Foraminiferal Analysis of the Late Miocene- Early Pleistocene Mangaopari Mudstone, South Wairarapa, New Zealand
The foraminiferal content of thirty-two samples from the late Miocene-early Pleistocene Mangaopari Mudstone within the southern Wairarapa region have been examined with the aim of determining the age and depositional environment of the unit. In particular the study addressed whether or not there were glacioeustatic cycles present in the unit. Integrating foraminiferal faunal distributions and sedimentological analysis provided geological, paleoclimactic, and paleoceanographic evidence to aid in the reconstruction of the paleoenvironment. The data was then compared with conclusions from previous studies. The section was divided into two different parts (upper and lower) based on changes in foraminiferal assemblages and grainsize distributions. The age and depositional environment of the Mudstone is suggested by the presence of several genera and species of foraminifera which is supported by grainsize analysis. The presence of Martinottiella communis and Karreriella cylindrica between 0-157.1m stratigraphically suggest that accumulation began in bathyal conditions at depths greater than 400m between. This is supported by grainsize analysis which indicates a medium silt with a high percent mud content ranging from 91.5-100%. This demonstrates deposition beginning in the late Miocene-early Pliocene at bathyal depths greater than 400m. The upper part of the mudstone (157.6-216.3) illustrates a regressive sequence with a distinctive shift to a much shallower depositional environment at outermost shelfal depths likely of 150-200m. This is represented with the presence of Truncorotalia sp. and Zygochlamys delicatula. Grainsize also support this discovery with a shift to very fine sandy silts with a percent mud content ranging from 83-93%. Previous findings conclude that this distinctive shift was caused by glacioeustatic cycles yet our data do not correlate with our glacioeustatic findings. Therefore, this shift is believed to be triggered by a tectonic event.