Oceanography Field Trip One Key

Transcription

1 Oceanography Field Trip One Key If you are using this key as a make-up guide, you should get a COLOR version of this field trip guide by going to and clicking on Field Trip Key. Makeup assignment: Please follow this field trip guide to each of the field trip stops visited by the class. At each of the field trip stops, take photographs of your smiling face standing in front of the rocks, beach, overlook, or other subjects of interest. Describe, in a written paragraph, what you see at each stop. Turn in your photographs and written descriptions by an agreed upon deadline. Please be forewarned that this makeup option is a poor substitute for participating in the actual field trip with the instructor. You will still be responsible on tests for all material covered or discussed during the regular class field trip. If you do not turn in this make-up assignment by the agreed upon deadline, you will fail the class.

2 Introduction Exploring sediments and rocks of the seafloor is the theme of the two on-land field trips. As we travel from stop to stop imagine yourself traveling around on the seafloor. Through the combination of the Saturday and Pt Lobos field trips, we ll be able to visit many of the seafloor areas we ve talked about in class, including the Monterey Canyon, a mid-ocean ridge, and the deep sea floor of the abyssal plain. Another theme of the weekend field trip is the present day depositional settings of dunes and beaches, and the contrasts between the rocky emergent coastline of Big Sur and the smooth, sandy coastline of a more stable environment like Marina and Ft Ord. The last theme of the weekend field trip is the collection of beach profile data. Please bring this field trip guide with you to the next Oceanography lab. Monterey Formation on Olmstead Road. The most important thing to notice here is the Monterey Formation Diatomaceous Shale. To the naked eye, this rock looks like mudstone. But if we had powerful microscopes, we could see all of the diatoms that comprise this rock. Since there is very little terrigeneous material in the rock, we know that it was formed far from continents. It was probably formed in the deep oceans of the abyssal plains. What is the evidence that tectonics moved these rocks up to this spot on the hill rather than the sea level dropping to expose these rocks? Folding and Faulting in the rocks. Rocks higher on hill would mean that sea level was once several hundred meters higher, which is unlikely because the current inter-glacial period suggests we are now at a sea-level high. See pictures of the Monterey Formation on the next page.

3 Monterey Formation on Olmsted Road. Note continuous, coherent beds. The Monterey Formation is composed of clay-sized particles and diatoms, silica-rich organisms that die and fall to the bottom of the sea and accumulate there. Ike looking closely at the Monterey Formation. Or maybe he s looking at a spider. Cherty layers in the Monterey Formation. The cherty layers probably contain more diatoms than the layers above and below (buff colored layers at left), which contain more clay. Chert, almost entirely SiO 2, weathers in a more brittle manner than the more clay-rich layers. Through tectonic processes. When plates or blocks of rocks converge, the rocks in the middle deform in such a way that they get thicker. Getting thicker means creating mountains and mountain ranges. Imagine continents colliding and mountain ranges forming. Mountaineers at the top of Mt Everest in the Himalayas have found fossils of creatures that lived in the oceans.

4 Marina State Beach Dunes and sandy beach in a stable environment that is not rapidly emerging. The dunes here are big and extend for a few miles inland. In fact, on the beach side of the sand dunes we can see evidence for previous episodes of dune formation. What is the evidence for more than one episode of dune formation? Layers of soil formation within the dunes. Evidence for soil formation includes a layer with a dark color from the addition of organic material. What is the evidence that waves are currently eroding the dunes? A very steep sea cliff is the main evidence. It is clearly over steepened for the coherency of the sand. Monastery Beach A nearby canyon causes deep water to occur very close to shore here. Also, good views of marine terraces. What differences can you see in grain size of the beach? Grain size of the sand here is bigger than at many other beaches. What is distinctive about the slope angle of the beach as it reaches the water? The slope angle of the beach is steeper than at many other beaches. It probably is caused by the way that the waves crash on a very steep offshore area. There is a canyon just offshore.

5 Hurricane Point Excellent view of the Big Sur coastline, one of the worlds best examples of an emergent coastline. It just doesn t get any better than this! The rock here is a marble, which is a metamorphosed limestone. Limestone is deposited in warm shallow seas where coral and tropical fish make it nice to snorkel. Then the limestone gets metamorphosed into marble. What is the difference between a marine terrace and a wave-cut platform? A wave-cut platform is at sea-level, whereas the marine terrace is above sea level. Image at left is a great example of a wave-cut platform. It is located near the town of Bolinas, north of San Francisco. This image shows a wave-cut platform in the making at Hurricane Point. In this image, the waves are breaking on the wave-cut platform. Note the house on the cliff built on landslide deposits.

6 Andrew Molera State Park We ll walk to the mouth of the Big Sur River here and see the results of volcanism and sedimentation at a midocean ridge. Note the pillow basalts and chert. We ll also note wave refraction as the waves move in towards the beach. Can you find any pink sand? If you can it is composed of garnet that occurs abundantly in rocks found upstream along the Big Sur River. What is the evidence that this group of rocks were deposited near a midocean ridge or on a seamount? The occurrence of pillow basalts and chert, a deep sea sediment. In this image, I m bending down to get a good look at some pillow basalts. The purple sand is probably garnet eroded from rocks washed down the Big Sur River.. Upon close inspection it looks crystalline and translucent.

7 Garrapata Beach Two main things to see here: the distal turbidites and the San Gregario Fault. The sedimentary rocks here are distal turbidites. That means that they are deposited by turbidity currents, but far out on the submarine fan. Remember these rocks and compare their grain size to the rocks that we ll see (or have seen) at Pt Lobos. What is the evidence that the turbidites here were deposited far from shore on the continental slope? Relatively fine grain size. Coarse sand is the largest grain size. Evidence for turbidites is the graded bedding. At left are the thinner turbidites. Graded bedding is difficult to see in the photograph. You can see however, that the rock consists of interbedded fine sands and shale. At left are some of the thicker turbidite deposits. Note the thicker sandstone layers. Shale layers are fairly thin here. A close examination of the sandstone layers revealed that lower right. Thus, stratigraphic up is to the upper left. they are coarser grained to the

8 Also at Garrapata Beach, we ll see the San Gregario Fault. The San Gregario Fault is part of the San Andreas Fault system and is one of the faults that separates the Pacific plate from the North American Plate. It is the same kind of fault as the San Andreas Fault. What kind of fault is the San Gregario Fault? It is a right-lateral strike-slip fault, just like the San Andreas. Evidence for this kind of slip motion comes from the slikensides just beneath the old stairs. How were you able to locate the San Gregario Fault on Garrapata Beach? Where is it? I hope everybody was able to locate it. The fault is located between the granite and the turbidites, just beneath the old stairs. Here s the granite near the stairs. See the evidence of springs, or groundwater, both of which are very common near fault outcrops. Here s some very disrupted mudstone/shale near the fault. Movement along the fault has probably removed any coherency from these fine-grained sediments. Here are some slickensides, or striations caused by movement along the fault, in granite very close to the stairs. The horizontal orientation of these striations indicates that the SanGregorio fault is a strike-slip fault. Regional studies indicate it is a right-lateral fault.