T.L. Plawman, P.I. Hagar, 1983. "Impact Structure", 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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Probable impact structures or "astroblemes" have been reported from several areas, based on seismic and borehole data (Sawatzky, 1977). This seismic line shows one example of such a feature located in central Montana. The data is 24-fold CDP, with a dynamite source, and is displayed in AGC format.
Mapping indicates that the structure is roughly circular in plan view. It has a central uplift surrounded by a syncline. Avery slight uplift forms an outer rim around the structure. These features are similar to those reported from the Red Wing Creek structure in North Dakota which is a well documented probable astrobleme (Brenan, 1975).
An impact origin for this structure is of course difficult to prove. However there seems to be no plausible alternative hypothesis. Reflections below the structure are flat and continuous. This seems to rule out a tectonic origin. For the same reason it seems unlikely that the feature is the result of igneous activity. There are no significant evaporites in this part of the stratigraphic section, which eliminates solution collapse as a probable cause. The borehole data show no evidence of carbonate mounding.
Numerous normal and reverse faults can be interpreted around and above the structure. Some of these are indicated on the interpreted section. Those which do not extend above the Jurassic Rierdon are probably a direct result of the impact. Other faults which cut higher in the section are probably due to rebound after the impact, or differential compaction of sediments over the structure. The impact zone is interpreted on the basis of much weaker and less coherent reflections. Correct identification of brecciated zones is critical for hydrocarbon exploration in this type of structure.
The borehole data is subject to several interpretations. The impact may have occurred before deposition of the Jurassic Rierdon formation or possibly as late as Jurassic Morrison time. The Jurassic Piper Lime (or "Firemoon") seems to be thoroughly brecciated. Some deformation extends at least as far down as the Mississippian Charles. No shock metamorphism was detected in samples; however no cores were cut.
Following impact, deposition of sediments was affected by the resulting structure until after the Lower Cretaceous Lakota was deposited. On the seismic, the Lakota is between the Morrison and Dakota Silt reflectors.