Palynology, sequence stratigraphy, and palaeoenvironments of middle to upper Jurassic strata, Bayu-Undan field, Timor Sea region

Abstract

Detailed palynological analysis of 230 core and sidewall core samples from the uppermost Plover, Elang, and lower Frigate formations, Bayu-Undan Field, Timor Sea, reveals the presence of diverse and moderately well preserved palynomorph assemblages (predominantly dinoflagellate cysts, together with acritarchs, spores, and pollen grains) that facilitate biostratigraphic, sequence stratigraphic, and palaeoenvironmental interpretations. The palynomorph assemblages, detailed systematically herein, comprise 96 spore and pollen species, 84 dinoflagellate species, and 15 acritarch species. Of these, the following 14 dinoflagellate cyst species are newly instituted: Ampulladinium aiax, Batiacasphaera macbethiae, Broomea fusticulum, Cauca bayuiana, Egmontodinium elongatum, Ellipsoidictyum fenestellum, Evansia? lacryma, Leberidocysta? strigosus, Leptodinium? ancoralium, Meiourogonyaulax baculata, Nannoceratopsis reticulata, Paragonyaulacysta? feneseptata, P. helbyi, and Yalkalpodinium playfordii. A further five species are not formally named due to a lack of adequately preserved specimens. One genus, Broomea COOKSONE 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) provide a means to identify 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 gradient from very nearshore to offshore environments or possibly an increase in salinities from euryhaline to stenohaline conditions.