Middle Ordovician (Darriwilian) conodonts from the Weemalla Formation, south of Orange, New South Wales

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Abstract

Detailed study of twenty species of conodonts from the basal Weemalla Formation (exposed in the Panuara district south of Orange, New South Wales) enables clarification of the concept of the unit and its regional stratigraphic relationships. The Weemalla Formation is of early Darriwilian to Gisbornian age, broadly comparable with the poorly fossiliferous Coombing Formation against which it is fault-juxtaposed. Both formations are overlain by the Forest Reefs Volcanics, of Late Ordovician age. Co-occurrence of Periodon macrodentatus, Drepanodus? bellburnensis, Paroistodus originalis? and Dzikodus hunanensis is indicative of an early Darriwilian (Da2) age for the basal part of the Weemalla Formation. Well-preserved specimens of Dzikodus hunanensis represent the first confirmed Australian record of this genus, which differs significantly from the superficially similar and often-confused Polonodus (previously recorded from basinal sequences with Da3 graptolites in Victoria). Domination in the Weemalla samples of Periodon macrodentatus, and its association with rare Spinodus sp. cf. spinatus, is interpreted as representing a deep-water (lower slope) biofacies typical of the Open-Sea Realm. This is consistent with the turbiditic siltstones, sandstones, and occasionally graptolitic shales comprising the Weemalla Formation.