A Study of the late Quaternary history of Poukawa Basin

Abstract

A number of Late Quaternary events are recorded in the deposits of the Poukawa basin, which is enclosed, tectonically controlled, and contains a lake surrounded by peat swamp. A drilling programme sponsored by .University Grants Committee, provided information about the sediment beneath the peat. Fieldwork provided details on cover bed stratigraphy around the swamp. Deposits of loess, tephra and alluvial fan material as well as peat, occur within the basin. Below the peat are fine grained sediments of unknown thickness. Eight tephras are recognised in the Poukawa basin; Taupo Pumice, Whakaipo Ash, Waimihia Lapilli, Whakatane Ash, Hinemaiaia Ash, Okareka ash, Kawakawa Tephra and Hauparu Tephra. From youngest to oldest they range in age from 2039±45 years B.P. to C.37,000 years B.P. The Okaraka and Hauparu tephras are recorded for the first time in Hawke's Bay. The Kawakawa Tephra is the most widespread Late Quaternary tephra marker horizon in the North Island, and is used as a datum for basin stratigraphy. The loess found in the basin is mostly non-calcareous and has originated from sources to the west of the basin (greywacke axial ranges). It ranges from a poorly sorted-medium sorted silt with similar grainsize characteristics as loess in other parts of New Zealand. Alluvial fan deposits are calcareous, derived from older deposits within the basin. Using tephras and the base of the peat as time planes, an accumulation rate 1 mm/year, is suggested for the sediments beneath the peat. Land pulmonates (landsnails) are found in the sediments beneath the peat. Their presence is used to imply past vegetation and climatic conditions. Two previously un-recorded species of Laoma. sp. are found. A new species of freshwater gastropod is found in the basin sediments. Pollens in the peat and organic-rich horizons also enabled the past vegetation to be inferred. For the period in time C.40,000 years to C.10,000 years, Poukawa basin experienced cool to cold climatic conditions. These conditions governed the vegetation pattern (shrubland/grassland mosaic) and sedimentation (loess deposition, fan building and fan deposits). Circa 10,000 years, peat formation began indicating gradual warming and moister conditions. Vegetation patterns changed, soil formation began and fan building stopped.