Basin Analysis of the Porter Group, Castle Hill Basin, Canterbury: Implications for Oligocene Tectonics in New Zealand.

A basin analysis of the Oligocene Porter Group rocks in Castle Hill Basin, Canterbury, was completed. The Porter Group contains the Coleridge Formation which comprises a lower sandstone unit and an upper micritic limestone unit, and the Thomas Formation which consists of biosparite limestone and interbedded tuffs. Basin analysis provided evidence that the Coleridge Formation lower sandstone unit was deposited in an inner shelf setting based upon its moderate sorting, large grain size range, laterally continuous geometry and lack of bedforms due to intense bioturbation. The upper micritic limestone is a mid shelf deposit composed of micrite and minor clastic grains. Provenance analysis has classified the lower sandstone unit as a quartz arenite. Both metamorphic and plutonic source areas are likely for the sandstone, along with reworked grains from underlying Formations based on QFL, SEM-CL, heavy mineral and glauconite analysis. The Thomas Formation limestone is a typical New Zealand cool water biosparite deposited on the inner shelf as a result of storms and debris flows, with the upper cross-bedded limestone lithofacies being reworked by currents in shallow water. Petrographic data showing multiple stages of diagenesis at the upper contact of the Thomas Formation provides evidence for a major tectonic event. The interbedded tuffs are a result of basaltic marine volcanism on the inner to mid shelf. The tuffs are reworked and deposited by turbidity current, debris flow and storms. Analysis of a dike within the Thomas Formation volcanics showed a weakly alkaline geochemical signature that is indicative of volcanism related to extension. A regional synthesis compared the Porter Group rocks in Castle Hill Basin with Oligocene rocks in North Canterbury, West Coast and North Otago. Oligocene quartz-rich sandstones are found in Castle Hill Basin, Harper Valley, Avoca and Culverden while micritic limestone is found on the East Coast from Marlborough to Otago. Oligocene basaltic volcanics interbedded with limestone and karst unconformities are found in Castle Hill Basin, Culverden and Otago. Normal faulting may be responsible for thickness variations and several regional karst unconformities in the eastern South Island. Plate reconstructions based on sea floor magnetic anomalies also suggests the New Zealand region was tectonically active during the Oligocene. Mounting evidence, including Eocene-Oligocene faulting and volcanism in the South Island, suggests that New Zealand may not be best described as a passive margin during the Early-Mid Tertiary