Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Description of the Route from Denver to Raton

Denver to Castle Rock

South of Denver on I-25, the route is underlain by the Denver Formation (Maastrichtian and Paleocene); about 5 mi south of the I-225 bypass around the city, the Denver Formation intertongues with rocks of the Dawson Arkose (Paleocene and Eocene), which is overlain by the Castle Rock Conglomerate (Oligocene). At Castle Rock, the small butte northeast of town is capped by the conglomerate and is the type section.

The structure of the Denver Basin is largely concealed by the relatively flat-lying Denver and Dawson Formations. The basin is asymmetric, with the deepest part located only a few miles east of the mountain front. Its axis lies beneath Denver and extends south-southeastward to Castle Rock and through points about 15 mi east of Colorado Springs and Pueblo. The Front Range, which consists mainly of Precambrian igneous and metamorphic rocks, was uplifted and thrust eastward over the west flank of the basin during the Laramide orogeny. Between Denver and Castle Rock, the thrusts are located a short distance east of the mountain front, where they are poorly exposed due to the soft nature of the Denver Formation, or concealed by the Dawson, which is post-Laramide.

The town of Kiowa, Colorado, lies about 20 miles to the east of Castle Rock and is situated approximately above the deepest part of the Denver basin. In March of 1999, a 2,200-foot well was core-drilled at the Elbert County Fairgrounds in Kiowa. This 2.5-inch-diameter core was obtained as part of the Denver Basin Project, which was a Denver Museum of Natural History project funded in part by the National Science Foundation. The goals of the project were to reconstruct the sedimentary and tectonic history of the Denver basin during the Late Cretaceous and early Tertiary. The hydrogeology of the Denver basin was also a primary focus of the project and detailed studies of the aquifers cored in the Kiowa well will provide valuable information about water resources in the Denver basin. The multidisciplinary project includes hydrogeology, paleontology (palynology and paleobotany), stratigraphy, sedimentology, paleomagnetism, fission-track dating, and other disciplines. Results from these various fields will be integrated to provide a basis for interpreting the history of the Denver basin and implications of that history for topics ranging from groundwater use to the extinction of the dinosaurs.

In this project the K-T boundary in the Denver basin and extinction patterns across it were examined. The study integrates paleobotanical and palynological data within the general sedimentological and stratigraphic framework to document the evolution of terrestrial vegetation from the Late Cretaceous to the early Tertiary in the Denver basin (Barclay and others, 2003; Nichols and Fleming, 2002).

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