Petrified wood from the Miocene volcanic sequence of Coromandel Peninsula,northern New Zealand

Petrified (silicified) wood is found throughout the Miocene sub-aerial volc anic sequence of Coromandel Peninsula. The identification of 160 samples, m ostly collected from geological formations of known age, shows that over th e period from 18-6 Ma the majority of trees within local forests were angio sperms, particularly southern beech (Nothofagus spp.) and casuarina (Casuar inaceae). A species close to modem Weinmannia and Laurelia was also relativ ely common. Conifers, including kauri (Agathis), celery pine (Phyllocladus) , and other podocarps (Podocarpaceae), were subordinate. Only the stems of larger forest tree species, with more durable heartwood, appear to have been preserved, and angiosperm species in general are poorly represented. This is attributed largely to the selective destruction of sm aller, mainly angiosperm, trees by fire, decay, and abrasion during transpo rtation within volcanic flows. Charred wood does not seem to have been sili cified . Analysis of the wood assemblage suggests there were some significant chan ges in the relative abundance of certain tree species during the Miocene. K auri apparently constituted 10-20% of the forest canopy around 13-15 Ma ago , but may have almost disappeared by latest Miocene time. Casuarina increas ed dramatically about 9-10 Ma ago, probably as a result of localised destru ction of the existing forest by volcanic (pyroclastic) eruptions, and rapid colonisation of the devastated area. It had declined to low numbers by the end of the Miocene. In contrast, Phyllocladus was relatively common betwee n 6 and 7 Ma ago, possibly indicating a change to cooler climatic condition s. The study has shown that various factors need to be taken into account in t he interpretation of petrified wood assemblages, including sampling bias, s elective preservation, the local derivation of wood (versus the more region al representation of pollen), and the influence of differing environmental conditions on forest composition.