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

Abstract

The Waikari Formation and Mount Brown Formation of North Canterbury represent inner to outer shelf deposits of siltstones, sandstones and limestones. The aim of this thesis was to integrate field observations, qualitative macrofossil and trace fossil data, and quantitative foraminiferal data to determine biostratigraphy, palaeoenvironments and reconstruct the palaeogeography of the Waipara/Waikari area through the Early Miocene. Multivariate data analysis of foraminifera data using Bray-Curtis two way and detrended correspondence analysis was 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 Gowan Hill Sandstone. A key feature of the Pahau Siltstone is the high glauconite content and heavy bioturbation caused by the trace Zoophycos. The Scargill Siltstone is recognised by the lower glauconite content compared to the Pahau Siltstone and a change in dominant trace to Ophiomorpha. The Gowan Hill Sandstone contains light grey siltstones and brown fine sandstones. The faunal component of the Gowan Hill Sandstone includes brachiopods, bryozoa and bivalves; and is distinguished by higher faunal component compared to the two other members of the Waikari Formation. Multivariate analysis indicated that the Waikari Formation was deposited in mid to outer shelf water depths. The Mount Brown Formation is dominated by yellow sandstone and also contains the Whiterock Limestone, the discontinuous Onepunga Shell Beds, North Dean Limestone and Red Crag Limestone members that are late Otaian to Altonian in age. The Whiterock Limestone is a bryozoan rich limestone of Otaian age. Celleporaria papillosa are abundant along with branching bryozoans, and the abundance of bryozoa in the limestone is typical of a cooltemperate environment. The Onepunga Shell Beds are lenses of molluscan dominated cast limestone that was deposited in a mid-shelf environment that is Altonian in age. The North Dean Limestone is characterised by cross bedding and is Altonian in age. The limestone represents a higher energy depositional environment in inner shelf water depths with non crossbedded units from the midshelf. Foraminifera identified in the North Dean Limestone include Elphidium crispum crispum and Amphistegina sp, typical of inner to mid shelf environments in a warm subtropical, temperate depositional environment. The Red Crag Limestone members are Altonian in age and are characterised by brachiopod and molluscan fauna. Red Crag Limestone 1 has limited fauna occurs discontinuously. Red Crag Limestone 2 is characterised by brachiopod and molluscan fauna. The brachiopods are more dominant in this unit. Red Crag Limestone 3 reflects subtle changes from Red Crag Limestone 2 with molluscan fauna being more prominent. Overall, the Waipara-Waikari region, North Canterbury experienced a shallowing from outer shelf to mid shelf environment during the Otaian stage reflected in the Waikari Formation deposits, and mid to inner shelf depositional environment during the Altonian stage, reflected in the Mount Brown Formation deposits.