ABSTRACT

The regional structure and stratigraphy of the entire Williston Basin remain a focus of research at the University of Saskatchewan. Prior work in the Canadian part of the basin, on 620 mi (1000 km) of seismic data, established a basin scale stratigraphic interpretation and revealed fluctuating subsidence rates. The study continues with a regional profile in the U.S. segment of the basin.

This combined regional seismic interpretation across the Williston Basin illustrates how regional profiles allow better analysis of tectonic and stratigraphic characteristics of the basin than borehole information alone. Regional seismic sections, on this basin-wide scale, require a display that takes the curvature of the earth into account. This display shows the basement with a shorter sediment/basement contact (relative to the surface) in contrast to conventional cross sections.

The data set used for this regional profile is the reprocessed sedimentary sections of the COCORP Montana and North Dakota lines. Not only do the results of the U.S. regional profile support the earlier findings of the Canadian regional profile, but they also provide a better perspective of the regional interrelationships of the large geologic features such as folds and faults. It is clearly evident that the Williston Basin did not subside continuously but with intermittent compressive stages that led to basin scale folding. The folding was greatest in the middle of the basin and decreased toward the flanks. The structural and stratigraphical patterns are influenced primarily by shortening of the basement/sediment contact during subsidence (vertically driven stresses) and, to some extent by the tectonism at the plate margins (horizontally driven stresses).