Late Ordovician allochthonous limestones in Late Silurian Barnby Hills Shale, central western New South Wales

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Abstract

Allochthonous limestone blocks exposed in the Eurimbla area, west of the Mitchell Highway between Molong and Wellington, are substantially older than the enclosing Barnby Hills Shale of Late Silurian age. Nine of the blocks yielded a diverse Late Ordovician conodont fauna, dominated by Panderodus gracilis, Belodina confluens, Periodon grandis, Paroistodus? nowlani and Yaoxianognathus? tunguskaensis. Occurrence of Taoqupognathus blandus in seven sampled blocks indicates a middle Eastonian (Ea2) age, although rare Taoqupognathus tumidus in one suggests an extension into the upper Eastonian (Ea3). These age determinations are confirmed by the presence of a silicified brachiopod fauna with typical elements (predominantly Mabella halis and Doleroides mixticius) of the previously defined fauna B of Eastonian 2 age. The conodont and articulate brachiopod faunas from the Eurimbla blocks are comparable with those from autochthonous limestones of Eastonian age elsewhere in the Molong Volcanic Belt, in particular the Bowan Park Group, except for occurrence of the conodont Webbygnathus munusculum and brachiopod Sowerbyella billabongensis which, in the Lachlan Orogen, are otherwise known only from the Junee-Narromine Volcanic Belt to the west. The allochthonous blocks may have been subject to one or more episodes of erosion and redeposition, prior to final emplacement in the Barnby Hills Shale.