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Electrical accessory curves facilitate recognition of subsurface formations from electrical logs and more accurate physical correlation of rock units. Two types of electrical accessory curves, which show the variation in percentage of thickness of sandstone (SP), and low resistivity mudstone as a function of depth, have been used to examine Laramide deformation in the Powder River basin of Wyoming during deposition of the thick (8,000 ft), nonmarine Late Cretaceous and Tertiary deposits which filled the basin.
The Laramide orogeny did not start in this part of Wyoming in late Maestrichtian, during deposition of the Lance Formation, and was only weakly active in early Paleocene time, during deposition of the Tullock Formation. Strong deformation started in middle Paleocene with subsidence along the axis of the basin and deposition of fine-grained Lebo mudstones. Other Laramide structures associated with basin deformation probably were started at this time. Strong subsidence continued into late Paleocene when coarse clastics of the Tongue River Formation first were deposited, indicating uplift and erosion of the adjacent mountains. Deformation continued through part of the Eocene, but ceased before Oligocene time.
Cross sections using electrical accessory curves illustrate their use in other thick nonmarine rock sequences and for detailed studies of subtle marine shale correlations. Electrical accessory curves are a new tool to help solve difficult subsurface rock correlation problems and they should be helpful in many other areas.
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