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Fault With Reversalof Displacement, Central Montana

T.L. Plawman
T.L. Plawman
ARCO Exploration Company
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January 01, 1983


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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AAPG Studies in Geology

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
A. W. Bally
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American Association of Petroleum Geologists
ISBN electronic:
Publication date:
January 01, 1983




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