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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.
Acknowledgements
For permission to conduct fieldwork on their properties, and providing access, we thank Ian and Helen Cathles of Cookmundoon (Wee Jasper), and Chris Barber and Neil Blasford (Corradigbee). Access to the area for the 1969 field mapping was facilitated by Dudley and Graham Barber. For assistance in 2003–2008 fieldwork, we thank B. Opdyke, K. S. W. Campbell, I. Cathles, C. Klootwijk, R. Hunt and L. Bean. Dr B. Opdyke (ANU) first proposed the humid alluvial fan interpretation, and Drs Brigitte Meyer-Berthaud (Montpellier), Elizabeth Truswell (ANU) and Greg Retallack (Oregon) gave advice on fossil plant material. The late Professor Stewart Edgell (died 13 April 2010) gave much helpful advice and provided a copy of his thesis and 1949 geological map. Professor Ken Campbell provided guidance and knowledge on numerous occasions and, with Dr Brad Opdyke, gave detailed comments reviewing an earlier version of the manuscript. Val Elder (ANU) assisted in specimen curation; B. Young helped with photography (ANU V3505); and we thank Xiong Cuihua for superb mechanical preparation of osteolepid specimens (ANU V3338, ANU V3133) at the Institute of Vertebrate Palaeontology & Palaeoanthropology, Beijing, and Professor M. Zhu and Ms Lu Jing for facilitating this, and providing images during preparation. We also thank the following for general advice on structural (M. Rickard, S. Cox) and stratigraphic geology (K. Crook, A. Felton, D. Strusz), and G. Retallack for a very helpful review which greatly improved the manuscript. This research was supported by ARC Discovery Grant DP0558499, as a contribution to IGCP Project 491. Provision of facilities at ANU in the Frank Fenner Building, College of Science, and D. A. Brown Building, Research School of Earth Sciences, is gratefully acknowledged.