The Rogue Valley region in SW Oregon: displaying 300 million years of geologic time

Typically, I post blogs when traveling away from my home in Ashland, Oregon. But with COVID-19 keeping us all at home, it seems a good time to investigate the landscape of my local region. Ashland is a tourist town best known as the home of the Oregon Shakespeare Festival (OSF), an eight-month season of 11 plays—four by Shakespeare and seven by classic and contemporary playwrights—in rotating repertory in three theaters.

The outdoor Elizabethan theater, although all three theaters are sadly closed for most of the 2020 season.

Ashland is also known for its scenic beauty; its valley location between steep mountain ranges provides stunning vistas and many opportunities for outdoor activities such as hiking and skiing. You will not be surprised to learn that the geologic makeup of the region is responsible for this natural grandeur. The valley where Ashland is located is the Bear Creek Valley, which feeds into the Rogue Valley farther north near Medford. The geologic setting is the same, and for simplicity I will refer to the whole region as the Rogue Valley region.

The valley is oriented SE–NW; see the map below. This view is looking across the valley (underlain by the Hornbrook Formation—Kh) from the NE slope (on the Tpc unit) to the SW mountains (Ashland pluton). Mt. Ashland (with snow) is the highest peak in the region (2300 meters / 7533 feet) and has a downhill ski resort on the north slope.

There is certainly geologic complexity in the region, but the basic distribution of rock units is straightforward and clearly displays nearly 300 million years of geologic history. The map below shows the distribution of geologic units as viewed from above, as if you were in a plane surveying the landscape. Each rock unit is denoted by a different color. The cross section below it shows the distribution of geologic units in a vertical profile—imagine using a huge knife to cut through the surface layers to expose what they look like beneath Earth’s surface.

The geologic units are as follows (from oldest to youngest): JTkt=Jurassic/Triassic Klamath terrane, including the Ashland pluton (blue, purple and burgundy colors); Kh=Cretaceous Hornbrook Formation (green colors); Tpc=Tertiary Payne Cliffs Formation (beige colors); Tv=Tertiary volcanic rocks (tan/brown colors); Ti=Tertiary intrusives (orange color); Qal=Quaternary alluvium (yellow color); Qls=Quaternary landslide (light tan color). The Qal/Qls units do not show up on the cross section below because they are very thin layers of modern-day sediments that are being deposited in streams and on slopes between the mountains and the valley. Mostly, they are covering the Kh unit in the valley.

At the bottom of this post is a geologic time scale that shows events in Oregon and (in red) the specific rock units in our region. Map and cross section are from Wiley et al., 2011
The cross section, a cut from SW to NE perpendicular to the valley (at the yellow line on the map above), shows how the rock units are all tilted toward the NE. Make a stack of books and imagine that each book is a rock unit, such as Kh. Then take the stack and tilt it so your books are inclined to the right (to the NE on the cross section). When you look at your stack from the side, you should see each book (i.e., rock unit), now with the oldest on the left (SW end) and the youngest on the right (NE end). When you look down on the top of your stack, you should see how each book (i.e., rock unit) is exposed at the surface and creates the “striping” of colors on the map view. The beauty of this distribution—rock units tilted to the NE and then eroded—is that we can start at the SW end and “walk through time” to the NE end.

There is a lot of complexity in the western (oldest) part of the region but we will dispense with it quickly in this post! The Klamath terrane is all of the area between the Rogue Valley and the Pacific Ocean. [Note that for geologists the Klamaths are specifically the colored area that extends from Redding to Roseburg on the map below. In contrast, the Siskiyous are a broader geographic region that also includes younger rocks of the Western Cascades that are not shown on the map below.]

A map of the Klamath terrane; note the location of Medford, the Rogue Valley (Kh unit), and the Ashland Pluton (pink Jp unit next to Kh). The Klamath terrane is actually a complex of terranes; each terrane is a fault-bounded chunk of Earth’s crust that has been moved from where it originally formed and, in this case, has collided with the continent. Unlike the units in the cross section across the Rogue Valley, the units within the Klamath terrane are oldest to the east and younger to the west. The reason for this is shown in the figure below.
The rocks in the Klamath terrane complex started life as igneous rocks—mostly volcanic islands that formed offshore—and sediments that were deposited on the seafloor adjacent to the volcanic islands. Because of active subduction processes (where oceanic lithosphere descends beneath continental lithosphere), the volcanic islands eventually collided with the continent and, in the process, were metamorphosed and deformed by the forces involved in the collision. The yellow “suture” lines mark the collision zones between successive volcanic islands. In some places bits of oceanic lithosphere got caught up in the collision, which is why we sometimes see serpentinite in the suture zones in the Klamaths. Notice that the terranes are younger toward the west—imagine a large piece of play-doh and then successively add thin strips to one end to emulate colliding terranes.

This process of colliding volcanic arcs went on for about 150 million years, from about 300–150 million years ago (Ma). Toward the end of this time, in the Jurassic period, granitic magma (red blobs in the diagram) intruded into the metamorphic rocks; the Ashland pluton is one example of these granitic intrusions.
Here is an example of a rock exposure in the Klamath terrane. It is difficult to tell what the original rock type was because the minerals have been altered by metamorphism and the layers have been distended and faulted by the compressional forces produced as the terranes collided with the continent.

We need to examine the basics of an important rock type in the region—igneous rocks, which are a primary rock type because they form from liquid rock (called magma) that rises up through Earth’s crust (see diagram below). The other two rock types are mostly secondary. Metamorphic rocks form when pre-existing rocks are altered through increases in applied temperature and/or pressure. Most sedimentary rocks form from erosion of pre-existing rocks, although a few are primary (e.g., salt deposits).

Magma rises upward through Earth’s crust. If the magma solidifies beneath the surface it forms igneous rocks called plutonic (after Pluto, god of the underworld) or intrusive (they intrude into pre-existing rocks). If the magma travels all of the way to Earth’s surface, it forms igneous rocks called volcanic (after Vulcan, god of fire) or extrusive (they are extruded up onto Earth’s surface). If we see plutonic rock at Earth’s surface (e.g., the Ashland pluton), we know that there has been a lot of erosion. While the pluton was forming deep underground, magma was also feeding volcanoes at Earth’s surface. Later, over millions of years, the land could be uplifted, causing the overlying volcanic ediface to be eroded away and eventually exposing the underlying plutonic rocks.
Here is a piece of granitic rock from the Ashland pluton. When magma cools beneath the surface, it cools slowly, and there is time for large crystals to form. Granite is a type of igneous rock that is rich in silica. The primary minerals are quartz and feldspar (light colored) and biotite and hornblende (dark colored). This piece shows two other common features of plutonic rocks: (1) The larger dark pieces are xenoliths, meaning “foreign rocks”. They are pieces of the rock that the magma intruded into. Sometimes the surrounding rock does not become completely absorbed by the magma and although it gets metamorphosed (i.e., changed) by the heat of the magma, it retains some of its original character. (2) The lighter-colored strip (upper right of photo) is a vein. It is a later phase of magma that intruded into the already solidified granite; it is lighter in color because it contains only the high-silica minerals of quartz and feldspar. In general, more silica-rich minerals have light colors, and silica-poor minerals have dark colors.
In the area around Ashland, the granitic rocks form beautiful landscapes. Typically, plutons are uplifted due to compressive stresses that cause fractures and faults to develop within the granitic rock. Erosion occurs preferentially along the fractures (i.e., cracks) and the chunks between the fractures become rounded by the effects of physical and chemical weathering. Soils are known as DG (decomposed granite); they are thin, well drained and nutrient poor.

Referring back to the geologic map and cross section above, recall that the rocks in our region are tilted toward the NE, which means the oldest rocks are to the SW and the youngest rocks are to the NE. Why are the rocks tilted? Because they are one limb of a very large-scale fold. To learn about folds, go to the Geology Page site: http://www.geologypage.com/2015/12/geological-folds.html. You can search this site to find basic information about many geologic processes.

This Google Earth image from NW Argentina shows a large-scale fold. This type of fold is called a syncline because the layers are tilted toward the center of the structure (arrows show direction of tilt). Take a stack of papers and fold them so that the center of the stack is down and the edges are up (like a cup). To make an anticline, where the layers tilt away from the center of the structure, fold your stack of papers so that the center is up and the edges are down (like a ridge). What we see in the Rogue/Bear Creek Valley region is a monocline; that is, only one side (called a limb) of a fold, analogous to the southern limb above. Imagine that A is the oldest rocks of the Klamath terrane to the SW; B is the younger Hornbrook Formation in the valley; and C is the even younger Payne Cliff Formation and overlying volcanic rocks on the slope and ridge to the NE. We do not see anything like the north limb of the syncline in our region because the High Cascade volcanoes are being built to the east.

In the next post, we will examine the reasons for topographic differences: why is the valley a low part of the landscape and why are the ridges high parts of the landscape, even though they are all tilted in the same direction.

The Rogue Valley region has rocks that span the past ~300 million years. The oldest rocks (Jurassic and older) are found in the Klamath terrane, west of the valley. The Jurassic-aged plutons (e.g., Ashland pluton) are the youngest parts of the Klamath terrane. The Kh/Tpc/Tv units all overlie the Klamath terrane and are all tilted toward the NE.

Important references

Wiley, T.J., et al., 2011, Geologic database and generalized geologic map of Bear Creek Valley, Jackson County, Oregon: DOGAMI Open-file report 0-11-11. The full version of the map (partially shown above) and the report can be accessed at: https://www.oregongeology.org/pubs/ofr/p-O-11-11.htm

Miller, Marli B., 2014, Roadside geology of Oregon, 2nd edition: Mountain Press Publishing Company. Source of time scale and Klamath terrane map. Marli also maintains a web site with her fabulous geology photos: https://geologypics.com/

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