In the Front Range of the Colorado Rockies, smack in the middle of a cliff that overlooks U.S. Highway 24, resides a very unusual geological formation. This reddish gray, sharp-edged, and erosion-resistant swath of sandstone stands in stark contrast to the crumbling, heavily weathered granites that lie on either side. Now, scientists say they have narrowed down when this anomaly and others like it in this region formed—a discovery that may give researchers new clues about the breakup of an ancient supercontinent hundreds of millions of years ago.
Many outcrops of the “Tava sandstone”—derived from a Native American name for Pikes Peak, a local landmark—are found along the Ute Pass fault, which runs along the Front Range near Colorado Springs. First noted by geologists more than 130 years ago, these deposits have long been recognized as strange, says Christine Siddoway, a geologist at Colorado College, Colorado Springs. Many sandstone formations show layers of some type, signs they were laid down over time in distinct episodes by wind or flowing water. But the individual grains in the Tava sandstone, which typically are bits of quartz measuring from 125 to 250 micrometers across, are well mixed, and they’re peppered with larger bits of quartz up to 3 millimeters in diameter. Once free-flowing but now firmly cemented together with an iron-bearing mineral called hematite, the sand grains were apparently injected into cracks in ancient granite—some of them as much as 6 meters wide—under high pressure. The now-solid Tava deposits apparently flowed from vast reservoirs of once-waterlogged sand, some of them containing more than 1 million cubic meters of material.
“This is a very unusual [sandstone],” says Arlo Weil, a structural geologist at Bryn Mawr College in Pennsylvania, who wasn’t involved in the new study. “It must have been formed by a very rapid, chaotic process.”
The Tava sandstone is unusual for another reason: It may be the only large-scale deposit in the world where sedimentary rocks such as sandstone have been injected into crystalline rocks such as granite, Siddoway says. Normally, molten material flows into cracks in sedimentary rocks and then solidifies, she explains.
For well over a century, geologists have debated the age of the Tava sandstone, Siddoway notes. Although the deposit obviously must be younger than the surrounding granite, which has an estimated age between 1.03 billion and 1.09 billion years, some teams have suggested that the Tava sandstone might have formed as recently as 280 million years ago. Now, analyses by Siddoway and George Gehrels, a geologist at the University of Arizona (UA) in Tucson, shed new light on when the deposits may have formed.
For their study, the researchers analyzed samples of Tava sandstone collected at six sites near Colorado Springs. First, they extracted between 100 and 125 tiny zircons—tiny bits of erosion- and chemical-resistant mineral—from each sample. Then they used uranium/lead dating to determine the age of each zircon (the time when its parent rock crystallized). Most of the zircons were between 1.33 billion and 970 million years old, and none were younger than 850 million years old, Siddoway says. Sometime after the zircons crystallized, possibly millions of years later, these bits of mineral eroded out of their parent rock and ultimately ended up being washed or blown into what is now central Colorado. So although the zircon ages provide clues to the age of the Tava sandstone, they aren’t definitive.
To help narrow the range of possible ages for the sandstone, Siddoway and Gehrels compared the age distributions of the Tava zircons with the age distributions of zircons found in other sandstone formations in Colorado, Utah, northern Arizona, and southern California. Statistically, the Tava distribution most closely matched the patterns in sandstones that had been deposited between 680 million and 800 million years ago, the researchers will report in an upcoming issue of Lithosphere.
The paper’s results are “strong, compelling evidence that this age range [for the sandstone’s deposition] is correct,” says Peter Reiners, a geophysicist at UA who was not involved in the new study.
That was an interesting era in Earth’s history, Weil says. An ancient supercontinent called Rodinia was breaking up, he notes, and what is now western North America was being stretched apart—a process that likely cracked the Colorado granites apart, creating voids that were suddenly filled with immense amounts of waterlogged sand that had accumulated atop the granites or nearby. The new findings “will change [geologists’] perspective on the Rodinia breakup,” especially regarding when the event occurred and whether it happened in several phases, he notes. They also suggest that the Ute Pass fault formed during that era, hundreds of millions of years before the Rockies were even born, making it much older than researchers have previously suspected, he adds.
If the fractures hosting the Tava sandstone were indeed formed during the breakup of Rodinia, the stretching and rifting of that supercontinent extended farther east than previously suggested, Reiners notes.
It's not yet clear where the reservoirs of sand that flowed into the granite fractures millions of years ago were situated, Siddoway says. She and her colleagues are now trying to figure that out. But the sands "almost certainly percolated downward from the surface into the older, underlying bedrock," Reiners says. Scientists have suggested that similar formations in Sweden formed when the immense weight of glacial ice forced sand and other loose material into fissures in underlying rocks. "These rocks are unusual and not very common, but they're not unheard of," he says.