GeoNotes

Paisano Peak shield volcano, caldera and dikes

The Paisano Peak shield volcano is located 5-15 miles southwest of Alpine along U.S. Highway 90 going to Marfa. The complex was injecting, erupting and exploding from 36-35 million years before present (middle Oligocene). Today’s landforms are erosional remnants of the volcanic flows and a later stage of intrusions.

As you drive through Paisano Pass, among the striking landforms south of the highway are dikes. The lava flows and ash flows coming out of the volcano are named the Decie Formation (2400-2600 feet thick). The shield cone stood 1500-2000 feet above the surrounding landscape. On the southwest edge of the volcano there are deposits from a 3-mile-diameter caldera explosion. The Paisano Pass volcano was one of the last eruptive centers in the Davis Mountains. Mauna Kea and Mauna Loa in Hawaii are modern examples of shield volcanoes.

One of the best ways to learn about the Paisano shield volcano is to visit the rest stop on the north side of Highway 90 about 5 miles west of Alpine. The Brewster County Historical Commission sign there describes the geology of the area very well. These excellent reference signs are scattered along the highways of the county and are worth a stop wherever you encounter one in your travels.  

Paisano Pass is a long traveled animal and human migratory route. Traders bringing goods to the rich silver mines near Ciudad Chihuahua and transporting Mexican silver to the U.S. passed through from 1823-1883. Ranching by European settlers began around Paisano Peak in the 1870s preparatory to the coming of the Southern Pacific. In 1882, Chinese laborers of the Southern Pacific Railroad built their way across the natural passageway from west to east between the Marfa and Alpine basins. At 5000 feet, Paisano Pass is the highest elevation on the transcontinental line from Los Angeles to New Orleans. The Chinese crews building eastwards met the line coming west from Del Rio near the Pecos River in January 1883. In 1915, groups of ranchers began to meet to hold a revival camp in the pass. That tradition continues today as Paisano Baptist Encampment, which will be meeting the last full week of July. The encampment sits within the caldera, portending sermons of fire and brimstone.

Resources:

Petrology and eruptive history of an Oligocene trachytic shield volcano, near Alpine, Texas (Parker, D.F., 1976, University of Texas Austin Dissertation).

GeoChronology of Magmatism in the Tertiary Volcanic Field, Trans-Pecos Texas (Henry, C.D. and McDowell, F.W., Bureau of Economic Geology Guidebook 23, 1986).  

Brewster County Historical Commission sign located 5 miles W of Alpine on Highway 90 (Bones, Jim)

Three Roads to Chihuahua: The Great Wagon Roads that Opened the Southwest 1823-1883 (Swift, R.L. and L. Corning, 1988)

Paisano: Story of a Cowboy and a Camp Meeting Revisited (Stokes, Katy, 1980 and 2005).