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

TY - JOUR

T1 - 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

AU - Stouge, Svend

AU - Harper, David A. T.

AU - Sevastopulo, George D.

AU - O'Mahony, Darren

AU - Murray, John

PY - 2016/7/1

Y1 - 2016/7/1

N2 - The Middle Ordovician Rosroe Formation consists of some 1350m 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 5m 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.

AB - The Middle Ordovician Rosroe Formation consists of some 1350m 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 5m 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.

KW - Allochthonous limestone blocks

KW - Ireland

KW - Laurentian affinity

KW - Lower and Middle Ordovician conodonts

KW - Palaeobiogeography

KW - Rosroe Formation

KW - South Mayo Basin

U2 - 10.1002/gj.2659

DO - 10.1002/gj.2659

M3 - Journal article

AN - SCOPUS:84923454665

SN - 0072-1050

VL - 51

SP - 584

EP - 599

JO - Geological Journal

JF - Geological Journal

IS - 4

ER -