The Grand Canyon is a spectacular record of bygone eras, the tales of which are written in ancient sedimentary layers along the formation’s walls. But for more than a century, scientists have been puzzled by a vast window of time that is mysteriously missing from this geological chronology.
In some places, rocks that were deposited 520 million years ago sit directly on top of layers laid some 1.4 billion years in the past, with no sign of the billion-year stretch in between those immensely different periods. This conspicuous gap in Earth’s history, known as the Great Unconformity, hints at tumultuous and erosive processes that essentially deleted these sediments, but the exact mechanisms behind this geological brainwipe remain unclear.
Now, scientists led by Barra Peak, a graduate student in geological sciences at the University of Colorado Boulder, have pioneered new techniques “to decipher the origin of this feature in its iconic Grand Canyon exposure,” according to a study published this month in Geology.
The team used a dating method called zircon thermochronology to reveal evidence of numerous tectonic movements, known as faulting events, that occurred on very small scales of just tens of miles. The findings both deepen our understanding of the Great Unconformity’s complexities while also opening up new avenues of research for other fields.
“This study is one of the first to use this method, zircon thermochronology, to actually be able to distinguish faulting on this scale,” Peak said in a call. “This method has been used pretty widely, but generally over a much larger spatial scale.”
“I think it is definitely, in some ways, a case study for how this could be hopefully applied in other areas as well to try to look at this small-scale faulting structure,” she added. “It does heavily impact local topography, and interpretations that can be drawn from that about conditions under which rocks formed, and potentially even environmental conditions related to what was going on with the ecosystems at the time.”
The Great Unconformity is dramatically exposed at the Grand Canyon, but it has been identified in rock layers in other parts of the world. As a result, scientists have suggested that this weird memory lapse may have been caused by an erosive event that spanned the globe, such as the worldwide glaciation associated with the “Snowball Earth” period some 700 million years ago or tectonic activity driven by the evolution of the ancient supercontinent Rodinia.
By analyzing minerals collected in seven locations across the Grand Canyon, Peak and her colleagues aimed to find evidence that might support one of the various explanations that scientists have proposed to account for the missing gap in time.
“Part of what we're doing in this study was trying to see if we could pinpoint, with dating of the rocks below the Unconformity, whether or not one of those hypotheses was more likely, or corresponded to the dates that we were able to measure in the Grand Canyon,” she said.
The results revealed distinct temperature patterns in the ancient rock samples, even within the relatively small region that the team sampled. For instance, Peak and her colleagues found that samples from the western side of their field studies contained rocks that had risen to the surface of Earth about 700 million years ago, whereas samples from the eastern areas were still miles underground at that time, buried in hotter and denser layers of the planet.
“This does definitely impact our understanding of the Unconformity itself, because it's pretty clear from our results that you had erosion happening in one part of the canyon and there was not erosion happening in another part, on a very small geographical scale, which we were not necessarily expecting,” Peak said.
“Because this erosion is so widespread, and it's clear that it was a lot of erosion, the simplest explanation is that it's one big thing happening over a very large area—maybe not a global area, but still we were expecting one history for the relatively small region that is the Grand Canyon,” she continued. “But instead, we found multiple erosion periods related, we think, to faulting.”
These lopsided regional patterns could support the idea that the Great Unconformity was linked to the breakup of Rodinia, an event that caused roiling tectonic activity. The complex fracturing of the landmass could account for the small spatial variations that the team observed in the Grand Canyon. Meanwhile, on a global scale, Rodinia’s dissolution likely disrupted the deposition of sediments so that they were never preserved as geological strata, producing the Great Unconformity.
While the new study yields valuable new insights about this huge gap in time, there are still many more mysteries about this geological feature to untangle. Peak and her colleagues hope to sample a wider range of locations in North America, and the world, that might provide a more detailed look at the complicated events that erased so many important epochs in Earth’s history.
“This [study] is really pointing to a Rodinia supercontinent breakup cause for the Great Unconformity in this location,” Peak said, adding that her research group is “trying to have a more holistic and global view of how this unconformity formed” by “not looking at a single location, but trying to get a sense of it throughout North America and we hope, eventually, globally.”
“I think at this point, the question of whether or not it was a single global event has been answered and that it wasn't,” she concluded. “It was really dependent on local conditions and what was going on in different places. But it's still kind of an open question about how extensive similar conditions were from place to place on what was then a supercontinent.”