Climatic and tectonic control on sandstone composition in the Permo-Triassic Sydney foreland basin, eastern Australia

The Permo-Triassic Sydney basin is located between a craton to the west and a volcanic source to the east. Systematic variations in sandstone composition, both in space and time, are characteristic patterns in the Sydney basin. A sharp change in sandstone composition is observed in the Late Permian from a quartzo-feldspathic subarkose to volcanic arenite. This change is attributed to the syndepositional development of a volcanic source along the eastern margin of the basin. Throughout the latter part of the Late Permian until Early Triassic time, the sediments were primarily derived from the volcanic source with a secondary mode from the craton. During this time, sandstone composition shows a gradual change in maturity from volcanic arenite to quartz arenite. This maturation was mostly related to chemical weathering in a temperate-humid climate in the source area. Petrographic data from the Sydney basin show that even temperate-humid climate can generate quartz arenite from a mixed cratonic-volcanic provenance and obliterate the plate tectonic signatures. In the Middle Triassic, the volcanic source was virtually shut off, and a quartz arenite unit was derived from the deeply weathered craton. Spatial variations in sandstone composition within correlative units and cyclicity within the broad maturity trend are also observed. Sandstones rich in volcanic grains to the east and sandstones rich in cratonic quartzose grains to the west, are related to the proximity of the respective source terrain. The cyclicity in compositional trends is related to a subtle variation in the duration of chemical weathering and fluvial dynamics.