Palaeoenvironments and biostratigraphy of early Miocene Waikari Formation and Mt Brown beds, North Canterbury, New Zealand.

摘要

The Waikari Formation and Mount Brown Formation of North Canterbury represent inner toouter shelf deposits of siltstones, sandstones and limestones. The aim of this thesis was tointegrate field observations, qualitative macrofossil and trace fossil data, and quantitativeforaminiferal data to determine biostratigraphy, palaeoenvironments and reconstruct thepalaeogeography of the Waipara/Waikari area through the Early Miocene. Multivariate dataanalysis of foraminifera data using Bray-Curtis two way and detrended correspondence analysiswas key to determining original depositional environments.The Waikari Formation consists of blue grey siltstones and brown fine sandstones of Otaian age.In the study area three members were identified; Pahau Siltstone, Scargill Siltstone and GowanHill Sandstone. A key feature of the Pahau Siltstone is the high glauconite content and heavybioturbation caused by the trace Zoophycos. The Scargill Siltstone is recognised by the lowerglauconite content compared to the Pahau Siltstone and a change in dominant trace toOphiomorpha. The Gowan Hill Sandstone contains light grey siltstones and brown finesandstones. The faunal component of the Gowan Hill Sandstone includes brachiopods, bryozoaand bivalves; and is distinguished by higher faunal component compared to the two othermembers of the Waikari Formation. Multivariate analysis indicated that the Waikari Formationwas deposited in mid to outer shelf water depths.The Mount Brown Formation is dominated by yellow sandstone and also contains the WhiterockLimestone, the discontinuous Onepunga Shell Beds, North Dean Limestone and Red CragLimestone members that are late Otaian to Altonian in age. The Whiterock Limestone is abryozoan rich limestone of Otaian age. Celleporaria papillosa are abundant along withbranching bryozoans, and the abundance of bryozoa in the limestone is typical of a cooltemperateenvironment. The Onepunga Shell Beds are lenses of molluscan dominated castlimestone that was deposited in a mid-shelf environment that is Altonian in age. The North DeanLimestone is characterised by cross bedding and is Altonian in age. The limestone represents ahigher energy depositional environment in inner shelf water depths with non crossbedded unitsfrom the midshelf. Foraminifera identified in the North Dean Limestone include Elphidiumcrispum crispum and Amphistegina sp, typical of inner to mid shelf environments in a warmsubtropical, temperate depositional environment.The Red Crag Limestone members are Altonian in age and are characterised by brachiopod andmolluscan fauna. Red Crag Limestone 1 has limited fauna occurs discontinuously. Red CragLimestone 2 is characterised by brachiopod and molluscan fauna. The brachiopods are moredominant in this unit. Red Crag Limestone 3 reflects subtle changes from Red Crag Limestone 2with molluscan fauna being more prominent.Overall, the Waipara-Waikari region, North Canterbury experienced a shallowing from outershelf to mid shelf environment during the Otaian stage reflected in the Waikari Formationdeposits, and mid to inner shelf depositional environment during the Altonian stage, reflected inthe Mount Brown Formation deposits.