Geology of the Waikato area : scale 1:250,000 Digital Download

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Edbrooke, S.W. 2005 Geology of the Waikato area : scale 1:250,000 Institute of Geological & Nuclear Sciences 1:250,000 geological map 4 

Abstract: The Waikato 1:250,000 geological map covers about 11,000 km2 of the Waikato, King Country and north Taranaki, in the west of the North Island, New Zealand. Extensive plains of the Hamilton lowland area in the north are bordered by dissected hills and ranges, formed mainly on Mesozoic basement rocks and a generally thin cover of Tertiary sedimentary rocks. The cover rocks include areas of limestone that form karst landscapes near Waitomo. The cones of Karioi, Pirongia and Kakepuku volcanoes rise above surrounding hills and plains in the north. Southeast of the lowlands, the hills are capped or buried by Quaternary ignimbrites that are the main landscape-forming feature. Dissected hill country formed on Miocene sedimentary rocks is characteristic of southern and southwestern parts of the map area. Extensive coastal dunes lie north of Marokopa, particularly around the entrances to Kawhia, Aotea and Raglan harbours. Offshore, the seafloor slopes gently westward for 50 km in the north and over 150 km in the south, to the shelf edge at a depth of about 150 m. Beyond the shelf edge the seafloor deepens rapidly to over 1000 m. Gannet Island, 20 km west of Aotea Harbour, is the only offshore island. Late Paleozoic to Mesozoic basement rocks in the onshore area of the map form parts of three tectonostratigraphic terranes. The Late Triassic to Late Jurassic Murihiku terrane rocks, broadly folded into the Kawhia Regional Syncline, underlie the area west of the Waipa Fault. The Dun Mountain-Maitai terrane, represented by a serpentinite body at Wairere, is faulted against the Murihiku terrane by the Waipa Fault. Mainly volcaniclastic sandstones of the Waipapa (composite) terrane lie east of the Waipa Fault. Basement rocks underlying the offshore area immediately west of Taranaki Fault are probably part of the Brook Street terrane, while those further west are part of the Median Batholith. Onshore, the basement terranes are overlain by discontinuously preserved Eocene to Late Miocene, mainly marine, sedimentary rocks of the Te Kuiti, Waitemata, Mahoenui, Mokau, Wai-iti and Whangamomona groups. In the Early Miocene, carbonate-dominated sediments of the Te Kuiti Group gave way to predominantly terrigenous sediments deposited in rapidly subsiding basins, as the convergent Australian-Pacific plate boundary propagated through northern New Zealand. Pliocene and Quaternary sediments comprise coastal sand deposits of the Awhitu and Karioitahi groups, and dominantly terrestrial alluvium of the Tauranga Group. Pliocene and Early Quaternary back-arc volcanism gave rise to the Orangiwhao Intrusive Group, and basalts of the Alexandra and Kerikeri volcanic groups, mainly in the west. Quaternary caldera-forming eruptions in the Taupo Volcanic Zone deposited voluminous and widespread ignimbrites and tephra deposits of the Pakaumanu and Whakamaru groups, and the Taupo and Oruanui formations, mainly in the east. The Tauranga Group is largely derived from reworking of silicic volcanic materials produced by these eruptions. Offshore, west of the Taranaki Fault, an almost continuous Cretaceous-Cenozoic sequence, up to 8000 m thick, overlies basement rocks. The map area contains a variety of geological resources, the most significant being the extensive ironsand deposits on the coast, sub-bituminous coals of the Waikato and Taranaki coalfields, large aggregate, limestone and sand resources, and locally extensive groundwater aquifers. Hot water springs and warm groundwater are known in the map area but there are no high temperature geothermal fields. The offshore area has significant oil and gas potential but has been only partly explored and further discoveries are expected. The Waikato is an area of low to moderate seismicity compared to other areas of New Zealand. Only one active fault, the Turi Fault located offshore, is known within the map area. Local damage has arisen from infrequent moderate magnitude, shallow earthquakes located within or very close to the map area and larger magnitude earthquakes located further away. The Waikato area faces a significant threat from future volcanic activity in the Taupo Volcanic Zone (TVZ), and a lesser threat from eruption of Mayor Island and Taranaki Volcano. Volcanic ash deposits would have a significant impact on the environment and human activity, and pyroclastic flows from the most violent TVZ eruptions would be extremely destructive. Landsliding on weak rocks, coastal damage from tsunami, and flooding of low-lying areas following prolonged high-intensity rainfall are additional geological hazards affecting the Waikato map area. (auth)