ABSTRACT

Seismic profiles matched by a true-scale structural cross section and a photographed clay model analog can be used to illustrate the two-dimensional geometry and developmental history of oil-field structures of the Big Horn basin. From these real and simulated examples, a two-dimensional model of the typical oil-field structure of the foreland is constructed. These structures have the following characteristics. (1) They involve faulted Precambrian basement. (2) The basement generally acts as a brittle, homogeneous material and does not fold appreciably. (3) The folding in the overlying sedimentary section results from rotational movement on a thrust fault that develops at 30° to the sediment-basement contact. The fault is propagated upward into the sedimentary section, producing an anticlinal fold, and downward into the deeper basement. The amount of shortening determines the displacement on the fault and extent of vertical uplift. (4) The folds are flexural-slip and asymmetric, but not concentric, because the ductile sedimentary layers are differentially stretched and thinned over the steep flank and along the overturned underside of the causal fault zone where layer-parallel extension is greatest. Thinning in these domains is compensated by thickening in the subthrust “synclinal” block. (5) Since the basement surface remains essentially planar and continuous “plateau uplift” cannot occur, a hinge must develop behind the primary thrust zone so that rigid-body rotation can take place. The dip on the fault surface increases upward with continued rotation. Longitudinal and transverse extensional faulting occur at shallow levels. (6) The original point of the basement thrust wedge is blunied under the constriction of the overlying sedimentary section by localized fault imbrication.

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