Contrasting carbonate depositional systems for Pliocene cool‐water limestones cropping out in central Hawke's Bay, New Zealand

Abstract

Pliocene limestone formations in central Hawke's Bay (eastern North IslandSepNew Zealand) accumulated on and near the margins of a narrow forearc basin seaway within the convergent Australia/Pacific plate boundary zone. The active tectonic setting and varied paleogeographic features of the limestone units investigated, in association with probable glacioeustatic sea‐level fluctuations, resulted in complex stratigraphic architectures and contrasting types of carbonate accumulation on either side of the seaway. Here, we recognise recurring patterns of sedimentary facies, and sequences and systems tracts bounded by key physical surfaces within the limestone sheets. The facies types range from Bioclastic (B) to Siliciclastic (S) end‐members via Mixed (M) carbonate‐siliciclastic deposits. Skeletal components are typical cool‐water associations dominated by epifaunal calcitic bivalves, bryozoans, and especially barnacles. Siliciclastic contents vary from one formation to another, and highlight siliciclastic‐rich limestone units in the western ranges versus siliciclastic‐poor limestone units in the eastern coastal hills. Heterogeneities in facies types, stratal patterns, and also in diagenetic pathways between eastern and western limestone units are considered to originate in the coeval occurrence in different parts of the forearc basin of two main morphodynamic carbonate systems over time. The depositional system of the eastern (highest accretionary ridge) sector carbonates is balanid‐dominated, detached from a landmass, and associated carbonate factories developed upon and about actively growing thrust‐cored antiforms. By contrast, the depositional setting for the western siliciclastic‐rich limestone occurrences was a continent‐attached platform. Here, moderate to high contents of Mesozoic basement‐derived siliciclastics indicate that carbonates probably formed and accumulated on a narrow shelf regularly provided with terrigenous sediments, and influenced by strong storm and tidal currents, which locally promoted balanid growth. A similar morphological system is envisaged for the youngest cyclothemic formations deposited in the central part of the forearc basin.