P.L. Inderwiesen, 1983. "The Stuart City Trend—South 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 Stuart City Reef is located in LaSalle County, Texas (Figure 1). The line is oriented in a north-to-south direction and is approximately 9.8 mi (15.8 km) long.
Summarizing the introduction in Bebout and Loucks (1974), the Stuart City Trend represents a narrow, linear zone of biogenic carbonate buildup which occurred on the margin of a broad shelf in South Texas during the Lower Cretaceous time (Figure 1). The shelf-margin sediments making up the Stuart City Trend average less than 2 mi (3.2 km) in width and separate the open-marine environment of the ancestral Gulf of Mexico to the east from a 400-mi-wide (644-km-wide) carbonate shelf to the west. The carbonate shelf is divided into several paleogeographic regions discussed in detail by Fisher and Rodda (1969). The abundance of marine organisms over the shelf area is attributed to the warm-water environment resulting from shallow-water depths of a few to 200 ft (61 m). On a larger scale this broad shelf and carbonate accumulation are found to encircle most of the Gulf of Mexico.
Bebout and Loucks (1974) have classified five major depositional environments across the Stuart City Trend based on facies relationships found from core samples. These environments, from east to west, are:
(1) an open-marine environment consisting of planktonic foraminifer wackestone accumulated in water greater than 60 ft (18.3 m) deep;
(2) a lower shelf-slope environment consisting of intraclast grainstone, echinoid packstone, and echinoid-mollusk wackestone accumulated in water 30 to 60 ft (9.1 to 18.3 m) deep;
(3) an upper shelf-slope environment consisting of localized patch reefs composed of coralstromatoporoid boundstone which are isolated from the shelf-margin environment by caprinidcoral wackestone. Water is 10 to 30 ft (3.0 to 9.1 m) deep;
(4) a shelf-margin environment consisting of (a) reed and banks composed of requienid and coralcaprinid boundstones formed in water 5 to 15 ft (1.5 to 4.6 m) deep, (b)various sand bodies including beaches, tidal bars, spits, and channel fill composed of rudist grainstone formed in water depths less than 10 ft (3.0 m) in depth, and (c) stable grain flat composed of algae-encrusted miliolid-coral caprinid packstone formed in water 1 to 5 ft (0.3 to 1.5 m) deep; and
(5) a shallow-water shelf-lagoon environment consisting of miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone formed in water less than 20 ft (6.1 m) deep.
The Cretaceous stratigraphic section of the Stuart City Trend is given in Figure 2. The overall regional dip is toward the Gulf of Mexico in response to basinal subsidence. The Stuart City formation corresponds to the shelf-margin environment discussed previously and has a total sediment accumulation of 2,000 to 2,500 ft (609.6 to 762.0 m) (Bebout and Loucks, 1974). Also, the Stuart City carbonate facies is progradational and is terminated by deep-water shelf carbonates deposited in response to a major transgression due to subsidence which lasted through Late Cretaceous (Bebout and Loucks, 1974).
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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.