Carboniferous to tertiary geology of the Airlie Block, northeast Queensland

The Airlie Block is a volcanic dominated geological province, located in coastal northeast Queensland, and comprising part of the northernmost New England Orogen. Twelve geological units define a stratigraphy including the Early Carboniferous, Early Permian, Late Triassic, Early Cretaceous and Tertiary. The Airlie Block thus records an almost continuous perspective on the development of the northern New England Orogen during the Late Palaeozoic (believed to be a period of predominantly crustal convergence) and Mesozoic (a period of major crustal extension). Exposure in the Airlie Block is dominated by three volcanic units, the Edgecumbe Beds (associated sediments are of Early Carboniferous age), the Airlie Volcanics (of either Early Permian or Early Cretaceous age), and the Proserpine Volcanics (of Early Cretaceous age). The Proserpine Volcanics are the most extensive unit, and the best understood. They comprise a sequence of basalt to rhyolite lavas, dykes, and pyroclastic deposits. The lavas formed small or low relief deposits (mainly domes and lava fields or mesa flows). Dykes are numerous and extensive, locally forming over 40% of exposure. They show a systematic variation in strike direction through the Airlie Block, interpreted to reflect original rift-fissure systems that controlled eruption and deposition. The pyroclastic deposits are mainly large volume ignimbrites (minimum of 300 km'), but include co-ignimbrite deposits as well as distinct ash fall, scoria fall and base surge deposits. In addition to these volcanic rocks, minor amounts of volcanic and basement derived sediments are exposed. These include conglomerate and megabreccia (minor volcanic debris flows), volcanic gravels and arkosic sandstones, and lacustrine mudstones and shales. Four major volcano-sedimentary facies are defined from these lithologies, describing the evolution of the Proserpine Volcanics within a sag basin or depression. These volcano-sedimentary facies are consistent with an extensional tectonomagmatic environment, also suggested by reconstructions of the eastern margin of the Australian plate for the Early Cretaceous. The Proserpine Volcanics are subdivided into three major petrographic groups basalt - andesite, dacite and rhyolite. These groups are further subdivided into nine petrographic types based on differing phenocryst assemblages. Important phenocrysts are plagioclase (anorthite through to andesine), orthopyroxene (invariably uralitised), augite, amphibole, and Fe-Ti oxides. Quartz and sanidine are rare phenocryst phases amongst the rhyolites. Chemically, the Proserpine Volcanics define a calc-alkaline to marginally alkaline range from basalt through to rhyolite. However they do not represent a singe fractionated series. Trace element chemistry suggests that a combination of crystal fractionation and mixing modified a range of basaltic and rhyodacitic parental magmas. The Edgecumbe Beds and Airlie Volcanics have similar calc-alkaline geochemistry and petrography to the Proserpine Volcanics, and are only distinguished by a combination of structure, volcano-sedimentary facies and relative stratigraphic level. The Edgecumbe Beds consist of two distinctive vertically dipping sequences, a lower sedimentary sequence (including lithic and arkosic sandstones, mudstones, shales, and fossiliferous clastic and oolitic limestones), and an upper volcanic sequence (including basalt to rhyolite lavas and ignimbrites, as well as minor lithic sandstones and limestone). The volcano-sedimentary facies of the Edgecimibe Beds indicate that the lower sequence was deposited in a combination of subaerial and marine-shelf environments. The upper volcanic sequence has a similar facies to the Proserpine Volcanics, and is also envisaged to have been erupted within a rift or depression. The Airlie Volcanics consist of interbedded basalt to andesite lavas, voluminous derived debris flow deposits, and minor basalt to dacite pyroclastics (ash fall tuffs and thin ignimbrites). Lithologies are envisaged to represent the near vent to proximal facies of an andesitic stratovolcano, with minor distal input from a separate felsic volcano. The Airlie Volcanics are tentatively related to a series of Early Permian diorite stocks (the Dingo Diorite), that may represent magma chambers to the volcanics. In addition to the three main volcanic units, field criteria and dating have distinguished nine other units. These include: granitic batholiths of Carboniferous/Permian (Gloucester Granite) and Triassic age; small exposures of Triassic rhyolite lavas (Mt Rooper Rhyolite); a series of Triassic granite stocks (Passage Islet Granite); a thin sedimentary unit of Early Permian or Early Cretaceous age (Wetruth Sandstone); Early Cretaceous diorite to granite stocks associated with the Proserpine Volcanics (Dryander and Ben Lomond Granite); and at least one basaltic dyke of Tertiary age. These units also show similar geochemistry to the Proserpine Volcanics (volcanics show similar petrography), irrespective of age or interpreted tectonic environment. This indicates a common or geochemically similar source region(s) for all of these magmas. An important characteristic of the Airlie Block is its fault defined boundaries. These were created as a result of complex Cretaceous and Tertiary tectonics related to the breakup of the eastern Australian Plate. Initial extensional faulting in the Early Cretaceous induced graben and rift formation, while subsequent sinistral strike-slip faulting partially reactivated these structures. This latter episode of faulting resulted in tilting of parts of the Airlie Block, as well as formation of the Hillsborough Basin along the southwestern margin of the Airlie Block. The present day geomorphology of the Airlie Block reflects some of the geological features mentioned above. The critical factor is the relative resilience of key units to erosion, and landforms are often shaped by the orientation of strata, fault zones, and dyke swarms. Additionally, active sag subsidence and sedimentation in the Hillsborough Basin has resulted in southwestern tilting of the central Airiie Block, deflecting drainage accordingly