Mafic enclaves in the Bernasconi Hills Pluton in southern California. Emily Paine for scale.
Students in 322 go on a geology trip across southern California led by their professor Cin-Ty Lee and graduate student Hehe Jiang. This is a mid-term field trip that the department has used to give a students a chance to apply some of their classroom knowledge to the field. When it comes to looking at rocks and minerals, there is nothing better than going to the field where one has context. It makes learning about rocks much more meaningful and more memorable.
So this year, the class did a transect across southern California. We arrived Friday afternoon in Ontario, California. Our first stop was in the northern Peninsular Ranges Batholith, part of a continental volcanic arc that extended from Mexico all the way up through Canada during the Cretaceous. This volcanic arc would form the backbone for much of our trip, so-to-speak. We visited the Bernasconi Hills pluton, where we examined outcrops showing extensive mafic-felsic mingling in the form of mafic xenolith swarms and highly attenuated schlieren. We then made our way across the San Jacinto Valley, a late Miocene-Pliocene graben formed by extension in the vicinity of the San Jacinto Fault, a splay of the San Andreas Fault. Around us, rising above the valley floor were large knobs of Cretaceous granitoids. We made our way down to Green Acres, where we had a chance to examine olivine-gabbros, examples of cumulates in a shallow mafic magma chamber.
We found ourselves the next morning in San Diego. After a quick breakfast at McDonald’s, we headed out to Point Loma to examine the Point Loma and Cabrillo formations. We picked the perfect place to look at these outcrops because we were right along the ocean, with waves crashing, sea gulls squawking, pelicans diving and the cool breeze blowing against us. These are late Cretaceous sediments. We examined them under our hand lens and discovered that they consisted of quartz, feldspars, and lots of fresh biotite and hornblende. Normally, biotite and hornblende don’t last long in the weathering regime, so their presence suggests a very juvenile sediment. These sediments, it turns out, were being shed off the Cretaceous volcanic arc, most likely while the arc was still active, given its age. Yesterday, we were looking at the eroded plutons and today, we are seeing their eroded tops in an ancient basin!
Cretaceous fore-arc sediments at Point Loma, California.
It was hard to pry ourselves from the ocean, but we had to because we a schedule to stay on. Our goal was to head east. On the way, we stopped by some Eocene forearc sediments exposed in a roadcut. These sediments were completely devoid of biotite and hornblende and were formed by the erosion of deeply weathered surfaces of the batholith, well after magmatism and mountain building had ended. A few of the students found some shell fossils in the sediments, indicating a shallow marine origin. After our brief foray into the Eocene, our drive along I-8 took us back into the Cretaceous plutons. We turned north to Cuyama Valley, where we looked at more olivine-gabbro cumulates along the shore of Lake Cuyama. But our excitement quickly turned to a large area of exposed migmatites. Here, the metasediments had been metamorphosed, deformed, and recrystallized to such a degree that they looked like a gneiss and in some cases looked like a highly foliated granite, but the tell-tale signs of migmatization were the abundant quartz- and feldspar-rich veins and dikes that appear to have formed during ductile deformation. This was the birth place of some of the granites that contributed to the Cretaceous batholith. There were a lot of mortar holes on the outcrop, left behind by Native Americans. How nice it must have been to grind food on migmatites!
Our next stop on Saturday was high up in the San Jacinto Mountains, just above Palm Springs. We stopped specifically at the top of Deep Canyon, where if one looked south, we could see a large dip slope composed of late Cretaceous mylontinized granites and tonalites. We crawled all over the mylonites in search of two things: pseudotachylites and titanites. Pseudotachylites are melts of the rock generated by intense frictional heating during an earthquake. These melts were everywhere, some in foliation and many crosscutting. The titanites, aka sphene, were large pistachio-green porphyroblasts formed during ductile deformation. All of these deformational features are associated in intra-arc thrusting and exhumation during the late Cretaceous and early Paleogene. Our day was soon coming to an end. We descended into the Coachella Valley, but not before stopping at a vista point to look at the San Andreas Fault and the Salton Sea. We ended our day in Palm Desert with a big Italian dinner!
We started Sunday morning with some unfortunate news. One of our students had his camera bag, tootbrush, and some clothes stolen out of his room, which had been inadvertently left slightly ajar in the chaos of organizing 18 students. We spent several hours searching for the bag, looking at security videos, and calling the police, but deep down, we knew that we weren’t going to see his bag again. The good thing, if there was a good thing, was that he still had his wallet and identification. So we moved on, but being a few hours behind, we decided to skip some of the stops. We ended up driving down the west side of the Salton Sea, stopping briefly at North Shores to touch the sea and to talk about the opening of the Gulf of California. The sea keeps receding each year as water supplies to agriculture have been steadily declining, so each year, we seem to have to walk further out across the barnacle beach to reach the water.
After we got our fix of dead fish, we drove another hour south and found ourselves at the mud volcanoes. The high water table and high heat flow here is what causes these mud volcanoes to form. Geothermal power plants have taken advantage of this combination. The mud volcanoes gave us an analog for many of the different volcanoes and lava flows we talked about in class: spatter cones, pahoehoe, aa, etc. It was great to see some of these mud volcano eruptions in action! Our next stop was Obsidian Butte, a Pleistocene rhyolitic lava flowwhich generated a large obsidian dome. We marveled at all the beautiful flow banding and we discussed why obsidian flows tend to be big blobs rather than long, thin lava flows. Viscosity!
Talking about mud volcanoes in the Salton Sea. Emily Paine, Detao He, Cin-Ty Lee, Josh Crozier.
The rest of the day was spent getting from the Salton Sea into the Mojave Desert. We stopped briefly at Salt Creek, where we crossed over the railroad tracks to look at the San Andreas Fault up close and personal. Unfortunately, while we were on the other side, the longest train in the world came up and decided to park itself there. We decided it was too dangerous for us to climb through the train back to our cars as there was no telling when the train might start up again. The only way was to go around or, in this case, we knew there was a bridge we could go under, but the bridge was quite a ways to the north. Should we wait or walk? We had no idea how long the train was going to stay, so we decided to do the long hike. By the time we got back to our cars, delayed by an additional hour, the train still had not moved, so it was a good decision.
Re-energizing with a good dose of fluids, we drove up out of the Salton trough, crossing the San Andreas Fault to the north and passing through Pliocene deltaic sediments, variably deformed and tilted. We stopped here to look at a wonderful assortment of textbook sedimentary structures (sorting, cross-bedding, soles, soft-sediment deformation), and then further up the canyon, we found the contact between these sediments and the basement, only here the basement was a new rock for the trip – Pelona greenschists associated with subducted sediments or basaltic crust during the Cretaceous. We ended our long Sunday trip in Joshua Tree National Park, with the students climbing up and down the giant quartz monzonite boulders.
On top of a boulder in Joshua Tree National Park. L to R: Detaho He, Michale Farner, Evan Neustater, Kate Nicholson, Elizabeth Finlay.
students examining pebble lag deposits in the Mecca Hills. Yunong Xu, Beineng Zhang, Stephanie Zou, Farah Ashraf.
Monday would be our last day out in the field. Our first stop would be Amboy Crater in the Mojave Desert on the North American plate. This was our opportunity to discuss why basaltic lava flows can flow much further than rhyolitic flows. Close inspection of the lava flows revealed tiny euhedral crystals of olivine and lots of vesicles! Off in the distance, we could see a pristine cinder cone. This must have been quite the sight when it was erupting. After our Amboy Crater stop, our goals were to look for xenoliths at Dish Hill and then trilobites in the Marble Mountains, but due to all the rains and flash floods that had happened earlier in the season, the highways were all closed. We had no choice but to change our plans on the fly. Our professor, Cin-Ty, decided then to try a place he had never been to before. Off in the distance, one could see limestones jutting up against some granitic basement, so we parked our cars and walked across the alluvial plain. We had no idea what to expect, so we were told to fan out and just explore. With 18 pairs of eyes, we were bound to see something, and soon, students were coming up with little fragments of Vesuvianite, a clear indicator mineral for skarns! But where was the actual skarn? So we fanned out again, falling the trail of bread crumbs and looking around for the contact. And there it was, all the vesuvianite was forming right up against the contact!
Vesuvianite with Elizabeth Finlay and Sarah Gerenday
With our pockets filled with new minerals, we continued north, deep into the Mojave. We stopped briefly at the Kelso Sand Dunes and then had lunch at the Kelso train depot, a nice green spot in the middle of the desert. We then headed to the Cima Volcanic field, where there are numerous cones, all erupted in the Late Miocene to present. We stopped at well-known but difficult to find lava tubes, and then we drove a little further up a rough road to look for xenoliths. Normally, we don’t do this latter part because the road is so rough, but since our plans at Dish Hill were diverted, we thought it would be a nice way to end the trip. Unfortunately, the road became far too rough to continue further, so we hiked the last stretch. And xenoliths we did find! Peridotites everywhere!
Hunting xenoliths in Cima. L to R: Cin-Ty Lee (hideen), Evan Neustater, Kate Nicholson, Yunong Xu, Farah Ashraf.
Our luck, however, would not hold out. As we turned our cars around and headed back down the slope, one of our cars hit a rock hard, twisting its axle. Definitely not good because it made it hard to drive. What to do now? The sun had set, the students were tired and hungry, and we had early flights out the next morning, but all the way back in Ontario, a three hour drive with a good car. We convened at the Mad Greek in Baker and it was decided that the everyone would pile into the other working cars and just go back to Ontario so they could get a good night’s sleep. Our professor and a couple of visiting grad students would drive the disabled vehicle back, slowly. It took the five hours and a tire blow out in the last 20 miles, but they arrived home safely.