Review of the Cambrian–Ordovician Wagonga Group, Lachlan Orogen, New South Wales South Coast, Australia: submarine slides, mélange and subduction

The Cambrian–Ordovician Wagonga Group of the Narooma and Batemans Bay regions, coastal southeastern New South Wales in the southeastern Lachlan Orogen, consists of oceanic rocks that were affected by major deformation during the Late Ordovician to early Silurian Benambran Orogeny. At Narooma, the stratigraphy is based on Cambrian–Ordovician conodonts in the Narooma Chert, which has a lower part containing bedded chert and interbedded siliceous mudstone and an upper part of black siliceous mudstone and interbedded white–grey mudstone/siltstone. The Narooma Chert is associated with the Kianga Basalt that includes pillow lava but also common mafic breccia/conglomerate. Overlying the upper Narooma Chert is the unfossiliferous Bogolo Formation containing mudstone and mélange of potentially diverse origins. The structure is complex with tight-isoclinal folding, mélange and potential thrust repetition of the succession, and is faulted against the Ordovician Adaminaby Group. The same stratigraphy is mapped at Batemans Bay in addition to interfingering of the Narooma Chert and Adaminaby Group turbidites at Burrewarra, Melville and Pretty points. The late Cambrian limestones at Burrewarra Point are associated with mafic breccia/conglomerate and are interpreted as ocean-island submarine slide deposits analogous to those on the modern deep-sea floor surrounding the Hawaiian Islands. These deposits are overlain by Ordovician quartz turbidites and Narooma Chert. Around Batemans Bay quartz turbidites and lithic turbidites are interstratified in sections of the Narooma Chert attesting to the interplay of the Ordovician turbidite fan and its interaction with ocean-island sources. The ancient ocean basin was closed by subduction in the Benambran Orogeny and associated with the later phase of magmatic activity in the Macquarie Arc. A subduction complex setting for this final accretionary deformation has been proposed but lacks a well-developed forearc basin compared with other subduction complex-forearc basin couplets of ­circum-Pacific orogenic belts. The Wagonga Group is an oceanic succession that includes pelagic/hemipelagic deposits, turbidites, ocean-island fragments, submarine slide deposits and mélange. The succession is strongly deformed with intense ductile deformation in the Late Ordovician–early Silurian Benambran Orogeny. The Wagonga Group and Adaminaby Group quartz turbidites were accreted to a subduction complex during the Benambran Orogeny.