Southwest Ireland Research Articles

Detrital magmatic muscovite from the Lower Carboniferous of southwest Ireland: buried granites uncovered?

Lower Carboniferous limestones from the Kilmaclenine area, Co. Cork, contain magmatic muscovites, individual granitic minerals and granitic fragments. These constitute primary evidence for the presence of buried granites to explain various geological and geophysical observations in southwest Ireland.

Abundance and spatial overlap of gobies (Gobiidae) in Lough Hyne, Ireland

The densities of gobies (Gobiidae, Pisces) were counted by scuba divers on 13 sublittoral transects in Lough Hyne, southwest Ireland. The characters used for underwater identification are described. Densities ranged up to 6.5 m−2 for Pomatoschistus pictus (Malm), 4.0 m−2 for Gobiusculus flavescens (Fabricius), 0.9 m−2 for Gobius niger L., 0.3 m−2 for G. cruentatus Gmelin, and 0.3 m−2 for Thorogobius ephippiatus (Lowe), while G. paganellus L., G. couchi Miller & El-Tawil and Lesueurigobius friesii Malm were rare on the transects. Gobius cruentatus and T. ephippiatus were largely confined to holes under rocks, while P. pictus and G. niger were most abundant on more open terrain. G. flavescens was hyperbenthic and largely confined to water less than 5 m depth. Significantly less P. pictus, G. flavescens and G. niger were recorded at night, and ‘sleeping’G. flavescens were observed. A working model of the spatial overlap of gobies in Lough Hyne is presented, and the possible roles of diet, predation and temporal activity patterns in interspecific competition are discussed.

Microhabitat utilisation by an assemblage of temperate Gobiidae (Pisces: Teleostei)

An assemblage of Gobiidae in Lough Hyne, southwest Ireland, was investigated to assess interspecific and seasonal differences in microhabitat utilisation. Five microhabitat guilds were distinguished using canonical correspondence analysis, comprising 2 semi-hyperbenthic guilds and 3 epibenthic guilds. Differential preferences for each environmental variable measured were examined using Jacobs' version of Ivlev's electivity index. Members of different guilds exhibited differential microhabitat use, whilst members of the same guild exhibited only subtle differences in microhabitat utilisation. Few seasonal variations were observed, with guilds maintaining their separation seasonally.

Basin history deduced from subtle changes in fluvial style: a study of distal alluvium from the Devonian of southwest Ireland

AbstractTwo detailed sections through 4 km of distal alluvium of the Upper Devonian Old Red Sandstone in the Munster Basin, southern Ireland, display a limited number of lithofacies. There is little ordering of these lithofacies on a small scale but changes in the proportions of lithofacies through time define a sequence of stages of basin evolution. The depositional environments changed progressively from sheetflood dominant, via mobile ephemeral channels and floodplains to a more fixed channel–overbank system and eventually to a coastal plain. On the basis of the sedimentary record a progressive reduction in subsidence rate with time is deduced as the main control on the evolution of the basin fill. This deduction is consistent with predictions from previously applied extensional basin models, but is insufficiently refined at present to distinguish between the different models.

The Effect of Cereal and Grass Management on Staphylinid (Coleoptera) Assemblages in South-West Ireland

(1) The effects of different cereal and grass management regimes on summer staphylinid assemblages were examined in hay meadows, lightly grazed pastures, silage fields, spring cereals, and winter cereals with routine pesticide application in south-west Ireland. (2) Forty-nine sites were sampled for adult Staphylinidae using pitfall traps and a D-vac suction sampler in June and July 1986. Comparisons between sites using species composition data were carried out using detrended correspondence analysis. (3) Large differences in staphylinid assemblages occurred between undisturbed meadows and pastures (without cultivation, cutting, heavy grazing and fertilizer use), and disturbed silage fields and cereals. The latter were rapidly recolonized after physical disturbance such as cutting and cultivation, once a vegetation layer was re-established. The assemblages of second-cut silage fields, spring cereals with a welldeveloped canopy, and of winter cereals, were very similar. (4) Dimethoate application resulted in a reduction in species richness in cereals, and there were more species in small fields, although the latter was due to the occurrence of species which were rare overall in cereals. No effect of propiconazole, the most frequently used fungicide, was detected. (5) Rapid recolonization in heterogeneous landscapes, as in this study, may lessen the perceived impact of disturbance factors, which may have a much greater impact in more homogeneous landscapes. Nonetheless, the results indicate that, with careful site selection, multivariate analysis of staphylinid species composition data can be a useful supplementary technique to experimental plot studies.

Stratigraphy of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) from the Fastnet Basin, Offshore South-west Ireland

The stratigraphy of the Triassic, Lower Jurassic and Middle Jurassic (Aalenian) of the Fastnet Basin is described. The sequence has been divided using wireline log criteria. Six rock units are recognized in the Triassic and 12 in the Lower Jurassic and Middle Jurassic (Aalenian). None of these major rock units shows any significant diachroneity. Apparent breaks in the sequence due to unconformities or condensed sections have been identified within or bounding three of the rock units: the Liassic Limestone, Liassic Marl and Liassic Sandstone. Dating is provided by both microfaunas and microfloras extracted from ditch cuttings and sidewall cores. The ages of the Triassic rock units are mainly determined by correlation with dateable sequences in the North Celtic Sea Basin, whereas the Jurassic rock units yielded abundant short-ranging microfossils.

A polygenetic palaeosol from the Silurian (Wenlock) of southwest Ireland

A problematic palaeosol unit caps a thick basalt at the top of the Clogher Head Formation, Dunquin Group (late Wenlock, Silurian) in the Dingle Peninsula, southwest Ireland. The main part of the unit consists of an altered calc-alkaline basalt with a fine clay assemblage of chlorite, corrensite and smectite probably resulting from weathering during Wenlock times. The uppermost part of the unit consists of detrital silicates which were admixed by bioturbation and well-defined burrow-fills are present. The underlying, weathered basalt was possibly truncated before the detrital horizon was deposited, and finally the whole unit underwent a phase of surface-water gleying. A catenary relationship existed locally with the pseudo-gley developed lower in the landscape while contemporaneous basalts in topographically higher, better drained areas, largely escaped the effects of gleying; these now exhibit reddened tops. The complex history and the possiblity of the incompleteness of the profile makes accurate assessment of the soil/climate function difficult. However, the occurrence of calcrete-bearing palaeosols in the overlying Mill Cove Formation supports the view that the basalt was weathered under a sub-humid or semi-arid climate. Considering the alteration of the Dunquin Group to actinolite-prehnite grade, the preservation of the smectite is highly unusual. There is no evidence for its origin due to more recent weathering or interstratal neoformation; it is thus interpreted as a relict product of the original weathering phase.

Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits?

The late Devonian Toe Head Formation crops out extensively in southwest Ireland and has been considered previously as the topmost Old Red Sandstone (terrestrial) formation in a conformable non-marine to marine sequence. The most characteristic lithofacies, the flat laminated and inclined parallel laminated sandstones, displays a range of structures from flat to gently inclined laminae and includes sets which meet the four criteria of Harms et al. (1982) for hummocky cross-strata. Associated facies include desiccated mudrocks, palaeosols and ripple cross-laminated sandstones which lack the flaser-linsen dominated bedsets characteristic of the overlying marine strata. Only non-marine fossils are recorded and numerous palynological preparations lack the marine indicators which appear at the top of the formation. Palaeocurrents show considerable spread with indications of overall easterly transport of sand. There is no preferred vertical sequence of lithofacies. The balance of evidence strongly favours a non-marine environment with the implication that hummocky cross-strata, as presently defined, are not good environmental or even process indicators. The fine to very fine sand sizes and possibly the amount of suspended load were important controls on the resultant structures. A depositional model of essentially fluvial deposition on a low-gradient coastal plain is proposed.

Time Partitioning of Foraging in the Limpet Patella vulgata

SUMMARY (1) The behavioural ecology of the limpet, Patella vulgata L., a common grazing intertidal mollusc, was investigated at Lough Hyne in south-west Ireland. Foraging in limpets has previously been shown to be a simple three-phase system: an outward travelling phase, a middle phase of intense foraging, and a period of return travel to the permanent home scar. (2) A predictive model was developed and considered the following strategies as the controlling factors for limpet foraging: (i) while foraging, limpets maximize their net energy gain; (ii) while foraging, limpets minimize their energy costs; (iii) while foraging, limpets minimize the time spent away from the home scar. Data were collected from the field to provide the morphological and behavioural parameters, and from various literature sources for the energetic parameters used in the model. (3) Limpets do not appear to forage so as to maximize their net energy gain; this is probably due to the predictability of foraging periods for limpets. The strategy of cost minimization also provided poor estimates for the time budgets observed; probably this is because the animals require energy for growth of the somatic and gonadal tissues. The partitioning of time between travelling and intense foraging by a limpet while foraging can, however, be explained in terms of strategy (iii), that of time minimization. (4) The possible environmental constraints which could influence limpet foraging trips and how these change with increasing height up the shore are discussed. The possibility that the 'feeding' trips may, in addition, maintain the algal biofilm in a highly productive phase is also discussed. The foraging behaviour of limpets varies between different sites around the British Isles, and it is suggested that this may be due to varying combinations of predation and desiccation risks.

Social characteristics and destinations of recent emigrants from selected regions in the West of Ireland

This paper examines the social characteristics and destinations of just under 1200 recent emigrants from selected urban and rural areas in the west and south-west of Ireland. It calls for a community-centred, regional approach to counteract the aspatial element in national accounts and cost-benefit analyses of recent emigration. It suggests that the incidence of emigration may vary regionally and socially, and that young adults in the west of Ireland may now regard this country and Great Britain as an integrated labour market for skilled and unskilled labour. Media coverage and recent literature suggest that contemporary emigration is qualitatively different from earlier outflows. The findings of this survey suggest that the qualitative aspects of recent emigration can be exaggerated and that it may be premature to talk of the modernisation of Irish emigration.

A deep seismic reflection transect across the Irish Caledonides

New reflection profiles recorded to 18s ( c .60km) along a 600 km north-south transect West of IREland (WIRE lines) provide a cross-section of the Irish Caledonides from Lewisian basement in the north to the Variscan fold-and-thrust belt in the south, and constrain the offshore extrapolations of major Caledonide structures. We interpret the profiles as showing the Great Glen fault and associated, NE-trending strike-slip faults as near-vertical structures which cut the whole crust and the uppermost mantle. Beneath the surface trace of the Fair Head-Clew Bay line we image a north-dipping zone which soles into horizontal layers just above the reflection Moho. The north-dipping zone separates south-dipping reflections above and to the north from north-dipping reflections below and to the south. From the Fair Head-Clew Bay line as far south as the Iapetus suture the lower crust contains north-dipping packages of reflectors possibly related to Acadian accretionary underplating. The Iapetus suture zone can be identified as a zone, more than 50 km wide, dipping north at 30° from 5 km depth offshore Dingle peninsula to sole into the Moho at c . 30 km depth west of the Shannon Estuary. In the upper crust the shallow part of the Iapetus suture zone appears to be transported to the north by late Caledonian retrocharriage or by Variscan thrusting. The Iapetus suture does not penetrate through the present-day reflection Moho, but there are scattered reflectors in the mantle not continuous with crustal reflectors, perhaps relics of a subducted slab. We recognize ‘lower-crustal’ reflectivity and the reflection Moho along almost all the WIRE profiles. Regionally the travel-time to the reflection Moho varies from 9 s ( c . 28 km?) offshore Donegal (northwest of Ireland) to 12 S ( c . 35 km?) offshore Kerry and Cork (southwest of Ireland). Apparent Moho offsets (rapid lateral changes in the travel-time to the reflection Moho) north of Donegal and north of Mayo are probably due to the Great Glen fault and associated faults. The ‘lower-crustal’ reflectivity varies considerably in thickness and prevalent dip direction along the WIRE profiles. In Donegal Bay this reflectivity is observed at depths less than 5 km ( c . 1.5s) whereas west of Kerry prominent reflectivity is only observed beneath 6s ( c . 17 km). Though the age of crustal reflectors cannot in general be determined, beneath central Ireland from Donegal to Dingle the lower-crustal reflectors mimic in dip direction the structural vergence of the exposed Dalradian and early Palaeozoic rocks, so that the lower-crustal reflectivity patterns, and probably much of the crust, must be Caledonian in age.

Analysis and tectonic interpretation of gravity data over the Variscides of southwest Ireland

The gravity data set for southwest Ireland comprising 5500 stations with a spacing of about 1–3 km has been filtered using standard wavenumber domain techniques. A band pass filtered map, emphasizing gravity anomalies originating to a maximum depth of 3–4 km, correlates well with surface geology. It shows major faults and density contrasts among six lithostratigraphical units, the most important being between the Old Red Sandstone and underlying basement. A low pass filtered map, emphasizing gravity anomalies from a maximum depth of 10 12 km shows major negative gravity anomalies at Killarney and Fermoy and positive anomalies beneath the Shannon estuary, on the Dingle peninsula and along the south coast. These data suggest that Caledonian basement was involved in Variscan deformation and any detachment lies at mid-crustal levels. Two north-south gravity profiles are modelled along Eastings 100 and 200, constrained by surface geology and published seismic refraction data. On Line 100 the source of the negative anomaly at Killarney is modelled as either a granite within basement or a deformed Devonian basin. The source of the Shannon positive anomaly is modelled as a large dense body within the crust whose southern limit coincides with the Iapetus Suture Zone. On Line 200, the Fermoy low is modelled as a laccolithic granite within basement. The positive anomaly along the south coast is a consequence of Mesozoic crustal thinning.

The deep geology and geophysics of Ireland and its continental margin

A two day meeting on 28th and 29rh October 1989 attracted an international audience of over 90 scientists principally from academic institutions and the oil and minerals industries to hear 24 papers on the deep geology and geophysics of Ireland and its continental margin. The aim of the meeting was to present and discuss the results of a number of recent geophysical experiments and to relate them to models of the deep geology Gravity and magnetic data for Ireland were reviewed by Murphy (Dublin Institute for Advanced Studies) whilst Lowe, Jacob, Prodehl and Ruthardt (Dublin Institute for Advanced Studies and University of Karslruhe gave an account of two land seismic refraction experiments across and parallel to Caledonian trends (COOLE and ICCSP). They demonstrated that the Moho beneath Ireland lies at a depth of about 30 km, and a clear change of velocity structure occurs across the Iapetus suture zone. Two magnetotelluric experiments were described by Brown & Whelan (University College Galway) and by Meju (University of Leicester). Both indicated zones of anomalously high crustal conductivity, the most prominent of which is associated with the Iapetus suture zone. A petrological model for such zones involving free water in the lower crust was described by Sanders (Trinity College Dublin; this volume). Ford, Brown & Readman (Plymouth Polytechnic, University College Galway and Dublin Institute for Advanced Studies; this volume) used gravity data from southwest Ireland to elucidate basement structures in the Variscides. Offshore potential field data were described by Readman, Gohl, Liebe & Makris

Geochemical character of Silurian volcanism in SW Ireland

The Dunquin Group (mid-Silurian) of Dingle, southwest Ireland is a sequence of basaltic to rhyolitic lavas and pyroclastic rocks interbedded with shallow- to non-marine sediments that records one of only four known outbreaks of Silurian volcanicity south of the Iapetus Suture in the British Isles. New geochemical, petrographic and field evidence indicate that the volcanic sequences exposed in the Dunquin inlier, and the islands of Beginish and Inishvickillane could have formed by fractionational crystallization in a layered, repeatedly replenished basaltic magma chamber, the site of which is probably coincident with an intense aeromagnetic anomaly to the southwest of Inishvickillane. The volcanic suite is subalkaline, transitional between calc-alkaline and tholeiitic and is non-bimodal. High LILE/HFSE ratios and high LREE concentrations suggest that the parent basaltic magmas were generated at a destructive continental margin, although subduction may have ceased before eruption.

Fine resolution pollen analysis of vegetation history in the Lough Adoon Valley, Co. Kerry, western Ireland

The Lough Adoon Valley on the north of the Dingle Peninsula, in southwest Ireland, has been occupied by humans, perhaps continuously, since Neolothic times. Since about 5000 B.P. the valley has been essentially deforested, apart from local reinvasions of Betula, and much of the landscape has been covered by blanket bog and heath. Time series analysis of pollen and charcoal influx from two sections of peat sampled at contiguous 1 cm intervals was carried out to explore the vegetation shifts associated with Betula decline and charcoal dominated phases. Cereal cropping and fire were important vegetational controls resulting in an interplay between heath, wetland, grassland and woodland but forest decline was not a result of fire. The number of pollen taxa interrelationships differed between the phases with a greater number of interrelationships in the Betula decline phase than when fire was more prominent in the environment.

Hydrography and phytoplankton of waters off south-west Ireland

Distributions of physical, chemical and biological characteristics were recorded for coastal waters off south-west Ireland during the summers of 1985–1987. A number of thermal fronts were identified from both shipboard measurements and satellite imagery. Upwelling was shown to be an important process in the area and enhanced phytoplankton biomass and productivity were associated with areas of upwelling and the fronts. Phytoplankton species composition, although varying between cruises, was associated with differences in hydrographic features.

The stratigraphy and structure of the Galley Head culmination zone: An area of enhanced shortening related to basin geometry within the Irish Variscides

AbstractIn southwest Ireland an Upper Devonian to Lower Carboniferous clastic succession was deposited in an ENE–WSW trending half‐graben, known as the South Munster Basin. Across the Galley Head peninsula on the south coast, this stratigraphical succession is attenuated due to the presence of a palaeogeographical feature called the Glandore High. Evidence suggests that the Glandore High was an east–west feature, faulted to the north and east, which was part of the southern flank (hangingwall rollover) of the South Munster Basin.During post‐Carboniferous Variscan deformation the relatively thin stratigraphy of Galley Head underwent prolonged folding, causing a local periclinal fold pair to develop within the hinge zone of a regional syncline. The main cleavage then developed parallel to bedding on the overturned south limb of the anticline of this fold pair. The local enhanced shortening caused the development of a structural culmination, and south facing, tight to isoclinal folds. The culmination was enhanced and tightened by a fault system of contractional, strike‐parallel faults linked by cross faults. Secondary folds occur across the hinges of regional anticlines and also on major fold limbs as isolated fold pairs and in monoclinal fold zones, some of which may have nucleated on irregular sandstone bodies. Local crenulation cleavages are related to late fault movements. Syn‐cleavage, conjugate, wrench faults record 10 per cent to 15 per cent strike‐parallel extension in the culmination.The deformation chronology of the Galley Head area is somewhat anomalous for the Irish Variscides in that the folds were well established before the onset of the main cleavage development. The enhanced shortening across the area was compartmentalized by major cross faults and a minor component of north–south sinistral shear was also active across the area causing a swing in strike and a late set of minor cross faults.Structural facing directions in southwest Ireland appear to be directly linked with the geometry of the deformed basins. Hence the southward facing along the south coast is due to the proximity of the southern margin of the South Munster Basin. Structural facing directions fan northwards across the basin and major folds are overturned to the north at the northern margin of the basin.

Prehistoric Copper Mining in South-West Ireland: The Mount Gabriel-Type Mines

This paper examines the Mt Gabriel-type exploitation of sedimentary mineralization in the West Cork region of south-west Ireland. It includes a consideration of the physical setting of the mines, their distribution, morphology, technology, dating and cultural affinities. Mt Gabriel, the highest mountain of the Mizen Peninsula in south-west Cork, was the setting for extensive mining of surface copper beds during the closing stages of the Early Bronze Age. Recent radiocarbon dating of wood and charcoal from these sites confirms that the main period of mining activity on this mountain was between 1700–1500 cal. BC, making these the oldest copper mines presently known in north-west Europe. The importance of these sites lies not only in their antiquity, but in their unique preservation in upland blanket bog environments.A recent programme of survey and excavation culminated in the investigation of the mine 3/4 site on the eastern slopes of the mountain. The removal of a large peat-based infill deposit from mine 3 revealed an 11 metre deep inclined opening, in which the transition from a strata-bound drift-mine to a cross-cutting tunnel was visible. Waterlogged sediments within this mine contained a large assemblage of roundwood fuel and wooden mining equipment, including a shovel, pick, wedges, lighting chips and handles for the many stone-hammers which were also found. The excavation of a large spoil mound outside this mine has provided important evidence on ore treatment in these operations, an important element of which involved the repeated crushing of rock using stone cobble-hammers. Activity areas located on the periphery of this dump included further dressing floors, concentrations of broken stone tools and a posthole structure with associated water trough.This mining, together with an expanded Early Bronze Age metal production in south-west Ireland, may be directly associated with an important intensification of agricultural production and human settlement in this region, which also finds expression in the adoption of new sepulchral and ceremonial traditions. The ore sources constituted an important resource during this period, contributing to the emergence of controlling élites who had an important managerial role in the production and distribution of metal.

Breeding periodicity and sex ratios in epifaunal marine amphipoda in Lough Hyne, Ireland

Thirteen species of amphipod were recorded from sponges and associated biota, notably algae and tunicates, in Lough Hyne (Ine), south-west Ireland. The sex ratio rarely approached unity in any species. No male Corophium bonnellii were collected. Mature male Lysianassa ceratina were not collected between late July and early September. Ovigerous females occurred throughout the year in five species. Lysianassa ceratina had the most restricted breeding period (May to September). Breeding activity, based on this criterion of reproduction, was generally greatest between April and July, but this period was extended in some species. A secondary autumn peak in breeding activity was recorded in four species. A comparison between the data from the present study and those from a similar study in Scotland suggests that in the case of two of the species, Corophium bonnellii and Lembos websteri, egg production throughout the year may be related to differing summer maximum temperatures.

Stream‐driven, high‐density gravelly traction carpets: possible deposits in the Trabeg Conglomerate Formation, SW Ireland and some theoretical considerations of their origin

ABSTRACTWithin high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition.