Early Palaeozoic quartz turbidite fan and volcaniclastic apron, Mudgee district, northeastern Lachlan Fold Belt, New South Wales∗

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Abstract
Early Palaeozoic rocks of the northern Capertee Zone in the northeastern Lachlan Fold Belt consist of: (i) the Early Ordovician Adaminaby Group that is elsewhere widespread throughout the eastern half of the fold belt and contains a monotonous quartz turbidite succession, and (ii) the Late Ordovician‐Early Silurian Coomber Formation, a predominantly mafic volcaniclastic unit with subordinate mafic igneous rocks that represents a deep‐marine apron of island‐arc rocks farther west. In the Bara Creek area, the Adaminaby Group is divided into three informal units: unit A is a thick succession of thick‐bedded, graded sandstones with basal chert and slate; it is overlain by unit B with slate and bedded chert of late Darriwilian age; and is in turn overlain by unit C, a thin succession of thin‐bedded turbidites. The Coomber Formation conformably overlies the Adaminaby Group; the contact is sharp with no interfingering between the two units and indicates inundation of the terrigenous turbidite fan by the growing volcanic‐island chain. Concordantly overlying the Coomber Formation are Late Silurian silicic volcanics, clastics and limestone of the Dungeree Volcanics indicating shallowing of depositional environments either due to uplift or infilling of the basin of deposition. The Ordovician tectonic setting reflects west‐dipping subduction in the New England Fold Belt with arc magmatism in the northeastern Lachlan Fold Belt and deposition on the evolving arc crust of a giant submarine fan derived from the Delamerian mountain chain to the west. As elsewhere in the northeastern Lachlan Fold Belt, only one regional cleavage (S2) is developed and affects all units, including the nearby Late Devonian succession, consistent with an Early Carboniferous timing of the deformation. Folding and thrusting accompanied cleavage development with tectonic transport from west to east. D2 was imposed on F1 that is indicated by steeply plunging folds (F2), intersection lineations and some downward‐facing F2. D2 cleavage, folds and thrusts are folded in areas affected by significant D3 deformation resulting in locally subhorizontal S2 and common kinking of S2. Early Carboniferous deformation occurred in a back‐arc setting and was synchronous with the development of a huge subduction complex in the New England Fold Belt.