary rocks" refers to sediments deposited after initiation of sea-floor spreading between North America and Africa. The term "synrift sediments" refers to sediments deposited in extensional grabens formed during continental rifting, but prior to the initiation of sea-floor spreading (during the Triassic through the Early to Middle Jurassic). The term "prerift sediments" refers to Paleozoic and older sediments that were deposited before the rifting between North America and Africa began. We use the term "postrift unconformity" to identify the break between undisturbed postrift sediments and the underlying faulted and tilted synrift sediments. Our "postrift unconformity" correlates with what Falvey (1974) has called the "breakup unconformity." However, we prefer the use of the term "postrift unconformity" because of the extremely time-transgressive age character of this unconformity, especially near the landward flanks of the basins.

Two 300-km-long (186-mi-long) profiles, USGS lines 32 and 25 (Figure 1), are displayed in both time and depth to illustrate the major sedimentary and structural features of the Carolina Trough and Baltimore Canyon Trough. Although most of the data display and discussion in this paper will concentrate on lines 25 and 32, shorter profiled over grabens landward of the Baltimore Canyon Trough, which are filled with prerift and/or synrift sediments, are shown in USGS line 9, 24, 28, and 35 (Figures 1, 12, 13, and 14).

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