Damning Del Puerto Canyon, a Geological and Natural Treasure in our County

I awoke this morning mildly astonished to see my own words making up the headline of a Modesto Bee article about a proposed dam in Del Puerto Canyon, a deep gorge cutting into the heart of the Diablo Range in the western part of Stanislaus County. Del Puerto is one of the most unique landscapes of California's Coast Ranges, and as I noted, a geological and natural treasure. I was hugely dismayed to find that a proposal exists to build a large reservoir in the lower canyon, and my email to a colleague ended up being quoted in the Modesto Bee article linked above (hence my surprise at being quoted; I wasn't directly interviewed). The article accurately describes my concerns about the project. There are large landslides in the lower canyon that canyon that would almost surely be reactivated (or accelerated; they show evidence of recent motion) if the base is inundated by lake water. There are definite seismic concerns, as a probable active fault lies just east of the dam site. But my biggest concern is the effect the dam will have on the natural environment of the canyon.

The environmental impact report was published recently (read it here.). Comments on the Environmental Impact Report can be made at a public meeting from 4 p.m. to 6 p.m. Jan. 15 at the Hammon Senior Center, 1033 W. Las Palmas Ave., in Patterson. Written comments will be accepted until Jan. 27 at Del Puerto Water District, 17840 Ward Ave., Patterson 95363. If you appreciate the intrinsic value of our precious local canyon, I hope you will comment and make your voice heard.

I've written often about Del Puerto Canyon over the years (see many of the articles here), and to give you a feel for the unique nature of the canyon, I'm adapting an article from last May.

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world.

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon. This will be the site of the proposed dam. It will be more than 200 feet high.

I guess I should be a bit more specific. We journey to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We can't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we are exploring have once been close to the Earth's core.

This is an active landslide that will be partially inundated by the proposed reservoir. I am concerned about the effect of adding water to the slip plane. California's first discovery of dinosaur bones was at the top of this slope.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationships, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

There is a spot in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon we penetrate the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we are there. In the uppermost part of the canyon, we reach the netherworld of mantle rock that was far less altered, so it retained some of its original appearance. In places we can see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They try to invade the surface realm, but they are defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the Great Valley. It can also be reached by way of Mines Road out of Livermore, a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

I know that there is a need for water in the Central Valley. But no matter how many dams get built, there will never be enough to meet the expressed needs and desire of agribusiness. But I feel that we need to keep some of the wild places, and Del Puerto is one of those especially unique places to learn about our planet. I hope you will make your voice heard about this project.

What to do on a Saturday? Let's Go to the Middle of the Earth (via Del Puerto Canyon)!

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world. Our Geology Club made the trip a few weeks ago as an ending of the semester celebration. Odd way to celebrate? Did you simply party? We traveled half-way to the center of the planet!

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon.

I guess I should be a bit more specific. We journeyed to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We couldn't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we explored had once been close to the Earth's core.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationship, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

We stopped in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon and we penetrated the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we were there. In the uppermost part of the canyon, we reached the netherworld of mantle rock that was far less altered, so that it retained some of its original appearance. In places we could see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They tried to invade the surface realm, but they were defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

We turned around and headed back to more familiar habitats.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the
Great Valley. It can also be reached by way of Mines Road out of Livermore, or  a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

A Look Back at Ten Years of Geotripping: Driving Through the Most Dangerous Plate Boundary in the World

Driving through the most dangerous (kind of) plate boundary in the world is actually not very easy to do. Subduction zones, with the exception of the volcanoes, are mostly deep under the sea. But Central California is a unique case, being an ancient subduction zone that has been uplifted and exposed by erosion, so that interested parties can literally drive through what once was miles underground or at the bottom of the deepest oceans. I got the idea for this blog series when I spent an afternoon driving the winding road that travels over the Coast Ranges at Lick Observatory, and down through Del Puerto Canyon into the Great Valley. It's the equivalent of driving twenty or thirty miles into the Earth's crust. Looking back over the titles, I'm worried that I am using up my lifetime supply of bad jokes...

I've been reviewing the archives this week to find some of my favorite posts from ten years of geoblogging. This compilation appeared on July 4, 2015.

Without a doubt, subduction zones are the most dangerous plate boundaries on the planet. Divergent plate boundaries produce earthquakes and occasional volcanoes, but nothing on the fearsome scale of the calderas and stratovolcanoes and magnitude 9 earthquakes experienced at convergent boundaries. Transform boundaries produce earthquakes, but they are magnitudes smaller than those produced at convergent boundaries (despite what certain Hollywood movies have asserted recently). Hot spots, while not a plate boundary, can produce huge caldera complexes like Yellowstone, but such monsters have not had much of an effect on human history of the last few thousand years. It is the subduction zones of our planet that have caused the most human misery, in the form of massive earthquakes, tsunamis, and violent volcanic eruptions.

We have been driving through an example of one of the most dangerous plate boundaries in the world, but our particular example has been inactive for a very long time. Central California was a subduction zone complex for more than 150 million years, primarily during the Mesozoic era, but it changed into a transform boundary only a few tens of millions of years ago. The San Andreas fault is the resulting feature, and it is capable producing damaging earthquakes, but even the most destructive quakes, like 1906 in San Francisco (death toll 3,000), is but 1/30 of the energy of a magnitude 9 quake like that of Indonesia in 2004 (where the resulting tsunami killed 200,000 people).

It's taken a couple of months to work through our journey, so I've compiled all of the posts here so one can catch the continuity of the story. Here goes...

A New Blog Series

The introduction to the new series, a geological transect from the California Coast to Yosemite Valley, crossing an ancestral subduction zone that once caused geological havoc in a zone from Mexico to Canada (and still is in a few places).

Reconnaissance

An overview (in the most literal sense) of the lands we will traverse on our journey. We have a look at central California from above.

These Rocks are All Wrong!

Granite is exposed in the rocks of the Point Reyes Peninsula. But the arrangement of rock and sediment in subduction zones suggests that granite shouldn't be anywhere near here. It's the San Andreas fault. In California, it's always the San Andreas' fault.

Looking for the Big One

The peace and serenity of Tomales Bay belies a violent past. The San Andreas fault slices right through the bay, and produces large earthquakes with disturbing irregularity. The epicenter of the San Francisco quake in 1906 was not far from here.

Welcome to Geology's Junk Drawer

The Marin Headlands began as a giant collector of geological flotsam and jetsam from the crust of the Pacific Ocean. The jumble of rocks accumulated in an accretionary wedge, and were later lifted up into the mountains of the Marin Peninsula.

Geology's Junk Drawer on the Marin Headlands

Exploring the hidden corners of the Marin Headlands, we find Redwood forests, beautiful views of San Francisco, and disturbing reminders of World War II.

Terra Fatale on the Marin Headlands

Although the subduction zone that formed the rocks of the Marin has been extinct for a long time, there are still plenty of hazards remaining in the region, both geological and nautical. The Point Bonita Lighthouse has been present in one form or another for 160 years. It hasn't always worked, as there are upwards of 300 shipwrecks in the area. There are other hazards too.

The Alien Bursts Forth in the Diablo Range!

We head across San Francisco Bay and start an arduous journey through the Diablo Range. The range formed as an exotic (read "alien") terrane pushed upwards through the sediments of the Great Valley Group, piercing the surface and rising into the sky. John Hurt would no doubt approve...

Exploring the Belly of the Beast in the Diablo Range

We make the first part of a long drive through a rugged portion of the Diablo Range, one of the largest sub-ranges in the Coast Ranges of California. Along the way we cross through two exotic terranes of rocks that had been carried miles deep into the crust in the accretionary wedge of the subduction zone.

At the Portal of Hell in Diablo Range

Making our way down Del Puerto Canyon in the Diablo Range, we do the equivalent of traveling through the crust of the Earth all the way into the mantle, finding outcrops of peridotite and dunite, rocks containing the mineral olivine among others. For the mercury miners, this truly was a portal to Hell.

Exploring the Ocean Crust without Unobtanium

Del Puerto Canyon cuts a swath through the Coast Range Ophiolite, the remnants of the ocean crust that once lay at the bottom of the Pacific Ocean. Though mostly in private ownership, the canyon is one of the most scenic in the Coast Ranges, and a pair of county parks in the upper reaches invite exploration.

Into the Realm of the Drowning Dinosaurs

Sediments accumulated in a deep trough along the west coast of North America called a forearc basin for upwards of 100 million years. The layers reached a depth of 5 miles! In those waters swam mosasaurs (yes, like in the recent Jurassic World movie, but they forty feet long, not a hundred), plesiosaurs, ammonites, and occasionally a drowning dinosaur. The first discovery of a dinosaur in California happened here in 1936.

The Sea Floor that became the Greatest Agricultural Region on Earth

The American Serengeti is a vast plain 400 miles long and 50 miles or so wide that once was the sea floor. The grasslands of the Great Valley once supported millions of migratory birds and grazing animals. It still supports millions of organisms, but these days, those organisms are humans. 95% of the original prairie has been developed for agriculture and the region produces a quarter of the nation's produce. And all of the almonds and walnuts.

In the Pleistocene, a Different Kind of Danger

The Great Valley would have been a most dangerous region for a different reason in the Pleistocene. Among the great herds of grazing animals there were predators, and they were adapted to bringing down giant prey, not the small game we find today. Gigantic Short-faced Bears, Saber-tooth cats, American Lions, Jaguars, and Dire Wolves.

The Dr. Who of Mountain Ranges

At least three Sierra Nevada ranges have existed throughout time. They might have even once been higher than today. The distorted deformed metamorphic rocks tell the story of the earlier ranges.

A Gentle Landscape Belies a Fiery Past

The Valley Springs formation, exposed throughout much of the Sierra Nevada foothills region, forms gentle grass and oak covered slopes, but the rock is made of volcanic ash that originated in monumental explosions millions of years ago. The ash came from gigantic calderas, some of which were hundreds of miles away in central Nevada.

A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

A lot of volcanoes in the world are made of mud. Lots and lots of mud. But this mud formed in violence. The Mehrten formation of the Sierra Nevada has other surprises too: gigantic tortoises, and six-foot long salmon...with fangs...

A Tale of Two Subduction Zones

There have actually been at least two subduction zones in California. Remains of the older one still make up the rocks of the Sierra Nevada Mother Lode, and those rocks were the source of gold in the Gold Rush.

Exploring the Underside of the Volcano

We wrap up the series in the heart of the ancient magmatic arc: Yosemite Valley. Walking among the towering cliffs, we are reminded that the rocks were actually formed within the magma chambers of volcanic systems, perhaps similar to Lassen Peak, Mt. Shasta, or even at times, Yellowstone.

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...