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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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