Year

1995

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

The Permian (Sakmarian-Artinskian) Talaterang and Shoalhaven Groups form the basal part of the Sydney Basin succession at its southernmost onshore extremity. A new stratigraphic model is proposed for the southern Sydney Basin and, although the previous group division is retained, considerable rearrangement of formations within and between the groups has been necessary as a result of recent field work. The Talaterang Group now includes the Clyde Coal Measures (incorporating the previous Pigeon House Creek Siltstone) and the Wasp Head Formation. The lower units in the overlying Shoalhaven Group are the Yadboro and Tallong Conglomerates, Pebbley Beach Formation, Yarrunga Coal Measures and Snapper Point Formation. The Yarrunga Coal Measures is now considered to be a completely separate unit from the Clyde Coal Measures and overlies the Tallong and Yadboro Conglomerates.

Five distinct depositional systems are recognised. Within the Talaterang Group, north-directed sediment dispersal in the mud-rich alluvial Clyde Coal Measures sequence and high energy east-directed debris flows in the Wasp Head Formation (system 1) suggest axial and transverse drainage related to possible north-trending extensional (rift) sub-basins similar to units in the Gunnedah and Bowen Basins. A succeeding phase of passive thermal subsidence initiated deposition of the Shoalhaven Group comprising a high energy west-directed alluvial braidplain to coastal succession (Yadboro and Tallong Conglomerates, Yarrunga Coal Measures and Pebbley Beach Formation; system 2), a broadly transgressive fluvial to marine sandstone and siltstone succession (Snapper Point Formation and Wandrawandian Siltstone; system 3), a progradational (regressive) shoreface succession (the lower Nowra Sandstone; system 4) and a transgressive marine succession (upper Nowra Sandstone and Berry Siltstone; system 5). Two tectonic phases correspond with the lithostratigraphic group divisions. The Talaterang Group was deposited within fault-bounded sub-basins (grabens or half grabens) as a result of an extensional phase which is widely documented at this stratigraphic level elsewhere in the Sydney-Bowen Basin. The Shoalhaven Group was deposited during a second phase represented by thermal sag and embryonic foreland development. Major flooding events which culminated in maximum flooding surfaces within the Wandrawandian Siltstone and the Berry Siltstone resulted from foreland accretion events during early basin development. This is indicated by a change from wave-dominated to longshore tidal facies within shoreface sediments near the top of Snapper Point Formation, and the presence of a tuffaceous unit in the Wandrawandian Siltstone, indicating that the foreland may have become emergent at this time. This inferred tectonic activity may represent the onset of the Hunter-Bowen Orogeny which came to a climax much later during the Late Permian.

The Nowra sandstone represents a progradational shoreface system which was derived from the cratonic western margin of the basin. This progradation may have resulted from uplift and subsequent erosion of the foreswell or forebulge of the basin, related to the tectonic loading event at the onset of deposition of the Wandrawandian Siltstone.

Three orders of sea level change are recognised within the succession. Third order change is the result of dominantly tectonic processes related to thermal subsidence and foreland accretion events. Fourth and fifth order sea level changes resulted in facies changes and parasequence development within the Yadboro and Tallong Conglomerates, Pebbley Beach Formation and Snapper Point Formation. There is good evidence within the Pebbley Beach Formation that these fourth and fifth order sea level changes were accompanied by climatic changes. On this basis fourth and fifth order sea level changes are attributed to Milankovitch Orbital forcing mechanisms. Fourth and fifth order cyclicity is superimposed on the larger third order tectonic trend.

The Talaterang and Shoalhaven groups provides insight into sedimentation at the cratonic margin of a foreland basin during embryonic stages of development. The succession reveals the complex interaction between tectonic affects (subsidence and uplift) and eustatic sea-level changes.

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