A biostratigraphic investigation of the Pukenui limestone formation, Southern Wairarapa, New Zealand

Abstract

The Pukenui Limestone Formation is a mid Nukumaruan succession of shallow marine limestones and fine grained terrigenous strata which outcrop in central and southern Wairarapa, New Zealand. In an area south of Martinborough, the adjacent Makara and Ruakokopatuna River valleys contain disparate numbers of limestone layers. To enable their correlation, and to construct a record of sea level change for the Pukenui Limestone, a 2.5km2 area was mapped, and eighteen sections were measured and described. Selected samples were analysed for grain size, and their content of foraminifera, ostracods, and molluscs. Using biostratigraphic and lithologic correlations, strata from the two river valleys were placed into five lithostratigraphic units. A series of depth biofacies were used to reconstruct the depositional environments and provide a relative sea level curve for each of the measured sections. The measured sections were then divided into three groups based on similar lithology and the proximity of the sections to each other. Composite relative sea level curves of the Pukenui Limestone were produced for three parts of the study area which contain a dissimilar record of sedimentation. Comparisons of these three sea level curves revealed six cycles of sea level change. Cycles C1, C2 and C6, show fluctuations that were substantially larger than the other three. The relative sea level curves were also compared with the oxygen isotope record of Naish et al. (1997). This comparison was inconclusive, because of the uncertain length of time over which the Pukenui Limestone was deposited, and the absence of a biostratigraphic or chronostratigraphic datum. The environmental reconstruction revealed the evolution of the Ruakokopatuna Anticline against a background of rising and falling sea level. The Ruakokopatuna Anticline was growing by the time Unit 4 was being deposited. The Windy Peak Anticline appeared to be growing when the earliest sediments of the Pukenui Limestone were being deposited.