Facies and allostratigraphy of high-latitude, glacially influenced marine strata of the Early Permian southern Sydney Basin, Australia

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Abstract

The Sydney Basin of New South Wales, Australia is a foreland basin containing a thick (up to 10 km) Permo-Triassic succession. The southern margin of the basin exposes strata deposited during Late Palaeozoic glaciation of south-eastern Gondwana. The Early Permian Wasp Head, Pebbley Beach, Snapper Point Formations and Wandrawandian Siltstone were deposited between 277 and 258 Ma on a polar, glacially influenced continental margin adjacent to ice sheets located over East Antarctica and eastern Australia. Sedimentary facies, together with related ichnofacies and fauna, can be grouped into six facies associations that record marine sub-environments ranging from high energy, storm-dominated inner shelf to turbidite-dominated upper slope settings. Cold marine conditions, with near-freezing bottom water temperatures, are recorded by glendonites. Ice-rafted debris, most likely deposited by icebergs, occurs in almost all facies associations.

An allostratigraphic approach, emphasizing the recognition of bounding discontinuities (i.e. erosion surfaces and marine flooding surfaces), is used to subdivide the Early Permian stratigraphy into facies successions. Three types of succession can be identified and record changes in the relative influence of allocyclic controls such as basin tectonics, sediment supply and glacio-eustatic sea level variation.

Together, sedimentological and allostratigraphic data allow reconstruction of the depositional history of the south-western margin of the Sydney Basin. Initial marine sedimentation, characterized by sediment gravity flows and storm-deposited sandstones of the lower Wasp Head Formation, occurred adjacent to a faulted basin margin. Overlying successions within the upper Wasp Head, Pebbley Beach and Snapper Point Formations, record aggradation in inner to outer shelf settings along a storm- and glacially influenced continental margin. Tectonic subsidence and basin flooding is recorded by deeper water turbidites of the Wandrawandian Siltstone.

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