Is “The Great Unconformity” a Misnomer?

Nathan Brown, GSA Science Communication Intern, University of Texas at Dallas

500 million years ago, the rocks that once lay under the area that is now Grand Canyon were eroded away along with others around the world to create one of the unifying features of global geology: the Great Unconformity. Or was it 700 million years ago? Is it possible the erosion happened earlier in the Grand Canyon than in what is now Canada? Considering those missing layers represent as much as 1.2 billion years in some areas and possibly eroded during the largest transition life has seen, these events may hold clues to understanding the evolution of life on Earth.

While geologists are able to use radio-decay geochronology to date the age of the rocks, sorting out when the erosion took place has proven to be much more elusive. At GSA Connects 2022, Dr. Rebecca Flowers, professor at the University of Colorado Boulder, presented her collaborative research using thermal history models and geologic relationships in the field to constrain some of the timing surrounding The Great Unconformity. Based on the findings, she suggests the name “the Great Unconformities” would be more accurate.

When rocks of non-sequential age come into contact due to erosion or lack of deposition, it’s known as an unconformity. The Great Unconformity, as it is currently known, represents a time in which hundreds of millions to over a billion years of sedimentary layers were wiped from the rock record or failed to be preserved in locations around the globe. It’s thought that this widespread erosion all happened at nearly the same time.

An example can be seen in the strata of the Grand Canyon, where sedimentary rocks directly contact granite more than a billion years older. The most popular theory for the cause of such an event is the global glaciations that occurred during the Cryogenian period, between 710 and 640 million years ago, informally known as “Snowball Earth.” This period directly precedes the Ediacaran and Cambrian, during which life evolved in ways that left a much larger footprint in the fossil record than the species that had come before them. This overlap has pertinent implications for the emergence of life as we recognize it today. In the process of eroding so much bedrock, the glaciation may have freed and delivered important nutrients to life in the sea, stimulating the evolutionary boom we observe. But was the erosion truly synchronous?

One method showing promising results in dating erosional events is tracing the temperature of the rock through time, called thermochronology. Because rocks get hotter as they are buried deeper, cooling after burial suggests uplift and possible exposure to weathering. Using a physical index of these thermal changes, it is possible to glean the depth of the rock through time by creating potential models of its thermal history. Thermochronologists create these histories by making use of the progression of the uranium decay chain preserved in mineral grains as an index. When uranium and thorium undergo radioactive decay, they produce helium-4 at a predictable rate. These isotopes are then trapped within the crystal at a proportional rate, but only if the uranium/thorium-bearing mineral is below a specific temperature, known as the closure temperature. If we know the expected rate of helium production and the closure temperature, we can then determine how long it has been since a mineral was at a depth that would cause a temperature above closure, releasing the built-up helium.

An important factor to consider when analyzing the retention of helium in a mineral is how much radio decay has damaged the crystal lattice. When energetic particles break links in this structure, they allow for more helium to escape than otherwise expected. If this is not corrected for, the sample appears to have cooled later than it did in reality. With these components accounted for, thermal models make a powerful tool for a variety of geologic problems. They are not, however, definitive. There are multiple time-temperature paths to arrive at the same data points, some of which make more sense than others. This requires interpretation to come to conclusions about the actual history.

To learn about how thermochronology is applied to the Great Unconformity, I had the pleasure of sitting down with a pioneer in the field, Dr. Rebecca Flowers. She advocates for a holistic approach to the timing question, combining geologic facts with refined thermal models. Along with her co-authors and contributors, Francis Macdonald, Christine Siddoway, Rachel Havranek, Barra Peak, Colin Sturrock, and Rich Ketcham, she used these analytical and observational techniques to piece together the asynchronous nature of the Great Unconformity. Flowers proposes that the single-event analysis of the timing and cause is missing a vital component: geologic constraints. She used observations such as physical relationships to confine the characteristics of the location in question. This limits the number of models that fit the situation. For example, there is a phenomenon in which overburdening pressure from glaciers can force sand into fractured bedrock, eventually forming what are known as sandstone injectites. One locality of these are the Tavakaiv injectites within the Pike’s Peak granite. This granite was eroded by the event that created the Great Unconformity in Colorado. Because the injectites were emplaced when the granite was exposed, we can say that the layers above the Pike’s Peak granite were eroded away before the injection during the Sturtian Glaciation. This eliminates the glaciation as being the cause of the extensive erosion. Using this geologic information to restrict prospective thermal models, we can better decipher the possible histories of the unconformity by excluding any model that has the Pike’s Peak granite buried at this time.

In Grand Canyon, we know that sediment was deposited on exposed basement during the Cambrian, meaning it was cool and at the surface. Later, the underlying basement rock was further buried during the Paleozoic. These geologic facts rule out thermal models that suggest heating occurring 500 million years ago or cooling during the Paleozoic. In sum, Flowers and her collaborators assert that the unconformities of the Grand Canyon happened both before and after Snowball Earth. This approach was also extended to outcroppings of the Great Unconformity in south and central Canada, for which they concluded that erosion happened after 650 and 570 million years ago, respectively.

With age gaps so distinct, it is hard to see these unconformities as one consistent event. An asynchronous explanation is much more likely, meaning the name “the Great Unconformities” is more accurate. This shows how events that appear simple and singular are not always so, and require the integration of multiple approaches to discern their complexity. It opens the door for more nuanced and possibly regional causes of massive erosion, such as tectonic uplift.

So what does this mean for the influx of nutrients hypothesis for the Ediacaran ecosystem? Is it still possible that these erosion events played a role in shaping evolution? If so, can we see local changes that reflect the regional Great Unconformities? We can only begin to discuss these with the refinement provided by Flowers and her coworkers’ thermochronological data and fieldwork. These ever-evolving methods may play a part in answering the questions they raise.

Acknowledgment

Special thanks to Dr. Rebecca Flowers for her GSA Connects 2022 presentation and consultation on this article. 

Reference

Rebecca M. Flowers, Francis A. Macdonald, Christine S. Siddoway, and Rachel Havranek (2020). Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth: PNAS, v. 117, no. 8, p. 10,172-10,180, https://doi.org/10.1073/pnas.1913131117.