R. Rae Teal, 1983. "The Triangle Zone at Cabin Creek, Alberta", 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 presented in this paper lies approximately 300 km (190 mi) west of Edmonton, in the Cabin Creek area, Alberta, Canada (Figure 2). Based on the excellent seismic data of this line and the well control provided by Shell Cabin Creek 4-9-55-3 W6M, a depth converted version of the line and a balanced structural cross section are constructed to show the geometry, at this location, of the transition between the disturbed belt of the Alberta Foothills and the undisturbed strata of the Alberta basin.
For the past 20 years, the term "Triangle Zone" was frequently applied in an informal fashion to the leading edge of the disturbed belt of the Southern Canadian Foothills. The name was originally derived from the triangular arrangement of surface dips and seismic reflections found in early seismic profiles (Figure 1). Subsequent seismic collection and drilling have shown that the arrangement of reflections arises from the cross-sectional shape of a large wedge of rock bounded by two planes of discontinuity: a lower one dipping to the southwest, and an upper one dipping to the northeast (Figure 5). These two planes are the expression of one and the same thrust (that is, they are the bounding surfaces of a northeastward facing wedge forced into the adjacent rock package to the northeast). The emplacement, or injection, of the wedge displaces the sediments above the wedge only in a vertical sense; in a lateral sense these sediments can be regarded as still autochthonous.
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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.