Recent stratigraphic studies of Upper Cretaceous rocks in southern Wyoming and northern Colorado indicate that a large delta formed along the west margin of the seaway during the Campanian. The delta was deposited on what is now regarded as the tectonic foreland of the Cretaceous geosyncline. The axis of the delta (area of thickest non-marine and transitional deposits) extends from central Moffatt County, Colorado, in a northeast direction passing near Rawlins, Wyoming. The delta is 80-120 miles wide and was built seaward for distances ranging from 100 to 250 miles. Thus, the size of the delta is comparable with the present Mississippi River delta. The deltaic deposits range in thickness from 1,500 to 3,000 feet.

Much of the delta has been removed by erosion but parts are now found in 7 separate structural basins. Formations comprising the delta are as follows: (1) Iles and lower Williams Fork in Sand Wash basin; (2) lower part of Mesaverde in Piceance basin; (3) upper sandstones of Pierre in North Park-Middle Park basin; (4) Mesaverde in Hanna-Laramie basin; (5) Parkman and Teapot in southern Powder River basin; (6) Mesaverde in southeast Wind River basin; (7) Mesaverde (Rock Springs, Ericson, and lower Almond) in Washakie-Great Divide basin.

There are several reasons for believing that these formations are part of a single delta. Facies trends that can be traced from basin to basin show a large bulge of non-marine beds protruding into the marine basin. A complex association of intertonguing non-marine and marine sediments is present. Shoreline sand trends exhibit rapid changes from one time stratigraphic unit to another. Isopach maps show that the time-stratigraphic unit containing the delta deposits is thicker in the area of the delta than elsewhere along the coast line or in the marine basin. All formations were deposited in a dominantly reducing environment.

The delta theory explains the following anomalous stratigraphic conditions: (1) the northeast shoreline sand trends across the southern Piceance basin which are an exception to the overall north-south trends in the Cretaceous basin of deposition, (2) the marine embayment, west of the delta, in which the Ericson sandstone and associated marine beds were deposited (area of Washakie-Great Divide basin), and (3) the thin nature of the Mesaverde in the Lost Soldier area resulting from the intertonguing of the Mesaverde formation in the southeast with the marine Cody shale in the north-west.

Most of the important gas production from this stratigraphic interval in the area of discussion is associated with the shoreline zone surrounding this delta. Oil production has not been found associated with the deltaic deposits. Oil from the upper Almond in Sweetwater County, Wyoming, is largely from shoreline sandstones deposited along the west margin of the marine embayment that formed immediately after deposition of the delta.

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