Higgs, K.E.; Strogen, D.P.; Munday, S. 2023 Rift-related sedimentation patterns and sandstone provenance in the Aotea Basin. Part 2: mineralogy and provenance. Lower Hutt, NZ: GNS Science. GNS Science report 2022/45. 96 p.; doi: 10.21420/S6B1-PJ55
Abstract
The main aim of this study was to investigate how the composition and provenance of early basin fill in Aotea Basin changes with respect to Cretaceous tectonics and basin evolution. To date, only three wells have been drilled in Aotea Basin, all of which are included in the study (Romney-1, Hoki-1, Tane-1). An initial report (Part 1) addresses Cretaceous stratigraphy with an aim to build a stratigraphic framework (Higgs et al. 2022); this second report (Part 2) documents the chemical and mineralogical features with a discussion on source-to-sink composition. This report presents data analysis and interpretation of downhole bulk-rock geochemistry and downhole bulk-rock (QEMSCAN) mineralogy from the Cretaceous intervals at Romney-1 and Hoki-1, respectively. Raw data are from Chemostrat (2017) and Fugro-Jason (2011) and have been integrated with available thin section and detrital zircon U–Pb geochronology data. Datasets from Romney-1 are from the Romney Formation and North Cape Formation (S_K80–85 and S_K90–96a, respectively); geochemical data does not extend into the older syn-rift interval and therefore comparisons cannot be made between the older Zealandia rift phase of basin development (S_K30–S_K50) and the younger West Coast – Taranaki rift phase (S_K80–S_K96). Data from Hoki-1 are from the North Cape Formation (S_K90b–96b). We have also reviewed available QEMSCAN and thin section data from spot samples at Tane 1 and evaluated all datasets with respect to the stratigraphic framework described in Higgs et al. (2022). The oldest samples from the northern well, Romney-1, are from the Merino Member, Romney Formation (S_K80), which are interpreted to represent first-cycle sands with a plagioclase-rich composition. The geochemical signature of cuttings through the S_K80 at Romney-1 is indicative of a fairly consistent provenance, largely from the Median Batholith (probable Darran and Separation Point suites). Overall, both the elemental and petrographic data at Romney-1 support a stratigraphic change from more plagioclase-rich sands with higher illite/mica and siderite in the Merino Member S_K80 to slightly more K-feldspar rich sands with a higher dolomite content in the Coopworth Member S_K85. However, sandstone cuttings from S_K85 have a more variable geochemistry compared to the older S_K80 Merino Member strata, most of which is interpreted to reflect a locally strong diagenetic overprint in the coastal facies. Sandstones occurring below and above the K_90 sequence boundary (SB) at Romney-1 are characterised by very low Na concentrations, which are shown to be due to extensive plagioclase alteration and locally associated concentrations of the secondary alteration products kaolinite and carbonates. These locally high-porosity samples occur below an interval of volcanic rocks within the lowermost North Cape Formation (S_K90a), which may also have provided a source of sediment through parts of the overlying strata. The volcanics are characterised by a very distinctive geochemical signature indicative of a plagioclase- and heavy-mineral-rich, quartz- and K-feldspar-poor composition. Sandstones from above the volcanics at Romney-1 display a relatively consistent geochemical signature, whereby deposition during marine transgression has resulted in the mixing of sediments from different sources. Most of the cuttings samples from this interval (S_K90–S_K96) have high S and P contents and are relatively enriched in K, Fe, S, P and certain trace elements (e.g. U, Rb, Cs, Th) and relatively depleted in Ca, Na, Sc/Y and Ti/Nb compared to sandstone from the older Romney Formation. These data are interpreted to reflect an up-hole facies change to more open marine, offshore deposits (e.g. higher S, Cs) in conjunction with a slightly higher sediment contribution from granitic source rocks (e.g. Karamea Suite biotite granite). A change above the K90 SB to higher P and light rare earth elements might also be indicative of variable input from Cretaceous volcanics throughout the lower North Cape Formation (S_K90). Mineralogical data from the southern well, Hoki-1, are broadly consistent with results from Romney-1 and suggest that the S_K90–96 (North Cape Formation) has a dominant quartzo-feldspathic composition. Immature (plagioclase-pyroxene) sediment at this well is interpreted to represent significant primary derivation from a relatively mafic-rich granitoid, possibly with some input from Cretaceous volcanics. The Separation Point Suite is likely to be the dominant source, with subordinate input from the Karamea Batholith and Western Province metasedimentary rocks. This provenance is consistent with wellsite proximity to the Buller/Takaka terranes and detrital zircon U–Pb age spectra from equivalent samples in the nearby Tane-1 well. A subtle shift in provenance is interpreted across the Cretaceous–Paleocene boundary based on a change in the heavy mineral suite, with the overlying Turi Formation relatively depleted in pyroxene and dominated by hornblende and apatite. Some temporal variations in sandstone composition are also observed through the Cretaceous section at Hoki-1, but these are primarily interpreted to be related to changes in lithofacies, depositional facies and diagenesis. Broadly, the younger S_K96 sequence at Hoki-1 (excluding S_K96 Sand) is characterised by a higher abundance of plagioclase feldspar, biotite, pyroxene, garnet and rutile compared to the older S_K90 sequence. This is not interpreted to reflect a significant shift in sediment source but instead is thought to be related to facies, with better preservation of unstable grains (plagioclase and mafic heavy minerals) in the overall finer-grained, more distal and clay-rich marine facies. Overall, results from this study indicate that the composition of Cretaceous deposits in the Aotea Basin is likely to be strongly related to the proximal basement geology. This may have evolved to include sediment derived from more mixed sources over time, tapping into hinterlands further afield during maximum regression in the S_K85. The start of marine transgression above the K90 SB subsequently led to more consistent sandstone compositions due to mixing of sediment in the offshore environment. There are no significant changes in the type of rock-forming minerals through the Cretaceous interval in the Aotea Basin. This is mainly thought to be due to primary derivation from granitoids, which are typically difficult to distinguish from bulk-rock geochemical or mineralogical data. The most significant differences observed in sandstone composition relate to Na (and plagioclase) depletion, most of which has been shown to have resulted from diagenetic alteration. Preferential alteration has occurred at or in close proximity to sequence boundaries (e.g. K90 SB) and is associated with secondary mineral precipitation. Based on these results, the main factors controlling significant shifts in present-day geochemistry and mineralogy are therefore interpreted to be related to lithology, depositional facies and secondary alteration (auth)
