Geology of the Cardrona Basin

Author:

McDonnell, Michael Peter.

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Abstract:

The late Tertiary stratigraphy of the Cardrona basin, Central Otago, gives evidence for a large river system that drained the Torlesse terrane to the north and flowed through Central Otago. The late-Miocene -Pliocene river gravel sediments in the Cardrona Valley contain up to 80% greywacke clasts. These gravels are located in the middle of the Haast Schist at least 80km from the nearest greywacke outcrop. None of the present drainage in Central Otago carries greywacke into the Cardrona Valley. Late Tertiary stratigraphy of the Cardrona Basin overlies Otago schist basement. The late Tertiary stratigraphy consists of late Miocene Manuherikia Group lake sediments conformably overlain by the Cardrona Group (new). The Cardrona Group consists of two formations, the Tuohys Breccia (new) and the Waiorau Gravel (new). The Tuohys Breccia is a schist dominated sedimentary breccia unit. The Waiorau Gravel is a greywacke rich river gravel. The Waiorau Gravel conformably overlies the Tuohys Breccia. The Cardrona Group is interpreted as being Pliocene in age on the basis of stratigraphy and correlation with similar units in Central Otago. The Tertiary sediments in the Cardrona Basin are overlain unconformably by recent schist dominated landslide and river deposits. Imbrication of pebbles indicates that the river that carried the greywacke into the Cardrona Basin flowed from the north. The petrography and geochemistry of the greywacke clasts show that the clasts are derived from the Torlesse terrane. Evidence from the active Manuherikia River in Central Otago indicate that it is possible to transport greywacke clasts at least 50km across schist without adding a lot of schist to the river sediments. The likely process by which the greywacke is concentrated in a river is by the preferential break up of the weaker schist clasts enabling the schist to be washed away as fine material. The size of the clasts in the river gravel indicates that the river has a flood flow of approximately 200cumecs. The two possible sources for the greywacke clasts in the Waiorau Gravel are the Central Otago ranges to the east of the Cardrona Basin or the Southern Alps near the Landsbrough river.

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103 leaves : ill (some col.), maps (some folded) ; 30 cm.

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2001McDonnell

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POLYGON ((168.942784876315329 -44.936511738108031,168.952541640227139 -44.897916419810521,168.996091232647956 -44.900525858967612,168.989925071073174 -44.938536718380192,168.942784876315329 -44.936511738108031))

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http://download.otagogeology.org.nz/temp/Abstracts/2001McDonnell.pdf

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Citation

McDonnell, Michael Peter., “Geology of the Cardrona Basin,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/383.

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