Middle Ordovician (Darriwilian) conodonts from allochthonous limestones in the Oakdale Formation of central New South Wales

Abstract

Twenty species of late Middle Ordovician conodonts including two new forms, Erraticodon bellevuensis sp. nov. and Ansella? haustra sp. nov., are described and illustrated from allochthonous limestone blocks in the Oakdale Formation of the Bell River Valley, N.S.W. The co-occurrence of Paroistodus horridus, Histiodella kristinae and Appalachignathus delicalulus in the fauna suggests the limestone was originally deposited in late Darriwilian (most likely early-mid Da3) time, providing a maximum age constraint on its inclusion in the Oakdale Formation. Some faunal similarities are apparent between this conodont fauna and that from (1) allochthonous limestone clasts within the Fairbridge Volcanics below the Wahringa Limestone Member and (2) contemporaneous deep-water cherts in basinal turbidite facies of the Adaminaby Group. The latter similarity suggests that prior to downslope displacement into the Oakdale Formation, the original depositional environment of the limestone was on an unstable outer shelf edge rather than in shallow water depths.