Provenance and porosity analysis of the Greymouth Basin, New Zealand.

• Main Author: Steadman, Ryan David
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: University of Canterbury 2017
• Subjects: Greenland; New Zealand
• Online Access: https://dx.doi.org/10.26021/6698; https://ir.canterbury.ac.nz/handle/10092/15157

The coal and lacustrine deposits of the Greymouth Basin have been explored for their economic potential. However, the associated coarse clastic sediments have not been as thoroughly investigated. Thus, there is continuing uncertainty about the provenance of the sediments and tectonic setting of the basin. This study uses conglomerate clast counts, sandstone point counts and geochemical analyses of clasts to examine the provenance of the Paparoa Group. Results show a dramatic eastern vs western lithological difference with conglomerates primarily on the west side of the basin, sandstones on the east, and mudstones inter-fingering both. The clasts encountered were predominantly metasedimentary with granite, hornfels, and rare unusual volcanic clasts. Aplite was recorded in the lowermost conglomerates and faded out with the introduction of granitic clasts in the middle Paparoa Group. Trace element geochemistry on basaltic clasts in the basin shows a tholeiitic composition, a typical rift signature. Geochemistry analysis of the granites was inconsistent with either Rahu or Karamea Suite granites and best fits a new A-type granite, low barium (<5 to 80ppm) and Strontium (18 to 42 ppm), located somewhere offshore. The sandstone porosity was variable ranging from 1% to 37% with grainsize, location and stratigraphic position in the basin. The degree of weathering in the sandstones was also variable with feldspar alteration ranging from minor to major clays (5% to 30%). Provenance and Geochemistry analysis show the sediment sources of the basin changed throughout time with results showing two main sources, an eastern granitic source, likely Buckland granite and the western Greenland Group metasedimentary sources. This contradicts some previous interpretations. Clast counts also show evidence for the un-roofing of a granitic source with the presence of aplite clasts lower in the basin conglomerates replaced by granite clasts stratigraphically higher. The volcanic clasts are evidence of active volcanism in the area which could be attributed to the rift setting. Porosity in the sandstones was variable with some good hydrocarbon reservoir potential. The lack of trap and cap rock in the Greymouth Basin being an issue. The Takutai Basin offshore contains similar sediments and