Correlation of some Pliocene tuffs in Southern Wairarapa, New Zealand, and comparison with biostratigraphic and magnetostratigraphic data

Abstract

Several megascopic silicic tuffs occur in Pliocene (Opoitian) calcareous mudstone of two well-studied Neogene sections in Southern Wairarapa, New Zealand. Electron microprobe analysis has been used to chemically characterise the glass from four tuffs from Hinakura Road (named Spooner, Sedd el Bahr, Missing, and Hikawera Tuffs) and several tuff exposures from two parallel sections in the Mangaopari Stream area. All are calcalkaline rhyolites (73.1–79.0 wt% SiO₂) and each consists of a distinguishable single glass population.

The Spooner and Hikawera Tuffs are recognised in the Mangaopari area providing a rare opportunity to directly correlate magnetostratigraphy between sections and to test the synchroneity of bioevents. Benthonic and planktonic foraminiferal bioevents are generally synchronous. They, along with the accepted magnetostratigraphic interpretation and fission-track dates on the tuffs, form a stratigraphic framework that is consistent with the tuff correlations. However, the last appearance of the nannoplankton Reticulofenstra pseudoumbilica (reported at 3.75 Ma) occurs 35 m above the Spooner Tuff (radiometric date 2.46 Ma) at Hinakura Road, but 56 m below the correlative of the tuff at Mangaopari Stream, suggesting significant diachroneity of this bioevent, which may be the result of reworking. The use of nannoplankton bioevents to the exclusion of other data at these sections results in erroneous ages. With definitive tuff correlations, it is concluded that the mid to lower part of the Pliocene sequence exposed at the sections examined is Gauss Normal age.

Keywords:

• silicic tuff
• tephra
• glass chemistry
• Mangaopari
• Neogene
• biostratigraphy
• magnetostratigraphy