P.L. Inderwiesen, 1983. "“Serpentine” Plug—Texas", 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 seismic line depicting the serpentine plug is located in Dimmit and Zavala counties, Texas (Figure 1). The orientation of the seismic line is in a northeast-southwest direction and is approximately 4.9 mi (7.9 km) long.
The name "serpentine plug" is a misnomer given to submarine volcanoes which formed on the Austin sea floor during the Late Cretaceous Period. Simmons (1967) attributed the name to the findings from the first discovery of such an igneous body bearing hydrocarbons near Thrall, Texas, in 1915 (Figure 1). Petrographic analyses showed the igneous rock to be of volcanic origin with serpentine the predominant mineral, formed by alteration of the original mafic rock (Udden and Bybee, 1916). Development of the field showed that the igneous rock was confined to Upper Cretaceous sediments leading some geologists to erroneously call it an igneous plug. Thus, the name "serpentine plug" was coined and is still used today.
Lonsdale (1927), in a study of both surface and subsurface serpentine plug samples, stated that their occurrence is genetically related to the Balcones Fault Zone. Sandlin (1980) suggested that the serpentine plugs associated with the Balcones Fault Zone represent a "volcanic province associated with a tensional zone." Roy et al (1981) mentioned that the tensional zone and resultant volcanic province are probably related to the opening of the Gulf of Mexico.
Sandlin (1980) pointed out the geographical relationship of serpentine plugs and the Balcones Fault Zone (Figure 1). The zone in which the serpentine plugs and Balcones faults coexist is approximately 200 mi (322 km) long trending northeast-to-southwest, and is 10 to 40 mi (16 to 64 km) wide. The serpentine plug outcrops occur in the same zone as the exposed Balcones faults, and fields associated with subsurface serpentine plugs (which are downdip or to the south of the outcrops) occur in the inferred zone of buried Balcones faults. For unknown reasons, the volcanic activity seems to be confined to the ends of the Balcones Fault Zone shown in Figure 1.
Summarizing Sandlin (1980), the Balcones Fault Zone consists of normal, en echelon faults which are downthrown mostly on the Gulf Coast side or to the southeast. The faulting is thought to have occurred in Late Cretaceous with a second minor movement in the Eocene (Collingwood and Rettger, 1926; Collingwood, 1930). The faults and fractures associated with the Balcones Fault Zone are thought to be the avenues by which magma makes its way to the Austin surface (Sellards, 1932). The reader is referred to Caran et al (1981) for an in-depth discussion of the Balcones structural trend.
Regional stratigraphy of the Upper Cretaceous is shown in Figure 2. The overall regional dip is in the direction of the Gulf Coast Geosyncline. Lewis (1977) stated that the volcanic activity occurred in the section from post-Austin time through Olmos. The serpentine plugs are found to outcrop with the early Taylor formations and attain depths of about 5,000 ft (1,524 m) below sea level on the southeasterly or downdip limit of the Balcones Fault Zone (Simmons, 1967).
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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.