geologictimepics

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Archive for the category “Unconformities”

Devil’s Punchbowl –Awesome geology on a beautiful Oregon beach

You could teach a geology course at Devil’s Punchbowl, a state park just north of Newport, Oregon. Along this half-mile stretch of beach and rocky tidepools, you see tilted sedimentary rocks, normal faults, an angular unconformity beneath an uplifted marine terrace, invasive lava flows, and of course amazing erosional features typical of Oregon’s spectacular coastline. And every one of these features tells a story. You can click on any of the images below to see them at a larger size.

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View southward from Cape Foulweather to the Devil’s Punchbowl.

 

180629-58ceThe rocks. They’re mostly shallow marine sandstones of the Astoria Formation, deposited in the early part of the Miocene, between about 16.5 to 22 million years ago. The rocks are tilted so you can walk horizontally into younger ones, which tend to be finer grained and more thinly bedded than the rocks below. This change in grain size suggests a gradual deepening of the water level through time. In many places, you can find small deposits of broken clam shells, likely stirred up and scattered during storms –and on the southern edge of the first headland north of the Punchbowl, you can find some spectacular soft-sediment deformation, probably brought on by submarine slumping. Later rock alteration from circulating hot groundwater caused iron sulfide minerals to crystallize within some of the sandstone.

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Soft-sediment deformation in the Astoria Fm expressed as folded beds between unfolded ones.

There’s also basalt –of the Columbia River Basalt Group –which originated more than 300 miles away in eastern Oregon. As if that weren’t unusual enough, these basalts intrude the sedimentary rocks! On reaching the coast, the basaltic lavas were somehow able to inject downward into the Astoria Formation to form dikes and sills. You can see a dike and sill next to the waterfall around the first headland, as well as some car-sized blocks of basaltic breccia. The breccias likely formed during explosions as the hot lava interacted with the wet sediment. They’re called “peperites”. At low tide, you can look southward from Cape Foulweather to see two long arcuate dikes that extend offshore from the beach.

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Basaltic breccia, broken during explosive interactions with wet sediment

The youngest “rock” isn’t a rock at all –it’s coastal sand and gravel. You see modern-day sediment on the beach and older, Pleistocene-age sediment capping the Astoria Formation beneath the marine terrace, about 60 feet above. The surface between the Pleistocene sediment and the Astoria Formation makes for a beautiful angular unconformity where the tilting and erosion of the Astoria Formation had to occur before the flat-lying sediment was deposited. I really love unconformities, by the way –if you want to read more, I blogged about unconformities in the Grand Canyon –and have tons of photos of unconformities as well!

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Angular unconformity between tilted Astoria Fm below flat-lying marine terrace deposits

 

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Erosion along a normal fault

Uplift. That uplifted marine terrace gives the most direct evidence for uplift along the coast because like today’s beach, the Pleistocene sediment was also deposited at sea level. Moreover, the terrace’s deposits are only about 80,000 years old so it speaks to very recent uplift. Expressions of older periods of uplift are the tilted sandstone beds themselves, having originated as a series of horizontal sheets of sediment below sea level and a number of faults visible in the cliffs that offset these beds. These faults are called normal faults because the block beneath the fault, called the footwall, moved up relative to the block above the fault called the hanging wall. As breakage along faults naturally enhances weathering and erosion, you can sometimes find small caves along them, a good example being near the Punchbowl arch.

Erosion. Naturally paired with uplift, erosion shapes the land surface. In coastal environments, wave action and gravity– by rockfall and landsliding– do most of the work. At headlands, waves eat away at the bases of cliffs to undercut and destabilize the rocks above. Naturally, some parts of a retreating headland are left behind, sometimes still connected to the headland by an arch. Once the arch fails, the remaining rock is left as a sea stack, itself to be gradually consumed by the waves. At Devil’s Punchbowl, a sea stack marking the edge of the former coastline sits a quarter mile out from shore. Another one stands right on the beach. The Punchbowl itself formed from a collapsed sea cave and now offers views through two arches, one directly out to sea while the other looks northward up the beach. You can explore the Punchbowl at low tide.

Breaking wave as seen through sea arch, Oregon

view out to sea from within the Punchbowl

Landslides actively move coastal material downwards towards the beach. You can see several slides at Devil’s Punchbowl, one of which lies along the main trail to the beach. Look for a hummocky landscape as you walk down the trail. The area around Devil’s Punchbowl is especially prone to landsliding because the sedimentary bedding slopes towards the ocean, providing natural surfaces for sliding.
170909-10If you visit this amazing beach at low tide, you can actually enter the Punchbowl –be careful though, it’s very slippery, and if the tide comes in, you’ll be in real trouble. You can also walk around the first headland to the north to visit the peperites, and check out the many tidepools along the way. Even when the tide’s in, you can see the Astoria Formation, the landslides, some of the faults, fossils, and the uplifted terrace. You can also view the waves crashing in the Punchbowl from above. And up there, you can see a beautiful coastal soil profile. Oh how I love this place!

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View into the Punchbowl, a collapsed sea cave. Note the inclined bedding.


 

For more photos of the Oregon Coast, check out my website geologypics.com and type “Oregon Coast” into the search.

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.

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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!

 

 

 

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