Abstract

The Pennsylvanian and Permian Denver Embayment in eastern Colorado was a shallow marine extension of the petroliferous Hugoton Embayment and Anadarko Basin. Systems tracts in these contiguous troughs record eustatic sea-level cycles, faulting and regional tilting associated with the Ouachita orogeny, and sequential basin filling as sediment overflowed the proximal basins and was swept downdip. Epeirogenic tilting to the southeast increased accommodation space as recorded by stratal thickening. Two orders of cyclicity are recognized: 1) shorter term cycles are recorded by Midcontinent-type cyclothems in the Virgil (Pennsylvanian) and by less well- organized sequences in the Wolfcamp (Permian); and 2) a superimposed, longer-term cycle governed a maximum transgression in Virgil (Shawnee) and again in Wolfcamp (Chase) time. The intervening lowstand (Wabaunsee) resulted in a regional unconformity.

Virgil highstand sequence tracts are dominated by a series of rimmed carbonate platforms stacked over the Transcontinental Arch. These barriers, in combination with lowered regional gradient during highstands, dammed arkosic detritus derived from the Ancestral Rocky Mountains to the west. In the deeper water southeast of the barrier, highstands are marked by condensed sections of anoxic black shale. Subsequent lowering of sea level increased the gradient, enabling siliciclastics to move seaward through and around the often-exposed carbonate shoals. Typically, the resulting deeper-water cyclothems consist of basal, lowstand terrigenous shale overlain by limestone, deep-water black shale, and regressive limestone capped by bypassed clastics of the succeeding cycle. In the regressive Wabaunsee phase of the long-term cycle the updip limit of both the carbonate and the black shale facies shifted basinward (to the southeast).

During the Wabaunsee major sea-level drop, exposed older Virgil sequences in the north end of the Denver Embayment were deeply weathered and beveled. The pulsing return of marine deposition is recorded in the onlap of Admire sequences northward from the Hugoton Embayment into the Alliance Basin. In succeeding Admire cycles, deposition was continuous from the Hugoton Embayment northward into the Alliance Basin as it had been during earlier Pennsylvanian time.

New depositional patterns developed in the Permian. At highstand, a carbonate rim and its associated evaporite basin expanded southeastward from the Transcontinental Arch to partly cover older reciprocal lowstand ramp sequences. Arkosic clastics overflowed the half grabens bordering the Ancestral Rocky Mountains and spread eastward, segregating carbonate deposition in the Denver Embayment from that in the Hugoton. This clastic sequence is informally referred to as the “Apishapa” delta complex. The Wolfcamp maximum transgression during Chase time is represented by an expanded tripartite cyclothem containing 80 feet of dolomite (Amazon Dolomite of Nebraska). In the overlying, landward-stepping sequences, evaporates predominate in the northwest and dolomites thin southeastward into the Apishapa clastic facies.

Hydrocarbon maturation was accomplished during deep burial under thick Mesozoic sediments. The Laramide orogeny rotated the gentle southeast plunge of the Late Paleozoic Denver Embayment, creating the modern Denver Basin and Las Animas Arch. At the north end of the Laramide Denver Basin, late Paleozoic hydrocarbon traps were largely unaltered but to the south where the dip was reversed, new traps were formed for possible late migrating hydrocarbons.