A tentative New Zealand chemostratigraphy for the Jurassic‐Cretaceous based on terrestrial plant biomarkers

Abstract

The previously erected chemostratigraphy for Late Cretaceous‐Eocene coaly sediments in Taranaki Basin, New Zealand, has been extended to the Jurassic, primarily using samples from the South Island. Biomarker analysis by selected ion GC‐MS indicates that the abundance of angiosperm‐derived triterpanes and gymnosperm‐derived diterpanes are lower relative to hopanes in samples from the mid‐Cretaceous Trichotomosulcites subgranulatus Assemblage (miospore superzone TS) than younger coaly sediments, and exhibit a significant degree of variation in diterpane distributions and ratios of angiosperm to gymnosperm biomarkers. The two samples from the mid‐Cretaceous Lycopodiacidites bullerensis‐3 Zone (LB3) have similarly relatively low amounts of woody plant derived terpanes, but almost identical diterpane distributions. Jurassic coals show the expected absence of biomarker contribution from angiosperms. They exhibit the simplest diterpane distributions, dominated by isopimaroids, and suggest a fairly uniform woody gymnosperm community. The characteristic Jurassic biomarker zone is designated CD*, after the Australian Callialasporites dampieri Superzone that spans the mainly mid‐Jurassic interval sampled. The angiosperm‐gymnosperm index is not readily extended to include samples older than the Late Cretaceous (Phyllocladidites mawsonii Assemblage, PM), because of the variable and sometimes high values exhibited by samples from the TS and LB3 miospore zones. However, various parameters can be derived from the distributions of woody‐plant terpanes that, when used in combination, permit reasonable discrimination of the CD*, LB3, and TS Zones from each other and three younger groups of zones—PM2, PM3‐MH1, and MH2‐NM. The chemostratigraphy can be applied to correlating terrestrial oils with their sources, with four main groupings of floral zones being recognised in New Zealand—LB3+TS, PM2, PM3+MH1, and MH2+MH3+NM. The previously tentative identification of 18‐norisopimarane is revised.