Late Quaternary shellbeds, western shelf, New Zealand

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Abstract
Extensive buried shellbeds and shelly zones occur over much of the western shelf between Cape Foulwind and the mouth of the Waikato River. Maximum thickness of shell penetrated by cores is about 2 m, but the deposits may be as much as 5 or 10 m thick. Radiocarbon dates show that shellbeds were deposited during the last marine transgression between about 20 000 and 5000; years ago. During the transgression, but especially during the past 5-6000 years when sea level reached its present position, shellbeds were gradually buried by muddy or sandy deposits. Radiocarbon dates were obtained for specific molluscan genera, for various states of shell preservation, and for different levels in a single core. Dating shows that some beds have experienced intermittent stirring or mixing for periods as long as 7000 years, during which time some shell was added from other areas. Alternatively, dates cast serious doubts on the reliability of radiocarbon dating based on shell material. Late Tertiary and Quaternary deposits exposed near Wanganui provide a record of at least seven earlier transgressions and are believed to provide an onshore analogue of modem shelf stratigraphy. The sea-level curve deduced from the dated shellbeds indicates that the western shelf has been downwarped during the last 20 000 years or so at a rate of from about 2.3 to 2.8 m per thousand years.