Pliocene‐Pleistocene (Nukumaruan) lithostratigraphy of the Tangoio block, and origin of sedimentary cyclicity, central Hawke's Bay, New Zealand

Abstract

Abstract Eleven formations are defined within the Pliocene‐Pleistocene Petane Group in a 450 km2 uplifted area of central Hawke's Bay (Tangoio block). In ascending order these are: Waipunga Formation (new), Esk Formation, Tutira Formation (new), Aropaoanui Formation (new), Darkys Spur Formation (new), Mairau Formation (new), Tangoio Formation, Te Ngaru Formation, Waipatiki Formation, Devils Elbow Formation, and Kaiwaka Formation. Petane Group strata are cyclothemic and alternate between coarse‐grained lithofacies (greywacke gravel, siliciclastic and carbonate sand/ sandstone and bioclastic limestone) ascribed to nonmarine and inner shelf depositional environments, and sandy/muddy‐silt ("papa") ascribed to mid‐shelf deposition. The Petane Group is characterised by remarkable lateral continuity of formations, contrasting with conspicuous vertical changes. This stratigraphic style is consistent with glacio‐eustatic sea‐level fluctuations as proposed by Vella and Beu & Edwards, whereby coarse‐grained lithofacies were deposited during sea‐level low stands and fine‐grained lithofacies were deposited during sea‐level high stands. Foraminifera extracted from low‐stand lithofacies display opposite ä18O signatures to that predicted for glacio‐eustasy, a result of synsedimentary seawater dilution during sea‐level low stands, and/or isotopic exchange during meteoric diagenesis. Stratigraphic and sedimentary criteria indicate that the Petane Group was deposited under unique circumstances of rapid basinal subsidence and a high rate of sedimentation. This offers an explanation for the general rarity of cyclothemic successions in the Pliocene‐Pleistocene stratigraphic record.