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Geology of the western Mamaku Plateau and variations in the Mamaku ignimbrite

Pleistocene ignimbrites in the western Mamaku Plateau east of Putaruru consist of the following successively younger formations: (?) Ongatiti Ignimbrite, Ahuroa Ignimbrite, Whakamaru Ignimbrite, Waihou Ignimbrite (new), Waimakariri Ignimbrite (new), Mamaku Ignimbrite. They can be distinguished on the basis of their compositional-, welding-, textural-, and field characteristics. The ignimbrites are separated by unconformities, and intercalated fluvial sedimentary deposits. The three most important units are the Whakamaru, Waimakariri and Mamaku Ignimbrites, which are well exposed and widespread. The Mamaku Ignimbrite covers a surface area of nearly 4300 km² on the Mamaku Plateau. Corresponding vertical changes in lithology, petrography and physical properties near the Rotorua Caldera define two flow members: a lower Sheet 1, and an upper Sheet 2, each of which is about 60 m thick. The ignimbrite constitutes a simple cooling unit which indicates that Sheet 2 was emplaced shortly after Sheet 1. Only one sheet (? Sheet 1) is evident in the western Mamaku Plateau. Welding zonation near-to-source is characterised by a gradual change from a basal glassy lenticulite zone to a nonwelded top. Smith's (1960b) zonal classification ranks the ignimbrite as being partially welded. This condition suggests that total thickness is not necessarily a major factor in the degree of welding of ignimbrites. The phenocryst assemblage of plagioclase, quartz, pyroxene, opaques, hornblende and biotite, is set in a devitrified fine ash matrix, which is an ubiquitous feature of the ignimbrite. Higher modal phenocryst content at the base of each sheet is attributed to compaction. Overall the phenocrysts are uniformly distributed and there is little vertical variation. Seven bulk chemical analyses reveal only minor vertical variations in composition. The near uniform chemical and mineralogical trends suggest nondifferentiation of the source magma. Texturally the Mamaku Ignimbrite is a poorly sorted lapilli ash. The grading of fragments like that shown in "the standard ignimbrite flow unit" is absent. However, the presence of a zone of aligned pumice fragments indicates that laminar flow operated during the later stages of deposition. Two major inferred faults in the western Mamaku Plateau define a western horst, a central graben, and an eastern horst. The eastern horst is postulated to be the southern extension of the axial median horst in the Hauraki Depression and of the Hauraki Rift. Repeated and extended intervals of erosion followed the emplacement of the ignimbrites. Detritus from the eroding sheets was removed by rivers flowing into the Hauraki Depression. Construction of the Mamaku Plateau has largely taken place since the eruption of the Whakamaru Ignimbrite, c.300,000 years ago, and terminated with the eruption of the Mamaku Ignimbrite, c.l40,000 years ago. Excavation of the long deep valleys in the plateau occurred mainly in the latter 42,000 years.
Type of thesis
Fransen, P. J. B. (1982). Geology of the western Mamaku Plateau and variations in the Mamaku ignimbrite (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11529
The University of Waikato
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