High Relief Topography on the Jurassic Unconformity (Alabama)
Published:January 01, 1983
J.R. English, J.S. Oslund, 1983. "High Relief Topography on the Jurassic Unconformity (Alabama)", 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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Along the edge of the interior salt basin in the southeastern United States there is a high relief unconformity. This unconformity was developed during deposition of the Jurassic Louann Salt and was buried by Jurassic sediments during the subsequent marine transgression related to the divergence of the South and North American continents and subsidence of the basin. The morphology of the unconformity may be characteristic of early stages of continental plate divergence. A seismic line near the edge of the salt basin in south Alabama demonstrates the acoustic response along the unconformity.
The area described in this paper is located in the southeastern United States along the edge of the interior salt basin (Figure 1). The unconformity is Jurassic in age and can be identified from Florida to Texas. Because it is regionally extensive, it truncates a wide variety of older rocks. The major units it truncates are Paleozoic sediments of the Appalachian and Ouachita orogenic belts, metamorphic rocks associated with these orogenic events, and Mesozoic continental sediments of the Neward group which were deposited during the early stages of divergence. There were four Jurassic units younger than the unconformity in the area shown on the cross section (Figure 2) and interpreted record section (Figure 3). These are the Norphlet, Smackover, Haynesville, and Cotton Valley. In Alabama, the Norphlet is a shallow marine and eolian sandstone; the Smackover is a shallow marine carbonate; the Haynesviller is a prograding sabkha and continental sequence; and the Cotton alley is a continental sandstone sequence. There are unconformities at top of Haynesville and Cotton Valley which modified the topography of the older unconformity where is was not buried. The seismic section (Figures 3, 4, 5) is located in south Alabama just north of the updip edge of the salt. It is a 24-fold, common-depth-point (CDP) dynamite line shot in 1980. The line parallels regional dip and is 5 mi (8 km) in length. Well control indicates that the rocks beneath the unconformity are granulites and mica-rich metamorphic rocks. The area is interpreted to be the subsurface equivalent to the Brevard zone of the Appalachians.
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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.