Materials for improved assessment of the petroleum source potential of New Zealand coaly rocks, 1: mid-Cretaceous stratigraphy, coal abundance, flora, and climate

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Raine, J.I.; Kennedy, E.M.; Griffin, A.G.; Sykes, R.; Clowes, C.D. 2018 Materials for improved assessment of the petroleum source potential of New Zealand coaly rocks, 1: mid-Cretaceous stratigraphy, coal abundance, flora, and climate. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/07. 99 p.; doi: 10.21420/G20D20

Abstract: Mid-Cretaceous coal-bearing rocks in New Zealand contain valuable records of coal-forming flora and past climate. They are not widespread in outcrop, but are more extensive in the subsurface of offshore sedimentary basins. The mainly alluvial and lacustrine sediments were deposited during development of rift basins before separation of Zealandia from Antarctica-Australia and seafloor spreading in the Southern Ocean and Tasman Sea. The stratigraphy and correlation of these strata is reviewed, with emphasis on palynological data (much of it unpublished). Palynological data compilation identified a pronounced change from lycopod-dominated to conifer-dominated assemblages within the Motuan Stage (late Albian), reflecting a change from a predominantly herbaceous vegetation to a podocarp-rich forest vegetation which prevailed through the rest of the mid-Cretaceous. Although this change may be partly due to climatic warming during the later part of the Early Cretaceous, identified by proxy records elsewhere in the world, other possible factors include the northwards drift of New Zealand due to plate tectonics, and development of a more humid climate through lacustrine and marine inundation of rift valleys. Other trends observed in the palynological data include a possible acme of araucarians during the Ngaterian-Arowhanan, and, beginning in the late Motuan, an increasing abundance of angiosperm pollen, reflecting the adaptive evolutionary radiation of the flowering plants. Previous work on the diverse and well-preserved Ngaterian (Cenomanian) Warder Formation flora from Marlborough is briefly summarised and updated. Climate estimates based on leaf morphology of the angiosperm component of the flora confirm a warm temperate climate and moderately high annual precipitation in this southern high latitude region. The abundance of coaly rocks (coal, shaly coal, and coaly mudstone) is assessed in mid-Cretaceous strata from four wells within Taranaki, Canterbury, and Great South basins, using geophysical well logs in conjunction with cuttings analyses. The greatest net thickness and relative abundance of coaly rocks (10.2 m, 6.2%) is seen in Te Ranga 1 (Taranaki Basin), which has two coaly intervals in the Taniwha Formation of TS1 miospore zone age (late Motuan–Ngaterian; late Albian–early Cenomanian). In Clipper 1 (Canterbury Basin), the coaly interval in the upper Clipper Formation is apparently within the TS2 to PM1a 1 zone range (late Ngaterian–Mangaotanean; late Cenomanian–Turonian); net coaly rock thickness through the whole Clipper Formation amounts to 4.1 m (0.86%). In the Great South Basin wells, coaly rocks of the main part of the Hoiho Group in Hoiho 1C (3.1 m, 1.1%) are also of TS1 zone age, the unconformably overlying uppermost Hoiho Group strata of PM1a-2 age (Teratan–Piripauan; Coniacian–Santonian) being barren of coaly lithologies, while the coaly interval in Kawau 1A is mainly within the TS1 zone (2.1 m, 0.66%), although coaly rocks also occur in strata of TS2 zone (0.6 m, 0.50%) and PM1a-2 zone (0.6 m, 0.40%). Strata of the early mid-Cretaceous lycopod-dominated phase were not encountered in the wells, but within the later podocarp-rich forest phase, climate does not appear to have been a limiting factor in coal formation. Instead, the occurrence of coaly rocks in the wells appears to be controlled mainly by depositional environment and rate of accommodation space formation. This work is part of an ongoing project on coaly source rocks. These data and inferences will be put into context of the broader project outcomes once associated work on Late Jurassic to earliest Cretaceous, latest Cretaceous, and Paleogene coal-bearing rocks is completed. (auth)