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A Structural Transect Across Kirby Ride of the Marin Healands Terrane of the Franciscan Complex, Marin County, CA Carl Marin Field Methods Spring 2011 Introduction The Marin Headlands Terrane of the Franciscan Complex is located in southernmost Marin County, California, about 2 km west of the Golden Gate Bridge, and 4 km northwest of San Francisco (figure 1). The Franciscan Complex of is composed of Cretaceous-Jurassic greenstone, radiolarian chert, sandstone and shale. The Franciscan Complex is found throughout the Coast Ranges of California. Franciscan rocks were accreted to the western edge of the North American Plate along a subduction zone in the late Mesozoic or early Tertiary. The orientations and types of the faults and folds at ,informally named, Kirby Ridge (figure 2 Field Map) can be used to better understand Mesozoic convergence of the plate boundary. This study was conducted during the spring of 2011 by Professor Caskey’s field method class at SFSU. The study consisted of mapping of road cuts along Conzelman Rd. The purpose of the study was to produce a geologic map and a cross section along Kirby ridge. This report contains a rock unit map and cross-section of the study area, macroscopic fold data from the chert, descriptions of the map units, and structural geology, and interpretations of the possible relations of the faults and folds in the map area, with reference to the cross-section. Stratigraphy The major units compose the Fransican complex along the roadcuts of Conzleman Rd; Mid Triassic pillow basalts; radiolarian chert; and sandstone- shale. The greenstone basement was extruded at the East Pacific Rise spreading zone during the Triassic. The radiolarian chert formed in deep ocean on top of the greenstone from the accumulation of siliceous skeletons of plankton. The shale and sandstone was deposited by turbidities on top of the chert. A. Greenstone: The Franciscan greenstone along Conzleman road is highly fractured and sheared. The greenstone is brown and contains pillow structures. The greenstone is a slope-forming unit found in many of the saddles along Kirby ridge. The greenstone is the basement rock. B. Radiolarian Chert: The radiolarian chert is a dark red to brown and is very hard and fine grained. The chert is highly folded and forms slopes very well due to its hardness. The maximum exposed thickness is ~200m note that this is a rough estimate due to the highly folded nature of the unit. The chert is deposited directly onto the greenstone at several contacts and due to reverse thrust faults found under the greenstone at other locations. (see cross section figure 3) C. Shale and Sandstone: The sandstone is yellow brown and consists of fine to medium sand grains. The shale is dark grey –black moderate to weakly cemented very fine grained. The maximum exposed thickness is ~40m. Structure Faults The rock units along Conzleman rd contain three thrust faults that are exposed in the road cuts. About ~300 m south west of Kirby Peak the reverse thrust places greenstone over the chert. This thrust dips moderately to the south and prominent shear fractures in the greenstone with a width of over 3m are subparallel to the fault contact. Another reverse thrust fault ~100m southwest of Kirby Peak places the greenstone over the chert. The third reverse thrust fault mapped is ~150 m northeast of Kirby peak dipping gently to moderately southwest and placing greenstone over the sandstone and shale unit. There is boudinage structure in the less competent sandstone/shale footwall. Folds The chert beds are highly and tightly folded throughout much of the mapped area figure 4 stereo net of poles of planes and trend and plunges. The hinge lines of the folds at the two outcrops generally trend westward. Conclusion The Marin headlands terrane consists of greenstone, radiolarian chert, and sandstone and shale. These sediments formed during the Late Mesozoic to early Tertiary. They were uplifted and accreted onto the North American Plate during the Tertiary. The faults and shearing of the Franciscan Complex may be due to larger scale faulting and uplift of the surrounding area.
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