Stratigraphy and sedimentology of Pliocene limestones, Wairoa district, northern Hawke's Bay

This project documents the sedimentary geology of a shallow-marine, limestone-bearing Pliocene succession (Mangaheia Group; up to 1.5 km thick) within the Wairoa Syncline, northern Hawke’s Bay. The primary focus is three cool-water “Te Aute type” limestone units in the succession – the early Opoitian age Opoiti Limestone (5-10 m thick), the late Opoitian age Whakapunake Limestone (30-100m thick) and the Waipipian age Tahaenui Limestone (10-30 m thick) – that form locally spectacular outcrops in the Wairoa district. Lithostratigraphically, the limestones are given formational status while the encasing siliciclastic sandstones and mudstones are lumped into a newly defined Wairoa Formation which can be informally classified as A, B, C or D depending on position with respect to the three limestone formations. Three field lithofacies groups – limestone (L), sandstone (S) and mudstone (M) – are erected, and variability within these groups is recognised by subdivision into three lithofacies types, L1-3, S1-3 and M1-3. The massive Opoiti Limestone (35-65% CaCO₃; L3 and S2) comprises poorly sorted, variably sandy, barnacle, epifaunal bivalve and locally brachiopod debris well-cemented by low-Mg calcite (LMC) (micro)sparite and micrite. Opoiti Limestone is a bioclastic quartzofeldspathic arenite to biosparrudite, occasionally dolomite-bearing. Occasional calcite infilled biomoulds are testament to former aragonite molluscan skeletons. Opoiti Limestone unconformably overlies Wairoa Formation A (2-200 m thick), and grades up into differentially cemented mudstone of Wairoa Formation B (700-800 m thick) which is unconformably overlain by Whakapunake Limestone. The late Opoitian Whakapunake Limestone (up to 85% CaCO₃; L1, L2 and S1) comprises (cross) bedded and differentially cemented interbeds of fossiliferous and more siliciclastic-rich facies. The limestones are typically poorly sorted bivalve-barnacle biomicrite to biosparrudite cemented by variable amounts of fringe and later equant LMC (micro)spar, sometimes including abundant zoned stoichiometric dolomite rhombohedra. Fabrics are moderately open to tight with some conspicuous limonitised glauconite. Whakapunake Limestone passes conformably up into massive calcareous mudstone of Wairoa Formation C (latest Opoitian; >10 m thick). A shallow angular unconformity separates the Whakapunake Limestone and/or Wairoa Formation C from the overlying well-cemented flaggy Tahaenui Limestone (50-80% CaCO₃; L1), a bivalve-barnacle biosparite to biosparrudite with some neomorphosed former aragonite skeletons and a moderately open fabric. In this limestone early formed dull luminescent isopachous fringing cements (host specific) precede variable pore occlusion by dirty LMC (micro)spar and/or micrite. The three limestone formations accumulated on the eastern side of a narrow forearc basin seaway (Ruataniwha Strait) within the Hikurangi subduction complex atop and about antiform upthrust ridges in an active convergent margin setting. Tectonics was the main control on the location and mainly lensoidal geometry of the limestone units, while Pliocene glacioeustatic sea-level fluctuations played a secondary role. Cross-bedded and interbedded limestonesandstone facies attest to deposition under the influence of strong tidal flows and repeated storm events that produced high-energy hydraulic conditions and reworking of variably mixed terrigenous-skeletal sands in the shallow (