OF-04-05 Geologic Map of the Castle Rock South Quadrangle, Douglas County, Colorado


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Citation: Thorson, Jon P. “OF-04-05 Geologic Map of the Castle Rock South Quadrangle, Douglas County, Colorado.” Geologic, 1:24,000. Open File Report. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2004.


This publication describes the geologic setting and mineral resource potential of this 7.5-minute quadrangle located in Douglas County, Colorado. Includes description of map units, structural geology, mineral resources, cross sections, oblique view as well as shapefiles with metadata. (1:24,000). Digital ZIP/PDF download. OF-04-05D

From the authors notes:
The Castle Rock South 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.

Most of the exposed bedrock in the Castle Rock South quadrangle is the assemblage of lithologies shown on the geologic map as 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 those 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 of the Dawson 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.

Following the erosion of some of the upper part of the Dawson Formation, the conglomerate of Larkspur Butte was deposited in a series of channels and broad valleys occupied by streams which drained the newly rejuvenated mountains. In the western part of the Greenland quadrangle the conglomerate of Larkspur Butte was deposited in narrowly confined, steep-walled stream valleys. These valleys became broader towards the east.

The Wall Mountain Tuff, an ignimbrite or glowing hot volcanic ash flow, was erupted in the late Eocene and poured across the landscape. This ash flow blanketed the eroded surface of the Dawson Formation and the valleys which contained the conglomerate of Larkspur Butte. Because of its great heat the ash compacted into a viscous plastic which flowed for short distances before it cooled into welded tuff. Erosional remnants of the Wall Mountain Tuff overlie the Dawson Formation on the higher buttes in the western part of the Castle Rock South quadrangle. The ash flow ponded in a depression in the east-central part of the quadrangle (sec. 27, T. 8 S., R. 66 W.) where it overlies the conglomerate of Larkspur Butte. The Castle Rock Conglomerate was deposited near the end of the Eocene in paleovalleys on an erosion surface that cuts across the upper Dawson, conglomerate of Larkspur Butte, and Wall Mountain Tuff. One of the Castle Rock paleochannels extends from the southwestern corner to the center of the quadrangle. Deposition of this conglomerate extended over most of the northeastern part of the Castle Rock South quadrangle filling a depression near the center of the Denver Basin.

Since the deposition of the 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.