lower carboniferous peritidal carbonates and associated evaporites adjacent to the leinster massif,...

GEOLOGICAL JOURNAL

Geol. J. 40: 173–192 (2005)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.999

Lower Carboniferous peritidal carbonates and associated evaporitesadjacent to the Leinster Massif, southeast Irish Midlands

ZSOLTR.NAGY1*, IAND. SOMERVILLE2, JAYM.GREGG1, STEPHENP. BECKER1

andKEVINL. SHELTON3

1Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, Missouri, USA2Department of Geology, University College Dublin, Belfield, Dublin, Ireland

3Department of Geological Sciences, University of Missouri-Columbia, Columbia, Missouri, USA

Analysis of a 275m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusualsequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Visean, LowerCarboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part ofthe sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. Theseinclude a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolitesand. A 68m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternatingstromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizonscontaining pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units arerecognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstoneswith minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons.The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in

the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that aredeveloped on the southern margin of this landmass, and similar depositional facies exist further to the east in the South WalesPlatform, south of St. George’s Land, and in Belgium, south of the Brabant Massif.The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster

Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyanstrata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits.Copyright # 2005 John Wiley & Sons, Ltd.

Received 25 August 2003; revised version received 20 April 2004; accepted 28 April 2004

KEY WORDS peritidal; microfacies; evaporites; biostratigraphy; lithostratigraphic correlation; Lower Carboniferous; Ireland

1. INTRODUCTION

Identifying evaporative environments, including peritidal facies, is important for interpreting depositional

sequences (Shinn et al. 1969; Lucia 1972; Purser 1973) and may have considerable economic importance.

Evaporite sequences can supply chloride ions and/or sulphur to the brine systems that are involved in the formation

of ore deposits (Warren 1999). Evaporites commonly act as sources, seals and reservoirs for hydrocarbons

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: Zs. R. Nagy, Schlumberger Data and Consulting Services, 1325 S. Dairy Ashford Road, Houston, TX 77077, USA.E-mail: [email protected]

(Shinn 1983; Evans and Kirkland 1988). These investigations, demonstrating the importance of evaporative sedi-

ments in the depositional sequence and relating them to ore formation, may help in elucidating the process of ore

formation in the Irish Midlands (see discussion in Banks et al. 2002), as the Waulsortian Limestone hosts world-

class base-metal (Zn–Pb–Ba) deposits (Ashton et al. 1986; Anderson et al. 1995).

This paper focuses on the peritidal sequence deposited adjacent to the Leinster Massif in southeastern Ireland.

Evaporative sedimentation in this area was observed first by Philcox (1994) from the Geological Survey of Ireland

(GSI-91-25) borehole (Figure 1). This paper aims to: (1) describe more fully the peritidal facies found in this bore-

hole; (2) identify the various depositional environments; (3) accurately date the various units in this core and cor-

relate them with other units within the Lower Carboniferous succession in the Irish Midlands; (4) redefine the

Milford Formation; (5) compare these strata with age-equivalent peritidal sequences in South Wales and East

Belgium; and (6) ascertain the influence of dense saline brines in the Leinster Massif area on regional diagenetic

patterns and sulphide mineralization in Lower Carboniferous strata in the southeast Irish Midlands.

By establishing the fundamental depositional environments proximal to the Leinster Massif, this study reveals

the possible link between restricted, evaporative environment and Zn–Pb base-metal mineralization in the Irish

Midlands.

Figure 1. Simplified geological map of southeastern Ireland showing the distribution of the Lower Carboniferous sedimentary rocks, majormineral deposits and the location of the Milford Borehole (GSI-91-25).

174 zs. r. nagy et al.

Copyright # 2005 John Wiley & Sons, Ltd. Geol. J. 40: 173–192 (2005)

2. STRATIGRAPHIC FRAMEWORK

In the study area, Lower Carboniferous (Mississippian) rocks are exposed in the southeastern flank of the

Castlecomer Syncline and northwest of the Leinster Massif. The areal distribution of these rocks is presented

in Figure 1, and a schematic cross-section of the Mississippian stratigraphy of the Irish Midlands is shown in

Figure 2.

The basement rocks of the southeast Irish Midlands are composed of massive syn- to post-orogenic intrusives of

late Silurian to early Devonian age including the Leinster Granite batholith which intruded Lower Palaeozoic

(Cambrian to Silurian) metasedimentary rocks (McConnell et al. 1994). The basement rocks are unconformably

overlain by the Old Red Sandstone facies consisting of clastic fluvial sediments that are generally less than 350m

thick in the Irish Midlands and thin rapidly northward (Philcox 1984; Phillips and Sevastopulo 1986). The clastic

sediments were inundated by a northward-directed marine transgression during the late Devonian–early Tournai-

sian (Philcox 1984). The transgression resulted in deposition of extensive ramp deposits of thin bioclastic lime-

stones, siltstones and calcareous shales (Lower Limestone Shale). Evaporites were also deposited during the Lower

Courceyan in the northwest part of the Dublin Basin (North Midlands Province) and central Ireland (Philcox 1984;

Phillips and Sevastopulo 1986), in the successions at Silvermines (Boyce 1990) and at Navan (Ashton et al. 1986;

Rizzi and Braithwaite 1996; Anderson et al. 1998). The succeeding well-bedded, argillaceous limestones were

deposited on the outer ramp during the proceeding transgression (Ballysteen Formation). In southern and central

Ireland, the Ballysteen Formation grades upward into massive, pure limestones of theWaulsortian mudbank facies.

The Waulsortian complex comprises coalesced mudbanks that average 150–250m in thickness in the Dublin Basin

Figure 2. Lithostratigraphic chart showing the Lower Carboniferous strata and facies relationships of the studied area west of the LeinsterMassif. A simplified correlation of the Chadian to Asbian sequence with South Wales and southwest England is also shown (after George et al.1976; Wright 1986). The stratigraphic range of the Milford Borehole (GSI-91-25) is indicated by the vertical bar. The formations recognized inthis study are based on lithological and petrographical observations of drillcore and comparison with the lithostratigraphy of the DurrowBorehole (Dur-2) in the Rathdowney Trend further to the west, and replace the previous nomenclature of Philcox (1994), Tietzsch-Tyler et al.(1994b), McConnell et al. (1994), and Archer et al. (1996). Abbreviations: Penn., Pennsylvanian; Lmst., Limestone; Fm., Formation; Mb.,

Member; Waul., Waulsortian. Grey areas represent hiatus.

lower carboniferous carbonates and evaporites, irish midlands 175


and Rathdowney Trend (Somerville et al. 1992; Lees and Miller 1995; Somerville 2003). The Waulsortian Lime-

stone is overlain by well-bedded, crinoidal, bryozoan limestones (Crosspatrick Formation) in the south-central

Irish Midlands, which represents a shallowing event (Archer et al. 1996). In the south Wexford (Duncormick) area

(Figure 1) a series of exploration boreholes has intersected a thick micrite and breccia interval (Wexford Forma-

tion), probably forming Waulsortian-equivalent strata, overlying a dolomitized, crinoidal, oolitic limestone

(Ballysteen Formation) (Sleeman and Tietzsch-Tyler 1988; Tietzsch-Tyler et al. 1994a; Nagy 2003). Evidence

for evaporites in the Wexford Formation occurs mostly as silicified nodules and clasts representing pseudomorphs

of former gypsum within the breccia unit and also as distinct anhydrite/gypsum beds, over 2m thick, within the

breccia and as rare gypsum veins (Sleeman 1980; Carter and Wilbur 1986; Nagy et al. in press).

Strong differential subsidence controlled the distribution of most Visean sediments, which resulted in a complex

facies mosaic consisting of juxtaposed basinal and shallow-marine shelf environments in the Irish Midlands. The

sedimentary sequences adjacent to the Leinster Massif suggest a restricted nearshore sedimentary environment

proximal to the Massif, whereas an open marine ramp environment was predominant in the Rathdowney Trend

area further to the west (Figure 2). Shallow-water and peritidal conditions were established during the late Chadian

around the Dublin Basin including the area west of the Leinster Massif. In the Milford Borehole (GSI-91-25), 6 km

south-southwest of Carlow (Figure 1), these strata are represented by dark grey peloidal grainstones, pale calcar-

enites with subordinate shales, mudstones and peritidal units (¼Milford Formation of Philcox 1994; McConnell

et al. 1994; Tietzsch-Tyler et al. 1994b; Nagy et al. 2001; Gatley et al. in press).

Avery shallow-water depositional environment with a restricted fauna developed in the distal areas (Rathdowney

Trend) of the ramp during the late Chadian (Aghmacart Formation) (Figure 2). These rocks are very similar to those

recorded in the Milford Formation and typically are composed of peloidal or micritic limestones and dolomites,

oolitic grainstones, and minor thin shale interbeds (McConnell et al. 1994; Tietzsch-Tyler et al. 1994b; Archer

et al. 1996). The overlying Durrow Formation represents a change to an open marine environment with the first

appearance of archaediscid benthic foraminifers, dasycladacean green algae, and colonial and solitary rugose corals

during the early Arundian (Gatley et al. in press). The lower part of the Durrow Formation (Arundian) is composed

mostly of skeletal intraclastic packstones and grainstones, whereas in the upper part (Holkerian), fossiliferous shales

are common, and oolitic grainstones and birdseye mudstones also occur (Nagy 2003). This type of lithofacies

sequence also appears in the GSI-91-25 borehole (Nagy et al. 2001) in the upper part of the Milford Formation

(Philcox 1994) and in the basal 27m (145–171m) in the nearby Tankardstown Borehole (GSI-89-10, Figure 1)

(McConnell et al. 1994; Nagy 2003).

The overlying Ballyadams Formation, composed of well-bedded, coral–algal-bearing grainstones, was depos-

ited uniformly over the entire south Irish Midlands during the late Holkerian–Asbian (McConnell et al. 1994;

Tietzsch-Tyler et al. 1994b; Archer et al. 1996; Cozar and Somerville 2005). The type section for the upper part

of the formation is in Ballyadams Quarry, County Laois, but the lower part is best developed in the Durrow bore-

hole, Dur-2 (Figure 1). The Ballyadams Formation passes conformably up into the thin-bedded cherty bryozoan

crinoidal limestones of the Clogrenan Formation. This unit is well exposed at Clogrenan Quarry, County Carlow

(Figure 1) at its type locality (Tietzsch-Tyler et al. 1994b; Cozar and Somerville 2005).

The Upper Carboniferous (Namurian–Westphalian Series) rocks, separated by a pronounced unconformity,

consist mostly of sandstones and shales with coal seams, which record deltaic environments. The outcrops of

these units are located in the Leinster Coalfield, west of the study area (Figure 1; Nevill 1956; Higgs 1986;

Tietzsch-Tyler et al. 1994b; Archer et al. 1996).

3. METHODS

A continuous 275m-thick core from the Milford Borehole (GSI-91-25) (National Grid: S7001 7014) was examined

and sampled in detail for this study. The lithological column, shown in Figure 3, was compiled based on macroscopic

examination of polished core and observations on 126 standard thin sections using transmitted light microscopy. Sam-

ples were stained with Alizarin red S and potassium ferricyanide for discriminating calcite from dolomite.



4. RESULTS OF LITHOFACIES ANALYSIS

The succession in the Milford Borehole has been divided into depositional environments containing several micro-

facies. Genetically or environmentally related microfacies are grouped together as a microfacies association. When

a series of microfacies pass gradually from one into other, it is called a microfacies sequence.

The formations described below are the same as those recognized in the Durrow Borehole (Dur-2) further to the

west in the Rathdowney Trend (Somerville et al. 1996; Nagy 2003; Nagy et al. 2004). Note that the Milford

Formation of previous authors (Philcox 1994; McConnell et al. 1994; Tietzsch-Tyler et al. 1994a, b; Archer et al.

Figure 3. Summary log of GSI-91-25 borehole showing the main lithological units, sedimentary and petrographic textures, and diageneticcharacteristics. The ranges of selected calcareous algae/calcimicrobes, foraminifers and corals are also shown. Dip of the section is insignificant.

Abbreviations: C.P. FM., Crosspatrick Formation.



1996) is here regarded as the proximal equivalent of this succession (Figure 2). Description of formations will

begin with the oldest unit first, followed by younger units.

The lower half of the core is dolomitized to varying degrees, whereas the upper half of the core is undolomitized

with some argillaceous interbeds, and limestones abundant in skeletal and non-skeletal grains.

4.1. Crosspatrick Formation

4.1.1. Subtidal ramp microfacies sequence (242.5–275m)

This basal unit is dominated by poorly stratified, massive, argillaceous dolomite and chert nodules (Figure 3). In

several horizons, non-destructive dolomitization reveals reworked crinoid and bryozoan debris, which are locally

concentrated in discrete bands (Figure 4A). Biomoulds and intercrystal porosity are partially filled with fibrous,

length-slow chalcedony (Figure 4B). At several horizons, large (3–5 cm) spherical cavities were observed display-

ing stylolitic argillaceous halos around the cavity containing abundant fine-grained (c. 50 mm), angular quartz hav-

ing diffuse or corroded outer crystal faces (Figure 4C). The cavities, from edge to centre, contain ‘rip-up’ clasts of

the halo, coarse-crystalline (100 mm to 2mm), euhedral blocky quartz with undulatory extinction and coarse-crys-

talline (100 mm to 3mm) saddle dolomite. Geopetal features are commonly observed where ‘rip-ups’ of the halo

and euhedral quartz characterize the bottom half of the cavity and the upper half is cemented by saddle dolomite

(Figure 4C). Similar cavities were described from probably time-equivalent rocks, i.e. the Lower Visean of the

Vesdre Formation, Belgium by Swennen and Viaene (1986, plate 2H).

Interpretation. The absence of sedimentological features indicative of supratidal or hypersaline conditions and

the abundance of a stenohaline fauna in this unit suggest a subtidal, normal marine depositional environment. The

sediments in the unit were deposited in a low- to moderate-energy setting, where crinoids and bryozoans were

frequently resedimented. Early dolomitization of the calcite matrix was probably penecontemporaneous with

the formation of anhydrite. Length-slow chalcedony is interpreted to be an anhydrite replacement (Folk and

Pittman 1971; Siedlecka 1972, 1976; Chowns and Elkins 1974; Tucker 1976; Milliken 1979), and small anhydrite

inclusions found within the quartz (Nagy et al. 2004) may confirm that interpretation. Anhydrite was probably

precipitated in large biomoulds as cement from fluids migrating through the unit. Large cavities found at several

horizons are interpreted as replacement of anhydrite nodules. Similar nodules were described by previous authors

from a deeper subtidal setting (e.g. Elorza and Rodriguez-Lazaro 1984; Maliva 1987; Alonzo-Zarza et al. 2002)

and served as an indicator of hypersaline porewater during early diagenesis (Maliva 1987).

4.2. Aghmacart Formation

4.2.1. Oolitic peloidal mobile shoal facies association (210.8–242.5m)

The Aghmacart Formation conformably overlies the Crosspatrick Formation and is dominated by pale grey, well-

stratified dolomite with subordinate unfossiliferous dolomitized mudstone interbeds. The pale grey dolomite com-

pletely replaces, cross-stratified, oolitic and peloidal grainstones (Figure 4D). The foresets locally dip at more than

30� and reach thicknesses of 0.2–0.6m. The shape of ooids is preserved and their size ranges between 0.3 and

0.6mm with varying degrees of sorting. Peloidal grainstones replaced similarly by dolomite frequently alternate

with oolites (Figure 4E) and are more common in the lower part of the unit.

The oolite unit is interbedded with a fine-crystalline (<4 mm) dolomite unit at c. 225m (Figure 3) that replaces

unfossiliferous, bioturbated mudstone with rhizoliths, rare detrital angular quartz and organic matter. Pseudo-

morphs of former anhydrite, such as megaquartz spherules (Figure 4F; cf. Siedlecka 1972, figure 4A; Milliken

1979, figure 9B) were observed within the laminated dolomitized mudstone unit at c. 211m.

Interpretation. Oolitic and peloidal grainstones indicate a well-agitated, high-energy environment within the

intertidal zone. Cross-stratification indicates strong water movement and sediment redistribution. Wave and tidal

agitation were probably important factors, which localized the ooid sand bodies near the margin of the landmass.

Replacement of anhydrite nodules by quartz was very early and pre-dates the brecciation (i.e. broken quartz

nodules).



4.2.2. Bioturbated lagoonal facies association (197.8–210.8m)

The oolitic and peloidal units are overlain by more argillaceous, medium grey dolomites replacing bioturbated

wackestones and packstones (Figure 3). Original sedimentary structures of the unlithified sediment were disrupted

by bioturbation. The bioturbated parts of this unit have distinctly coarser crystal size. Abundant detrital quartz

grains (silt size), intercrystal fluorite and a resedimented dolomite clast with quartz pseudomorph of a ‘stair-step’

anhydrite crystal (e.g. Scholle et al. 1992, figure 5) were observed (Figure 4G).

Figure 4. Petrographic features of the subtidal shelf deposits (A–C, Crosspatrick Formation), oolitic mobile shoal (D–F) and the overlying subtidallagoonal unit (G, Aghmacart Formation). Large white arrows point towards stratigraphic top. (A) Resedimented crinoidal wackestone to packstonedisplaying preservation of original sedimentary texture in an intensively dolomitized unit, 268.18m, scale bar¼ 1 cm. (B) Broken reworked crinoidfragment replaced by length-slow, fibrous chalcedony within fine-crystalline dolomite, 274.34m, cross-polarized light (cpl), scale bar¼ 500mm. (C)Rock slab showing an internally brecciated replaced anhydrite nodule within dolomitic crinoidal wackestone. Nodule with geopetal fabric issurrounded by argillaceous halo (a), late epigenetic dolomite (d) occupies the upper part, 258.38m, cm scale. (D) Replacement texture of cross-laminated oolite in dolomite unit, 233.6m, scale bar¼ 1 cm. (E) Alternating dolomitized oolite (lighter bed) and peloids (darker bed), 222.3m, scalebar¼ 1 cm. (F) Quartz replacing former anhydrite nodule within a brecciated interval, cpl, 210.65m, scale bar¼ 1mm. (G) ‘Stair-step’ quartz

pseudomorphs of former anhydrite within dolomitized intraclasts in the lagoonal unit, cpl, 204.58m, scale bar¼ 150mm.



Interpretation. Bioturbation by benthic organisms indicates deposition in a well-oxygenated, low-energy envir-

onment, where salinity of the seawater was probably normal. The sediments were deposited below the wave base,

but common intraclasts and concentrations of bioclasts suggest frequent reworking of sediment.

4.2.3. Tidal flat facies association (143–197.8 m)

The unit is composed dominantly of non-destructive, fabric-preserving fine-crystalline (<15 mm) planar-s dolo-

mite and, rarely, fabric-destructive medium-crystalline (50 to 200 mm) planar-s and -e dolomite (Nagy et al.

2004). The most common lithofacies include laminated former microbial mats (Figure 5A) displaying desiccation

cracks that occasionally enclose lenticular (lozenge or lath shaped) gypsum/celestite casts (Figure 5B; cf. Friedman

and Shukla 1980, figures 2 to 6; Geeslin and Chafetz 1982, figure 10). The former evaporite crystals show displa-

cive growth suggesting penecontemporaneous precipitation with the early substratum. They were subsequently

replaced by polycrystalline calcite (Figure 5B; cf. Kendall 2001, figure 8). The fine-crystalline dolomite post-dates

evaporite precipitation, as indicated by the corrosion of the replacements by the early dolomite. Dolomite rhombs

were found in the breccia cement displaying gravity-driven settling on clasts forming ‘snow-on-the-roof’ texture

(Sangster 1988). The microbial mats commonly alternate with unfossiliferous birdseye mudstones. The fenestral

pores are commonly filled with geopetal vadose dolosilt and blocky calcite. The mudstones frequently contain

rootlets, mud aggregates and glaebules (nodules) with circum-granular cracking (Figure 5C). Large root-hole

infillings are suggested by the presence of patches of dense microspar, 1–3 cm in size, displaying horizontal to

gently oblique fissures (Figure 5D; cf. Sanz-Rubio et al. 1999, figure 5). This feature is similar to the root-mat

horizon found in a calcrete bed in South Wales (Figure 5E; cf. Wright et al. 1988, figure 3). Some horizons in

the mudstone contain abundant detrital quartz silt and organic matter. Other important lithofacies within the unit

include peloidal, dolomitized mudstone with abundant detrital quartz, intraclasts, mud aggregates and microbial

mat or rhizolith casts. The diversity of biota is generally very low, only ostracods and gastropods occur. Rarely,

some beds display ‘salt and pepper texture’ containing a few blackened components (possibly ooids), mud aggre-

gates or former microbial mat clasts (Figure 5F). A similar texture was described from the Persian Gulf by Shinn

(1973, figure 4). The bioturbated dolosilts commonly contain abundant detrital quartz silt and vermiform gastro-

pods (microconchids) were observed. In the upper part of the section remnants of ooids were observed, which have

been replaced by medium-crystalline planar-s dolomite.

Interpretation. The petrologic features indicate sediment accumulation in the upper intertidal and lower supra-

tidal environment with occasional subaerial exposure horizons (Shinn et al. 1969; Lucia 1972; Shinn 1983).

Rhizoliths and glaebules indicate pedogenesis in the sequence (Esteban and Klappa 1983; Wright 1994). Pseudo-

morphs of former evaporites suggest that the area was temporarily inundated by hypersaline water. Increased sali-

nity is also supported by the almost complete absence of fossil assemblages. Soluble evaporites have been subject

to dissolution causing collapse structures and brecciated horizons. Terrigenous quartz in the mudstones indicates

deposition proximal to the Leinster Massif; however, siliciclastic influx was rather subordinate during the deposi-

tion of the unit.

The contact of the tidal flat unit with the underlying lagoonal deposit at 197.8m is probably discordant. This may

be indicated by a pedogenic horizon (Figure 5C) immediately above the lagoonal deposits (Figure 3). A

——————————————————————————————————————"Figure 5. Petrographic features of the tidal flat deposits (A–F), oolitic peloidal barrier sand (G–I, Aghmacart Formation). Large white/blackarrows point towards stratigraphic top. (A) Organic-rich algal laminites with probable synsedimentary displacement (arrow) resulting inbrecciated textures, 191.35, scale bar¼ 1 cm. (B) Calcitic pseudomorphs after probable gypsum or celestite within fine-crystalline dolomite, cpl,179.43m, scale bar¼ 150mm. (C) Palaeosol surface with large mud aggregates and circum-granular cracking (arrows), 197.17m, scalebar¼ 1 cm. (D) Calcified root-mat horizon with large horizontal root mould-filling microsparite (lower part), 171.17m, scale bar¼ 1 cm. (E)Close-up view of a small root mould. Note the micritic coating around the cylinders, scale bar¼ 300 mm. (F) ‘Salt and pepper’ texture consistingof dolomicrite, blackened components, and mesh of broken gastropod shells (not visible), 185.92m, scale bar¼ 1 cm. (G) Oolitic peloidalgrainstone with abundant loosely coiled vermiform gastropods, 142.04m, scale bar¼ 1 cm. (H) Close-up view of the crust in an ooliticgrainstone showing fibrous surface film as a result of microbial activity. Note how the film follows the outline of oolitic components, cpl,

120.89m, scale bar¼ 1mm. (I) Close-up view of ‘meniscus-type’ bridging of ooids in a grainstone, cpl, 120.89m, scale bar¼ 500mm.



distinctive karst surface is, however, not present, suggesting either a short-lived subaerial exposure event or arid/

semi-arid climate where chemical weathering is slow.

4.2.4. Backshoal oolitic and peloidal sand facies association (117.5–143.0m)

This unit is made up of well-stratified, mostly unfossiliferous limestone with minor replacement dolomite between

121 and 123m. The lower part of this section consists of alternating oolitic and peloidal grainstones with abundant

vermiform gastropods (Figure 5G) and ooid aggregates. The oolite unit is shale-free, but organic stringers were

observed. Micritization of ooids by boring cyanobacteria is common, where preserved ooids display radial or

tangential ultrastructure. In one horizon, a thin crust is formed by fibrous calcite on the top of ooids (Figure 5H).

A similar fibrous biofilm coating ooids has been described from modern subtidal stromatolites of the Exuma Cays,

Bahamas (MacIntyre et al. 2000; cf. Reid et al. 2000, figure 4; Visscher et al. 2000, figure 2). Ooids are commonly

linked by uneven micrite coating (‘meniscus-type’ bounding) to form aggregates (Figure 5I).

Oolitic and peloidal beds are overlain by stromatolitic peloidal mudstone with fenestral fabric (Figure 6A).

A small microerosion surface occurs at c. 132m within the oolitic pack-grainstones (Figure 6B). The lithofacies

below the erosion surface includes ooids and peloids displaying mottled texture, in which micritic carbonate was

precipitated in a dispersed manner to form weakly defined nodules (Wright 1994). Above the sharp irregular sur-

face, clasts with circum-granular cracking appear to be floating within pale calcareous microspar (Figure 6C).

Other microfacies in the unit consist of (i) intensively burrowed oolitic peloidal packstones with numerous

encrusting calcimicrobes (Ortonella and Girvanella), vermiform gastropods and ostracods, and (ii) unfossiliferous

peloidal grainstones. The upper part of the oolite unit is more fossiliferous and records the first appearance of colo-

nial corals at 124.5m (Figure 3).

Interpretation. Petrographic features observed within the oolitic sand unit suggest sedimentation in the upper

subtidal to lower intertidal environment. The ooids and peloids probably accumulated behind the extensive shoal

complex that existed on the ramp (Nagy 2003; Gatley et al. in press).

The processes forming the coating in samples from southeast Ireland may have been microbial in origin and

probably played an important role in the initial stabilization and cementation of the carbonate sand. Grain aggre-

gation by ‘meniscus-type’ micritic grain-to-grain bridging (Figure 5I) found in Mesozoic and Recent carbonates

was interpreted by Hillgartner et al. (2001) as firm- to hardgrounds, formed within a subtidal environment. Fibrous

biofilm, similar to that in Figure 5H, was interpreted as the result of microbial activity and sulphate reduction by

MacIntyre et al. (2000), Reid et al. (2000) and Visscher et al. (2000). These authors also suggest that the crusts

represent a discontinuity surface in sedimentation. Micritization or micritic coatings are, however, also a well-

known phenomenon in palaeosols (Wright 1994) and may relate to direct bioerosion by micro-organisms, for

example, fungi (James 1972).

The contact with the overlying Durrow Formation is transitional, with the return of oolitic facies (see below).

In addition, the first appearance of colonial rugose corals (see below) suggests that the upper part of this unit was

already normal saline and deposited in an open marine environment.

——————————————————————————————————————"Figure 6. Petrographic features of the peritidal deposits of the Durrow Formation. Large white or black arrows point towards stratigraphic top.(A) Peloidal micritic laminae with well-developed fenestral porosity, 139.84m, scale bar¼ 1 cm. (B) Microerosion surface with oolite showingmottled texture (bottom) and corroded oolite clasts floating in pale grey microspar matrix (above), 132. 41m, scale bar¼ 1 cm. (C) Brecciatedmicritic clast floating in pale microsparitic matrix, enlarged from (B). Clast has numerous micritic glaebules bounded by circum-granularcracking (arrows), scale bar¼ 750 mm. (D) Birdseye mudstone showing complex irregular tubular fenestrae, 67.0m, scale bar¼ 1 cm. (E and F)Reticulate pattern of micrite threads, 31.9m, scale bar¼ 300 mm. (G) Well-developed mottled texture in oolite in lower part overlain by ooliticgrainstone with intraclasts. Note the irregular scour surface at the contact (arrow), 30.7m, scale bar¼ 1 cm. (H) Close-up view of the mottledtexture (arrows) in oolitic peloidal grainstone, enlarged from (G). Note the sharp contrast between micritic and sparitic fields, scale bar¼ 2mm.(I) Transgressive lag sediment with dolomitic argillites (bottom), in situ thrombolites (th), coarse skeletal debris, and skeletal mudstone. Notegastropods (g) at top, 29.63m, scale bar¼ 1mm. (J) Close-up view of the thrombolites, enlarged from (I). The thrombolites display a clotted

texture, scale bar¼ 750 mm.



4.3. Durrow Formation (8.42–117.5m)

The Durrow Formation consists of a variety of different limestone lithofacies and a few thin dolomitized beds. The

lower part of the section is rich in skeletal and non-skeletal grains with lesser volume of argillaceous components.

At two horizons (c. 67m and c. 30m) the formation becomes shaly. Based on the recognition of petrographic fea-

tures, seven different microfacies have been defined. Facies sequences form two basic types of facies associations:

(1) open lagoonal association consisting of skeletal packstones and grainstones, and (2) cyclic peritidal association

dominated by oolites and calcrete horizons with pedogenic features.

4.3.1. Open lagoonal facies association

(1) Coarse-grained bioclastic packstone and grainstone microfacies. This microfacies consists of poorly sorted,

intensely micritized bioclasts, such as Koninckopora (dasycladacean green algae), kamaenids and aoujgaliid

algae. Other skeletal grains include crinoids, brachiopods, bivalves, foraminifers, fenestellid bryozoans and

coral fragments. The less common packstones are more argillaceous and occur from the middle to the upper

part of the formation. Stylolitic contacts between grains are common in both the packstones and grainstones.

Wavy organic solution seams in the packstones appear to post-date early dolomitization as indicated by the

corrosion of dolomite by the solution seams. The origin of these stringers may be related to mechanical

compaction instead of chemical dissolution (Shinn et al. 1977; Shinn and Robbin 1983) because they usually

terminate at the transition between dolomitized and undolomitized rocks.

(2) Oolitic grainstone microfacies. This coarse-grained microfacies is similar to the uppermost oolitic facies of the

Aghmacart Formation and contains well sorted, micritized ooids, oolite intraclasts, peloids and infrequently

large mud intraclasts. It is mostly unfossiliferous and contains only scattered large encrusting calcimicrobes

such as Ortonella and Girvanella.

(3) Fine-grained peloidal skeletal packstone to grainstone microfacies. This facies is composed of peloids and

abundant foraminifers, especially Brunsia, Earlandia, calcispheres, ostracods, Koninckopora and Girvanella

and rarely, some crinoid, brachiopod and coral fragments.

(4) Very coarse-grained crinoidal algal peloidal grainstone microfacies. Large crinoids and Koninckopora are the

main constituents in this facies. In addition aoujgaliids, large endothyrid foraminifers and brachiopods, and

large intraclasts are also rarely present. The size of the components fines upward, with the microfacies grading

into the peloidal skeletal grainstone microfacies. Bioturbation is less common, if present; the burrow structure

may be filled with peloidal grainstone.

(5) Algal (Kamaena) crinoidal argillaceous packstone microfacies. This microfacies is well developed in the GSI-

89-10 drillcore (Figure 1) and only subordinate in the GS-91-25. Kamaena is the predominant alga, but

Koninckopora and aoujgaliids are also present in low to moderate abundance. Foraminifers (including

archaediscids), crinoids and brachiopods are common, and Syringopora fragments can be found. The

microfacies is argillaceous with organic dissolution seams and is intensely micritized.

4.3.2. Cyclic peritidal facies association

(6) Skeletal argillaceous packstone (coquina) microfacies. The microfacies consists of fine-grained, laminated

packstones with a resedimented mesh of small crushed bioclasts, such as Earlandia, ostracods and

calcispheres. Small organic solution seams are common in this facies. The contact with the underlying bed,

commonly bioclastic skeletal wackestones and packstones, is sharp, but the contact with overlying beds is

gradational. Skeletal argillites are most common between 60 and 70m in the borehole.

(7) Fenestral peloidal mudstone microfacies. Peloidal mudstones with well-developed fenestrae (Figure 6D) occur

above argillaceous packstone beds at 67m. This is similar to the microfacies found in the tidal flat facies

association in the Aghmacart Formation. Infrequently, ostracods are a major component of the mudstone, but it

is otherwise rather unfossiliferous. Large micritic aggregates and calcitized gypsum/celestite pseudomorphs

also occur. The fenestral mudstone may grade upward concordantly into peloidal or intraclastic grainstone



containing numerous encrusting Ortonella. The absence of evaporite pseudomorphs in this microfacies

suggests a predominantly subtidal setting. A similar conclusion was reached by Wright and Wright (1985) for

the laminar structures found in the Llanelly Formation, South Wales. Similar fenestrae associated with

evaporites in the peritidal setting were interpreted by Wright (1982, 1983) as solution fenestrae in calcrete.

(8) Algal stromatolite microfacies sequence. The limestone unit consists of several subunits between c. 28 and

32m, overlying skeletal argillaceous packstone. Unfossiliferous mudstones or micritic aggregates containing

small, 200–400 mm long, anastomosing micritic wall structures occur at the base (Figure 6E and F). Wright

and Wright (1985) described similar micritic wall structures with reticulate pattern from the Lower

Carboniferous Penllwyn Oolite Member in South Wales and interpreted them as preserved stromatolitic

microstructures. Philcox (1994) described vertical calcite-filled tubes, 2–3mm across, from 31.9m in the

Milford Borehole and interpreted them to be associated with columnar stromatolites. This facies is directly

overlain by micritized oolitic packstones and grainstones. Within this subunit there is a sharp irregular surface

(Figure 6G), which is micro-erosional, but does not truncate the ooids. The mottled oolite microfacies below

this horizon is very similar to the one described from the tidal flat environment, but the mottled texture is more

pronounced (Figure 6H). Above the horizon, micritized ooids are larger and cemented by isopachous blocky

calcite cement. Numerous large Girvanella–Ortonella encrusted grains and intraclasts also occur in this

interval (Figure 6G). The uppermost facies begins at its base with peloidal mudstone, slightly dolomitized in

situ thrombolites at 29.5m (Figure 6I). The thrombolites are<1 cm in size (Figure 6J), though Philcox (1994)

described 4–5 cm high ‘bioherms’ with steep margins from 28.7m. They are characterized by a macroscopic

clotted fabric and a lack of lamination (Figure 6J). The thrombolites are overlain by very coarse, resedimented,

micritic and encrusted clasts, interpreted as a transgressive lag deposit, grading up into a gastropod-rich

mudstone with scattered cerithid-type gastropods (Figure 6I). The contact with the overlying intraclastic

skeletal grainstone is sharp and stylolitic.

Interpretation. The first appearance of a stenohaline fauna and flora in the Durrow Formation marks the onset of

an open marine environment with increasing water depth in the lower part of the section (c. 117m). This change

possibly caused the diversification of microfacies. The overall depositional environment may be envisaged as a

broad lagoon, where skeletal limestones alternating with peritidal carbonates were deposited behind skeletal peloi-

dal shoals.

5. BIOSTRATIGRAPHY

A total of 36 rugose and tabulate coral samples were collected from the GSI-91-25 by one of the authors (I.D.S.).

Standard thin sections prepared for petrographic analysis were used for microfossil examination from approxi-

mately 126 rock samples. The ranges of selected calcareous algae/calcimicrobes, foraminifers and corals are pre-

sented in Figure 3.

The Crosspatrick Formation lacks stratigraphically important micro- and macrofossils. This is mainly due to

intense dolomitization, but adverse environmental conditions may also have been partly responsible. The lowest

recorded microfossils are in the lower oolitic unit (oolitic peloidal mobile shoals, Aghmacart Formation) and

include Endothyra sp., Earlandia sp. and Kamaena sp. The uppermost unit of the Aghmacart Formation (back-

shoal oolitic peloidal sand) marks the appearance of the colonial rugose coral, Dorlodotia pseudovermiculare at

124.7m suggesting late Chadian age (Rugose Coral Assemblage A of Mitchell 1989; Jones and Somerville 1996).

This unit also contains encrusting oncoids of Ortonella, Girvanella and Pseudosolenopora sp. The succeeding

strata (Durrow Formation) mark the first appearance of biostratigraphically important taxa and include

late Chadian to Arundian corals (Dorlodotia pseudovermiculare, D. briarti, Syringopora sp.), foraminifera

(Consobrinella sp., Palaeotextularia monolaminar), Palaeospiroplectammina mellina at 117.5m and a possible

Paraarchaediscus? sp. (a poorly preserved specimen with doubtful generic identification) at 115.9m. These data

confirm that the Crosspatrick and Aghmacart formations are of early to late Chadian age; the Chadian/Arundian

boundary is at c. 117.5m. The succeeding strata yielded Arundian algae Koninckopora tenuiramosa (bilaminar),



foraminifera Eoparastaffella simplex, Eostaffella sp. and colonial rugose corals Siphonodendron martini at

96.14m; Arundian foraminifera Uralodiscus sp. and coral Siphonodendron sociale at 80.93m; and Arundian-

Holkerian foraminifera Glomodiscus sp., Pojarkovella sp., and Paraarchaediscus concavus stage at 55m. These

data suggest that the Durrow Formation is of early Arundian to Holkerian age and the Arundian/Holkerian bound-

ary is at c. 54m. In the uppermost part of the section, the solitary rugose coral Axophyllum vaughani was recorded

at 12.38m indicating a Holkerian or younger age (see Mitchell 1989). No diagnostic fauna of Asbian age have

been found in the drillcore.

6. DISCUSSION

As a result of the detailed microfacies analysis and biostratigraphy, it was possible to (1) assign the former Milford

Formation to two major lithostratigraphic units (Aghmacart and Durrow formations) recognized throughout the

Rathdowney Trend (Figure 2), (2) characterize the microfacies features of the Crosspatrick, Aghmacart and

Durrow formations, and (3) outline the geological evolution of the study area and make a comparison with the

Chadian-late Holkerian sequences of other areas in the region.

6.1. Geologic evolution of the Chadian-late Holkerian sequence adjacent to the Leinster Massif

The Milford Borehole (GSI-91-25) proximal to the Leinster Massif records one regressive and one transgressive

cycle ranging from (mid?) late Chadian to late Holkerian age (Figure 3). The regressive cycle includes at the base,

the sediments of the Crosspatrick Formation containing mid- to outer ramp deposits (Figure 4A), which were

found both in the study area and basinward to the west in the Rathdowney Trend. Extensive oolitic shoals formed

during the late Chadian along the Rathdowney Trend (Nagy 2003; Gatley et al. in press) may have effectively

restricted the water movement from offshore to lagoonal areas. The shoaling of the sequence resulted in the appear-

ance of the tidal flat environment of the Aghmacart Formation representing backshoal peritidal deposits. The

microfacies characteristics suggest that the sediments were deposited in the inner ramp environment and the pre-

dominance of oolitic sand units (Figure 4D and E) near the landward margin is typical of a ramp setting (cf. Read

1985; Burchette and Wright 1992). Petrographic characteristics indicate that microbial activity was an important

factor in stabilizing the carbonate sediment. The Durrow Formation represents a transition between the underlying

Aghmacart Formation containing peritidal deposits and the overlying Ballyadams Formation (not present in the

GSI-91-25 drillcore) showing typical open shelf characteristics (Nagy 2003; Cozar and Somerville 2005). The

appearance of an open marine stenohaline environment during the early Arundian indicates an increase in water

depth and establishment of normal saline conditions. The stratigraphical relationship of lagoonal facies association

(packstones and grainstones) and peritidal facies association (palaeosols) is indicative of a shoaling-upward cycle,

which is typical of high-energy intertidal units (James 1979) and similar cycles were described from South Wales

(see below; Wright 1986, figure 10).

6.2. Regional correlation of Chadian-late Holkerian proximal sequences

The regional palaeogeography during the Lower Carboniferous was dominated by St. George’s Land, a landmass

covering most of what is now mid-Wales and southeast Ireland (in Ireland its western extension is referred to as the

Leinster Massif) (Figures 7 and 8). The sedimentary sequence deposited proximal to this landmass overlying the

Old Red Sandstone facies in southeast Ireland suggests a correlation to the age equivalent strata in South Wales

(Clydach Valley and Gower) and southwest England (Mendips/Burrington area) (cf. George et al. 1976; Wright

1986) (Figure 2).

In South Wales, the Chadian–Arundian succession comprises alternating oolites and cyclic limestones (Gilwern

Oolite, Caswell Bay Oolite) in the lower part, and peritidal carbonate sequences with subaerial exposures contain-

ing clays and fluvial sandstones (Caswell Bay Mudstone, Llanelly Formation) in the upper part (Figure 2). The

particular section that is exposed in the Milford Borehole (GSI-91-25) may be lithologically equivalent to the units



described by Riding and Wright (1981) from Miskin Quarry, South Wales. The Caswell Bay Oolite (similar to the

lower oolite in the Aghmacart Formation) is a patchily dolomitized, cross-bedded oolite. It is overlain by the

Caswell Bay Mudstone (similar to the tidal flat mudstone in the Aghmacart Formation) consisting of breccia con-

glomerate, mottled and peloidal mudstones, laminated mudstones, paper shales and thin limestones that are locally

dolomitized (see Ramsay 1987; Hird et al. 1987). The upper oolite unit of the Aghmacart Formation in the Milford

Figure 7. Palaeogeography during the Chadian in southern Ireland after Hitzman (1995), Gatley et al. (in press), Carter and Wilbur (1986) andNagy (2003). Abbreviation: MMS, Moate–Moyvoughly Shelf; TB, Tynagh Basin; FT, Ferbane Trough; BS, Borrisokane Shelf; STT,

Silvermines–Tullamore Trough; LKBP, Longford–Kingscourt–Balbriggan Platform.

Figure 8. Regional palaeogeography during the Chadian of Britain (after Walkden 1987), Ireland (after Hitzman 1995), and Belgium (afterSwennen et al. 1990). Locations of Chadian peritidal facies comprising evaporites are also shown (checked pattern): 1, Aghmacart Formation,west of Leinster Massif (this study); 2, Ballysteen and Wexford Formations, south of Leinster Massif (Carter and Wilbur 1986; Nagy et al.in press); 3, Ballynahone Micrite Formation, Northern Ireland (Somerville et al. 2001); 4, Llanelly Formation, South Wales (Bhatt 1975); 5,Belle Roche and Vesdre Formations, east Belgium (Swennen and Viaene 1986; Swennen et al. 1990). Inset box shows the area of Figure 7.



Borehole shows similarities with the succession typical from the northern zone of the SouthWales Coalfield, where

the peritidal unit is also capped by oolites (Penllwyn Oolite Member) (Wright and Wright 1985; Wright 1986).

Within the peritidal unit (Llanelly Formation), Bhatt (1975) and Wright (1986, figure 9B) described and illustrated

evidence of limited amounts of early replacement of evaporites from Pwll-Du Quarry, Clydach area. This included

dolomite and calcite pseudomorphs after early diagenetic gypsum and anhydrite, and evaporite solution breccias in

peloidal mudstones and grainstones. The bioclastic skeletal grainstones of the lower part of the Durrow Formation

are very similar to those of the High Tor Limestone in the southern zone of the South Wales Coalfield (Gower

Peninsula and southeast Wales). The Holkerian succession comprises a major oolitic complex (Hunts Bay Oolite,

Gower area) and backshoal open lagoonal, oolitic peritidal carbonates (Clifton Down Limestone, Mendips/

Burrington area), which may be equivalent to the upper part of the Durrow Formation.

6.3. Regional correlation of Chadian–Arundian peritidal sequences

The peritidal sequences described above include evaporites; however, these were subsequently replaced during

diagenesis. As a result, certain petrographic characteristics are commonly the only source of information about

the sedimentary environment (see Warren (1999) and references therein).

The occurrence of length-slow chalcedony in large crinoids (Figure 4B) in the Crosspatrick Formation suggests

replacement of anhydrite cement. Oxygen isotope data from the dolosilt matrix of the crinoidal wackestones show

compelling evidence for the presence of an 18O-enriched, highly evaporated brine during anhydrite cementation

and dolomite precipitation (Nagy et al. 2004). Some other nodules containing megaquartz and saddle dolomite

cement (Figure 4C) indicate that the process of dissolution of sulphate and subsequent open-space filling also

occurred. The latter process was also described by Swennen and Viaene (1986) for the occurrence of dolomite

nodules found in the Lower Visean Vesdre Formation, east Belgium. The nodules are rarely present within the

tidal flat of the Aghmacart Formation, and absent in the peritidal deposits of the Durrow Formation. Together with

sedimentological evidence including desiccation cracks, root moulds (Figure 5A, D and E) and other pedogenic

features (Figure 5C), the lack of biota, and very common laminated former microbial mats with replacements of

gypsum (Figures 5A, 6A) indicate an evaporative environment adjacent to the Leinster Massif.

Evaporative conditions also dominated other parts of the peritidal area adjacent to St. George’s Land (and its

southeastern extension, the Brabant Massif, in Belgium) during the Chadian (Figure 8). Evaporite pseudomorphs,

former anhydrite nodules, tabular gypsum and anhydrite crystals unaffected by dissolution were described from

drillcores in south Wexford (Sleeman 1980; Carter and Wilbur 1986; Nagy et al. in press). One of the breccia types

described from the drillcores represents dissolution and subsequent collapse of large thickness of evaporites

(Sleeman 1980; Nagy et al. in press). The Wexford area was also located marginal to the Leinster Massif.

In the eastern part of Belgium (Figure 8), the Lower Visean carbonates consist of a sequence of dolomites with

pseudomorphs of former gypsum and anhydrite crystals. Nodules were found within finely laminated, dark-

coloured dolostones (Vesdre Formation), which represent formation within a restricted environment (Swennen

et al. 1981; Swennen and Viaene 1986; Peeters et al. 1992), bordered to the north by the Brabant Massif and to

the south by the Venn Shoal. This is followed by the Belle Roche Breccia, whose evaporite dissolution collapse

origin was demonstrated by Swennen et al. (1990). Succeeding the breccia unit are peritidal limestones of Lower

Visean age (Terwagne Formation), which consist of palaeosols and rendzina (Maes et al. 1989).

6.4. Implication for Irish base-metal mineralization

The Waulsortian Formation hosts one of the world’s major base-metal (Zn, Pb, Ba and Ag) districts, associated

with late diagenetic dolomitization (Anderson et al. 1995), in the Rathdowney Trend area (Figure 1). It has been

suggested that the presence of at least two geochemically distinct fluids was necessary to form these mineral

deposits (Everett et al. 1999; Gleeson et al. 1999; Johnson et al. 2001; Banks et al. 2002; Johnson 2003). Both

of these fluids originated from seawater and at least one of them was evaporated to high salinities, in some cases,

reaching or exceeding gypsum or halite saturation. Mixing of these fluids probably occurred during mineralization



(Samson and Russell 1987; Banks and Russell 1992; Wright et al. 2004). Banks et al. (2002) suggested that areas

of evaporated seawater formed the recharging fluids responsible for mineralization.

Extensive peritidal platform and lagoonal facies existed north of the Dublin Basin and around the Leinster

Massif during the Tournaisian–Visean (Figures 7 and 8), which resulted in the widespread precipitation of evapor-

ite minerals (Philcox 1994; Carter and Wilbur 1986; Nagy et al. in press). Based on the results of petrologic obser-

vations and stable isotope geochemistry, Nagy et al. (2004) postulated that evaporated seawater with increased

salinity, generated on the tidal flat and lagoons proximal to the Leinster Massif, moved through the mid- to outer

ramp westward and reached the area of mineralization. The source of magnesium to dolomitize the Crosspatrick

Formation in the studied area was provided by this refluxing brine.

7. CONCLUSIONS

A detailed petrographic and biostratigraphic study of the Milford Borehole (GSI-91-25) has elucidated sedimen-

tary processes and depositional environments adjacent to the Leinster Massif during Chadian to late Holkerian

times. A peritidal sedimentary environment was maintained close to the landward margin of the ramp, which

includes subtidal carbonate deposits (Crosspatrick Formation), oolite mobile shoals, subtidal lagoon, tidal flat

and backshoal oolite peloidal sands (Aghmacart Formation), and open lagoonal and peritidal deposits (Durrow

Formation). The characteristics of these depositional facies are typical of sediments deposited in an inner, middle

and outer ramp environment.

Comparison with the age-equivalent strata in South Wales and southwest England indicates a close similarity

of depositional style in the areas. The peritidal sequence is also similar to the Chadian sequence known in south

Wexford, which is also marginal to the Leinster Massif, and in east Belgium south of the Brabant Massif.

Petrographic and diagenetic features suggest that evaporative conditions were dominant during the deposition of

the tidal flat unit. This high salinity seawater, generated on the peritidal area by evaporation, may be a candidate for

the recharging fluid, which was responsible for supplying magnesium for regional dolomitization and excess chlor-

ide ions for the base-metal ore-fluids responsible for Zn–Pb mineralization in the Rathdowney Trend further to the

west.

ACKNOWLEDGEMENTS

We wish to acknowledge support from the American Association of Petroleum Geologists and the Jefferson

Smurfit Corporation to Zs.R.N., support from the National Science Foundation (NSF-INT-9729653 and NSF-

EAR-0106388) to J.M.G. and K.L.S, the donors of the Petroleum Research Fund, administered by the American

Chemical Society (PRF 35893-AC8) to J.M.G. and K.L.S. We thank W. Wright (Robertson Research), P. Cozar

(University of Madrid), A. Sleeman (Geological Survey of Ireland) for allowing us access to sample subsurface

drillcores and to publish the results. We thank T. Culligan, J. Kennedy, A. Keogh (UCD) and M. Roberson (UMR)

for help in thin section preparation, and T. McIntyre (GSI) for logistical help in sampling core.

An earlier version of this manuscript has greatly benefited from the constructive reviews by R. Swennen, A.

Sleeman and an anonymous referee.

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Scientific editing by Alastair Ruffell.



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