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

Abstract

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 300o to max. 480 °C; some found in the Lough Nafooey area have abnormally high indices (CAI 6), which correspond to temperatures of about 360o 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, mixing with other rock types now found in the coarse polymict clastics of the Rosroe Formation. The faunas fill the stratigraphical ‘gap’ between the Lower Ordovician Lough Nafooey Volcanic Group and the upper Middle Ordovician Rosroe Formation in the South Mayo Trough and represent a brief interval conducive to carbonate accumulation in an otherwise siliciclastic‐ and volcaniclastic‐dominated sedimentary environment. Copyright © 2015 John Wiley & Sons, Ltd.

Bioestratigrafía y provincialismo de conodontes del tramo medio-superior de la Formación San Juan en el cerro Viejo de Huaco, Precordillera, Argentina

“…En el archipiélago de Svalbard, Lehnert et al (2013) estudian la bioestratigrafía del Grupo Oslobreen, en la Formación Valhallfonna, registrando la Zona de Oepikodus evae. Stouge et al (2016) analizan bloques de calizas que presentan evidencias de transporte, contenidas en la Formación Rosroe, Área de Lough Nafooey, oeste de Irlanda, reconociendo la Zona de Oepikodus evae.…”

“…Al analizar muestras de la Formación Rosroe, Killary Bay Little, oeste de Irlanda, Stouge et al (2016) recuperan una asociación de conodontes que indicaría una correspondencia con la Zona de P. macrodentatus, que a su vez se correlacionaría con la Zona de Lenodus variabilis.…”

Conodont biostratigraphy and correlation of the San Juan Formation at the Cerro La Silla section, middle Tremadocian-lower Dapingian, Central Precordillera, Argentina

“…In limestone blocks showing evidence of transport, contained in the Rosroe Formation, Lough Nafooey Area, Western Ireland, Stouge et al (2015) recognize an association dominated by Oepikodus evae, Bergstroemognathus, Periodon and Protopanderodus.…”

Deciphering the geology of some Darriwilian–Sandbian (Ordovician) ‘ghost’ formations in the UK and North America using olistoliths in marine debris flows

“…This is illustrated by the fact that very little has been published on British faunas from the Tremadocian, Floian, Dapingian and lower and middle Darriwilian (cf. Bergström & Orchard, 1985; Stouge et al , 2016). Sandbian and Katian conodont faunas are much better known, having been described from several, but geographically rather scattered, localities as summarized in Figure 1 (see also Ferretti, Bergström & Barnes, 2014; Ferretti, B ergström & S evastopulo, 2014; Bergström & Ferretti, 2015).…”