Stratigraphy, composition, and environment of deposition of the Whangai Formation and associated Late Cretaceous-Paleocene rocks, eastern North Island, New Zealand

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    Moore, P.R. 1988 Stratigraphy, composition, and environment of deposition of the Whangai Formation and associated Late Cretaceous-Paleocene rocks, eastern North Island, New Zealand. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey bulletin 100. 82 p.

    Abstract: Whangai Formation is a non-calcareous to calcareous siliceous mudstone (or shale) or Haumurian-Teurian (Maastrichtian-Danian) age, generally 300-500 m thick, present throughout eastern North Island. It constitutes part of a Late Cretaceous-Paleogene transgressive sequence. In western parts of the region it conformably overlies a transgressive sandstone or fining-upward sandstone-mudstone interval of Piripauan-Haumurian (Campanian-Maastrichtian) age which rests unconformably on older rocks; in eastern areas the unit conformably or disconformably overlies a thick Late Cretaceous flysch facies. The formation shows complex vertical and lateral facies changes, but overall there is an upward change from non-calcareous siliceous mudstone to calcareous siliceous mudstone. Carbonate (calcite) content also increases eastward, to a maximum of 40%. The formation is almost everywhere conformably overlain by the Waipawa Black Shale (Formation) of Paleocene age. The Whangai Formation is generally highly bioturbated, but contains few macrofossils and a restricted, low-diversity microfauna (including rare radiolaria). The fauna indicates deposition mainly at shelf depths, possibly under reduced oxygen levels and sub-normal salinity. Compositionally, the Whangai Formation is a moderately to poorly sorted siltstone, consisting mainly of detrital quartz, plagioclase, and muscovite, with minor glauconite and biotite, in a clay-rich siliceous matrix. Stratigraphic and geographic variations in clay mineral assemblages are evident. Chemical analyses indicate the Whangai Formation is very similar in composition to the average shale and average offshore mud (excluding silica), and also to some other siliceous shales; it is essentially a "normal" shale with excess silica. There is no evidence for a significant acidic volcanic component, and excess (non-detrital) silica was almost certainly derived from a biogenic source, principally diatoms and/or radiolaria. Quartz is the only silica phase present. Vitrinite reflectance and organic geochemical data indicate the Whangai Formation is immature to marginally mature over much of the region, and has little petroleum source potential onshore. Deposition of the Whangai siliceous shale probably resulted from development of a slowly subsiding basin, coincident with a reduction in the supply of a coarse terrigenous sediment and increased productivity of siliceous organisms. A remarkably similar facies was deposited over a large part of the Southwest Pacific in the Late Cretaceous and Paleocene in response to opening of the Tasman Sea and separation of the New Zealand sub-continent from Antarctica (auth)


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