Abstract

The lower Oligocene Wall Mountain Tuff and the Castle Rock Conglomerate cap mesas and buttes of the High Plains in the southern Denver Basin between Denver and Colorado Springs, Colorado. The 35 million year old rhyolitic tuff forms the distal portion of a slightly welded ash-flow which traveled over 140 km (87 mi) eastward along a low relief late-Eocene erosion surface from a source in the southern Sawatch Range to the study area in the High Plains. The tuff covers 800 km² (300 mi²) in the southwest Denver Basin and 8000 km² (3000 mi²) in the southern Front Range and South Park. In the Denver Basin, the thickness decreases from 12 m (40 ft) in the west to about 4 m (13 ft) at the eastern limits of outcrop. Phenocryst content, composed of sanidine, plagioclase, and biotite, decreases eastward from 15 to 3 percent across the Denver Basin. Outcrops in the Denver Basin are aligned and branch, replicating eastward and southeastward flowing immature drainage patterns present at the time of eruption.

The Castle Rock Conglomerate was deposited either on the Wall Mountain Tuff or, more commonly, on a surface eroded up to 90 m (300 ft) into the underlying early Tertiary (late Laramide) Dawson Formation. The Castle Rock Conglomerate, a granule to gravelly conglomerate some 70 m (230 ft) thick, trends northwest-southeast in an elongate discontinuous band (80 km (50 mi) long and up to 30 km (19 mi) wide) across the central Denver Basin from near Castle Rock to Calhan, Colorado. The Castle Rock Conglomerate is distinguished from the underlying, somewhat finer-grained, arkosic, Dawson Formation by the presence of clasts of the early Oligocene Wall Mountain Tuff. The Castle Rock Conglomerate is highly crossbedded and contains large angular slump blocks of Wall Mountain Tuff, exceeding 2 m (6 ft) in length, as well as rounded boulders and cobbles of typical Front Range basement rocks. Crossbedding dips east to southeast, a direction parallel to the length of the outcrop belt. Planar crossbedded gravel (facies Gp) is predominant over trough cross-bedded gravel (facies Gt) and massive gravel (Gm). Fining-upward gravelly sequences 2.5–9 m (8–29 ft) thick are observed locally. Cut and fill structures are very common within the fining-upward sequences. Epsilon cross-stratification is rare. Deposition is interpreted to be from low to moderately sinuous streams which flowed in valleys up to about 90 m (300 ft) deep. Periods of torrential flooding are indicated by the presence of boulders 55 km (31 mi) from the nearest edge of the Front Range. Because the outcrop belt and paleocurrents are aligned with the point where the present South Platte River emerges from the Front Range, the Castle Rock Conglomerate is believed to mark the course of the South Platte River in Chadron time.

The Castle Rock Conglomerate differs significantly from the underlying Dawson Formation which was controlled by late Laramide tectonics. The Castle Rock was controlled by divergence of a river in the Front Range by a volcanic eruption, and by the gentle east to southeast regional paleoslope of the High Plains. The Dawson is a synorogenic deposit; whereas, the Castle Rock Conglomerate is a non-orogenic deposit.

The present north to northwesterly regional dip of the Castle Rock Conglomerate, opposite to the paleocurrents, suggests post-Castle Rock rotation of the Denver Basin and diversion of the Platte River to its present course. Post-Castle Rock deformation of the Front Range is indicated by offset of the late Eocene erosion surface in the Front Range from outcrops of the Wall Mountain Tuff and the Castle Rock Conglomerate in the Denver Basin.