Tectonostratigraphic evolution of the Williston Basin
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In the Williston Basin five regional seismic profiles, covering $\sim$3090 km were utilized for a comprehensive study of this complex geologic feature. 2300 km field data were added to the existing 790 km profile. The novel seismic information in conjunction with a sizeable number of wireline data and incorporation of structural and isopach maps provided a unique data environment for development of a new elaborate tectonostratigraphic model of this major continental depression. Standard reflection seismic processing procedures were implemented with special emphasis on regional perspectives, including "Earth curvature correction", to generate images of the basin fill. The latter helped to reveal the true nature of this large scale cratonic basin. This novel information permitted new approaches in establishing the deformation styles in the Williston Basin. Structural studies of the newly reprocessed regional seismic profiles revealed the compressional nature of the radially arranged tectonic elements in the center of the basin, and the extensional character of the peripheral regions. The results suggest that axisymmetric deformation controlled the early stages of the Williston Basin area, and was the causal factor of the oval shape of the basin. In the first, "pre-Williston" phase, the region was uplifted by an axisymmetric lithospheric intrusion creating radial extensional signatures in the central zone and compressional structures in the surroundings. Erosion and thermal cooling and/or phase change of the mantle material led to the initiation of the basin subsidence. Consequently, in the "intracratonic phase" (Sauk - Absaroka), the pre-existing radial and circumferentially arranged structures were periodically reactivated in the opposite sense. The active periods were unrelated to global orogenic events of the continent. The exception is the Kaskaskia I (Devonian) interval, when the territory was tilted to the northwest and the axisymmetric cause of the subsidence was overprinted. The subsequent "foreland phase" (Zuni - Tejas), was dominated by lateral forces of the Sevier and Laramide orogenies. This plate-margin-related major tectonic development was associated with the NNW-SSE elliptical elongation of the basin and the related highly prevalent NE-SW/NW-SE faulting and fracturing. Additional consequences of this process were offsetting and rotation of the pre-existing radial and circumferential structural features. These radial and circumferential structural features of the Williston Basin may be recognizable in comparable cratonic environments (e.g., Michigan Basin, Paris Basin). Comprehensive seismic/sequence stratigraphiy was developed throughout the basin. In the Sauk - Absaroka interval the sequence stratigraphic and the lithostratigraphic boundaries are generally identical. In the Zuni - Tejas interval, when the clastic sedimentation was dominant, the two subdivisions are not identical. In these younger strata 16 sequence stratigraphic units were identified. More detailed subdivision of the interval containing the Eagle Sandstone revealed that two major sources of the terrigenous sediments are directly recognizable on the seismic profiles, beyond 500 km east of the shorelines.