Tertiary geology of Birchwood area, near Ohai : South Island, New Zealand.


Arifin, Maximon Shah, 1947-


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Early Oligocene to middle Miocene sedimentary rocks of the western Ohai district are mapped, subdivided into lithostratigraphic units. The geology structures are interpreted, based on field evidence and aerial photo study. Tertiary strata are folded and faulted, in the east of the study area they are faulted against the Twinlaw basement. Two major sedimentary sequences are recognized; a transgressive sequence represented by the Orauea Mudstone, the Birchwood Lower, Middle and Upper Members, and a regressive sequence represented by the Rannock Siltstone, the Feldwick Formation, the Reipihi Formation and the Lentile Formation. The relation between these two sequences is gradational. Evidence of hard ground submarine erosion occurs in the lower part of Feldwick Formation during deposition of the regressive sequence.
The sedimentology of the Birchwood Middle Member is described in terms of submarine fan, a middle-fan depositional environment is interpreted to this Member.
A biofacies analyses of benthonic and planktonic foraminiferal assemblages (after methods of Phleger and of Vella) is used to assist further the interpretation of depositional environments.
Clay fractions of the fine-grained sediments were analysed by X-ray diffraction methods, and show that mixed two or three layers clay mineralogy is predominant in most of the samples analysed.
Petrographic analyses of sandgrade sediments in the Birchwood Middle Member permit the conclusions that the main provenance of terrigenous components are plagioclase-rich intrusive and volcanic rocks of the Takitimu and Longwood massifs.
The rocks of Feldwick Formation are described and grouped into lithofacies. Local variations in sea-floor relief and several transport mechanism involved during the deposition of Feldwick Formation in inner shelf environments are interpreted by megascopic and microscopic characteristics of the lithofacies.
Petrographic analyses of some specimens of the Reipihi and the Lentile Formations suggest that two main provenances can be identified, i.e. a schist provenance and a granite provenance, and that these vary in relative importance within the formation.

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148 leaves : ill., maps ; 30 cm.


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Arifin, Maximon Shah, 1947-, “Tertiary geology of Birchwood area, near Ohai : South Island, New Zealand.,” Otago Geology Theses, accessed April 13, 2024, https://theses.otagogeology.org.nz/items/show/152.

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