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Stratigraphy and Sedimentology of Early to Middle Miocene Strata, Western Taumarunui Region, King Country Basin

The study area of this thesis is located in the King Country Basin, North Island, New Zealand. It contains a 400 m thick marine sedimentary succession of Miocene age and Quaternary ignimbrites. The field area comprises 900 km2 of steep to rolling farmland and some large patches of native forest, and includes the towns of Ohura in the west and Taumarunui in the east. There is limited outcrop exposure in the study area, and the sedimentary succession is often weathered. The study area is cut by numerous faults that have formed in an extensional regime behind the modem volcanic arc (Taupo Volcanic Zone). The major fault is the Ohura Fault, which has changed its sense of displacement (reverse to normal) in response to changing stress regimes during the Neogene. No basement is exposed in the field area, which lies wholly in Early Miocene (Otaian Stage) Mahoenui Group and Mokau Group sediments, and Middle Miocene Mangarara Formation, Otunui Formation, and Mt Messenger Formation sediments. The Mahoenui Group comprises the Taumarunui Formation, characterised by flysch deposits, and the Taumatamaire Formation, a massive mudstone that interfingers with the Taumarunui Formation in the field area. These Early Miocene units represent rapid subsidence and basin formation, with accumulation in outer shelf to slope environments. The Mokau Group comprises the Bexley Sandstone, Maryville Coal Measures, and Tangarakau Formation, which represent an overall marine transgression. The Mangarara Formation is unconformable on the Mahoenui Group, and represents marine transgressive onlap across a land surface cut into the Mahoenui Group during the late-Early Miocene. This unit is overlain by the Otunui Formation, which represents the development of a shelf succession, and which in tum is overlain by the Mt Messenger Formation which represents deposition in slightly deeper waters. Quaternary ignimbrites have a scattered distribution on hill tops in the study area. Seven lithofacies have been identifed in the study area, and these have been subdivided into 14 sub-facies. These range from massive mudstone deposited in a bathyal environment, to coal measures deposited in estuarine/swamp/flood plain environments. The facies have been described and interpreted within formations, and the environments for facies associations are inferred. The Mahoenui Group sediments represent deposition in a bathyal setting, and the Mokau Group was deposited in environments ranging from shoreface to inner shelf environments. The Mangarara Formation was deposited in a near shore environment, and the Otunui Formation accumulated in a shelf to uppermost slope setting. The Mount Messenger Formation, which overlies the Otunui Formation mainly south of the field area, occurs at one locality in the study area. The King Country Basin developed in response to transpression and crustal shortening related to the inception of the modem plate boundary system through New Zealand during the Early Miocene. The sedimentary succession of the Mahoenui Group, Mokau Group, Mangarara Formation, Otunui Formation and Mt Messenger Formation has developed in response to the coupled effects of tectonism associated with this plate boundary system, and changes in relative sea level.
Type of thesis
Evans, T. P. H. (2003). Stratigraphy and Sedimentology of Early to Middle Miocene Strata, Western Taumarunui Region, King Country Basin (Thesis, Master of Science). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10571
University of Waikato
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