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Archive for the tag “Basin and Range Province”

Touring the geologic map of the United States

Published in 1974 by the US Geological Survey, the geologic map of the United States beautifully lays out our country’s geology. If you’re stuck at home these days –as most of us are—you can gaze at this masterpiece and go anywhere! USAGeolMap-allllr

S Willamette Valley

Southern Willamette Valley. Eugene’s in the center of the map

At their simplest, geologic maps read like road maps: they tell you what rock unit or recent deposit forms the ground at a given place. So right here where I am in Eugene, Oregon, I can see that I live on “Q” –which stretches north up the Willamette Valley. “Q” stands for Quaternary-age material (2.5 million years to present), which is typically alluvial material –or sediment deposited by rivers and streams.  Just to the south of me lie a variety of older volcanic and sedimentary rocks. You can look them up in the map’s legend using their symbols. Here’s Rule #0: white areas, being mostly alluvium, mark low areas; colorful areas indicate bedrock, so are typically  higher in elevation.

The beauty of this map is that you can see the whole country at once, and using just a few rules, can immediately glean the underlying structure of a region.

For any of the maps, photos, and diagrams here, you can see a larger size if you click on the image.

x-cut block diagramlr

Rule 1: Rock bodies without internal contacts appear as color swatches (or “blobs”) as seen on the block diagram below. These rocks include intrusive igneous, undifferentiated metamorphic, and flat-lying sedimentary rock. (If you’re not sure what geologic contacts are, please see my recent post on the nature of geologic contacts.)

Rule 2: Rock bodies with internal contacts appear as stripes, which you can see from the diagram above and the photos below. These rocks are typically inclined sedimentary rocks –simply because the contacts between different rock units appear as lines where they intersect the Earth’s surface.  And notice that the steeper the rocks dip (their angle of inclination), the narrower the stripe becomes. In fact, the outcrop widths of vertically dipping beds equals their thickness!

aerial dipping beds

Aerial view of tilted Paleozoic-Mesozoic sedimentary rock at the edge of the Front Range, Colorado

Rule 3. You really can read a map like a cross-section –in fact, maps are basically the same things as cross-sections except they represent horizontal slices of the crust rather than vertical ones. As an example, you get an incomplete geologic history if you only use the cross-section in the block diagram above. The map view actually has all the information because it not only depicts the fold in map view, but also the fault.

Rule 4. Folded rocks typically appear on maps as zig-zags, or in the case of the single anticline in the block diagram above, as a half zig-zag. You get a full zig-zag if you have a paired anticline and syncline, like in the diagram below. The map expression results from the hinge of the fold being inclined –or plunging—rather than simply being horizontal. plunging folds

And here’s an air photo of a plunging fold in the Montana Fold-Thrust Belt.

Plunging anticline, SW Montana

Aerial view of north-plunging anticline in the Fold-Thrust Belt of SW Montana

Time Scale2020lr

Geologic Time Scale. If you don’t know the geologic time scale, click on the image to the side to see it at full size (like all the images here). As far as reading geologic maps, it’s very helpful to know the time scale and the abbreviations of the different time periods. So, “X, W, Y, Z” are all Precambrian (X=Archean; W, Y, Z=progressively younger Proterozoic); “Pz” means Paleozoic (541-252 million years ago), “Mz” means Mesozoic (252-66 million years ago) “Cz” means Cenozoic (66 million years ago to present). Of course, the Paleozoic, Mesozoic, and Cenozoic have further subdivisions with their own abbreviations that you can see on the full-size image.

 

The map.

boxes and regionslrNow for some features on the geologic map! This post is just an introduction, so I’ll stick to the boxed areas to illustrate some geological relations –and maybe throw in a few photos along the way. Then I’ll briefly discuss the localities shown in blue –and the rest of the map is for your own entertainment!

1. SnakeRPlain, IdahoBathlrMap 1. Snake River Plain and Yellowstone. The Snake River Plain, beneath that long flat stretch of Interstate 84, is covered by Quaternary age basalt flows. The landscape is flat because the lavas are flat –and so they show up as a big swatch of pink. Compare it to the ribbon-like bands to the north and south, mostly steeply inclined, folded, and faulted sedimentary rock of the Fold-Thrust Belt. The white stripes between the ribbons mark valleys (they’re filled with alluvial deposits) so we can infer that the ribbons, which mark bedrock, are mountains–which they are.

Notice how the Snake River Plain cuts across the ribbons of the Fold-Thrust Belt. It’s much younger: the lavas all erupted over the top of the older, deformed rock. You can actually see that relation if you drive US20 along the north side of Craters of the Moon National Monument.

And just beyond the Snake River Plain to the northeast, you can see Yellowstone, marked by rhyolites that also cover the older, surrounding rocks. Yellowstone is on trend with the Snake River Plain because they are both products of the southwestern drift of the North American Plate over the Yellowstone Hot Spot. Lying beneath the Snake River Plain Basalts are former rhyolitic calderas like Yellowstone. You can see some of these rhyolites at Shoshone Falls in Twin Falls, Idaho.

Shoshone Falls, Idaho (Pan)

Shoshone Falls spills over light-colored rhyolites. Rocks on the skyline are Snake River Plain Basalt, Idaho

Then there’s the Idaho Batholith, shown as the green swatch, which formed because of subduction along the western margin of North America during the Cretaceous Period. It’s of about the same age and origin as the Sierra Nevada in California.

2. Basin and RangelrMap 2. The Basin and Range Province. Described by the geologist Clarence Dutton in 1884 as “An army of caterpillars”, the Basin and Range consists of alternating basins, shown by the white color for alluvium, and ranges, shown by the colors, indicating various types of bedrock.

Geologically, the province is varied and complex, and I can’t begin to do it justice here except to say that much of the modern day landscape is a product of crustal extension –so many of the ranges are bound by normal faults, in a manner akin to the illustrations below.  It’s quite a wonderful part of the world and includes Death Valley, my go-to place for all things geology.

Basin and Range

Aerial view across the Basin and Range Province in Nevada.

Tilted-F-blocks-and-DV

Tilted fault blocks. The photo of Death Valley region as tilted fault blocks coincides with the inset on the cross-section.

And notice how the southeastern part of Map 2 includes the ribbon-like patterns of the Fold Thrust Belt –and southeast of there, the broad swales of the Colorado Plateau! (Map 3).

Map 3. Colorado Plateau. In preparation for a field trip to the Colorado Plateau, my grad school thesis advisor described the region as “an island of tranquility surrounded by tumultuous Cenozoic deformation.” I’ll never forget that –so accurate a description and such flowery language!3. ColoPlateaulr

170703-20

Aerial view of incised meanders of the Green River cutting through flat-lying rock, Utah.

From the map, you can see that the region is marked by wide color swatches edged by narrow ribbons. The swatches mark approximately flat-lying sedimentary rocks and many of the ribbon-like edges mark monoclines, where the rock flexes from flat-lying to steeply dipping to flat-lying again, like the photo below of the San Rafael Swell. The monoclines formed in the early Cenozoic because of faulting in the underlying basement rock.

Monocline on the Colorado Plateau, Utah

Aerial view of the southern San Rafael Swell, Utah

In some places, such as Black Mesa (location “a”) in northern Arizona, you can see the ribbons surrounding the swatch. These areas typically mark individual plateaus, capped by flat-lying rocks with older rocks exposed on the slopes below. And in the Grand Canyon, the ribbon-like patterns similarly form by older rocks exposed downwards towards the river.

3a. GrandCyn+ pic

Geologic map and and photo of the Grand Canyon, AZ

And those little red dots? Those are bodies of intrusive igneous rock, including the La Sal, Abajo, and Henry mountains that intruded the Paleozoic and Mesozoic sedimentary rocks at various times during the Cenozoic.

2734

La Sal Mountains rise over flat-lying Paleozoic and Mesozoic rock of Canyonlands National Park, Utah

 

4. BlackHillslrMap 4. The Black Hills rise dramatically from the western plains because they’re a structural dome, a fold in which the rocks dip outwards in all directions, away from the oldest rock in the core. You can see that the rocks get younger out from the core if you click on the image to see it at full size. The core consists of bluish-gray rocks labeled “X” –which indicates an age from 2.5-1.6 billion years and the surrounding rocks are labeled “lPz” then “uPz” then “Tr”, “J”, and “K” to indicate “lower Paleozoic”, “Upper Paleozoic”, “Triassic”, “Jurassic”, and “Cretaceous” respectively. Notice how the rocks dip the steepest on the east side of the dome.

Dome

Now if you look at the state of Michigan on the USA map, you can see that it’s the same thing but in reverse. It’s a structural basin, with the youngest rock in the core!

Map 5. Transect across the Appalachians. Starting near the Ohio-Pennsylvania border, you can see that rock is pretty flat-lying because it consists of lots of irregular swatches. The irregularity mostly comes from topographic changes that expose different rocks at different elevations, in a manner similar to some of the features in Map 3. But moving eastward, you hit the Valley and Ridge Province of the Appalachians: long ridges of folded Paleozoic sedimentary rock.5. NAppalachiantransectlre

Sed-08

Channel deposit in the Triassic Newark Group, Connecticut.

And then you hit the teal-colored Newark Group, a series of Triassic sedimentary rocks deposited in basins as North America rifted away from Africa. It’s part of the Piedmont, a  section of the Appalachians that’s of lower topographic relief than the Valley and Ridge Province, but much more variable in its bedrock geology. It even includes fragments of oceanic lithosphere and mantle, shown in the navy blue color, and Proterozoic metamorphic rock, labeled “Ygn”.

Finally, you reach the Cretaceous sedimentary rocks of the Coastal Plain (shown in green), deposited after the Appalachians had formed and largely eroded. These rocks, as well as the overlying Tertiary rocks are relatively undeformed. Notice how the basal contact of the green “lK” cuts off the contacts within the Piedmont!

5a. NAppalachianCloseupThis inset of a close-up of the Valley and Ridge shows a classic example of the zig-zag pattern that results from plunging folds. You can determine the direction the folds plunge by looking at the ages of the rocks: anticlines contain the oldest rock in their cores whereas synclines contain the youngest rocks in their cores. Because anticlines plunge in the direction of their “noses” (where the outcrop pattern zigs or zags), we can infer that these folds are plunging to the northeast.

Map 6. Another view of the Appalachian Coastal Plain. Here’s a great example of a regional, angular unconformity, between the undeformed Cretaceous and younger rocks of the Coastal Plain and the highly deformed rocks of the Appalachians.6. S Appalachianslr

And of course, there’s so much more!

You can download a full-sized version of this map from the United States Geological Survey at https://pubs.er.usgs.gov/publication/70136641. It comes in 3 parts: West half, east half, legend.

In the meantime, here are some of my favorite other features:

The Columbia River Basalt Group (orange swatch) that erupted between 16-6 million years ago and covers some 70,000 square miles of Washington, Oregon, and western Idaho.

Basalt flows of the Columbia River Basalt Group, Imnaha Canyon,

Lava flows of the Columbia River Basalt Group in eastern Oregon

The Lewis Thrust along the east edge Glacier National Park. The fault brings Proterozoic sedimentary rock of the “Belt Supergroup” over Cretaceous sandstone. It’s a very low angle fault so displays a very irregular trace as it winds in and out of the valleys, kind of like a contour line.

color reversal: KODAK UNIVERSAL K14. SBA settings neutral SBA off, color SBA on

Lewis Thrust, which lies just above the tree line, places Proterozoic rock over Cretaceous rock.

The Ogallala Formation (yellow swatch), only about 6-2 million years old and up to 1000 feet in thickness, covers more than 170,000 square miles of the Great Plains –and provides drinking and irrigation water to the vast majority of people in the region.

Deposits of the Mississippi River (gray). From the Mississippi delta all the way to Cairo, Illinois, the river’s deposited Quaternary alluvium on top rocks as old as Eocene.

Meander loops on Mississippi River, Louisiana

Aerial view of meander loops on Mississippi River, Louisiana

Woohoo!


Incidentally, all the photos shown here are available for free download from my geology photo website. It has a search function that accesses more than 3500 images I’ve taken over more than 35 years.

 

Geologic field trip from Yellowstone Lake to Portland, Oregon at 30,000 feet

What a start to the new year!  January 1, I flew home to Oregon with a north-facing window seat on a spectacularly clear day.  So much incredible landscape!  So much incredible geology!  Here are nine photos I shot out the plane window, keyed to the geologic map below.

Yel-PDX + US map

Photo 1.  Absaroka Range, northern Wyoming and southern Montana.  You can see that these mountains consist of layered rocks (see bottom of photo especially)–but they’re not sedimentary.  They are basaltic to dacitic lava flows and pyroclastic rocks of the Absaroka Volcanic Field,  erupted from about 53-43 million years ago.  Much of the present topography is the result of glacial erosion during the Pleistocene.

Absaroka Range, east edge of Yellowstone Lake on left.

Absaroka Range, east edge of Yellowstone Lake on left.

 

Photo 2.  Yellowstone Lake.  As you can see on the map, Yellowstone Lake fills only a fraction of the caldera created by Yellowstone’s Lava Creek Eruption, 600,000 years ago.  Since then, rhyolite lavas, shown in pink, filled in the caldera.  Notice the oval-shaped bay at the end of the lake’s western arm.  It’s called West Thumb, and is a younger caldera that erupted about 150,000 years ago.  It’s a caldera within a caldera!  It’s pretty big too– almost identical in size to Crater Lake in Oregon –but compared to the main caldera, it’s tiny.

Photo and geologic map of Yellowstone National Park

Photo and geologic map of Yellowstone National Park. The dashed red line marks the caldera edge.

 

Photo 3. Recent faulting of the Basin and Range Province. In this photo, the Pahsimeroi River flows northwestward to its confluence with the Salmon River, near the left side of the photo –and the Salmon continues flowing northward for about 100 miles before it turns westward and eventually joins the Snake River.

Recent faulting along western edge of Lemhi Range, Idaho.

Recent faulting along eastern edge of Pahsimeroi Valley, Idaho–and western front of Lemhi Range.

But what I think is so cool about this photo is that it so clearly shows the abrupt western edge of the Lemhi Range, which runs diagonally from the right (east) side of the photo to just above the center.  The range literally rises right out of the ground.  That abruptness is caused by faulting that takes place recently and frequently enough that erosion doesn’t keep up with it.  The fault is a normal fault, caused by crustal extension.  Notice the linear nature of the ranges to the northeast (upper right) –More normal faulting!  This is a northern expression of the Basin and Range Province.  Woohoo!

 

Photo 4. Mountains of the Idaho Batholith.  Granitic rock of the Idaho Batholith underlies a huge area of Idaho, some 14,000 square miles of it. On the geologic map, it’s the big green area.  The rock intruded as a series of plutons during the Late Cretaceous, from about 100 – 65 million years ago.  Similar in age and composition to the Sierra Nevada Batholith, the Idaho Batholith was fed by magma created during subduction along the west coast of North America.

Mountains of the Idaho Batholith

Mountains of the Idaho Batholith

 

Photo 5. Hell’s Canyon.  Not only does the north-flowing Snake River in Hell’s Canyon form the boundary between Idaho and Oregon (Yay, we made it to Oregon!), and not only is it the deepest canyon in the conterminous United States, but it’s also incredibly important from a geologic-history-of-western-North-America point-of-view.

Notice the flat areas above the canyon–they’re especially visible on the west (left) side, but you can also see them on the east.  Those places are flat because they’re made of flat-lying basalt of the Columbia River Basalt Group. These basalts erupted mostly between 17-14.5 million years ago, but kept erupting off and on until about 6 million years ago –and they cover ALL of northern Oregon and ALL of southeastern Washington State.  In fact, they flowed all the way to the Pacific Ocean.

Hell's Canyon and the Snake River.

Hell’s Canyon and the Snake River. The Imnaha River forms the next deep canyon to the left (west).

Those basalt flows overlie rock of the Wallowa accreted terrane: mostly volcanic and sedimentary rock that formed in an island arc setting, far offshore from North America.  It was added (accreted) to the North American continent during the Mesozoic –probably some 150 million years ago.

 

Photo 6. Wallowa Mountains, Oregon. Just west of Hell’s Canyon are the Wallowa Mountains, Oregon’s premier alpine country outside of the Cascades.  Like Hell’s Canyon, the Wallowas contain the accreted Wallowa terrane overlain by Columbia River Basalt –but the Wallowas also host the Wallowa Batholith, a Jurassic-Cretaceous granitic “stitching pluton”.  It’s called a stitching pluton because it intrudes across accreted terranes and “stitched” them together.

Glacial valleys and frontal fault zone on the north side of the Wallowa Mountains, Oregon.

Glacial valleys and frontal fault zone on the north side of the Wallowa Mountains, Oregon.

You can see a bunch of other things in this photo though.  First off, the mountains end suddenly in a line: a recently active fault zone that has uplifted them more than 5000′ relative to the valley floor. Also, you can see how glaciers carved the landscape.  Notice the deep U-shaped valleys, cirques, and knife-edged ridges called aretes.  And see the lake in the upper right corner of the photo?  It’s Wallowa Lake, dammed by a glacial moraine!

(at this point, the folks in the seats next to me wanted to throw me out of the airplane)

 

Photo 7. View of Washington High Cascades over The Dalles.  That’s Mt. St. Helens on the left (west), Mt. Adams in the middle, and Mt. Rainier in the far distant right.  Mt. Rainier is 90 miles away!

Looking north over the Dalles to Mts. St Helens, Rainier, and Adams.

Looking north over the Dalles to Mts. St Helens, Rainier, and Adams.

These volcanoes are dormant –which means that they’re …sleeping?  And they can awaken at any time.  I remember a college friend of mine wanted to climb Mt. St. Helens in 1979.  It was dormant then, and nobody worried about it.  Then in May, 1980 it erupted violently, blowing off its top 2000′.  Both St. Helens and Mt. Rainier have erupted many times in the past several thousand years; Mt. Adams though, erupted only twice in that period.

 

Photo 8.  Columbia Gorge, the Washington High Cascades, and the Bonneville Landslide.  From left (west) to right, the volcanoes are Mt. St. Helens, Mt. Rainier, and Mt. Adams.  You can see the Bonneville Landslide along the river on the right side of the photo, directly below the left base of Mt. Adams.  It detached from the cliffs directly behind it about 1450 A.D. and slid right into the river –and it pushed the river about a mile to the south! Just downriver from the landslide, you can seevthe Cascade Locks zig-zagging across the river.

View northward over the Columbia River Gorge to the Washington High Cascades.

View northward over the Columbia River Gorge to the Washington High Cascades.

The ridges at the bottom of the photo lead up to Mt. Hood, another dormant stratovolcano and Oregon’s highest peak.  Apparently, the view out the south side of the plane was even more ridiculously cool.

 

Photo 9. Columbia River, just below Portland.  Right near Portland, the Columbia River turns northward for about 40 miles before it heads west again out towards the Pacific–and it drops only 10 feet in elevation for the whole distance.  The northward deflection of the river is probably the result of uplift of the Portland Hills, which likely began as long as 16 million years ago (they also deflect 16 million year old lava flows of the Columbia River Basalt). That town along the river in the background is St. Helens, Oregon.

View northward, down the Columbia River.

View northward, down the Columbia River, Washington on the right, Oregon on the left.


See more geologic photos of Oregon by typing “Oregon” into the geology search engine on my website –or type “Oregon, aerial” if you want to see aerial shots!  And if you’re suddenly really excited about Oregon geology, please check out the new edition of Roadside Geology of Oregon!

 

 

 

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