Geologists rewrite textbooks with new insights from Cambrian Rocks of Grand Canyon

Since soon after our planet formed, Earth's 4.6 billion-year-long history was dominated by single-celled life. Something dramatic happened about 500 million years ago called the Cambrian "explosion," during which an incredible diversity of life forms became preserved in the rock record. These fossils included major groups of diverse animals that evolved into animal groups still living today (including humans).

A recent paper by UNM researchers and an expanded collaborative team is featured as the lead science article in GSA Today's November 2024 issue titled The Cambrian of the Grand Canyon: Refinement of a Classic Stratigraphic Model.

"The Tonto Group of Grand Canyon holds a treasure trove of sedimentary layers and fossils chronicling the Cambrian Explosion some 500 million years ago, when the first animals with hard shells rapidly proliferated and sea levels rose to envelop continents with emerging marine life," said Carol Dehler, professor at Utah State University.

Dehler was lead author with UNM adjunct professor and paleontologist Fred Sundberg as co-lead, and coauthors including UNM Distinguished Professors Karl Karlstrom and Laurie Crossey, James Hagadorn from Denver Museum of Nature and Science, Mark Schmitz from Boise State University, and Steve Rowland from the University of Nevada Las Vegas.

In addition to research advances, the paper has geoscience education advances that build on the classic model for marine transgression (advance of the seas across the continents) developed 50 years ago on these same rocks by Eddy McKee. The McKee Model, taught to many geology students worldwide, posits gradual deepening and gradual shifts in sedimentary environments.

"Our new model for deposition of the Tonto Group is much more nuanced, showing a mixture of marine and non-marine settings, breaks or unconformities when no sediment was being deposited, and a much faster tempo of evolution," said Karlstrom. "Even more than before, the Tonto Group of Grand Canyon remains one of the most important Cambrian type sections in the world because of its complete exposure."

The National Science Foundation Division of Earth Sciences supported the work. "Our findings are a reminder that science is a process," Hagadorn said. "Our work in the Grand Canyon, one of the world's most well-known and beloved landscapes, connects people to this science in a very personal way."

Further, advanced tools provided new insights into rock sedimentation speeds and offered clues on how rapidly marine species, such as trilobites and other early animals, diversified.

"Our new tandem U-Pb dating methods are refining precise ages for each layer in the succession and for the transitions between trilobite biozones," said Schmitz, "we are finding that different trilobite species radiated, then went extinct at a very fast, sub-million year, tempo."

This method involves grinding up the sedimentary rock sample and separating out hundreds of sand-sized zircon crystals that were deposited within the sediment. The U-Pb ratios of the grains are first acquired using a preliminary method for rapid dating, then the youngest grains are precisely dated with a more elaborate (and precise) laboratory method.

"Sedimentary rocks are hard to date" said Crossey "but the deposition of the sediment and fossils entombed within it, must be younger than the age of the youngest grain so with many dates we can bracket sedimentary ages."

Dehler says the team's new model, compared to McKee's model, offers key pathways for both students and researchers to reach deeper understanding of the Cambrian explosion.

"From the Tonto Group's 500-meter-thick strata, we're learning about sea-level rise and the effects of catastrophic tropical storms - probably more powerful than today's devastating hurricanes. This occurred in a world without land plants and during a period of very hot temperatures when the Earth was ice free. Sea levels were so high that sediments like the Tonto Group were deposited atop most continents in environments that allowed rapid expansion of animal diversity on Earth."