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AAPG Bulletin

Abstract


Volume: 51 (1967)

Issue: 10. (October)

First Page: 1929

Last Page: 1947

Title: Salt Solution and Subsidence Structures, Wyoming, North Dakota, and Montana

Author(s): John M. Parker (2)

Abstract:

Salt beds with a thickness range of a few feet to 600 ft are common in rocks of Middle Devonian and Permian ages in the northern Rocky Mountain area. Variations in thickness of these salt beds in short horizontal distances require consideration of the hypothesis that the salt-thickness changes are due to post-depositional salt removal by solution and not to original variations in the thickness of salt deposited. Thin, regionally persistent sedimentary units directly above and below the salts prove that, locally, there was little depositional change of salt thickness. Therefore, within the regions of salt deposition, the salt must have been removed after deposition in the present thin or zero salt areas.

Local removal of Middle Devonian salt (Prairie Formation) in the Williston basin occurred during later Devonian time and continued through the time of deposition of the Mississippian Lodgepole and Tilston Formations. Local removal of Permian salt (Minnelusa Formation, Opeche Shale, and Goose Egg Formation) in both the Williston and Powder River basins took place near the close of the Jurassic. Compensating deposition took place in the collapse depressions created by underlying salt removal, thus dating the age of solution.

In the examples of salt removal described in this paper, a few to several hundred feet of sediment was deposited over the salt before salt removal took place. It is postulated that salt removal was accomplished by the formation of local and regional fracture systems which allowed ascending water to escape from regional aquifers below the salt beds, and that this water dissolved the salt and carried it in solution to the ocean floor.

Oil accumulations in some places are related to the formation of local subsidence structures by salt removal. Geologists have postulated the presence of regional lineaments along and above which salt has been removed. Locally, the salt-removal areas may be irregular in size, shape, and distribution, although, as a whole, these areas may occur within a regional band.

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