Jurassic coal in Western Australia and its depositional environment

The Jurassic coal in Western Australia is represented by the Cattamarra Coal Measures of the Cockleshell Gully Formation, located in the Hill River area. The area occupies the Hill River Shelf of North Perth Basin, situated approximately 225 km north of Perth. Six drill core samples from the coal measures comprising coal seams G 1 to G 6 were examined for lithotype and maceral analyses. The coal measures subcrop in a half-graben, bounded by the Lesueur-Peron Fault in the west, and the Warrada!ge Fault in the east. Predominantly, the coal comprises banded, dull banded, and dull lithotypes, with minor bright, bright banded, and fusainous types, or is regarded as clarain and durain, with minor amounts of vi train and fusain. Tiny specks of pyrite are present in the coal. Maceral analysis reveals that the coal is rich in vitrinite and inertinite, whilst the exinite and mineral matter are minor in content. The vitrinite, composed mainly of telocollinite and desmocollinite, varies from 4l.6% to 73.0%. Meanwhile, the inertinite having a range from 10.4% to 24.8%, is dominated by semifusinite, fusinite, and inertodetrinite. The exinite group content which is relatively low with 6.2%-20.4% in range, is represented by sporinite, cutinite, alginite, and resinite. The mineral matter, dominated by clay and pyrite ranges from 2.8% to 35.6%. The microlithotypes analyses show that the vitrite plus clarite content varies between 42.5% to 64.8%, intermediates from 25.4% to 50.7%, whilst inertite plus durite content is relatively low, ranging from 4.8% to 18.5%. Based on lithotype, maceral, and microlithotype analyses, the depositional environment of the coal is interpreted as a telmatic wet forest swamp of a brackish to upper-lower delta plain environment. Furthermore, trace element distribution, especially Boron content in the coal supports the presence of a marine influence during peat deposition in the mire. Thereby, the environment of coal deposition is postulated to be an upper to lower delta type.