The Sequence Architecture of Mid-Pleistocene (c. 1.1–0.4Ma) Cyclothems from New Zealand: Facies Development during a Period of Orbital Control on Sea-Level Cyclicity

  1. P. L. de Boer2 and
  2. D. G. Smith3
  1. S. T. Abbott and
  2. R. M. Carter

Published Online: 29 APR 2009

DOI: 10.1002/9781444304039.ch23

Orbital Forcing and Cyclic Sequences

Orbital Forcing and Cyclic Sequences

How to Cite

Abbott, S. T. and Carter, R. M. (1994) The Sequence Architecture of Mid-Pleistocene (c. 1.1–0.4Ma) Cyclothems from New Zealand: Facies Development during a Period of Orbital Control on Sea-Level Cyclicity, in Orbital Forcing and Cyclic Sequences (eds P. L. de Boer and D. G. Smith), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304039.ch23

Editor Information

  1. 2

    Utrecht, The Netherlands

  2. 3

    London, UK

Author Information

  1. Department of Geology, James Cook University of North Queensland, Townsville, Q4811, Australia

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 28 JAN 1994

ISBN Information

Print ISBN: 9780632037360

Online ISBN: 9781444304039

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Keywords:

  • sequence architecture of mid-Pleistocene cyclothems - New Zealand;
  • correlation to the mid pleistocene isotope scale;
  • Heterolithic association (nearshore shelf);
  • Well sorted sand association (shoreface);
  • Silty sandstone association (innershelf)

Summary

South Wanganui Basin, located in western North Island, New Zealand, and about 200 × 200 km in area, contains a fill up to 4 km thick of mainly shelf facies sediment of Plio-Pleistocene age. The eastern edge of the basin has undergone gentle upwarping along the adjacent Pacific/Indo-Australian plate boundary, resulting in excellent coastal exposures through late Pliocene to early Pleistocene strata. The markedly cyclic mid-Pleistocene (c.1.1–0.4 Ma) section exposed at Wanganui comprises 10 superposed cyclothems with a total thickness of c.160 m corresponding to fifth (100 ka)- and sixth (40 ka)-order sequences equivalent to odd-numbered oxygen isotope stages 11–31. Each cyclothem comprises three parts: (i) a basal suite of shoreface and innermost shelf sediments containing intertidal and shallow subtidal molluscan faunas and, more commonly, transported shell gravels (type A shellbeds); (ii) a mid-cycle shellbed which includes in situ offshore molluscs in a matrix of muddy fine sandstone or siltstone (type B shellbeds); and (iii) an upper unit of terrigenous siltstone, either bedded and barren of macrofossils, or massive, bioturbated and containing a sparsely scattered in situ fauna, commonly similar to that of the subjacent shellbed. The threefold subdivision of each cyclothem occurs within both fifth- and sixth-order sequences, and corresponds to the transgressive systems tract, mid-cycle condensed shellbed and highstand systems tract of the Exxon sequence stratigraphic model, respectively. Compared with sixth-order cyclothems, fifth-order cyclothems may contain thicker transgressive systems tracts and mid-cycle condensed shellbeds, deeper water faunas in their mid-cycle shellbeds, and deeper water sedimentary facies in their high-stand systems tracts. Because the Wanganui coastal cyclothems were deposited many tens of kilometres east (inshore) from the contemporary shelf-edge, the stratigraphic record at Wanganui contains no marine low-stand systems tract sediment. The section therefore comprises dominantly periods of interglacial deposition, representing the late transgressive and high-stand portions of each sea-level cycle. Glacial periods are represented only at surfaces of marine planation and bioerosion at the base of each cyclothem, which mark the sequence boundaries. Marine ravinement and in situ boring pholad bivalves (type C shellbeds; equivalent to Glossifungites ichnofacies) have removed any former traces of non-marine conditions at the sequence boundaries in the coastal section, but subaerial surfaces (including soils) are preserved inland towards the eastern edge of the basin. The facies architecture of the Wanganui Basin is of compelling interest because (i) the sediments were deposited during a period of known sea-level fluctuation driven by Milankovitch orbital controls, and (ii) the invertebrate faunas used to assist the sequence interpretation comprise living species, i.e. are of known habitat significance. For these reasons, the sedimentary interpretation at Wanganui possesses a degree of certainty which is lacking from sequence stratigraphic studies of ancient, pre-Pliocene sections, for which dating, palaeoecologic reconstruction and the role of sea-level controls are of necessity highly inferential.