An isopach map of the Frontier Formation and the lower part of the Cody Formation in parts of Wyoming shows (1) a north-trending area of thickening in a western paleobasin associated with the thrust belt, (2) a north-trending area of thinning on a western arch, and (3) a northwest-trending area of thickening in eastern Wyoming that includes a composite of several different thickening patterns. Paleotectonic activity during Turonian time was most intense in the area of the western basin and the western arch and caused varying thicknesses of preserved strata in the lower part of the Frontier below a regional unconformity at the base of the Wall Creek Member. Less intense paleotectonic deformation is recorded in central Wyoming by the unconformities at the base of the Wall C eek Member and by the subtle variations of thickness of a bentonite-bounded unit of marine shale in the lowermost Cody Formation. These variations in thickness in the lower Cody may have started late during the time of Wall Creek sandstone deposition, and they may be related to deposition of the uppermost sandstones of the Wall Creek, which produce hydrocarbons from stratigraphic traps in the deep part of the Powder River basin.