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

Description of the Route from Denver to Raton

Walsenburg to Trinidad

Leaving Walsenburg, the route continues on Pierre Shale for about the next 37 mi, past the town of Aguilar to Trinidad. Aguilar is located adjacent to coal-bearing Cretaceous and Tertiary rocks of the Raton basin. These rocks (Fig. 2), which define the eastern limits of the Walsenburg and Trinidad coal fields, are marked by abrupt cliffs formed by the Trinidad Sandstone (Upper Cretaceous). Overlying the Trinidad are the coal bearing Vermejo Formation (Upper Cretaceous) and the Raton Formation (Upper Cretaceous and Paleocene). The Raton is overlain by the Paleocene Poison Canyon Formation. The Dawson Arkose of the Denver Basin is roughly equivalent to the Poison Canyon Formation of the Raton basin, and the Denver Formation is approximately equivalent to the Raton Formation. The scenery along this part of the route is dominated by the Spanish Peaks to the west. The Spanish Peaks, in part held up by intrusives, are well known for many dikes that radiate from them for several miles. The Spanish Peaks plutons have been dated at 21.7 Ma ± 1.0 Ma (Stormer, 1972, East Peak Granite) and 22.9 Ma ± 2.0 Ma (Smith, 1975, West Spanish Peak stock). Other dates ranging from 19.8 ± 1.6 Ma to 39.5 Ma have been reported from intrusive rocks in the Spanish Peaks area; the 39.5 Ma date appears questionable and most dates fall into the 20-25 Ma range (Marvin and others, 1974, p. 3233). Ages obtained by Penn (1994) indicate continued intrusive activity in the Spanish Peaks region from 26.6 Ma to 21.3 Ma with the middle period of activity (24.6-22.8 Ma) involving West and East Spanish Peaks and the radial dikes. Most of the sills and dikes in the Raton and Vermejo Park areas were intruded at about the same time (J.D. Obradovich, C.W. Naeser, and H.H. Mehnert, USGS, written commun., 1976-1983).

West of the interstate and along the route to Trinidad, the Pierre Shale is exposed beneath cliffs formed by the Trinidad Sandstone. Just past the turnoff to Ludlow, the view to the south is dominated by what appears (to some) to be the silhouette of an early 1920's Franklin automobile, outlining the hood and trunk, which is formed by the basalt flows on Fisher's Peak, the northern segment of Raton Mesa. Though not dated as yet, the flows that form the crest are considered to be about 3.5 Ma, the same as Bartlett Mesa north of Raton, New Mexico (Stormer, 1972 and Stroud, 1998).

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