MOLECULAR FOSSILS IN COASTAL BITUMENS FROM SOUTHERN AUSTRALIA - SIGNATURES OF PRECURSOR BIOTA AND SOURCE-ROCK ENVIRONMENTS

Weathered waxy crude oil and black asphaltite (tar) are regularly wash ed ashore along the coastlines of South Australia and western Victoria . These bitumens generally strand on medium to high-energy, sandy beac hes in west or southwest facing bays. Some very weathered bitumens hav e been discovered in Holocene dune deposits up to 200 m from the prese nt-day coastline. Differences in the sulphur content and carbon, hydro gen and sulphur isotopic compositions of the coastal bitumens suggest that they belong to at least four oil families, one of which may be em anating from submarine seeps along Australia's highly faulted southern continental margin. The remainder appear to be flotsam carried south from S.E. Asia, possibly Sumatra. The waxy bitumens (Families 1-3) ran ge from paraffinic to aromatic-intermediate in bulk composition and co ntain up to 2.6% S. Their density (13-38-degrees API) dictates that th ey are near-surface drifters. They have low pristane/phytane ratios (p r/ph less-than-or-equal-to 2), abundant 4alpha-methyl steranes (C30 wi th 24-ethyl substitution) and, in most cases, high concentrations of b otryococcane. This biomarker assemblage indicates accumulation of fres hwater algal source material (including the remains of Botryococcus sp . and dinoflagellates) under anoxic to suboxic conditions. The additio nal presence of 28,30-bisnorhopane, dinosterane and 24-n-propylcholest ane among the molecular fossils in Family 3 bitumens suggests derivati on from a lacustrine organic facies subject to marine incursions; or, alternatively, mixing in the reservoir of two end-member (lacustrine a nd marine) oil types. Trace amounts of oleanane, and isomeric bicadina nes in the waxy bitumens constrain the age of their source rocks (Late Cretaceous or younger) and also preclude their local derivation; an I ndonesian origin is considered likely, implying long-distance transpor t by surface ocean currents. The Family 4 bitumens (approximately 5-de grees API) are bottom drifters. These sulphur-rich (3-6% S), aromatic- asphaltic crudes have molecular signatures that include: pr/ph approxi mately 1; C27 > C29 > C28 > C30 desmethyl steranes; dinosterane domina nt over 4alpha-methyl-24-ethylcholestane; an enhanced concentration of 28,30-bisnorhopane relative to hopane; and the absence of 2- or 3-met hylhopanes. A distal, anoxic marine facies of the Late Cretaceous Belf ast Mudstone in the eastern Otway Basin, or its lithostratigraphic equ ivalent in the Duntroon Basin, are possible local sources of the aspha ltic bitumens.