Crouch EM, Kennedy EM, Griffin AG, Raine JI, Clowes CD, Reichgelt T, Sykes R. 2022. Materials for improved assessment of the petroleum source potential of New Zealand coaly rocks, 3: Paleocene–Eocene stratigraphy, coal abundance, flora and climate. Lower Hutt (NZ): GNS Science. 145 p. (GNS Science report; 2021/02). doi:10.21420/MZQ1-R764.
Abstract
This report focuses on Paleocene, Eocene and relevant latest Cretaceous strata of the Zealandia Megasequence and provides a review of paleofloral, paleoclimate and coaly source rock abundance assessments. It is the third report in a series, with previous reports focusing on the Jurassic to Early Cretaceous and mid-Cretaceous. Paleocene and Eocene strata (c. 66–34 Ma) are deposited widely throughout New Zealand’s sedimentary basins as a result of post-rift transgression and long-term passive margin subsidence. Non-marine strata of this age are predominantly present in western and south-eastern basins. A review is provided of the stratigraphy and correlation of New Zealand’s Paleocene and Eocene non-marine strata, with selected outcrop and wells discussed in more detail. There is an emphasis on palynological data, as these provide relevant information for dating and correlating coaly rock strata and understanding past vegetation and climate. Marine strata are included where important terrestrial palynomorph data have been obtained. The nearest living relative (NLR) paleoclimate reconstruction method is applied to miospore data from Tui-1 exploration well, Taranaki Basin, to reconstruct mean annual temperature (MAT) from the early Paleocene to late Eocene. New paleoclimate estimates are produced from a Paleocene leaf fossil assemblage from Strongman Mine, West Coast, and a review of known Paleocene and Eocene plant macrofossil sites and associated climate proxy data is provided. Geophysical log-derived estimates of the net thicknesses of coaly source rock lithologies are provided for four exploration wells, three in Taranaki Basin (Maui-4, Pukeko-1, Waihapa-1/-1A/-1B) and one in Great South Basin (Tara-1), and existing geochemical data are used to calculate petroleum source potential index for the three Taranaki wells. Using new and available paleotemperature data from non-marine and marine sites, and known plant micro- and macro-fossil information, an overview of New Zealand Paleocene to Eocene climate and vegetation is discussed. The Paleocene has relatively warm temperatures, with background sea surface temperatures (SSTs) of 20–22°C, and two intervals of cooler temperatures: the post-Cretaceous/Paleogene (K/Pg) boundary, where SSTs drop to c. 13°C, and late Paleocene decreases in both SST and MAT associated with deposition of the Waipawa organofacies (c. 59 Ma). Vegetation was severely affected by the K/Pg boundary event, with mixed forest vegetation replaced by ferns that then transitioned into gymnosperm-dominated assemblages that characterise the remaining Paleocene. Sparse precipitation estimates in the lower Paleocene suggest high rainfall, but these estimates are lower than from Eocene assemblages. Thermophilic vegetation is first seen in the latest Paleocene prior to short-term warming of the Paleocene–Eocene thermal maximum (c. 56 Ma) and continues into the longer-term warming of the early Eocene. Temperatures peak in the early Eocene climatic optimum, with SSTs up to c. 35°C and MAT up to c. 24°C, and vegetation is characterised by increasing angiosperm and declining gymnosperm abundance. Through the late early to middle Eocene, SSTs show notable cooling (>10°C), which contrasts with minimal cooling in MAT estimates. The reappearance of Nothofagidites pollen is seen in the middle Eocene, and miospore assemblages in the upper middle to late Eocene continue to reflect a cooler climate, with an increasing abundance and diversity of Nothofagidites taxa, disappearance of some thermophilic taxa and varying gymnosperm abundance. There is evidence of developing regional vegetation in plant communities in the late Eocene, perhaps the result of cooler climates or geographical barriers. This report provides contextual information to help understand the underlying relationships between coaly source rock formation, coal oil and gas potential, depositional controls, vegetation and paleoclimate. (The authors)
