Provenance analysis of the Late Cretaceous to Paleocene Rakopi, North Cape, and Farewell formations, northwest Nelson, New Zealand (2018)
AuthorsSmithies, Sarah Lynetteshow all
The Taranaki Basin of New Zealand first developed in the middle of the Cretaceous as Zealandia rifted from Gondwana. Rift basins were active depocentres throughout the Late Cretaceous to Paleocene when rifting slowed. The only outcrops are located in northwest Nelson in the Pakawau sub-basin, a northeast trending graben bound by the Wakamarama Fault to the southeast and the Kahurangi Fault to the northwest. This study focuses on the provenance of the Late Cretaceous Rakopi and North Cape formations of the Pakawau Group and the Paleocene Farewell Formation deposited within the Pakawau Sub-basin. There are a range of potential basement sources, including the metasedimentary units of the Buller and Takaka terranes, granitoids of the Separation Point Suite and Karamea Suite, volcanics of the Takaka Terrane, and a variety of schists.
The depositional environments of the Rakopi, North Cape, and Farewell formations provides the context for the provenance analysis undertaken in this thesis. Previous studies were reviewed, and additional data added from outcrop description, paleoflow measurements, and measured stratigraphic columns. The oldest unit is the Late Cretaceous Rakopi Formation which contains sandstones, carbonaceous mudstones, siltstones, minor conglomerates, and coal, interpreted as a meandering river and floodplain environment. Locally at the base of the Rakopi Formation is the Otimataura Conglomerate; conglomerates, breccias, and sandstones interpreted as alluvial fans fed from the Wakamarama fault scarps. The Late Cretaceous North Cape Formation conformably overlies the Rakopi Formation and contains sandstones, siltstones, carbonaceous mudstones, thin coal seams, and rare conglomerates. It is interpreted as a marginal marine deposit with sediment contributions from fan deltas prograding from the Wakamarama Fault into a shallow marine embayment. New work on the Paleocene Farewell Formation has led to three lithofacies associations being proposed. Association FM has trough cross-bedded sandstone beds in channels interbedded with thick carbonaceous mudstone units, interpreted as sandy meandering river deposits. Association FS has complex stacked trough cross-bedded sandstone channels sometimes capped by thin siltstone and carbonaceous mudstone beds, interpreted as sandy braided river deposits. Association FG has thick conglomerate beds, sometimes with tabular cross-bedding, interbedded with sandstone and rare siltstone, interpreted as gravelly braided river deposits. The north to northeast paleoflow direction shows drainage down the axis of the Pakawau sub-basin, and flow to the west away from the fault in outcrops adjacent to it.
For all formations, conglomerate composition was analysed by clast counts, followed by comparison of clast petrology and whole-rock geochemistry to potential basement sources. The Otimataura Conglomerate is dominantly composed of local basement. The North Cape and Farewell formations contain abundant metasedimentary clasts and minor vein quartz, likely from the Takaka Terrane Ellis Group; minor granitoid clasts from a mixture of Separation Point Suite intrusives and a previously unknown A-type granite; minor volcanic clasts from the Takaka Terrane; minor intrabasinal sedimentary clasts; and rare schist and chert. The granitoid and volcanic clasts become less abundant and more weathered towards the top of the Farewell Formation.
The composition of the sandstones was investigated by petrographic point counts. The Rakopi Formation Otimataura Conglomerate has litharenites and sublitharenites, with lithics dominated by schist in the southern field area (Paturau River), and sedimentary lithics further north (Pakawau Bush Road), reflecting the local basement composition. The Rakopi Formation (excluding the Otimataura Conglomerate) and the North Cape Formation sandstones are feldsarenites and lithic feldsarenites. The lithics are dominantly metasedimentary and sedimentary, with some volcanics, plutonic fragments, and schist. The Farewell Formation has mostly lithic feldsarenites and becomes more quartzose upsection. It has more metasedimentary and sedimentary lithics, which become more abundant up the stratigraphy.
The whole-rock geochemistry of fifty-four sedimentary samples was analysed by ICP-MS, ICP-OES, and XRF. The geochemistry shows elevated S and P2O5 in the North Cape Formation, indicating a marine-influenced depositional environment. A marine influence is also suggested for the Rakopi Formation by elevated P2O5 content and the presence of rare glauconite. The Rakopi Formation has elevated chemical index of alteration (CIA) values, and high Rb/Sr suggesting the presence of illite/smectite clays, likely diagenetic. The North Cape Formation has very low clays. The Farewell Formation has increasing CIA values and Ga/Rb up-section, suggesting increasing amounts of kaolinite clays likely due to weathering. A more granitic provenance for the North Cape Formation and upper Rakopi Formation is suggested by elevated Na2O and K2O showing the presence of plagioclase, lower Zr indicating less zircon, and higher light rare earth element concentration.
When rifting began in the Late Cretaceous, deposition was highly localised resulting in local provenance of the Otimataura Conglomerate. The remainder of the Rakopi Formation and the North Cape Formation have a mixed provenance of Takaka Terrane metasedimentary and volcanic units, Separation Point Suite, and A-type granites, showing drainage away from the Wakamarama Fault. The Farewell Formation provenance is also dominated by Takaka Terrane metasedimentary units, showing drainage from the Wakamarama Fault, although high degrees of weathering show that fault movement was slowing creating greater residence times for sediments in the system. The composition of the Rakopi, North Cape, and Farewell formations has implications for their hydrocarbon reservoir potential offshore, for example high clay contents in the Rakopi and North Cape formations are expected to decrease their permeability and reservoir potential.