J.A. Austin, Jr., 1983. "OBC 5-A: Overthrusting in a Deep-Water Carbonate Terrane", 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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The geology of the Bahamas has been studied for decades. We know that steady subsidence since the Jurassic and the associated continuous upbuilding of carbonate banks have resulted in a thick (more than 5 km; 3.1 mi) sequence of shallow-water limestones and evaporites overlying postulated rift basins filled with terrigenous clastics and volcanics (Dietz et al, 1970; Meyerhoff and Hatten, 1974; Sheridan, 1974; Tator and Hatfield, 1975; Case and Holcombe, 1980; Sheridan et al, 1981). However, the complete sedimentary section was never sampled and seismic reflection profiles collected over the banks are characterized by poor penetration and persistent ringing caused by competent shallow sediments.
Consequently, a continuing controversy exists as to whether the Bahamas are underlain by continental or oceanic crust. Based on a variety of evidence, Uchupi et al (1971) hypothesized that the northwestern Bahamas are an extension of the North American continent, while the southeastern Bahamas rests on oceanic crust with structural trends controlled by Atlantic fracture zones. In contrast, Mullins and Lynts (1977) postulated that the Bahamas are completely underlain by rifted continental crust which was originally part of Africa but became attached to the North American plate when the modern Atlantic began to develop. Gravity surveys in the region do suggest the presence of fairly low-density (continental) rocks (Talwani et al, 1960), and magnetics data also support a continental origin in that anomaly trends are continuous with those of peninsular Florida (Klitgord et al, in press). However, major difficulties arise in computing accurate layer thicknesses and velocities for the Bahamas because of the similar acoustic properties of basement rocks and well-lithified flat-bedded carbonates. For example, Ball et al (1971) used seismic refraction techniques to estimate an average crustal thickness of 23.5 km (14.6 mi) for the Bahamas, whereas Rayleigh wave dispersion studies suggest an oceanic-type crust perhaps only 14 km (8.7 mi) thick (Sheridan, 1972).
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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.