Jill Antoinette Kenny
Published: 1980
Total Pages: 0
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The 75 km2 Ihungia catchment is located half way between Gisborne and East Cape, on the Raukumara Peninsula, in the extreme east of the North Island of New Zealand. It includes part of the northeast-trending, complex belt of Cretaceous and Tertiary rocks occupying central and eastern Raukumara Peninsula, and the purpose of this project was to map and study both Cretaceous and Tertiary rocks. The "autochthonous" rocks of the Hikurangi beds and Tapuaeroa Formation, exposed in the north of the region, were deposited in the Late Cretaceous as thick mudstone and flysch accumulations, and subsequently deformed. The area then became a repository for three sheets, emplaced from the north over the "autochthonous" material. The first is thought to have been emplaced in Late Waitakian to Early otaian times (earliest Miocene), and is composed predominantly of Mokoiwi Formation mudstones (Motuan age). Blocks of Mangatu Group lithologies (Dannevirke to Arnold), enclosed in bentonite, are subordinate in volume. The sheet is exposed as a wide belt, north of the Ihungia road. The second thrust slice, occupying most of the catchment south and northwest of the road, contains thick accumulations of Upper Tertiary mudstones and flysch of the Ihungia Formation (including the igneous Ihungia Conglomerate Unit) and a bryozoan limestone (Kouetumarae Member - new member). It was probably emplaced during mid-Wanganui times (latest Pliocene). Whangai Formation shales and the Black Hills sandstone/flysch Member (new member) of the Wanstead Formation (both part of the Mangatu Group) are found in the third thrust sheet (emplaced sometime after the second thrust), which today occupies the Black Hills - Puketiti area in the southeast of the study area. Each thrust has been influenced by local faulting and folding. Twelve phases of megascopic faulting, including thrusting, have been recognised within the Ihungia catchment. Only the Mangarakeke Member (new member) of the Hikurangi beds, and the Ihungia Formation contain fossils. The Mangarakeke Flysch was dated as Ngaterian by Inoceramus hakarius, and an upper otaian to lower Lillburnian age was determined for the Ihungia Formation from foraminifera and mollusca. Ages of other units were taken from the literature. Structural details of the massive Ihungia mudstones were deduced only after extensive foraminiferal analysis. Inspection of spores and pollen grains from the Rangikohua Member (new member) of the Hikurangi beds and Wahingamuku unit (new unit) of the Black Hills Member, Wanstead Formation, by the Palynology Section of the Geological Survey, indicates that these lithologies are Ngaterian and Dannevirke to Arnold respectively. "Badlands"-type erosion is widespread in the Ihungia Valley. Deforestation by European settlers last century has lead to severe erosion of some lithologies of the Mokoiwi and Wanstead Formations, and shallow regolith failures cover much of the Ihungia Formation terrain. The more resistant Taitai Sandstone (Mokoiwi Formation), Rangikohua Sandstone, Wahingamuku Sandstone and Ihungia Sugar Loaf Greensand (new member of Wanstead Formation) and Kouetumarae Limestone, stand out in the landscape. Stabilisation of the land, by reafforestation and/or waterflow control, is needed urgently. Quaternary deposits, oil and gas seepages, and recent evaporite deposits are described briefly.
