COASTAL BITUMENS FROM WESTERN-AUSTRALIA - LONG-DISTANCE TRANSPORT BY OCEAN CURRENTS

Samples of weathered bitumens were collected from granite rocks and quartz-sand beaches near the high-water mark along the west and south coasts of Western Australia. Samples were subjected to analysis using GC and GC MS techniques. The proportions of n-alkanes and branched and cyclic compounds in the samples were variable and the variation was attributed to differences in sample weathering. More detailed analysis of the bitumens for biomarkers revealed the presence of botryococcane, 4-methylsteranes, bicadinanes and oleanane. The occurrence of these compounds has also been observed in crude oils from the S.E. Asian region. The presence of bicadinanes has been attributed to a source contribution from an angiosperm currently confined to tropical regions and indicates that the source rocks from which the bitumens were derived were deposited after Late Cretaceous times. The absence of Late Cretaceous and Tertiary sediments deposited in lower latitudes in the northwest and western offshore basins of Australia suggests a non-local source of the bitumens. A mechanism involving the Leeuwin Current has been proposed to account for transport of these bitumens which are presumably sourced from locations in S.E. Asia.