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Grand Canyon Unconformities –and a Cambrian Island

A prominent ledge punctuates the landscape towards the bottom of the Grand Canyon. It’s the Tapeats Sandstone, deposited during the Cambrian Period about 520 million years ago, when the ocean was beginning to encroach on the North American continent, an event called the Cambrian Transgression. Above the ledge, you can see more than 3000 feet of near-horizontal sedimentary rocks, eroded into cliffs and slopes depending on their ability to withstand weathering and erosion. These rocks, deposited during the rest of the Paleozoic Era, are often used to demonstrate the vastness of geologic time–some 300 million years of it.

View of the Grand Canyon from the South Rim trail. Arrows point to the Cambrian Tapeats Sandstone.

View of the Grand Canyon from the South Rim trail. Arrows point to the Tapeats Sandstone.

But the razor-thin surface between the Tapeats and the underlying Proterozoic-age rock reflects the passage of far more geologic time  –about 600 million years where the Tapeats sits on top of the sedimentary rocks of the Grand Canyon Supergroup. Those rocks are easy to spot on the photo above because they contain the bright red rock called the Hakatai Shale. Even more time passed across the surface where the Tapeats sits on top of the 1.7 billion year old metamorphic basement rock. You can put your thumb on the basement and a finger on the Tapeats –and your hand will span 1.2 billion years!

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An exposure of the Great Unconformity in Montana. Notice how it’s a 3D surface that extends back into the wall.

These surfaces, where sedimentary or volcanic rock was deposited upon much older rock are called unconformities. This particular one, beneath the Tapeats Sandstone, is called the Great Unconformity. You can visit other exposures of the Great Unconformity in many places throughout the American West, including Wyoming, Montana, Colorado, and even Wisconsin.

Unconformities –and all sedimentary bedding surfaces in general– are visible where erosion has cut across them and exposed them to view. Being three-dimensional surfaces, they extend underground in all directions from there. What’s more, before the overlying rock was deposited, each of those surfaces used to be Earth’s surface. Think about it. Sedimentary rocks form from sediments, and sediments are deposited on the seafloor or land–the Earth’s surface. So if you could strip away the Tapeats and everything above it, you’d be looking at Earth’s surface just before the Tapeats was deposited.

Faulting and tilting
And depending on where you are, the Tapeats sits on different rock –because different bedrock characterized the Earth’s surface during the early Cambrian just as it does today. If you look north up the long and straight Bright Angel Canyon, you see the red-colored Hakatai Shale beneath the unconformity on the west side of the canyon; on the east side, you see basement rock.

Grand Canyon, Arizona

View north up Bright Angel Canyon. Tapeats Sandstone overlies Hakatai Shale on west side and basement on east.

Faulting and erosion, before deposition of the Tapeats, explains some of these differences according to the diagram below. The first cross-section, Stage A, shows the scene after deposition of the Grand Canyon Supergroup atop the basement. Notice that the surface between these sedimentary rocks and the basement is also an unconformity. At stage B, the fault that extends up Bright Angel Canyon causes tilting and lifts up its eastern side. Later erosion (stage C) removed the uplifted Grand Canyon Supergroup to expose the basement. On the down-dropped west side, those rocks were largely preserved. Notice that in stage D, the base of the Tapeats Sandstone (in green) corresponds to the land surface of stage C. Stage E represents today’s landscape as viewed across Bright Angel Canyon.GCynsequencehoriz


Types of unconformities
We have different names for unconformities, depending on the rock beneath the surface. If it’s metamorphic or intrusive igneous rock, such as east of Bright Angel Canyon, it’s called a nonconformity. If it’s sedimentary or volcanic rock that’s tilted at a different angle from the overlying rock, it’s an angular unconformity. If it’s sedimentary rock that’s parallel to the overlying rock, it’s a disconformity.

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Grand Canyon unconformities. D: Disconformity; A: Angular Unconformity; N: Nonconformity

The Grand Canyon is full of disconformities that you don’t notice because the rocks are parallel. One of the more impressive ones is between the Mississippian Redwall Limestone and the Cambrian Muav Limestone (top of the green layer on the right). Along that surface, rocks of both the Ordovician and Silurian periods are missing!

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Paleozoic Rocks in the Grand Canyon. Redwall Limestone (R) rests disconformably on the underlying thin-bedded Muav Limestone.

There are also buttress unconformities. To see one of those, you get to go for a hike!

Cambrian Island
You can see the Great Unconformity up close if you hike to it –by following any of the trails that descend from the Grand Canyon’s rim to the river. The steep grade makes both the descent and ascent unusually arduous, so it’s a far more enjoyable trip if you do it as an overnight backpack. Earlier this month, I hiked to the unconformity via the New Hance Trail, one of the national park’s less used trails. Much of the hike involved scrambling and looking for the trail –so I was thoroughly spent (and grateful) when I made it back to the rim. But in Red Canyon, which was my destination… Wow! I practically lived and breathed the Proterozoic!

Rip-up clasts in sandstone

Rip-up clasts of algal limestone (notice thin laminations) in sandstone of Bass Formation

For one thing, the rocks are beautiful. There’s the red Hakatai Shale, deposited in coastal plain and deltaic environments, with its thin beds and occasional ripple marks and mudcracks. And there’s the underlying Bass Formation –which is mostly limestone but has this amazing maroon-colored sandstone where I had lunch. There you can see fragments of an algal mat that were ripped up by a storm and distributed throughout the overlying sand bed. And these rocks form the canyon walls and make everything red. Above them? Flat-lying Paleozoic rock sitting on the Great Unconformity!

Angular Unconformity, Grand Canyon  (Pan)

Angular unconformity between the Cambrian and tilted Proterozoic rock in the Grand Canyon; the buttress unconformity exists where the Tapeats Sandstone pinches out and the overlying Bright Angel Shale abuts the brown knob of Shinumo Quartzite on the right. The knob persisted as an island during the Cambrian Transgression–close-up below.

And that’s where you can see the Cambrian island, right next to a buttress unconformity. The Shinumo Quartzite, a Proterozoic rock that overlies the Hakatai Shale, is such a strong and resistant rock that it defied erosion during the Cambrian Period. A large block of it stood above the landscape as an island that persisted all through Tapeats time and into Bright Angel time. The ancient island towers over Red Canyon, with the Tapeats Sandstone and Lower Bright Angel Shale thinning and then pinching out against its sides.

You can imagine the scene, with a rising sea surrounding the island and depositing Bright Angel Shale around it. As the sea rose over the island, more shale was deposited, eventually burying the island. Now that the Colorado River and its tributaries are eroding the canyon, the island’s exposed.

Angular Unconformity, Grand Canyon

This block of Shinumo Quartzite persisted as an island during much of the Cambrian.

That’s the buttress unconformity— where the younger rock is deposited against –and so abuts– the older rock.

Phew!


These photos –and more of the Grand Canyon–are all freely downloadable from my website Geologypics.com. Just type “Grand Canyon” into the keyword search!

 

 

 

Cove Palisades, Oregon: a tidy short story in the vastness of time

If I were a water skier, I’d go to Lake Billy Chinook at Cove Palisades where I could ski and see amazing geology at the same time. On the other hand, I’d probably keep crashing because the geology is so dramatic! Maybe a canoe would be better.

Lake Billy Chinook, Oregon

View across the Crooked River Arm of Lake Billy Chinook to some of the 1.2 million year old canyon-filling basalt (right) and Deschutes Fm (left). The cliff on the far left of the photo is also part of the 1.2 million year basalt.

The lake itself fills canyons of the Crooked, Deschutes and Metolius Rivers. It backs up behind Round Butte Dam, which blocks the river channel just down from where the rivers merge. The rocks here tell a story of earlier river canyons that occupied the same places as today’s Crooked and Deschutes Rivers. These older canyons were filled by basaltic lava flows that now line some of the walls of today’s canyons.

CovePalisades2From the geologic map, modified from Bishop and Smith, 1990, you can see how the brown-colored canyon-filling basalt, (called the “Intracanyon Basalt”) forms narrow outcrops within today’s Crooked and Deschutes canyon areas. It erupted about 1.2 million years ago and flowed from a vent about 60 miles to the south. You can also see that most of the bedrock (in shades of green) consists of the Deschutes Formation, and that there are a lot of landslides along the canyon sides.

The cross-section at the bottom of the map shows the view along a west-to-east line. Multiple flows of the intracanyon basalt filled the canyon 1.2 million years ago –and since then the river has re-established its channel pretty much in the old canyon. While the map and cross-section views suggest the flows moved down narrow valleys or canyons, you can actually see the canyon edges, several of which are visible right from the road.

Read more…

Great Unconformity in Montana –and rising seas during the Cambrian

Here’s yet another picture of the Great Unconformity –this time in southwestern Montana.  Once again, Cambrian sandstone overlies Precambrian gneiss.  You can see a thin intrusive body, called a dike, cutting through the gneiss on the right side.  You can also see that the bottom of the sandstone is actually a conglomerate –made of quartzite cobbles derived from some nearby outcrops during the Cambrian.

Great unconformity in SW Montana.

Photo of Cambrian Flathead Sandstone overlying Proterozoic gneiss in SW Montana.

 

And that’s me in the photo.  My left hand is on the sandstone –some 520 million years or so old; my right hand is on the gneiss, some 1.7 BILLION years old.  There’s more than a billion years of missing rock record between my two hands.  Considering that the entire Paleozoic section from the top of the Inner Gorge in the Grand Canyon to the top of the rim represents about 300 million years and is some 3500′ thick… yikes!

And… just like in the Grand Canyon and elsewhere, there is Cambrian age shale and limestone above the sandstone.  This rock sequence reflects rising sea levels during the Cambrian.  It’s called the “Cambrian Transgression”, when the sea moved up onto the continent, eventually inundating almost everywhere.  If you look at the diagram below, you can see how this sequence formed.

Marine transgression

Sequence of rock types expected during a transgression of the sea onto a continent.

If you look at time 1, you can see a coastline in cross-section, with sand being deposited closest to shore, mud a little farther out, and eventually carbonate material even farther out.  As sea levels rise (time 2), the sites of deposition for these materials migrates landward, putting mud deposition on top the earlier sand deposition and so on.  At time 3, the sequence moves even farther landward, resulting in carbonate over mud over sand.  If these materials become preserved and turned into rock, they form the sequence sandstone overlain by shale overlain by limestone –just what we see on top the Great Unconformity.

 

 

 

Great Unconformity –in the Teton Range, Wyoming

As it turns out, the “Great Unconformity”, the contact between Cambrian sedimentary rock and the underlying Precambrian basement rock, is a regional feature –it’s not only in the Grand Canyon, but found all over the Rocky Mountain West –and for that matter, it’s even in the midwest.  As an example, here are a couple photos from the Teton Range in Wyoming.

The yellow arrow points to the contact between the Cambrian Sandstone and underlying Precambrian metamorphic rock... the Great unconformity.

This top photo shows the Grand Teton (right) and Mt. Owen (left) in the background… in the foreground, you can see a flat bench, which is underlain by flat-lying Cambrian sandstone.  Below that are darker-colored cliffs of Precambrian metamorphic rock.  The unconformity is right at their contact (arrow).

Also notice that the Grand Teton and Mt. Owen are made of metamorphic (and igneous) rock –but they’re much much higher in elevation than the sandstone.  That’s because there’s a fault, called the “Buck Mountain fault” that lies in-between the two.  The Buck Mountain fault moved the rock of the high peaks over the ones in the foreground during a mountain-building event at the end of the Mesozoic Era.  Because the metamorphic and igneous rock is so much more resistant to erosion than the sandstone, it stands up a lot higher.

Precambrian metamorphic and igneous rock of the Teton Range and overlying sedimentary rock.

This lower photo shows the view of the Teton range from the top of the sandstone bench (appropriately called “Table Mountain”).  As you look eastward towards the range, you can pick out the Buck Mountain fault (between the metamorphic and igneous rock of the high peaks) and the Cambrian sedimentary rock (the layered rocks).  Significantly, the Cambrian rocks, just like in the Grand Canyon, consist of sandstone, overlain by shale, overlain by limestone.

And geologic time… remember… for the sandstone to be deposited on the metamorphic or igneous rock, the metamorphic and igneous rock had to get uplifted from miles beneath the surface and exposed at sea level.  And since then, it’s been uplifted to the elevation of The Grand Teton (13370′) and Mt. Owen (12, 928′) !

Click here to see more photos of unconformities.
or… click here to see a geologic map of Grand Teton National Park, Wyoming.

Great Unconformity –Grand Canyon, Arizona

So just like intrusive igneous rocks, metamorphic rocks require great lengths of time to accomplish the uplift and erosion in order to be exposed at Earth’s surface.

So what do we make of this photograph?  It shows a sequence of sandstone, shale, and limestone sitting on top metamorphic rock (called the “Vishnu Schist”) in the Grand Canyon.  The sandstone was deposited right on top the schist.

Great unconformity, Grand Canyon, Arizona

Sequence of Cambrian sandstone (the ledge across the middle of the photo), shale (the overlying slopes) and limestone (the upper cliffs) deposited on top the Vishnu Schist in the Grand Canyon.

 

Since sedimentary rocks, like sandstone, shale, or limestone, are deposited at Earth’s surface –and metamorphic rock forms beneath the surface, this photo shows that BEFORE the sedimentary rocks were deposited, the metamorphic rock (schist) had to have been uplifted and exposed.  So all the time required to bring the schist to the surface had to take place before the sandstone was even deposited.

The surface of contact between the sandstone and the schist is called an unconformity because it is here that we see evidence for a great deal of missing rock record.  The sandstone must be much younger than the schist –for the very reason that the schist first had to get uplifted and exposed at the surface before the sandstone was deposited on top of it.  So… because the sandstone is so much younger, but it was deposited right on top the schist, there must be a gap in the rock record between them … an unconformity.

And here is where we see evidence for even LONGER periods of time.  Overlying the sandstone?  Thousands and thousands of feet of more sedimentary rock.  And much of that sedimentary rock was marine… formed at sea level.  It is now over a mile above sea level.

And the schist itself?  The people who’ve studied it have determined that much of it was originally volcanic –which means that it originally formed at the Earth’s surface.  So… over geologic time, it must have been buried to the depths needed to turn it into a metamorphic rock BEFORE it was uplifted and exposed.

So… how old is Earth?  Some say 6 or 10,000 years… I think we’re looking at 10s of millions in this photo.  And if we consider the numerical ages for these rocks, 1.7 billion is the age of metamorphism of the schist –its original volcanic rock must have been older!

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