Cape Blanco/Sixes River Terrane

For a Google Earth .kmz file to explore Cape Blanco and the Sixes River Terrane, scroll to the very bottom of the page.

There are so many geologically interesting things happening along the southern coast of Oregon, but most people don’t tend to travel farther south than Coos Bay. What most people don’t know is that the southern coast geology is completely different from the northern coast geology. The very southern corner of Oregon is composed mostly of accreted terranes (Figure 1). A terrane is a group of rocks that formed together, show similar composition and environmental characteristics to each other, but do not show environmental or compositional relationship to the other rocks surrounding them. The rocks are also physically separated from the other rocks surrounding them by faults. The southern Oregon Coast terranes were accreted or added to the North American continent during one or even both of the following processes: 1) During subduction the oceanic plate that gets subducted (the Juan de Fuca plate) contains sediments and rocks that can be thrust up onto the land as the plate sinks below the continental plate (the North American plate). A good way to think of this is if you open up and oreo, filling sides up and scrape the filling off from one cookie onto the other as you slide the frosting topped cookie underneath the other. 2) A transform fault can carry a portion of land to a new location where it is sutured, or added, onto that new location. A modern analogue of this would be the San Andreas Fault. Anything east of the San Andreas Fault is part of the North American plate, anything west belongs to the Pacific plate. These two plates are moving in different directions, so if you think about it, places like San Diego and Los Angeles are moving northward and if we project their movement into the future (very, very far in the future) that portion of California west of the San Andreas fault will eventually collide with Alaska and get added to that area.

The Southern Oregon Coast's accreted terrane was either accreted through subduction or was transform faulted to its current location. Either way, there is some foreign and strange geology happening down on the south coast, but what exactly can be seen? We are going to focus mostly on two different processes; the accretion of the Sixes River Terrane and the formation of the marine terraces along Cape Blanco (Figure 1).

The Sixes River Terrane was accreted before the formation of the marine terraces of Cape Blanco, so we will start there. The small terrane is actually part of a much larger physiological province of Oregon called the Klamath Terrane. The Klamath Terrane includes many different terrane slices that start at Redding in northern California and spread in a northwesterly direction ending with the Sixes River Terrane just south of the town of Bandon, Oregon. These terranes are made up of slivers of rock that were formerly from the ocean floor and each one has been thrust underneath the one previously accreted, with the oldest in the east and youngest to the west. The Sixes River Terrane is the youngest and last accreted terrane that is part of the greater Klamath Terrane. The terrane is exposed just north of Cape Blanco and also just south of Roseburg, Oregon (Orr and Orr, 1999). The Sixes River Terrane is a melange, meaning the entire terrane is composed of mappable geological units consisting of rock blocks of different ages and origin, commonly embedded in argillite (a hard rock similar to shale or mudstone), sandstone, or serpentinite with a chaotic internal structure (Wakabayashi, 2011). In fact, the phrase melange is French for “to mix or to mingle”. So, basically a melange is a group of rocks that are all mixed up and not well bedded. There are basically two rock formations that can be found in the Sixes River Watershed, the area in which we are going to travel; the Galice Formation and the Otterpoint Formation.

The Sixes River Terrane is almost split in half with the northern part mostly containing the Otterpoint formation, a highly sheared melange of mudstone, sandstone, volcanic rocks, chert, serpentinite, and blueschist (http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5315612.pdf). The phrase “highly sheared” suggests that as these rocks were emplaced both by subduction and northward transfrom faulting to their current position, meaning that these rocks have been squashed, and pulled, and rolled around to the point where exposed rocks show jointing, inclusions mixed in from other formations, and hydrothermal alterations. In other words, these rocks are a mess (Figure 2). The Otterpoint Formation is equivalent to the Franciscan Formation of the northern California Coast Range, a formation that stretches all the way down to San Francisco (http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5315612.pdf).

The southern half of the watershed is composed of the Galice Formation, which is part of the Klamath Mountain Terranes, with some younger formations mixed in like the Cretaceous Rocky Point and Humbug Mountain Formations (http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5315612.pdf). The

Galice Formation is composed of metamorphosed sedimentary and igneous rocks, like blueschist, that were later intruded by volcanic diorite. The intrusions of diorite are very important in the area because its emplacement led to the mineralization of gold in the area. The heat of the molten diorite dissolved the gold in surrounding rocks as well as other ions, such as quartz. These dissolved ions were then moved along fractures in the rocks by hot fluids. When these fluids came to rest so did the gold within quartz vein deposits (http://www.sbs.com.au/gold/story.php?storyid=128, Figure 3).

Because of the many different rocks that make up the Sixes River Terrane, you can stand in the river's valley and see a multitude of different rock types and colors. There are igneous, metamorphic and sedimentary rocks and within those types of rocks I found cherts, conglomerates, limestones, serpentinites, blueschists, diorites, and several other samples. The shear number of rock types had me collecting there for several hours and saying the phrase “awesome” a lot.

Evidence of the active tectonics in the region can be seen closer to the beach in the form of marine terraces exposed at Cape Blanco. Marine terraces start as an the old beach surface that is eroded almost flat or at an incline by wave action. Over time these surfaces are raised above the modern beach by tectonic activity creating a steep beach cliff and corresponding flat ledge above the active beach. At Cape Blanco there are four terraces and several tectonic processes responsible for raising the marine terraces to their current height about the modern beach (Figure 4). First, subduction raises these terraces above the modern sea level by compression. As Juan de Fuca and North American plates collide in subduction, the uppermost portion of the plate boundary is locked in place due to friction. However, even though the two plates are locked together, the Juan de Fuca Plate is still moving northeast as the North American Plate is moving southwest causing compression and crumples the leading edge of the North American plate. This wrinkling of the plate raises marine terraces above sea level and causes the greater Coastal Mountains of the Pacific Northwest to increase in height. However, Cape Blanco shows a little extra lift and this can be seen in the tilt of the sedimentary beds on either side of the cape. To the north of Cape Blanco the sedimentary rocks in the beach cliff dip to the north and to the south of the cape the sedimentary rocks dip to the south suggesting that the cape is actually the axis of an anticline fold feature (Kelsey, 1990). The fold causes Cape Blanco to rise but the surrounding area to fall or subsiding as part of the accompanying syncline. The rising of terraces has exposed several rock formations in the area of Cape Blanco including the Otterpoint Formation at the very bottom and the Empire Formation at the top. The Empire formation is composed of massive, poorly bedded, light colored (white) sandstone with minor interbeds of siltstone, giving the cape name (Ehlen, 1967; Figure 5). As you move to the south from the

Cape Blanco lighthouse other rock deposits are exposed including Quaternary gravels and finally the former beach sands of Cape Blanco Terraces at the top(Hladky et al, 2013). The beach cliff at Cape Blanco is one of the tallest in Oregon at around 245 ft from sea level.

The Southern Oregon Coast s truly unique in its ability to show so many different tectonic processes, surface landforms, and rock types in such a small area. There are few other places where one can see all of the types of rocks on display and within only a few feet of each other. I highly recommend the trip.