Geology of the Waikato Coal Measures, Waikato Coal Region, New Zealand

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Edbrooke, S.W.; Sykes, R.; Pocknall, D.T. 1994 Geology of the Waikato Coal Measures, Waikato Coal Region, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 6 236 p., 8 maps

Abstract: Waikato Coal Region comprising 13 coalfields between Drury in the north and Mangapehi in the south, is subdivided into northern and southern subregions by the Hakarimata and Taupiri ranges. It is New Zealand's principal coal producing region and coal exploration drilling since the early 1900s, particularly during the 1974-1987 New Zealand Coal Resources Survey, provides the major source of data for this study, which includes studies of coal measure palynology and coal petrography. Waikato Coal Measures are the basal formation of the predominantly transgressive Te Kuiti Group, unconformably overlying an undulating surface cut into deeply weathered Mesozoic basement rocks, and grading up into shallow marine formations of lower Te Kuiti Group. The coal measures are typically less than 100 m thick but exceed 200 m locally. Eight subbituminous coal seams are recognised, four in the northern subregion (Taupiri, Kupakupa, Renown and Ngaro seams) and four in the southern subregion (Pirongia, Waipa, Okoko and Mangapehi seams). The distribution of palynomorph biozones recognised in the lower Te Kuiti Group shows that Waikato Coal Measures young southward from late Eocene (Kaiatan-Runangan) in the Huntly/Rotowaro area to middle Oligocene (late Whaingaroan) at Benneydale. The coal measures were deposited within a N to NNW-trending valley system, about 35 km wide and 200 km long, parallel to the regional structural grain. Tectonic faulting is inferred during earliest deposition at Rotowaro, but did not persist, the major influences on the pattern of deposition being regional subsidence and basement topography, the latter strongly influenced by regional structures in basement rocks. Continued subsidence, interrupted by uplift and minor tilting in the earliest Miocene, produced maximum burial depths for the coal measures of between 1000 and 3000 m, inferred from coal rank variation, by early Miocene (Altonian). Middle to late Miocene tectonism produced regional uplift, tilting to the NW, and the subrectangular pattern of N-NNW and NE-ENE normal faults which dominate the structure of the region today. Waikato Coal Measures are dominated by fine-grained lithologies, principally mudstone, with lesser proportions of sandstone and coal; conglomerate is rare. Most sediments were derived from the thick regolith, rich in pedogenic kaolinite, formed on basement rocks. Waikato coals vary in rank on a regional scale and within individual coalfields, primarily as a result of variation in maximum depth of burial. Macro- and micro-petrographic and palynofloral compositions of the seams are described from selected seam profiles. Coal measure basin history is summarised in a series of paleogeographic maps representing intervals based on palynomorph biozones. These show the early development of anastomosed and meandering fluvial drainage systems, and accumulation of thick peats, on the northern alluvial plain in the late Eocene (Kaiatan-Runangan). Fluvial sedimentation began in the south during earliest Oligocene (early Whaingaroan) marine transgression across the northern subregion. Transgression continued southwards, strongly influencing late coal measure deposition on the southern coastal plain and tidal coast during the early Oligocene (early Whaingaroan). Coal measure deposition was ended by marine transgression across Mangapehi Coalfield and the establishment of marine Te Kuiti Group deposition over the entire Waikato region by middle Oligocene (late Whaingaroan). (auths)

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