Early Cretaceous and early Late Cretaceous deltaic sediments of the Denver basin include the oil-producing "D" and "J" sandstones (upper Dakota Group). The very fine- to coarse-grained quartzose sandstones are in part cross-bedded and ripple marked, in part marine, and in part nonmarine; contain coal and carbonaceous or lignitic beds, mud pellets, siderite concretions, interbedded claystones; and in some areas grade laterally and vertically through siltstone into marine shale.

Within the "J" sandstone there is a well-defined delta extending northwestward into the Denver basin in Colorado. There are smaller deltas in western Nebraska, and, between these two areas, there is a narrow marine embayment. A similar deltaic pattern is developed in the "D" sandstone. Petroleum is concentrated in the parts of the deltas that have the greatest interfingering of marine and deltaic deposits.

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