Introduction

A few months ago, we followed US101 along the Washington coast and reported on Eocene (56-34 Ma) sedimentary rocks that were deposited in the trench that fronts the Cascadia subduction zone. Today I am expanding my investigation into the heart of the Olympic Mountains, which comprise the northern extent of the sedimentary wedge known as the Coast Range (see Fig. 1).

Figure 1. This schematic representation of a subduction zone beautifully encapsulates the Olympic Mountains, which are labeled on it as well–even Olympic National Park. The Coast Range (shown in green) is the exhumed remains of the sedimentary wedge, as shown in purple and labeled as the Trench. It is useful to note that, as subduction has continued (at least since the Eocene), the trench moves seaward and older, previously buried, ocean sediments are pushed up over the subducting ocean plate. This motion is indicated by the white arrows on the front of the block diagram.

Figure 2. (A) Map showing Lake Crescent (outlined with a box) and Hurricane Ridge, the two locations I’ll be discussing in this post. I presented some of the geology of Victoria, British Columbia, and Vancouver Island in a previous post.

(B) Geologic map from RockD of the Lake Crescent area. Two rock types dominate the area: Eocene marine sediments and slightly younger basalt flows and volcanic breccia.

(C) Detail map of the area outlined in panel B, showing the Spruce Railroad Trail. The circle outlines the two locations discussed in the next section.

Lake Crescent

Figure 3. Lake Crescent fills a deep (~600 feet) valley created by glaciers over the last two-million years, which was blocked by a landslide several thousand years ago. The shoreline in this image is broken by a series of linear ridges. These are resistant layers of sedimentary rock that has been rotated to a high angle.

Figure 4. The entrance to McPhee tunnel has been reinforced with rock bolts, which can be seen throughout this bedding plane. The left side of the photo shows why the tunnel was dangerously unstable: the ridge through which it was cut is composed of thin layers of interbedded sand, silt and clay.

Figure 5. McPhee tunnel is only a couple hundred yards long (see Fig. 2C), but the exit reveals completely different rocks. There is a red line in the geologic map (Fig. 2B); this represents a contact between the marine sedimentary rocks (Fig. 4) and the volcanic rocks.

Figure 6. (A) Outcrop scale photo of basaltic rocks near the exit to McPhee tunnel. I have tentatively labeled one round boulder as a pillow basalt. Pillow basalts result when lava erupts on the sea floor in deep water (~1000 feet deep) because of the immense water pressure. When the rock weathers, they retain their shape because of a more homogeneous composition than the lava surrounding them.

(B) Detail showing the wide variation in clast size and shape. This mix of material, all of it basalt, is indicative of a tectonic breccia. Imagine what happens to undersea lava flows as they weather through seawater action and collapse into the trench (see Fig. 1).

Hurricane Ridge

Figure 7. This photo shows Hurricane Ridge (see Fig. 2A), which leads to Hurricane Hill at about 5500 feet above sea level. The next few photos show the kinds of rocks visible along Eagle Point trail, which leads to the summit.

Figure 8. These sedimentary rocks resemble those at McPhee tunnel’s entrance (Fig. 4). They are tilted to nearly vertical, consist of dark and bright layers (mud and sand/silt, respectively), and are fairly continuous; however, they have an additional feature that might escape one’s attention at first: some of the darker layers contain rounded forms that are brighter. If I’m right, these are boudins, concentrations of silica-rich minerals that result from the rocks being squeezed under high pressure.

Figure 9. (A) This outcrop (about 20 feet across), located a few hundred yards from Fig. 8, is dominated by sandy sediments. The surface is littered with angular blocks, which suggests that these rocks were subjected to intense deformation after they became lithified (turned to rock).

(B) Detail of area indicated by the rectangle in (A). Now we can see bedding planes, e.g. in the center of the photo. These beds are also nearly vertical. These are reminiscent of the sandstones we saw along the coast; I’ll bet they contain turbidites, layers of coarser sediment deposited by submarine landslides in deep water.

Figure 10. (A) These thin beds have been folded into a chevron. This implies extreme horizontal stresses.

(B) The bedding is almost obliterated in this exposure, where coarser sediment (lighter colored) is mixed with shale. Furthermore, the entire assemblage is fractured into small pieces like those lying on the ground.

Figure 11. This view from the top of Hurricane Hill is looking northward towards Vancouver Island, British Columbia. The main features seen here are the knobs that protrude from the summit of this adjacent ridge. Promontories like this were ubiquitous along the ridges comprising the Olympic Mountains. Compare this topography to the narrow ridge leading up to Hurricane Hill (Fig. 7). The fractured and folded rocks we saw along the trail (Figs. 9 and 10) have weathered faster than these, but what are they?

Figure 12. (A) Closer view of the left side of Fig. 11, showing several interesting features: a series of knobs to the left, then a deep ravine cutting across the ridge, then massif to the right. These are basalt that has not been as deformed as the sedimentary rocks we saw lower down the trail. Seafloor basalts erupt into layers of marine sediments, but they are harder and more resistant to both chemical and physical alteration during burial and deformation. These layers are unusually strong layers.

(B) Close-up of a knob on Hurricane Hill. I didn’t approach closer because signs asked visitors to remain on the trail, and it was a pretty steep slope into a deep canyon. I don’t see any possible basalt pillows (see Fig. 6A); instead, it seems to be fractured into rectangular blocks. However, some of the rounded boulders seen in the foreground look like lava pillows. RockD calls this formation, Crescent Formation, pillowed…

Final Thoughts

The Coastal Range extends from Alaska to Mexico but not as a continuous mountain range; instead, it comprises a series of mountain chains like the Olympic Mountains. These individual mountain chains are lumped together because they all formed during subduction of multiple tectonic plates underlying the Pacific Ocean during the last 200 million years. The Olympic Mountains are simply the most volcanically active area in recent times.

The subduction zone trench (see Fig. 1) is continuously filled with sediment eroded from the adjacent highlands in a process that doesn’t cease until plate convergence ends. Sediment is carried into the trench, buried, then exhumed ; there is simply more volume of sediment/rock than the trench can contain. Today the trench is approximately 100 miles west of Seattle whereas it was located within the Olympic Mountains between 55 and 34 million-years ago, a difference of about 70 miles; in other words, the subduction zone has moved about 30 miles westward in less than 60 million years.

I am bothered by one discrepancy, as I understand the picture of marine sedimentation and eruption of seafloor basalt in the westward-propagating trench: Where is the modern analog of the thick sequences of basalt that occur within the Olympic Mountains?

Things to consider (I don’t have the answers):

1. Volcanism within the Cascades Range (see Fig. 1) is overwhelmingly andesitic, which results from mixing of the sinking ocean plate (gabbro and basalt) with continental crust (granitic) as magma rises.
2. Massive outpourings of basalt occurred in eastern Washington between 17 and 5 Ma, known as the Columbia River Basalt Group.
3. The North American plate has been moving over a hot spot in the upper mantle currently located beneath Yellowstone National Park.
4. Pillow basalt is a definitive indicator of submarine eruption, as seen occurring today in Hawaii. Thus, these features are consistent with either (a) eruption of flood basalt in the trench or (b) collision of a sea mount with North America.

Every subduction zone is unique and overprinted with whatever else is going on in the earth’s crust and upper mantle. I wonder if the Olympic Mountains basalts are the result of mixing of marine sediments with flood basalts, and possibly incorporation of islands or seamounts.

Food for thought…