Upper Jurassic (Oxfordian) dinoflagellate cyst taxonomy, palynostratigraphy and biosequence, stratigraphy of the Jansz-Io gas field, North West shelf, Australia

Abstract

High-resolution palynological analysis was conducted from four wells of the Jansz-Io Gas Field (Exmouth Plateau, Northern Carnarvon Basin) to refine the Australian Oxfordian Wanaea spectabilis microplankton biozone. The refined biozonation scheme was then utilised to produce integrated biostratigraphic, sequence stratigraphic and palaeoenvironmental interpretations of the Jansz Sandstone reservoir sequence. Palynological preparations from 155 conventional core samples yielded a highly diverse assemblage of dinoflagellate cysts and acritarchs of good to excellent preservation. The assemblage comprises 112 genera with 194 microplankton species and varieties. Of these, 2 new genera and 26 new species are formally described, 2 species are emended with 1 species generically reattributed, and an additional 11 genera and 37 species are informally described. For future reference of this well preserved material, each species is illustrated in a large catalogue of plates. Key taxa identified through quantitative analysis are utilised to refine the intersected portion of the Wanaea spectabilis microplankton zone. Proposed zonal subdivisions are defined on first and last stratigraphic occurrences, first and last common occurrences and species acmes which can be correlated across the Jansz-Io Field. The subzones are tentatively correlated to the Poulsen & Riding, 2003 Northern Hemisphere Dinoflagellate Cyst Zones; Bown & Cooper, 1998 Nannofossil Zones; the Groupe Francais d'Etude du Jurassique, 1997 Tethyan, Sub-Boreal and Boreal Ammonoid Zones; and the ICS 2008 Geologic Time Scale. Lithofacies and qualitative palynofacies analyses are integrated with petrophysical wireline logs to produce bio-sequence stratigraphic and palaeoenvironmental interpretations for the Jansz Sandstone reservoir sequence. Bioevent-derived zonule boundaries are identified to reflect 'biostratigraphic parasequences' arranged in transgressive/regressive couplets as indicated by an inversely oscillating AOM/wood ratio. These biostratigraphic parasequences provide valuable information regarding sequence and reservoir architecture in the absence of lithostratigraphic controls. The application of selected dinocyst species as palaeoenvironmental proxies is examined. Extreme intraspecific variability of some dinocyst taxa may reflect salinity and/or temperature changes caused by a high-frequency, cyclical climatic regime, such as a seasonal monsoonal climate.