The San Juan Basin
By Larry Larason
There are two San Juan Basins. One is hydraulic, the other is structural. A hydraulic basin concerns water flow. All the streams, washes, and gullies that contribute water to the San Juan River are considered part of the San Juan River Basin. The San Juan River watershed encompasses 24,600 square miles. In turn, because the San Juan flows into the Colorado River, the San Juan River Basin is part of the Colorado River Basin of more than 200,000 square miles. The Colorado River flows 1470 miles, cutting through several uplifted plateaus to create Grand Canyon. Calling such an area a basin is purely metaphorical; the term watershed would seem a better descriptor. Locally, the San Juan [hydraulic] Basin drains northwesterly from the Zuni Mountains to the San Juan River. For example, Crownpoint is within the San Juan River basin, but the Rio Puerco flows through Gallup to the Little Colorado River, so we are not.
On the other hand, a structural basin is just what the name implies: the rocks are warped down to a center. The structural San Juan Basin lies within the river’s watershed and occupies much of the southeastern part of the Colorado Plateau. It stretches over 4600 square miles from Cuba to north of Durango, including Farmington, and south to near the Zuni Mountains. It is nearly circular and, in part, is up to two and one-half miles deep. It is mostly filled with Mesozoic rocks.
How did the basin form? During the Cretaceous Period the supercontinent Pangaea was breaking up. The northern portion, Laurasia, also split from north to south, sending North America drifting westward away from Europe. As oceanic crust was subducted beneath the western continental boundary, the compression caused a period of mountain building, which geologists call orogeny. This one was named the Laramide Orogeny. It began about 80 million years ago and lasted till roughly 50 million years ago – from the late Cretaceous Period into the Oligocene Epoch. As the mountains were pushed up, basins subsided. The San Juan Basin wasn’t the only big one; three in Wyoming, the Wind River, Powder River, and Big Horn, are even larger.
One of the most interesting features of the San Juan Basin is the hogback that bounds the northwestern and northern edges. As you drive US 491 from Gallup to Shiprock, you begin noticing a dip in the rocks along the route beyond Newcomb at about milepost 59. The angle of dip increases the farther north you go until it culminates in Hogback Mountain west of Farmington, where the dip is nearly vertical.
To consider what this means, first think about the hogback [Nutria Monocline] at the east end of Gallup. This hogback was formed as the Zuni Mountains were pushed up during the Laramide Orogeny. As the mountains rose, the rock in the hogback was tilted up. Erosion stripped away the softer strata leaving the Gallup Sandstone and some other hard rocks in the hogback standing tall. In other words, this hogback was created by uplift.
Hogback Mountain is just the opposite. Along the edges of the basin, rocks dipped inward as the basin sank. The basin began filling with sediments washed off the rising mountains. Even after the vast inland sea disappeared, the basin remained a swampy region for some time, inhabited by crocodiles and turtles. Later, erosion removed about 3000 feet of the surface west of the basin, leaving the eastern face of Hogback Mountain [an erosion resistant layer called Cliff House Sandstone] standing up to mark the rim of the basin.
Also remarkable, the hogback continues along the northern edge of the San Juan Basin, although it may not be as dramatic in most places as it is in Hogback Mountain. However, along US 160 between Pagosa Springs and Durango, at about milepost 122 near the bridge over the Piedra River, some vertical strata mark the basin’s rim. The hogback in this place also marks the edge of the Colorado Plateau where it abuts the San Juan Dome, another Laramide feature which is the underlying part of the San Juan Mountains, which is now obscured by extensive volcanism. The dome was the source of much of the sediment that filled in the basin.
Water in the Cretaceous inland sea lacked oxygen; organic matter that fell to the sea floor did not decay normally. Instead, over time it became petroleum or natural gas. Oil and gas exploration began in the San Juan Basin as early as 1890. Although small amounts of oil and quite a bit of gas were found, no operation was truly successful until 1921, when natural gas was piped into Aztec from a nearby well. The initial system had no pressure regulators in it. If you left your stove burning on Sunday morning and went to church, and some of your neighbors turned their stoves off, the flow of gas in your stove would increase. You might return home to find your house in flames. Still, the Aztec system was the first commercial methane operation in New Mexico. The first commercial oil well was drilled near Hogback in 1922. Although oil exploration is on-going, the focus in the basin has shifted to coalbed methane. One of the Cretaceous formations, the Fruitland, is the largest source of coalbed methane in the U. S. It has produced well over a trillion cubic feet of gas, and development is still continuing.
There are at least nine Badlands scattered across the surface of the San Juan Basin. These areas of banded strata and weird erosional forms [hoodoos] are interesting places to hike in good weather. They are also fine places to search for fossils. Finding fossils is easier in such landscapes because they lack a lot of vegetative cover and digging in the mudstone is easier than slowly chipping bones out of tough sandstone. Fossil hunting began here in the early days of the 20th century. The basin hosts a treasure trove of fossils, notably dinosaur specimens. The most recent was found in the Bisti Badlands in 1997 and excavated a year later. It was nicknamed The Bisti Beast until it could be studied and given a scientific name [Bistahievorsor sealeyi]. Because the Badlands are a federal wilderness area, the diggers had to walk into the site every day carrying their tools with them. After the bones were excavated the New Mexico Army National Guard volunteered a helicopter to pick up the load without landing and deposit it on a flatbed trailer waiting on the highway to be taken to the Museum of Natural History and Science in Albuquerque. The beast was a tyrannosaurid with a skull over 3 feet long. When alive it was probably thirty feet long and weighed approximately 3 tons.
The San Juan Basin is a significant feature on the Colorado Plateau, one that has had great economic and scientific impact on the Four Corners.