Underfilled-Overfilled Cycles in a Foreland Basin
OR
About the Course
PESA members, view the course for free!
The Permian and Triassic strata of the Sydney Basin provide an ideal opportunity to better understand the relative roles of climate and tectonism during deposition. The strata were deposited in the foreland basin of the New England Fold Belt during the Hunter-Bowen Orogeny. During deposition the world experienced a transformation from icehouse conditions in the Permian to greenhouse conditions in the Triassic. The basin covers an area of over 36000 km2 onshore and the Permian and Triassic strata are over 3 km thick with the depocenter close to the city of Sydney.
A detailed facies analysis and sequence stratigraphic study was undertaken of the Late Permian and Early Triassic sediments from core and outcrop data and by constructing regional cross-sections from well data. New paleogeographic reconstructions were made for various stages. The sediments were deposited in alluvial plain, coastal plain and shallow marine environments. Bay and estuarine deposits are common. Numerous coal beds in the Permian indicate that mires were extensive at times whereas extensive and thick red paleosols in the Triassic suggest substantially drier conditions. Much of the sediment entering the basin appears to have been derived from the New England Fold Belt and related uplifts, including
alluvial fan deposits close to the basin margin. Minor amounts of sediment were derived from the southwest during the Late Permian but a large influx of quartzitic sediment from that direction entered the basin in the Middle Triassic to form the Hawkesbury Sandstone.
A model of episodic thrusting in the New England Fold Belt, with related variations in subsidence patterns, is key to understanding the stratal geometries of the basin. A series of unconformities within the Permian and Triassic are more pronounced towards the southwestern part of the basin and are interpreted to have formed as a result of periodic uplift of a foreland bulge related to active thrusting in the New England Fold Belt that lies to the northeast of the basin. The late Permian coal measures were deposited while the basin was underfilled, with axial drainage and the most distal deposits present in the centre of the basin. By contrast, the basin was overfilled during several episodes during the Early Triassic with drainage across the entire basin away from the fold-belt but the basin returned to an underfilled pattern in the Middle Triassic.
The Sydney Basin is a globally significant location for understanding the nature of the Permian to Triassic transition and related extinction event and climate change. The new model of underfilled-overfilled cycles in the foreland basin requires a more nuanced interpretation than just invoking climate change to explain the changes in sedimentation. The red bed paleosols, for example, appear to have formed in well drained floodplain environments rather than the coastal water-logged mires in which the Permian coals accumulated.
Your Instructor
Chen Yu obtained a Master’s degree in Petroleum Geoscience at the University of Adelaide and is now close to finishing a PhD thesis on the topic: “Sedimentology and basin evolution of the Sydney Basin, New South Wales, Australia” at the Australian School of Petroleum and Energy Resources, University of Adelaide.