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Mesozoic Deformation of the Franciscan Complex in the Marin Headlands Terrane Alexandra Allen ERTH 620: Field Methods Geology Department San Francisco state University The Marin Headlands, located in Marin County, California, directly north of San Francisco, is a scenic highland exposing a mélange of rocks from the Franciscan Complex in what is known as the Marin Headlands terrane, as shown in figure 1. This terrane block is one of eight formed millions of years ago through intense tectonic activity that deformed a sequence of depositional units called the Franciscan Complex. The three main rocks ranging from 80 to 200 million years old that make up the sequence exposed in the Marin Headlands include greenstone altered from basalt, radiolarian chert, and alternating beds of sandstone and shale. More than 200 million years ago, during the Jurassic period, greenstone and chert were deposited in the central Pacific Ocean, near the equator, in a mid-oceanic ridge on the Farallon plate (Elder). As the ridge pushed the plate away, it moved northeast until it reached a latitude close to Mexico, where it then began to subduct underneath the continental crust of the North American plate’s westernmost side during the Cretaceous period (Leech). Because the density of the mafic-basaltic ocean crust is greater than that of the continental crust, the Farallon plate was pushed beneath the North American plate completely, forming a marine trench in the process of subduction (Konigsmark). As the oceanic plate scraped along the continental crust, marine sediments along the continental margin filled in much of the trench. The stress and strain caused by the friction as these plates slid past each other caused high grade metamorphism as well as intense deformation and folding. The lightest material scraped off and along the plates, became wedged between the plates as part of an accretionary wedge that was uplifted in an eastward direction by major normal faults (Leech). Prior to the formation of the San Andreas Fault, in the Cenozoic, 30 million years ago, the deformation caused by subduction had fractured the wedge into eight terranes in the form of “shingles”. As the San Andreas Fault formed and extended from south to north, it pulled the terrane blocks northward along the continental coast, rotating the blocks along the way. During the Cenozoic period, the San Andreas transform carried the terrane all the way to the Marin Headlands, rotating the block an estimated 90° from its origin (Elder). This specific block of terrane made up of heavily deformed Franciscan Complex rock was again uplifted by thrust faults to its current location and named the Marin Headlands terrane. On the dates of February 12th and 19th of 2016, my fellow students in ERTH 620: Field Methods in Geology at San Francisco State University and I traversed to the Marin Headlands in Marin County, California to map and collect field data in order to understand the geologic structures and units in the Franciscan Complex exposed in this location through cross sections and geometric data of folds and faults. Our research site was located about 4.5 miles north-west from the northern peninsula of San Francisco, along Conzelman Road. We analyzed and collected data on an exposed outcrop of the complex ranging from where the road becomes oneway to the intersection of Conzelman Road and McCullough Road along the Cliffside to the northern side of the road below Kirby Ridge. Using Bruntons, we collected a series of strikes and dips, as well as plunge data of macro and micro folds, unit contacts, and faults while mapping geologic units of the Franciscan complex along the way. This data will be used to make figures for a report on the structures of the Marin Headlands, including a cross section across the three major points on Kirby Ridge (figure 3), a stratigraphic column of the units exposed (figure 6), a topographic map depicting the units in the sequence as well as their contacts and any faults cutting through (figure 2), and stereonets showing the orientation of folds (figure 5). This report will contain these figures as well as interpret the Franciscan Complex’s paleo-depositional environment, and any relations between faulting and folds located and mapped in the sequence. The three units of the Franciscan Complex that comprise the Marin Headlands terrane were formed in the ocean, but different processes created the rocks, in order from oldest to youngest: greenstone, chert, and sandstone/shale, as shown in figure 6. The rocks in this sequence range from 200 to 90 million years old and have a maximum thickness of 15,240 meters thick (National Park Service). Greenstone The oldest unit, greenstone, is a highly metamorphosed igneous basalt of a yellowish-green color with evidence of bedding with a thickness of 5-10 m. The greenstone in this location does not have its base exposed. Pillow basalts that have an average width of 10 cm to 1 foot can be found in some of the layers, in imbrication that show stratigraphic up, as well as the flow direction and evidence that it was in a fluid (seawater) (Leech). When basaltic lava poured out of a midoceanic ridge volcano, it immediately came into contact with seawater that combined with the heat from the magma, altered minerals in the basalt into chlorite and pumpellyite, creating greenstone (Golden Gate National Recreational Area). The greenstone in the Marin Headlands is discontinuous and because of its vulnerability to weathering, generally is slope forming where it has been thrust above younger chert beds that form lenses. Along Kirby Ridge, as seen in figure 1 and 2, the greenstone sits along saddles along the ridge. The greenstone has been aged to be older than 100 million years (Elder). Radiolarian Chert The middle-aged unit is an open ocean formed, radiolarian, red chert. The chert sits above greenstone as a depositional contact. Radiolarians, single-celled organisms with silica shells, die and fall to the ocean floor where they decompose and their shells remain in the form of silicate- rich chert. This is a sedimentary rock with layers anywhere between 2-12 cm thick, and a general thickness of 80 m with some small layers of red clay throughout. These layers are parallel to bedding and show stratigraphic up (Leech). Because of the intense Z-folding, that resulted from grinding against the continental margin while being transported in the terrane blocks, in the bedding, it has been called a ribbon chert. Folds have a general orientation, striking southwest and dipping west as shown in figure 2 and 3. The exposed chert is red due to oxidation of silicates and very brittle. Fossils of douvilleiceras mammillatum and mantelliceras date the unit between 200 and 100 million years ago between the Early Jurassic and Late Cretaceous (National Park Service). Sandstone/Shale The youngest unit in the Franciscan Complex that is observable in the Marin Headlands is a sandstone and shale unit. Sandstone, the most common clastic rock found in the complex, is coarse-grained with angular grains of K-feldspar, quartz, and basalt. The sandstone is a yellowish-brown and heavily weathered though it is well-cemented, as shown in figure 6. The layers of sand were deposits from the continental margin of the North American plate that filled a marine trench during subduction through turbidites and submarine landslides (Leech). Because of the rapid flow of sediment, particles settled out of suspension with the smallest differentiating to the top, forming shale. Shale layers are a very fine-grained rock of a greyish-black color that are also subject to intense weathering and erosion and therefor a slope forming unit with the sandstone (Elder). Beds of the sandstone/shale exposed in the Marin Headlands range from 1-5 m thick but the bedding has been estimated to be about 20 m thick. Shale layers are thin, between 1-5 cm thick as shown in figure 2 and 3. The sandstone/shale unit is a depositional contact sitting above the radiolarian chert and estimated to be 100-90 million years old, deposited during subduction of the Farallon plate in the Early Cretaceous (National Parks Service). Faulting in the Marin Headlands is common with normal and thrust faults prominent along Kirby Ridge as seen in figure 2. Four main thrust faults were observed along Conzelman Road and orientations were measured for each one to better understand the source of faulting here. The first fault exposed along Conzelman Road, moving from the southern to northern end, is approximately 200 meters north from the one-way road as seen in figure 2. This fault pushes older greenstone above younger chert and is a reverse-thrust fault with the hanging wall of greenstone moving upward relative to the footwall of red chert, as seen in figure 3. The fault is a contact between the two units, with slick and sides along the shear zones parallel to the fault, expressed in the greenstone. The fault itself dips steeply to the southwest, turning due west as it moves towards the surface and higher topography. Shear fabric in the chert is dipping 50°-70° southwest. The second thrust located about 300 meters north of the previous fault, as shown in figure 2, is a fault contact between all three of the units in the Franciscan Complex exposed in the Marin Headlands. This fault is also a reverse-thrust with the hanging wall of older greenstone has been pushed above and over the younger units of sandstone/shale and chert that make up the dropping footwall as seen in figure 3. The units dip steeply to the southwest with the chert striking S40°E and dipping 40°SW with its layers parallel to the fault contact. Layering in the greenstone has a strike of N20 ° W and a dip of 30 ° SW. The orientation of the fault contact between sandstone/shale and greenstone is 176 °, 36 ° NW. Shearing is apparent in greenstone unit. The third fault along Conzelman Road is not exposed but is oriented about 110 meters north from the previous, and is a vertical strike-slip fault, marking a contact between chert and younger sandstone/shale units, shown in figure 2. Though we cannot exactly see the fault because of heavy vegetation and topsoil, we can infer its existence due to the disrupted continuity of the chert unit. To the northeast of the hidden contact, chert sits higher than the sandstone/shale unit on top of the exposed ridge, striking N40 ° W. As seen in figure 3, there must be a vertical fault to explain the movement of chert directly perpendicular to the orientation of layering. The fourth fault, another reverse-thrust, along this area of the Marin Headlands is another fault contact that pushes older greenstone above younger sandstone/shale and chert units of the Franciscan Complex. This fault is exposed at the intersection of Conzelman Road and McCullough Road as seen in figure 2. Folding in the Marin Headlands terrane is extensive and intense, with several categories exposed throughout the greenstone, chert, and sedimentary units. Chert layers are the most obviously altered beds with Z-folds and Chevron folds throughout as seen in figures 2, 3, and 6. Most macroscopic folding is oriented southwest with Chevron folds plunging southwest on a smaller scale. On the eastern side of the exposed ridge, folds dip only gently south (Konigsmark). As seen in figures 4a,b, and c, orientations of macrofolds in the chert have similar characteristics in stereonet data collected in the previous semester in a structural geology class. Folding in sandstone/shale units is not common and not as well defined due to heavy erosion. General folding in the Marin Headlands terrane can be attributed with synclines and anticlines (Elder). One measured anticline between sandstone/shale and chert, located about 500 meters from the intersection of the one-way road and Conzelman Road as shown in figure 2, is dipping at a low angle. Plumose structures from mode 1 fractures are evident in the chert as well as columnar joints. The axis of the anticline itself is parallel to faulting on either side with its shorter limbs oriented northward. Synclines in the area, on average, plunge southwest, oriented about 40 ° S25 ° W with the shorter limbs between 155 ° and 160 ° (Konigsmark). Thrusting and folding in the area have similar orientations and cut through the same units, leading me to believe that they are coeval and most likely formed during the late Mesozoic during intense deformation from the movement of the Farallon plate northward. This is most likely why folds are parallel to bedding with axis perpendicular to bedding, and oriented southwest. Some folding does not follow the general orientation and it can be assumed that these folds are younger and may be the result of the major normal fault in the Marin Headlands that uplifted the terrane block to its current position. This paper concludes that Mesozoic deformation from the movement of the Marin Headlands terrane, composed of the Francsican Complex, resulted in intense faulting and folding, especially in the brittle chert units. The orientations and fault data included in this report can be attributed to the overview detailing the history of the complex structures found throughout the Marin Headlands. Reference Page Elder, William P. "Geology of the Golden Gate Headlands." Usgs.gov. National Park Service, Golden Gate National Recreation Area, Calif., 2001. Web. 16 Apr. 2016. "Geology of the Golden Gate Headlands." Golden Gate National Recreational Area. Nps.gov, 2016. Web. 26 Apr. 2016. Konigsmark, Ted. "Geologic Trips San Francisco and the Bay Area." Geologictrips.com. Geopress. Web. 26 Apr. 2016. Leech, Mary. "Geology of the Marin Headlands and the Half Moon Bay Coast." Sanandreasfault.org. Stanford University, 19 May 2001. Web. 26 Apr. 2016. United States. National Park Service. "Geology Resources." National Parks Service. U.S. Department of the Interior, 2016. Web. 26 Apr. 2016.