Cretaceous stratigraphy of Raukumara Peninsula, part 1: Cretaceous stratigraphy of Koranga (parts N87 and N88); part 2: Geology of the Lower Waimana and Waiotahi Valleys (part N78)

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Speden, I.G. 1975 Cretaceous stratigraphy of Raukumara Peninsula, part 1: Cretaceous stratigraphy of Koranga (parts N87 and N88); part 2: Geology of the Lower Waimana and Waiotahi Valleys (part N78). Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey bulletin 91 70 p.

Abstract: Part 1 : Cretaceous Stratigraphy of Koranga. At Koranga complexly folded rocks classed in Torlesse Supergoup, possibly Later Jurassic to early Cretaceous in age, are in places unconformably overlain by fossiliferous sandstone, conglomerate and intra-formational breccia of Koranga Sandstone (Korangan Stage; Aptian, probably Upper Aptian). Richly fossiliferous grit, sandstone, and siltstone of Te Wera Sandstone (Urutawan Stage, Albian) unconformably overlie Torlesse Supergoup over most of the area mapped but locally overlies, probably with low angle unconformity or disconformity, Koranga Sandstone. Te Wera Sandstone grades verticaly and laterally into Karekare Siltstone Formation (Urutawan to Ngaterian stages, Albian-?Cenomanian) and its Choveaux Sandstone Member. In the west Karekare Siltstone is overlain, possibly disconformably or unconformably, by a sandstone unit of Arowhanan age (Cenomanian-Turonnian) which west of the area mapped overlies Torlesse Supergroup with angular unconformity. These older Cretaceous units are overlain unconformably by Rakauroa Formation (Haumurian Stage, Maastrichtian), which at Koranga transgresses successively older untis towards the north-east. Rakauroa Formation consists of a thin basal glauconitic, quartzose sandstone phase grading into siltstone which in places is siliceous, contains large concretions and lacks diagnostic macrofossils. Thus the succession contains two regional unconformities, a probably local unconformity, and possibly a second local unconformity or disconformity. Extensive new fossil collections, coupled with a re-examination of previously made collections, show four biostratigraphic zones : the Aucellina cf. radiatostriata Assemblage Zone, the Aucellina cf. gryphaeoides Assemblage Zone, the Aucellina euglypha Range Zone and the Inoceramus hakarius Range Zone. A fifth zone, the Inoceramus rangatira Range Zone, is present but was not studied. These zones are related to the New Zealand Cretaceous stages established by Wellman, and problems of absent index species, discontinuous geographic distributions, and facies faunas are discussed. The presence of unconformities, and possibly disconformities, all of which are followed by transgressive sandstone sequences, and the absence of regressive sequences, indicate a complex geological history. This conflicts with the model of continuous deposition throughout the Cretaceous accepted by many New Zealand geologists. Unconformities or disconformities, particularly at the base of sandstone or conglomerate units, should be critically searched for in Cretaceous sections. Significant intervals of Cretaceous time may not be represented at Koranga. The regional extent of unconformities implies that they may have a profound effect on biostratigraphic and time stratigraphic subdivision and on overseas correlation. Part 2 : Geology of the Lower Waimana and Waiotahi Valleys. Complexly folded and faulted, steeply dipping unfos siliferous rocks of Torlesse (Late Jurassic-? Early Cretaceous) are overlain unconformably by some 350m of late Early Cretaceous fossiliferous shallow shelf sandstone, siltstone and minor conglomerate and breccia. The low dipping, open folded Early Cretaceous sediments form a north striking anticline. No top to the formation is known and its stratigraphic position is uncertain, although numerous species common to the Korangan (Aptian) and Urutawan (Albian) stages at Koranga indicate a Korangan-Urutawan age. The presence of species, including Maccoyella incurvata Waterhouse, Maccoyella n.sp. B, and Inoceramus cf. neocomiensis d'Orbigny, not known from elsewhere in New Zealand, suggest the presence of a new biostratigraphic unit equivalent to some part of the time represented by unconformity between the Korangan and Urutawan stages at Koranga and, perhaps, overlapping the Aucellina cf. gryphaeoides (Sowerby) Assemblage Zone (Urutawan Stage) at Koranga. Extensive Late Quaternary and Holocene deposits blanketing older rocks include river gravels flanking the major valleys and ash showers derived from the Taupo Volcanic Zone.