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An Analysis of the Morphology and Physical Properties of Pillow Lavas

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An Analysis of the Morphology and Physical Properties of Pillow Lavas An Analysis of the Morphology and Physical Properties of Pillow Lavas of the Nicasio Reservoir Terrane, Marin County, California: Implications for Seamount Formation and Structure Susan R. Schnur Senior Integrative Exercise March 9, 2007 Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota. Table of Contents Introduction 1 Geologic Setting 4 Nicasio Reservoir Terrane - Specific Study Area 10 Pillow Forming Parameters 12 Methodology 19 Pillow Size and Morphology 19 Physical Properties 20 Pillow Chemistry and Mineralogy 24 Geochemistry and Mineralogy 25 General Stratigraphy Based on Flow Morphology 30 Pillow Size and Morphology 36 Pillow Density and Porosity 46 Discussion 51 Depth 51 Slope 53 Lava Viscosity 54 Cooling Rate 55 Effusion Rate 56 Tectonic Location 57 Model for the Nicasio Reservoir Seamount 61 Distinguishing Between Multiple Flow Units 61 Stratigraphic and Chronologic Relationship between Flows 62 Seamount Formation and Structure 63 Future Work 67 Conclusions 69 Acknowledgements 71 References Cited 71 List of Figures 75 List of Tables 75 Appendix A – Digitized Photos of Pillow Exposures 76 Appendix B – Raw Pillow H and V Measurement Data 85 Appendix C – Complete XRF Analyses for 3 Samples 88 An Analysis of the Morphology and Physical Properties of Pillow Lavas of the Nicasio Reservoir Terrane, Marin County, California: Implications for Sea- mount Formation and Structure Susan R. Schnur March 9, 2007 Faculty Advisors: Lisa Gilbert, Williams College Cam Davidson, Carleton College Abstract – The physical and morphological properties of pillow lavas exposed in outcrop are useful in determining the original chemical and environmental conditions under which the pil- lows formed. Pillow size, morphology, density, specific gravity, and porosity can be used to determine the depths at which pillows were emplaced and the gradient of the paleoslope. Varia- tions in flow morphology throughout a section indicate changes in parameters such as effusion rate and cooling rate and may indicate flow transitions within a single eruption event. Chemistry and mineralogy of pillows can help indicate viscosity of lavas, the nature of vesicle formation, and tectonic location of formation. These concepts are applied in a multi-parameter approach to understanding the formational history of the pillow lavas of the Nicasio Reservoir terrane, an exotic terrane of the subduction-related Cretaceous Franciscan formation of Northern Califor- nia. Results indicate that rocks exposed near the Nicasio reservoir at the Northern extent of the terrane represent at least two separate eruption events, one of which likely formed on the steep upper flanks of a seamount at ocean depths between 1.6 and 2.5 km. The second flow, repre- sented by larger pillows with higher vesicularity, is thought to have been emplaced over the first flow in shallower water between depths of 0.6 to 1.25 km, potentially within a collapsed caldera. These interpretations are used to construct a model for formation of the Nicasio Reservoir terrane pillows and provide insight into the general structure of seamounts. Keywords: pillow lava, lava flows, ophiolite, volcanic rocks, Franciscan complex. 1 Introduction A significant amount of volcanic activity occurs in submarine environments, fo- cused at seamounts and at mid-ocean ridges. Active volcanic vents extrude basaltic lava to form flow units that build on top of each other to create thick extrusive piles of basalt on the ocean floor. This extrusive pile forms the upper 1.5 km of the igneous portion of a typical section of oceanic crust and overlies a downwards progression from sheeted dikes to layered gabbros at depth (Carlson and Herrick, 1990). As ocean drilling is costly, time-consuming, and presents a limited one-dimensional view of crustal layers, much of our knowledge concerning oceanic plate composition, stratigraphy, and horizontal variation is derived from studies of ophiolites. Ophiolites provide useful opportunities to examine oceanic crust in cross-section on land, and a growing body of work points to significant similarities between obducted sections of oceanic crust and in-situ oceanic crust (Dilek et al., 2000). Ophiolites also provide cross-sections of the upper extrusive pile, allowing more detailed observation of solidified flow morphologies and contacts between multiple flow units. However, ophiolites are typically deformed, metamorphosed, and chemically al- tered during emplacement onto continental crust. These processes disrupt original strati- graphic relationships, making it difficult to determine the chronological order in which exposed lava flow units were erupted. However, various properties, including pillow morphology, density, vesicularity, mineralogy, and chemistry can still be used to differen- tiate between multiple flow units in an outcrop as well as to better understand the ambient conditions under which the flows formed. Lava flows are often composed of several different flow morphologies that repre- sent different ambient and lava conditions. General categories of morphology determined from sub-aerial and submarine observation of lava are pillowed, lobate and sheet flows (Gregg and Fink, 1995). Pillowed flows are characterized by inflated tubular lobes of lava that often form branching patterns and are connected by narrow necks through which 2 hot lava is delivered from tube to tube on its way to the flow front (Vuagnat, 1975). Apart from these necks, each tube is distinct, separated from other tubes by a fine-grained chill margin formed by rapid cooling of lava exposed to the ambient environ- ment. Pillows appear in outcrop as circular, irregular, and elliptical masses surrounded by a glassy chill-margin, and often display a typical v-shape where the basal surface of each pillow has sagged downwards prior to cooling to conform to the v-shaped valleys between underlying lobes. Other flow morphologies arise when lava has cooled slowly enough that indi- vidual pillows have merged into each other and hot lava is to free to flow laterally be- neath an broader chilled crust rather than being funneled into tubes. This process is often evidenced as lobate flows, which represent pillows that have cooled just slowly enough that separate pillows have collapsed into each other. Sheet flows represent the opposite extreme of lava morphologies, and are evidence of either low viscosity lava or extremely slow cooling, during which lava spreads out into a thin layer with no distinct rim-bound structures save for a single overlying chilled skin (Gregg and Fink, 1995). Pillow lavas are by far the most common lava flow morphology, dominating the distribution of morphology types at both seamounts and mid-ocean ridges such as the Mid-Atlantic ridge, where virtually all lava erupted forms pillows (Moore, 1975). As well as being visually appealing, the physical and morphological properties of pillow lavas vary across a range of ambient and lava conditions. Thus, field studies of exposed pillowed flows in ophiolites can help us work backwards to derive these specific forma- tion conditions, making them ideal for the type of multi-discipline analysis described in this paper. The current study uses physical and chemical data collected at a superb sequence of pillow lavas exposed as part of an upper ophiolite sequence in Marin County, Cali- fornia to make connections between these measured and inferred parameters. The goal of this work is to draw together evidence from both field studies and more quantitative 3 laboratory work and to use this evidence to describe the formation history of the pillow lavas. These results can help answer the question of how many chronologically distinct lava flows resulted in the pillow lavas at the Marin County study site and may provide information on methods of differentiating between lava flows in outcrop. This knowl- edge is then used to construct a model describing the formation process of the Nicasio seamount, shedding light not only on the formation of the Nicasio Reservoir terrane but on the three-dimensional structure of seamounts in general. This information is important in the context of modern marine geology as little is currently known about the three-di- mensional structure of seamounts. 4 Geologic Setting The western coast of California, with its history of active subduction and terrane accretion, presents an interesting geologic puzzle. The variety of different rock com- plexes that are found next to each other yet often have very different geologic histories, provide an exciting location for mapping and fieldwork. The Nicasio Reservoir terrane is one such accreted complex, identified as being of oceanic origin due to the layer of pillow lavas that forms its base (Blake, 1984). The terrane is a member of the Franciscan complex, one of three Mesozoic rock complexes that dominate the lower stratigraphy of the San Francisco Bay region. The Franciscan is overlain by the Great Valley complex, composed of the Jurassic Coast Range ophiolite (serpentinite, gabbro, diabase, basalt) and the Jurassic Great Valley sequence, composed predominantly of sandstone and shale (Blake, 2000). This group is bounded on the West by the Salinian complex, composed predominantly of granitic and gabbroic plutonic rocks emplaced in a batholith system that has been carried northwards by movement along the San Andreas fault (Blake, 2000). Figure 1 shows a general distribution of Mesozoic complexes in the Bay Area of North- ern
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