Sediments from two Queensland Tertiary lacustrine oil shale deposits, the Lowmead and Duaringa Basins, have been petrologically characterized from standard X-ray powder diffraction analyses, backscattered electron microscopy and electron microprobe data. The results presented illustrate the potential of backscattered electron imagery for the study of diagenesis in fine-grained, organic-rich sediments.
The Lowmead Formation consists of lamosites, claystones and brown coals deposited in freshwater lacustrine and fringing peat-swamp environements. Their dominant constituents are kaolinite and quartz, with opaline silica and siderite important as eogenetic phases. Despite its similar geological setting, the Duaringa Formation contains a more varied lithological assemblage with less organic matter being preserved. Freshwater lacustrine claystones and lamosites are interbedded with carbonate sediments deposited during periods of saline/alkaline conditions in the lake. Within these carbonate sediments, major growth of eogenetic mineral phases occurred, principally dolomite and analcite. As in the Lowmead Basin, siderite is an important eogenetic phase in the Duaringa Formation. Two types of sideritic ironstone band occur throughout both basins; sphaerosiderites within claystone/coal sediments and fine-grained bands of siderite within lamosites. The bulk mineralogy of the siliciclastic detritus shows no significant trends with depth within either basin.