The Great Unconformity (GU) is one of geology’s deepest mysteries. It is a gap of missing time in the geological record between 100 million and 1 billion years long, and it occurs in different rock sections around the world. When and how the GU came to be is still not totally resolved.
Now a team of researchers studying the unconformity as it occurs on the Ozark Plateau in the United States has found chemical evidence in rocks suggesting that the GU began forming toward the end of the Precambrian, between about 850 and 680 million years ago. Their evidence implies a culprit behind all of the missing rock: global tectonic uplift associated with the breakup of the ancient supercontinent Rodinia.
Forces of nature seek to even out large differences in topography, the researchers explain in a recent paper published in the journal Geology. Any sudden large-scale uplift, they posit, would have exposed relatively more Rodinian rock than normal to weathering and erosion.
The new evidence points to 6–8 vertical kilometers of fresh rock material uplifting at the end of the Precambrian. “This means there was probably a boatload of erosion,” explained Michael DeLucia, tectonicist of the University of Illinois at Urbana-Champaign and lead author of the work. As time passed, this weathering and erosion carved the GU.
How to Erase Time
Where the GU horizon exists on the planet, the difference in rock type above and below the horizon is striking: In the Grand Canyon, the Precambrian Vishnu Schist is warped and twisted compared to the Cambrian Tapeats Sandstone that overlies it. On the Ozark Plateau, at the team’s field site in a region called the St. Francois Mountains, 1.4-billion-year-old granite and rhyolite lies directly underneath 500-million-year-old sandstone.
“We drove down to the St. Francois Mountains, and we sampled a bunch of granites and rhyolites, and then separated out zircon crystals from the samples,” said DeLucia. Those zircons, he explained, were key in figuring out when the rocks began exhuming, or uplifting and then eroding.
In hot environments like those deep in Earth, zircon crystals steadily lose helium atoms, which, DeLucia explained, form at a constant rate from the radioactive decay of the elements uranium and thorium. “Deeper in the crust, helium is readily released out of the zircon,” he said. “But once you pass a certain temperature threshold as the rock rises and cools, the crystal lattice of the zircon cools enough to act basically as a jail, and you start retaining all of this helium.” When the relative amounts of uranium and thorium compared to the now retained helium atoms are known, researchers can rewind the clock to when the helium “jail” in the zircon formed—and thus when a supercontinent uplifted.
The team’s rewind of the zircon they sampled revealed that the rocks uplifted and cooled between 850 and 680 million years ago. “The results indicated that there was widespread exhumation of the craton [the large, stable nucleus of continents]—not just mountain belts,” said Stephen Marshak, a structural geologist at the University of Illinois at Urbana-Champaign and one of the paper’s coauthors.
But how much exhumation? The researchers estimate that because the zircon jail starts to close at temperatures prevalent about 6–8 kilometers below the surface, 6–8 vertical kilometers of rock would have needed to erode to expose the rocks that we see today.
The team also detected an uplift pulse—dated from 225 to 150 million years ago—timed with Pangea’s assembly and breakup. This window of uplift serves as a reality check for their method: It matches well with dates for Pangea’s evolution gleaned by other established methods for teasing out the timing of geological events.
Diving into Deep Time
The team’s paper is a prime example of “deep-time thermochronology,” as Marshak called it—a new technique for dating ancient uplift events.
Before, it was thought that very old zircon grains could not provide reliable dates with this method. Such ancient crystals tend to “leak” helium atoms because their crystal lattices are damaged by radioactive decay, said coauthor William Guenthner, a thermochronologist also at the University of Illinois at Urbana-Champaign and a pioneer of the method. However, the team was able to quantify this leaking, which, depending on the amount of damage, occurs at a steady rate.
One factor that may muddy the team’s conclusions is that hot fluids moving through the crust might have reset the zircon grains’ internal clocks, according to Shanan Peters, a sedimentologist at the University of Wisconsin–Madison who was not involved in the work. Peters explained that there are deposits in the team’s field area that formed thanks to hot fluids moving through the rock.
But if hot fluids did, indeed, reset the clocks, then the change would not have occurred across the board, DeLucia noted—there would be a predictable signal in their data set, with certain grains having reset to the time of fluid flow. “And we don’t see that,” he said.
Making a Snowball
The timing of the GU’s formation may also help explain what triggered the so-called “snowball Earth” glaciations, an episode beginning about 720 million years ago in which much of the planet likely became covered in ice sheets.
Chemical weathering associated with the erosion that formed the GU likely pulled the greenhouse gas carbon dioxide out of the atmosphere, sequestering vast quantities in the ocean and lithosphere. This “primed the pump for the glaciations,” said Guenthner.
The sequestration would have likely helped cool the planet to such an extent that a snowball state could initiate, he explained.
But some questions remain unanswered, Peters noted. For instance, why didn’t the uplift of Pangea also lead to the formation of something like the GU or lead to glaciations akin to the snowball Earth events?
“From the point of view of sediments, the Great Unconformity is completely unique,” Peters said. And therein lies the next mystery to solve—determining what made Rodinia’s uplift so different.
—Lucas Joel, Freelance Writer; e-mail: [email protected]m