ABSTRACT

The Williston Basin is a virtual casebook of basement tectonic features repeatedly affecting sedimentation and hydrocarbon accumulation. Archean (2.5 billion years old) and Early Proterozoic Trans-Hudson structures (1.85-1.95 billion years old) that were subtly reactivated during the Paleozoic affect deposition, and at least by Laramide time (Late Cretaceous to Eocene) they were significantly rejuvenated as faults and folds that control the distribution of important modern hydrocarbon pools such as the Cedar Creek and Nesson anticlines. Basement features in this area are clearly expressed in gravity and magnetic data. Because of the demonstrable reactivation of these lithologic contacts as structures, we can make some predictions about possible structural development in other, less obvious areas. The Cedar Creek anticline is a prime example that shows repeated rejuvenation of an Archean lithologic contact. The Laramide down-to-the-southwest fault along this line juxtaposes relatively magnetic material on the downthrown side of the fault; the magnetic high there correlates with a gravity low. Analogs to the Cedar Creek basement feature can be identified in several locations in the basin, and Ordovician Red River facies trends may be affected by basement features similar to those at Cedar Creek. The Nesson anticline also follows old basement lithologic boundaries. Its north-south trend indicates that the contacts are part of the Trans-Hudson Orogen. Details of the Nesson complex suggest that segmentation of the field is related to transverse NE-SW disruptions in the magnetic data. Subtle northeast-trending disruptions in the gravity and magnetic data correlate with lines of small fields and may be related to facies such as oolite shoals in the Mississippian carbonates. Basement tectonics, inferred from gravity and magnetics, can be used as a guide to exploration in multiple plays in the Williston Basin.