Pliocene limestones and their scallops : lithostratigraphy, pectinid biostratigraphy and paleogeography of eastern North Island Late Neogene limestone

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Beu, A.G. 1995 Pliocene limestones and their scallops : lithostratigraphy, pectinid biostratigraphy and paleogeography of eastern North Island Late Neogene limestone. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 10; New Zealand Geological Survey paleontological bulletin 68 iv, 243 p.

Abstract: Bioelastic limestone previously mapped as ''Te Aute limestone'' occurs very widely in eastern North Island, from the Gisborne district to South Wairarapa, over a longitudinal distance of 375 km and a width of up to 60 km, i.e., limestone comprises about 3000 km2 of the surficial rocks. Pectinid biostratigraphy of limestone beds, combined with foraminiferal and molluscan biostratigraphy of the interbedded siliciclastic rocks (largely mudstone), show that rock previously mapped as Te Aute limestone is a recurrent lithofacies of no formal lithostratigraphic significance, ranging in age from Tongaporutuan (Late Miocene) to late Nukumaruan (earliest Pleistocene). Te Aute lithofacies limestone, in the strict sense, is a coarse, porous, cream to yellow, relatively pure, bioclastic limestone compositionally dominated by (in decreasing order) barnacle plates, calcitic bivalve fragments, and bryozoa, and limited in age to the late Opoitian to early Nukumaruan stages (Early to latest Pliocene). Hawke's Bay Group (of Hochstetter 1864) is adopted in place of Te Aute Group for Opoitian to early Nukumaruan rocks of central Hawke's Bay. Other limestone formations are assigned to Petane, Napier, Poporangi, Mangamaire and Onoke Groups. Forty five limestone formations and 12 members are described throughout eastern North Island; 27 of these are new (one being proposed informally) and five other new formations are proposed in Petane Group. Limestone distribution is displayed on a new map at 1:250 000 (three sheets, from East Cape to South Wairarapa). In a new Pliocene pectinid biostratigraphy. Phialopecten ongleyi (Opoitian), Kaparachlamys mariae (type species; Nukumaruan) and T. katieae n.sp. (Mangapanian) are referred to the new genus Towaipecten. Phialopecten marwicki occurs in both Opoitian and Waipipian stages, increasing gradually in size with time (the stage boundary is defined at the point where P. marwicki reaches a maximum height of 115 mm). P. thomsoni is restricted to Mangapanian rocks. The early Nukumaruan P. triphooki occurs as several ecomorphs, two of which are common and widespread: P. triphooki (sensu stricto) is limited to Nukumaruan Te Aute lithofacies, whereas P. ''narrow-ribbed triphooki is limited to aragonitic shellbeds. The primary datum adopted for base Nukumaruan is the appearance of Zygochlamys delicatula. Apart from Z. delicatula, Pliocene index taxa all belong in Tribe Palliolini; their phylogenetic classification and evolution are briefly reviewed. A palinspastic map of eastern North Island at 2.5 Ma, drawn as a precursor to constructing paleogeographic maps, restores dextral offset across the North Island shear belt (NISB) by 105 km (an average offset rate of 42 mm/year). Restoration of tectonic offsets in the NISB is based on the assumption that the two types of early Nukumaruan limestone, each with a discrete form of Phialopecten triphooki , originally were deposited in separate environments that have bee n displaced by dextral strike-slip motion. During early Nukumaruan time (2.5 Ma) the southern North Island was much less elevated and much more widely open to the south than it is now. Scinde Island Limestone and other basal Nukumaruan formations of Te Aute lithofacies were deposited on both sides of a Ruataniwha strait, which was open from northern Wairarapa into the present Hawke Bay. In a coeval, widely open basin encompassing the present Wanganui and South Wairarapa ''basins'' and lying across the present east coast highlands, terrigen-rich aragonitic limestone was deposited in shallow areas (including the rising Tararua-Ruahine shoal) during the transgressions between glacial sea-level minima and interglacial sea-level maxima. Hawke Bay is a large pull-apart structure, which only began to open in its present form after propagation of the Wairarapa Fault zone at 2.5 Ma. During Mangapariian time (c. 2.8 Ma), all of southern North Island was submerged. Te Onepu/Te Waka limestone was deposited on both sides of the Ruataniwha strait, while strong currents over the Wanganui-Wairarapa sill eroded away some earlier Mangapanian rocks and prevented limestone deposition. The congruence of Mangapanian and early Nukumaruan paleogeographies shows that dextral offset did not commence on the Wairarapa-Poukawa fault zone until 2.5 Ma. A sea-level control on periods of limestone deposition appears at least as likely as a tectonic one. Limestone appears to have been deposited in eastern North Island during four Pliocene third-order sea-level cycles and during early postglacial transgressions of at least four Nukumaruan sixth-order cycles. Limestone porosity and permeability data show (as might be expected) that the formations most suitable to act as hydrocarbon reservoirs are the younger (Waipipian to early Nukumaruan) and purer Te Aute lithofacies units of central and southern Hawke's Bay: Awapapa, Rotookiwa, Te Onepu, Pakipaki, and Scinde Island limestones. (auth)

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