LITHOFACIES, ORGANIC MATTER, DEPOSITIONAL ENVIRONMENTS, LAKE-FACIES ASSOCIATIONS, AND LAKE-BASIN TYPES OF THE RUNDLE AND CURLEW FORMATIONS (MIDDLE-LATE EOCENE), QUEENSLAND, AUSTRALIA IN THE ERD-110 CORE

The Rundle and Curlew Formations in the ERD-110 core contain a wide range of lithofacies and organic matter that record varying conditions of sedimentation and preservation in an evolving lacustrine system. There is a strong correlation of organic matter type and quality with lithofacies, sedimentary structures, bedding style, and stacking patterns. Major lithofacies include kerogenite, kerogenous shale, calcareous claystone, carbonaceous claystone, and lignite, along with siltstone, sandstone, and dolomite. Body and trace-fossil content varies systematically among the members: ostracods are abundant in most units, gastropods and fish, turtle, and crocodilian fragments are common to sparse, and large coprolites and inclined to horizontal burrows are locally common. Organic-matter ranges from 0.5 to 40.3 wt% TOC, hydrogen indices range from 5 to 1296 mg/g, and Fischer assay yields range from 0 to 212 l/T. Chemical and visual examinations reveal 3 major types of organic matter: 1)well-preserved algal-amorphous with high TOC and HI, 2)moderate TOC-HI amorphous kerogen probably derived from herbaceous debris, and 3)low TOC-HI, poorly preserved terrigenous (herbaceous/woody/coaly).

These formations accumulated at low paleolatitude under persistently warm humid conditions. The hydrology of the lake basin appears to have evolved from persistently open in the underlying Worthington Formation (fluvial) to intermittently to persistently closed in the Rundle Formation (freshwater and saline lacustrine) 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. Given the relatively constant climatic conditions, it appears that major changes in behavior of the lake system were driven mainly by potential accommodation (subsidence) changes interacting with rates of supply of water and sediment.