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In an area having perhaps more relief on the top of the Precambrian surface than any other structural province in the world, attention understandably has been focused on the basement-cored structures of the Rocky Mountain foreland. Deep-seated compression interpreted as responsible for the shortened basement features has also created detached structures in the overlying sedimentary cover. These latter structures have never been the object of systematic study, yet they provide important additional evidence of a compressional origin for the Rocky Mountain foreland and other forelands, and are also prospective for oil and gas.
Detached structures can be attributed to at least two types and stages of deformation. First, compression operating early in the development but prior to the differentiation of the foreland created small-scale fold and thrust structures. Probable examples of early compressional structures include those formed on what are now the gentle tilted flanks of the Owl Creek Range in Wyoming and the Cara Cura Range in Argentina. Second, after differentiation of the foreland into blocks, the flexural slip mode of folding in competent sedimentary layers dictates that space problems in both anticlines and synclines be accommodated by the creation of decollement surfaces and associated detachment structures. Examples have been documented from virtually every Rocky Mountain foreland basin. Specific lly cited are the North Park basin in Colorado, Elk Mountain area, northwestern Wind River basin, and Big Horn basin in Wyoming, and the Cara Cura mountains in Argentina. Prominent detachment horizons in the Rocky Mountain foreland are shales of Cambrian, Triassic (Chugwater), and Cretaceous (Mowry, Cody, and equivalent) age.
Oil and gas have been produced from detached folds. A negative aspect is that otherwise prospective beds beneath a completely detached structure do not have closure unless they are affected by deeper faults. More optimistically, closures related to local detachment are prospective.
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