A Revised Mississippian Lithostratigraphy of County Galway

Journal of Palaeogeography 2015, 4(1): 1-26 DOI: 10.3724/SP.J.1261.2015.00065

Lithofacies palaeogeography and sedimentology

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

Markus Pracht1, Ian D. Somerville2, * 1. Geological Survey of Ireland, Beggars Bush, Dublin 4, Ireland* 2. UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland

Abstract An integrated study of borehole data and outcrop of Mississippian (late Tour- naisian 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 Ba- sin 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

* Corresponding author. E‑mail: [email protected]. Received: 2014-07-29 Accepted: 2014-09-12 2 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015

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. Ac- tive syn-sedimentary faulting influenced deposition in the Viséan and interfingering of basinal sediments with slumps and shallow-water shelf carbonates are recognized.

Key words Carboniferous, Mississippian, western Ireland, carbonate microfacies, biostratigraphy, foraminifers, shelf and basin environments, palaeogeography

1 Introduction and geological setting Slieve Dart Fault in the north to Craughwell and Loughrea in the south (Figure 1). It represents part of an extensive The study area includes a large terrane of flat-lying carbonate platform (“Galway-Roscommon Shelf ” in Lower Carboniferous (Mississippian) limestone in Co. Morris et al., 2003; Nagy et al., 2005 or “Tuam Block” Galway, western Ireland, approximately 2000 km2, in Long et al., 2004), adjacent to the older Precambrian extending from Lough Corrib in the west to the River and Lower Paleozoic Connemara Massif to the northwest Suck (Ballinasloe) in the east, and from Ballymoe and the of Galway city. Exposures are very limited and much of

Figure 1 Geological map of western Ireland (adapted from Long et al., 2004; Pracht et al., 2004). O.R.S. = Old Red Sandstone. Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 3 the area is covered by glacial and fluvioglacial sediments; data from the limestones has been gathered and used to the best sections are in the limestone quarries close to help characterize and date the various formations, and Galway city (Pracht and McConnell, 2012). The most to correlate them within the region and with other type recently published maps of the area (1:100,000; Pracht et sections and boreholes outside the study area. These are al., 2004; Long et al., 2004) show that the majority of the principally from the Burren region of Co. Clare and Gort exposed limestones are of Viséan age and mainly depicted Borehole (G-1) in southern Co. Galway in the southwest, as undifferentiated shelf carbonates, with small isolated and around Boyle, Co. Roscommon and Carrick-on-Shan- patches of Viséan basinal limestones. non, Co. Leitrim to the northeast (Figure 1). Finally, analysis Recent investigations have attempted to differentiate is made of the carbonate depositional environments and this >800 m-thick Viséan carbonate succession into palaeogeographic maps are reconstructed for the region. constituent formations, based on analysis of drillcore from several deep boreholes (>300 m thick) and many shorter 2 Lithostratigraphy, biostratigraphy boreholes (c. 100 m thick) drilled by the Geological Survey and microfacies of Ireland (GSI). In total, during the period from 2005 to 2012, 30 boreholes have been drilled within this region Succeeding the basal Carboniferous siliciclastic and logged in detail. Sampling of selected boreholes has units and Tournaisian limestones, four distinct Viséan been undertaken by the authors to determine the age of lithostratigraphic successions have been recognized within different stratigraphic units based on their macrofauna the study area (Table 1). In the northeast and central part of (corals) and microfossils (principally foraminifers and the region is the Dunmore-Ballymoe succession, identi‑ calcareous algae). Correlation between the deeper bore‑ fied in the Esker (GSI‑12‑01), Fartamore (GSI‑12‑02), holes has recognized several distinct Viséan stratigraphic Lisna (GSI‑11‑01), Corancarton (GSI‑11‑315) and successions that clearly show lateral facies changes across Bullaun Bridge (GSI‑07‑178) boreholes (Figures 2, 3). the region. These comprise in the west the Oughterard- The stratigraphic units identified are closely comparable Headford succession around Lough Corrib, the Dunmore- to the succession defined by Caldwell (1959) from the Ballymoe succession in the northeast and centre, and Carrick-on-Shannon Syncline, 40 km to the northeast in further south the Craughwell-Burren succession (Figure counties Roscommon and Leitrim (Figure 1, Table 1), 2). Also, in the southeast of the area, a transition from which crops out between Boyle and Carrick-on-Shannon. shelf to basin facies can be recognized, as well as an older In the western part of the region is the Oughterard-Head- Tournaisian succession with Waulsortian mud-mounds in ford (Lough Corrib) succession, based on boreholes the Loughrea-Tynagh area (Figures 1, 2). (HS‑4, GSI‑02‑03, GSI‑92‑09, GSI‑92‑10; Figure 2) and The region has been affected by major NE‑ and ENE- outcrops around the shores of Lough Corrib (Pracht et al., trending Variscan faults with downthrows of hundreds 2004; Long et al., 2004). In the south of the region is the of metres. The Athenry and Craughwell Faults juxtapose Craughwell-Burren succession, identified in GSI‑06‑02, shelf and basinal successions, and the Loughrea Fault and GSI‑06‑387, GSI‑07‑179, GSI‑08‑239, GSI‑10‑04 and Slieve Dart Fault expose inliers of basal Carboniferous GSI‑96‑50 boreholes (Figures 2, 5) and comparable to the siliciclastic rocks and Tournaisian (Courceyan) limestones Burren-Gort succession in south Co. Galway and north (Pracht et al., 2004; Long et al., 2004; Pracht and Co. Clare (Pracht et al., 2004; Gallagher et al., 2006). McConnell, 2012; Figures 1, 2). In addition, thick Viséan basinal successions are present The main aim of this study is a synthesis of the newly in the southeast of the region recognized in GSI‑07‑01 acquired borehole data from the 30 recently drilled GSI (Cuddoo), GSI‑07‑03 (Gloves), 2099-2 and 01-1798/1 boreholes integrated with older borehole data on file in boreholes (Figures 4, 5) (Loughrea succession). the GSI (28 boreholes; comprising 14 drilled by GSI and The description of the various Lower Carboniferous 14 non-GSI commercial drillholes), together with new formations is from oldest to youngest, beginning with the field data we have collected. The result of collating data basal siliciclastic and limestone formations of Tournaisian from the 58 boreholes is a much improved and detailed age. These are only briefly described, as the main focus geological map and lithostratigraphy of the region (Fig‑ of this paper is on the younger Viséan formations, which ure 2), with an attempt to show the areal distribution of areally account for the majority of the region. More Viséan formations tied to key boreholes and outcrops. comprehensive descriptions of these Tournaisian units Also, a wealth of microfacies and micropalaeontological can be found in Pracht et al. (2004). Also, as the Viséan 4 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015

Figure 2 Geological map of Co. Galway and location of selected boreholes and localities mentioned in the text. The three main stra‑ tigraphic successions are also highlighted. Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 5

Table 1 Revised Mississippian lithostratigraphy for Co. Galway showing lateral equivalence of the main successions (Modified and updated from Morris et al., 2003; Pracht et al., 2004; Pracht and McConnell, 2012). Abbreviations: TOURN. = Tournaisian, COURC. = Courceyan, CHAD. = Chadian, ARUND. = Arundian, HOLK. = Holkerian, MFZ = Mississippian foraminiferal zone, Mbr. = Member, mb = mud‑mound (bioherm/buildup), O.R.S. = Old Red Sandstone, LST. = limestone, GRP. = Group.

FORAM LOUGH CORRIB DUNMORE-BALLYMOE LOUGHREA ZONATION SUCCESSION CRAUGHWELL-BURREN SUCCESSION SUCCESSION SUCCESSION SUB-

AGE (Poty et al. (Oughterard and

STAGE (Central and NE Galway) (SE Galway) STAGE 2006) Headford) (W Galway) (South and SW Galway) FORMATION FORMATION FORMATION FORMATION

TWO MILE DITCH Mbr. MFZ 13-14 KNOCKMAA BURREN BURREN BURREN mb

CORRANELLISTRUM CROGHAN MFZ 12 AUGHNANURE OOLITE LIMESTONE

ARDNASILLAGH BALLYMORE LUCAN MFZ 11 TUBBER VISÉAN (part) ILLAUNAGAPPUL LIMESTONE

MISSISSIPPIAN TUBBER

OLDCHAPEL LST. MFZ OAKPORT LIMESTONE 9-10 CREGG LST.

MFZ WAULSORTIAN LST. WAULSORTIAN OUGHTERARD LST. KILBRYAN LIMESTONE 6-8 LST. BALLYSTEEN GRP. BALLYSTEEN and MOATHILL BALLYSTEEN GRP. (part)

TOURN. TONWEEROE

COURC. CHAD. ARUND. HOLK. ASBIAN LOWER LST. SHALE GRP. LOWER LST. SHALE GRP. BOYLE SANDSTONE O.R.S. O.R.S. limestone formations defined around the shores of Lough type section near Boyle, Co. Roscommon (Caldwell, Corrib in the west of the region can be directly equated to 1959; Morris et al., 2003) where it is c. 105 m thick. It is the limestone succession further east (see Table 1), only represented in GSI‑12‑01 by >30 m of grey sandstone. Its details of the stratigraphy in the Dunmore-Ballymoe area, lateral equivalent in the western Lough Corrib succession which is regarded as the standard succession for most of (GSI‑92‑10 and HS-4) is the Tonweeroe Formation the Galway-Roscommon Shelf (see Morris et al., 2003), (>115 m thick) which shows a lower red-bed sandstone is documented here. More detailed descriptions of the and conglomerate sequence passing up to grey bioturbated Lough Corrib succession can be found in Pracht et al. sandstones and mudstones (Figure 2). In GSI‑92‑10 (2004). Biostratigraphic dating of the Tournaisian and borehole the Tonweeroe Formation rests directly on Viséan limestones has been achieved principally using the Silurian-Devonian Galway granite. The age of the foraminifers. The assemblages have been assigned to Boyle Sandstone Formation has been confirmed as late the standard Mississippian foraminiferal biozones MFZ Tournaisian based on spore data (CM Zone) from the 6-MFZ 14 of Poty et al. (2006; Table 1) (broadly equivalent type section (Morris et al., 2003). In GSI‑12‑01 the Boyle to the former Cf2-Cf6ɣ zones and subzones, as used in Sandstone Formation is followed by marine limestones Jones and Somerville, 1996). Important characteristics of the Moathill Formation (Figures 2, 3). In the southern of the limestone microfacies are summarized in Table 2, boundary of the study region a thicker and more complete based on an examination of over 400 thin sections. Tournaisian succession is preserved around Craughwell (2099-2) and Loughrea (Figure 1). There, continental red 2.1 Basal Carboniferous siliciclastic units bed sediments pass up to a transitional marine succession In the north of the region, the basal siliciclastic unit (Lower Limestone Shale Group) developed around the is identified as the Boyle Sandstone Formation, which margin of the Slieve Aughty Inlier (Figure 1), succeeded forms inliers in Slieve Dart and Castlerea and has its by an open marine limestone succession (Ballysteen 6 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 Abbreviations: e.o.h. = end of hole, LST. = Abbreviations: e.o.h. = end of hole, LST. = Arundian. Correlation of boreholes from west to east across northern Co. Galway with important coral taxa (in red) and microfossils indicated.

Chadian, ARUND. Limestone, CHAD. = Chadian, Figure 3 Figure Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 7 coral taxa (in red) and microfossils indicated. Abbreviations: e.o.h. = end of hole, COURC. Abbreviations: (in red) and microfossils indicated. taxa coral = Asbian. Holkerian, ASB. = Arundian, HOLK. Holkerian, Correlation of boreholes from north to south across Co. Galway with important Correlation

Chadian, ARUND. Courceyan, CHAD. = Chadian, Figure 4 Figure 8 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 transitions with important coral taxa (in red) and microfossils indicated. Abbreviations: (in red) and microfossils indicated. coral taxa with important transitions basin - = Asbian. Holkerian, ASB. = Arundian, HOLK. Holkerian, Correlation of boreholes in southern and southeastern Co. Galway to show platform Correlation

e.o.h. = end of hole, ARUND. e.o.h. = end of hole, Figure 5 Figure Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 9

‑ - low water water - - mounds gy shelf gy shelf gy mid gy mid gy outer - water - ener ener derived bio ener ener - - - - - gy shelf; coral bi ‑ , low to high water moderate‑ to gy shelf; protected - ener - water and high Depositional setting - ener - - Deep shelf to basinal facies with redeposited shelf moderate‑ ex ‑ subaerial with occasional mud posure; localised in slightly deeper shelf Low shelf clastic material; partial dys ‑ partial material; clastic aerobic environment Predominantly shallow to high moderate‑ shallow Predominantly ostromes Moderately shallow Moderately to high Shallow high lagoon ‑ Main biota and components Crinoids, sponge spicules (common), calcispheres os ‑ (sparse), cyanobacteria, foraminifers (abundant), pyrite, de ‑ bryozoans, kamaenids; tracods, fenestellid trital quartz and mica Abundant crinoids and kamaenids, foraminifers, bryozoans, fenestellid sponge spicules, calcispheres, trilobites Abundant crinoids, dasycladacean algae, red algae, algae, dasycladacean Abundant crinoids, foramini ‑ problematica, cyanobacteria, kamaenids, brachiopods, bryozoans, fenestellid calcispheres, fers, ostra ‑ spines, trilobites, bivalves, gastropods, echinoid cods, coral fragments; peloids, intraclasts, micritized bioclasts; ooids algae, red dasycladacean Abundant crinoids, calcispheres, foraminifers, cyanobacteria, kamaenids, ostra ‑ bryozoans, brachiopods, bivalves, fenestellid coral spines, trilobites, cods, gastropods, echinoid mud mounds with intraclasts; fragments; peloids, trepostome, peloids, containing cavities stromatactid sheets, ostracods and abun ‑ and fenestellid tabuliporid dant sponge spicules Abundant crinoids and kamaenids, sparse foramini and kamaenids, Abundant crinoids fenestellid calcispheres, sponge spicules, fers and algae; rare coated bryozoans, sparse dasycladacean grains and ooids Common foraminifers, crinoids, dasycladacean algae, Common foraminifers, crinoids, dasycladacean kamaenids, gastropods, problematica; cyanobacteria, peloids, intraclasts, sparse ooids sorted sorted - - grained - d skeletal, pe ‑ d skeletal, skeletal peloidal skeletal sorte - peloidal, intraclastic peloidal, Microfacies wackestone/packstone sorted coarse - Mostly fine-grained,sorted wackestone/packstone poorly grainstone with coarser skeletal bands Moderately sorted fine-grained wackestone/packstone skeletal and well and grainstone intraclastic grainstone Poorly sorted and well skeletal, skel ‑ packstone and grainstone; etal wackestone Poorly sorted and well skeletal Mainly poorly sorted skeletal sorted poorly Mainly with well‑ wackestone/packstone grain ‑ skeletal intraclastic sorted stone Mainly well packstone and intraclastic loidal and grainstone; lime mudstone - ‑ ‑ and lime ‑ inter grained, - to coarse bedded - - and massive and grained - gillaceous lime ‑ gillaceous grained light and - bedded and coarse - - bedded ar - and coarse - grey limestone, thick grey limestone, , medium - (macrofacies, microfacies and interpretation of depositional facies) of late Tournaisian and Viséan limestone formations in the Co. Galway region, arranged in arranged Co. Galway region, in the formations limestone Viséan and Tournaisian of late facies) of depositional and interpretation microfacies (macrofacies, Thin bedded mostly fine-grained lami ‑ mud ‑ with interbedded limestone nated grained coarser thin occasional stone; limestone beds; frequent chert bands vals; chert bands Medium Mostly light grey medium with dolomitic grained limestone dark grey well limestone, rare clay bands; frequent limestone, coral colonies and brachiopod bands; rare massive fine-grained limestone in ‑ dolo‑ some partial cavities; with tervals mitisation Mostly fine- to medium-grained, dark grey well stone ing with light grey coarse light ing with Mostly fine-grained, dark greyalternat limestone argillaceous bedded well- stone; shale interbeds; frequent coral frequent stone; shale interbeds; colonies Fine massive; occasional intervals of fenes ‑ massive; occasional occasional micrite; burrowed and tral dolomitic brachiopods; nodules; chert intervals light Viséan) approximately stratigraphical order (oldest at the base). The limestones are classified according to the Dunham, 1962 scheme (modified by Embry and Klovan, 1971). approximately stratigraphical order (oldest at the base). Characterization Characterization Late Lucan - Burren Tubber (Viséan)

(Late Viséan) (Late (Late Viséan) (Late (Early Viséan) (Early (Early Viséan) (Early oghan Limestone Formation (age) Macrofacies (core and outcrop) (Early Oakport Limestone Cr Ballymore Limestone Ballymore Table 2 Table 10 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 - shelf outer outer high water water water - - - Table 2, continued Table or inner ramp with or inner ramp gy, deep gy, deep gy, ener ener Depositional setting - - ramp Low or middle ramp Low shelf energy Deeper shelf/ramp for cleaner shelf/ramp Deeper limestones Nearshore shallow abundant siliciclastic input ‑ ‑ Main biota and components bryozoans, crinoids, sponge spicules, cal spicules, sponge crinoids, bryozoans, Fenestellid bryozoans, crinoids, sponge spicules, cal spicules, sponge crinoids, bryozoans, Fenestellid cyanobacte ‑ ostracods, sparse foraminifers, cispheres, ria, kamaenids; minor detrital quartz grains cispheres, very sparse foraminifers, cyanobacteria, Fenestellid bryozoans, foraminifers crinoids, fenestellid Large (sparse), brachiopods, gastropods, sponge spicules, calcispheres, cyanobacteria; silicified bioclasts com ‑ mon bryozoans, ostra ‑ fenestellid Brachiopods, crinoids, cods, foraminifers (sparse) and calcispheres

Crinoi ‑ grained - skeletal packstone/ skeletal Microfacies sorted medium sorted - - rich equivalent beds - Poorly sorted skeletal wacke ‑ stone and packstone Lime mudstone and skeletal wackestone/packstone; dal Well skeletal packstone/grainstone; fine-grained poorly sorted skel ‑ etal wackestone/packstone rudstone and grainstone Well ‑ ‑ - - nodules; Variable lithologies comprising mainly lithologies Variable thin bedded and nodular argillaceous limestone with interbedded bioclastic mudstone; in south of the region, sand ‑ cross stone and siltstone interbeds; Massive light grey fine-grained lime ‑ in mud cavities stone with stromatactid Mostly fine-grained dark greynodules, si ‑ stone and mudstone; chert lime licified limestone; dolomitic intervals mound facies chert bedding, bioturbation; brachiopods, crinoids and solitary cor brachiopods, crinoids and solitary als common An alternation of bioclastic limestone, of bioclastic An alternation mud ‑ and sandstone calcareous grey stone Ballysteen Waulsortian (Tournaisian) (Tournaisian) (Tournaisian) (Tournaisian) Formation (age) Macrofacies (core and outcrop) Moathill Formation Kilbryan Limestone Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 11

Group) (Table 1), typical of the Gort borehole (G-1) in region near Gort, the Ballysteen Formation together with the Limerick Province (see Philcox, 1984; Somerville and the Ballymartin Formation form the Ballysteen Group Jones, 1985; Sleeman and Pracht, 1999; Somerville and (Somerville et al., 2011). Waters, 2011; Somerville et al., 2011). Microfacies: Typically poorly sorted skeletal wacke‑ stone/packstone and grainstone. The main skeletal com‑ 2.2 Moathill Formation ponents are crinoids, brachiopods, fenestellid bryozoans, This formation comprises an alternation of bioclastic sponge spicules, algae (Kamaena), sparse foraminifers, limestone, grey calcareous sandstone and mudstone. It is gastropods and calcispheres. Partial silicification of bio‑ c. 15 m thick in GSI‑12‑01 in the north of the region (Fig‑ clasts is common. ure 2). It represents the local transitional unit into the suc‑ Depositional setting: The Ballysteen Formation was ceeding limestone-dominant Ballysteen Formation. The deposited mostly in an offshore shelf/middle ramp, be‑ Moathill Formation forms part of the Navan Group which low fairweather wave-base but above storm wave-base, is better developed east of the River Shannon in Co. Long‑ in which fine-grained siliciclastic sediment accumulated. ford (Morris et al., 2003; Somerville and Waters, 2011). The biota is rich and diverse with abundant large crinoids, Microfacies: The limestones typically comprise mod‑ brachiopods and solitary corals. The lack of sedimentary erately well-sorted, coarse-grained skeletal grainstone/ structures, poor sorting of bioclasts and abundance of rudstone (Figure 6A). The main skeletal components are bryozoans implies low-energy, relatively quiet water con‑ brachiopods, crinoids, fenestellid bryozoans, ostracods, ditions. Locally, though, in the Loughrea area, shallower algae (Kamaena), foraminifers and calcispheres. water, higher energy conditions developed with the pres‑ Depositional setting: The formation was deposited in ence of ooids and better sorting of bioclasts. a nearshore, shallow-water siliciclastic-dominant shelf in Age: The Ballysteen Formation is late Tournaisian in which both fine‑ and coarse-grained clastic sediments ac‑ age based on a sparse foraminiferal assemblage includ‑ cumulated. Periodically, shelf limestones with a rich and ing Eoforschia, Septatournayella, Earlandia, Eblanaia diverse benthic biota developed, when siliciclastic input (MFZ 6), and algal/cyanobacteria and microproblematica onto the shelf was reduced. High-energy conditions pre‑ (Parachaetetes, Girvanella, Salebra sibirica). vailed during the formation of the limestones, as suggested 2.4 Waulsortian Limestone Formation by the good sorting and rounding of skeletal material. Age: The Moathill Formation is late Tournaisian in Within the study area, the Waulsortian Limestone For‑ age, based on a sparse foraminiferal assemblage includ‑ mation mainly occurs around Loughrea in the southeast, ing Endothyra, Septatournayella, Earlandia, Eoforschia where it reaches a thickness of c. 500 m (Pracht et al., (MFZ 6), and algal/cyanobacteria and microproblematica 2004). Although no boreholes in the recent drilling pro‑ (Girvanella, Salebra sibirica). gramme encountered this unit, further to the north in the Slieve Dart Borehole (02-3471-01; Figure 2), 45 m of 2.3 Ballysteen Formation Waulsortian Limestone were recorded (Morris et al., This formation comprises light-grey and dark-grey, 2003). The dominant Waulsortian lithology is pale-grey, fine-grained gillaceousar bioclastic limestone alternat‑ massive limestone, which, at outcrop in eastern Galway, ing with calcareous mudstone. The limestones are often forms large mud-mounds several tens of metres in thick‑ bioturbated and chert nodules are commonly developed. ness (Lees, 1964, 1994; Lees and Miller, 1985, 1995). Rare large solitary rugose corals and crinoids are the main There is a sharp basal contact with dark grey argillaceous macrofaunal elements. Identified taxa include: Canino- limestones of the Ballysteen Formation. In the Loughrea phyllum patulum, Sychnoelasma, and Amplexizaphrentis. area it is succeeded by either the Tubber Formation (see In the northeast of the region, in GSI‑11‑01, the Ballys‑ below) or basinal facies (Lucan Formation), but in the teen Formation is nearly 100 m thick (Figure 4), but thins northern part of the region it is overlain by and is partly westwards around Dunmore (c. 35 m in MGR.82.35 and equivalent to the Kilbryan Limestone Formation (Mor‑ 20 m in GSI‑12‑01 respectively; Figures 3, 4). In the south ris et al., 2003; Table 1). In some boreholes (GSI‑12‑01, of the region near Craughwell (2099-2), the Ballysteen GSI‑11‑01, GSI‑11‑315 and MGR.82.35) where typical Formation is also thin (c. 56 m), but thickens southeast massive pale-grey micritic Waulsortian limestone is not to Loughrea where it is c. 168 m (Pracht et al., 2004) and developed, a thinly-bedded, fine-grained nodular lime‑ contains oolitic and sandy intervals. In the south of the stone and shale unit is present and referred to as ‘Waulsor‑ 12 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015

Figure 6 Photomicrographs of carbonate microfacies (all in plane polarized light) of late Tournaisian and early Viséan limesto‑ nes. A-Skeletal grainstone with crinoids (C) and brachiopods (B) in sparry calcite cement, Moathill Formation, Esker Borehole (GSI‑12‑01), 300.2 m; B-Spiculitic wackestone, with monaxin and hexactin calcitic spicules in lime mud matrix, Waulsortian equi‑ valent, Corancarton Borehole (GSI‑11‑315), 440.9 m; C-Skeletal wackestone with fenestellid bryozoans (F), sponge spicules (S), trilobites (T) and ostracods (O) in lime mud matrix, Waulsortian equivalent, Corancarton Borehole, 440.9 m; D-Skeletal wackestone with kamaenids and ostracods in lime mud matrix, Kilbryan Limestone Formation, Corancarton Borehole (GSI‑11‑315), 379.2 m; E- Skeletal peloidal intraclastic grainstone with moderately well-sorted micritized bioclasts in sparry calcite cement, Oakport Limestone Formation, Corancarton Borehole (GSI‑11‑315), 290.1 m; F-Skeletal grainstone with abundant tubular problematicum Luteotubulus in sparry calcite cement, Oakport Limestone Formation, Fartamore Borehole (GSI‑12‑02), 177.1 m; G-Skeletal intraclastic packstone with dasyclad alga Koninckopora (K) sp. (monolaminar) and foraminifers (F) in sparry calcite cement, Oakport Limestone Formation, Fartamore Borehole (GSI‑12‑02), 144.5 m; H-Skeletal packstone with abundant foraminifers and calcispheres in sparry calcite cement and lime mud, Oakport Limestone Formation, Esker Borehole (GSI‑12‑01), 64 m. Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 13 tian equivalent’ (Figures 3, 4). The latter unit is differenti‑ components are crinoids, fenestellid bryozoans, sponge ated from the Kilbryan Limestone Formation by its higher spicules, algae (Kamaena)(Figure 6D), foraminifers, cya‑ proportion of limestone to mudstone. nobacteria coating shells, trilobites and calcispheres. Microfacies: The characteristic lithology of the Waul‑ Depositional setting: The formation was deposited in sortian equivalent is wackestone with crinoids, fenestrate a moderately deep-water shelf/mid ramp, below storm bryozoans, sponge spicules, trilobites, brachiopods and wave-base, in which both fine-grained clastic mud and molluscs (Figure 6B, 6C). In the lime mudstone of the carbonate mud accumulated. Low-energy conditions pre‑ mud-mounds, large spar-filled cavity structures (stroma‑ vailed during the deposition of the fine‑grained muddy tactis), ranging up to many centimetres across, are com‑ limestones. The benthic biota is quite rich and diverse and monly developed (Lees, 1964, 1994; Lees and Miller, locally kamaenid algae and ostracods can be abundant. 1985, 1995; Somerville, 2003), and foraminifers are rare. Age: The Kilbryan Limestone Formation lacks diag‑ Depositional setting: The mud mounds characteristic of nostic microfossils. Based on its stratigraphic position this formation developed in a deep-water distal ramp set‑ between the older Tournaisian-aged Ballysteen Formation ting below storm wave-base. Low-energy conditions pre‑ (or Waulsortian Limestone Formation) and the overly‑ vailed during the formation of the massive limestones and ing lower Viséan Oakport Limestone Formation, it is late their laterally equivalent bedded limestones dominated by Tournaisian to earliest Viséan in age. crinoids, sponges and bryozoans. 2.6 Oakport Limestone Formation Age: The Waulsortian Limestone is late Tournaisian in age based on very limited foraminiferal and conodont data The Oakport Limestone Formation (Caldwell, 1959; (Sevastopulo, 1982; Pracht et al., 2004). Morris et al., 2003) consists of light-grey, massive, coarse- grained bioclastic limestone with occasional intervals of 2.5 Kilbryan Limestone Formation dark-grey fine-grained fenestral lime mudstone; rare black The Kilbryan Limestone Formation (Caldwell, 1959; shales and very rare green clay bands occur in the lower Morris et al., 2003) consists of dark-grey, fine-grained part. Chert nodules and bioturbation are occasionally de‑ argillaceous bioclastic limestone alternating with mi‑ veloped. Large solitary rugose corals, tabulate corals, gas‑ nor mudstone and some chert nodules. The lower part tropods, brachiopods and crinoids are the main macrofau‑ of the formation shows a nodular bedding, bioturbation, nal elements. Identified coral taxa include: Siphonophyllia geopetals and dewatering structures. It also contains two cylindrica, Michelinia megastoma, and Clisiophyllum sp. thin green clay bands in GSI‑12‑02 (at 262.7 m and 267.2 The base of the Formation has been placed at the first m) in GSI‑12‑01 (at 202.6 m and 223 m) and in GSI‑11‑315 appearance of light-grey bioclastic limestone above the (at 365.0 m and 389.5 m) (Figures 3, 4). One of these may underlying Kilbryan Limestone Formation. The top, also possibly correlate to the clay band in HS-4 (at c.75 m; gradational, is placed at the incoming of argillaceous li‑ Figure 3) and the one in MGR.82.35 (at c. 33 m; Figure thologies. The thickest interval of Oakport Limestone 4). These clay bands have been interpreted elsewhere as (140 m) is recorded in GSI‑11‑01 and GSI‑12‑01, similar decomposed volcanic ashes (tuffs) (Philcox, 1984; Morris to that in the Slieve Dart Borehole (02-3471-01; 137 m) et al., 2003). The upper boundary to the overlying Oakport (Figures 2, 3, 4; Morris et al., 2003). The Formation no‑ Limestone Formation is transitional, with light-grey lime‑ tably thickens and becomes more argillaceous from west stone increasing in proportion to argillaceous limestone for (GSI‑12‑02) to east (GSI‑12‑01) (Figure 3). The Oakport tens of metres. The Formation is 108 m thick in GSI‑12‑01 Limestone Formation is laterally correlated to the Cregg compared to c. 185 m in MGR.82.35. Further west near Limestone and Oldchapel Limestone Formations in the Headford (Long et al., 2004), the Kilbryan Limestone For‑ Lough Corrib succession (Figure 3, Table 1). In HS-4, mation is laterally correlated to the Oughterard Limestone only the lower 28 m of Oakport Limestone Formation was Formation of the Lough Corrib succession and is 166 m in drilled. In the central region near Tuam, the Oakport Lime‑ HS-4 (Figure 3, Table 1). The macrofauna comprises rare stone Formation thins to 110 m (GSI‑11‑315; Figure 4) but large solitary rugose corals (Amplexus coralloides) and was not intersected in boreholes further to the south. tabulates (Syringopora). Microfacies: Typically moderate to well-sorted, intra‑ Microfacies: Typically moderate to well-sorted skel‑ clastic, skeletal peloidal wackestone and packstone with etal wackestone (commonly with microsparite matrix) occasional intraclastic skeletal grainstone (Figure 6E) and and poorly sorted skeletal packstone. The main skeletal rare ooids in the upper beds. The main skeletal compo‑ 14 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 nents are locally abundant problematica (Luteotubulus, moderate to well-sorted skeletal peloidal packstone/grain‑ Figure 6F), dasycladacaean algae (Koninckopora (Figure stone, and spiculitic wackestone (Figure 7B). The main 6G) and Kamaena), endothyroid foraminifers (rich and di‑ skeletal components include: crinoids, fenestellid bryozo‑ verse in the lower beds, Figure 6H), cyanobacteria coating ans, sponge spicules, sparse dasycladacaean algae (mono‑ shells, calcispheres, crinoids and micritized bioclasts. laminar and bilaminar Koninckopora and Kamaena), Depositional setting: The Oakport Limestone For‑ mostly sparse foraminifers, although small archaediscids mation was deposited in a shallow-water shelf setting can be locally abundant (Figure 7C), rare red algae (aouj‑ above fairweather wave-base. The presence of rounded galiids), calcispheres, problematica (salebrids) and very intraclasts, ooids, micritized bioclasts and peloids in well- rare ooids/coated bioclasts. sorted coarse-grained limestones attests to high-energy Depositional setting: The Ballymore Limestone Forma‑ conditions. However, the formation also contains fenestral tion was deposited in a moderately deep-water shelf below and burrowed lime mudstones, which indicate deposition fairweather wave-base but above storm wave-base. Also, in low-energy intertidal lagoonal environments protected the formation comprises mainly fine-grained limestones by ooid and skeletal grainstone shoals. The biota is rich interbedded with thick mudstone intervals which would and diverse with abundant crinoids, foraminifers and dasy‑ have accumulated in a low-energy shelf setting. The biota cladacean algae, the latter suggesting growth in a shallow is rich and diverse with crinoids, fenestellid bryozoans and photic zone. sponge spicules, suggesting mainly quiet water, low-en‑ Age: The Oakport Limestone Formation is early Viséan ergy conditions. However, the development of occasional (Chadian) in age, based on the presence of the key fora‑ coral biostromes and sparse dasyclad green and red algae minifers: Eoparastaffella simplex, Pseuodolituotubella, implies more turbulent conditions in a shallower photic Brunsia, Pseudoammodiscus, Eblanaia michoti, Eostaffel- zone. la (an assemblage characteristic of MFZ 9), as well as the Age: The Ballymore Limestone Formation is early Vi‑ alga Koninckopora sp. (monolaminar wall) and abundant séan (Arundian) in age, based on the presence of the key Luteotubulus. foraminifers: Uralodiscus, Paraarchaediscus at involutus stage, Glomodiscus, Valvulinella, and Forschia (typical of 2.7 Ballymore Limestone Formation MFZ 10-11), as well as the dasyclad alga Koninckopora The Ballymore Limestone Formation (Caldwell, 1959; tenuiramosa, and species of the rugose coral Siphonoden- Morris et al., 2003) attains its greatest thickness (225 m) dron. At its type locality, the age of the Ballymore Lime‑ in GSI‑11‑315 (Figure 4), which is thicker than in the stone Formation is Arundian to Holkerian (Morris et al., type section (c. 170 m) at Ballymore House near Boyle 2003). (Caldwell, 1959). It consists predominantly of dark-grey 2.8 Croghan Limestone Formation well-bedded and nodular fine-grained argillaceous lime‑ stone showing common bioturbation, with occasional in‑ The Croghan Limestone Formation (Caldwell, 1959; tervals of massive, light-grey, coarse-grained limestone. Morris et al., 2003) is thickest in Gort Na Garrow Bore‑ The middle of the formation includes thicker calcareous hole (>77 m) (GSI‑07‑06), near Dunmore and is also pres‑ shale intervals. Chert nodules are commonly developed. ent in GSI‑11‑315 (Figure 4) in the north of the region. Large solitary rugose corals, fasciculate rugose and tabu‑ In the type section at Cavetown Lough near Boyle c. 107 late coral colonies, bellerophontid gastropods, brachio‑ m are recorded (Caldwell, 1959; Morris et al., 2003). The pods and sparse crinoids are the main macrofaunal ele‑ formation comprises dark-grey, well-bedded, fine-grained ments. Identified taxa include: Siphonophyllia garwoodi, argillaceous limestone showing frequent bioturbation, with Michelinia megastoma, Siphonodendron martini, S. soci- occasional intervals of massive, light-grey, coarse-grained ale, Palaeosmilia murchisoni, and Amplexus coralloides. limestone. Chert nodules are commonly developed. The The Ballymore Limestone Formation is laterally corre‑ main macrofaunal elements are large solitary rugose cor‑ lated to the Ardnasillagh Formation in the Lough Corrib als, fasciculate rugose and tabulate coral colonies, bel‑ succession and differs mainly in having less chert (Table lerophontid gastropods, brachiopods and sparse crinoids. 1). It is succeeded conformably by the Croghan Limestone Identified taxa include: Siphonophyllia garwoodi, Mi- Formation. chelinia sp., Siphonodendron martini, and Cravenia sp. Microfacies: Comprises poorly sorted intraclastic skel‑ The Croghan Limestone Formation is laterally correlated etal wackestone/packstone/grainstone (Figure 7A, 7D), to the Corranellistrum Formation in the Lough Corrib suc‑ Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 15

Figure 7 Photomicrographs of carbonate microfacies (all in plane polarized light) of early to late Viséan limestones. A-Skeletal packstone with closely packed bioclasts, mostly fenestellid bryozoans (F) and crinoids (C) in sparry calcite cement, Ballymore Lime‑ stone Formation, Corancarton Borehole (GSI‑11‑315), 256.5 m; B-Spiculitic wackestone, with monaxon calcitic spicules and ostra‑ cods in lime mud matrix, Ballymore Limestone Formation, Corancarton Borehole (GSI‑11‑315), 208.6 m; C-Skeletal packstone with crinoids (C) and archaediscid foraminifers (A) in sparry calcite cement, Ballymore Limestone Formation, Lisna Borehole (GSI‑11‑01), 2.5 m; D-Intraclastic-skeletal grainstone with intraclasts (I), foraminifers (F) and cyanobacterial coating on shells in sparry calcite cement, Ardnasillagh Limestone (Ballymore Limestone) Formation, Fartamore Borehole (GSI‑12‑02), 133.1 m; E-Skeletal packstone/ grainstone with bilaminar palaeotextulariid foraminifers (F) in sparry calcite cement, Burren Formation, Keekil Borehole (GSI‑10‑03), 99.7 m; F-Bryozoan-rich boundstone with trepostomes (Tr), encrusting tabuliporids (Ta) and fenestellids (F) in lime mud matrix with patches of sparry calcite cement, Burren Formation, Keekil Borehole (GSI‑10‑03), 129.75 m; G-Poorly sorted skeletal, intraclastic grainstone with crinoids, brachiopods, micritized bioclasts and peloids in sparry calcite cement, Tubber Formation, Killsolan Borehole (GSI‑10‑01), 109.8 m; H-Fine-grained spiculitic wackestone, with dense nework of spicules in lime mud matrix, Lucan Formation, Monivea Borehole (GSI‑06‑387), 152.5 m. 16 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 cession and succeeded conformably by the Burren Forma‑ the Aillwee Member is c. 150 m thick (Gallagher, 1992). tion (Table 1). The Burren Formation was previously referred to as the Microfacies: Comprises poorly sorted skeletal wacke‑ Knockma Formation (Long et al., 2004), but is here incor‑ stone and packstone (with microspar matrix), and rare porated in the Burren Formation. In the type area and much skeletal grainstone. The main skeletal components are cri‑ of south Co. Galway (e.g. Kiltullagh Borehole GSI‑06‑02; noids, fenestellid bryozoans, sponge spicules, trilobites, Figure 5) the Burren Formation succeeds the Tubber For‑ rare red algae (aoujgaliids), foraminifers, (small archae‑ mation (Pracht et al., 2004; Gallagher et al., 2006), but discids can be locally abundant), calcispheres, problem‑ over much of north Co. Galway it rests conformably on the atica (Draffania and salebrids) and kamaenids. Croghan Limestone Formation (as in GSI‑07‑06, Figure Depositional setting: The Croghan Limestone Forma‑ 4). However, in the southeast of the region it interfingers tion was deposited in a moderately deep-water shelf set‑ with and succeeds the basinal facies of the Lucan Forma‑ ting, similar to that of the Ballymore Limestone. Also, tion, as in Monivea Borehole (GSI‑06‑387) (Figure 5; see the formation comprises mainly fine-grained argillaceous Table 1). The top of the Burren Formation and its contact limestones which would have accumulated in a low-energy with the overlying Slievenaglasha Formation are not pres‑ shelf. The biota is rich and diverse with crinoids, fenestel‑ ent in the study area. lid bryozoans, sponge spicules, solitary corals, algae and Typically, the limestones of the Burren Formation are foraminifers. pale grey and thickly-bedded to massive, but there are also Age: The Croghan Limestone Formation is middle Vi‑ intervals that are darker grey, medium-bedded and cherty. séan (Holkerian) in age, based on the presence of the key The Formation also comprises black calcareous shales in‑ foraminifers: Paraarchaediscus at concavus stage, Eo- terbedded with bioclastic limestone in the lower part. In staffella, Valvulinella, Forschia (MFZ 12), the dasyclad the Two Mile Ditch Member, there are a number of clay alga Koninckopora tenuiramosa, and Draffania biloba. At bands that can be observed in quarry sections resting of‑ its type locality the age of the Ballymore Limestone For‑ ten on an irregular palaeokarstic surface (Figure 8A, 8B). mation is Holkerian to Asbian (Morris et al., 2003). In two boreholes (GSI‑07‑05 and GSI‑10‑03, Figure 2) mud-mound facies lithologies >35 m thick have been en‑ 2.9 Burren Formation countered, but these mounds are not exposed at outcrop. The Burren Formation (Pracht et al., 2004; Gallagher et The macrofauna in the Burren Formation is dominated al., 2006) is present in boreholes GSI‑10‑03, GSI‑10‑04, by numerous coral colonies of Siphonodendron sociale, GSI‑10‑05, GSI‑07‑05, GSI‑07‑06 and GSI‑07‑179 with S. pauciradiale and Lithostrotion vorticale (Figure 9A), its greatest thickness (>244 m) in Castlequarter Bore‑ with crinoids and large gigantoproductid brachiopods of‑ hole (GSI‑08‑239). The formation occupies most of the ten concentrated in bands at the base and near the top of low-lying ground north of Galway city towards Tuam. beds. These brachiopods often occur in a convex-upward It was formerly quarried around Angliham and Men‑ orientation, especially near the top of beds (Figure 9B). lough on the southern shores of Lough Corrib, and is cur‑ Microfacies: The bedded limestones comprise skeletal rently well exposed in the working quarries at Two Mile wackestone, packstone and grainstone (Figure 7E). The Ditch (GR 132870/229237), Harrington’s Quarry (GR mud-mounds are composed of fine-grained wackestone 138281/240201), Mortimer’s Quarry (GR 137553/248535) and have stromatactid cavities with radiaxial fibrous cal‑ and Esker Readymix Quarry (GR 153450/224105) (Figure cite (RFC) cement and peloidal mud. Locally the mounds 2). As a result of the recent drilling programme, it is now are rich in trepostome (Leioclema), fistuliporid (Tabu- recognized that the Burren Formation, although in places lipora) and fenestellid bryozoans and form boundstone more argillaceous than in its type locality in the Burren fabrics (Figure 7F). The main skeletal components in the region in Co. Clare, can be extended northwards into south non-mound facies are crinoids, dasycladacean green algae and central Co. Galway (Pracht and McConnell, 2012). (Koninckopora), red algae (aoujgaliids and ungdarellids), Furthermore, it is also recognized that the Two Mile Ditch fenestellid bryozoans, foraminifers (palaeotextulariids Member, representing the upper part of the Burren For‑ (Figure 7E) and small archaediscids can be locally abun‑ mation, with a type section in Two Mile Ditch Quarry, is dant), sponge spicules, calcispheres, problematica (Draf- laterally equivalent to the Aillwee Member of the Burren fania and salebrids) and kamaenids. region (Pracht et al., 2004; Gallagher et al., 2006). In the Depositional setting: The Burren Formation was depos‑ type section, the Burren Formation is c. 400 m thick and ited in a shallow-water shelf setting, mainly between fair‑ Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 17

A A

P P P P P P

P P

Figure 8 Well-bedded grey limestones of the Asbian Burren Formation (Two Mile Ditch Member) showing: A-An irregular pa‑ laeokarstic horizon (arrowed) with green clay wayboard (palaeosol, P) above infilling the topography; B-The palaeokarstic horizon is horizontal (arrowed) with conspicuous orange staining of limestones below the clay band (palaeosol, P). Two Mile Ditch Quarry. Scale: hammer is 40 cm long. 18 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015

Figure 9 A-Right way up and inverted colonies of Lithostrotion vorticale (L) with overturned convex-up gigantoproductid bra‑ chiopods (G); B-Band of overturned convex-up gigantoproductid brachiopods (G). Burren Formation (Harrington’s Quarry). Scale: hammer head is 20 cm long; coin is 2.5 cm in diameter. weather‑ and storm wave-base, but with periodic shallow‑ velopment of bryozoan-rich mud-mounds (buildups) sug‑ ing events leading to temporary subaerial exposure. The gests a quieter deeper water setting on the shelf. latter is indicated by the developments of palaeokarstic Age: The Burren Formation is late Viséan (Asbian) in surfaces and palaeosols, particularly in the upper part of age (MFZ 13-14), and the Two Mile Ditch Member is late the formation. The rich concentration of large brachio‑ Asbian in age (MFZ 14) based on the presence of the diag‑ pods near the top and base of beds implies more turbulent nostic foraminifers: Paraarchaediscus at angulatus stage, higher energy conditions, especially when overturned in Koskinotextularia cribriformis, Cribrostomum lecomptei, the hydrodynamically stable position. Abundant coral col‑ Palaeotextularia longiseptata, Lituotubella, Vissariotaxis, onies suggest relatively shallow water within the photic Pseudoendothyra and the red alga Ungdarella uralica. At zone, confirmed by the abundance of dasyclad algae and its type locality the age of the Burren Formation is Asbian large robust thick-walled foraminifers. The occasional de‑ and is succeeded conformably by the Brigantian Slievena‑ Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 19 glasha Formation (Gallagher et al., 2006). corded in boreholes GSI‑09‑381, GSI‑96‑51, GSI‑05‑152 and GSI‑10‑01 include Paraarchaediscus at involutus 2.10 Tubber Formation stage, Uralodiscus rotundus, U. settlensis, Glomodiscus, The Tubber Formation (Gallagher, 1996; Pracht et al., Planoarchaediscus and Paraarchaediscus at concavus 2004), attains a maximum thickness of >292 m in Kill‑ stage, which establish a late Arundian to Holkerian age asolan Borehole (GSI‑10‑01) in the east of the region near (MFZ 10-12). In the highest beds of the formation below Mount Bellew Bridge (Figure 1), and is c. 102 m thick the Burren Formation in boreholes GSI‑06‑02, GSI‑07‑179 near Craughwell (2099-2) in the south (Figure 4). It ex‑ and GSI‑08‑239, the presence of Paraarchaediscus at an- tends from the top of the Ballysteen Formation (or Waul‑ gulatus stage, Paraarchaediscus at concavus stage, and sortian Limestone Formation if present) up to the base of Nodosoarchaediscus, as well as Koskinotextularia, Lituo- the Burren Formation. It is well exposed in the type area tubella and the problematicum Draffania biloba imply a around Tubber (c. 8 km SSW of Gort) where it is c. 300 Holkerian to early Asbian age (MFZ 12-13). m thick (Pracht et al., 2004). It represents most of the Vi‑ 2.11 Lucan Formation séan Burren-Gort succession below the Burren Formation and occurs mostly in the central and southern part of the The Lucan Formation represents rocks of basinal fa‑ region, south of Harrington’s Quarry. Further north it is cies that are recorded in boreholes in the southeast of the probably laterally equivalent to the combined Oakport to region (GSI‑06‑02, GSI‑06‑387, GSI‑07‑01, GSI‑07‑03, Croghan Limestone Formations of the Dunmore-Bally‑ GSI‑95‑113, 2099-2 and 01-1798-1). The greatest thick‑ moe succession (Table 1). ness (>733 m) is recorded in 01-1798-1. In Kiltullagh The Tubber Formation is characterized by medium- Borehole (GSI‑06‑02) 195 m of the Lucan Formation is grey, crinoidal-rich limestone, with shaly partings at some recorded with rocks of the Tubber Formation above and levels and several chert horizons. Fasciculate colonies of below (Figure 5). In the Monivea Borehole (GSI‑06‑387), Siphonodendron are abundant in the upper part of the Tub‑ >200 m of the Lucan Formation is recorded below the ber Formation (Pracht et al., 2004). Burren Formation. The Lucan Formation consists mainly Microfacies: The Tubber Formation consists mostly of of well-bedded, fine-grained, weakly laminated and bio‑ fine‑ to medium-grained, moderately well-sorted, skeletal turbated argillaceous limestone alternating with dark grey and peloidal packstone and grainstone (Figure 7G) with to black calcareous mudstone. Some limestone beds con‑ some coarse-grained bioclastic grainstone intervals. The tain siliceous bands and pyrite is common. Rare slumped main skeletal components are crinoids, brachiopods, rare intervals are present. red algae (aoujgaliids), foraminifers, (small archaedisc‑ Microfacies: Limestones comprise mainly wackestone ids can be locally abundant), calcispheres, problematica and packstone with rare thin beds of grainstone. The main (Draffania and salebrids) and kamaenids. skeletal components are spicules (Figure 7H) and calci‑ Depositional setting: The Tubber Formation was depos‑ spheres, with crinoids, fenestellid bryozoans, ostracods ited in a moderately shallow-water shelf, below fairweath‑ and rare foraminifers, with an absence of algae, apart from er wave-base but above storm wave-base. The presence of kamaenids. coarser-grained limestones with intraclasts and rounded Depositional setting: The Lucan Formation was depos‑ bioclasts implies higher energy conditions. The biota is ited in a deep-water, low-energy basinal setting. The pres‑ rich and diverse with abundant crinoids and common fora‑ ence of fine-grained limestones with abundant spicules minifers, and elsewhere in the upper part of the formation and calcispheres, sparse foraminifers and absence of dasy‑ are recorded coral biostromes. clad algae confirms a depositional setting below the photic Age: The Tubber Formation ranges in age from latest zone in quiet water, low-energy conditions. The presence Tournaisian? to late Viséan (Chadian to Asbian substages). of detrital quartz and feldspar indicate a terrigenous input, In the lower part of the formation the presence of Kon- and pyrite suggests dysphotic to anaerobic conditions on inckopora sp. (monolaminar alga) and Eotextularia di- the basin floor. Much of the skeletal material has likely versa suggests a latest Courceyan to Chadian age (MFZ been derived from a neighbouring shelf. 8-9). Higher in the formation the presence of Eoparas- Age: The Lucan Formation ranges in age from Arundian taffella simplex, Pseuodolituotubella, Brunsia, Pseudoam- to Asbian. In GSI‑07‑01 a suite of archaediscid foramini‑ modiscus, Eostaffella and absence of archaediscids imply fers of Arundian age (MFZ 11) was recorded including a Chadian age (MFZ 9). Important foraminiferal taxa re‑ Paraarchaediscus at involutus stage, Planoarchaediscus 20 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 and Uralodiscus. In GSI‑07‑03 foraminifers of Asbian age Oughterard granite. It also contains a tuff band which may (MFZ 13-14) are recorded, characterized by small archae‑ correlate with one in the Kilbryan Limestone Formation discids of Paraarchaediscus at angulatus stage. farther east. Microfossil data in the Oughterard Limestone suggest that the tuff band lies close to the chronostrati‑ 3 Evolution of the depositional setting graphic Tournaisian-Viséan boundary (Sevastopulo and of Mississippian limestones in west- Wyse Jackson, 2009). ern Ireland In the early Viséan (Chadian) there was the initiation of shallow-water platform sedimentation with bioclas‑ The oldest Mississippian marine rocks in the study area tic limestones, oolites and fenestral limestones (Oakport are known from boreholes in southern Co. Galway, the Limestone Formation). This was followed in the Arun‑ Burren and Loughrea in southeastern Galway. The Gort dian by more argillaceous bioclastic limestones rich in Borehole (G-1) and boreholes around Loughrea contain crinoids and fenestellid bryozoans with abundant colonies a transitional succession of sandstone, limestone and shale of Siphonodendron (Ballymore Limestone Formation). (Lower Limestone Shale Group) identified as belong‑ Further to the south, bioclastic limestones of the laterally ing to the Limerick Province — a province recognized equivalent Tubber Formation developed, which have oc‑ throughout most of Munster (Philcox, 1984; Somerville casional oolitic grainstone intervals. In the southeast of and Jones, 1985; Sleeman and Pracht, 1999). It marks a the region, deep-water basinal facies were deposited (Lu‑ marine transgression which advanced northwards across can Formation) and over 700 m of mainly fine-grained, the region during the Courceyan (Pracht et al., 2004; Sev‑ spiculitic, laminated limestone and mudstone, devoid of astopulo and Wyse Jackson, 2009; Somerville and Waters, algae, accumulated during the Arundian to Asbian. East of 2011; Somerville et al., 2011). The succeeding Ballysteen Athenry there are rapid changes in thickness and facies Formation represents widespread, uniform, limestone changes recorded in the boreholes to suggest that syn-sed‑ sedimentation with a rich and diverse open-marine fauna imentary faulting on the NE-trending Athenry Fault and in the late Tournaisian, that developed as far north as Ba‑ ENE-trending Craughwell Fault (Figure 1) was probably llymoe (Figure 2). The area around Dunmore was desig‑ partly controlling carbonate sedimentation (Figure 5). nated by Philcox (1984) as a separate Tournaisian province Shallow-water limestone sedimentation continued over (Dunmore Province), distinct from the Limerick Province most of the region (Croghan Limestone and Tubber Lime‑ to the south, but similar to the North Midlands Province stone Formations) during the Holkerian, and in the Asbian, to the east (Figure 10), although forming a much thinner a uniform carbonate facies up to 400 m thick was devel‑ succession. oped throughout much of the region (Burren Formation). The Ballysteen Formation is followed in the later Tour‑ It is characterized by rich faunal bands of brachiopods and naisian by localized development of deeper water Waulsor‑ corals and abundant dasycladacean green alage. Most cor‑ tian mud-mounds, which typically can be around 200-300 al colonies are in situ but some are overturned. Many ho‑ m thick in local complexes, surrounded by bedded cherty rizons of brachiopod contain concentrations of overturned limestone (Lees, 1964, 1994; Lees and Miller, 1985, 1995; Gigantoproductus implying periodic strong storm currents Devuyst and Lees, 2001; Somerville, 2003). However, sweeping across the shallow water shelf. The upper part of in the Ballinasloe area in eastern Galway (Figure 1), the the formation (Two Mile Ditch Member) is also distinc‑ complexes amalgamated to form an interval >700 m thick tive, as it contains clay wayboards, interpreted as palaeo‑ (Gatley et al., 2005). In the north of the region, around the sols, which rest on irregular limestone surfaces considered Slieve Dart Inlier and Mount Bellew Bridge (Figure 2), to be palaeokarsts and comparable to those well-exposed thinner developments of Waulsortian mud-mound facies in the Burren region in the Aillwee Member (Pracht et al., occur (Morris et al., 2003). These interfinger with rocks of 2004; Gallagher et al., 2006). This unit shows a glacio- the Kilbryan Limestone Formation. The latter formation eustatically controlled shallowing-upward cyclicity lead‑ is characterized by the presence of thin decomposed vol‑ ing to subaerial emergence and karstification of exposed canic ash bands (tuffs) which form regional isochronous limestones, with terrestrial accumulation of soils. These marker horizons between boreholes. In western Galway, features are typical of many late Asbian successions in around Lough Corrib, the laterally equivalent Oughterard southern Ireland (Gallagher, 1996; Gallagher and Somer‑ Limestone Formation contains much siliciclastic detri‑ ville, 1997, 2003; Cózar and Somerville, 2005; Waters et tus derived from the local basal clastics and the exposed al., 2011). Locally, the Burren Formation contains mud- Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 21 mounds, over 35 m thick, which are only recognized in rich deeper water shelf succession of the Ballysteen Group boreholes. They are like the older, larger Waulsortian mud- (Somerville and Waters, 2011). The latter, including the mounds, which contain cavities with peloidal mud and Ballymartin and Ballysteen Formations, represents more radiaxial fibrous marine cements. However, locally, the open-marine conditions with a richer and more diverse Asbian mounds contain a boundstone framework provided fauna. It is recognized as far north as the Dunmore-Bally‑ by bryozoans. These are similar to bryozoan-rich mounds moe area. In the late Tournaisian, the deepest water facies (buildups) described from coeval rocks in North Wales was developed in the Loughrea area in the southeast (Fig‑ (Bancroft et al., 1988; Wyse Jackson et al., 1991) in which ure 10), where thick accumulations of Waulsortian Lime‑ the trepostome Leioclema and Tabulipora were important stone are recorded (Lees and Miller, 1995). These would framework components of the buildup. Nevertheless, the have formed prominent mud-mounds with substantial top‑ absence of light-requiring algae and a sparse foraminiferal ographic sea-floor relief of several hundred metres, most content suggest a quieter and probably deeper water shelf of which was below the photic zone, as dasycladacean al‑ setting than most limestones of the Burren Formation. gae are absent (Lees and Miller, 1995; Somerville, 2003). The youngest Mississipian limestone rocks exposed Low-energy sedimentation continued, in part coevally, in the Burren type section (Slievenaglasha Formation of with the deposition of the Kilbryan Limestone Formation Brigantian age) are not developed in the Galway area, and in the northern part of the region, although probably in if ever deposited were subsequently eroded. shallower water, with an abundance of sponge spicules, fenestellid bryozoans and ostracods, with occasional cor‑ 4 Palaeogeographic reconstructions als. This formation is also characterized by rare thin green clay bands considered to be decomposed volcanic ash bands (tuffs), which can act as isochronous surfaces. Simi‑ 4.1 Tournaisian lar horizons are recorded in the Oughterard Limestone in The Lower Paleozoic and Precambrian rocks which the west and the equivalent Argillaceous Limestone in Co. lie to the west and northwest of Galway city would have Longford, east of the River Shannon (Morris et al., 2003), formed a prominent high (Galway High) during the early marking a distinct episode of volcanicity throughout cen‑ Tournaisian, with lowland continental sedimentation oc‑ tral Ireland in the latest Tournaisian. curing in the Oughterard area (Cope et al., 1992; Figure 4.2 Early-Middle Viséan 10). This facies is recognized in boreholes, where red-bed sediments rest unconformably on the Oughterard granite. In the early Viséan (Chadian), a marked shallowing The Oughterard area forms part of the Connemara Massif event occurred which led to the development of a wide‑ (Figure 1), which was the source of much of the siliciclas‑ spread shallow-water platform in the northern part of the tic detritus brought into the west of the region, although its region. This was the initiation of the Galway-Roscommon contribution diminished east of Lough Corrib (Figure 10). Shelf (Figure 11), which is known to extend as far north In the southeast of the region, however, marine sedimenta‑ as Boyle and Carrick-on-Shannon (Morris et al., 2003; tion occurred, associated with a trangression that moved Sevastopulo and Wyse Jackson, 2009; Figure 1). It repre‑ northwards across Co. Galway during the early Tournai‑ sents the accumulation of high-energy bioclastic and oo‑ sian (Ivorian) (Sevastopulo and Wyse Jackson, 2009; Fig‑ litic grainstones rich in dasycladacean algae, alternating ure 10). with low-energy lagoonal fenestral micrites of the Oakport The transitional limestone, shale and sandstone suc‑ Limestone Formation. In the Arundian, slightly deeper cession of the Lower Limestone Shale Group, comprises water shelf facies developed (Ballymore Limestone For‑ cross-bedded sandstones, oolitic and skeletal grainstones mation) with more argillaceous bioclastic limestones and and mudstones with desiccation cracks (Somerville and coral biostromes. This facies is known throughout the Jones, 1985). These indicate an alternation of intervals northern part of Co. Galway, but is laterally equivalent to of sedimentation during low‑ and high-energy conditions the more variable Tubber Formation, which is known from with periodic exposure. They represent a marginal marine the Craughwell area in the south and contains oolites, de‑ to shallow-water open shelf succession that developed veloped on the Clare-Galway Shelf (Figure 11). In the east on the Clare-Galway Shelf which extended south into of the region, east of Athenry, deep-water basinal condi‑ the Limerick Province (Sevastopulo and Wyse Jackson, tions are recorded with the deposition of the Lucan Forma‑ 2009). It was succeeded by a predominantly limestone- tion in the Athenry Basin (Figure 11). This facies is known 22 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015

Figure 10 Palaeogeographic reconstruction of Co. Galway region for the late Tournaisian (based on Cope et al., 1992). to extend farther east and probably forms the western mar‑ deposition of the Burren Formation on the Clare-Galway gin of the Tynagh Basin (Sevastopulo and Wyse Jackson, Shelf (Figures 1, 11). This limestone facies has a rich and 2009) which lies south of Ballinasloe (Gatley et al., 2005; diverse fauna and flora with abundant foraminifers and red Figure 1). The initiation of these subsiding basins in the and green algae. Coral biostromes are also well developed, early Viséan (Chadian) where substantial thicknesses of as well as concentrations of large brachiopod bands. The sediment accumulated, was probably related to a phase of presence of palaeokarstic surfaces and overlying palaeo‑ extensional tectonics which is known throughout the re‑ sols is an indication of periodic subaerial exposure of the gion (e.g. Tynagh; Philcox, 1984) and the Curlew Fault, carbonates. A similar shallow-water carbonate shelf is de‑ north of the Curlew Mountains (Figure 1; Philcox, 1989; veloped in the northeast of the region near Mount Bellew Philcox et al., 1992), and eastwards into the Dublin Basin Bridge. However, the two shelves are separated by deeper (Nolan, 1989; Strogen et al., 1996). water basinal facies of the Athenry Basin marking a con‑ tinuation of the Lucan Formation (Figure 11). In some in‑ 4.3 Late Viséan stances, though, there is clear evidence in long borehole In the late Viséan, shallow-water shelf facies is well- sections (GSI‑06‑387, GSI‑06‑02) to demonstrate that the developed in the south and southwest of the region with the two facies interfinger. Markus Pracht and A revised Mississippian lithostratigraphy of County Galway Vol. 4 No. 1 Ian D. Somerville: (western Ireland) 23

Figure 11 Palaeogeographic reconstruction of Co. Galway region for the Viséan (Chadian-Asbian) (based on Cope et al., 1992).

South of Galway City and around Lough Corrib, rare margin mounds described from the northern margin of the bryozoan-rich mud-mounds occur within the bedded lime‑ Dublin Basin near Loughshinny (Somerville et al., 1992; stones of the Burren Formation (Figure 11). These build‑ Somerville, 2003) which have locally developed bound‑ ups probably represent a slightly deeper water shelf setting stone frameworks. but are surrounded by algal-rich bioclastic limestones that clearly formed in shallow water. These mounds are similar 5 Conclusions to the Waulsortian-type mounds in possessing stromatac‑ tis cavities with radiaxial fibrous marine cements, that are 1) Analysis of new borehole data integrated with older known west of Lough Ree at Lecarrow (30 km northeast borehole records and combined with outcrop study has en‑ of Mount Bellew Bridge) (Morris et al., 2003) close to the abled a more detailed comprehensive geological map and northwestern margin of the Dublin Basin between Athen‑ lithostratigraphy to be constructed for the Mississippian ry, Ballinasloe and Athlone (Figure 1). They are also simi‑ rocks of Co. Galway in western Ireland. lar to the Asbian mounds of Co. Sligo (Somerville, 2003; 2) Micropalaeontological investigations using fora‑ Somerville et al., 2009). They have affinity with shelf minifers and microfacies analysis of the limestones has 24 JOURNAL OF PALAEOGEOGRAPHY Jan. 2015 enabled them to be precisely dated and their components, comments which helped improve the final version. textures and fabrics documented. In addition, palaeogeo‑ graphic map reconstructions have been achieved for the References late Tournaisian, and early to late Viséan intervals in the region. Bancroft, A. J., Somerville, I. D., Strank, A. R. E., 1988. A bryozoan 3) In the southeast of the region the oldest marine Mi‑ buildup from the Lower Carboniferous of North Wales. Lethaia, ssissippian deposits are associated with a transgression 21: 51-65. that moved northwards across Co. Galway. These Tour‑ Caldwell, W. G. E., 1959. The Lower Carboniferous rocks of the Carrick-on-Shannon Syncline. Quarterly Journal of the Geologi‑ naisian rocks belong to the Limerick Province, and are cal Society of London, 115: 163-187. represented by the Lower Limestone Shale and Ballysteen Cope, J. C. W., Guion, P. D., Sevastopulo, G. D., Swan, A. R. H., Groups followed by Waulsortian Limestone Formation, as 1992. Carboniferous. In: Cope, J. C. W., Ingham, J. K., Rawson, part of an overall deepening-upward trend. In the north‑ P. F. (eds). Atlas of Palaeogeography and Lithofacies. Geological west and north of the region, marine Tournaisian rocks are Society, London, Memoir, 13: 67-86. much thinner with a shoreline located northwest of Gal‑ Cózar, P., Somerville, I. D., 2005. Stratigraphy of upper Viséan car‑ way City (Galway High). bonate platform rocks in the Carlow area, southeast Ireland. Geo‑ 4) The central region of Co. Galway has a standard logical Journal, 40: 35-64. Viséan succession that can be directly correlated with the Devuyst, F. X., Lees, A., 2001. The initiation of Waulsortian buildups Carrick-on-Shannon succession in counties Roscommon in western Ireland. Sedimentology, 48: 1121-1148. and Leitrim to the northeast and east as far as the River Dunham, R. J., 1962. Classification of carbonate rocks according to Shannon. It is dominated by shallow-water limestones depositional texture. In: Ham, W. E. (ed). Classification of Car‑ bonate Rocks. AAPG Memoir, 1: 108-121. (Oakport, Ballymore and Croghan Limestone Formations) Embry, A. F., Klovan, J. E., 1971. A Late Devonian reef tract on that formed the Galway-Roscommon Shelf. This succ‑ northeastern Banks Island, Northwest Territories. Bulletin of ession is laterally equivalent to the Tubber Formation to the Canadian Petroleum Geologists, 19: 730-781. south. In the southeast of the region basinal facies of the Gallagher, S. J., 1992. Lithostratigraphy, Biostratigraphy and Pa‑ Lucan Formation accumulated in the Athenry Basin, which laeoecology of the Late Dinantian Platform Carbonates in parts was probably connected to the Tynagh Basin to the east of Southern and Western Ireland. Unpublished Ph. D thesis, Na‑ (Figure 1) and persisted throughout most of the Viséan. tional University of Ireland. 5) In the late Viséan, shallow-water limestones of the Gallagher, S. J., 1996. The stratigraphy and cyclicity of the late Burren Formation extended across much of the southern Dinantian platform carbonates in parts of southern and western part of the region on the Clare-Galway Shelf. These lime‑ Ireland. In: Strogen, P., Somerville, I. D., Jones, G. L. L. (eds). stones exhibit distinctive palaeokarstic surfaces overlain Recent Advances in Lower Carboniferous Geology. Geological Society Special Publications, 107: 239-251. by terrestrial accumulations of palaeosols which formed Gallagher, S. J., Somerville, I. D., 1997. Late Dinantian (Lower Car‑ during prolonged regressive conditions of glacio-eustati‑ boniferous) platform carbonate stratigraphy of the Buttevant area cally controlled cyclicity. They also are characterized by North Co. Cork, Ireland. Geological Journal, 32: 313-335. the presence of rich concentrations of large brachiopod Gallagher, S. J., Somerville, I. D., 2003. Lower Carboniferous (Late shells and colonial coral horizons which developed in pre‑ Viséan) platform development and cyclicity in southern Ireland: dominantly high-energy conditions, along with abundant Foraminiferal biofacies and lithofacies evidence. Rivista Italiana foraminifers and calcareous algae. Locally, bryozoan-rich di Paleontologia e Stratigrafia, 109 (2): 152-165. mud-mounds developed in slightly deeper water, lower en‑ Gallagher, S. J., McDermot, C. V., Somerville, I. D., Pracht, M., ergy conditions on the shelf. Sleeman, A. G., 2006. Biostratigraphy, microfacies and deposi‑ tional environments of Upper Viséan limestones from the Burren - Acknowledgements region, County Clare, Ireland. Geological Journal, 41: 61 91. Gatley, S., Somerville, I. D., Morris, J. H., Sleeman, A. G., Emo, G., 2005. Geology of Galway and Offaly: A geological description Markus Pracht publishes with permission provided by of Galway-Offaly, and adjacent parts of Westmeath, Tipperary, the Director of the Geological Survey of Ireland. Brian Laois, Clare and Roscommon to accompany the bedrock geology McConnell is thanked for his insightful comments on an 1:100,000 Scale Map Series, Sheet 15 with contributions by W. earlier draft of the paper. The reviewers Markus Aretz, Cox, T. Hunter-Williams, R. van den Berg and E. Sweeney; A. G. Tadeusz Peryt and Colin Waters, and the Editor-in-Chief, Sleeman (ed.). Geological Survey of Ireland, 90pp. Prof. Zeng-Zhao Feng are thanked for their constructive Jones, G. L. L., Somerville, I. D., 1996. 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