Geology of the Clinton district


Bishop, David Graham.


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A map and description of the geology of 73 square miles of the Clinton District is presented. The area is underlain by sedimentary rocks of Paleozoic and Mesozoic age with a small area of Tertiary sediments occurring in the northern part of the district.
The oldest rocks are those of the Tuapeka Group, mainly indurated greywackes of the prehnite-pumpellyite metagretwacke facies. The structure is not clear but equal area projections reveal fold axes trending north-north-east and plunging at 45* in this direction.
The younger Paleozoic rocks have been divided into three groups. the oldest, Waipahi, is probably similar in age and content to the Waipahi Group established by Wood (1956) in the adjacent Gore Subdivision. The Arthurton Group corresponds to the lower part of Wood's Arthurton Group, and a new unit, the Popotunoa Group, is established for rocks probably equivalent to the upper part of the original Arthurton Group.
Fossils were found in rocks of the Arthurton Group and included the pelecypod Atomofesma, the brachiopod Lissochonetes, and several gastropods. A new brachiopod, Martinia clintonensis, is described. Most of the species recorded formed part of a shallow water biocoenosis which was exterminated by an influx of volcanic ash.
The three groups comprise a marginal eugeosynclinal sequence of volcanic greywackes and sandstones, shales, tuffs, and conglomerates and, in part, belong to the heulandite and laumontite zones of the zeolite mineral facies.
The structure and stratigraphic relationships of these groups us not absolutely clear and several possible interpretations are presented. A possibility exists that some of the rocks mapped as belonging to the Waipahi Group are actually younger than the Arthurton and Popotunoa Groups. The rocks are thrown into a series of overturned folds trending north-west south-east and are the oldest rocks of the overturned north-eastern limb of the Southland Syncline.
Several steeply plunging minor folds were also recorded, possibly aligned along a north-north-east trending zone. Available evidence suggests that these may be older than those trending north-west south-east.
The oldest Mesozoic rocks in the district have been placed in a new group, the Kuriwao Gorge Group, which is approximately equivalent to the Wairuna Peak Beds of Wood (1956). Two formations are recognised, the Kuriwao (lower) and Waiwera (upper) Formations. From rocks near the base of the Waiwera Formation specimens of a new species of Conchostracan were collected and these are described with four more from Etalian strata at Kaka Point. Preliminary work indicates that they may be useful index fossils in the older Triassic rocks of Southland.
The Kaihikuan Stage conformably overlies the Waiwera Formation but is very thin, and absent at some localities. It in turn is overlain by the Oretian Stage which forms the southern boundary of the district.
The Triassic rocks are conglomerates, volcanic greywackes and sandstones, and tuffs. They fall into the heulandite zone of the zeolite facies. They dip steeply, being sometimes overturned along the northern limb of the Southland Syncline. At Waiwera Gorge they are involved in a subsidiary anticline plunging south-east at approximately 25*.
Overall, the Paleozoic and Mesozoic rocks, with the exception of the Tuapeka Group, belong to the volcanic arenite suite, the nomenclature of which is herein defined. The amount of pyroclastic material supplied to the developing geosynclines progressively increased with the passage of time. Freshwater or estuarine coal measures, equivalent to the Gore Lignite Measures (Wood, 1956) and possibly Bortonian-Knintan age overlie rocks of the Tuapeka Group near the Pomahake River. Kaolinitic clays overlie and underlie a seam of low grade lignite, which at one locality is on fire. A description of the mineralogy of a resultant fused sediment is presented which suggests that temperatures in excess of 1500*C were attained for a short time.
Tertiary and later deformation has been mainly restricted to gentle warping about north-north-east and north-west axes and largely controls the present topography and drainage pattern. Reverse and normal faulting along the Murihiku escarpment probably occurred during the early Tertiary and resulted in the uplifted Triassic block and possibly in other smaller uplifted blocks in the Paleozoic rocks.
Finally a short discussion of the economic geology of the area is presented.

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POLYGON ((169.285327900529865 -46.074607818028326,169.292435549858567 -46.070671287935895,169.294341691818033 -46.067756312621569,169.305953117679024 -46.060392818481759,169.327564445005692 -46.056922574799692,169.346191534901862 -46.046803619578739,169.355349798486742 -46.037570414866785,169.371986992279972 -46.03155131644894,169.388078187088155 -46.033847768151773,169.397735171380049 -46.043083559745654,169.401853368040406 -46.071791361338022,169.389499829812053 -46.247629925799053,169.277877358020419 -46.223207773502679,169.285327900529865 -46.074607818028326))

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Bishop, David Graham., “Geology of the Clinton district,” Otago Geology Theses, accessed October 15, 2018,