Santa Fe National Forest
Introduction
The Gilman Tunnels are on NM 485 along the Rio Guadalupe in the southwestern Jemez Mountains, approximately 5 miles northwest of the intersection of NM 4 and NM 485. Two narrow and unusually high tunnels were cut through Precambrian granite in the 1920s to facilitate passage of logging trains through this particularly rugged and constricted section of Guadalupe Canyon, known as the Guadalupe Box. Logs that were harvested in the western Jemez Mountains in the 1920s were taken by narrow-gauge railroad to a sawmill in Bernalillo. The tunnels were enlarged in the 1930s to accommodate logging trucks. Logs were hauled out of the mountains and then loaded on trains at Gilman logging camp, which was established in 1937 about two miles south of the tunnels. The railroad was shut down by flooding along the Jemez and Guadalupe Rivers in 1941. The highway now occupies the old railroad bed. Aside from providing access to the Guadalupe Box itself, NM 485 provides an unparalleled view of the stratigraphy of Guadalupe Canyon.
Longitude: -106.763763428
Latitude: 35.7318124306
(WGS 84 or NAD 83)
Location
The Gilman Tunnels are part of the Jemez National Recreation Area, on the Santa Fe National Forest. Access to the tunnels is by way of NM 485, a paved but narrow road that branches to the northwest off NM 4 just north of the Walatowa Visitor Center. From the north the tunnels can be reached via Forest Road 376 (unpaved), which heads south off of NM 126 several miles east of Fenton Lake.
Regional Setting
The Gilman Tunnels lie in the transition between the Jemez Mountains and the Sierra Nacimiento. The Jemez Mountains are part of a volcanic field that has been active for the past 12 million years. The Sierra Nacimiento has a long, complex geologic history. The Sierra Nacimiento formed mainly during compressional Laramide deformation, starting about 75 million years ago, but has also been affected by extension along the Rio Grande rift. The Precambrian rock through which the tunnels are cut was brought to the surface along the Jemez fault, a major northeast-trending, rift-bounding fault, the same fault that formed Soda Dam to the northeast.
The Rock Record
Granitic rocks that formed in the roots of an ancient 1.6 billionyear- old mountain range are exposed at the tunnels, northwest of the Jemez fault. Also exposed in the vicinity is the most complete section of Mississippian to Pennsylvanian sedimentary rocks in the Jemez Mountains. Younger rocks, including brick-red sandstone and siltstone of the Permian Abo Formation, red-orange, cross-bedded sandstone of the Permian Yeso Formation, white sandstone of the Permian Glorieta Formation, brick-red mudstone and sandstone of the Triassic Moenkopi Formation, and yellowish-tan, cross-bedded sandstone of the Agua Sarca Formation in the Triassic Chinle Group, are exposed just downstream of the tunnels on the southeast side of the fault. Bandelier Tuff is preserved high on the cliffs.
Geologic History
The granitic gneiss at the Gilman Tunnels likely originated as granite that was intruded at great depth in a mountain range that formed the southwestern margin of the North American continent. The granite was subsequently deformed, causing platy micaceous minerals to become aligned or foliated, forming the gneissic texture we see today. Resting on the gneiss are gray Mississippian limestones that record the presence of a shallow ocean approximately 330 million years ago. A gap in the rock record of approximately 1.27 billion years, known as the Great Unconformity, occurs between the gneiss and the limestone. The shallow ocean persisted into Pennsylvanian time with the deposition of limestone, shale, and sandstone of the Osha Canyon Formation, Sandia Formation, and the Madera Group. All of these units are quite fossiliferous. Pieces of granitic gneiss in the upper beds of the Madera Group indicate that the Sierra Nacimiento was a mountain range even in Pennsylvanian time. The seas withdrew toward the south, to be replaced by the south-flowing river system preserved in the Abo Formation. The climate became increasingly dry during Permian time, which led to the deposition of the cross-bedded, eolian (wind-blown) sand dunes in the Yeso Formation.
Sea level rose again during late Permian time, causing sedimentation of white sand beaches and offshore sand bars in the Glorieta Formation. Sea level dropped and the river systems established themselves in the area. These river systems are recorded in the Triassic Moenkopi Formation and the Agua Sarca Formation of the Triassic Chinle Group. The yellow-tan, cross-bedded Agua Sarca Formation was deposited by a river that flowed from Texas to Nevada and contains abundant petrified wood. No rocks that were deposited between 225 million years ago and 1.6 million years ago elsewhere in the Jemez Mountains are preserved in this part of the range; these rocks were either eroded away during Laramide deformation or were never deposited here. The region that includes the Gilman Tunnels was blanketed by the 1.6 million-year-old lower Bandelier Tuff when it erupted from a precursor to the Valles caldera, about 13 miles to the northeast of this spot. The hot ash, pumice, rock, and gas cloud rolled over a rugged landscape, filling in old river valleys. River gravels from those old valleys are now preserved under the tuff high on the canyon wall to the east of the tunnels, a classic example of inverted topography. A second large eruption covered the area with tuff 1.2 million years ago. In the 1.2 million years since that eruption, the Rio Guadalupe, which flows into the Jemez River about 5 miles downstream, has cut the beautiful canyon that we see today. The Jemez fault zone is responsible for bringing the ancient 1.6 billion-year-old rocks to the surface and has offset the 1.2 millionyear- old Bandelier Tuff, dropping the tuff down about 50 feet to the southeast, indicating that this fault has been active in the relatively recent geologic past.