Macrofossil biofacies in the late Neogene of central Hawke's Bay: applications to palaeogeography

Abstract

The Late Miocene–Early Pleistocene (Tongaporutuan–Nukumaruan) sedimentary succession in the forearc basin in central and western Hawke's Bay, encompassed by the Tolaga and Mangaheia Groups, comprises a wide variety of variably fossiliferous lithofacies, ranging from non-marine greywacke-derived conglomerates to bathyal mudstone and flysch beds. Thirty molluscan biofacies, inferred to have accumulated in estuarine to outer shelf palaeoenvironments, have been identified and represent both in situ and transported assemblages. Although distributed through the late Neogene succession, most biofacies occur within the Late Pliocene–Early Pleistocene part of the Mangaheia Group. Using the spatial distribution of the different biofacies, we have constructed detailed palaeoenvironmental reconstructions for the Plio-Pleistocene of central Hawke's Bay. The stratigraphic occurrence of particular biofacies has been primarily controlled by relative sea-level positions and variations in sediment input during high-frequency glacio-eustatic sea-level oscillations. The distribution of biofacies reflects proximity to contemporaneous shorelines, localised sources of carbonate sediment or dominance of siliciclastic sedimentation, and records the interplay between tectonic and eustatic drivers of relative sea-level change.