Geologic Survey of the Northern Cascades: 4. Thrust Faulting within a Subduction Zone
Figure 1. This is my last post from our Cascades Loop drive. The geologic map shows a wide variety of rock types west of the Columbia River, some of which we’ve seen before. But we also see two unfamiliar lithologies: Tertiary basalts that erupted throughout eastern Washington between 17 and 6 million-years ago; and mixed Paleozoic/Mesozoic metamorphic rocks (539-66 ma), which appear as a sliver of gray pointing like an arrow towards Site 12. These older rocks have been thrust over younger Mesozoic metamorphic rocks along a thrust fault (dashed line with arrow indicating direction of relative movement).
Figure 2. Stop 11 is located along the Columbia River, where steep slopes conceal high-grade, metamorphic rocks generally assigned Mesozoic ages. This image shows a knob dissected by numerous intersecting dikes. Ages for metamorphic rocks indicate the time when they were transformed into gneiss, schist, etc. The description from the NGMDB map describes a jumble of rocks in complex interrelationships.
Figure 3. This outcrop is about 10 feet across (the tree is young); it shows what the geologic map’s metadata are saying. There is no continuity visible, yet thin layers of sandstone/mudstone are visible, adjacent to blocks of buff material in the lower left; meanwhile, the right side reveals quite different layering.
Figure 4. This image reveals an intrusive, igneous texture that has been modified into what I would call an orthogneiss. Large grains of feldspar have formed layers because dark minerals like amphibole and biotite have been remineralized into the dark laminae. None of these minerals (except perhaps an occasional zircon) is original. The entire outcrop appears to have been folded into a three-dimensional, origami form. The gray area exposes the core of this spheroidal structure.
Figure 5. These two photos from Stop 12 are so speculative that they could be considered as schematic. I included them to demonstrate the change in geomorphology when crossing something as subtle as a thrust fault. These images are from the intersection of US-2 and US-97; I don’t need GPS to know where I am. The NGMDB map shows a thrust fault crossing the Columbia River at this spot.
(A) This mountain forms steep, rock faces. It is composed of Mesozoic metamorphic and igneous rocks. (B) This mountain, only a few miles north, is predominantly covered with loose soil and has a few outcrops on the upper slope.
There is an identified thrust fault between them, pushing the northern rocks over the younger rocks to the south (some time in the past). At first glance, the two look alike but, on closer examination, we can identify differences even from a mile away. I should add that a thrust fault is not cut with a knife; instead, it is a general zone of brittle failure within colliding rock masses. The result is seldom obvious.
Figure 6. Stop 13 in East Wenatchee revealed deformed rocks similar to those at Chelan (Stop 11), which isn’t surprising; however, this road-cut revealed larger exposures that aren’t as criss-crossed by veins. The lighter colors are the result of illusive changes in chemistry that interact with the environment, but they seem to reflect subtle differences in lithology. Note the massive, layered section at the bottom that seems to have been cut off; perhaps this is an outcrop-scale expression of regional compression and thrust faulting.
Figure 7. This photo is difficult to explain. Thin layers of what was once an intrusive rock abut an intrusive rock that hasn’t been as altered. This is an opportunity to point out the fallacy of jumping to geological conclusions: this might look like a sedimentary rock laid down on a granite boulder, which once formed a shoreline–it is NOT.
Final Thoughts
These four posts have explored a subduction-zone orogen, from youngest to oldest. The rocks got progressively more complex during our transit. They tried to fool us repeatedly by revealing patterns that, at a glance, seemed familiar and could be mistaken for textbook schematic concepts.
The rocks are never wrong, but they are known to have a bit of fun at our expense.
Geologic Survey of the Northern Cascades: 3. Metamorphic and Igneous Rocks East of the Orogenic Core
Today’s post is a continuation of the last one, which explored the central core of the Northern Cascades. We followed the Methow River south until it joined the Columbia River through what is generally a region of marine sediments that have been dissected by faults (Fig. 1), and resemble the metasedimentary and metavolcanic rocks from the western region.
Figure 1. The Methow River follows the axis of a broad valley underlain by marine sediments (green area). We didn’t follow any of the side roads, so I can’t say anything about those rocks. We didn’t see any good outcrops until Site 7, so the story will pick up there. The eastern region is highly faulted (thin black lines on the geologic map), many of which are inferred to be thrust faults. One such prominent fault is outlined with a dashed yellow line because it delineates an area of anomalous high-grade metamorphic and igneous rocks similar to those within the central highlands. The curved regional contact with the Igneous province to the east reflects the resulting uncertainty.
Figure 2. This is a typical exposure along the Methow River, Site 7 near Carlton. There are several horizontal lines visible in the outcrop, but it is impossible to say if this is an example of metasedimentary rocks or jointed metamorphic rocks, both of which are present in this complex geological environment.
Figure 3. There were a lot of good exposures along State Route 153 so we stopped a lot. (A) This photo was taken a few miles SE of Fig. 2, where the geologic map indicates a sliver of Mesozoic (252-66 Ma) metamorphic and intrusive rocks. Of course, this boulder was in an old flood plain of the Methow River; the rounded nature of the sample (~two-feet across) suggests it was transported several miles from its origin.
(B) This boulder has a similar appearance but it was located on the western side of the Cascades, and is tens-of-millions of years older. Both samples are granitic in composition. It appears to be more weathered and beaten up by Skagit River, which is steeper than the Methow. I included it as an example of the continuation of several geological processes in the Northern Cascades for more than 100 Ma.
After a lengthy conversation with ChatGPT, the best I can say about the curious juxtaposition of different surface characteristics within these samples is that they may have resulted from differential weathering within fracture zones in the original igneous rock. The darker material is not a dike, but instead a zone of increased stress (possibly associated with faulting) where water preferentially altered some minerals into weaker products like clays.
Sometimes there isn’t a direct answer to a geological question.
Figure 4. This is a high-resolution image (~10x magnification) of the dioritic igneous rock found in some areas along the Methow river. Grayish blobs are quartz whereas the visible white grains are feldspar; it is impossible to tell if they are Na, K, or Ca feldspar without a microscope. The dark areas are amphibole.
Figure 5. This excellent exposure along the Methow River (to the left) was visible at Site 8. The appearance of this rock is determined by a strong joint set. There are two candidates for preliminary guesses about its lithology from the geologic map descriptions: 1) Mesozoic orthogneiss (high-grade metamorphic rock derived from granitic rocks); and 2) Tertiary intrusive rocks of granitic type. There is a difference in their appearance.
Figure 6. This outcrop looks like a gneiss to me. Note the near-horizontal layering (i.e. foliation); at high pressure, minerals become oriented parallel to the direction of least stress (i.e. they flatten). There are several large crystals protruding from the surface, however; these are porphyroblasts, which form when incompatible elements are squeezed out of the matrix.
Figure 7. This highly fractured outcrop at Site 9, near Pateros, caught my eye because the rocks didn’t form a vertical wall like we saw in Fig. 5. According to the map this is another example of undifferentiated Mesozoic orthogneiss, which occurs in small blocks throughout the region (purple areas in Fig. 1).
Figure 8. This face presents a pretty striking facade. The streaks of white against the dark surface look like sedimentary bedding; however, they are not bedding and never were. What I don’t see in this exposure are folds, where foliation has been warped into oscillating patterns that often reverse direction at this scale.
Figure 9. (A) Site 10 is located within a mile of the Columbia River. This road-cut is at least 60 feet high. This looks like more Mesozoic orthogneiss. All of those lineations are metamorphic foliations. Slight variations in local mineralogy create a complex pattern, especially on the surface.
(B) This close-up (~10x magnification) shows that some part of this mass of rock is undeformed diorite–that is, it hasn’t been smashed into orthogneiss.
(C) The edge of this sample was recently broken, revealing granitic rock beneath the surface. The dark lines in (A) are, in many cases, no more than surface weathering phenomena. We saw this a lot during this post. I can’t explain it, but it no-doubt entails surface chemistry, probably involving air temperature/humidity and imperceptible alterations in mineralogy during burial and exhumation over tens of millions of years.
Final Thoughts
We didn’t see any metasedimentary rocks on our trip along the Methow River, despite the preponderance of these rocks on the geologic map (green areas in Fig. 1). What we saw were a lot of slivers of granitic rocks and their metamorphic products. That’s probably because these rocks are very durable whereas lightly altered metasedimentary and metavolcanic rocks weather relatively quickly.
Glance back at Fig. 1 now that you’ve seen the rocks, noting the large number of faults (thin black lines), and ask yourself how Mesozoic rocks that were once buried miles beneath the surface pop up like wack-a-moles in this gently undulating terrain.
Maybe the Cooperative National Geologic Map is right to list every possible rock type in the geological metadata.
What a mess!
Geologic Survey of the Northern Cascades: 2. The Central Orogen
Figure 1. This magnificent vista of Washington Pass, at an elevation of 5477 feet, is representative of the core of the Northern Cascades. Highway 20 has to change direction to get out of this dead end, created by glaciers among massive mountains of high-grade metamorphic rocks, some of which have been exhumed from depths greater than twenty miles. Some of the intrusive rocks were emplaced at much shallower depths. Remember that it took tens of millions of years for this striking rock-scape to evolve on the surface of a dynamic planet.
Figure 2. I have annotated this piece of the USGS national geologic map to give an overview of the terrain we traversed. Today’s post will only be discussing Sites 4-6, which fall within the core of the orogen. We saw low-grade metamorphic rocks in my last post. The bright-colored area represents rocks that are classified as orthogneiss–a mixture of non-plutonic rocks that were subjected to high pressure and heat. The dark-purple shades are Mesozoic intrusive rocks and the lighter hues are Tertiary. Site 6 is approximately where Fig. 1 was photographed.
Figure 3. Steep peaks like this one (near Site 4 in Fig. 2) replaced the ridges seen in the sedimentary prism rocks to the west. When you see spires and domes, it is safe to assume that you are looking at high-grade metamorphic/intrusive igneous rocks.
Figure 4. The Skagit River canyon between Sites 4 and 5 is filled with boulders and gravel, but there are no exposures. This boulder is interesting because it is composed of two lithologies with different properties. I think this is a piece from miles upstream; the irregular part looks like a coarse-grained intrusive rock like granite whereas the smoother component is probably an intrusive rock that filled cracks in the host rock.
Extreme physical erosion in the river has emphasized several sets of intersecting dikes that were filled at different times in this rock’s history.
Figure 5. (A) Photo looking north from Site 6 in Fig. 2. According to the geologic map, this mountain comprises Tertiary (~66-2.3 Ma) intrusive rocks that don’t contain a lot of quartz (e.g. diorite).
(B) This is a closer view of the rectangle in A, showing vertical lineations that form ridges in the steep face of the mountain. At first glance these structures remind me of columnar jointing in basalts, but similarity in appearance doesn’t indicate a common origin. These lineations are irregular and wavy in some places. They probably followed jointing that developed as the massive rock cooled…under a continuously changing stress environment; e.g., uplift from erosion of overburden, sliding along a transform fault just to the west of here, and a jumble of motion along numerous tensional and compressive faults.
Figure 6. This view is looking southward towards a mountain comprising Paleozoic-Mesozoic (~250-66 Ma) intrusive rocks. The pluton in Fig. 5 was injected into this mass of far-older rock with a similar composition. This difference is striking at this distance: these older rocks present a curvilinear pattern of joints and fractures. I could speculate about the complex, almost turbulent, flow suggested by this view–but I won’t.
These rocks are so much older than those in Fig. 5 that they experienced several lifetimes before the tectonic stress field of the Tertiary (~66-2.3 Ma) developed; nevertheless, they reveal the cumulative effects of continuous deformation associated with ocean crust subduction.
Figure 7. I included this high-resolution (~20x) photo as a baseline for some rocks we’ll see later. This is the Tertiary intrusive rock, but the fabric (i.e. mineralogy and structure) has been modified from the original magma. Most of the minerals are feldspar, but only chemical and microscopic analysis can differentiate potassium and sodium end members. The anomalously large, dark crystal is probably the result of solid-state diffusion after the magma cooled. There is quartz, identifiable as the amorphous gray areas, throughout but not at high concentrations.
The description in the geologic map covers a wide range of quartz-poor plutonic rocks, so I won’t try to name it. What matters is that this is not a rock that simply cooled and was frozen in time.
Final Thoughts
Driving through batholiths, which are built up over millions of years from multiple plutons, gives plenty of opportunities for panoramic photography, but the real geological story is buried within the minerals themselves.
Washington Pass has exposed the beating heart of the Northern Cascades. We’ll have to look at the periphery to learn more.
Geologic Survey of the Northern Cascades: Deformed Trench Sediments
Figure 1. The ridges in the background of this photo represent the western foothills of the Cascades Range (Site 1 in Fig. 2). Today’s post will follow Highway 20 as it winds its way into the heart of the orogen, exploring sedimentary rocks that were deposited when Washington’s coast was located east of Puget Sound.
Figure 2. (A) We drove a loop around the Northern Cascades and stopped at the numbered locations. Today’s post will report on sites 1-3, which form the western flank of the central peaks. (B) The Northern Cascades are not primarily volcanic, although there are active volcanoes in the region, e.g. Mt. Baker. The box outlines the larger map area. Note that these mountains are not primarily composed of intrusive rocks; and they are northwest of the Columbia River volcanics.
Figure 3. This geologic map is derived from the Cooperative National Geologic Map (https://ngmdb.usgs.gov/nationalgeology). The sediment prism represents lithified sediments that collected within the subduction trench. These are called metasedimentary and metavolcanic rocks because they are only slightly modified by heat and pressure.
The wide range of ages results from the difficulty of applying radiometric dating to such deformed rocks, as well as the assortment of material that is mixed together within the trench. For example, the maroon area north of Site 2 is described as melange–a mixture of different rock types that have been folded and faulted as if put in a blender.
The dashed line is thought to be a strike-slip fault along which the eastern block has moved southwards; we don’t know which side moved, of course, without studying the regional distribution of rocks. That is beyond the sope of this post. The rocks to the east side of this fault are the core of the Northern Cascades: metamorphic rocks that have lost all evidence of their origins, and younger igneous rocks that we’ll see in the next post.
Figure 4. This is a typical exposure of these deformed sediments from Site 2 (see Fig. 3 for location). These are sedimentary rocks yet no bedding can be seen from this distance.
Figure 5. This photo from Site 2 was taken in a small quarry, which permitted examination of a fresh surface on a boulder that was removed from somewhere on the hillside. Note the absence of bedding. I am pointing to the next photo.
Figure 6. Close-up of the boulder in Fig. 5, showing foliation in lieu of bedding (top to bottom in photo). There is a thin quartz vein running from left-to-right near the top of the image, and several micro-veins throughout the sample. The original sediment was heated sufficiently, under pressure, for quartz to become mobile, but there wasn’t a lot of quartz in the original sediment. It may have been either mud or possibly volcanic layers.
Figure 7. This road cut near Site 3 is representative of the melange found in the western foothills of the Northern Cascades. There is a suggestion of bedding but nothing is persistent, and there are no straight lines. This is typical for deformed sedimentary prism sediments.
Figure 8. This close-up of the rocks in Fig. 7 is about 12 inches across. It reveals quartz filling a cavity within a larger mass of breccia. Quartz is very mobile under moderate heat and pressure.
Figure 9. This close-up is a few inches across. It reveals a mixture of mineral crystals (seen as rectangles and shiny grey areas throughout the image) that are feldspar (another mobile mineral) and quartz, but no lineation. Deformation was pervasive, from the outcrop scale (Fig. 7) to the microscopic scale. These rocks behaved like mush on geological time scales (e.g. millions of years).
Final Thoughts
This schematic block diagram of a subduction zone has to be advanced a few million years to explain the rocks we discussed in this post. The Trench (orange area with white arrows on the front of the diagram) in this photo represents the last few million years whereas the rocks we saw are located closer to the Cascade Volcanoes today.
The rocks haven’t moved that far from where they were deposited as sediment and lava in the submarine trench; instead the trench has moved westward as the thickening continental crust (right side of diagram) forced the ocean plate to sink further to the west. It’s a simple question of conservation of mass and energy; as powerful as plate spreading is, it has limits on geologic time and space scales. In other words, N. America was bigger and heavier.
The melange being created in the current trench will be exposed in tens of millions of years, and we probably won’t be around to observe it.
Industrial Archeology in the Northern Cascades
I’ve noticed a lot of abandoned industrial buildings and equipment molded into the landscape since I moved to Tacoma. For example, old sawmills and a smelter have become waterfront parks; pieces of railroad machinery have been transformed into artwork all over town; a gravel quarry was renovated into a golf course that includes several large structures; rusted valves and other maritime equipment adorn the marina; the list goes on. There are even abandoned and partially demolished structures that were once bars and fishing stores in nonindustrial areas.
Therefore I wasn’t surprised at what I found when we took a drive along Hwy 20 to explore the Northern Cascades before it closes for the winter.
A guidebook led us to a small town named Concrete. You can guess what its main industry was.
This was a concrete manufacturing plant built by the Washington Portland Cement Company (WPCC) in 1914. The company built a town to house the workers and stayed in business for ten years. The property was then purchased by Puget Sound Energy (PSE) and converted to a hydroelectric power generation facility.
The new owners recently began the archeological work.
Industrial archeologists have begun field work to reconstruct life in a company town more than a century ago.
The site has rest rooms and picnic tables, and a small visitor center housed in what was once a lab of some kind. However, the museum and rest rooms were closed for winter.
This view shows part of the tramway used to move hot concrete pellets from the kiln, which is gone but was to the left of the concrete structure to the left of the photo. The concrete silos were filled with–you guessed it–Portland cement by a screw device on top of them, which is apparently still installed but not open to the public.
This building was connected by the tramway but the signage doesn’t explain everything yet; this is a scientific investigation in progress. After what I’ve seen around the area, I am not surprised at all by PSE taking an interest in such a humble project.
PSE constructed this dam on Baker River. I think the building is the original generator house, but they haven’t installed detailed signs yet. There is a new facility just downstream that appears to be using the original plumbing. The transmission lines are on top of the hill, where this photo was taken, but I don’t know if this antique is still operating; there were several power lines leading down to it however. Hydroelectric generators last for decades, but this one would have been inefficient and probably not very useful today.
But you never know…
The National Park Service visitor center was closed for winter, but PSE had a small facility located a few miles further up Hwy 20. We stopped to stretch our legs and discovered a pedestrian suspension bridge crossing the Skagit River … and guess where the trail led.
About a half-mile walk led to a hydroelectric generation facility that was in use until recently. It was used to power a sawmill. Two water turbines/generators can be viewed through the window in the middle of the building, but they weren’t spinning and no water was flowing through the output spillway to the right.
This is one of the turbines that ran the generator.
PSE maintains a small sculpture garden that includes this representation of a salmon nest in a stream bed. In the background, you can see the edge of an arch built of electric insulators from the power station.
Final Thoughts
Industrial archeology wasn’t the principal reason for our journey through the Northern Cascades, but it was a welcome and rewarding addition to a great fall weekend in the Pacific Northwest.
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