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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 962

Last Page: 962

Title: Sedimentology and Tectonic Implications of Oligocene Castle Rock Conglomerate, Southern Denver Basin, Colorado: ABSTRACT

Author(s): David G. Morse

Article Type: Meeting abstract


The Oligocene Castle Rock Conglomerate, 70 m thick, trends northwest-southeast in an elongate discontinuous band (80 km long) across the central Denver basin from near Castle Rock to Calhan, Colorado. The conglomerate was studied to determine the depositional environment and any implications on the tectonic history of the southern Front Range. The Castle Rock Conglomerate is a nonmarine formation with large-scale cross-bedding. It contains angular slump blocks of Wall Mountain Tuff, some exceeding 2 m in length, as well as rounded boulders and cobbles of Front Range basement rocks. It fills valleys, up to 90 m deep, carved into the underlying Oligocene Wall Mountain Tuff and the early Tertiary Dawson Formation. The Castle Rock Conglomerate is interpreted as a deposit of a braided stream, possibly an ancestral South Platte River, which flowed in a broad valley toward the east then southeast.

Several tectonic implications were determined from this study: (1) the Front Range, after being eroded to a low plain in the Eocene, was regenerated after the middle Oligocene, as shown by structural offset of the Castle Rock at the edge of the Front Range, and by its interpretation as a restricted fluvial deposit rather than a broad fanglomerate; (2) the fluvial interpretation supports other suggestions that this formation resulted from the eastward diversion of the Rio Grande headwaters by the Thirtynine Mile volcanic field across the area which is presently the Front Range; and (3) north to northwesterly dipping beds, opposite to paleocurrents, indicate post-Castle Rock rotation of the Denver basin.

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