Upukerora Formation, Maitai Group, in western Otago and northern Southland

Author:

Pillai, DDL

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Abstract:

Mid Permian Upukerora Formation, predominantly a coarse clastic unit, lies at the base of the Maitai Group in western Otago and northern Southland. It is characterized by polymict volcanogenic breccias and conglomerates and also contains, sandstones, siltstones and atomodesmatinid limestone. The formation lies in depositional contact on the Early Permian Dun Mountain 'ophiolite and represents a transition from dominantly volcanic to dominantly sedimentary processes.
Upukerora Formation stratigraphy is documented in detail in a belt extending 120 km along the Dun Mountain ophiolite belt from the Bryneira Range in the north to the Gyzeh in the southeast. The thickness of the formation varies widely from less than 40 m in the Bryneira Range to 750 m in East Eglinton. Distinctive red and green polymict breccia and conglomerate beds typify the formation. Fine grained spilitic clasts dominate over all other breccia clast lithologies. Other clast types are locally abundant, e.g. siltstone clasts at the Gyzeh, plagioclase phyric basaltic-andesite clasts in the area between Bald Hill and the East Eglinton and purple keratophyre clasts at Serpentine Saddle. A new unit, the Heu siltstone member, composed of thin bedded sandstone and mud stone couplets, is informally proposed and described from the top of the Upukerora Formation in the Annear Creek-East Eglinton area.
Interbedded fossiliferous strata and hyaloclastite indicate a submarine origin for the formation. Clast rounding is weakly developed suggesting an origin below wave base. Tabular shaped breccia beds lie parallel to interbedded packets of thinly bedded sandstone and mudstone turbidites suggesting that these sediments are redeposited and not primarily talus deposits below submarine scarps. The predominance of clast supported breccia and conglomerate beds and generally low proportion of mud argues against a matrix rich debris flow origin. Alternatively breccias may have been transported by a type of marine hyper-concentrated flood flow.
New provenance data from modal sandstone compositions, sandstone geochemistry, breccia clast petrography and geochemistry and detrital grain mineralogy indicate derivation from mafic igneous rocks of the underlying ophiolite along with some intermediate-silicic input from a volcanic arc. In addition, basaltic-andesite to andesitic rocks of the underlying upper part of the Livingstone Subgroup (uppermost unit of the Dun Mountain ophiolite) are documented, and an arc related origin is proposed. Synthesis of these data and recently available data from other sources suggests deposition of the Upukerora Formation and at least part of the underlying Dun Mountain ophiolite in a back-arc environment.

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vii. 147 p. ill. (some colour), 30 cm.

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OU geology Identifier:

1989Pillai

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Location (WKT, WGS84):

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http://download.otagogeology.org.nz/temp/Abstracts/1989Pillai.pdf

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Citation

Pillai, DDL, “Upukerora Formation, Maitai Group, in western Otago and northern Southland,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/234.

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