A large basement "high," or structurally positive element, defined by isopach maps and Bouguer gravity

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anomaly maps, has been identified in the central part of the Powder River basin, Wyoming. By comparing productive areas and sandstone trends to the high, it is apparent that it has affected deposition of sedimentary units since at least Lower Cretaceous time and conceivably since marine foreland deposition began.

Sandstone sequences, such as the Muddy Formation, deposited in a primarily fluviodeltaic environment were deflected around the flanks of the high whereas units deposited in a predominantly marine system, such as the Shannon, had their best sandstone development on top of the high. Understanding the relations between the positive element and sandstone distribution is an obvious aid to exploration.

Similar highs in other Rocky Mountain structural basins are identified and exploration targets discussed. These highs provide early migration paths for hydrocarbons, platforms for the development of porosity in clastic rocks, and in some places, can be shown to be areas unfavorable for exploration because of the absence of sandstone.

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