Lower and Middle Ordovician conodonts of Laurentian affinity from blocks of limestone in the Rosroe Formation, South Mayo Trough, western Ireland and their palaeogeographic implication

• Published in: Geological Journal
• Main Authors: Stouge, Svend, Harper, David A. T., Sevastopulo, George D., O'Mahony, Darren, Murray, John
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2015
• Subjects: Canada; Newfoundland
• Online Access: http://dx.doi.org/10.1002/gj.2659; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fgj.2659; https://onlinelibrary.wiley.com/doi/pdf/10.1002/gj.2659

The Middle Ordovician Rosroe Formation consists of some 1350 m of coarse, mainly siliciclastic to volcaniclastic sedimentary rocks, deposited in a submarine fan environment, and is restricted to the southern limb of the South Mayo Trough, western Ireland. Discrete allochthonous blocks, reaching 5 m in size, are present in the formation at several localities. Conodonts recovered from these blocks, collected from two separate locations, are of late Early and mid Mid Ordovician age. The conodonts have high conodont‐alteration indices (CAI 5) indicative of temperatures as high as 300 o to max. 480 °C; some found in the Lough Nafooey area have abnormally high indices (CAI 6), which correspond to temperatures of about 360 o to max. 550 °C. The oldest fauna is dominated by Periodon aff. aculeatus and characterized by Oepikodus evae typical of the Oepikodus evae Zone (Floian Stage; Stage Slices Fl2–3, Lower Ordovician). The younger conodont assemblage, characterized by Periodon macrodentatus associated with Oistodella pulchra , is referred to the P . macrodentatus conodont Biozone (lower Darriwilian; Stage Slices Dw1–2). The Rosroe conodont assemblages are of Laurentian affinity; comparable faunas are well known from several locations along the east to south‐eastern platform margin of Laurentia and the Notre Dame subzone of central Newfoundland, Canada. The faunal composition from the limestone blocks suggests a shelf edge to slope (or fringing carbonate) setting. The faunal assemblages are coeval with, respectively, the Tourmakeady Formation (Floian–Dapingian) and Srah Formation (Darriwilian) in the Tourmakeady Volcanic Group in the eastern part of the South Mayo Trough and probably are derived from the same or similar laterally equivalent short‐lived carbonate successions that accumulated at offshore ‘peri‐Laurentian’ islands, close to and along the Laurentian margin. During collapse of the carbonate system in the late Mid Ordovician, the blocks were transported down a steep slope and into deep‐water by debris flows, ...