T.L. Plawman, 1983. "Fault With Reversalof Displacement, Central Montana", 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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Several major faults in the Central Montana Platform area are known to have had reversals of displacement during a complicated structural and depositional history. This seismic line across the Sand Creek Fault shows a particularly good example of such reversal. The data is 24-fold CDP, with dynamite as the source. The line is shown in AGC format, with northwest to the right. The identification of the Charles and higher reflections are fairly certain based on nearby well control. The Flathead reflection is somewhat more questionable due to a lack of deep Bell control in the immediate area.
Prior to the Mississippian, the sense of movement on the Sand Creek Fault was down to the north. This is evidenced by the thinner interval between the Mississippian Charles and Cambrian Flathead on the south (left) side of the seismic section. This movement probably occurred during the Ordovician or Devonian. The Mississippian sediments were deposited on this beveled Devonian surface. During the Pennsylvanian the movement on the fault reversed, the south side becoming the downthrown block. The Pennsylvanian and some older rocks were eroded off on the north side. This is clearly indicated on the seismic by the thicker interval between the Jurassic Piper Lime (or "Firemoon") and the Mississippian Charles on the south (left) side of the section.
Deposition was nearly continuous until the Upper Cretaceous Eagle was deposited. A minor uplift occurred during the deposition of the Jurassic Morrison, but does not seem to be associated with any significant movement on the faults in this area. The fault was reactivated during the Laramide Orogeny (latest Cretaceous to early Tertiary). Once again the sense of movement was reversed, the northern side being downthrown. On the seismic line all reflectors above the Piper are down on the north (right) side. The Laramide movement was insufficient to completely remove the Pennsylvanian (up to the north) displacement. Therefore the Charles and Flathead are still higher on the north (right) side of the seismic line. For a more detailed discussion of the geologic history of the central Montana area the reader is referred to Norwood (1965).
Above the Lower Cretaceous Dakota Silt the fault appears to bifurcate. As noted above, the Eagle shows the overall down-to-the-north Laramide movement. However, on the right hand branch of the fault the Eagle has a small, local, down-to-the-south displacement. This may be due to a slight clockwise rotation of the small block between the faults. An alternative interpretation is that it may be the result of strike slip motion along the Sand Creek Fault. An unknown amount of left lateral strike slip motion has been postulated for the east to west trending faults of central Montana (Smith, 1965). The later interpretation is supported by structure maps on the Eagle which show a complex pattern of en-echelon faults along the Sand Creek Fault, rather than the single linear fault present at deeper levels.
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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.