Lower Cretaceous strata (essentially the Dakota Group) of Wyoming and the Southern Rockies are of considerable economic importance. To date recoverable reserves of about 450 million bbl of oil and 1½ trillion cu ft of gas have been discovered in these rocks.

Lower Cretaceous sediments were deposited in the incipient Rocky Mountain geosyncline which seemingly began to subside in this region in Aptian time. The first sediments to be laid down were inland floodplain conglomerate, sandstone, and variegated mudstone (Cloverly, Lakota, Lytle, lower Cedar Mountain, etc.) transported into this depression by river systems from the Sioux uplift on the east and from the Mesocordilleran geanticline on the west.

As the transgressing arboreal sea entered this part of the trough, and ultimately joined the southern sea, environmental relations became more complex. Marine shale and siltstone (Skull Creek, lower Thermopolis, and Kiowa) formed in the deeper part of the trough occupied by the sea and graded landward (eastward and westward) into sandstone, shale, and coal of the transitional littoral-marine-lagoonal, paludal, and deltaic environments (Fall River, upper Cedar Mountain, Plainview, upper Cheyenne, etc.). These, in turn, graded laterally into sandstone and variegated mudstone of the now restricted floodplain environment.

Toward the end of Early Cretaceous time, a low arch rose in southern Kansas and southern Colorado, separating the trough into two parts. However, continuing subsidence allowed the sea to spread greatly both east and west. Floodplain deposits (now composed of sandstone and mudstone) continued to accumulate between the source areas and the arboreal sea and interfinger seaward with shale and sandstone of the transitional environments (Omadi, "Dakota," Naturita, etc.). These in turn interfinger with the marine upper Thermopolis and Mowry Shales. Tongues of these formations (Newcastle, Muddy, "J," etc.) project seaward into the marine shale and probably represent deltaic wedges formed during minor regressions of the sea.

Petroleum has been produced from nearly all the units mentioned, representing deposits of marine, littoral, deltaic, paludal, lagoonal, and floodplain environments. Most production has come from Newcastle, "J," and Muddy sandstones which probably are deltaic and littoral marine complexes.

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