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Precambrian igneous and metamorphic rocks underlie the Powder River basin. Their distribution is uncertain because of lack of subsurface control.
For the purpose of this paper the geologic section is divided into eight rock units. Some of these rock units cross time boundaries between periods.
Rock Unit 1 includes Cambrian and Lower Ordovician sediments deposited by a sea that began transgressing eastward during Middle Cambrian time and regressed at the close of Lower Ordovician time. Following this regression all sediments in Rock Unit I were eroded from the extreme southeastern part of the Powder River basin.
Rock Unit 2 is bounded by the "St. Peters Break" of post-Lower Ordovician time at the base and the Upper Silurian unconformity at the top. The sea in which these beds were deposited transgressed southward from the Williston basin. Sediments of this unit thin southward due to Upper Silurian erosion.
Rock Unit 3 includes the predominantly carbonate rocks of Devonian and Mississippian age. Four unconformities can be mapped within this unit; one at the top of the Devonian, one between the Lower and Middle Mississippian, one between the Middle and Upper Mississippian and one at the top of the Mississippian. Widespread karst topography characterizes the upper surface of the unit at the top of the Mississippian.
Rock Unit 4 includes the Pennsylvanian and Lower Permian rocks. Lower Permian tectonics and erosion breached folds subsequently buried under Unit 5.
Rock Unit 5 includes the Permian and Triassic redbeds and associated carbonates and evaporites. Large-scale uplift and erosion prior to Jurassic formed an unconformity at the top of this unit.
Rock Unit 6 includes all Jurassic rocks and the transition sediments at the base of the Cretaceous.
Rock Unit 7 begins with the transgression in Early Cretaceous time and the deposition of a dominantly shale sequence subdivided by four economically important regressive cycles. The top of Rock Unit 7 is the top of the Cretaceous, a major and early pulse of the Laramide orogeny.
Rock Unit 8 includes all beds of Tertiary age. The present-day configuration of the Powder River basin and the major structures and fault systems were formed at this time (Laramide orogeny). Tertiary sedimentation was controlled by the present basin outline. Previous sedimentation was related to tectonic features of a much broader area than the Powder River basin.
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