Tectonic subsidence analysis of the Williston Basin

Abstract

The Williston Basin is one of a number of North American intracratonic basins whose subsidence is poorly understood. The Basin is circular in shape and contains nearly 5 km of preserved sedimentary rocks at its center; ranging from Cambrian to Tertiary in age. The Basin's subsidence is investigated quantitatively using the backstripping method in order to gain a better understanding of its origin.

Data from 24 oil exploration wells located in central and northwest portions of the basin were correlated and used in the study. A computer program called UNPAK was developed to perform the backstripping analysis. Tectonic subsidence curves were calculated to evaluate episodic subsidence models and extrapolated across unconformities to evaluate continuous subsidence models. Extrapolated tectonic subsidence curves near the center of the basin display a relatively constant subsidence through the Paleozoic, at a rate of slightly less than 1 cm ka-1, and then decrease to the present-day.

Improbable behavior of subsidence curves was used to constrain the magnitude of the eustatic models used in the backstripping calculations. A Cambro-Ordovician first-order rise of between 100 and 160 m, and a Cretaceous first-order rise of approximately 150 m above present-day sea level are consistent with the stratigraphic record in the Basin. No bathymetric corrections were used in the backstripping calculations, however, results from this study indicate water depth during deposition of the Bakken Formation was probably in the range of 50 to 100m.

Calculated point-loads necessary to account for the deflection of the Basin (using an elastic plate model) ranged from 2.5 x 1018 N to 2.9 x 1018 N, for an episodic model. If a continuous subsidence model was invoked, then a load of between 5.3 x 1018 N and 6.5 x 1018 N is required to account for the deflection of the Basin.

Of the previously proposed models to account for the basin's subsidence, continuous subsidence models were favored. A phase-change model, involving progressive metamorphic reactions of gabbro to eclogite, seemed the most likely of these models. Evidence in favor of the continuous subsidence models includes: an excellent fit of the extrapolated tectonic subsidence data to a smooth curve (implying a single subsidence mechanism); an axis of sedimentation which shifted in a circular fashion around a relatively stationary center of deposition; very close correspondence between unconformities in the basin and falls in second-order relative changes in sea level from published curves; and evidence for uplift and erosion of the surrounding cratonic arches in between depositional cycles.

Future acceptance of a continuous subsidence model to explain the Williston Basin's subsidence will ultimately depend upon the outcome of the debate surrounding the existence and origin of short-term eustatic changes.