Raton Basin

Colorado-New Mexico

This outcrop is along Interstate 25 in Colorado USA about 4 miles (6.5 km) north of the New Mexico border. The horizontally layered rocks are in the Raton Formation, a thick (350-meter) package of strata that spans the Cretaceous-Tertiary boundary. The Raton Basin subsided at least in part because of the tectonic rise of the Sangre de Cristo Range to the west, which in turn was part of the compression in Late Cretaceous time (80 to 55 million years ago) we call the Laramide-Sevier Orogeny. That’s the mountain-building event that uplifted most of the Rocky Mountains we see today.

The Raton Basin was a closed, swampy environment, and the Raton Formation contains abundant coal, some of which you see in my photo (black powdery layer spilling down the slope). Rivers brought coarse sands and silts into the coal basin from the adjacent uplifts. Some of those sandstones are represented in the photo by the light buff to tan material.

The famous K-T boundary, when an asteroid impacted earth about 65 million years ago and extinguished the dinosaurs and much more, is visible in places in the Raton Formation (though I think not in this photo). It is represented by a thin (a few centimeters) clay that has a significantly elevated iridium content, one of the hallmarks of the strata deposited at that time.

The vertical dikes that cut the Raton Formation in the photo are probably feeder dikes that carried magma, molten rock, up to surface flows that now cap Raton Mesa a few miles east of the photo location. The basalt on Raton Mesa is probably between 8 and 2 million years old, so that would probably be the age of these dikes as well—geologically very young. Volcanism in this region has continued until quite recently (e.g., Capulin Volcano in northeastern New Mexico, which erupted about 60,000 years ago or less). The basalt flows and recent volcanism are probably related to a branch of the Rio Grande Rift, an active pull-apart zone that extends from south-central Colorado through New Mexico to El Paso.

The coal beds in the Raton Formation contain abundant methane, and the Raton Basin has been one of the more important producers of coalbed methane natural gas in the U.S. (Hoffman & Brister, 2003, New Mexico’s Raton Basin coalbed methane play: New Mexico Geology, v. 25, n. 4 pp. 95-110.)

Raton (Spanish for mouse) Mesa was named for the abundant rodents in the hills there. Bubonic plague still occurs in the area at times, with about 290 human cases in New Mexico over the past 75 years.