Most of the core sequences in this book are from the Cretaceous basin of the Western Interior, USA. This basin is one of the largest cratonic (foreland or back-arc) basins in the world. Because the strata contain economic products, mainly petroleum and coal, they have been thoroughly studied, both in outcrop and subsurface occurrence. The original basin was 500 to 1,000 mi wide and extended from the Arctic to the Gulf of Mexico (Fig. 2.1). The relationship of the basin to other structural elements in the western portion of North America has been outlined by Dickinson (1976; Fig. 2.2). The foreland basin formed on a thick continental crust and was bordered on the west by a fold-thrust belt and on the east by the Canadian shield.

During the Early Cretaceous, sediments were derived from both sides of the basin, though the thickest strata are along the western margin. During the Late Cretaceous, the dominant source of sediment was along the western margin and lithofacies were controlled by changes in environments from coastal plain to shoreline to marine shelf and to the deeper water of the basin. Intertonguing of nonmarine strata on the west with marine strata in the center of the basin is the dominant pattern of sedimentation (Fig. 2.3). Thickness of the Cretaceous strata varies from 2,000 to 20,000 ft. The thinnest sections occur from the geographic center to the eastern margin of the basin because of slower sedimentation rates. Total organic content is higher in these strata because of the slow sedimentation rates and because of anoxic conditions in deeper water that favored preservation of organic matter.