Biostratigraphy of Kora-4 offshore well, North Taranaki

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Strong, C.P.; Hayward, B.W. 1993 Biostratigraphy of Kora-4 offshore well, North Taranaki. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 93/24 42 p.

Abstract: Kora-4 offshore petroleum exploration well, located in north Taranaki, was drilled to a total depth of 3500 m during August and September 1988. Bottom-of-hole lies within rocks of Porangan (mid Eocene) age. Analysis of foraminifera from 28 cuttings and 12 side wall core samples provided the following biostratigraphy: 0-640 m undated; 640-1000 m Nukumaruan-Castlecliffian; 1210-1390 m Waipipian-Nukumaruan; 1510 m Waipipian; 1600-1650 m late Opoitian-Waipipian; 1680-1681 m late Opoitian; 1687.5 m Waiauan-early Tongaporutuan; 1685-1920 m volcanics; 1930 m Clifdenian; 1931.5-2100 m late Altonian; 2289.5-2412 m mid Altonian; 2500 basal Altonian; 2635 m basal Altonian-late Otaian; 2750-2876 m Otaian; 2905-2970 m early Waitakian; 2982-3030 m Duntroonian; 3090 m late Whaingaroan-Duntroonian; 3119-3129 m early Whaingaroan; 3173-3216 m Runangan; 3258 m Kaiatan-Runangan; 3327-3453 m Bortonian; 3500 m Porangan. A time gap is present between the top of the volcanics (Waiauan-early Tongaporutuan) and the overlying sediments (late Opoitian) at 1685 m. A small time gap could also be present between the bottom of the volcanics and the underlying sediments (early Clifdenian) at 1920 m. The late Waitakian is not well defined, and may be represented by a condensed interval. The foraminiferal assemblages indicate mid bathyal depths from Porangan through to Waiauan-early Tongaporutuan at the top of the volcanics. The site shallowed to upper bathyal depths by the late Opoitian and rapidly to outer shelf depths during the Waipipian. The Kora-4 site has remained at about outer shelf depths since the Waipipian. The subsidence and tectonic curves derived by geohistory analysis, suggest rapid early Paleogene tectonic subsidence followed by very gentle tectonic subsidence (greater actual subsidence due to loading) through to the Duntroonian. Actual and tectonic subsidence increased from Waitakian through to early Clifdenian. This was followed by a period of considerable uplift (c. 700m) during the Lillburnian-Waipipian (most rapid in Waipipian). Further rapid subsidence initiated in the Waipipian continues through to the present day. (auths)