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Important types of sandstone bodies in the Cretaceous sequence of the Rocky Mountain region include regressive shoreline sandstones and more restricted barrier-island and transgressive-marine deposits. Each is supplied by longshore transport and deposited in nearshore-marine environments. Differences depend, in part, on the rate of sediment supply in relation to the rate of subsidence. Recognition of type and knowledge of similarities and differences provide useful guides in exploration.
Regressive shoreline complexes were formed by seaward progradation of beach and shoreface deposits. They are as much as 100-ft. thick and sheet-like in geometry, extending tens of miles both parallel and perpendicular to the shore. They are replaced laterally and overlain by alluvial deposits, with channels locally scoured into the marine sequence. These sandstones are very common in the Rocky Mountain Cretaceous but rarely contain stratigraphic oil accumulations.
Barrier-island sandstone bodies were formed by upward (and seaward or lagoonward) accretion of beach and nearshore-marine deposits. They also are as much as 100-ft. thick, 10 mi. wide, and tens of miles long parallel to shore. They commonly overlie nonmarine or lagoonal deposits and are overlain by lagoonal or marine shales. Some contain stratigraphic oil accumulations.
Transgressive-marine sandstones occur in significant thicknesses only where transgression was slowed locally by topographic relief. In one example where the paleotopography is related to differential erosion of the truncated sequence ("strike valleys"), the sandstone bodies are as much as 50-ft. thick, a few miles wide, and tens of miles long. They rest directly on the erosion surface, thin laterally by onlap, and are overlain by marine shales. Sandstones of this type contain stratigraphic oil accumulations, but appear to be relatively uncommon.
Acknowledgments and Associated Footnotes
1 Marathon Oil Corporation, Littleton, Colo.
Copyright © 2006 by the Tulsa Geological Society