Wednesday, February 24, 2010

The Other California: The Flotsam and Jetsam of an Ancient Ocean Basin


After a busy couple of weeks in other places, I'm getting back to the Other California, the exploration of the places in my fair state that don't always show up on the postcards...

In most parts of the world, the geological story can be teased out by following some basic rules of stratigraphy: in a sequence of layers, the oldest is usually found at the base; if a layer appears in the side of a cliff, we can reasonably expect that it will found to continue at another location nearby. Sediments usually form in horizontal layers. These principles were realized three hundred years ago by Nicolas Steno, and ultimately led to the establishment of geology as a science. Geologists were able to use these principles and a growing knowledge of sedimentary structures and facies to work out a reasonably accurate story of the history of the crust in many parts of the world. In regions like the Alps of Europe and the Appalachian Mountains in the eastern United States, the early geologists were able to unscramble incredibly complex rock sequences because they were able to correlate formations from one mountain ridge to another. Fault lines were not necessarily a problem; walk far enough and you could find a continuation of your missing layers somewhere along the other side of the fault.

The rules seemed to apply, until the geologists arrived in California. There are places in the state where the rocks are so chaotic and discontinuous that geologists with the classic training in stratigraphy had to throw up their hands in frustration. Following an outcrop? Great, there's a fault. We'll look for a continuation of the layer. And look. And look some more. And never find it. What are these older rocks doing on top of these younger rocks? That doesn't make sense. And why can't we find any fossils in many of these rocks? They're sedimentary, they should have fossils. California was an enigma and a chaotic mess to the first geologists who tried to pry loose her secrets in places like the Coast Ranges, the Transverse Ranges, and the Sierra Nevada.

There is a region that is pretty much the epitome of the chaos that is California geology. Tucked between the Cascades and the Coast Ranges of Northern California, the Klamath Mountains rise to elevations of just over 9,000 feet, and essentially resemble a large plateau that has been deeply dissected by stream erosion, especially along the Trinity and Klamath Rivers. The Klamaths are divided by geographers into a number of subordinate ranges, the Siskiyous, the Trinity Alps, the Marble Mountains, and others, and most of the country is untrammeled wilderness.

The Klamaths are a lesson in geology contradictions. Almost every kind of marine environment shows up somewhere, shallow nearshore deposits, coral reefs, carbonate shelves, deep ocean muds and trench deposits, but in close proximity are found unusual igneous rocks derived from the deep mantle and the base of the ocean crust. Here and there the rocks are intruded by granitic plutons much like those of the nearby Sierra Nevada, but also there are granites that are much older. Everywhere there are faults. Mostly they are thrust faults but occasionally normal faults are found, indicating that both compressional and extensional forces have affected the rocks.

The key to unraveling the mysteries of the Klamath Mountains was to stop trying to make logical sense of the rocks! There was not a single sequential story to be deciphered for this part of the world at all, there were a multitude of stories, stories that unfolded in many different places around the planet. One of the first researchers to make this connection, W.P. Irwin (in the early 1960's), divided the rock sequences into a series of "belts" that seemed to have originated in places other than the Klamaths. This model evolved into the concept of accreted terranes, a pioneering idea that played a central part in the acceptance of "continental drift" and plate tectonic theory in the late 1960's.

A terrane is a term used to denote:
...a fault-bounded geologic region that differs from adjoining geologic regions by its distinctive stratigraphy, structure, tectonic history, and in some cases biota ...Terranes may be either allochthonous or autochthonous, far-traveled or near their point of origin. Terranes may amalgamate together to form a superterrane, and terranes become accreted terranes after they collide with continental crust along an active plate margin.*
Translated, a terrane is the crustal equivalent of the debris that collects in a river eddy: the floating branches, leaves, bits of styrofoam coolers, and plastic bottles. In this case, the Klamath Mountains were a collecting point of various bits of ocean crust and continental fragments in the eastern Pacific region during Paleozoic and Mesozoic time.

Four major terranes were originally identified in the Klamaths by Irwin: the Eastern Klamath, the Central Metamorphic, the Western Paleozoic and Triassic, and the Western Jurassic. With continued research, these have been subdivided into many subterranes. A hint of the complexity can be discerned in the geologic map shown above, which is part of an effort to correlate the terranes of the Klamath Mountains to those in the northern Sierra Nevada.

A bit of a plea: Geotripper needs your help! I've explored a lot of California, but not so much in the Klamaths. Do you have a favorite spot you would like to share? Be a Geotripper Geoblogger for a day, and tell us about it!

*The Terrane Puzzle: New Perspectives on Paleontology and Stratigraphy from the North American Cordillera, edited by Robert B. Blodgett, and George D. Stanley, Jr.; Geological Society of America Special Paper 442, page 2.

4 comments:

Lockwood said...

I would so like to help; I've taken a number of trips in the area. But when my apt. flooded a few years ago, I lost all of my undergrad notes and texts, along with all my slides.

Still, I can offer one anecdote: On a trip with Jad D'Allura from SOU (Ashland) we made a stop along the Klamath River. In one ~50 yard outrop, we saw a talc-chrysotile schist (peridotite parent), a felted amphibolite (basalt parent), and bedded cherts. That group was probably originally one broken chunk of sea floor. The next block contained mica schist and blocky beds of wollastonite, probably a carbonate shelf and shale facies. The third block was a somewhat metamorphosed turbidite series. To see all of these environments represented in 50 yards is hard to conceive.

Gaelyn said...

California really is a geologic mess, along with SW Oregon's Siskyous which run east and west and share some of the same marble found in Kings Canyon/Sequoia. Wasn't easy to talk about with visitors when working at Oregon Caves.

andrew said...

I didn't realize that the Klamaths extend so far east of Redding. I've spent very little time there, unfortunately, but last year I made a point of driving through the purple patch on that map, which is the largest exposure of peridotite in the 48 states, if not North America. I came home with some nice samples and photos.

Terry's world said...

read assembling california by john macphee, a great rendition(albeit without diagrams) of the accretionary history of our terranes.