Well. I certainly didn’t intend for that to be a one year coffee break. The last year has been very interesting, for many different reasons, but there haven’t been many road trips. However, I am now officially a Geology Student™ which means…field trips! I’ve been on a couple already, but I’m going to start blogging here about the second one. (And yes, I know, I still have a bunch of posts about my previous travels to write up. I will get to them eventually, I promise.)

This weekend field study trip was to Anza-Borrego Desert State Park. Here is an overview map of our stops for orientation. As usual, you can click to embiggen.

Orientation Map

The geology of this part of California is fascinating. And fascinating means complicated. But that’s true of pretty much all of California.

California hasn’t just always been here like some of the more boring parts of the country. Like, say, Kansas. It’s been assembled over millions of years by sticking all kinds of parts onto other parts and then shifting them around and ripping them apart. For an excellent read about this, I recommend Assembling California by John McPhee.

I’m going to go backwards through time, because that corresponds (roughly) with the order of our stops. So, starting in the present day:

Anza-Borrego Desert State Park is located in the Colorado Desert, which is part of the Sonoran Desert. It lies along the western edge of the Salton Trough, a pull-apart basin. This means it’s an area of crustal extension, where tectonic forces are pulling the crust apart, making it thinner, and causing subsidence. The surface of the Salton Sea, which was created (accidently!) in 1905, is 226 feet below sea level.

The Salton Trough is considered to be part of the Basin and Range Province, which I’ve written about before. And just like in the Basin and Range Province, the Salton Trough is laced with normal faults and detachment faults. But it’s also home to the San Andreas Fault (on its east side) and the associated Elsinore and San Jacinto Faults (on its west side). Elsinore and San Jacinto are part of the San Andreas Fault system. I’ve also written about the San Andreas before, but it’s a lot more complicated than I’d realized at the time. (There’s that word again.)

The boundary between the North American Plate and the Pacific Plate isn’t a simple line labeled “San Andreas Fault”. There are a whole series of faults that are roughly parallel to one another that allow the lateral motion that will one day make Los Angeles and San Francisco next door neighbors. And the southern end of the San Andreas Fault is right at the Salton Sea. Here’s a more zoomed in map with those faults labeled:


As I’ve mentioned before, the San Andreas is a right-lateral strike-slip fault; more specifically, it’s a transform fault. Transform faults connect two divergent (spreading) plate boundaries, or two convergent (colliding) plate boundaries, or a divergent plate boundary to a convergent plate boundary, and “transform” that pushing-together or pulling-apart motion to sliding-past motion.

So what are the two plate boundaries being linked by the San Andreas? At the southern end, it’s the Gulf of California Rift Zone, an extension of the East Pacific Rise (a divergent boundary). At the northern end, it’s the Mendocino Triple Junction, a place where three plates meet: the North American, Pacific, and Gorda plates. Triple Junctions are a bit more complicated, but this one basically consists of two transform boundaries and a divergent boundary. You can see the whole thing on this map from the USGS:

For our purposes, the fact that the southern end of the San Andreas intersects with a divergent boundary, or spreading center, is what’s important. In a nutshell, the Gulf of California Rift Zone is ripping Baja California away from mainland Mexico, and southwestern California away from the rest of the continent, like a zipper. Spreading centers make new seafloor, which moves away from the spreading center’s ridge, to be followed by even newer seafloor. It’s what’s pushing the east coast of North America away from Europe and widening the Atlantic Ocean, and it’s what’s pushing Baja California away from Mexico and widening the Gulf of California.

In fact, this spreading center extends northward past the shoreline of the Gulf of California, and lies underneath the Salton Sea. Spreading centers are associated with frequent, low-magnitude earthquakes. The southern end of the Salton Sea and its shoreline are called the Brawley Seismic Zone (labeled as such in my second map above) because of these frequent earthquake swarms. In fact, just such a swarm occurred last summer. So if this spreading center is connected to the one in the gulf, and the Salton Sea is below sea level, why doesn’t the Gulf of California extend all the way to Indio, at the northern end of the Salton Sea?

Because of this:

The Colorado River drains into the Gulf of California right in the area we’re talking about. And rivers carry sediment. (If you have any doubt about that, see my post about a thunderstorm in Zion. And that’s just one small tributary of the Colorado.) And where rivers meet oceans (or lakes, or reservoirs), they dump their sediment and form deltas. So the only thing keeping Indio from being oceanfront property is a very long history of sediment from the Grand Canyon (and the rest of the Colorado’s watershed) being dumped into the top of the gulf. I’ve labeled both the river and the delta in this map:


Incidentally, you can also see in this map the source of a huge problem: all that green between the Salton Sea and the delta. That’s agriculture. In a desert. That can only exist when you divert huge quantities of water from the river. Add to that the disruption in flow from dams (Hoover Dam, Glen Canyon, etc) which also make very effective sediment traps, and what you get is big problem for the ecosystems of that delta. But that’s a topic for another time.

To summarize the current geology of the park region, we have a junction between a spreading center and a transform fault zone, resulting in both extensional and shearing stresses, forming basins surrounded by mountains, all adjacent to a river delta that separates it from an ocean.

So our tour through Anza-Borrego begins with some exposures dating to this most recent period, that is the formation of the delta. Here’s a zoomed in map of our day one stops:

Day One

Here you see some subtly layered deposits along Arroyo Tapiado near Canyon Sin Nombre. You can see that most of the deposit consists of fine-grained sediment, but there are a few layers with noticeably larger pieces, as you can see more easily in this close up.

Since a stream’s competence, or the largest size particles it can carry, depends on its velocity, examining the size of the particles in the sediment gives you an idea of the stream’s history. So the flow that deposited these sediments was fairly consistently slow, with occasional periods of increased velocity. This is not at all surprising for a deltaic environment. When a stream enters a larger body of water, its velocity, and therefore its competence, drops abruptly. This, in fact, is why deltas form in the first place. All that sediment gets dumped right at the spot where the velocity drops.

As you might expect, all this sediment build up presents a problem to the stream: a stream depends on the gradient of its channel to move it along, and when the channel gets choked up with sediment, the gradient disappears. The result: the stream seeks another route to its destination, forming distributaries.

Distributaries have the effect of shifting the sediment deposition around to new locations as the streams course constantly changes. As long as the influx of sediment continues, the delta will continue to grow larger and larger.

Another feature to notice in this picture is that the layers are tilted. You might have noticed on the map that we are very close to the Elsinore Fault at this location. Although I said these faults were strike-slip faults, there are also compressional stresses in these faults. I’m looking toward the west in my photo, so these beds are dipping to the north. There is a quite a lot of north-south compression in the region, as we’ll see a bit later, but it’s not well understood why.

Canyon Sin Nombre is a slot canyon, and it was pretty fun to hike to the top. The view from here is called a “Walk Through Time” because you’re standing on top of very recently deposited sediment, looking over an area covered with sediment deposited 5-10 million years ago, to mountains covered with sediment deposited 15-20 million years ago. We’ll get to these spots soon.

Amongst all this nondescript sediment were some real beauties, including this piece of granite with some lovely biotite crystals.

Now let’s go back about 5 million years. If you remove 5 million years worth of Colorado River sediment, you no longer have that huge delta forming a barrier separating the Salton Trough from the Gulf of California. Instead you have a very long sea called the Imperial Sea.

Imperial Sea

But the river is still there, dumping its sediment load into the water. Without the enormous delta to so abruptly stop the river’s flow, very fine sediment is able to form thick, widespread deposits that eventually forms mudstone. Much, much later, after the sea has been cut off and the mudstone is high and dry, heavy rains cut channels through the rock, forming caves: Mud Caves.

The openings for some of these caves are pretty inconspicuous. You really need to know they’re there or you’ll just walk past them.

Once inside, you’ll find some pretty tight passages.

Most of the caves we explored are completely dark, but some have open areas such as the one above. Some even have skylights.

In some areas, the sediments that formed during this marine incursion contain fossils. We visited such an area on day two:

Day Two

I labeled the location “Oyster Beds” because, well, there are a lot of oyster fossils. It has been suggested that the oyster fossils accumulated in shallow marine distributaries of the nascent Colorado River delta where the stream flow was a little faster, while the smaller particles traveled further offshore to deposit as non-fossiliferous mudstones. So by comparing the location of the Oyster Beds to that of the Mud Hills/Mud Caves, you can get some idea of where the river delta was forming.

Unfortunately, I was so busy looking for fossils that I didn’t take any pictures of the ones I found. Suffice to say, finding them wasn’t difficult. They’re everywhere. I did, however, get this shot of the tilted strata at the oyster beds.

Again, I’m looking toward the west in this shot, and these layers are dipping to the south, thanks to the ever present faults.

Later during this transitional period, as the delta started to grow, sand was deposited. Sand, being larger particles than silt, needs a higher velocity stream to transport it. As the delta grew and exerted more of an effect on the river’s velocity, we see deposits of larger and larger particles. This “coarsening-up” trend (in other words, the particles get coarser as one moves up the stratigraphic column, where younger layers are on top of older layers) indicates that the basin was getting shallower as it filled with sediment.

Very near the Oyster Beds are the Wind Caves. These sandstone outcrops were eroded into some cool little “caves”, and I use that word generously, by the action of water, despite their name.

This is about as cave-y as the Wind Caves get. Unfortunately, there’s quite a bit of graffiti in the caves, but they’re kind of fun to climb around, if you’re into that kind of thing. I think it’s more fun to think about the fact that these caves might have once been part of the canyon walls of Zion, where they got eroded by a flash flood and washed down the Virgin River, and then the Colorado River, and through the Grand Canyon, and then ended up in California to create an adult sized playground.

In order to get to the Wind Caves and Oyster Beds, one must drive through Split Mountain. Split Mountain is split because it is a water gap. Water gaps form when a stream exists before a mountain. As the mountain is uplifted, the stream continues to cut down through its channel, resulting in a mountain with a gap.

While the split part of Split Mountain isn’t all that impressive, what you see along the walls of the wash/road is. Unfortunately, the white knuckle ride in a van that is decidedly not four wheel drive precluded me from taking pictures, except of this feature:

This is the Split Mountain/Fish Creek Anticline. I talked about synclines in a previous post. An anticline is, essentially, the opposite of a syncline. Both are folds due to compressional forces. A syncline is simply a downfold, and an anticline is an upfold. Sometimes you have an anticline right next to a syncline, as you’ll see in a future post.

Usually anticlines and synclines are formed by compression due to fault movements. And as I think you’re aware by this point, there is no shortage of faults in this area. However, I found quite a different explanation for the formation of this particular anticline. It has been suggested that rather than being the direct result of tectonic forces, this is the result of a landslide. An enormous landslide. A sturzstrom. Which is German for enormous landslide. Because why use two perfectly understandable English words when you can use one terrifying German one?

Actually, a sturzstrom is more than just an enormous landslide, it’s one that travels a much greater horizontal distance than would be expected for its vertical distance. And these would be quite terrifying. There is some debate about what causes this exceptional movement, but the story at Split Mountain goes something like this: about 5 million years ago, an earthquake dislodged 300,000,000 cubic yards of rock which flowed for more than seven miles and dug itself into the marine sediments that eventually formed these layers, buckling them.

There’s no question that there has been a lot of mass movement in this area. All you have to do look at the walls of this canyon to see that. As I said, I didn’t take any pictures, but this guy did. In his second photo, you can see what some people call fanglomerate: conglomerates that coalesce into an alluvial fan. Conglomerates are sedimentary rocks composed of large, rounded fragments. Alluvial fans are fan-shaped deposits of sediment that form at the bases of mountains.

Alluvial fans are very common in arid regions, where it doesn’t rain often, but when it does, it washes a lot of sediment down the mountains because there’s so little vegetation to stabilize the slopes. Conglomerates are commonly found where running water has been flowing. So it makes sense that the two would go together. What’s impressive about Split Mountain, though, is the size of the boulders you see in those layers. You can get an inkling of the size in his pictures, but there are some truly astounding ones in there.

Now we’re going to go back a bit further in time, to about 20 million years ago. Things look quite different now. The Colorado River is no longer a factor in the area. In fact, the Colorado River at this time was draining into the Pacific in Monterey. At this time, the separation of Baja California from Mexico and the opening of the Gulf of California is just beginning. The area is flooded with salt water, depositing shallow marine sediments.

But this area is not tectonically quiet. There is active volcanism and hydrothermal activity. Some of this hydrothermal activity creates calcite veins.

This calcite was mined by the Polaroid company during World War II for making bomb sights. Today the mine is at the end of a rather precarious jeep trail. On the hike back down from the mine, we took a slot canyon trail. Scattered among the sandstone were several of these:

They appear to me to be basaltic xenoliths in granodiorite. In other words, first there were basaltic rocks, formed from fluid lava of the kind associated with seafloor spreading centers. Chunks of this rock became incorporated into younger igneous rocks of higher silica content which formed deep beneath the crust.

This makes some sense given the tectonic history of this part of California. Going back more than 90 million years, the North American Plate was not in contact with the Pacific Plate. Its neighbor was the Farallon Plate. And it was eating its neighbor. Well, the Farallon Plate was subducting under the North American Plate. That’s what happens when oceanic plates (like Farallon) collide with continental plates (like North America). Because oceanic plates are made of dense basalt, they are less buoyant than continental plates, made of less dense granodiorite.

And the reason these plates were colliding was that the supercontinent of Pangaea was breaking up. Pangaea was assembled from all of Earth’s landmasses, and it began breaking up around 250 million years ago. At the time, the east coast of North America was stuck to the west coast of Europe, and the east coast of South America was stuck to the west coast of Africa. The Atlantic Ocean didn’t exist. In fact, you could say that Pangaea is still breaking up, because the Atlantic Ocean continues to widen to this day.You can check out an interactive globe of the world as it looked then here.

Once the Farallon Plate was consumed, its spreading center was also subducted. While seafloor spreading stopped, and North America got a new neighbor in the Pacific Plate, the Farallon Plate didn’t simply disappear. Some of it melted, rose up through the plate above and made volcanoes. That’s what happens at subduction zones. If one oceanic plate subducts under another, the result is a volcanic island arc, like Japan. If it’s an oceanic plate subducting under a continental plate, you get a continental volcanic arc, like the Cascade Ranges of the Pacific Northwest.

Volcanic island arcs don’t last forever. The slow march of time and tectonics carries them toward the continent and plasters them on. These are called terranes: chunks of crust that were created somewhere else and stuck onto a new home. California is full of them. The entire west coast of North America is, for that matter. Take a look:

Not all of the magma produced from the melting of a subducting plate comes out at volcanoes. Much of it cools and crystallizes at depth, forming huge masses of igneous rocks called batholiths. In the area of Anza-Borrego, this is called the Peninsular Range Batholith. While these rocks form at depth, subsequent uplift and erosion can expose them at the surface, and that’s what we have in my photo above.

As an aside, not all of the subducted plate is melted. Incredibly, even more than 50 million years after it disappeared from Earth’s surface, Farallon lives on!

Fragments of the Farallon plate are still hanging out way down at the core-mantle boundary. And it’s not just sitting there quietly anticipating death. It’s thought that plate fragments at the core-mantle boundary are responsible for the formation of mantle superplumes. which are responsible for volcanic activity at locations distant from plate boundaries (like at Yellowstone) and for initiating continental rifting (like at the East African Rift Valley).

There are still some remnants of the Farallon plate at the surface. The Juan de Fuca plate off the coast of Northern California, Oregon and Washington is one, as is the Cocos plate off the coast of Central America. As the Farallon plate disappeared, the spreading centers at its edges moved closer and closer to North America. They now live on as the Gulf of California Rift Zone and the Juan de Fuca Ridge.

With that in mind, let’s visit our last site, Rainbow Canyon.

Day Three

Before I describe what we saw there, I have to mention a sad fact about the geological record: it’s not complete. Sometimes when looking at all the rock layers present in a locale, there are huge periods of time missing. It might be that no rocks formed during that time, or it might be that rocks had formed and then eroded completely. Either way, these gaps in time are called unconformities. In Anza-Borrego, there is a huge gap between about 25 million years ago and about 500 million years ago. Because the 500 million year old rocks that we see are metamorphic, this type of gap is a nonconformity.

The place to see these 500 million year old metamorphic rocks is Rainbow Canyon:

The 500 million year old rocks are metamorphosed sandstone. About 80-90 million years ago, they were intruded by granodiorite as part of the emplacement of the Peninsular Range Batholith. I believe these rocks would qualify as migmatites, a kind of hydrid rock that’s part metamorphic and part igneous. For a good explanation of migmatites, to which I contributed some photos, see this blog.

In short, metamorphism occurs in the solid state, if a rock melts, it’s igneous. Migmatites have a dark metamorphic component which didn’t melt, and a light igneous component which did. Why does the color make a difference? Because the color is indicative of the silica content, which determines the melting temperature. Bowen’s Reaction Series is helpful for understanding this:

The felsic minerals are lighter in color and melt at lower temperatures. The mafic minerals are darker in color and melt at higher temperatures. So if the temperature gets to some intermediate point, the felsic minerals will melt but the mafic minerals won’t.

One interesting thing about the igneous intrusions here was that many of them had pegmatitic textures. These means they had exceptionally large crystals, indicating that they crystallized in the presence of large quantities of water:

Selected References: