U.S Fish & Wildlife Service
West of the flocks of birds and birdwatchers along the wetlands of the Rio Grande, a story of cataclysmic volcanic eruptions, ancient dune fields, and long-gone towering mountains lie quietly awaiting visitors to the Bosque del Apache National Wildlife Refuge. The Chupadera Wilderness Trail and the Canyon Trail traverse this geologic record in the contemplative solitude of a wilderness setting, affording “rockwatchers” a quick trip through millions of years of Earth history.
(WGS 84 or NAD 83)
From the north, take I–25 south to the San Antonio/US 380 exit (Exit 139). Go east on US 380 for about 0.6 miles to the junction with NM 1 (Old Highway 1). Turn right and take NM 1 south for approximately 3.3 miles to the refuge boundary. The Visitor Center is another 4.5 miles on the same road. From the south, one can follow the same directions or take Exit 124 (San Marcial). Go east on the dirt road 1.5 miles, then north on NM 1 to the southern boundary of the refuge.
Many pullouts are available along NM 1 for birdwatching, and larger, signed parking areas are provided for the two trails. The refuge also has about a 12 mile driving tour loop with birdwatching platforms and trails that is accessible from the Visitor Center area.
Geography and Geologic Province
The refuge straddles the Socorro Basin, between the Chupadera Mountains on the west and a string of discontinuous low hills and Little San Pasqual Mountain on the east. This setting is a product of Rio Grande rift crustal stretching, which broke the crust beneath central New Mexico into fault-bounded blocks, each several miles wide, that very slowly tilted like falling dominoes during progressive crustal extension. Domino blocks near the Bosque have been tilted to the east. Up-tilted edges of blocks became mountains and hills, while downtilted edges became basins. The rift here is superimposed on the eastern part of the Mogollon–Datil volcanic field. The Canyon Trail and Chupadera Wilderness Trail traverse rocks related to both the rift and the volcanic field.
The Rock Record
Volcanic rocks, dune sands, river sands, and flood gravels line the trails of the refuge’s western wilderness. The oldest rocks, 36- to 27 million-year-old lavas and welded tuffs, lie along the Chupadera Wilderness Trail at the base of Chupadera Peak. The tuffs, which are typically gray, brown, or reddish layers of fused ash and crystals, erupted from Mogollon–Datil calderas, supervolcanoes characterized by violent eruptions of large volumes of material and collapse of the land surface into the underlying magma chamber as they emptied. Features of the Mogollon–Datil field, both welded tuffs and calderas, are discussed in more detail in chapters of that section. The lavas, which are typically rubbly and black to dark reddish brown, erupted from a now-buried Mount St. Helens-type volcano in the vicinity of the refuge.
The lavas and tuffs are no more than a few hundred feet thick at the base of Chupadera Peak, but as the trail climbs westward, it ascends through 1,000 feet of a single tuff, the 29.2 million-year-old La Jencia Tuff. This unusual thickness is the result of the molten La Jencia ash filling the colossal depression left by the prior eruption of one of the largest of the Mogollon–Datil calderas, the 32.5 millionyear- old Socorro caldera. The caldera depression once extended 16 miles from Chupadera Peak northward to Socorro Peak, but subsequent filling by the La Jencia Tuff and disruption by Rio Grande rift faults now obscure most of its original extent.
Rocks of an entirely different character straddle the Canyon Trail, where 15 to 8 million-year-old, pale-brown sandstones interfinger with discontinuous, darker-brown conglomerates. The sandstones are characterized by large, curved crossbeds indicating the sands accumulated in eolian dunes that traversed eastward across the area in an arid or semiarid climate. In contrast, conglomerates were deposited by highenergy flood events. The nature of the gravels and various other features of the conglomerate beds indicate that the floods originated from the east. At the time, there was no Rio Grande here, and the low hills that today lie to the east must have once been formidable mountains to have shed such coarse material this far westward.
The youngest rocks occur along the lower reaches of the Chupadera Wilderness Trail, where uncemented gravel beds interfinger with loose, light-gray sands. The gravels clearly originated from the Chupadera Mountains to the west and not the ancient eastern mountains. The sands are deposits of the ancestral Rio Grande, which arrived in this area only about 5 million years ago.
Lavas and tuffs blanketed the area from 36 to 27 million years ago, constructing a high volcanic plateau extending westward into the Mogollon–Datil volcanic field. Rio Grande rift crustal stretching began around 25 million years ago, dissecting the plateau and uplifting mountain blocks while down-dropping the refuge area. From 15 to 8 million years ago, a large dune field occupied the southern Socorro Basin and lapped against an eastern mountain block, which shed gravels westward into the dunes during flood events. Continued stretching, fault-block motions, and erosion subsequently rearranged the topography, uplifting the Chupadera Mountains while eroding the eastern mountains. The rising western mountains blocked westerly winds, effectively killing the dune field. The Rio Grande spilled into the area from the north around 5 million years ago. Beginning about 800,000 years ago, the Rio Grande began to cut its valley, and the modern setting was established.
All that remains of the volcanoes, dunes, and ancient mountains are recorded in the east-tilted rock layers along the trails in the western refuge wilderness areas. Along the upper reaches of the Chupadera Wilderness Trail (9.5 miles round trip), examine the tilted lavas and tuffs, and while climbing to the top, notice the abundant elliptical pockets in the red cliffs of La Jencia Tuff; these pockets were produced by steam rising through the tuff shortly after eruption and before it solidified. Along the 2.2 miles Canyon Trail, notice the great thickness and continuity of the sandstone beds. The desert dune field in which these sands accumulated must have been quite extensive.