Sevilleta National Wildlife Refuge

Sevilleta National Wildlife Refuge (NWR) is located in central New Mexico about 15 miles north of Socorro. The refuge spans the Rio Grande valley from the Sierra Ladrones on the west to the Los Pinos Mountains on the east, an area of approximately 30 miles by 15 miles, encompassing 230,000 acres. The area was designated a Spanish land grant, Sevilleta de la Joya, in 1819. The land grant was sold in 1928 to Socorro County and then, in 1936, to General Thomas D. Campbell, under whom it became a ranch for sheep and cattle. The Campbell Family Foundation donated the land to the Nature Conservancy in 1973, which then passed it on to the U.S. Fish & Wildlife Service to become a national wildlife refuge dedicated to preservation and enhancement of the integrity and natural character of the land. The transfer stipulated that the land should undergo natural processes of succession, including floods and fires, without human interference. Thus, most of the refuge has very limited public access, but the Mesa View Trail west of the Visitor Center is open for hiking at least five days per week.

Location

Geological Overview

Sevilleta NWR straddles the southern edge of the Albuquerque Basin of the Rio Grande rift where it narrows into the Socorro Basin. Pre-rift rocks (older than 25 million years and ranging in age back to 1.6 billion years) are exposed on both sides of the Rio Grande valley in the southern part of the refuge. Rift-flank uplifts to the west are the Sierra Ladrones and the southeastern margin of the Colorado Plateau. To the east are the Los Pinos Mountains, where a complex zone of faults related to Laramide crustal shortening 75–45 million years ago is located. The Rio Grande rift is still expanding as western New Mexico moves very slowly away from eastern New Mexico, resulting in east-west stretching and north-south rending of the continent. Hot mantle rocks have risen to fill the space beneath the stretched crust, resulting in episodic volcanic eruptions within the rift. The most recent manifestation of this is the Socorro magma body, a pancake-shaped igneous intrusion located at a depth of about 12 miles. The Rio Grande itself is a relatively recent addition to the geologic story. It began to flow through the southern Albuquerque and Socorro basins about 5 million years ago.

Geologic Features

The Mesa View Trail starts and ends at Sevilleta NWR Visitor Center and forms two loops ascending, crossing, and descending the top of the mesa to the west, where panoramic views are stunning. The trail rises 0.8 miles and more than 200 feet to the mesa at the top of the cliffs and branches there. The north loop (3.8 miles) parallels the top of the cliffs along the edge of the mesa to a spectacular view of an eroded bowl exposing the downthrown and warped sediments west of the north-trending Cliff fault. The route continues to the north and east on the downthrown side of the fault, then crosses through a narrow gap cut into the upthrown side, and continues south to the Visitor Center over gentle ridges and drainages. The south loop, newly called the Ladrones Vista Trail (1.9 miles) follows the Cliff fault and adjacent eroded bowls cutting the downthrown block of sediments. Before turning east, the trail crosses a high terrace of the Rio Salado, today 3 miles farther south. The route descends across tilted sediments and crosses the fault onto deeply eroded sediments on the upthrown side of the fault. The trail then continues east and north to the Visitor Center.

The Mesa View Trail traverses weakly-consolidated sedimentary rocks, uplifted by the Cliff fault, that are about 2 to 4 million years old. Gravels exposed in the cliffs were carried here by the Rio Salado and the Rio Puerco. From the top of the trail, magnificent vistas of the vast surrounding landscape show the extent of the rift across Sevilleta NWR and the many fault blocks forming the distant hills and mountains. Much of the area between the Ladrones Vista Trail and San Lorenzo Canyon is currently being uplifted by the Socorro magma body, but so slowly that it is not noticeable without careful measurement.

Additional Reading

Block diagrams and cross sections illustrating geologic and tectonic evolution of the Sevilleta National Wildlife Refuge, Rio Grande rift, central New Mexico
— R.M. Chamberlin and D.W. Love, New Mexico Bureau of Geology and Mineral Resources, Open-File Report 579, 2016.