The Parnell Grit beds revisited: are they all the products of sector collapse of western subaerial volcanoes of the Northland Volcanic Arc?
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Allen, SR, The Parnell Grit beds revisited: are they all the products of sector collapse of western subaerial volcanoes of the Northland Volcanic Arc?, New Zealand Journal of Geology and Geophysics, 47, (3) pp. 509-524. ISSN 0028-8306 (2004) [Refereed Article]
The Parnell Grit encompasses a number of very thick, coarse, volcaniclastic, gravity flow deposits that were deposited within the early Miocene Waitemata flysch basin in Northland, New Zealand. The flysch was fed predominantly from the Northland Allochthon and contemporaneous volcanoes in the north and west. The Parnell Grit beds comprise basaltic to andesitic lava and pumice clasts set in a matrix of crystal-rich sand and clay. Lithofacies characteristics include stepwise grain-size and density grading from a lava pebble-rich base, a middle pumice-rich, weakly stratified to massive part, and an upper part composed of lithic and crystal-rich sandstone. Most beds include large intrabasinal rip-up clasts aligned parallel to bedding, and some are rich in shallow-water fossil fauna. Deposition was from high particle concentration gravity flows transitional between turbidity currents and debris flows. A study of 36 different Parnell Grit occurrences around Northland has revealed that lithofacies characteristics and composition can be related to position within the basin and enclosing flysch facies. Thick, pebble-cobble conglomerates dominate the beds that were shallowest and closest to source in the north. Those within the deeper, central part of the basin are finer grained and include significant components of intrabasinal siltstone. Most are rich in pumice. Provenance of the Parnell Grit beds enclosed within volcanic-rich and mixed flysch to the north and west shows strong evidence for a source from the volcanoes to the west. In comparison, those enclosed within the volcanic-poor flysch and basal Waitemata Group to the southeast were derived from a northern or eastern source. Most beds comprise reworked fragmental debris, suggesting derivation from a volcanic apron some distance from the cone. The large destabilisation events that generated the volcaniclastic gravity flows may have been related to progradation of the volcanic apron across the inherently unstable Northland Allochthon. © The Royal Society of New Zealand 2004.
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