About This Item
Share This Item
Sedimentation during the Lower and middle Cretaceous in the northern Great Plains (eastern Montana, North Dakota, South Dakota, and northeastern Wyoming) was influenced by recurrent movement of basement fault blocks. Regional thickness variations of four time-stratigraphic intervals are linear and have dominant orientations of northeast and northwest. Regional distribution of porous and permeable sandstone in the Muddy-Newcastle formation and equivalents (Albian), and in the Inyan Kara Group and equivalents (Aptian-Albian) are also linear and have dominant orientations of northeast and northwest. Similar thickness variations and directions of Lower Cretaceous units on the southwest flank of the Black Hills have been described in detail by Weimer and others in 1983.
A depositional model, incorporating structural and sea level changes, shows that marine and nonmarine units thicken in structurally low areas and thin over structurally high areas.
A tectonic model to account for the linear orientations, illustrates the effects of simple and pure shear stress systems. The stress systems were formed from the recurrent movement of basement faults possibly caused by movement of the North American plate during the Cretaceous.
Major surface expressed folds and faults and surface lineaments have dominant orientations of northeast and northwest. These features either overlap or are parallel to linear thickness trends of Cretaceous rocks. Both surface and subsurface features are thought to have resulted from common basement fault block movement.
Sedimentation and tectonic models can be used to predict vertical and lateral distribution of marine and nonmarine rocks. When integrated with these models, orientation of fractures and routes of fluid migration can be predicted by paleostructure analyses.
End_of_Article - Last_Page 1329------------