On 24 May 1869, a group of explorers led by John Wesley Powell set out on a 3-month-long adventure, during which they explored the gorges and chasms of the Colorado River. Multiple expeditions over the next few years culminated in Powell’s vivid 1875 report: “Dame Nature kneaded this batch of dough very thoroughly,” he wrote of Precambrian rocks in the Grand Canyon.
Powell’s report also includes a detailed drawing of the Great Unconformity, so named by geologist Clarence Dutton in 1882.
The Great Unconformity commonly marks the surface that separates rocks full of fossils, younger than about 500 million years, from largely fossil free rocks dating anywhere from hundreds of millions to even billions of years earlier, said Rebecca Flowers, a thermochronologist at the University of Colorado Boulder. This surface betrays a history of either nondeposition or erosion (or both) lurking in the boundary between the rock strata.
Unconformities are relatively common in the geologic record and can be thought of like pages missing from a book, explained Kalin McDannell, a postdoctoral geologist at Dartmouth College. “You can think of [the Great Unconformity] as missing chapters” from Earth’s geologic history, he said, with many of the same chapters missing from strata across the globe.
Because the rocks are gone, investigating unconformities is challenging. “You have to use other techniques…to tease out that missing history,” said Flowers. In a talk at the Geological Society of America (GSA) Connects 2022 meeting in Denver, she discussed whether the Great Unconformity needed to be produced by a synchronous, global erosional event. She argued that it may instead be better characterized as many Great Unconformities, based on modeling when these chapters were torn from the book of Earth history.
Missing Rocks, Missing Time
To figure out how much time is missing between old Precambrian rocks and younger, fossil-rich strata on top, scientists turn to thermochronology: the temperature history of a rock—when it was hot and when it was not.
Beneath Earth’s surface, the deeper the rock is, the hotter it is. (This somewhat predictable relationship is called the geothermal gradient.) Combining that information with when a rock was hot or not tells you when a rock was at a certain depth.
That information combination, said Flowers, “can give you insight into when overlying rocks were removed by erosion and when [the sampled rocks] came to the surface.”
However, a “cooling age” calculated from a single mineral taken from a particular rock does not yield a unique cooling history; it’s just one point on what could be a simple or complicated surfaceward journey. Determining the intricacies of a rock’s travels requires using multiple minerals from a single sample that record different parts of the path and consideration of geologic information, said McDannell, who was not involved in Flowers’s study but is actively applying thermochronologic methods to research the origin of the Great Unconformity.
Snowball Earth or Something Else?
If a global-scale phenomenon really did generate the Great Unconformity, Snowball Earth may be the culprit. In this famous scenario, ice sheets like those on Greenland and Antarctica today extended all the way to the equator. If a link between Snowball Earth and the Great Unconformity resulting from subglacial erosion—glaciers scouring the continents—exists, scientists should be able to see major erosion between about 720 million and 635 million years ago (the Cryogenian period), said McDannell.
The best places to look for evidence of Snowball Earth are in areas least affected by more recent events, said Flowers—a tall order considering that the Great Unconformity formed many hundreds of millions of years ago. Her approach involved strategically targeting locations that have other geologic constraints on when the unconformity was at the surface.
In her GSA talk, Flowers discussed how thermochronologic results from the east and west sections of the Grand Canyon suggest different thermal histories—different routes to the surface—at a scale of tens of kilometers. Faulting that resulted from tectonic activity, she said, most likely drove the rocks’ upward rise at disparate times. Moreover, her models, which incorporate geologic constraints, hint at both parts of the canyon cooling prior to the time of Snowball Earth. Similarly, Flowers and her colleagues argued that in Colorado, the rocks were already at the surface by Snowball Earth time. In this example, she said, “the Great Unconformity erosional surface [likely] formed before the Snowball Earth glaciations.”
Models and Observations
Scientists rely on different modeling programs that simulate thermal histories with the goal of reproducing the thermochronologic data, said Kendra Murray, an assistant professor at Idaho State University who was not involved in Flowers’s research.
“Fitting the [thermochronologic] data is the minimum requirement, and that does not prove a model right—just consistent,” explained Kerry Gallagher, a professor at the University of Rennes in France who wrote QTQt, one of the two most commonly used modeling programs.
These modeling programs can incorporate geologic observations as well and can explore whether the best-fitting models are driven by thermochronologic data or external geologic constraints. This kind of exploration “brings a lot more clarity to what parts of our information are most important to the story that we think we see,” Murray said.
In the Grand Canyon and Colorado examples Flowers discussed, she and her colleagues combined thermochronologic data with independent geologic constraints in a program called HeFTy. In her talk, she referred to geologic constraints that all geologists agree on as “geologic facts.” For instance, rocks overlying an unconformity define the time at which those rocks must have been at the surface.
However, McDannell said the conclusions of Flowers and colleagues on the Grand Canyon require the use of those geologic constraints. The thermochronologic data on their own aren’t driving these models, he said, which means that how those constraints are integrated into the modeling program becomes paramount.
In Flowers’s talk, she discussed geologic features, called injectites, in the Colorado example that helped her and her colleagues argue for pre–Snowball Earth erosion. Flowers said the injectites suggest that the rocks were near the surface when these curious features were emplaced.
But that interpretation and the associated temperature constraints are debatable, said McDannell.
“If everyone was happy with those constraints, there wouldn’t be any arguments going on,” said Gallagher.
Glaciers Versus Tectonics
Testing the connection between Snowball Earth and the Great Unconformity is simpler in places that weren’t undergoing active tectonic faulting at around the same time, said Brenhin Keller, an assistant professor in geology at Dartmouth. In the Grand Canyon, that tectonic history could obscure a glacial signature.
Instead, examining rocks far from semicontemporaneous tectonics, said McDannell, would mean a simpler history to unwind. For instance, he and Keller collected data from tectonically quiescent central Canada, and their modeling, with and without geologic constraints, suggested significant Cryogenian exhumation—and therefore a Snowball Earth–Great Unconformity connection.
Rocks with similar palimpsests around the world need to be studied, Keller said. If a global pattern of Cryogenian erosion appears, that would strengthen the idea that Snowball Earth glaciations caused at least some of the surfaces of the Great Unconformity.
Nevertheless, Flowers’s proposition of multiple Great Unconformities (with multiple causations) may be valid. Some causations could be tectonic, others could be glacial, and still others might be both.
“There’s more than one period of erosion that goes into any given unconformity surface,” said Keller. To address the proposed Snowball Earth–Great Unconformity link, he said, the question is, “Was there erosion in the Cryogenian?” If so, then perhaps that’s the Greatest Unconformity.
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer
31 January 2023: This story has been updated to clarify potential causes of the Great Unconformity.