Geology of the Raglan-Kawhia area

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Waterhouse, B.C.; White, P.J. 1994 Geology of the Raglan-Kawhia area. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 13 48 p.

Abstract: The oldest rocks in the Raglan-Kawhia district are late Triassic to late Jurassic sedimentary rocks of the Murihiku Super Group. They are mainly fine-grained indurated siltstones and sandstones with conglomerates, tuffs, rare shellbeds and carbonaceous layers in a sequence some 6500 m in thickness. Five groups are distinguished on the basis of lithology and fossil content: Newcastle (oldest), Rengarenga, Kirikiri, Apotu, and Huriwai. Pre-Tertiary deformation of the Murihiku terrane compressed these basement rocks into a series of broadly north-striking subparallel, asymmetric folds (Albatross Syncline, Toe Syncline, Kawaroa Anticline, and Kaimango Syncline), constituting a regional synclinal structure traceable over 150 km from Taranaki to Port Waikato. Strongly unconformable Tertiary coal measures, limestones, siltstones and sandstones lap on to the folded ''basement'' rocks and attain a combined thickness of up to 500 m. Over 95% of these rocks are within the Te Kuiti Group (Oligocene-Miocene) with the balance in the far northwest of the map area being referrable to the Waitemata Group (Miocene). Pliocene-Quaternary andesitic and basaltic volcanics, agglomerate, and ash, pumiceous sediments, dune sands and terrace deposits cover much of the western and southeastern part of the map area. Andesitic to dacitic dikes, sills, plugs, stocks, and plutons of the mid Pliocene Orangiwhao Intrusive Group intrude Triassic and Jurassic rocks near Albatross Point. Alexandra Volcanic Group includes low angle, composite, polygenetic cones or shields consisting of lava flows, volcanic breccia, dikes, lahar deposits and tuff. The Group is represented by Pirongia, Karioi (756 m), Koponui (554 m), and numerous minor centres (e.g. Wharauroa, Te Uku, Pukepuketetai) and flank vents. The age of the Alexandra Volcanic Group is Pliocene to early Pleistocene. Pumiceous sands, silts, lignite, and dune sands accumulated during complex eustatic and tectonic events and form the rocks assigned to the Kaihu Group. They are present alone much of the western coastline, to heights of over 100 m asl, and in the inner harbour areas of Raglan, Aotea, and Kawhia. The Tauranga Group includes alluvial, swamp, and terrace deposits, and volcanic ashes derived from local sources, Taupo Volcanic Zone, and Taranaki. The age of the Tauranga and Kaihu Groups is Pliocene to Holocene. Both pre- and post-Tertiary faults strike north-south, and northeast-southwest. Some exhibit dextral offset, particularly in the south of the map area, and all have normal components. Titanomagnetite ironsands, Tertiary limestones, and aggregate from hard sedimentary and volcanic rocks are important local products. Coal deposits in the southeast of the map area may be worked at some future date.

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