New Zealand's floral origins and the Oligocene land crisis

New Zealand land area decreased significantly during the Oligocene (34–23 million years ago), perhaps culminating in total submergence. The 2009-2012 Marsden-funded Oligocene land crisis project looked at a large number of terrestrial late Oligocene to early Miocene fossil localities concentrating on those sites where preservation was good enough to study in detail. Similarly, marine sedimentary rocks of the same age were investigated.

Analysis of the terrestrial plant ecology of localities dated as occurring at the time Zealandia was regarded as having been completely submerged below sea level, 22-25 Ma years ago, proved that diverse plant communities existed. This suggests that parts of Zealandia had always been above sea level and that numerous ecological niches occurred, providing support for a variety of coastal and inland plant communities.

An independent assessment of the existence of 22-25 Ma land masses was obtained from sedimentological and petrological analyses of marine sandstones and limestones within the same time period. Sandstones from the Murchison, Te Anau and Waiau basins indicate that they were derived from sub-aerial erosion of nearby persistent landmasses, rather than being recycled on a shallow marine shelf. The significant non-carbonate content of Oligocene limestones from across New Zealand also supports the presence of an enduring archipelago that acted as a sediment source during maximum submergence.

Main findings
  • No paleobotanical, stratigraphic or petrological evidence was found to indicate that Zealandia was totally below sea level between 22-25 Ma years ago – all evidence located suggested a persistent, well-vegetated landmass or islands.
  • Terrestrial diversity was found to be much higher than at the present day suggesting no environmental stress existed caused by climate or reduced land area. No evidence of a total floral turnover was found.
  • Detailed analyses of some fossil macroplants found at Newvale suggested that trans-oceanic dispersal from Australia, and possibly further afield, has been a continuous process through time. Furthermore, cladistics analysis shows that the presence of a fossil species is not necessarily evidence of the antiquity of any modern clade.
  • All marine sediments studied contained first order sub-aerial, terrestrially derived particles, from a landmass or landmasses with relatively low relief.
  • If Zealandia was ever totally below sea level it probably was not during the late Oligocene-early Miocene time period we investigated (22-25 Ma based on previous published literature).
  • Island refugia appear to have existed throughout the Oligocene and early Miocene allowing the survival of our present day Gondwanan relicts like the tuatara and kauri.
  • Known rates of tectonic movement probably are not fast enough to allow enough time to elapse to cause complete inundation of Zealandia for several million years prior to and after to dramatically change the paleogeography of the localities we studied in detail.

The results do not entirely negate the possibility that complete inundation was prior to the period of time examined, but do add support to the overall interpretation of there being persistent islands during the broad, Late Oligocene maximum marine transgression.

The GNS Science team contributed the above results to introductory text and five papers in a 2014 Special Issue of New Zealand Journal of Geology and Geophysics v. 57 no. 2.

Principal Investigators: Dallas Mildenhall and Nick Mortimer

Associate Investigators:

  • Dr Liz Kennedy, GNS Science
  • Ms Julie Palmer, Massey University
  • Dr Daphne Lee, Otago University
  • Dr Kathleen Marsaglia, California State Unviersity, USA
  • Professor David Cantrill, National Herbarium of Victoria, Royal Botanic Gardens, Melbourne, Australia
  • Dr Michael Knapp, Max Planck Institute for Evolutionary Anthropology, Germany