Controls on the Sedimentology and Paleoenvironmental Evolution of the Permian and Triassic of the Sydney Basin, New South Wales, Australia
Date
2021
Authors
Yu, Chen
Editors
Advisors
McCabe, Peter
Amos, Kathryn
Amos, Kathryn
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Abstract
The Sydney Basin covers an area of approximately 52000 km2 and is located around the city of Sydney in New South Wales, Australia. About 70% of the basin is onshore and the rest underlies the narrow continental shelf offshore. The Permian and Triassic strata that fill the basin provide an ideal opportunity to better understand the interplay of various allogenic controls during deposition of coastal to shallow marine strata. The early-middle Permian strata were deposited during a rift-sag phase whereas the later Permian and Triassic strata accumulated in a foreland basin related to the New England Fold Belt. During deposition of the strata, the world experienced a major transformation from icehouse conditions in the early Permian to greenhouse conditions in the Early Triassic. Observations and interpretations, based on a study of outcrops, well logs and cores, were made to provide new insights on the evolution of depositional systems, changes in climatic regimes, and the role of tectonism during evolution of the Sydney Basin. To this end, a series of regional cross-sections and paleogeographic maps were prepared. There is abundant evidence of cold climate conditions in the early Permian (Cisuralian) strata of the Sydney Basin. The basal formation in the southern part of the basin, the Wasp Head Formation, has abundant evidence of glacial activity including facies interpreted as tillites and glacial outwash. Large en echelon blocks are interpreted as glaciotectonic push moraines. Dropstones and glendonites are common in the Cisuralian sediments indicating cold marine waters with floating ice. Small ice-keel scours seen within shallow marine sediments also indicate the presence of at least seasonal floating sea ice. The abundance of dropstones appears to be more a function of sedimentation rates (more dropstones in sediments that accumulated slowly) than a function of the variations in floating sea ice over time. Most of the Cisuralian strata consist of shoreface-offshore facies with some estuarine deposits. The oldest marine strata suggest a wave-dominated shoreline with the development of thick units of hummocky and swaley bedding. The shoreline then became tide-dominated with abundant heterolithic strata. The coastline lay towards the west. In the late Cisuralian (Kungurian) a major transgression pushed the coastline further inland and much of the basin was covered by a shallow sea. At this time the fine-grained and fossiliferous Wandrawandian Siltstone was deposited in an extensive open embayment. The late Permian (Guadalupian-Lopingian) formations are coal-bearing and consist predominately of bay/lagoon facies with fluvial, tidal, estuarine and deltaic facies. The finest grained units have depositional facies that are similar to the Wandrawandian Siltstone and are interpreted as open bay deposits. Overall, the environment is envisaged to have been a vast bay system with associated coastal plains. The bay was connected to the open marine environment to the southeast, and rivers drained into the bay from the west, southwest and north. The Newcastle Coal Measures in the north-eastern part of the basin contain conglomeratic alluvial fan/pebbly channel facies derived from the adjacent New England Fold Belt that became a positive area at this time. The bay system shifted towards the southwest over time as t he r esult o f t he uplift of the orogenic belt and increased input of sediment from that source. The Lower Triassic (Induan-early Olenekian) strata of the Narrabeen Group consist predominantly of fluvial channel sandstones, heterolithic shallow lacustrine facies and red beds. The basin was dominated by a large alluvial plain with rivers draining towards the southwest away from the New England Fold Belt. The red beds thin away from the orogen and are interpreted as compound paleosols that accumulated in well drained areas. In the late Olenekian, a major transgression reintroduced marine conditions into the central part of the basin and the upper Narrabeen Group is dominated by heterolithic marineinfluenced channel and bay facies. Tidally-influenced channels ran east-west across the basin, as subparallel to the foreland basin axis but the northeastern lithic source remained dominant. A dramatic shift in the Middle Triassic (Anisian) introduced coarse-grained quartzitic sandstone from a source far to the south. The Hawkesbury Sandstone was deposited in a complex that has been compared to the Plio-Pleistocene sediment complex of the Rhine- Meuse Delta, with large estuarine channels connected to a major river system. The overlying fine-grained Wianamatta Group was deposited in depositional setting that transformed from freshwater lacustrine to brackish/marine, suggesting a long transgressive episode after the deposition of the Hawkesbury Sandstone. Abundant liquefaction and fluidisation features are present within the Anisian Hawkesbury Sandstone. Previously unrecognised large sand volcanoes have necks up to 27 m in diameter – the largest sand volcanoes documented from outcrops. Extensive massive sandstones, that are intimately related to the sand volcanoes, are interpreted as injectite complexes. The abundant evidence of liquefaction and fluidisation suggests that the basin experienced frequent seismic activity at this time, presumably related to thrusting in the fold belt but also possibly related to nearby volcanic activity. The fluidisation that created the injectites and ejectitites was probably related to water and gas expulsion from the underlying Permian coals. A new sequence stratigraphic model is proposed for the Guadalupian and Lopingian coal measures, with seven sequences that can be correlated regionally within the basin. The sequence boundaries are best developed in the southwestern part of the basin, which is interpreted to be the area of the forebulge. Five sedimentary cycles are also recognised in the Triassic Narrabeen Group. The cycles are better defined in the northwest, close to the orogenic front. Both the Permian sequences and Triassic cycles are interpreted to be due to variations in basin subsidence rates and sediment supply related to repeated thrusting events in the New England Fold Belt. The paleogeographic evolution of the basin during the late Permian to Middle Triassic is best explained by a model of tectonically-driven underfilled-overfilled cycles of the foreland basin. The late Permian coal measures were deposited during an underfilled condition, whereas the majority of the Early Triassic Narrabeen Group was deposited during an overfilled condition. Another underfilled phase was initiated during deposition of the uppermost Narrabeen Group. The concept of underfilled and overfilled basin fill suggests a more nuanced interpretation is required to understand the changes in sedimentation across the Permian-Triassic boundary. The red bed paleosols of the Narrabeen Group clearly formed in more arid conditions than the Permian coals but they also appear to have formed on a well-drained alluvial-fluvial plain with relative low water tables, rather than the coastal water-logged mires in which the coals accumulated. The absence of coals from the Triassic facies that were deposited in areas with high water tables may be best explained by a seasonal climate with extensive dry periods that prevented peat accumulation.
School/Discipline
Australian School of Petroleum and Energy Resources (ASPER)
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, Australian School of Petroleum and Energy Resources, 2022
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