Geological history of the Cairns-Townsville hinterland, North Queensland

A history of geological events has emerged from the regional geological mapping of 35,000 square miles of the Cairns-Townsville Hinterland of North Queensland from 1956 to 1959.The hinterland consists of a broad Precambrian shield - the Georgetown Shield -which is 200 miles long and 150 miles wide, and flanked to the east by a Palaeozoic geosynclinal zone 50 to 150 miles wide, containing 40,000 feet of sediments. The geosynclinal zone is the northern part of the Tasman Geosynclinal Zone, which occupies most of the eastern coast of Australia. The shield and geosynclinal zones are intruded by igneous rocks: 20,000 square miles of acid and 5,000 square miles of basic and ultrabasic crop out. The decipherable history of the Georgetown Shield began early in the Precambrian (Archaean?) when its central region was depressed into deep crustal levels. Sediments deposited in the depression were regionally metamorphosed to granulite, amphibolite, and migmatite, and during the Middle Precambrian (Lower Proterozoic?) uplifted to form a geanticline. At the same time the western flank of the central geanticline was down-warped to form the Etheridge Geosyncline, in which a total thickness of 30,000 feet of sediments most pelagic, with lesser amounts of terrigenous and organic sediments, was deposited. Also during this uplift 15,000 feet of fine-grained quartz and calcareous detritus were deposited on its eastern flank in the Paddy's Creek area. The Precambrian history of the Shield terminated with the intrusion of the Forsayth Batholith into the core of the geanticline, and of ultrabasic rocks along its eastern fractured edge. Steep-angled thrusting of the Archaean metamorphics over the Lower Proterozoic sediments may have accompanied both these intrusions. After the Precambrian, the shield was uplifted and fractured extensively during the deformation of the Tasman Geosyncline.The Tasman Geosyncline in North Queensland contains a maximum thickness of 409000 feet of sediments ranging from Upper Ordovician? to Permian, which were deposited in a belt 270 miles long and averaging 100 miles wide. The deformation of the geosyncline began in the late Silurian with the uplift of amass of early Palaeozoic sediments in its central region, which divided the original depositional area into two parts. During this uplift serpentine and gabbro were intruded along the south-western fractured edge of the central tectonic land. The uplift of the tectonic land continued in the Devonian and Carboniferous, and was one of the major events that controlled sedimentation in the geosyncline; it forced the depositional areas to contract and migrate, resulting in the replacement of the early widespread marine sedimentation by restricted freshwater sedimentation. The geosyncline was further deformed by fracturing along the south-eastern edge of the Shield, which gave rise to a rift - the Broken River Rift. The last sediments in the hinterland were deposited during the late Carboniferous and Permian; immediately after this period the whole of the Tasman Geosyncline and the Georgetown Shield was land, and the shield continued to arch upwards. This arching resulted in extensive fracturing of the central region of the shield and its north-eastern edges. The fractures formed rift valleys and cauldron subsidence areas, which were occupied by rhyolite and ignimbrite, coeval and comagmatic with two ages of granite. The extrusion of continental olivine basalt in the Cainozoic near the eastern fractured shield edges was the final igneous episode of the history of the Cairns-Townsville Hinterland.

Tags: AU-QLD, Earth Sciences, GA Publication, Published_External, Record, geology
Organization: Geoscience Australia Data