In the Heart of the Devil: The Damning of Del Puerto Canyon

This beautiful canyon is under serious threat

California geology is complicated. Unlike any other state, it is affected by the interactions of all three kinds of plate boundaries: divergent (the crust pulling apart), convergent (the crust compressing together), and transform (the crust sliding laterally). All of these forces have formed a complex landscape with incredible scenery unlike any other place in the world. 

The Coast Ranges province is one of those unique regions. Extending some 400 miles from the Oregon border to the Transverse Ranges near Point Conception, it is one of the least familiar parts of California (aside from the Pacific Highway 1 corridor through Big Sur and the Marin Headlands/Point Reyes region). The province is defined by a series of individual mountain ranges that trend roughly parallel to the coast, but the variation in rock types and structure is astounding. Within the province there are active volcanic fields, older inactive volcanoes, vast tracts of tilted sedimentary rocks, exposures of twisted and folded rocks formed deep within subduction zone complexes, and even a displaced section of Sierra Nevada granitic crust. Numerous active faults slice through the province, including many of California's most dangerous: the San Andreas, the Hayward, the Calaveras, and many others.

We just explored the heart of the devil: the so-called Diablo Range. It is one of the largest individual ranges in the province, running for around 150 miles from Mt. Diablo and the Carquinez Strait on the north to the Coalinga area in the south. The region is largely undeveloped, and few paved roads cross range. We followed one of those few roads, the one that traverses Del Puerto Canyon. It's a one-of-a-kind experience, the equivalent of driving into and through the crust of the Earth and into the mantle below. It is the path to the nether-world that has often been called the home of the devil.

Del Puerto Canyon lies just west of the Central Valley town of Patterson. A paved road, state route 130, connects Patterson with the Santa Clara Valley, but anyone thinking it would make a shortcut between the two localities is in for a rude awakening: curvy, narrow, with steep drop-offs, it is not a road for the faint-of-heart. It also has some extraordinary scenery and some absolutely fascinating geology. 

The lower canyon exposes 25,000 feet of late Mesozoic and early Paleogene marine sediments deposited in the forearc basin of the Cordilleran subduction zone that stretched the length of California. The middle stretches reveal the oceanic crust on which the sediments were deposited, the Coast Range ophiolite (the second-most complete section found in California). The uppermost canyon is the strangest environment of all, consisting of rocks that were once part of the Earth's mantle. The rocks are interesting, and so are the plants that survive on the ultramafic soils.

One of our students discovered an ammonite fossil on this trip!

The canyon also has a place in the history of California paleontology. The 25,000 feet of oceanic sediments provide an extensive record of fossil species, including the clams, snails, ammonites and shark teeth that are expected in such environments. Mesozoic marine reptiles have also been found in the region, including plesiosaurs, ichthyosaurs, and a new species of mosasaur, Plotosaurus bennisoni. The canyon was also the site of the discovery of California's first dinosaur, a species of duckbilled dinosaur called Saurolophus. It was discovered by 16-year-old Al Bennison of Gustine in 1936. We found a single fossil this trip, an ammonite. Someday, it'll be a dinosaur, right?

To me, the most interesting rocks are found in the upper canyon. The mantle of the Earth is a 1,800-mile-thick layer that starts at a depth of 15 or 20 miles beneath the continental crust. It is generally composed of a rock called peridotite or dunite, made up of the mineral olivine with varying amounts of pyroxene and various ores of chrome, mercury, magnesium, and copper. Peridotite is chemically unstable in surface conditions and alters mostly to serpentine. Many of the rocks we observed showed some degree of alteration. The rock below that looks like alligator skin (below) is composed of fractured chunks of pyroxene (the reddish-brown) and serpentine (the green fracture filling).

In a few spots one can find some relatively unaltered peridotite (below).

We also found some samples of chromite ore. Chrome contributes to the production of stainless steel and has applications in forming armor. During peacetime, there are cheaper sources of chrome overseas, but during wars the supplies may be cut off. During the world wars, chromite was mined in the upper canyon and transported by rail down the canyon to Patterson to be processed.

The little black grains are chromite

The upper canyon was also a source of cinnabar, a mercury sulfide mineral. Despite its toxic nature, mercury was a critical component in the processing of gold ores during the Gold Rush. Miners could often make a good wage mining the ores, but they would do so at great risk to their health. Luckily the temptation to gather mercury ore was not possible as the mine properties are fenced off.

The last part of our journey is the saddest. The lower canyon, the first five-and-a-half miles, is under threat. A local irrigation district is intent on building a reservoir that will flood much of the canyon under hundreds of feet of water. It will serve no purpose other than to store excess water from the California Water Project (in the rare years when such excess is available). It would then be used in subsequent drought years. In other words, it would be an evaporation pond, a waste of water. There would be no recreational facilities. Numerous archaeological sites would be flooded, and precious prairie and riparian ecosystems would be destroyed.

Del Puerto, ("the Gate") Credit: Elias Funez, Save Del Puerto Canyon

There are geological concerns. The dam itself would be constructed a quarter mile from a potentially active fault system (there was an earthquake swarm in the canyon earlier this year). There are seven gigantic earthflows and slumps, one a mile long, that would be partially inundated and potentially reactivated. And it's true I'm not an engineering geologist, but I question the stability of the shale, siltstone and sandstone that the dam abutments would be anchored in. I wasn't reassured by the environmental impact report.

In any case, there is community opposition to this misguided plan. It will do nothing to benefit the local community even while it threatens nearby cities. If you would like to learn more, and support efforts to stop this boondoggle project, please contact the organization Save Del Puerto Canyon. It would be such a shame to destroy yet another beautiful place in service to economic benefits for the very few.

 

Driving to the Center of the Earth in Del Puerto Canyon...Sideways

Okay, not really to the center, but deeper than any drill or mine shaft ever went. Del Puerto Canyon in the California Coast Ranges provides a window into the Earth's mantle, while passing through more than ten miles of crustal material. Allow me to explain! We were there last weekend, a group of our students gathered together after finals to explore the canyon.

The Great Valley began as a forearc basin, a sequence of sedimentary layers lying on oceanic crust between a subducting trench and the edge of the North American continent. The sediments were derived from the erosion of the Ancestral Sierra Nevada, which at the time was a string of volcanoes not unlike the Andes or Cascades of today.

Source: https://commons.wikimedia.org/wiki/File:Franciscan_subduction_model.gif

What a fascinating time this was! Huge volcanoes were erupting every few decades, while large rivers were constantly eating away at the mountains. A diverse group of dinosaurs roamed the forests on the flanks of the volcanoes including hadrosaurs (duckbilled dinosaurs). Raptors and carnivores similar to T-rex and Deinonychus (the American version of the velociraptors) lurked in the trees and along the rivers. Various species of Pterosaurs flew overhead.

The first dinosaur found in California was discovered on this ridge above the landslide.

The very first dinosaur fossil ever discovered in California was found in this very canyon, by a teen named Al Bennison in 1936. The creature was a Saurolophus, a plant-eater that had been caught in a river flood. The carcass floated out to sea, and was eventually entombed in the sediments of the Great Valley forearc basin.

Fall colors in Del Puerto Canyon. Unlike the rest of the country, we believe in enjoying the fall until winter actually begins on the calendar.

Like the adjacent coastline, the sea teemed with life. There were creatures much like those of today, clams and snails (pelecypods and gastropods), fish, and sharks. And then there were the sea-going reptiles: the Plesiosaurs (the suspected "Loch Ness Monster" creature, unfortunately), the Ichthyosaurs (large dolphin-like reptiles), and Mosasaurs (35-foot-long swimming Komodo Dragon relatives). A new species of Mosasaur was discovered by Al Bennison just a few miles away from Del Puerto Canyon.

The sediments accumulated to an incredible thickness, more than 25,000 feet. The sediments were later twisted upwards to form the eastern flank of the Coast Ranges. The road in Del Puerto Canyon winds for more than 10 miles through the sandstone, siltstone, and shale that once formed the ocean floor. We were driving up canyon, but deeper down into the crust. We were looking for the Tesla-Ortigalita fault that marks the boundary between the so-called Great Valley Group, and the underlying Coast Range Ophiolite, a slice of oceanic crust.

We soon reached the rocks near the base of the Great Valley Group, and found the exposure of the fault zone (it cuts diagonally upwards to the left in the picture above). I visited this spot nearly twenty years ago with Al Bennison, who by then was long retired. He described the structural relationships to us, and pointed out that ammonite fossils (cephalopods related to the Pearly Nautilus and the Octopus) could sometimes be found in the shale on the right side of the fault. He walked over, glanced at the cliff for a moment and said "here's one", handing me the delicate fossil. I've been to that darned outcrop another twenty times or so over the years, and I've never found one for myself!

So of course, what happens when I take our Geology Club students to the outcrop last Saturday, students who have never seen this spot, some of whom haven't even learned about ammonites yet? Yes, they found one. Durn it all....

At this point we've driven 10 miles into the canyon, and passed through five miles of seafloor sediments. We've reached the top of the oceanic crust, known in this area as the Coast Range Ophiolite, one of the most complete such sequences in all of California. For the next part of the journey we sought to pierce the crust and travel into the Earth's mantle below. We continued up the canyon...

Hunting for Fossils in the Sierra Nevada...Wait a Minute, What Fossils are there in Granite?

Photo by Mrs. Geotripper

Last night I attended a great presentation on the dinosaurs and other (more interesting) Mesozoic reptiles found in California. It was given by Dick Hilton, a former prof at my school, who is currently teaching at Sierra College in Rocklin. I was invited to head up to the Sierra Nevada to look for fossils with Dick and fellow prof Noah Hughes, and I jumped at the chance.

But...fossils? In the Sierra Nevada? Isn't the Sierra Nevada composed of granitic rock? Granite and other plutonic rocks develop from cooling magma deep in the Earth's crust, an environment that is neither conducive to life, nor to the preservation of fossils. A quick look at a geologic map reveals that the Sierra is only about three-quarters exposed granitic rock. Most of the remainder is composed of metamorphic slate and metavolcanic greenstone, with a fair amount of serpentinite (California's state rock).

Metamorphic rock is the product of taking pre-existing rock and subjecting it to extreme heat and pressure. The resulting rocks, with names like slate, phyllite, schist, marble and quartzite, often bear little resemblance to their previous form, their protoliths. Any fossils that might have been part of the original rock are often destroyed in the process. There is a rich record of tectonic events leading to the formation of the Western Metamorphic Belt, a story too complex to even summarize in a short blog post. Long story short, fossils shouldn't be found in the Sierra Nevada. The rocks have been too distorted and altered. For the most part...

The Mariposa formation is a deposit that formed on the bottom of a deep sea off the coast of California in Jurassic time. The shoreline lay east of where it is today, and the Sierra Nevada was a different place: a series of active volcanoes led to a coastal forest. Dinosaurs, pterosaurs, and primitive mammals roamed the forests and floodplains. In the sea, large swimming reptiles including plesiosaurs and ichthyosaurs were to be found. The Mariposa is somewhat less altered than many other rocks of the metamorphic belt, and because of this, a few fossils have been found, fossils that enabled geologist to figure out the age of the rocks, an important step in unraveling the complex geologic history of the region.

The Mariposa formation is exposed along Highway 49, and we were searching for fossils in the vicinity of Don Pedro Reservoir. To my great delight, after a bit of sweating and slipping down wet grassy slopes we found some interesting specimens.

The most common fossils include the bivalve Buchia, an important diagnostic fossil indicating a Jurassic age for the unit.

I almost missed a small belemnite, which looks a bit like a fossilized cigar. The cylindrical fossil is the internal shell of a squid-like creature.

The prize find for me on this day was a small ammonite. Ammonites are relatives to the pearly nautilus which lives in today's seas. They can be thought of as an octopi with a shell. I have a spotty history with ammonites in the sense that for the last quarter century, I haven't been able to find any, and it hasn't been for lack of trying. I once stood in front of a productive outcrop with a noted paleontologist, and I watched him walk up and pull an ammonite out of the cliff face. I've been back to that spot many times over the last twenty years, and I have yet to find another. But today, I flipped a rock over, and there it was, a little tiny ammonite.

I didn't make that best find of the day, though. That honor belonged to Noah, my fellow prof at Modesto Junior College. The sample below shows a sprig of a species of redwood tree. This is an astounding find to me. Redwoods survive today in just three places in the world, on the northern California coast, in the Sierra Nevada, and in a small grove in China. But the trees once ranged across the northern hemisphere, and as Noah's find shows, they have been around for more than 160 million years. Think of it this way: dinosaurs once roamed through forests of redwood trees. And here in the rock was a distant ancestor to the Sequoia trees that grow just a few miles up the hill from our fossil site.

We were reminded of dinosaurs one more time today as we were driving home. We passed a large herd of modern dinosaurs who were displaying the kind of herd behavior that we think the large plant-eating dinosaurs displayed during the Mesozoic Era. Luckily they didn't attack!

Dick Hilton wrote the guide about the history of Mesozoic reptiles in the California region, and the book remains the best source of info about mosasaurs, ichthyosaurs, plesiosaurs, and the handful of dinosaur species found in the state. You can get the book at http://www.ucpress.edu/book.php?isbn=9780520233157 or any other online seller. I give it my highest recommendation!

Accretionary Wedge #42 "Countertop" Geology and the rock outcrops in downtown Venice

This month's Accretionary Wedge is hosted by Volcanoclast and asks of us the following:

Have you seen a great countertop out there? Sure, everyone says it’s “granite”, but you know better. Take a picture, post it on your own blog or send it to me and I’ll post it for you. Do you think you know what it is or how it was formed? Feel free to include your own interpretation and I’m sure others will enjoy joining in the discussion. Ron Schott suggested that we expand the entries by including any decorative stone material that has been separated by humans from it’s source. This includes buildings, statues, etc. There’s a lot of really unusual stuff out there, so make sure to find a good one.

So...my countertops are made of reprocessed clay (tiles, in other words) and are thus unremarkable, so I will take advantage of Ron's suggestion. I am offering up the picture above. Here's the story of where I found it...

Several years ago we took a class of geologists and archaeologists to Italy and Switzerland for a taste of overseas geology and culture. I didn't have the connections to do our own chosen itinerary, so we were on an arranged tour with a few geological diversions. We made sure we got to Pompei, and climbed to the top of Vesuvius (above), and in the best moment of the trip we managed to find the site of the original Alvarez K/T boundary near Gubbio (the first place where evidence was found for the asteroid that ended the existence of the dinosaurs). That is one happy (and VERY relieved) professor in the picture below. I more or less knew where the outcrop was located, but we missed it on the first pass with our large bus on the very narrow road, and we had to go several miles before the bus could make an Austin-Powers-style back-and-forth U-turn.

Still, we were on a tour, and in the middle stretches of the trip we explored the very beautiful towns of Florence, Verona and Venice. Spectacular, but not a lot of actual geology (although we had a lot to say about the mountains in the distance). I tried to convince our crew to go see the marble quarries at Carrara, but they insisted on seeing some badly engineered tower in some coastal town called Pizza or something like that instead (below).

It was in Venice that I found my contribution to the Accretionary Wedge. Geology-wise, the city is interesting because it is sinking, in part for being built on mud, and partly because of the tectonic environment (it is near a convergent boundary, and is being compressed downwards between two mountain systems). But by its very nature it has no natural outcrops of rock at all.

No natural outcrops, but plenty of rocks! The sidewalks and squares are covered with marble tiles from quarries in the Alps and Apennines. And as I was walking in the shade at the edge of Piazza San Marco (St. Mark's Square) I noticed unique swirls in some of the tiles: ammonites!

Ammonites were cephalopods related to octopi and the chambered nautilus that populated the seas in huge numbers during the age of the dinosaurs. It was nice to see these fossils in the middle of the urban environment of Venice, in a place so removed from their place of origin!