Miocene wood from Kaipara Harbour, New Zealand

Abstract

Silicified woods of probable Altonian age, from south east Hukatere Peninsula, Kaiparatwere studied using techniques including study of thin-sections, scanning electron microscope micrographs. X-ray diffractograms and thermograms from differential thermal analysis. Techniques were often modified to suit the nature of the preservation of the wood. Pyroclastic flows, probably from a northerly direction, carried woods from a broad-leaf podocarp forest to mix with probable Avicennia growing in estuarine conditions close to the site of preservation. Many woods were partially or completely carbonised by the flow. The woods were probably silicified by silica in solution, formed by devitrification of volcanic glass in the pyroclastics. The sequence of silicification is likely to have been either opal-A → opal-CT → α quartz and chalcedony, or directly to α quartz and chalcedony depending on the local alkalinity of the pyroclastics. The mode of occurrence of α quartz, chalcedony, Cristobalite, tridymite, lussatite and amorphous opal is discussed, as is the degree of crystallinity of the quartz. Other minerals present in the woods include goethite, members of the sapponite group, and a range of organic compounds. The fossil woods resemble Recent species of Dacrydium and other gymnosperms, Nothofagus, Quintinia, Vitex, Leptospermum and a myrtacean similar to Metrosideros or Eucalyptus. Wood resembling that of modern Avicennia forms 93% of the sample studied. The fossil Avicennia grew much, larger than the local extant species and this is related to the warmer climate of the Miocene. A range of insect borings is described including the first occurrence in the New Zealand geological record of termites. Hexagonal pellets similar to those of recent Kalotermes brouni are silicified in burrows in some wood samples.