WILLIS, JAMES J., Department of Geology, Baylor Univ., Waco, TX

ABSTRACT: Structural Development of the Fold-Thrust Model, Rocky Mountain Foreland

The fold-thrust model was first proposed by Berg (1962) to explain the structural relationships in wells in the Rocky Mountain foreland that were initially drilled through Precambrian basement, then through a fault-bounded, thinned, overturned Paleozoic and Mesozoic section, and finally into normal Mesozoic and Paleozoic strata. Recent drilling activity and geophysical data have indicated that many of the large-scale, Precambrian-cored, mountain-front overhangs in the Rocky Mountain foreland are fold-thrusts (e.g., the Beartooth Mountains, Bighorn Mountains, and Immigrant Trail area). Some structures developed during early stages of development may represent potential sub-thrust hydrocarbon prospects. Furthermore, many basinal flank structures exhibit fold-thrust geometries as well, i cluding the Elk basin and Dallas-Derby oil fields. This relatively recent phase of drilling represents a new frontier in petroleum exploration in the Rocky Mountain foreland; hence, an understanding of fold-thrust geometries and deformational mechanics is critical.

Initial stages of fold-thrust development are characterized by a reverse (or thrust) fault that originates in basement and dies out upward into Paleozoic strata, forming an asymmetrical anticline and adjacent syncline (Early-Stage Fold-Thrust). Continued compression and uplift causes volumetric problems to occur in the progressively constricting synclinal hinge due to the increasing asymmetry of the fold-thrust structure. Initially, these volumetric difficulties may be relieved by the formation of minor backthrusts and forethrusts, which originate through bedding-plane slippage and result in localized structural thickening. However, as deformation progresses, continued volumetric problems in the tightening syclinal hinge cannot be relieved by this minor thrusting, and a second thrust ault commonly develops in the upper part of the stratigraphic section, near the synclinal axis subparallel to the initial basement-originated fault (Intermediate-Stage Fold-Thrust).

As deformation progresses, the initial reverse fault (and associated imbricates) continues its upward propagation through the stratigraphic section. The adjacent sycline becomes more tightly folded with the increasing fold-thrust asymmetry and volumetric constraints propagate downward into the lower portions of the synclinal hinge. The synclinal-hinge fault propagates both farther up and down into the stratigraphic sequence to resolve those volumetric problems, such that ultimately the entire stratigraphic section is sheared by the synclinal-hinge fault. The dual fault system of the fold-thrust is now complete (Late-Stage Fold-Thrust). Continued deformation of the fold-thrust results in severe rotation and overturning of the entire fault-bounded section. Structural thinning of the fau t-bounded section by shearing between the two faults also characterizes the final stages of fold-thrust development (Final-Stage Fold-Thrust).

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.