Materials for improved assessment of the petroleum source potential of New Zealand coaly rocks, 2: Jurassic to Early Cretaceous stratigraphy, coal abundance, flora and climate

• Main Authors: Raine, J. Ian, Griffin, Angela G., Kennedy, Elizabeth M., Clowes, Chris D., Sykes, Richard
• Format: Text
• Language: unknown
• Published: GNS Science 2020
• Subjects: New Zealand; Tundra
• Online Access: https://dx.doi.org/10.21420/t2nd-e424; https://shop.gns.cri.nz/sr_2020-32-pdf

Jurassic to Early Cretaceous strata of New Zealand were deposited at high latitude on the southern margin of Gondwana in several tectonic situations, now recognised as distinct tectonostratigraphic terranes within the Eastern Province of Zealandia. To develop a chronostratigraphic framework of coaly petroleum source rock potential in relation to vegetation and climate, we review the stratigraphy of non-marine strata, estimate relative abundance of coaly lithologies in available exploration wells and outcrop sections and discuss macrofloral and palynological evidence for paleoclimate. Marine strata with relevant paleobotanical data are also mentioned but covered in less detail. This work is part of a larger programme of research into petroleum source rocks. The Drumduan Terrane in Nelson City includes plant-bearing carbonaceous siltstone of mid Jurassic age in a non-marine sequence of volcanogenic breccia and finer clastics. Thin coal seams occur in similar strata further south, but metamorphic rank throughout is too high for miospore preservation. In Southland, Permian strata of the Brook Street Terrane are unconformably overlain by Barretts Formation, fossiliferous fluvial to estuarine strata of mid Jurassic age. This unit is predominantly sandstone and conglomerate, but finer sediments, including thin coal beds, occur locally. Good miospore preservation indicates a low degree of thermal maturity. The Torlesse Composite Terrane has yielded Jurassic and Early Cretaceous palynofloras at some localities, and non-marine strata with macroscopic plant fossils of mid Jurassic age at several Canterbury localities. Miospore preservation at some localities indicates low thermal maturity, but, at other places, carbonaceous material is of anthracite coal rank. In these terranes, lack of detailed lithological logs from outcrop sections prevented numerical estimates of the relative proportion of coaly rocks. The Murihiku Terrane includes extensive outcrop of Jurassic strata in western North Island and southern South Island, and Early Cretaceous in the subsurface of Northland. The relatively undeformed strata are largely of marine facies but include non-marine sediments of mid- and latest Jurassic age. Plant macrofossils are well-documented from the non-marine strata, and the whole sequence has provided palynological data. Lithological logs from six measured outcrop sections, and geophysical logs from three petroleum exploration wells (Waka Nui-1, Pluto-1 and Pukearuhe-1), were used to estimate relative proportions of coal, shaly coal and coaly mudstone. Coaly lithologies make up a very small percentage of the total Jurassic interval in the wells, ranging from <0.5% in Pukearuhe-1 (Early and Middle Jurassic) to c. 1.5% in Waka Nui-1 (Middle Jurassic). Higher percentages of coaly rocks occur in some outcrop sections but are mainly due to coaly mudstone, and coal or shaly coal percentages reach at most 3%. Synthesis of paleobotanical and palynological records supports predominance of coniferous forest vegetation, initially podocarp and then mainly araucarian, through the Middle Jurassic but reduced tree cover before and after this interval. Fern, or mixed fern-cycadophyte vegetation with scattered pteridosperms and conifers, appears to have been more characteristic of much of the Jurassic and Early Cretaceous. Jurassic climate was oceanic and temperate but possibly near a polar limit of tree growth. A marked vegetation change in the mid-Early Cretaceous is characterised by expansion of lycophytes, possibly signifying tundra vegetation. Higher bryophytic spore abundance during the Early and Middle Jurassic suggests greater moisture availability than in the Late Jurassic and Early Cretaceous, and this may be reflected in more abundant coaly lithofacies in the earlier interval. However, high sedimentation rates with attendant paleoenvironmental instability probably had a greater influence in depressing occurrence of coaly rocks, especially during the Late Jurassic to Early Cretaceous. (auth)