Clay petrology of Mesozoic to recent sediments of central western North Island, New Zealand

Abstract

NOTE: Missing pages 177-78 and 181-82.

Clay mineral, bulk mineral and chemical data for Mesozoic to Recent onshore sediments and Recent western shelf surficial sediments from central Western North Island are presented with the aim of determining the significance of areal and stratigraphic variations in clay mineral distributions. Sample analysis procedures using mainly X-ray diffractometry have been determined that are relatively simple, rapid and applicable to this wide variety of sediment types. Clay mineral analysis was carried out on the <2μ size fraction of all samples, and on the <1μ, 2-4μ, 4-8μ and 8-16μ size fractions of selected samples to compensate for any mineralogic bias imposed through the selection of the traditional <2μ fraction. The predominance in the same samples of illite and chlorite in coarser fractions but of montmorillonite and kaolinite in finer fractions emphasises that the geologic interpretation of clay mineral distributions in sediments may be significantly influenced by the grain size fraction chosen for analysis. Clay mineral mounts prepared using dropper-on-glass slide and smear-on-glass slide techniques gave identical qualitative and similar quantitative results so that the former simpler technique was used. Mesozoic to Recent sediments in Hamilton, Taranaki and Wanganui Districts contain varying amounts of kaolinite, illite, montmorillonite, chlorite and mixed-layer clay minerals. Clay mineral assemblages of mudstones, limestones or concretions, represent best those assemblages existing at the time of sediment deposition; clay minerals in sandstones are prone to diagenetic alteration because of their generally greater porosity. The clay mineralogy of Mesozoic to Pleistocene sediments bears the imprint of a complex set of factors, including provenance, climate, tectonism, depositional environment and diagenesis. The illite, chlorite and mixed-layer clay mineral suite of Mesozoic sediments is consistent with derivation from an igneous/metamorphic provenance, partly modified by transformation during burial diagenesis. The abundant kaolinite in Upper Eocene Waikato Coal Measures is in part detrital from soils developed by prolonged chemical weathering of Mesozoic sediments, and in part diagenetic from clay mineral transformations under acid leaching conditions associated with carbonaceous sediments. The abundant crystalline montmorillonite in Oligocene sediments (Te Kuiti Group) was primarily neoformed in shallow marine basins characterised by extremely slow deposition and carbonate sedimentation. The abundant crystalline illite and chlorite characteristic of most Miocene to Lower Pleistocene sediments were mainly detrital from Mesozoic sediments under conditions of increasing topographic relief and more rapid physical erosion as a result of accelerating tectonic activity during this period. Montmorillonite in Lower Miocene sediments (Mahoenui and Mokau Groups) was largely detrital from uplifted Oligocene sediments. However, montmorillonite in Middle Miocene sediments (Mohakatino Group) was derived primarily from the alteration of contemporary volcanic products while much of the montmorillonite in Upper Miocene to Pliocene sediments (Mt Messenger to Tangahoe Formations) was detrital Mohakatino lithologies. The increased abundance of montmorillonite in Plio-Pleistocene sediments results from the alteration of increasing quantities of volcanogenic material derived from contemporaneous volcanism in central and western North Island. The clay mineralogy of western shelf sediments is dominated by illite and montmorillonite, with lesser amounts of chlorite and mixed-layer clays, and some kaolinite. On the basis of the distribution and origin of these clay minerals, five petrologic zones are distinguised: (1) Hamilton Shelf - Abundant montmorillonite, rare to common poorly crystalline illite, and lesser kaolinite, mixed-layer clays and poorly crystalline chlorite; primarily detrital from Oligocene mudstones, some montmorillonite neoformed from Quaternary volcanogenic material. (2) North Taranaki Shelf - Abundant crystalline illite, common mixed-layer clays and crystalline chlorite, minor montmorillonite; primarily detrital from Miocene mudstones. (3) Central Taranaki Shelf - Abundant crystalline illite, very common montmorillonite, common chlorite and mixed-layer clays, and some kaolinite. Neoformation of montmorillonite from Egmont derived volcanogenic material contemporaneous with deposition of detrital illite, montmorillonite, chlorite and mixed-layer clays from northwestern South Island and perhaps North and South Taranaki Bight terrestrial provenances. (4) South Taranaki Shelf - Abundant crystalline illite, and common mixed-layer clays and crystalline chlorite; primarily detrital from northwestern South Island sources. (5) North Cook Strait Basin – Abundant crystalline illite, common mixed-layer clays and crystalline chlorite, and some montmorillonite; primarily detrital from northwestern South Island sources and, to a lesser extent, from Plio-Pleistocene mudstones in south Tarangaki-Wanganui Districts. Clay mineral distributions on the western shelf are the product of a variety of interrelated physical, chemical and biochemical processes that have produced both detrital and authigenic sediments. Moreover, distribution patterns are further complicated by the existence of both modern and relict material.