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Archive for the tag “Angular unconformity”

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!

 

 

 

Geologic history of the western United States in a cliff face in Death Valley National Park

Of the many geologic events that shaped the western United States since the beginning of the Paleozoic Era, five really stand out.  In approximate chronological order, these events include the accumulation of tens of thousands of feet of sedimentary rock on a passive margin, periods of compressional mountain building that folded and faulted those rocks during much of the Mesozoic–likely driven by the accretion of terranes, intrusion of subduction-related granitic rock (such as the Sierra Nevada) during the Jurassic and Cretaceous, volcanic activity during the late Cenozoic, and mountain-building by crustal extension during the late Cenozoic and continuing today.  This photo on the western edge of Panamint Valley in Death Valley National Park of California, captures all five.

View of canyon wall on west side of Panamint Valley in SE California --part of Death Valley National Park.  See photo below for interpretation.

View of canyon wall on west side of Panamint Valley in SE California –part of Death Valley National Park. See photo below for interpretation.

The photograph below shows an interpretation.  Paleozoic rock is folded because of the Late Paleozoic-early Mesozoic compressional mountain-building; it’s intruded by Jurassic age granitic rock, an early phase of Sierran magmatism that took place just to the west; the granitic rock is overlain by Late Cenozoic basalt flows, and everything is cut by a normal (extensional) fault.  And there is also a dike that cuts the Paleozoic rock –probably a feeder for the basalt flows.

Interpretation of top photo.

Interpretation of top photo.

So this is all nerdy geology cross-cutting relations talk –but here’s the point: in this one place, you can see evidence for 100s of millions of years of Earth History.  Earth is old old old!  THAT’S why I love geology!

And for those of you who crave geologic contacts?  This photo has all three: depositional, between the basalt and underlying rock; intrusive, between the Mesozoic granite and the folded Paleozoic rock; fault, the steeply dipping black line between the basalt and the Paleozoic rock.  Another reason why I love geology!


click here to see photos and explanations of geologic contacts.
or click here for a slideshow of Death Valley geology.

Cambrian rock

–the last posting, (March 21) had a photo of granite of the Cretaceous Sierra Nevada Batholith intruding Cambrian sedimentary (now metamorphosed) rocks.  These photos show more Cambrian rock.  The Cambrian Period (542-488 million years ago) is the bottom of the Phanerozoic Eon –and one reason Cambrian rocks are significant, is that they are the oldest rocks to contain shelly fossils.  Older rocks, called “Precambrian” may contain fossil impressions or fossilized algae, but don’t contain any shells.

At the risk of being too repetitive (see post March 13) the upper photo shows Cambrian limestone in the Death Valley region –there are thousands of feet of Cambrian Limestone in the Death Valley region.   The lower photo shows Cambrian sandstone, shale, and limestone overlying tilted Precambrian sedimentary rock in the Grand Canyon.

My point is that the Cambrian section is traceable over great distances.  That’s important, because the base of the Cambrian provides a common datum over much of the western US –certainly from the Sierra Nevada to Death Valley to the Grand Canyon –but in later posts, you can see that it’s also in Colorado, Wyoming, Montana… and so on!

Cambrian limestone in the Nopah Range, SE Californi

Thousands of feet of marine limestone make up many of the mountain ranges in the Death Valley area of SE California. Click here to see a geologic map of Death Valley National Park...

The photo above shows the Cambrian Bonanza King Formation (gray) on top the Cambrian Carrara Fm (orange).

And the photo below shows the near-horizontal Cambrian and younger rocks of the Grand Canyon over tilted Precambrian sedimentary rock.  It’s really thin here… the Cambrian only goes up through the arrow.

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