Palynology, sequence stratigraphy, and palaeoenvironments of Middle to Upper Jurassic strata, Bayu-Undan Field, Timor Sea region Part Two

摘要

This second part of a detailed palynological study of 230 core and sidewall core samples from the uppermost Plover, Elang, and lower Frigate formations, Bayu-Undan Field, Timor Sea, concludes the dinoflagellate (Rigaudella to Woodinia), acritarch, and prasinophyte phycomata taxonomy, and examines the biostratigraphy, sequence stratigraphy, and palaeoenvironments from this stratigraphic interval. The palynomorph assemblages, detailed systematically herein, comprise 55 dinoflagellate species and 17 acritarch and prasinophyte phycomata groupings. Of these, the following six dinoflagellate cyst species are newly instituted: Ampulladinium aiax, Batiacasphaera macbethiae, Cauca bayuiana, Ellipsoidictyum fenestellum, Leberidocysta? strigosa, and Yalkalpodinium playfordii. A further species Is not formally named due to a lack of adequately preserved specimens. The palynostratigraphic sequence has previously been assessed as latest Bathonian to early Oxfordian in age, based mainly on correlations with European dinoflagellate assemblages dated by ammonite faunas. Ten subzones (TB1 through 5 and M through 5) are informally proposed, collectively encompassing the Ternia balmei and Voodooia tabulata dinoflagellate cyst Interval Zones. The subzones are defined variously on first and last appearance datums, first and last common occurrences, and species acmes. The acme events are coincident with marine flooding surfaces and enable precise correlation across the Bayu-Undan Field. The Elang Formation - the prime focus of this study - is divided into three third-order sequences, each comprising a lowstand, transgressive, and highstand systems tract. These tracts and their delimiting stratal horizons or flooding surfaces possess distinctive palynomorph or palynodebris assemblages. Qualitative and quantitative variations of these assemblages correlate very closely with gamma-ray log profiles and can consistently be placed within a sequence stratigraphic framework across the Bayu-Undan Field. Palynofacies analyses provide additional means to distinguish among the systems tracts; viz., microphytoplankton diversity increases through transgressive systems tracts to a peak diversity at the maximum flooding surface; atypically brown wood frequencies may also peak at maximum flooding surfaces; and black debris frequencies increase higher in the highstand systems tracts. Palaeoenvironmental interpretations based primarily on palynological assemblages (especially microphytoplankton groupings and palynodebris distribution) enable identification of depositional settings in non-cored intervals. For instance, the occurrences of Ternia balmei, the Meiourogonyaulax group, the Ctenidodinium group, and the Rigaudella group represent an approximate gradation from very nearshore to offshore environments or possibly an increase in salinities from euryhaline to stenohaline conditions.