Abstract

Isopach maps of upper Paleozoic and Mesozoic rock units in Wyoming show that there are long zones in which many of these units are thin. Younger zones of thinning are not always superimposed on older zones but most zones trend northwest or north. Each zone may be the result of slight folding and subsequent erosion prior to deposition of the next younger rocks, or it may represent merely non-deposition, slower deposition, or deposition of sediments that differ slightly in volume from those in the surrounding areas. Many zones may be the result of a combination of these factors.

These zones of thinning are of possible economic significance where they are crossed by Laramide folds, for the amount of closure in older rocks beneath the Laramide folds may be materially increased. In areas where the rocks have very gentle monoclinical dips at the surface, some zones of thinning are of such magnitude that there is closure in older rocks at depth.

Zones of thinning developed as early as Pennsylvanian time and as late as Montana time and can be recognized in rock units of most of the intervening epochs. The basic data necessary for construction of adequate isopach maps are lacking in many areas, so not all the outlines of even the major zones of thinning are known. After the zones are located, detailed studies of lithologic changes, porosity and permeability changes, grain-size changes, and other features of the rocks present on and around the zones of thinning would be necessary to determine the effect of zones of thinning on oil and gas possibilities. Such studies will help determine whether most of the oil and gas has accumulated as a result of Laramide folding or whether some may have accumulated earlier along zones of have accumulated earlier along zones of thinning whose structural expression has been obscured by Laramide folding.

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