By Larry Larason
It should be spring by the time you read this – time to get out and about. Pack a picnic lunch and take a trip to see some diatremes and other things close to home in the Chuska Mountains. This is a loop drive up 491, across Narbona Pass, and back to Gallup on Navajo 12. The pass was originally named Washington Pass after Col. James Washington, civil governor and military commander of New Mexico Territory, who led an expedition in 1849 that resulted in the death of the Navajo leader Narbona. Until the 1990s most Navajos assumed that the pass had been named for President George Washington. Students in Martin Link’s Navajo history class at Diné College were disgusted when they learned the truth and undertook a project to have it renamed in honor of Narbona [1766-1849]. It took six years, but the change was made in 1992. Oddly enough, the Mexican military leader, who came across the pass in 1805 to lead a punitive expedition into Canyon de Chelly, was Lt. Col. Antonio Narbona. His force killed more than one hundred men, women and children who were hidden in what is now called Massacre Cave in Canyon del Muerto.
Drive north out of Gallup on US 491. At Tohatchi the highway makes a broad curve around the end of the Chuska Mountains. When it swings north again [at about milepost 29] you begin seeing debris from a landslide between the road and the mountains. This landslide extends at least 25 miles along the eastern side of the peaks and reaches out in some places to eight miles from the cliffs at the top. We don’t know whether this slide happened all at once or over time, but it represents a significant collapse on the steep eastern side of the Chuskas. Some geologists believe that the slide happened during a wet period because some of the large blocks of stone seem to have slid, rather than tumbled, down slope. No one knows when the slide occurred. It was probably before humans lived in the area.
At Sheep Springs turn west onto NM 134. Notice that to the north you can see Bennett Peak and Ford Butte. Both of these are diatremes in the Navajo Volcanic Field. You might be able to see the tip of Ship Rock, as well. Narbona used to station lookouts atop Bennett Peak to watch for enemy incursions.
After about a mile on NM 134 you will see more evidence of the landslide. It looks almost like construction debris piled in heaps. At a couple of places along the drive look for outcrops of the Tohatchi Formation, a gray, layered mudstone. The Tohatchi was laid down during the upper Cretaceous and contains some petrified wood and isolated dinosaur bones.
The Chuska Mountains separate the San Juan and Black Mesa Basins. You can view the San Juan Basin behind you.
It was on this pass in 1835 that Narbona, leading 200 warriors, ambushed an expedition of almost 1000 men from Santa Fe who were seeking captives for the slave trade. The leader, Blas de Hinojos, and many of the slavers were killed.
Near the top of the pass you enter the crater of a diatreme. A tall rock [a volcanic plug] stands like a sentinel at the eastern portal. This and the Sonsela Buttes are among the few places in the Navajo Volcanic Field where lava made it to the surface. The flows here were mostly just that – flows, not explosive eruptions like those you see at volcanoes.
I’ve written about diatremes before: GJ 5/’06; 8/’08; and 2/’11. But I’ll recap some of the information about them. Diatremes are gaseous eruptions. The gas erupts in successive surges with such force that rock is shattered or even pulverized by the explosions. Breccia falls into a tuff ring around the crater and back into the pipe. Most diatremes are created when ascending magma encounters ground water, which flashes to steam, triggering the eruptions. Although lava may not break through the surface, it often flows upward through the weakened crust to create dikes around the necks of diatremes. These dikes, like those seen prominently at Ship Rock, are exposed by subsequent erosion. There are more than 300 diatremes in the Four Corners, the best known being Ship Rock and Agathla Peak.
The Narbona Pass crater, or maar, is about two miles in diameter. The walls around it are 700 feet high. We can’t date the maar, itself, but basalt magma probably flowed shortly after the eruption. It came in three phases from 27.5 to 24.3 million years ago. Some of the basalt ponded on the floor of the crater, but it is hard to see because it is mostly covered by alluvium.
Near Milepost 11 are snow equipment sheds. This road has to be kept open in winter because the FAA maintains a radar station just south of here.
In the crater, look for a basalt outcrop with columnar jointing on the north side above the highway. Here and there you will notice greenish soil. This is the pulverized rock brought up by the diatreme explosions.
This maar is eroded, but not as much as many of them in our region. For example, on the southeast you can see a wall of layered eruption debris that was part of the tuff ring. Each layer in this wall probably represents another eruptive surge.
In a road cut you can see light-colored Chuska Sandstone. The Chuska Formation occurs nowhere but on top of these mountains and in a couple of outliers, such as White Cone near Wheatfields Lake. Notice the interesting cross bedding in the sandstone, which indicates that it was deposited in sand dunes. But dunes form on flats and in basins, not on the tops of mountains! How did they get here? In 2003 geologists proposed that they formed on a piedmont extending out from the San Juan Mountains, which were pushed up during the Laramide Orogeny 80-75 million years ago – the same time that the Chuska Mountains raised. The San Juans shed alluvium at a rapid pace partially filling the San Juan Basin. Then about 34 million years ago the climate cooled and dried in North America; wind began whipping sand into dunes, not just here but in the Mogollon-Datil region, in Wyoming, on the Great Plains, and so on. The Chuska Sandstone is composed of fossilized dunes from that period. It probably covered much of the San Juan Basin, as well. Erosion has since stripped about 1500 meters from the basin, leaving remnants of the Chuska Sandstone isolated here on top the mountains. More recently it has been proposed that the dunes were part of a massive erg, or sea of sand, that covered 100,000 square kilometers and extended continuously as far as the Rio Grande Rift. Again, erosion removed this desert except for what remains on the Chuska Mountains.
The day use area is a good place for a picnic in the shade of pines and aspens. [Note: when I was scouting this article in early March, there was still a lot of snow on the ground, but it should be clear in April.]
After lunch continue on NM 134 to the intersection with Navajo 12. Turn south. At milepost 43 look ahead on the left for Green Knobs. It’s just beside the road on the east. Green Knobs is a large diatreme that has intruded through the Triassic Chinle Formation and Jurassic sandstone. The knobs are less than one kilometer in diameter, and there are no basalt dikes associated with them. The green color comes in part from the mineral olivine. The rock in Green Knobs resembles kimberlite, the source of diamonds in Africa. Several writers still call it that, but more recent analysis has demonstrated that the constituent minerals are wrong. It is serpentinized tuff – that is, it was chemically altered after emplacement. I wrote about Green Knobs in January, so I won’t say any more.
The volcanic neck on the left in Navajo N. M. rises out of the red sandstone to loom over town like something out of a Godzilla movie. It is called appropriately “the Beast.” Some call it “the Frog”, although I’ve never seen the resemblance. There is another less obvious diatreme, or volcanic neck, at the North end of town, which has been cut down for road fill. The drive to Fort Defiance is one of the prettiest in this region. Enjoy!
South of Fort Defiance, Black Rock, a dike-like volcanic neck, is on the west.
Continue through Window Rock and go on to Gallup.