The geology of the Fairfield Group, Canning Basin, Western Australia

The Canning Basin, which covers some 450,000 km2, is the largest sedimentary basin in Western Australia. The earliest Palaeozoic rocks are widespread marine Ordovician limestone, shale and sandstone up to 3260 m thick which unconformably overlie Precambrian metamorphic and sedimentary rocks. Overlying the Ordovician are Devonian rocks, consisting of evaporites and redbeds (probably Early and Middle Devonian) confined to the centre of the basin, and extensive Late Devonian reef carbonates on the northerly and, perhaps, the southerly margin, together with associated shale and sandstone in the centre of the basin. The Devonian sediments are overlain by Carboniferous, Permian, M e s o z o i c and Cainozoic sediments. During the Late Devonian and Carboniferous, sedimentation was virtually confined to the northern half of the Basin: on the Lennard Shelf, in the Fitzroy Trough, and on the Jurgurra Terrace (Fig. 2). The trough contains up to 6000 m of Carboniferous rocks. The Upper Devonian Lower Carboniferous platform sediments consist of alternating beds of limestone, shale, sandstone, and marl. Generally, they postdate the Devonian reef-complex (Playford &amp; Lowry, 1966) and are poorly exposed; consequently they have received less geological attention than the well-exposed reef carbonates. Previous work has resulted in differing interpretations of the formal stratigraphy; including different nomenclatures for the same rock bodies. The presence of an unconformity has also been questioned. During May - August 1972 a joint Bureau of Mineral Resources (BMR) Western Australian Geological Survey (WAGS) Field Party carried out detailed fieldwork in the vicinity of Fitzroy Crossing, Western Australia. Comprehensive collections of petrographic, palaeontological, and geochemical samples were made from the Upper Devonian and Lower Carboniferous to increase the understanding of the sequence. The area of outcrop is situated on the northern margin of the Canning Basin in the Kimberley Division of Western Australia (Fig. 1). Detailed fieldwork was concentrated in three areas; a narrow belt (up to 10 km wide) extending along the southwest face of the Napier and Oscar Ranges from Station Creek in the northwest to Fitzroy Crossing in the southeast, a distance of about 160 km; the Horseshoe and Burramundi Ranges, 65 km east of Fitzroy Crossing; and the Red Bluffs area, 135 km southeast of Fitzroy.  METHODS OF STUDY: The outcrop areas were mapped on air photographs at a scale of 1:80,000. Comprehensive sections were measured and samples for petrological, palaeontological, and geochemical studies were collected at intervals of 1.5 m, or more, depending on outcrop. Thirty sections were measured, from which some 500 samples were collected. Detailed reports have been published or are in press dealing with the distinction of limestone's in the sequence (Radke, 1976), the conodonts (Nicoll &amp; Druce, in prep.), and geochemistry (Druce &amp; Radke, 1977). In addition to surface sampling, three holes were drilled to provide information on the sequence between the oldest known Carboniferous rocks and the subsurface equivalents.

Tags: AU-WA, Bulletin, Earth Sciences, GA Publication, Published_External, geology
Organization: Australian Ocean Data Network