Some Aspects of Sedimentology of the Wanganui Basin, North Island, New Zealand

Abstract

The thesis comprises studies of the marine Pleistocene sediments of the Wanganui Basin, North Island, New Zealand. Part I deals with the chronology of the sediments and correlation of horizons within and outside the basin, by dating glass shards from tephra horizons using the fission-track method. Correlation to similar tephras from Hawke’s Bay, to deep-sea cores taken 1000km east of New Zealand and to the central North Island volcanic district is attempted. These fission-track ages fill a dating gap that previously existed in the New Zealand marine Quaternary sequence. Thirteen tephras were examined in the Wanganui Basin and were found to range in age from 1.50±0.21m.y.B.P. (Ohingaiti Ash) to 0.28±0.05m.y.B.P. (uppermost Finnis Road Ash). These tephras record major rhyolitic eruptive phases in the central volcanic region. The most significant eruptive phase began 1.06±0.16m.y.B.P. with the deposition of the Makirikiri Tuff sediments, continued to 0.88±0.13m.y.B.P. and is tentatively associated with the older ignimbrites of the King Country, west of Lake Taupe. A volcanically quiet period followed when no volcanic glass was deposited in the sediments, until 0.74±0.09m.y.B.P. Several large eruptions then occurred between 0.74 and 0.28m.y.B.P. The age of the Plio-Pleistocene boundary, at the base of the Hautawan Stage in the Wanganui Basin is 1.87m.y.B.P. The age of the base of the Nukumaruan is 1.55m.y.B.P., the Okehuan, 1.06m.y.B.P., the Castleclifflan 0.45m.y.B.P., and the Hawera Series is less than 0.38m.y.B.P. Palaeomagnetic stratigraphy was determined for the upper Nukumaruan and lower Okehan sequence in the Rangitikei River. Viscous components of magnetism were removed from the samples by thermal demagnetising, extreme care being needed to obtain consistent results. Independent dates from the palaeomagnetic stratigraphy substantially confirm the fission-track dates. The Bruhnes-Matuyama boundary is clearly defined between the Rewa and Potaka Pumice Members (aged 0.74 and 0.61m.y.B.P. respectively) of the Kaimatira Pumice Send Formation. The Jaramillo event was not recognised and is probably represented in part of the sequence where sediments are too coarse and friable to yield palaeomagnetic cores. Part II deals with the detailed sedimentology of the lower Okehuan Stage sequence which is composed of two volcaniclastic formations, the Makirikiri Tuff and Kaimatira Pubmice Sand, separated by a non-volcaniclastic siltstone formation, the Okehu Siltstone. Interpretations of the Sedimentary structures in the Makirikiri Tuff and the Kaimatira Pumice Sand Formation confirm previous conclusions of shallow water deposition based on palaeontological evidence. Some structures also indicate the high rate of sediment accumulation during deposition of the volcancic sediments. Size analysis statistics show influence of source material and processes acting on the sediment during transport and deposition. Rapid sediment accumulation is emphasised by poor sorting, and processed inferred from the sedimentary structures are confirmed by the grain size analyses of the same structures. Analysis of the attitude of large and small scale cross-stratification reveals a complex polymodal palaeocurrent pattern, as might be expected of shallow water to intertidal sequences. Although often bipolar-bimodal, the dominant sediment transport appears to have been from west to east, similar to the direction of current movement along the Wanganui coast today. Size and petrography of clasts from the conglomeratic horizons indicated sediment sources both from the central volcanic region of North Island and from the Mesozoic “greywackes” of the axial mountain ranges which were emergent and probably significantly elevated at the time when the sediments were accumulating. No volcanic debris was deposited with the Okehu Siltstone. The mineralogy of the sands points to the same sediment sources but also indicates that some metamorphic material was being introduced most likely from South Island. Part III of the thesis represents a pilot study undertaken to determine whether isotopic differences in fossil shell composition could be used to distinguish shells that grew in fully marine water from those that grew in less saline conditions. Carbon and oxygen isotope ratios were determined on shells from three formations whose environments had been adequately studied by paleontologists. The horisons chosen were the Waipuru Shellbed, the Tewkesbury Formation and the Tainui Shellbed. Agreement with the palaeontological evidence and thus distinction between the fully marine and the fresh water contaminated marine environments was possible with the technique.