Facies analysis, source rock geochemistry, tectonic evolution, and sequence stratigraphy of the Greymouth Basin, South Island, New Zealand. (2020)
Type of ContentTheses / Dissertations
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsMaitra, Mrinmoyshow all
The Greymouth Rift Basin is part of the Late Cretaceous to Early Palaeocene West Coast-Taranaki Rift System in New Zealand that includes the petroleum producing deeply buried Taranaki Basin. The Greymouth Basin comprises alternating fluvial and lacustrine deposits with an abundance of high-quality coals and coaly mudstones. These sedimentary rocks, unlike most parts of the rift which are deeply buried and only available via seismic analysis, are accessible in outcrops and through extensive drill cores. This research aims to develop tectonic-sedimentary models and sequence stratigraphic framework to understand the evolution of the Greymouth Rift Basin from Late Cretaceous to Early Palaeocene as well as to infer the petroleum potential of the lacustrine deposits. The results can be used as an analogue for deeply buried Late Cretaceous sedimentary basins in New Zealand, including hydrocarbon producing Taranaki Basin.
The sedimentary facies analyses of the Greymouth Basin indicate that the north-western side of the basin was dominated by alluvial fans alternating with fan deltas whereas the southern and eastern sides of the basin were dominated by meandering rivers and floodplains alternating with muddy low gradient deltas. Deep, organic-rich, thick lacustrine facies were deposited in the centre of the basin commonly replaced by thick, low ash, mires during alluvial phases. The overall facies distribution suggests a half-graben geometry for the Greymouth Basin with the primary basin-bounding fault located to the north-west. This contradicts with previous tectonic models of the Greymouth Basin where a basin bounding fault was postulated on the eastern side of the basin based on isopach maps of the coal-bearing strata from drill cores.
Better mapping of shoreline and shallow subaqueous facies has led to revised isopach maps and cross- sections allowing interpretation of the basin’s tectonic development through time. The Greymouth Rift Basin widened through time as indicated by westward stepping of the basin bounding fault and younger basal bounding surface with ever younger sediments sitting directly on the basement. The basin’s sedimentary fill also thickened through time as indicated by the thickening of the conglomerates and lacustrine mudstones up-section. Results show that the oldest alluvial-lacustrine phase of the Jay and Ford members records small isolated lakes separated by alluvial fan-fan delta conglomerate facies marking the locations of small, discontinuous normal faults. As the basin widened and deepened through time, new faults formed to the west, small intra-basinal faults became inactive or amalgamated to form a major basin bounding fault. This major border fault controlled the subsidence of the late syn-rift phase of the basin. Applying the information about the evolution of the Greymouth Basin, it can be concluded that the West Coast-Taranaki Rift basins experienced the same basin development history initiating from small sub- basins that widened and deepened through time from the amalgamation of active normal fault segments.
Sequence stratigraphic analysis of the Greymouth Basin indicates that a complete depositional sequence likely records episodes of increased subsidence rates relative to sediment supply that increases the accommodation condition. This was followed by a longer period of tectonic quiescence when decreased subsidence rates in the basin permitted deltaic systems to prograde into the lake eventually completely infilling it and transitioning to alluvial systems across the basin. The onset of each phase of more rapid subsidence in the Greymouth Basin was marked by a sequence boundary. The sequence boundaries correlate with subaerial unconformities as the basin widened due to the progressive creation of new faults to the northwest. The results indicate that the cyclic variation of the alluvial-lacustrine deposits was associated with the rift basin development where the primary driving force was episodic tectonic activity through time compared to other dominant factors like climate variation or outflow of streams.