PRELIMINARY CONODONT CORRELATION OF THE LOWER-MIDDLE ORDOVICIAN (FLOIAN-DARRIWILIAN) ROSROE FORMATION WITH THE TOURMAKEADY FORMATION, SOUTH MAYO TROUGH, WESTERN IRELAND

Conodont samples from two new localities support the idea that there exists an upper and a lower conodont assemblage in rocks mapped as the Rosroe Fm. in western Ireland. Stouge and others (2015) described the conodont fauna from two Rosroe Fm. localities, the Rosroe Peninsula and Lough Nafooey. A Floian (Lower Ordovician) lower conodont assemblage was described from the Lough Nafooey locality and a Darriwilian (Middle Ordovician) upper conodont assemblage was described from the Rosroe Peninsula. Both localities were recollected and the ages confirmed. A new locality on the south side of Killary Bay contains small carbonate clasts (<5 cm in diameter) in a dominantly clastic conglomeratic debris flow. One of the clasts yielded a conodont fauna that is dominated by Parapanderodus striatus along with Protopanderodus cf. varicostatus, Paraoistodus? sp., and indeterminate oistodiform and coniform elements. This fauna is consistent with the Darriwilian age for the upper conodont assemblage described by Stouge and others, and correlates with their Rosroe Peninsula samples. A new locality on the south side of Lough Nafooey in a unit mapped as the Rosroe Fm. yielded a fauna dominated by Periodon aff. aculeatus from one initial sample. This fauna is consistent with the Floian age for the lower conodont assemblage described by Stouge and others, and correlates with their Lough Nafooey sample. The preservation of the elements is unusual as the elements are coated by red quartzitic silt and slightly deformed, stretched and cracked, just like the conodonts described by Stouge and others. Two samples from the Tourmakeady Fm. yielded a fauna consisting of Oistodus aff. lanceolatus, Periodon cf. flabellum, Protopanderodus aff. gradatus, and Paraoistodus? sp. This fauna is consistent with the few previously reported conodont faunas from the Toumakeady Fm., and is consistent with a Floian age that is at least in part correlative with the lower conodont assemblage reported from the Rosroe Fm. These new conodont data support Stouge and others’ idea that the carbonates at the base of the unit mapped as Rosroe Fm. on the south side of Lough Nafooey are contemporaneous with the Toumakeady carbonates approximately 10 kilometers northeast of Lough Nafooey.