Hydrocarbons in commercial quantities are produced within this province from rocks ranging in age from Cambrian to Eocene, and from many different types of traps. The geologic environment which led to the origin and entrapment has been influenced by structural history, by climatic factors resulting from crustal movement, and by evolution of organisms.
Some extant mountain ranges and intermontane basins began to evolve in middle Precambrian time. Intermittent vertical oscillation of the foreland contemporaneous with the dominant subsidence of the trough in western Wyoming and southeastern Idaho characterized the events of Paleozoic time up to Pennsylvanian. A northwest-trending structural grain related to the Ancestral Rockies evolved in Pennsylvanian time. Major structural relief developed in central and northern Montana along the Sweetgrass and Big Snowy arches before Late Jurassic time, and gave rise to conditions conducive to hydrocarbon accumulations in stratigraphic-type traps.
Major deformation of the Central Rocky Mountain province resulted from a series of episodes initiated in the Cordilleran trough during middle Cretaceous time and continuing to a climax in early Eocene time. Intermontane basins were filled with rocks of local derivation of both continental and lacustrine character at least 18,000 feet thick.
The present overthrust belt of southeast Idaho, western Wyoming, and southwestern Montana developed from a geosynclinal prism of sediments which had accumulated during Paleozoic and earlier Mesozoic time.
The structural elements of the province can be classified in three general categories. The first consists of the east-west trending fold and fault complexes such as the Big Snowy arch, the Lake Basin fault zone, and the Uinta Mountain uplift, all of which are located along sites of late Precambrian subsidence. The second comprises large northwest-trending crustal folds, the crests of which have been eroded to the Precambrian cores and the troughs of which have been the sites of accumulation of thick sedimentary sequences of local derivation. The third includes the low-angle thrusts and long, sinuous folds of the overthrust belt in which only the sedimentary veneer is involved.
The mechanisms responsible for the formation of these three categories of structure are not readily ascertainable. The structures may have resulted from differing responses to a regional tangential stress system, in which case the patterns of Precambrian deformation influenced the Laramide deformation to a very large degree. The overthrust belt may have had a separate and unique origin not dependent upon lateral compression. Behavior of the crust beneath the relatively thin sedimentary veneer has been more important in structural evolution than heretofore reported.
Figures & Tables
The first article in this book is the address that introduced the technical program of the 46th Annual Meeting of the AAPG. The organization and presentation of this symposium volume was developed in an orderly geographic continuity. Modern concepts of structural form and the sequence of tectonic events are carefully reported all along the mountainous western margins of the American continents. The relationship of this structural knowledge to the accumulation of oil and gas is constantly emphasized in the 26 papers contained herein.