Paleofloral, Paleofaunal, and Organic Geochemical Record of a Full Spectrum of Evolving Lake-Basin Types In the Oil Shales and Associated Lithofacies of the Rundle and Curlew Formations (Middle-Late Eocene), Queensland, Australia

Oil shales and associated lithofacies of the Rundle and Curlew Formations in the ERD-110 core contain a wide range of lithofacies and biofacies that vary systematically with lake-basin type.

The lowermost interval of the Rundle Formation (Telegraph Creek and lower Ramsay Crossing seams) and uppermost Curlew Formation represent fluvial-lacustrine lithofacies association (overfilled lake basin type); they contain Pediastrum, with some Cleistosphaeridium algae and Azolla capricornica megaspores, disarticulated osteoglossid fish remains, common Planorbis and sparse, poorly preserved ostracods, including Cadona sp. and Cypridopsis sp. Organic carbon content ranges from 0.9 to 19.15%, and Fisher assays from 3 to 188 l/t.

The overlying Brick Kiln, Humpy Creek, and Munduran Creek seams along with the Kerosene Creek seam just below the Curlew Fm comprises fluctuating-profundal lithofacies (balanced filled) with common fern spores, angiosperm pollen, and cf. Typha, in addition to Pediastrum and Cleistosphaeridium; vertebrate remains include: osteoglossid, percoid, theraponid, and lungfish, and turtles and crocodillians (Lutajanus, Percalates, Scleropages, Neroceratodus, Epiceratodus, Crocodilus). Invertebrates include gastropods (Planorbis, Pedinogyra) and ostracods (Cadona aequalis, Virgatocypris tenuistriata, Cadona subtriangularis, Cypridopsis linearis, Cypridopsis compressa, Cypris munduranensis, Stenocypris lowmeadensis). Organic carbon content ranges from 1.7 to 34.77%, and Fisher assays from 5 to 233 l/t.

The central Telegraph Creek seam represents evaporative lithofacies association (underfilled), with abundant desiccation cracks and insect burrows and minimal preserved paleoflora or fauna, restricted to only Botryococcus algae. Organic carbon content is restricted to a few thin intervals; it ranges from 1.4 to 15.27%, and Fisher assays from 0 to 142 l/t.

The hydrology of the basin appears to have evolved from persistently open to intermittently to persistently closed in the Rundle Formation to persistently open in the Curlew Formation (mire-floodplain). This evolution probably represents a full cycle of lake-basin fill: from fluvial to overfilled to balanced-filled to underfilled to balanced-filled to overfilled to fluvial.