U-Pb zircon geochronology of silicic tuffs and chronostratigraphy of the earliest Old Red Sandstone in the Munster Basin, SW Ireland

Abstract

Newly acquired U-Pb magmatic zircon dates from silicic tufts within the Old Red Sandstone (ORS) magnafacies of the Munster Basin (SW Ireland) are intercalibrated with newly discovered (late Givetian) and reappraised (mid-Frasnian) miospore assemblages to provide the first biostratigraphically constrained numerical ages in the Irish Devonian succession. The weighted mean 2~176 isotopic age determined for the Keel Tuff Bed (385.0 + 2.9 Ma) is indistinguishable from that of the previously investigated Enagh Tuff Bed (384.9 + 0.7 Ma). In conjunction with very similar rare earth element (REE) signatures, this confirms their correlation, placing a minimum age of 384.9 + 0.4 Ma on the newly discovered Reenagaveen microflora, which is assigned to the late Givetian TCo Oppel zone. The equivalence of the Keel and Enagh Tufts constrains a vertebrate fauna containing Bothriolepis and the Valentia Island tetrapod ichnofauna to pre-date this event. Isotopic dating of thickly bedded subaerial tufts from the Lough Guitane Volcanic Complex, a major accumulation of rhyolites and silicic volcaniclastic rocks, reveals ages of 384.5 +_ 1.0 Ma (Killeen Volcanic Centre), indistinguishable from the Keel-Enagh Tuff Bed, and 378.5-t-0.2 Ma from the Horses Glen Volcanic Centre, previously considered to be the oldest of the complex. The Horses Glen Centre post-dates the Moll's Gap Quarry microflora, the only current biostratigraphical control on the age of the early ORS in the east of the basin depocentre, thus indicating a minimum age for the (mid-Frasnian) IV Oppel zone, the revised biostratigraphic age of this assemblage. These controls on the early ORS (1) suggest that Munster Basin initiation occurred before late Givetian time and (2) give time-averaged (compacted) accumulation rates of c. 0.17-0.25 and 0.18 mm a -1 for eastern and western Iveragh, respectively. The minimum basin duration time was c. 23 Ma to the end of the Devonian period. The implications of these data for the depocentre stratigraphy, volcanic events, proposed ORS cyclicities and the geohistory of the Munster Basin are examined.

The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland

“…A review of the potential causes of resetting has been provided by Ennis et al (2015) and the extension associated with Lough Guitane volcanism in the Munster Basin was briefly mentioned. Given that Williams et al (2000) obtained a U-Pb zircon age of 378.5 Ma for the Horses Glen Volcanic Centre and 384.5 Ma for the Killeen Volcanic Centre (which lie to the SE of the Dingle Basin), it is likely that the younger detrital mica ages represent resetting by high heat flow associated with extension and volcanism, which may have been caused by emplacement of a granitic body at depth below the Munster Basin (Avison 1984). Alternatively, these younger ages could represent partial resetting owing to low-grade Variscan metamorphism at the end of the Carboniferous Period.…”

Follow the footprints and mind the gaps: a new look at the origin of tetrapods

“…2010). Together with the secure dating of the long-recognised tetrapod footprints from Valentia Island in Ireland as late Givetian (Williams et al 2000; Stössel et al . 2016), this discovery implies that tetrapods have a ghost lineage extending back almost 20 million years before the earliest known tetrapod body fossils, and ten million years before the earliest elpistostegids.…”

Structure of the Dingle Peninsula, SW Ireland: evidence for the nature and timing of Caledonian, Acadian and Variscan tectonics

“…Russell (1978), Higgs & Russell (1981) and Williams et al (1997, 2000); 9. Higgs & Russell (1981) and Williams et al (2000). AN – Annascaul Formation; BF – Ballyferriter Formation; BH – Bull's Head Formation; BM – Ballymore Formation; BR – Ballyroe Group; BS – Ballinskelligs Sandstone Formation; CD – Carrigduff Group; CG – Coosglass Formation; CO – Coumeenoole Formation; CP – Cappagh Sandstone Formation; CS – Chloritic Sandstone Formation; DQ – Dunquin Group; EK – Eask Formation; GB – Glashabeg Conglomerate Formation; IC – Inch Conglomerate Formation; KM – Kilmurry Sandstone Formation; LK – Lack Formation; LLS – Lower Limestone Shales; LS – Lough Slat Conglomerate Formation; PG – Pointagare Group; SF – St Finan's Sandstone Formation; SH – Slea Head Formation; SM – Slieve Mish Group; SW – Smerwick Group; TB – Trabeg Conglomerate Formation; VS – Valentia Slate Formation.…”