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The Grand Canyon tells one of the world’s greatest geologic stories. Its distinctive features allow researchers to piece together the history of this unique location, one of America’s treasures and a UNESCO World Heritage Site. Thinking of the geologic record as a book is helpful to understand each page of Earth’s history. The beginning of the story starts at the bottom of the canyon and moves forward in time as you get closer to the rim.

One way he does this is by pointing out the discrepancies in Radiometric dating, specifically of Grand Canyon rocks. He begins by suggesting that Radiometric dating has many flaws and does not make a good clock because the initial conditions are not known and the rate of decay may not be constant (Austin 1994: 129, 2005). Anthony Fauci has left his mark on medical science and, it appears, singles looking for love as well. That’s because President Joe Biden’s chief medical adviser has apparently inspired a new term for a particular type of dating: “Fauci-ing.” Urban Dictionary defines “Fauci-ing” as.

(Credit: Annie Scott, USGS. Public domain.)

Introduction to Grand Canyon Geologic Principles

Stratigraphy is the study of the rock layering, and reveals a wealth of information about what Earth was like when each layer formed. In the Grand Canyon, there are clear horizontal layers of different rocks that provide information about where, when, and how they were deposited, long before the canyon was even carved. The Law of Superposition states that sediment is deposited in layers in a sequence, the oldest rocks are on the bottom and the youngest rocks are on the top, similar to the way that sand piles up in an hour glass. This principle is a key part of determining the relative age of a rock layer. The three main rock layer sets in the Grand Canyon are grouped based on position and common composition and 1) Metamorphic basement rocks, 2) The Precambrian Grand Canyon Supergroup, and 3) Paleozoic strata. These three main sets of rocks were first described by the explorer and scientist John Wesley Powell during his expeditions of the Grand Canyon in the late 1860s and early 1870s. To learn more about the Powell expeditions, visit http://www.usgs.gov/Powell150. A USGS geologic field photograph map of the Grand Canyon can be viewed or downloaded here.

The image on the right is a stratigraphic section of Grand Canyon by John Wesley Powell (1875). “A” is the metamorphic basement complex (Early Proterozoic Vishnu Group), with igneous intrusives labeled “a”; “B” is the Grand Canyon Supergroup (Middle and Late Proterozoic); “C” indicates the Paleozoic strata; “x” and “y” delineate the major unconformable contacts. The image on the left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity that was first noted by Powell (credit: Annie Scott/USGS).

Unconformities are gaps in the geologic record that occur when rocks or sediments are eroded away and time elapses before new deposition occurs. New sediment eventually forms new rock layers on top of the eroded surface, but there is a period of geologic time that is not represented. You can think of unconformities as missing “pages” in the book of the geologic record. Missing layers may seem like a problem, but the very fact that there is this gap in the record provides information to geologists, indicating changing ocean levels or changes in the Earth’s crust. In the Grand Canyon, unconformities are common in the Grand Canyon Supergroup and the Paleozoic Strata.

The three main types of rock are igneous, sedimentary and metamorphic. Igneous rocks are cooled magma (melted rock found underground) or lava (molten rock found above ground). Granite (cooled from magma, known as an intrusive igneous rock) and basalt (cooled from lava, known as an extrusive igneous rock) are two types of igneous rocks. Sedimentary rocks are formed by smaller pieces of sand and mud stick together in layers. Examples include: sandstone, mudstone ,shale , siltsone, chert, limestone, and more. Sedimentary rocks often contain fossils that can be used to help identify the age of the rock. Certain fossils, called index fossils, are particularly useful because they are abundant in a relatively narrow time range. Over time, pressure increases as sediment increases, and minerals help form these rock layers. Metamorphic rocks are formed when sedimentary or igneous rocks change due to exposure to heat and/or pressure. All three rock types can be found in the Grand Canyon, and each layer adds an important understanding to the geologic history of the region.

Metamorphic Basement Rocks

The oldest rocks in the Grand Canyon, found at the bottom of the canyon, areprimarily metamorphic, with igneous intrusions (the name given to when magma or lava enters or cools on top of previously formed rock). The intrusive igneous rocks here are called Zoroaster granite. The name given to this rock set (the combination of metamorphic and igneous rock of a certain age found at this location) is Vishnu Basement Rocks. Primarily schist (metamorphic) with granite (igneous), these rocks have visible crystals and are about 1.7 billion years old, from an era early in Earth history known as the Proterozoic. On Powell’s expedition to explore and map the Grand Canyon, he named this part of the exposed rock “The Granite Gorge.” This rock set tells the story of the creation of North America, when volcanic islands collided with the continental landmass, forming metamorphic rocks through the intense heat and pressure. Volcanism continued after the collision and igneous intrusions continued after metamorphosis. [1]

(Public domain.)

The Grand Canyon Supergroup

The middle rock set, the Grand Canyon Supergroup, is primarily sandstone and mudstone, both sedimentary rocks, with some areas of igneous rock. They are from the late Proterozoic, only slightly younger than the metamorphic basement rocks. These rocks do not contain many fossils, because they formed before complex life on Earth was common. The few fossils that are present include stromatolites, columns of sediment formed by cyanobacteria. The composition (sandstone) and presence of stromatolites indicate that this area was previously a very shallow sea. The rock layers in the Grand Canyon Supergroup have been tilted, whereas the other rocks above this set are horizontal. This is known as an angular unconformity. The top of these sediment layers was then eroded away, forming the Great Unconformity.

Paleozoic Strata

These layers are sedimentary, and primarily sandstone. This set makes up most of the typical reddish layers that you often see in images, and which come to mind when thinking of the Grand Canyon. Following the Great Unconformity, this set is much younger than the other rock layers, and fossils are prevalent. The most common fossils are small sea creatures, such as brachiopods, bryozoans, coral, and crinoids. Combined with the sandstone, this tells us that the region was a warm, shallow sea when these sediments were deposited.

Old Rocks, Young Canyon

A key feature of the Grand Canyon is, unsurprisingly, the canyon itself. The width and depth make it truly remarkable, and expose the rock layers that were discussed above. After all the rocks were deposited, there was a period of uplift (where plate tectonics literally force a section of the Earth upward), setting the stage for canyon formation. It provided a high enough elevation that water could flow downward, cutting through the rock as it went.

This incredible formation was carved over millions of years by the Colorado River. The canyon itself has formed much more recently than the deposition of rock layers, only about 5 million years ago (as opposed to the rocks, the youngest of which are a little less than300 million years old). The canyon has since been forming at varying rates, with periods of intense erosion carving the canyon. The river must have had periods of quick movement, carving deep, not only wide. To view or download a 2018 report about the Colorado River downstream of Grand Canyon, click here.

The confluence of the Colorado and Little Colorado rivers, showing their paths through the rock

(Public domain.)

The river continues to be an agent of change, reshaping the canyon over time. The canyon isn’t fully formed as long as there is water flowing. There is ongoing research about river flow, sediments, and geomorphology. The Glen Canyon Dam controls the Colorado River now, providing electricity to six states and changing the natural flow patterns. Since the construction of the dam in 1963, researchers have been studying how changes in river flow affect the erosion and deposition of sediment along the Colorado River and the changes to riparian vegetation and food webs.

More Information about Fossils

The Paleozoic Strata contain many fossils that help scientists learn about the geologic history of North America. Most of the fossils are ocean-dwelling creatures, telling us that the area now in the middle of Arizona was once a sea. Some of the most common fossils found in the Grand Canyon are listed below.

Trilobites were invertebrates that lived in shallow marine environments and varied widely in size. They are index fossils for the Paleozoic, and were particularly prominent during the Ordovician.

Tracks and burrows are known as trace fossils, because they are not preservations of the actual organism, but instead show where the organism moved and lived. They are commonly tunnels dug by trilobites and worms in muddy ocean sediment.

Brachiopods left shells behind that are quite common in Paleozoic rocks.

A photo of Grand Canyon rocks, displaying superposition, the Great Unconformity, and the angling of the Grand Canyon Supergroup rocks

(Public domain.) https://education.usgs.gov/lessons/schoolyard/superposition.html

The North Rim of the Grand Canyon, showing a tree growing on a ledge overlooking the canyon

(Public domain.)

Visualization of map of riverbed and canyon walls near Navajo Bridge, 4.5 miles downstream from Lees Ferry, Arizona. River bathymetry was measured with multibeam sonar and topography was measured with a boat-mounted laser scanner. The data from this survey collected in April 2016 will be used to measure changes in sand storage on the river bed and to model streamflow and sand transport.

(Public domain.)

Learn More

Geologic Map of the Grand Canyon: https://pubs.usgs.gov/imap/i-2688

The Grand Canyon Monitoring and Research Center: https://www.gcmrc.gov/about/about_default.aspx

Geologic Maps of the Grand Canyon: https://geomaps.wr.usgs.gov/arizona/

Information about John Wesley Powell and his expeditions: https://pubs.er.usgs.gov/publication/pp669

Grand Canyon Geology Training Manual: https://www.nps.gov/grca/learn/nature/geology_manual.htm

NPS Introduction to Grand Canyon Geology: https://www.nps.gov/grca/learn/nature/grca-geology.htm

USGS Educational Videos: https://education.usgs.gov/videos.html

Citizen Science: Find projects to get involved in with the USGS https://txpub.usgs.gov/myscience/

The Trail of Time, an exhibit at the Grand Canyon showing different rocks and their ages: http://tot.unm.edu/w-elves.html

NPS Geologic Tours: https://www.nps.gov/subjects/geology/geologic-tour.htm

Grand Canyon Fact Sheets: https://www.nps.gov/grca/learn/education/learning/upload/GeoArticle-11-1-11-2017.pdf

Fossils in the Grand Canyon: https://www.nps.gov/grca/learn/nature/fossils.htm

Geologic Field Photograph Map of the Grand Canyon Region: https://pubs.usgs.gov/gip/0189/gip189.pdf

References

Timmons, Stacey. Grand Canyon Geology Training Manual. Dec 16, 2013.

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The history of the Earth and of life on Earth is written in sedimentary rock layers. To understand the history, you must understand the rocks. Sedimentary rock layers of 541- 485 million years old, from the Cambrian Period, provide a record of incredible mystery—what caused the fairly abrupt, in geologic terms, appearance on Earth of a great diversity of early animal forms and their preservation as fossils?

This question is tied to several other questions about the environments the animals lived in: how much oxygen was there in the atmosphere and oceans, how was carbon cycled between organisms and the oceans, and how were the continents arranged in terms of the plate tectonic assembly and break up of supercontinents?

A new research project, with investigators from Boise State University, The University of New Mexico, Utah State University, University of Nevada Las Vegas, Denver Museum of Nature and Science, and Museum of Northern Arizona, has been funded with a three-year, $815,000 grant by the National Science Foundation’s Sedimentary Geology and Paleobiology program.

This team of scientists, including Professors Karl Karlstrom and Laura Crossey from UNM’s Department of Earth & Planetary Sciences, is undertaking a detailed study to find new occurrences of fossils such as trilobites, brachiopods, and microfossils, to measure meter-by-meter how carbon-isotope values changed through time, and to apply new technologies for precise dating of rock layers.

“The Earth is 4.56 billion years old, and life appeared on our planet very early, at least by 3.8 billion years ago,” said University of New Mexico Earth and Planetary Sciences Professor Karl Karlstrom. “Life during the first 85 percent of life's history on Earth (from 3.8 billion to 0.63 billion years ago) consisted of single celled organisms.”

James Hagadorn from the Denver Museum of Nature and Science is coordinating a team of paleontologist to study the fossils. “The timing, causes, and impacts of the debut of animal life, known as the Cambrian “Explosion,” are poorly understood”, said Hagadorn. “Yet fossil clues to understanding this event are entombed in Cambrian strata that formed as oceans flooded the world’s continents, and as coastal environments blanketed the landscape with vast swaths of sand, mud, and fossils.”

This explosion in the diversity of complex multicellular animal life happened very rapidly during the Cambrian Period between 539 and 485 million years ago and the reasons for this giant jump start towards modern animal diversity are still being debated, as well as why some of the early animals, including the trilobites, began to develop hard shells that allowed them to be preserved better in the rock record.

Boise State University Professor Mark Schmitz is the project lead and person responsible for the U-Pb dating. “The pace and tempo of the evolution of life on Earth, even after centuries of advances, remains a major detective story for geologists,” said Schmitz. “The Cambrian explosion was an extroardinary event and rocks from this time period are well exposed in Grand Canyon. These strata and their fossils contain the clues for why life developed so fast and so far towards what we know to be modern animal diversity. As in many detective stories, knowing the when and where provides essential clues for figuring out the how and why.”

Utah State University Professor Carol Dehler will coordinate the geochemical analyses of the rock strata. “The carbon encased in these amazingly well exposed strata will provide scientists with a “barcode’ of changes in the carbon cycle alongside the changes in biota. These records, when calibrated by U-Pb dating, will provide a window into the causes, styles, and rates of evolutionary change in the Grand Canyon area and will also be compared to other records around the world. One of the best parts, though, is collecting the samples, bed by bed, and documenting the promising information that is recorded through these thousands of feet of strata.”

Major results of this study were published (May issue, 2020) in a pair of articles in the peer-reviewed journal “Geology.”

The Karlstrom paper, Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale, now redefines the Tonto Group to include layers above and below that were previously poorly understood by scientists; the Sixtymile Formation is added to the bottom and the Frenchman Mountain Dolostone to the top based on new radiometric dating.

This change has global ramifications because the new age of 508 to 497 million years old for most of the Tonto Group is much younger than was previously thought. This indicates that seas flooded North America very quickly and that other continents experienced the same event. The Tonto Group dating has also recalibrated the international Cambrian timescale by adding a new location where the time of key extinctions and appearances of fossil groups is precisely and accurately dated.

One of the best-preserved records of these events is in the bottom of the Grand Canyon—in a package of sedimentary strata known as the Tonto Group. The Tonto Group is spectacularly exposed in 3 dimensions (4 counting the time dimension) through much of the 270-mile-length of Grand Canyon and its tributaries. The last time these layers and their fossils were studied in detail was about 75 years ago, by Grand Canyon geologist Edwin McKee.

The second paper, Asynchronous trilobite extinctions at the early to middle Cambrian transition, led by F.A. Sundberg from the Museum of Northern Arizona, also includes other team members as coauthors. This paper addresses the question of whether trilobite radiations and extinctions were globally synchronous, versus geographically restricted and diachronous.

They studied the traditional lower to middle Cambrian boundary across where two major groups of trilobites were thought to go extinct at the same time as a third group appeared. Instead, based on the new U-Pb dating these three trilobite groups, paradoxidids, olenellids and redlichiids, and their associated biotas overlapped for about 3 million years, indicating that trilobite transitions were progressive and occurred in different places at different times rather than being globally synchronous.

“It was once thought to be a global turnover, where extinctions of two trilobite groups were synchronous with appearance of a third group of trilobites at 509 million years ago,” explained Sundberg. “Instead, the new calibration of the Cambrian timescale shows that they coexisted in time (in different parts of the globe) from 509 to after 506 million years ago, a significant percentage of the time of early trilobite evolution during the Cambrian explosion.”

Sedimentary rocks are hard to date, so even where fossil transitions are well-studied in the rock record around the globe, it has not been possible to assign an accurate numeric age in millions of years. The key to unlocking this problem was to find and date the youngest sand grains in the form of the mineral zircon, which is amenable to U-Pb geochronology.

“In Grand Canyon, the strata that were thought to span tens of millions of years are now known to have been deposited in just a few million years. This change has been made possible by precise U-Pb radioisotopic dating of detrital zircons,” added Schmitz. “It reveals surprisingly fast changes at a time when trilobite groups were appearing and going extinct very rapidly. Because of this new dating, Grand Canyon has, again, become a globally significant field laboratory for studying globally important debates.”
The NSF grant titled, “Collaborative Research: Constraining the tempo and dynamics of Cambrian Earth systems in western Laurentia,” will further investigate this important time in Earth history with a study that will characterize and date layers above and below key fossil layers throughout the succession in Grand Canyon and all across the western U.S. First steps involve measuring and characterizing, in teams, where the key fossil horizons exist in the cliffs of Grand Canyon. Samples are collected for later laboratory analysis, then sent to the different universities to determine the age, chemistry and paleontology of the layers.

“This promises to further refine the precise timing of unconformities and depositional episodes to test hypotheses about how marine flooding episodes relate to global biologic, tectonic, and ocean/atmosphere changes,” said Crossey.

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The broader impacts of this research are far-reaching and include mentoring a suite of post- doctoral, graduate, and undergraduate scholars, recruiting and training Hispanic and Native American students, and outreach and distance learning to help this science inspire younger audiences, including 4th-12th graders in rural, first-generation, first-nation, inner-city and culturally diverse settings.

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The research will also reach many of the six million annual visitors to Grand Canyon National Park through Park programs, the Trail of Time exhibit, media, and NSF-sponsored field forums on Grand Canyon geology.