Representative Multichannel Seismic Profiles Over the U.S. Atlantic Margin
J.A. Grow, D.R. Hutchinson, K.D. Klitgord, W.P. Dillon, J.S. Schlee, 1983. "Representative Multichannel Seismic Profiles Over the U.S. Atlantic Margin", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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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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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.