The structural basement in the Rocky Mountains foreland deforms by faulting and rigid-body rotations. The faults at the interface between the sedimentary and crystalline rocks can be anything from low-angle reverse to normal faults. Despite the range of geometries, the total assemblage of faults and rotations is best explained by a movement system that is dominated by vertical motions along faults, many of which are curved in cross section. The first causes deep within the crust are not sufficiently well documented in the geophysical record to justify a firm interpretation. However, there are certain conditions recorded in the geologic history of the region and in the surface structures that place constraints even on speculations. With these constraints, vertical movements seem more likely to dominate than horizontal movements. The sedimentary layers are deformed primarily by forced folding. Their final geometry is a product of several parameters such as welding and stratigraphic make-up. Measurements on natural folds demand that the section either thinned or detached. Detachment without appreciable thinning further requires that (1) large displacements occur within the sedimentary section and (2) at the termination of these folds, the movement must be in several directions. Geologists’ intuition as to how the layered rocks achieved their shapes is not always correct, but field data combined with experimental and theoretical data provide a basis for understanding these folds. It is concluded that the structural style in the Rocky Mountains foreland is not unique, but rather it is only an excellent example of a more universal class of deformation, namely, forced folding.

Excellent exposures in an area of multiple drape folds in the northwest corner of the Big Horn Mountains provide data for the construction of a three-dimensional model of the upper surface of the Mississippian Madison Formation. In the model, the vertical and horizontal scales are the same. Comparison of deformed and undeformed Madison surfaces indicate that displacements required by the deformation vary from place to place on the surface in both magnitude and direction. Calculations made from field measurements show that there is not enough fracturing or thinning in the Madison layer to account for the indicated displacements. Also, reasonable shortening on the basement faults underlying the forced folds cannot account for the calculated displacements in the sheet. Any solution to the mechanics of the drape-folding process must, therefore, meet the described constraints and still account for the indicated nonuniform displacements. Detachment of the sedimentary layers from the basement and intrastratal slip are two processes that meet the constraints and still account for the indicated nonuniform displacements.

This paper presents conceptual models of basement configurations for several structural types in the Rocky Mountains foreland—upthrusts, rotated basement blocks, and plateau uplifts. These hypotheses of structural development are supported by calculated stress states and fracture patterns derived from them. The goodness-of-fit between field observations and the final geometries of the models leads to the suggestion of possible loading conditions for the initiation of Laramide structural development of uplifts in the foreland. These suggested loading conditions for individual features lead, in turn, to a hypothesized stress state consisting of a regional component of crustal horizontal compression superposed on the controlling stress fields caused by more local load variations at depth beneath the brittle upper crust.

Abstract In recent years three developments which have evolved more or less independently, when related, may be of value to the petroleum industry. First is the recognition, through normal oil field development, that fractures are significant to both reservoir capacity and performance. Second is the fact that controlled laboratory experiments have produced, in increasing quality and quantity, empirical data on rupture in sedimentary rocks. These data have been segregated to demonstrate the individual control on rupture of several important parameters: rock type, depth of burial, pore pressure, and temperature. The third development consists of the discovery of new methods to recognize, evaluate, use, and, in some cases, see fractures in the subsurface. This discussion of these three developments may help geologists and engineers to find new approaches to exploration and exploitation of fractured reservoirs. Reservoir and production engineers presently make the greatest use of fracture data, but geologists should find this information useful in exploration for oil and gas trapped in subsurface fractures. Except in the search for extensions to proved fracture reservoirs, there is in the literature a paucity of clear-cut examples of the use of fracture porosity data in advance of drilling. For this reason, several speculative exploration methods discussed herein implement mapping of fracture facies as well as stratigraphic facies.