During the Late Cretaceous, a basin of deposition covered the western interior of the North American continent, extending from the present Gulf of Mexico to the Arctic. Large quantities of detrital sediment were transported from a source area along the western margin of the basin and deposited in coastal plain, shoreline, and marine environments. Rivers carrying the sediment load formed three large delta complexes: one in central Montana; a second in central Wyoming; and, a third in northern Colorado and southwest Wyoming. These three deltaic centers formed dispersal points from which sediment was carried and distributed within the basin by marine processes. Of the three delta complexes, the northern Colorado-southwest Wyoming delta contains the greatest volume of sedimen , representing 2 to 3 times the volume of each delta to the north.

Deltas are defined by establishing time-stratigraphic units in the marine shale formations, either by faunal or physical criteria, and by tracing these units into the shoreline sandstone and coastal plain deposits. Lithofacies maps, isopachous maps, and restored sections are prepared for each time-stratigraphic unit. The recognition of a delta is based on two or more of the following criteria: 1) an arcuate lithofacies pattern of coastal plain strata (nonmarine) protruding into the marine basin; 2) for a designated time-stratigraphic unit, thickest deposits in the general shoreline zone (area of topset and foreset strata) associated with this lithofacies pattern; 3) a complex intertonguing of marine (foreset and bottomset) and nonmarine (topset) strata; 4) rapidly changing shoreline s ndstone trends from one time-stratigraphic unit to another; 5) abundance of stream deposits over deposits of other environments of coastal plain; 6) biological criteria in marine strata, especially lack of fossils (absence of pelagic calcareous foraminifera); arenaceous benthonic forms tend to dominate; 7) persistence of above criteria in vertical stratigraphic sequence indicating semi-permanency of drainage systems responsible for deltaic deposits.

In Upper Cretaceous strata, topset strata are characterized by lenticular sandstone, siltstone, claystone, shale, and coal. Depending on the nature of the river load and the energy of the basin, the foreset strata may be interbedded siltstone and shale, or large sheets of sandstone forming a partial aureole around the nose of the delta. Bottomset strata are shale or interbedded shale and siltstone with minor amounts of sandstone.

The Upper Cretaceous (Campanian and Maestrichtian) of southern Wyoming and northern Colorado illustrates deltaic sedimentation and its influence on oil accumulation. Localized deltaic loading caused penecontemporaneous differential deformation of the subsiding basin floor. Incipient structural highs between the more rapidly subsiding delta areas controlled early migration of petroleum in stratigraphic traps. Where subsequent geologic history has favored preservation of these early traps, large petroleum accumulations have been discovered.

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