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Volcaniclastic Rocks of the Orton-Bradley Formation, Banks Peninsula, New Zealand. Abstract
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Chapter 3: Terminology and Stratigraphy Chapter 5: Physical Volcanology Chapter 6: Interpretations and Lithofacies Analysis Chapter 7: Discussion and Geological History Acknowledgements and References
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Volcaniclastic
Rocks of the Orton-Bradley Formation, Banks Peninsula, New Zealand.
A Thesis submitted in fulfilment of the requirements of the Degree of Master of Science in Geology in the University of Canterbury by Richard Sutton. 1993.
Abstract In this thesis, the physical volcanology and the petrography of the two volcaniclastic members of the Orton Bradley Formation on Banks Peninsula, are described. The Mt. Bradley Volcaniclastic Member is largely pyroclastic, with the upper deposits being attributed to relatively dry surge eruptions resulting from the explosive interaction of ground water and hawaiite magma (though amount of variation in the various units indicate that the water:magma ratio was quite variable), from a tuff ring vent on the northern flanks of Mt. Bradley. The lower, largely obscured, units are thought to be wet base surge deposits, resulting from the interaction of magma with a crater lake which had formed within the crater. There are also epiclastic deposits, mainly lacustrine mudstones and associated alluvial deposits, associated with breaks in the eruptive cycle. The Tablelands Volcaniclastic Member is totally composed of surge deposited, pyroclastic material and was probably erupted over a shorter period of time. The deposits from the tuff cone vent on the northern flanks of Mt. Bradley, are derived from relatively wet basaltic phreatomagmatic eruptions causing a higher degree of fragmentation and forming different bedding structures than those associated with the drier Mt. Bradley Volcaniclastic Member. Deposits associated with the second tuff cone vent on the eastern flanks of Mt. Bradley, indicate somewhat drier vent conditions than those at vent on the northern flanks. Three distinct eruptive modes are recognised and there lateral lithofacies sequences are described. Mode 1 results in the eruption of dry base surges, these have a water:magma ratio of less than 0.3 and steam is their primary fluidising medium. Mode 2 produces wet base surge eruptions with water:magma ratios of greater than 0.3, condensed steam is the major fluidising medium. The third and relatively minor mode, consists of crater lake discharges, possibly driven by phreatic eruptions occurring within the lake. The petrography of the deposits indicates that they are predominantly composed of type B sideromelane (with alteration to palagonite in some units) and lithic clasts (predominantly hawaiite in the Mt. Bradley Volcaniclastic Member and porphyritic basalt in the Tablelands Volcaniclastic Member). This website is abridged version of my masters thesis with the graphic elements removed. |