Diagenesis in Early Miocene Waitemata Group sediments, Upper Waitemata Harbour, Auckland, New Zealand

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概要

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• The Waitemata Group in the Upper Waitemata Harbour area is a turbidite sequence with interbedded volcaniclastic sandstones, rhyolitic granule-sand and tuff horizons, and conglomerate lenses containing metamorphic, volcanic, plutonic and sedimentary clasts. Clastic debris in the sediments is little altered except for devitrification of volcanic glass. Zeolites and clay minerals encrust clastic grains and fill pores. There is a strong lithologic control on diagenetic assemblages. Lithic-rich sandstones poor in volcanic debris have calcite cement. Volcaniclastic sandstones have zeolite cement with the paragenetic sequence clinoptilolite ( + mordenite) → chabazite and/or erionite, and finally rare analcime. Rhyolitic granule-sand horizons contain the paragenetic sequence analcime → clinoptilolite → chabazite and/or erionite. Conglomerate horizons, which frequently lack any vitric debris, have analcime cement. In rhyolitic tuff only mordenite occurs crystallising directly from devitrified glass shards. Clay minerals present are smectite and illite, and chlorite in highly altered volcaniclastic sediments. Interbedded mudstones sealed fluids into volcaniclastic sandstone and tuff horizons and diagenesis occurred in a closed hydrologic system. The original pore fluid, believed bicarbonate-rich, precipitated calcite cement in the absence of vitric debris. In volcaniclastic and vitric rocks devitrification of glass liberated alkalies and silica into pore fluids leading to crystallisation of zeolite. Conglomerates filling channels cut into other sediments were an open hydrologic system and analcime crystallised from Na-saturated fluids flowing through them.

著者

• BLACK Philippa

  Department of Geology, University of Auckland

• Black Philippa

  Department Of Geology University Of Auckland

• DAVIDSON Kevin

  Kawakawa Engineering Ltd.

関連論文

• Mid-Tertiary magmatic events in New Caledonia : K-Ar dating of boninitic volcanism and granitoid intrusives
• Diagenesis in Early Miocene Waitemata Group sediments, Upper Waitemata Harbour, Auckland, New Zealand
• Metamorphism and metamorphic K-Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

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