Mesozoic geology of the Matawai district, Raukumara Peninsula

Abstract

At Matawai, Raukumara Peninsula a high angle unconformity separates Cretaceous conglomerates, sandstones and siltstones from the underlying Urewera Greywacke, at least a part of which is upper Jurassic. This unconformity is believed to be of considerable significance. It represents a considerable time gap, a major deformation phase, and the initiation of a new regional sedimentation cycle. Mapping and sedimentological studies enable the recognition of two major lithostratigraphic units within Cretaceous strata at Matawai. The Aptian - Coniacian MATAWAI GROUP (new name) includes the Koranga, Te Wera and Karekare Formations. It is overlain unconformably by the Maastrichtian - ?Landenian WHAREKOPAE GROUP (new name), consisting of the Tahora (revived name) and Rakauroa Formations. The low angle unconformity between the Matawai and Wharekopae Groups is believed to represent only a minor phase of deformation and renewed marine transgression. It is local, not regional, and does not mark a significant time gap. A further unconformity separates Koranga Formation shallow marine coarse clastic sediments from texturally more mature sediments of the overlying Te Wera and Karekare Formations. It too is interpreted as being of only minor significance; it marks only a short period of non-deposition and not a major change in sedimentation pattern. Six biostratigraphic zones, all based on short-ranging species of either Inoceramus or Aucellina have been mapped in Matawai Group strata. Diverse shallow water faunas of the Koranga, Te Wera and lower Karekare Formations are succeeded by upper Karekare assemblages containing few fossils other than Inoceramus. Koranga Formation sandstones are texturally and compositionally immature. Like thick-bedded Urewera sandstones, they are of very mixed provenance, being derived from acid plutonic - high grade metamorphic, sedimentary and volcanic sources. Thus although the oldest Matawai Group strata are stratigraphically discrete from the Urewera Greywacke, they were derived largely from similar sources. Petrographic study of sandstones and X-ray study of siltstones suggest Karekare Formation sediments were derived largely from underlying Urewera and Matawai strata. Reworking of unlithified Matawai Group sediments provided much of the detritus for the glauconitic quartzose Tahora sandstone and the siliceous Rakauroa siltstones and shales. Distribution and composition of clays and feldspars in Matawai and Wharekopae rocks indicate a proportion of the sediment was derived from a granitic source to the south or southeast. Although zeolites and authigenic epidote are common in Urewera rocks at Matawai, extensive zeolitisation is virtually restricted to the sandstones and conglomerates overlying the Urewera Greywacke - Matawai Group unconformity. Development of zeolites is not, therefore, wholly a function of depth of burial. Volcanogenic Urewera and Koranga sandstones are both suitable hosts for zeolite formation, and restriction of extensive zeolitisation to the latter indicates the importance of permeability in controlling zeolite distribution. The relatively permeable Koranga Formation was a paleoaquifer because of its stratigraphic position between relatively impermeable Urewera and Karekare strata. From consideration of regional facies patterns, Wharekopae strata at Matawai and their coal measure and marginal marine equivalents in the South Island are believed to be the compositionally mature end products of a sedimentation cycle initiated in Aptian (Korangan) time. Matawai Group strata are considered to post-data the major deformation pulse known as the Rangitata Orogeny. Study of published and unpublished work on coeval rocks elsewhere in New Zealand suggest a fundamental and regional break between Aptian, Albian or even younger sediments, and older rocks. The Matawai Group and its coeval equivalents should, therefore, be included in the Kaikoura Sequence, and not, as has been proposed, in the underlying Rangitata Sequence.