Journal of the Royal Society of New Zealand abstracts
Petrified wood from the Miocene volcanic sequence of Coromandel Peninsula,
northern New Zealand
P. R. Moore*, R. Wallace**
*Peninsula Research, P.O. Box 120, Waihi, N.Z.
**Anthropology Department, University of Auckland, Private Bag 92019,
Auckland, N.Z.
Petrified (silicified) wood is found throughout the Miocene sub-aerial volcanic
sequence of Coromandel Peninsula. The identification of 160 samples, mostly
collected from geological formations of known age, shows that over the period
from 18-6 Ma the majority of trees within local forests were angiosperms,
particularly southern beech (
Nothofagus spp
.) and casuarina
(Casuarinaceae). A species close to modern
Weinmannia and
Laurelia was also relatively 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
smaller, mainly angiosperm, trees by fire, decay, and abrasion during
transportation within volcanic flows. Charred wood does not seem to have been
silicified.
Analysis of the wood assemblage suggests there were some significant changes
in the relative abundance of certain tree species during the Miocene. Kauri
apparently constituted 10-20% of the forest canopy around 13-15 Ma ago, but may
have almost disappeared by latest Miocene time. Casuarina increased
dramatically about 9-10 Ma ago, probably as a result of localised destruction
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 between
6 and 7 Ma ago, possibly indicating a change to cooler climatic conditions.
The study has shown that various factors need to be taken into account in the
interpretation of petrified wood assemblages, including sampling bias,
selective preservation, the local derivation of wood (versus the more regional
representation of pollen), and the influence of differing environmental
conditions on forest composition.
Keywords petrified wood; Miocene; Coromandel Peninsula; New
Zealand; volcanic units; silicification; wood anatomy; tree size;
paleovegetation; Nothofagus; Agathis; Phyllocladus;
Laurelia; Weinmannia; angiosperm; gymnosperm; conifer; kauri;
beech; casuarina; podocarp.
(c) Journal of The Royal Society of New Zealand,
Volume 30, Number 2, June 2000, pp 115-130
PDF file of entire paper: medium quality (1844K); (scanned from paper original: notes about this process)
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