Paleoenvironment of the Gore District

Tam, Cecilia W. S. (Cecilia Wing Sze)

This project undertakes a study of the Tertiary sediments of the Gore District, in the south of the South Island, New Zealand. Areas studied include the Copland Pit, Chatton, Balfour, Pomahaka, and Freshford localities. Tertiary sediments within the study area were analyzed to reconstruct the region paleoenvironment. These sediments include the Chatton Formation of the Copland Pit and Chatton area, Balfour Limestone, Pomahaka Estuarine Beds, and the Freshford Sandstone. Ages of these sediments were determined to be Duntroonian, indicated by biostratigraphic correlation of fossils. Lime concentration, grain-size analysis, fossil identification and diversity has been carried out, to interpret the paleoenvironment of each individual area. A paleogeography and reconstruction is inferred by this study for these five areas during the Duntroonian. The depth paleoecology of the Chatton Formation, in the Copland Pit, is inferred to be shallow marine (<30m), with the depositional environment a very fine sandy bottom situated in a relatively high-energy environment. While at Chatton, this sandstone with its storm-concentrated beds, indicates a shallower marine depth (<20m) than that at the Copland Pit, in an environment that was situated in the vicinity of the storm base. The depositional environment and water activity is inferred to be similar to that of the Copland Pit. The Balfour Limestone, situated at Balfour, indicates a depositional environment of inner to mid continental shelf. The depositional environment was a fine sandy bottom, within an increasing high-energy environment, with little input of terrigenous material. Pomahaka was an estuary (~0-1 Om) and the depositional environment was a muddy bottom. Freshford was a very shallow marine (<20m) and the depositional environment was of a fine sandy bottom. The terrigenous sediments from each locality appear to have been eroded from proximal local basement rocks. The Chatton Formation, in the Copland Pit, was derived from the nearby Murihiku Terrane; the Chatton Formation in Chatton was eroded from the nearby Maitai Terrane; while the Balfour limestone had little sediment input from the Murihiku Terrane. Interbedded relationship between the Gore Coal Measures and the Chatton Formation suggests several episodes of marine transgression and regression having occurred in the study area. The paleo-water temperature is inferred to have been~ 19-23°C, based on interpreted fossil ecology. The Copland Pit, Chatton, and Freshford localities were situated close to the paleoshoreline, while Balfour was located in an open sea and middle shelf environment. Pomahaka is interpreted to have been an estuarine environment at the mouth of a meandering, low energy river that flowed from east to west or alternatively, north to south. Topographic highs are inferred to have existed to the east of the Copland Pit, Chatton, Freshford, and Pomahaka localities, with open seas to the west or southwest.

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

2001Tam