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

Description of the Route from Denver to Raton

Pueblo to Walsenburg

In Pueblo, one passes from the Pierre Shale into the underlying Cretaceous Niobrara Formation (table 2) at about the Arkansas River. South of the river the old Colorado Fuel and Iron (CF&I) steel-making plant is on the left and the new Comanche power generating plant is to the southeast. This power plant was constructed in large part to provide power for electric steel making furnaces, which totally cut off the demand for Raton basin coking coal that had been produced for many years at CF&I's Allen and Maxwell mines (more recently called the New Eagle and Aztec mines, now closed), west of Trinidad. Coal for the steam plant is brought by unit train from the Belle Ayr mine near Gillette, Wyoming.

Table 2. Generalized stratigraphic section of Cretaceous and Tertiary rocks seen from Colorado Springs to Raton.

South of Pueblo, drive across gently rolling, dissected terrain underlain by marine Upper Cretaceous rocks (table 2) of the Niobrara, Carlile, and Greenhorn Formations, past ridges and mesas supported mainly by the Fort Hays Limestone Member of the Niobrara and the Bridge Creek Limestone Member of the Greenhorn. Benches formed by these limestone units can be seen in the breached anticline a few miles south of Pueblo and across the drainage divide at the abandoned rest stop on the left that is mostly on the pinon and juniper covered Bridge Creek Member beneath mesa crests formed by the Fort Hays Member.

At the Colorado City Exit from I-25, cross the structural divide, the Apishapa arch, that separates the Denver and Raton basins. Just south of the excellent road-cut exposure of the Fort Hays Limestone at the exit, the route descends the dip slope of the Fort Hays Limestone, which crops out at the bottom of the hill along Greenhorn Creek, east of the bridge. After crossing the creek, continue up section through the Smoky Hill Member of the Niobrara Formation to the Apishapa fault near the top of the hill. The sandstone beds exposed in roadcuts of the highway to the left are on the south side of the fault and consist of the Dakota Sandstone, the oldest Cretaceous unit in this vicinity; the large ridge at the crest of the hill is supported by Dakota Sandstone. The lower part of the Dakota Sandstone (table 2) represents the transgression of the Cretaceous epeiric sea, which extended across the North American continent from the Arctic Ocean to the Gulf of Mexico in the Early Cretaceous and existed during most of the Late Cretaceous (Cenomanian to Maastrichtian). The Dakota shows several hundred feet of reactivated Laramide movement along the fault, which originated during the late Paleozoic (Ogden Tweto, USGS, oral commun., 1980). The Apishapa fault trends about east across Las Animas County and forms the northern boundary of the late Paleozoic Apishapa arch. The fault is a major fracture zone and water wells in the zone provide a large part of the water supply for Colorado City.

South from the top of the Dakota ridge, leave the Denver Basin and proceed across the valley back up section through the progressively younger south-dipping marine shales, limestones, and limy muds of the Graneros, Greenhorn, and Carlile to the next ridge, capped once again by Fort Hays Limestone (table 2). At the top of this ridge, look south into the Raton basin, a large arcuate structural basin that extends south into New Mexico (Fig. 1). The Fort Hays and Smoky Hill Members of the Niobrara Formation form the dip slope for several miles to the south from the ridge crest. Farther along, the Pierre Shale overlies the Smoky Hill and underlies the route into Walsenburg and on south to Trinidad.

The Wet Mountains, which dominate the skyline to the west of the Interstate highway, are composed mainly of Precambrian crystalline rocks. The summit of Greenhorn Mountain, which is the prominent peak at the southern end of the range (elevation 12,334 ft), is unusual in that a remnant of Oligocene volcanic rocks uncomformably overlies the Precambrian. Several prominent pediment surfaces are visible to the west along the route.

East of the highway and visible for several miles is Huerfano Butte, a conspicuous conical peak that rises about 100 feet above the surrounding plain. According to Penn (1994), the butte is a biotite olivine alkali-gabbro plug bisected by two east-trending dikes rather than a volcanic edifice of some sort as indicated in popular literature. The age of the intrusives is about 25.2 Ma, similar in age to East and West Spanish Peaks intrusives.

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