GEOLOGY OF THE NORTH-EAST MARGIN OF THE GREAT ARTESIAN BASIN QUEENSLAND, AUSTRALIA

Abstract

The north-east margin of the basin as described herein comprises some 40,000 square miles situated regionally between the Bowen basin and the Drummond Range on the east and north-east respectively and the towns of Longreach, Charleville, and Mitchell on the west and south. The limit of the present-day Great Artesian basin in this area is marked by south-east striking outcrops of the Jurassic Precipice Sandstone.The stratigraphy of the area can be grossly subdivided into three time-rock units as follows:Pre-PermianPermian-TriassicLate Mesozoic and Tertiary.Pre-Permian rocks of importance include thick Devonian and Carboniferous units. Marine and evaporite facies are present in the Devonian in the south-west (Adavale basin), changing to more continental facies with volcanic and some marine incursions to the north-east (Drummond basin). Carboniferous units are dominantly paralic to continental types. The Devonian and Carboniferous systems were subjected to various degrees of pre-Permian tectonism and removal and, because of this, the existing occurrences of these systems are only fragments of their original extent. Permian and Triassic, marine to continental units were deposited across the area, thinning from east to west in possible cycles of transgression and regression. In addition to thinning regionally, these systems show marked variation in thickness due to transgression and drape and compaction over major, pre-Permian structural trends.Subsequent to deposition of the Triassic, a pronounced tectonic change was introduced. Previously negative areas to the north-east (Drummond basin—Springsure Shelf) began to rise and the Middle to Late Mesozoic subsidence and filling of the Great Artesian basin to the south-west commenced. Therefore, Triassic and Permian strata thicken regionally to the north-east under the Great Artesian basin margin, while post-Triassic strata thicken south-westward, in the opposite direction, into the basin.Three dominant structural trends are reflected in the sedimentary rocks of the area. These are:North-east trends under much of the Great Artesian basin margin.North-westerly trends of the Drummond basin to the north-east of the Great Artesian basin margin.Northern trends of the Denison trough east of the Great Artesian basin margin.