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Stratigraphy and fluvial sedimentary facies of the Neocomian lower Strzelecki Group, Gippsland Basin, Victoria

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Abstract

The Strzelecki Group incorporates Berriasian to Albian, fluvial sediments deposited in the Gippsland Basin during initial rifting between Australia and Antarctica. Neocomian strata of the lowermost Strzelecki Group are assigned to the Tyers River Subgroup (exposed in the Tyers area) and the Rhyll Arkose (exposed on Phillip Island and the Mornington Peninsula). The Tyers River Subgroup incorporates two formations: Tyers Conglomerate and Rintoul Creek Formation. The latter is subdivided into the Locmany and Exalt Members. Ten fluvial sedimentary facies are identified in the lowermost Strzelecki Group: two gravelly facies; four sandy facies; and four mudrock facies. Associations of these facies indicate: (i) prevalence of gravelly braided-river and alluvial-fan settings during deposition of the Tyers Conglomerate; (ii) more sluggish, sandy braided to meandering fluvial systems during Locmany Member sedimentation; and (iii) a return to active, sandy, braided-river settings for deposition of the Exalt Member. The Tyers Conglomerate and Rhyll Arkose rest on an irregular erosional surface incised into Palaeozoic rocks of the Lachlan Fold Belt. The overlying Rintoul Creek Formation incorporates more mature sediments where lithofacies associations varied according to base-level change, variations in subsidence rates, and/or tectonic uplift of the principal sediment-source terranes to the northwest.

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