The stratigraphy and faunas of the permian marine sediments of the Gigoomgan area, Queensland

Geological work covering 50 square miles in the Gigoomgan area south west of Maryborough, Queensland, has established that marine Permian rocks in the area are folded into a broad anticline, the Gigoomgan Anticline. East of the axis of the anticline, Permian strata dip gently to the north east and are unconformably overlain by the widespread Glen Olive Conglomerate of Lower Triassic age, which also dips gently to the north east. The conglomerate passes up into the Brooweena Formation, which is known to contain Lower Triassic Marine fossils. West of the axis of the Gigoomgan Anticline a thick sequence of Permian marine rocks dip steeply to the South west. Permian and Triassic rocks are intruded by diorites of Late Permian age and these and the Permo-Triassic rocks are unconformably overlain by a Late Triassic sandstone, the Sunny Glen sandstone. This sandstone is intruded by the Sunshine Andesite possibly of Jurassic age. The Marine Permian rooks have been divided into formations, the lowest of which is the highly fossiliferous marine Lower Kolbar Beds. These are overlain by a sequence of vesicular basalts and pillow lavas, the Nant Basalts which in turn are overlain by the Upper Kolbar Beds with lithology and fauna very like the Lower Kolbar Beds. These beds are succeeded by a thin discontinuous greywacke, the Gundish Bridge Greywacke, which passes up into the Gigoomgan Limestone. The Gigoomgan Limestone described by Richards and Bryan (1932) as an algal limestone is now found to be comprised or fragments and prisms from the pelecypod Atomodesma Beyrich. The limestone is divided into three members; the lower and upper members represent the banks on which the Atomodesma shells lived and these were deposited in water near wave base. The middle limestone member consists mainih of single Atomodesma prisms and represents the basin limestone facies associated with the banks. Deposition of the Gigoomgan Limestone started with a marine transgression involving the underlying Gundish Bridge Greywacke and terminated with a marine regression at the top of the limestone involving the overlying Teebar Greywacke. Both greywackes cotain a fauna of Lower Permian age. Permian sediment at Gigoomgan correlate well with similar sediments at Gympie described by Runnegar (1963). The top of the Gigoomgan Limestone is found to contain a fauna (The Ingelarella ingelarensis zonule) which correlates well with a similar fauna at the top of the South Curra Limestone (The Ingelarella ingelarensis zone). This fauna, formerly described by Runnegar as middle Permian in age, is now demonstrated to be Lower Permian in age. Five species of Lower Permian brachiopods are described (and further contributions are made to the know-ledge of each species). For the first time the Lower Permian Spiriferid genus Attenuatella is described from Queensland.