Kora : a study of a miocene, submarine arc-stratovolcano, North Taranaki Basin, New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science with Honours at Massey University
Kora is a relict submarine arc-stratovolcano buried offshore in north Taranaki Basin. New Zealand. Kora was active on the seafloor in middle to upper bathyal water depths from the late Early Miocene to Late Miocene times. Post-eruptive burial of the volcanic edifice by Mohakatino Formation and Giant Foresets Formation sediments has preserved the edifice and its flanking volcaniclastic deposits. Arco Petroleum New Zealand Inc. drilled the Kora feature in 1987 and 1988. Core recovered from the Kora-1A, Kora-2, and Kora-3 wells contain lithologies derived entirely from fragmented volcanic rocks, with no evidence for massive lavas or pillow lavas. Typical lithologies are interbedded tuffs, hyaloclastite tuffs, volcanic conglomerates, and tuff breccias. The framework clasts in the tuff breccias and conglomerates are porphyritic andesite lithic clasts and andesite eruptives. The lithics were derived from subvolcanic intrusions that formed prior to the main period of edifice construction between 16 Ma and 12 Ma. The round porphyritic conglomerate framework clasts were shaped in transit through the volcanic conduit during volcanic eruptions. Conglomerates lack a planar clast fabric and have a polymodal matrix. They were deposited as density modified grain flows. The tuff breccias are the suspended tails of these deposits. The interbedded tuffs and sparse pebble trains are interpreted to be suspension deposits derived from primary subaqueous eruptions. The fragmental volcaniclastic rocks erupted from Kora were formed entirely at the water-magma interface from fuel-coolant interactions, and cooling-contraction granulation. In contrast, modern volcaniclastic rocks on the southern Kermadec submarine arc-volcanoes, Rumble IV and Rumble V, commonly form from collapsing proximal pillow lava outcrops and small eruptive vents. Like Kora, epiclastic redeposition of volcaniclastic debris on Rumble IV and Rumble V include avalanche slides, debris flows, and grain flows, with little evidence for large-scale channel deposits. Seismic facies comprising the Kora edifice were determined from seismic reflection profiles. The individual apron facies reflectors are identified. These comprise a downlapping terminal wedge that marks the downslope limit of volcaniclastic debris, or the surface along which they travelled. Long continuous, subparallel, individual apron facies reflectors typify northwestern aspects of Kora; these reflectors can be traced laterally from the crest of the edifice to the long thin terminal wedge at the toe of the edifice. The southeastern aspect consists of individual apron facies reflectors that are hummocky, discontinuous and intertwined, with short thick terminal wedges. The edifice has been subject to a sector collapse on NW slopes, where a slump scar occurs. The eastern slopes dip more steeply than the western slopes. The edifice has a conical morphology and is some 10 - 12 km in diameter. The major element geochemical analyses from Kora have been compared to geochemical anlayses from the Coromandel, Waitakere, Rumble IV, Wairakau, Egmont, Titiraupenga, Alexandra, Kiwitahi, and Tongariro volcanic centres using discriminant function analysis. Results have identified four assemblages of volcanic centres with comparable major element geochemistry. Kora, which fits in to the Waitakere, Wairakau and Alexandra volcanic assemblage is a southward extension of the Northland volcanic "trend".