Great Unconformity – a billion years of no rocks?

Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland, or that observed by John Wesley Powell in the Grand Canyon in 1869. Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.

Background

Unconformities tend to reflect long-term changes in the pattern of the accumulation of sedimentary or igneous strata in low-lying areas (often ocean basins, such as the Gulf of Mexico or the North Sea, but also Bangladesh and much of Brazil), then being uplifted and eroded (such as the ongoing Himalayan orogeny, the older Laramide orogeny of the Rocky Mountains, or much older Appalachian (Alleghanian) and Ouachita orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of tectonic uplift are generally periods of mountain building, often due to the collision of tectonic plates. The “great” unconformities of regional or continental scale (in both geography and chronology) are associated with either global changes in eustatic sea level or the supercontinent cycle, the periodic merger of all the continents into one approximately every 500 million years.

Hutton’s Unconformity

Hutton’s Unconformity at Siccar Point, in county of Berwickshire on the east coast of Scotland, is an angular unconformity that consists of gently dipping, reddish, Upper Devonian and Lower Carboniferous breccias, sandstones, and conglomerates of the Old Red Sandstone overlying deeply eroded, near-vertical, greyish, Silurian (Llandovery) greywackes and shales. The Llandovery greywackes and graptolite-bearing shales of the Gala Group were deposited by turbidity currents in a deep sea environment about 425 million years ago. The overlying Devonian strata were deposited by rivers and streams about 345 million years ago. Thus, this unconformity reflects a gap of about 80 million years during which deep sea sediments were lithified, folded, and uplifted; later deeply eroded and weathered subaerially; and finally buried by river and stream sediments.

Exposures of the unconformity at Siccar Point, provided James Hutton, accompanied by John Playfair and Sir James Hall, the clearest example of an unconformable relationship between two sets of sedimentary strata that involved a complex geological history. The clear truncation of near-vertical Silurian sedimentary strata by well-bedded conglomerates and sandstones belonging to the Upper Old Red Sandstone allowed Hutton to demonstrate the existence of significant breaks in the geological record, in this case a break separating strata that were then called alpine schistus and secondary strata. This and other unconformities provided evidence for Hutton’s ideas about the recycling of geological materials and for unconformities representing very large time periods. He argued that these concepts pointed to the great antiquity of the Earth and the vastness of the geological time-scale.

Powell’s Unconformity, Grand Canyon

The Great Unconformity of Powell in the Grand Canyon is a regional unconformity that separates the Tonto Group from the underlying, faulted and tilted sedimentary rocks of the Grand Canyon Supergroup and vertically foliated metamorphic and igneous rocks of the Vishnu Basement Rocks. The unconformity between the Tonto Group and the Vishnu Basement Rocks is a nonconformity. The break between the Tonto Group and the Grand Canyon Supergroup is an angular unconformity.

Powell’s Great Unconformity is part of a continent-wide unconformity that extends across Laurentia, the ancient core of North America. It was first recognized twelve years before Powell’s expedition by John Newberry in New Mexico, during the Ives expedition of 1857-1858. However, the disruption of the American Civil War kept Newberry’s work from becoming widely known. This Great Unconformity marks the progressive submergence of this landmass by a shallow cratonic sea and its burial by shallow marine sediments of the Cambrian-Early Ordovician Sauk sequence. The submergence of Laurentia ended a lengthy period of widespread continental denudation that exhumed and deeply eroded Precambrian rocks and exposed them to extensive physical and chemical weathering at the Earth’s surface. As a result, Powell’s Great Unconformity is unusual in its geographic extent and its stratigraphic significance.

The length of time represented by Powell’s Great Unconformity varies along its length. Within the Grand Canyon, the Great Unconformity represents a period of about 175 million years between the Tonto Group and the youngest subdivision, the Sixtymile Formation, of the Grand Canyon Supergroup. At the base of the Grand Canyon Supergroup, where it truncates the Bass Formation, the period of time represented by this angular unconformity increases to about 725 million years. Where the Tonto Group overlies the Vishnu Basement Rocks, the Great Unconformity represents a period as much as 1.2 to 1.6 billion years.

Frenchman Mountain, Nevada

A prominent exposure of Powell’s Great Unconformity occurs in Frenchman Mountain in Nevada. Frenchman Mountain exposes a sequence of Phanerozoic strata equivalent to those found in the Grand Canyon. At the base of this sequence, the Great Unconformity, with the Tapeats Sandstone of the Tonto Group overlying the Vishnu Basement Rocks, is well exposed in a manner that is atypical and scientifically significant in its combination of extent and accessibility. This exposure is frequently illustrated in popular and educational publications, and is often part of geological fieldtrips. There is a gap of about 1.2 billion years where 550 million year old strata of the Tapeats Sandstone rests on 1.7 billion (1700 million) year old Vishnu Basement Rocks.

As a widespread phenomenon

The term “Great Unconformity” has also been used to refer to the anomalous concentration of unconformities, including basement nonconformities, below the base of the Cambrian. Charles Walcott was among the first to note this phenomenon, remarking in 1910:

I do not know of a case of proven conformity between Cambrian and pre-Cambrian Algonkian rocks on the North American continent. In all localities where the contact is sufficiently extensive, or where fossils have been found in the basal Cambrian beds or above the basal conglomerate and coarser sandstones, an unconformity has been found to exist. Stated in another way, the pre-Cambrian land surface was formed of sedimentary, eruptive, and crystalline rocks that did not in any known instance immediately precede in deposition or origin the Cambrian sediments. Everywhere there is a stratigraphic and time break between the known pre-Cambrian rocks and Cambrian sediments of the North American continent.— Charles D Walcott, “Abrupt Appearance of the Cambrian Fauna on the North American Continent”, Cambrian Geology and Paleontology (1910)

A potential link has been proposed between such sub-Cambrian unconformities and glacial erosion during the Neoproterozoic Snowball Earth glaciations. Alternatively, it has been proposed that multiple smaller events, such as the formation and breakup of Rodinia, created many unconformities worldwide. Evidence indicates that the Pikes Peak unconformity was formed before the Snowball Earth glaciations.

Possible causes of the Great Unconformity

There is currently no widely accepted explanation for the Great Unconformity among geoscientists. There are theories that have been proposed; it is widely accepted that there was a combination of more than one event which may have caused such an extensive phenomenon. One example is a large glaciation event which took place during the Neoproterozoic, starting around 1 billion years ago. This is also when a significant glaciation event known as ‘Snowball Earth’ occurred. Snowball Earth covered almost the entire planet with ice. The areas that underwent glaciation were approximately those where the Great Unconformity is located today. When glaciers move, they drag and erode sediment away from the underlying rock. This would explain how a large section of rock was taken away from widespread areas around the same time.

The Great Unconformity (GU) is one of geology’s deepest mysteries. It is a gap of missing time in the geological record between 100 million and 1 billion years long, and it occurs in different rock sections around the world. When and how the GU came to be is still not totally resolved.

Now a team of researchers studying the unconformity as it occurs on the Ozark Plateau in the United States has found chemical evidence in rocks suggesting that the GU began forming toward the end of the Precambrian, between about 850 and 680 million years ago. Their evidence implies a culprit behind all of the missing rock: global tectonic uplift associated with the breakup of the ancient supercontinent Rodinia.“This means there was probably a boatload of erosion.”Forces of nature seek to even out large differences in topography, the researchers explain in a recent paper published in the journal Geology. Any sudden large-scale uplift, they posit, would have exposed relatively more Rodinian rock than normal to weathering and erosion.

The new evidence points to 6–8 vertical kilometers of fresh rock material uplifting at the end of the Precambrian. “This means there was probably a boatload of erosion,” explained Michael DeLucia, tectonicist of the University of Illinois at Urbana-Champaign and lead author of the work. As time passed, this weathering and erosion carved the GU.

How to Erase Time

Where the GU horizon exists on the planet, the difference in rock type above and below the horizon is striking: In the Grand Canyon, the Precambrian Vishnu Schist is warped and twisted compared to the Cambrian Tapeats Sandstone that overlies it. On the Ozark Plateau, at the team’s field site in a region called the St. Francois Mountains, 1.4-billion-year-old granite and rhyolite lies directly underneath 500-million-year-old sandstone.

“We drove down to the St. Francois Mountains, and we sampled a bunch of granites and rhyolites, and then separated out zircon crystals from the samples,” said DeLucia. Those zircons, he explained, were key in figuring out when the rocks began exhuming, or uplifting and then eroding.Researchers can rewind the clock to when the helium “jail” in the zircon formed—and thus when a supercontinent uplifted.In hot environments like those deep in Earth, zircon crystals steadily lose helium atoms, which, DeLucia explained, form at a constant rate from the radioactive decay of the elements uranium and thorium. “Deeper in the crust, helium is readily released out of the zircon,” he said. “But once you pass a certain temperature threshold as the rock rises and cools, the crystal lattice of the zircon cools enough to act basically as a jail, and you start retaining all of this helium.” When the relative amounts of uranium and thorium compared to the now retained helium atoms are known, researchers can rewind the clock to when the helium “jail” in the zircon formed—and thus when a supercontinent uplifted.

The team’s rewind of the zircon they sampled revealed that the rocks uplifted and cooled between 850 and 680 million years ago. “The results indicated that there was widespread exhumation of the craton [the large, stable nucleus of continents]—not just mountain belts,” said Stephen Marshak, a structural geologist at the University of Illinois at Urbana-Champaign and one of the paper’s coauthors.

But how much exhumation? The researchers estimate that because the zircon jail starts to close at temperatures prevalent about 6–8 kilometers below the surface, 6–8 vertical kilometers of rock would have needed to erode to expose the rocks that we see today.

The team also detected an uplift pulse—dated from 225 to 150 million years ago—timed with Pangea’s assembly and breakup. This window of uplift serves as a reality check for their method: It matches well with dates for Pangea’s evolution gleaned by other established methods for teasing out the timing of geological events.

Diving into Deep Time

The paper is a prime example of “deep-time thermochronology,” a new technique for dating ancient uplift events.The team’s paper is a prime example of “deep-time thermochronology,” as Marshak called it—a new technique for dating ancient uplift events.

Before, it was thought that very old zircon grains could not provide reliable dates with this method. Such ancient crystals tend to “leak” helium atoms because their crystal lattices are damaged by radioactive decay, said coauthor William Guenthner, a thermochronologist also at the University of Illinois at Urbana-Champaign and a pioneer of the method. However, the team was able to quantify this leaking, which, depending on the amount of damage, occurs at a steady rate.

One factor that may muddy the team’s conclusions is that hot fluids moving through the crust might have reset the zircon grains’ internal clocks, according to Shanan Peters, a sedimentologist at the University of Wisconsin–Madison who was not involved in the work. Peters explained that there are deposits in the team’s field area that formed thanks to hot fluids moving through the rock.

But if hot fluids did, indeed, reset the clocks, then the change would not have occurred across the board, DeLucia noted—there would be a predictable signal in their data set, with certain grains having reset to the time of fluid flow. “And we don’t see that,” he said.

Making a Snowball

The timing of the GU’s formation may also help explain what triggered the so-called “snowball Earth” glaciations, an episode beginning about 720 million years ago in which much of the planet likely became covered in ice sheets.

Chemical weathering associated with the erosion that formed the GU likely pulled the greenhouse gas carbon dioxide out of the atmosphere, sequestering vast quantities in the ocean and lithosphere. This “primed the pump for the glaciations,” said Guenthner.

The sequestration would have likely helped cool the planet to such an extent that a snowball state could initiate, he explained.

But some questions remain unanswered, Peters noted. For instance, why didn’t the uplift of Pangea also lead to the formation of something like the GU or lead to glaciations akin to the snowball Earth events?

“From the point of view of sediments, the Great Unconformity is completely unique,” Peters said. And therein lies the next mystery to solve—determining what made Rodinia’s uplift so different.

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