A revised Mississippian lithostratigraphy of County Galway (western Ireland) with an analysis of carbonate lithofacies, biostratigraphy, depositional environments and palaeogeographic reconstructions utilising new borehole data

Abstract

An integrated study of borehole data and outcrop of Mississippian (late Tournaisian to late Viséan) rocks in Co. (County) Galway, western Ireland has enabled a more detailed geological map and lithostratigraphy to be constructed for the region. Several carbonate formations have been distinguished by microfacies analysis and their precise ages established by micropalaeontological investigations using foraminifers and calcareous algae. In addition, palaeogeographic maps have been constructed for the late Tournaisian, and early to late Viséan intervals in the region. The oldest marine Mississippian (late Tournaisian) deposits are recorded in the south of the study region from the Loughrea/Tynagh area and further south in the Gort Borehole; they belong to the Limerick Province. They comprise the Lower Limestone Shale Group succeeded by the Ballysteen Group, Waulsortian Limestone and Kilbryan Limestone Formations. These rocks were deposited in increasing water depth associated with a transgression that moved northwards across Co. Galway. In the northwest and north of the region, marginal marine and non-marine Tournaisian rocks are developed, with a shoreline located NW of Galway City (Galway High). The central region of Co. Galway has a standard Viséan marine succession that can be directly correlated with the Carrick-on-Shannon succession in counties Leitrim and Roscommon to the northeast and east as far as the River Shannon. It is dominated by shallow-water limestones (Oakport, Ballymore and Croghan Limestone Formations) that formed the Galway-Roscommon Shelf. This facies is laterally equivalent to the Tubber Formation to the south which developed on the Clare-Galway Shelf. In the southeast, basinal facies of the Lucan Formation accumulated in the Athenry Basin throughout much of the Viséan. This basin formed during a phase of extensional tectonics in the early Viséan and was probably connected to the Tynagh Basin to the east. In the late Viséan, shallow-water limestones of the Burren Formation extend across much of the southern part of the region. They are characterized by the presence of rich concentrations of large brachiopod shells and colonial coral horizons which developed in predominantly high-energy conditions. These limestones also exhibit palaeokarstic surfaces and palaeosols which formed during regressive conditions of glacio-eustatically controlled cyclicity. Locally, slightly deeper water, lower energy conditions developed on the shelf with the formation of rare bryozoan-rich mud-mounds. Deep-water basinal facies were maintained in the central and southeastern parts of the region between the two shelves with the persistence of the Lucan Formation. Active syn-sedimentary faulting influenced deposition in the Viséan and interfingering of basinal sediments with slumps and shallow-water shelf carbonates are recognized.

Early Mississippian evaporites of coastal tropical wetlands

“…In the south of Ireland, Famennian to Tournaisian times saw a diachronous marine transgression north‐eastward onto non‐marine strata of ‘cementstone’ facies (Clayton & Higgs, ; Clayton et al ., ; Pracht & Somerville, ). Evaporites were proved in the subsurface in the Lough Allen Basin south‐west of Enniskillen and near Killeshandra on the margin of the south‐west limit of the Longford‐Down Massif (Sheridan et al ., ; Clayton & Higgs, ; Mitchell, ; Philcox et al ., ; Waters et al ., ).…”

A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland

“…The entire area is underlain by Lower Carboniferous wellbedded and pure limestones ranging between the Tubber formation (early Viséan), the Lower Burren and Upper Burren formation (mid-Viséan, Asbian), and the Slievenaglasha Formation (late Viséan, Brigantian). The presence of irregular limestone surfaces is interpreted as palaeokarst (Pracht et al 2015). The total thickness of limestone reaches at least 510 m. The strata dip generally to the south, ranging between 2 and 3°.…”

Submarine and intertidal groundwater discharge through a complex multi-level karst conduit aquifer

“…Palaeokarst horizons occur at the top of the Maumcaha member below the overlying Aillwee member, which is distinguished between a Lower and Upper Aillwee member (MacDermot et al 2003), and at the top of the Aillwee member. In addition, several impermeable clay Bwayboards^are located within the Aillwee member, interpreted as palaeosols, which rest on irregular limestone surfaces considered to be palaeokarsts (Pracht et al 2015). The dip of the strata is uniformly to the south, in the range of 2-3° (Fig.…”

“…Dolomitic layers are known to exist at the top the Finavarra member (Pracht et al 2004) and between the Lower Burren formation from the Upper Burren (Gallagher et al 2006). Palaeokarst horizons occur at the top of the Maumcaha member below the overlying Aillwee member, which is distinguished between a Lower and Upper Aillwee member (MacDermot et al 2003), and at the top of the Aillwee member.…”

“…There is one fault in the catchment, i.e. MacDermot's fault in the western part of the study area, striking into the NNE sub-parallel to the Fergus Shear Zone, with a slight (< 200 m) sinistral displacement of members of the Burren and Slievenaglasha formations (Pracht et al 2004). Joints strike NW-SE and E-W, and the age of joint formation is postVariscan.…”