A mechanical and thermal model for the evolution of the Williston Basin

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Abstract
The Williston Basin is a large, circular, intracratonic basin containing over 4 km of sediments ranging in age from Cambrian to Tertiary. Data on seven formations from over 1300 wells in Montana, South Dakota, and North Dakota were used to determine the shape of the basin over time. By assuming that the lithosphere behaves like an elastic plate on a fluid substrate, the shape of the basin could be closely matched using a point load. The match was improved by adding the equation of a plane to the equation for elastic deformation before fitting the model to the data. The planar adjustment allows for the effects of regional sedimentation or erosion, or eustatic sea level changes. The mechanical modeling indicates that the elastic strength of the lithosphere in this area has generally increased from the Ordovician to the present. The geometrical center of the basin migrated less than 20 km over 450 m.y. The direction and magnitude of tilt of the plane removed from the data is related to tectonic events in western North America. The increasing deflection of the basin over time can be explained by a simple thermal model (which would explain the thickening of the lithosphere) consisting of conductive cooling of a hot region in the lithosphere. Depths to formations in a single well differ from the subsidence predicted by the elastic model with regional tilt removed because of global sedimentation events and the development, in the Mesozoic, of a foreland thrust basin of which the Williston Basin became part. The mechanical and thermal models constrain possible initiating mechanisms for this basin. The thermal modeling and the absence of volcanics in the stratigraphic column rule out a shallow (crustal) heat source. Neither wrench faulting nor crustal stretching were likely to have been significant in the formation of the Williston Basin.

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