PETROLOGIC EVIDENCE FOR ISLANDS DURING THE 23-26 MA MAXIMUM MARINE INUNDATION OF ZEALANDIA

New Zealand is part of Zealandia, a continent in the SW Pacific Ocean that is now >90% submerged. From widespread shelf limestones in the onland stratigraphic record, it is known that Zealandia experienced maximum marine inundation (MMI) in the interval 23-26 Ma (latest Oligocene to earliest Miocene). To date, there is a demonstrable lack of nonmarine strata confined to this specific age range, so there is uncertainty as to whether Zealandia was 100% submerged at this time (Landis et al. 2008) or if there was partly emergent land with discrete islands (King et al. 1999).

To test these alternate hypotheses, a new reconnaissance petrologic study of marginal marine to deep marine sandstones from three South Island basins (Waiau, Te Anau and Murchison) was made. Samples range in age from Late Eocene to Early Miocene i.e. from below, within, and above the MMI interval. Sand grains in the examined samples are generally poorly sorted and subangular. Furthermore, petrographic modes and geochemical compositions of the sandstones show specific matches with local basement plutons and terranes. Thus the studied sandstones are not significantly reworked or of mixed provenance, but are interpreted as immature first-cycle clastic deposits.

The results suggest the presence of persistent local sediment sources (island landmasses) adjacent to at least some New Zealand marine sedimentary basins. Thus it appears that, during MMI, Zealandia was not totally submerged. This interpretation has important implications for the vicariance versus dispersal origins (Gondwana cargo versus recolonisation debate) of New Zealand’s modern day flora and fauna.