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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 961

Last Page: 962

Title: Paleogeography and Tectonic Implications of Late Cretaceous to Middle Tertiary Rocks of Southern Denver Basin, Colorado: ABSTRACT

Author(s): David G. Morse

Article Type: Meeting abstract


In the southwestern Denver basin, the lower part of the Dawson Formation consists of point-bar sequences composed of andesitic detritus. Eastward, it thickens and becomes predominantly mudstone with subordinate thin point bars. The upper Dawson consists of a basal, eastward-thinning, wedge of feldspathic (braided stream) conglomerates followed by feldspathic or andesitic point-bar deposits, overlain by braided-stream arkoses. Dawson cross-bedding dips eastward. During lower Dawson deposition, the Front Range is interpreted to have only minor topographic expression because andesitic debris came from a source west of the Front Range and was deposited in meandering streams. Early Paleocene erosion of the first Laramide exposure of the Front Range basement produced the lower rkosic unit. Following meandering-stream deposition, a second major arkosic pulse of coarse braided-stream deposits prograded eastward (late Paleocene-early Eocene). Extensive Eocene erosion and stability followed until the Oligocene when the extensive Wall

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Mountain Tuff (35 m.y.) was ejected form the southern Sawatch Range and flowed up to 30 km east of the Front Range. The Oligocene Castle Rock Conglomerate forms an elongate, northwest-southeast, discontinuous band across the southern Denver basin. Cross-beds dip east and southeast. Thick tabular cross-beds and large granite boulders derived from the Front Range indicate torrential flood deposition. North to northwesterly dipping bedding, and up to 1,000-m vertical offsets between Front Range Wall Mountain Tuff outcrops and those in the Denver basin suggest significant post-Castle Rock uplifting of the Front Range.

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