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Geology of the northern Mamaku Plateau

The geology of the northern Mamaku Plateau is dominated by the rhyolitic Mamaku and Waimakariri lgnimbrites, although exposures of the dacitic Waiteariki Ignimbrite are common in river gorges. The Mamaku lgnimbrite comprises 4 flow units (sheets 1 to 4). The base of sheet 2 (the bulk of the ignimbrite) is an uncompacted pyroclastic breccia to lapilli tuff grading into a moderate to strongly welded zone, often lenticulitic. The upper zone is incipiently welded, often light pinkish grey, with poor to moderate crystal contents. Fossil fumeroles are common in the upper middle of the flow, devitrification and vapour phase alteration is extensive. The upper zone of the Waimakariri Ignimbrite is usually light grey and incipiently welded. Towards the lower middle of the flow this grades down into a welded lenticulite, occasionally eutaxitic. The base is usually an incipiently welded pyroclastic breccia enriched in lithics. This ignimbrite has a poor to moderate crystal content, is composed of at least three flow units and often has an underlying Plinian airfall. Gas escape structures appear in distal outcrops. The unwelded top of the Waiteariki Ignimbrite is often eroded. Usually only the middle of the flow, a densely welded lenticulitic with eutaxitic texture is present. The base of the flow is a moderately devitrified, incipiently welded, pyroclastic breccia to fine tuff. Evidence suggests the Waiteariki Ignimbrite comprises 2 cooling units both of which are crystal rich. With increasing distance from source plagioclase crystal percentages, pumice size and pumice percentages decrease in the Mamaku Ignimbrite (sheet 2). For the Waimakariri Ignimbrite, there is a decrease in crystal lengths with distance from source (assumed to be the Taupo Volcanic Zone). There is an increase in crystal percentages from a medial to distal position, and a moderate increase in pumice percentages. This is explained by elutruation of vitric material from the Waimakariri as a co-ignimbrite air fall ash concentrating pumice and crystals. No relationship exists between crystal length and vertical position in the Mamaku ignimbrite. Pumice percentages often increase towards the top of the flow, while pumice numbers increase towards the middle of the flow. Welding is at a minimum at the base, at a maximum in the lower middle of the outcrop, reducing towards the top of the flow. Phenocryst increase towards the base of sheet 2 is attributed to welding/compaction. No relation exists between crystal length and vertical position in the Waimakariri Ignimbrite; pumice percentages and size increase towards the base of the flow, while pumice numbers often increase towards the base and top of the flow. Horizontal sections over distances of 194 and 350 m for the Mamaku and Waimakariri Ignimbrites respectively, demonstrated that they are relatively homogeneous over short distances. Both the Mamaku and Waimakariri Ignimbrites traveled as moderately fluidized laminar flows, but the surge deposits separating Mamaku flow units were turbulent as were parts of distal Waimakariri. The surges (probably foward jetting from the front of the flow) imply the Waimakariri flowed faster than the Mamaku Ignimbrite. A new flow unit is presented, perhaps more applicable to distal large ignimbrite eruptions, than the standard ignimbrite flow unit. Most field evidence indicated the Mamaku and Waimakariri Ignimbrites were emplaced en masse. The diversity of facies in both Mamaku and Waimakariri Ignimbrites implies a single large pyroclastic flow can operate under several flow regimes, dependent to some extent on local paleo-surface conditions.
Type of thesis
Morgan, M. D. (1986). Geology of the northern Mamaku Plateau (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14455
The University of Waikato
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