OF-03-06 Geologic Map of Black Forest Quadrangle, El Paso County, Colorado


The Black Forest Quadrangle is located in El Paso County. Includes cross section, map unit correlation, map with geology overlay, booklet with extended descriptions of map units and geologic setting (1:24,000). Digital PDF download. OF-03-06D

From the Author’s Notes:

The Black Forest quadrangle is located near the western edge of an asymmetrical, oval-shaped, geological structural depression called the Denver Basin. This structural basin lies immediately east of the Front Range and covers a large part of eastern Colorado north of Pueblo, southeastern Wyoming, and southwestern Nebraska. Bedrock in the Black Forest quadrangle dips gently northeast towards the axis of this basin.

The bedrock in the Black Forest quadrangle is the assemblage of lithologies shown on the geological map as facies units of the upper part of the Dawson Formation (TKda). At the time of deposition of this unit during the Paleocene and Eocene Epochs (about 65 to 50 million years ago) the uplift of the Front Range was well underway. Braided streams were delivering to the basin a mixture of gravel, sand, silt and clay derived from weathering and erosion of that uplifted area. The source of the granitic arkosic materials was mostly the Precambrian Pikes Peak Granite located immediately to the west of the Rampart Range mountain-front fault system. Stream flow was generally towards the east. The pebble conglomerate and arkosic sand beds are cross-bedded and fill broad channels generally cut into finer-grained deposits of clayey sandstones and sandy claystones. Interbedded with the channel deposits are occasional structureless beds deposited by mudflows. Also interbedded between the coarse-grained beds are finer-grained and thinner-bedded strata of light-gray to gray-green clayey sandstone and brown or brownish-gray sandy claystone containing fragments of organic material and plant fossils. The fine-grained parts of the upper Dawson were deposited by gentler currents in areas between the braided stream channels and probably were covered with vegetation.

In a period of intense weathering during upper Dawson deposition in the earliest Eocene a strongly developed paleosol horizon was locally developed across the Denver Basin. This paleosol was proposed to be the boundary between the Paleocene and Eocene Epochs by Soister and Tschudy (1978), but recent work has shown that the paleosol formation occurred in the early Eocene shortly after the formation of an unconformity that is now recognized as the Paleocene-Eocene. In the present work the projected location of the strongly developed paleosol has been used as the boundary between Dawson facies units four and five, although this major paleosol is not well developed in the quadrangle. After the period of landscape stability and weathering, mountain uplift resumed and Dawson facies unit five (TKda5) was deposited in the basin. The uppermost part of the Dawson Formation was removed by an extended period of erosion in the Late Eocene.

Following the erosion of some of the upper part of the Dawson Formation, the Conglomerate of Larkspur Butte (Thorson, 2003) was deposited in a series of channels and broad valleys occupied by streams which drained the newly rejuvenated mountains. In the northwest part of the Black Forest quadrangle the Conglomerate of Larkspur Butte was deposited in a broad paleovalley. The late Eocene-age Wall Mountain Tuff, an ignimbrite or glowing hot volcanic ash flow, was erupted in the late Eocene and caps the Conglomerate of Larkspur Butte in the Greenland quadrangle. Erosional remnants of this volcanic unit are still present in the Cherry Valley School quadrangle about 2 mi to the north of the Black Forest quadrangle. The Castle Rock Conglomerate was deposited near the end of the Eocene on an erosion surface that cuts across the upper Dawson and Larkspur conglomerate strata and is younger than the Eocene surface upon which the Wall Mountain Tuff was deposited.

Since the deposition of these Eocene rocks, the area experienced continued periods of erosion and deposition. During the Miocene, the Ogallala Formation was deposited across much of eastern Colorado and probably once covered the quadrangle but has since been removed by erosion. During the Quaternary, deposits of unconsolidated sands and gravels were left in paleochannels, former flood plains along stream courses, and on various upland erosion surfaces as streams eroded the landscape.