Depositional environment, stratigraphy, structure and paleobiology of the Hatchery Creek Group (Early- Middle Devonian) near Wee Jasper, New South Wales

Abstract

A revised depositional model of predominantly swampy rather than lacustrine conditions is proposed for the upper dark shales and mudstones (Corradigbee Formation) of the Hatchery Creek Group. The whole sequence is interpreted as a humid alluvial fan deposit, conformable on underlying limestones, with a total thickness of about 1800 m. Cyclic sedimentation probably resulted from climatic fluctuations much longer than seasonal events and may reflect Milankovitch cyclicity. The most recent Devonian time-scale calibrations indicate that much of the Hatchery Creek sequence could have been deposited during the Emsian, giving adequate time for subsequent folding during the Middle Devonian Tabberabberan episode. The Corradigbee Formation contains a unique fossil fish assemblage, not represented elsewhere in eastern Australia, but sharing features with Early Devonian faunas from Yunnan, China, and the Middle Devonian Aztec Siltstone fish fauna of Victoria Land, Antarctica. The first invertebrate fossils are recorded from the Hatchery Creek Group (freshwater gastropods, indeterminate arthropods). Abundant plant remains at some localities include lycopsids, some early leaf-like structures, and deep root systems preserved in paleosols, the earliest record of such features from Australia. The new data are inconsistent with Northern Hemisphere fossil evidence linked to a modelled dramatic drop in CO2 levels and rise in O2 during the Devonian Period, but comply with some other evidence that the first forests may have evolved somewhat earlier in East Gondwana than elsewhere.