Late quaternary transgressive/regressive sequences from Taranaki continental shelf, western New Zealand

Abstract

Interpretation of seismic reflection profiles (3.5 kHz, airgun), and piston core and borehole stratigraphic logs, collected from mid-outer shelf depths (50–130 m) on the Taranaki continental shelf, western New Zealand, have delineated at least 4 alternating late Quaternary seismic and lithologic couplets. Each couplet comprises an acoustically transparent, upper silty unit that overlies a highly reflective sandy unit, characterised by a channelised erosive base. Radiocarbon and biostratigraphic nannofloral dating of selected samples provides chronostratigraphic control for the succession, indicating that the entire sequence is probably late Quaternary in age (< 600 kyr B.P.). Based mainly on nannofossil paleoenvironmental interpretations and seismic reflection characteristics, it is proposed that the silty units represent interglacial highstand deposits and that the intervening sandy units were deposited either as glacial lowstand or transgressive nearshore deposits that developed under rising sea-level conditions. Erosive, irregular basal unconformities were probably developed during relative sea-level fall and represent Type 1 sequence boundaries; there may also have been a component of wave planation during the subsequent transgression. High-amplitude channel-fill seismic units are possibly fluvial and/or paralic sediments, deposited during the sea-level lowstand and/or the ensuing transgression. Tentative correlation of late Quaternary units on the Taranaki shelf is made to the deep-sea oxygen isotope stage curve to provide temporal resolution of the stratigraphic section. Four possible sedimentation models are considered, based on different temporal interpretations of the observed litho- and seismic stratigraphy. Estimated sedimentation rates for each model are compared to existing data-sets as a validation tool. The data are inconclusive, but appear to support scenario 1 which is characterised by moderate average sedimentation rates over the past 300,000 years, typically 0.57 m/kyr, with an anomalous phase of slow deposition (or non-deposition and/or erosion) during stage 6.