The Denver basin is one of the largest structural basins in the Rocky Mountain area and extends across portions of Colorado, Nebraska, and Wyoming. The basin is typically asymmetrical with its axis paralleling and close to the Front Range. The deepest portion of the basin lies near Denver, where more than 12,500 feet of sediments are present.

Recently discovered outcrops suggest that during pre-Pennsylvanian time, the Denver basin area was a normal marine shelf receiving sediments from early Paleozoic seas. Post depositional uplift along Siouxia exposed this area to deep erosion removing nearly all the early sediments.

In Early Pennsylvanian time, transgressive seas entered the Denver basin area from the Anadarko basin, depositing a predominantly clastic terrane. Near the end of the Atoka, the first major pulses of the Ancestral Rockies occurred. This uplift reached maximum proportions during the Des Moines. Clastic material eroded from the uplifted mountains was deposited contemporaneously with marine sediments deposited in the expanding seaway. This expansion reached its maximum during the Missourian and was followed by a slight Virgil regression. Continued Permian regression left a full suite of environments and facies ranging from normal marine through an evaporitic sequence to terrestrial deposits. Late Permian and Triassic sediments indicate the Ancestral Rockies were weakly positive and suppl ed sediments to a shallow hypersaline sea. Non-depositional conditions persisted from Upper Triassic through Lower Jurassic, and into Mid-Jurassic time. During Middle and Upper Jurassic time, transgressive seas fluctuated across the basin from the northwest. As these seas regressed at the close of the Jurassic, a broad inland flood plain developed.

Early Cretaceous seas inundated the area from the north and south reworking earlier sediments and obscuring the Jurassic-Cretaceous boundary. Initial basin-forming movement occurred at this time. Fluviatile material from exposed areas to the east and northeast developed a complex deltaic pattern as it interfingered with marine sediments basinward. Deltaic deposits also extended into the area from the southwest merging with sediments from the east. Another cycle of transgression and regression developed similar depositional patterns. It is within these two Early Cretaceous sedimentary

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cycles that significant reserves of oil and gas have been discovered. In the Upper Cretaceous, a major transgression joined the northern and southern seas into a large seaway crossing the downwarping basin, and lapping against the uplifted Front Range. This Laramide tectonic activity reached its peak during the Eocene with the basin acquiring its present configuration.

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