CORRELATION OF THE LOWER PART OF THE ROSROE FORMATION WITH THE TOURMAKEADY FORMATION, WESTERN IRELAND

The age of the Rosroe Formation in western Ireland has been variously interpreted to range from as old as the Floian (Lower Ordovician) to as young as the latest Katian (Late Ordovician). The published age disparities and difficulties in correlation of the Rosroe are due to the poor fossil content of the unit. The unit does contain rare carbonates that occur as clasts within debris flows, and these carbonates contain conodonts and other fossils. Samples from exposures along the Rosroe Peninsula, exposures on the south side of Killary Bay, and from two exposures on the south side of Lough Nafooey yield a bimodal age association of conodonts. A Floian conodont assemblage is present from the Lough Nafooey area and a Darriwilian (Middle Ordovician) conodont assemblage is present from the Rosroe Peninsula and the south side of Killary Bay. Conodonts and brachiopods from the Tourmakeady Formation exposed approximately 10 kilometers northeast of Lough Nafooey support a Floain, or perhaps slightly younger, age for this rather poorly exposed unit. The Tourmakeady consists of a series of discontinuous exposures that contain carbonate breccia similar in lithology to the carbonates in the exposures of the Rosroe nearby on the south side of Lough Nafooey. New conodont data support Stouge and others’ (2015) 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. The mapping of these areas leaves open the possibility that these units may be more than just contemporaneous, but indeed may be correlative, or that the Floian carbonates in the debris flows were sourced from the same carbonate platform that is now completely eroded.