Abstract

Lower Cretaceous strata of Wyoming and the Southern Rockies constitute one of the most complex stratigraphic sequences in the Rocky Mountain region. They consist of many different lithologic types formed in various depositional environments which existed during the early history of the Rocky Mountain Geosyncline. Because of the stratigraphic complexity, correlations are difficult to make, boundaries are not generally agreed upon and terminology is unwieldy.

The geologic history of this region can be subdivided into four phases: (1) an early fluvial phase with sediments derived mainly from the Mesocordilleran highland in the west; (2) an early transgressive phase in which aboreal and austral seas filled the trough and eventually joined to make a continuous seaway from Arctic to Gulf; (3) a regressive phase in which the seas retreated in response to floods of sediment resulting from orogeny in western source area and Souixia Arch to the east; and (4) a late transgressive phase accompanied by much volcanic activity to west.

Sediments of the fluvial phase are conglomeratic sand-stones and variegated mudstones of the inland floodplain environment. Early transgressive phase sediments include conglomerates and mudstones of the floodplain environment; tabular and lenticular sandstones, carbonaceous shales, coals and thin oyster beds of transitional environments (coastal plain, littoral, lagoonal, estuarian, deltaic, etc.); and gray to black calcareous shales and siltstones of the marine environment. Regressive phase and late transgressive phase sediments are like those of the transgression but facies differ in relative importance and geographic distribution.

As of May 1, 1967 more than 725 million barrels of oil and 1,970 billion cubic feet of gas have been produced from Lower Cretaceous strata in 550 fields in this region. It is estimated that ultimate production from these rocks will range from 1,300 to 2,500 million barrels of oil and 3,500 to 7,000 billion cubic feet of gas. Production has been from sediments of all environments but has been greatest from transitional sediments of regressive and late transgressive phases.