S. Jaunich, 1983. "The South-Western African Continental Margin—Part I", 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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Distinctive lithotectonic units are recognizable on the south-western African continental margin, namely the prerift, rift-valley, and drift sequences. Four seismic horizons (T, R, P and L) are mapped throughout the area and they mark unconformities which range in age from Mesozoic to Tertiary.
The T-to-R interval is regarded as the rift-valley sequence, the rifting stage being a major component in the evolution of the west African continental margin.
Horizon T, the rift-onset unconformity, forms the base of the Jurassic/Cretaceous succession and where present is considered the top of prerift rocks of diverse age and lithologies. A great number of grabens and half-grabens varying in length and width have been recognized. These trend parallel and subparallel to the present coastline. These are evident on parts 1 to 4 of the composite seismic profile across the south-western African continental margin. Horizon R is an erosional surface generally regarded as the drift-onset unconformity. Where horizon T is not present, horizon R marks the base of the Lower Cretaceous sediments and the top of the prerift rocks. Further to the west, approximately 100 km (62 mi) from the coastline, the T-to-R interval thickens rapidly in a westerly direction. Horizon T descends steeply and soon becomes unrecognizable as a distinct reflector. The eastern pinchout point of this T-to-R wedge is subparallel to the present coastline (illustrated on profile 4 of composite line). Both the eastern grabens and the T-to-R wedge further offshore are related to the "breaking apart" of the continents and therefore are significant as tectonic features.
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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.