Shannon Basin Research Articles

Flow transformations, mud partitioning and the variable stratigraphic architecture of basin-floor fan fringes

Highly efficient sediment gravity flows can bypass mid fan channels and lobes and deposit significant volumes of sand, mud and particulate organic matter in outer fan and basin plain settings. The Serpukhovian to Bashkirian fill to the Shannon Basin, western Ireland, includes deep-water fan deposits (Ross Sandstone Fm) that gradationally overlie basin floor shales (Clare Shale Fm). As part of a broader progradational succession, the upward transition from muddy basin floor to sandy fan preserves the stacked deposits of settings present prior to and outboard of mid-fan channels and lobes. Three fully cored boreholes and associated wireline data constrain the facies tracts in an 18 km long panel orientated oblique to original depositional dip. Two distal successions dominated by hybrid event beds (HEBs) are recognised, separated by a prominent condensed section. The lower Cosheen system includes m-thick, tabular HEBs with prominent linked debrites that pass down dip into much thinner sandstones overlain by sand-speckled mudstone caps that thicken distally before thinning. The latter are interpreted as secondary mudflows released following reconstitution of more thoroughly mixed sections of the up-dip linked debrites. Significant bypass and accumulation of mud by this mechanism helped heal local topography and maintain a relatively flat sea floor promoting an overall tabular geometry for the deposits of larger volume hybrid flows reaching the distal sector of the basin. The overlying distal Ross system fringe is characterised by very fine to fine-grained sandstones and is lateral to compensationally-stacked lobes further to the west. It has a progradational (at least initially) stacking pattern, facies transitions developed over shorter length scales, and includes outsized event beds but these are thinner than those in the Cosheen system. Common banding and evidence for turbulence suppression by dispersed clay rather than entrained mud clasts indicate these were transitional flows. In this case, event beds are inferred to taper distally, with significant mud emplaced by plug flow retained as caps to sandy event beds rather than bypassing down-dip. Different flow transformation mechanisms thus impacted how mud was partitioned across the fringe of the two systems and this influenced bed geometries, larger scale bed stacking patterns and stratigraphy. Whereas the flow efficiency concept stresses the ability of flows to carry sand in a basinward direction, it is also imperative to consider the variable efficiency of mud transport given the operation of clay-induced flow transformations. These can either promote bypass or trigger premature fallout of mud with implications for how systems fill accommodation, bed -scale facies transitions and the burial and preservation of particulate organic carbon fractionated along with the clay in deep-water system fringes.

Trends in reconstructed monthly, seasonal and annual flows for Irish catchments (1900–2016)

Trends in reconstructed monthly, seasonal and annual flows for Irish catchments (1900–2016)

Early burial mud diapirism and its impact on stratigraphic architecture in the Carboniferous of the Shannon Basin, County Clare, Ireland

AbstractDelta fronts are often characterized by high rates of sediment supply that result in unstable slopes and a wide variety of soft‐sediment deformation, including the formation of overpressured and mobile muds that may flow plastically during early burial, potentially forming mud diapirs. The coastal cliffs of County Clare, western Ireland, expose Pennsylvanian (Namurian) delta‐front deposits of the Shannon Basin at large scale and in three dimensions. These deposits include decametre‐scale, internally chaotic mudstone masses that clearly impact the surrounding sedimentary strata. Evidence indicates that these were true mud (unlithified sediment) diapirs that pierced overlying strata. This study documents a well‐exposed ca 20 m tall mud diapir and its impact on the surrounding mouth‐bar deposits of the Tullig Cyclothem. A synsedimentary fault and associated rollover dome, evident from stratal thicknesses and the dip of the beds, define one edge of the diapir. These features are interpreted as recording the reactive rise of the mud diapir in response to extensional faulting along its margin. Above the diapir, heterolithic sandstones and siltstones contain evidence for the creation of localized accommodation, suggesting synsedimentary filling, tilting and erosion of a shallow sag basin accommodated by the progressive collapse of the diapir. Two other diapirs are investigated using three‐dimensional models built from ‘structure from motion’ drone imagery. Both diapirs are interpreted to have grown predominantly through passive rise (downbuilding). Stratal relationships for all three diapirs indicate that they were uncompacted and fluid‐rich mud beds that became mobilized through soft‐sediment deformation during early burial (i.e. <50 m, likely <10 m depth). Each diapir locally controlled the stratigraphic architecture in the shallow subsurface and potentially influenced local palaeocurrents on the delta. The mud diapirs studied herein are distinct from deeper ‘shale diapirs’ that have been inferred from seismic sections worldwide, now largely disputed.

Deep-water clastic systems in the Upper Carboniferous (Upper Mississippian–Lower Pennsylvanian) Shannon Basin, western Ireland

The Upper Carboniferous Shannon Basin of western Ireland contains a more-than-2300-m-thick (7540 ft) basin-fill succession, shallowing upward from deep-water to deltaic and incised fluvial deposits. The deep-water basin floor and slope succession is world renowned as an analog for hydrocarbon-bearing deep-water sandstones on several continental margins such as the basins in East and West Africa, South America, the Gulf of Mexico, and not least offshore northwest Europe. The Shannon Basin is frequently visited by both academia and industry for research and training purposes. A series of behind-outcrop research boreholes reveals the subsurface expression of the deep-water rocks and is complemented by seismic-scale cliff exposures. The succession is interpreted as a first-order basin-scale linked sedimentary system. This system can be analyzed using the principles of source-to-sink analysis and leaves the visitor with a complete picture of the basin fill, enhanced by spectacular sedimentological and stratigraphic detail.

Land use scenarios and projections simulation using an integrated GIS cellular automata algorithms

Over the years, urban growth models have proven to be effective in describing and estimating urban development and have consequently proven to be valuable for informed urban planning decision. Therefore, this paper investigates the implementation of an urban growth Cellular automata (CA) model using a GIS platform as a support tool for city planners, economists, urban ecologists and resource managers to help them establish decision making strategies and planning towards urban sustainable development. The area used as a test case is the River Shannon Basin in Ireland. This paper investigates the spatio-temporally varying effects of urbanization using a combined method of CA and GIS rasterization. The results generated from Cellular automata model indicated that the historical urban growth patterns in the River Shannon Basin area, in considerable part, be affected by distance to district centres, distance to roads, slope, neighbourhood effect, population density, and environmental factors with relatively high levels of explanation of the spatial variability. The optimal factors and the relative importance of the driving factors varied over time, thus, providing a valuable insight into the urban growth process. The developed model for Shannon catchment has been calibrated, validated, and used for predicting the future land use scenarios for the future time intervals 2020, 2050 and 2080. By involving natural and socioeconomic variables, the developed Cellular automata (CA) model had proved to be able to reproduce the historical urban growth process and assess the consequence of future urban growth. This paper presented as a novel application to the integrated CA-GIS model using a complicated land use dynamic system for Shannon catchment. The major conclusion from this paper was that land use simulation and projection without GIS rasterization formats cannot perform a multi-class, multi factors analysis which makes high accuracy simulation is impossible.

Mid-Carboniferous ammonoids from the Shannon Basin, western Ireland: identification of crushed material

In the Carboniferous Shannon Basin, Western Ireland, the majority of ammonoids spanning the Serpukhovian-Bashkirian Stages (E1–R2 biozonal indices) are preserved as flattened, partial 2D impressions, for which identification is problematic. Ammonoid ornament changes through successive developmental stages of the animal, but the changes of ornament with ontogeny have not been studied for many of the taxa that occur in the Shannon Basin. An approach involving the ontogenetic study of all available 3D material from concretions and museum collections can be used to improve the confidence of identification of flattened material. This is described here for the following Bashkirian ammonoid species: Phillipsoceras circumplicatile, Vallites henkei and Reticuloceras subreticulatum. Systematic ontogenetic changes in ornament are described here for these taxa for the first time. This represents a first step towards advancing 2D ammonoid taxonomy, which will improve biostratigraphic resolution, with implications for large-scale ammonoid diversity and evolutionary studies.

Conodont biostratigraphy of the mid-Carboniferous boundary in Western Ireland

AbstractTwo stratigraphic sections spanning the mid-Carboniferous boundary were examined in the Clare Shale Formation of the Shannon Basin in Western Ireland. Calcareous nodules intermittently occur within these generally non-calcareous organic-rich shales, and these have yielded Serpukhovian and Bashkirian conodont elements. The biostratigraphic range of the Irish material is illustrated here for the first time. Results show that the mid-Carboniferous and Arnsbergian–Chokierian boundaries are coincident at Ballybunion.Gnathodus girtyiis restricted to the lower part of the Serpukhovian Stage (Late Mississippian), whileG. bilineatus bollandensispersists into much younger strata, close to the first occurrence ofDeclinognathodus noduliferus s.l. One element belonging toG. postbilineatusis also present. These findings support the argument thatD. noduliferus s.l. developed fromG. b. bollandensis, rather than fromG. girtyi. The biostratigraphical tool for the identification of the mid-Carboniferous boundary globally, and the suitability of the section at Arrow Canyon, USA as a GSSP, may therefore need to be reassessed.

Sub-aqueous sand extrusion dynamics

Extruded sandstone mounds and sandstone sheets are increasingly recognized in the geological record and seismic data, yet the underlying mechanisms that control these features, and determine whether deposits form discrete accumulations or areally extensive sheets, remain poorly understood. Here we address these issues through study of a well-exposed ancient sand sheet in the Shannon Basin, Ireland. Sand volcanoes are shown to have erupted contemporaneously following liquefaction and fluidization of delta-front mouth-bar sands and silts, the expelled sediment forming an extruded sheet of sand. The sand sheet formed during a single prolonged eruption event with the interaction of radial gravity currents from the hundreds of vents exerting control on the internal architecture of the extrudite. Based on our observations, here we develop a process-based model that demonstrates that sand extrusions can form sheets only if (1) multiple vents are extruding coevally, causing gravity currents to interact, or (2) topographic forcing, such as channelling, redirects the otherwise radial gravity currents, resulting in sheet-like deposition of extruded material away from the vent site. This study provides a new model of sand extrudite formation, and examines the potential for identifying extrudites from core and bed-scale studies and differentiating them from liquefied beds.

Construction of an evolutionary deglaciation model for the Irish midlands based on the integration of morphostratigraphic and geophysical data analyses

AbstractAlternative, established models for the deglaciation of the midlands of Ireland are tested against an interpretation of a suite of deglacial sediments covering an area of 600 km2 in the east central midland area. Interpretation of the sediments is based on geomorphological mapping, lithostratigraphic characterization of exposures and geotechnical data supported by electrical resistivity tomography (ERT) and ground penetrating radar (GPR). GPR depicted small‐scale sedimentological and deformational structures within low‐conductivity soft sediments, such as cross‐bedding, planar bedding, channel‐like features and faulting planes, and revealed the internal architecture of eskers, glaciodeltas, subaqueous fans and raised bogs. ERT data permitted the detection of depth to bedrock and the lithological characterization of unconsolidated sediments. The ten sites presented were surveyed by traditional mapping methods and/or geophysical techniques. This allowed the construction of a local model of the deglaciation of the area which recognized five main stages. An ice sheet covering most of Ireland withdrew as a single body as far as the midlands. At this stage, two main directions of ice retreat are identified from the spatial distribution of meltwater/overflow channels, esker and morainic ridges, and ice‐marginal glaciolacustrine deposits. A pattern of deglacial sedimentation into an expanding ice‐marginal glacial lake is depicted. The glacial lake was dammed to the west by two ice dome fronts, one decaying to the north‐west and another to the south‐west, and by the Shannon Basin watershed to the east. Glacial lake outlets identified along the watershed and the altitude of the topset/foreset interface zone depicted in glaciodeltaic deposits allowed the identification of three lake water levels. The highest level is at 87–89 m Ordnance Datum (OD), the second lake level at 84 m OD and the third at 78 m OD. Copyright © 2012 John Wiley & Sons, Ltd.

FIRST REPORT OF INTERSEX ROACH RESIDING IN IRISH RIVERS DOWNSTREAM OF SEVERAL WASTEWATER TREATMENT PLANTS

Endocrine disrupting chemicals (EDCs) are chemicals that have the ability to mimic or disrupt the endocrine system in wildlife. Many of these EDCs are present in treated Irish sewage effluents and landfill leachate and have recently been identified in Irish rivers. The effects of exposure to EDCs were investigated in Irish brown trout (Salmo trutta), sampled from selected Irish rivers downstream of Irish wastewater treatment plants (WWTPs) with control fish sampled upstream of the same WWTP. The effects were also investigated in Irish roach (Rutilus rutilus) sampled downstream of Irish WWTPs. Control roach were sampled from a pristine salmon fishery river (River Deel) located in the west of Ireland. Histopathological and macroscopic analyses of downstream trout showed no adverse reproductive effects, compared to control fish sampled upstream of the WWTP. Statistically significant (p <0.05) populations of wild roach from three sampled Shannon basin rivers were, however, observed to be phenotypically intersex (ovo-testis) when compared to the control river. This, it is hypothesised, is as a direct result of exposure to WWTP effluents. The results reported herein are the first observations of intersex fish in Irish rivers.

Multiscale stratigraphic analysis of a structurally confined submarine fan: Carboniferous Ross Sandstone, Ireland: Discussion

In promoting the Ross Formation (Carboniferous Shannon Basin)2 as an excellent outcrop analog for Gulf of Mexico, oil-rich, Pliocene–Pleistocene, salt-withdrawal minibasins, Pyles (2008) reaffirmed the popular deep-sea-turbidite model for the Ross Formation (Collinson et al., 1991; Chapin et al., 1994; Elliott, 2000; Martinsen et al., 2000; Lien et al., 2003) without mentioning a detailed published reinterpretation of the Ross Formation as lacustrine, river-fed turbidites (hyperpycnites) and wave-modified turbidites (Higgs, 2004). Oil field development in technologically challenging deep-water settings can have costly economic consequences if based on predictions emanating from inappropriate outcrop analogs. Such consequences include, in order of increasing costliness, (1) selection of nonoptimum perforation intervals, causing lower production flow rates and lower ultimate recovery; (2) nonoptimum placement, spacing, and number of development wells, with the same effects; and (3) inaccurate predictions of reserves volume and production rates, leading to unwarranted declaration of field economic viability (hence major expenditures such as platforms, development drilling programs, and pipelines) or nonviability (Higgs, 2004). For an outcrop to be considered analogous to any given subsurface example, the two facies associations should be essentially indistinguishable, insofar as this can be judged from the existing core control; in other words, the interpreted depositional processes should be the same, resulting in near-identical sand-body (reservoir) architecture. Given the passive margin context and present deep-water (below storm wavebase) slope setting of the Gulf of Mexico minibasins (e.g., Pyles, 2008), a similar deep-marine setting can be inferred for the Pliocene–Pleistocene. In contrast, the Ross Formation may be neither marine nor of deep-water origin. Sedimentological evidence summarized below suggests (1) lowered salinity, amenable to much greater frequency and duration of hyperpycnal flows than in …

Multiscale stratigraphic analysis of a structurally confined submarine fan: Carboniferous Ross Sandstone, Ireland

An integrated, multiscale analysis of the Ross Sandstone documents important evolutionary patterns of a structurally confined submarine fan. Through time, the following are evident: (1) depocenters of high-frequency stratigraphic cycles stack vertically, (2) the depositional area increased, (3) regional paleocurrent patterns define a fanlike pattern, (4) the Ross Sandstone correlates in the landward direction to a bypass surface, (5) the Ross Sandstone inherited the basin physiography from the preceding Visean carbonates, and (6) the Ross Sandstone is ponded in the Carboniferous Shannon Basin. These observations demonstrate that the submarine fan filled an actively subsiding, structurally confined basin in an aggradational pattern. A high-resolution geologic map and a cross section of the Ross Sandstone at Loop Head Peninsula depict the location and stratigraphic architecture of every exposure of the Ross Sandstone. These data reveal that (1) lobes are the dominant architectural element (by area) at all stratigraphic positions in the Ross Sandstone; (2) although channels are the most commonly studied features in the Ross Sandstone, they occupy 7% or less of the total cross sectional area of the formation; (3) channels increase upward at the expense of lobes; (4) slumps increase upward; and (5) percent sandstone decreases upward mostly because of an increase in slumps. These trends define an increasing architectural diversity upward within the Ross Sandstone. Observations in the Ross Sandstone coupled with regional patterns for younger formations in the shallowing-upward, basin-fill succession are used to introduce a unifying model for the stratigraphic evolution of the Namurian strata in the basin. The model proposes that each formation in the shallowing-upward succession reflects its own depositional system related to an evolving landscape, and each was sourced from a different direction. This study quantifies vertical trends in stratigraphic features and analyzes them in a statistical manner to test the significance of external controls on local stacking patterns. The statistical analysis reveals that temporal changes in regional conditions, including sediment supply, source area, and basin shape, are statistically and empirically related to local changes in stratigraphy, including architectural element associations and percent sandstone. These relationships suggest that fans with a larger area have more architectural diversity and heterogeneity than their smaller counterparts. Regardless of fan area, lobes are the dominant architectural element. Also, source area is related to the type of architectural element present and percent sandstone. Lobes and their genetically related channels were sourced from the south, whereas slumps and heterolithic strata are mostly sourced from the west. Physical similarities, including climate, eustasy, size and shape of the basin, thickness of stratigraphic cycles, regional stacking patterns, subsidence, percent sandstone, and architectural elements between the Ross Sandstone and the ponded phase of the northern Gulf of Mexico minibasin-fill successions, imply that the Ross Sandstone is an excellent outcrop analog for these successions.

Facies and sequential organisation of a mudstone-dominated slope and basin floor succession: the Gull Island Formation, Shannon Basin, Western Ireland

The lower part of the Carboniferous Shannon Basin of Western Ireland contains a deep-water succession which exceeds 1200 m in thickness that comprises five lithologically different units deposited within a confined, relatively narrow basin: (i) a calciclastic debris-flow and turbidite unit formed by resedimentation from nearby carbonate platforms, (ii) a siliciclastic black shale succession with former source potential which onlaps basin margins (Clare Shales), (iii) a sandstone-dominated turbidite formation, controlled by ponded accommodation and deposited axially in the basin (Ross Formation), (iv) a mudstone-rich turbidite-bearing succession, which onlaps basin margins (lower Gull Island Formation), and (v) a mudstone-dominated prograding slope succession (upper Gull Island Formation and lower Tullig Cyclothem), which grades transitionally upwards into deltaic deposits. The top unit records progradation at a time when basin differential subsidence had diminished significantly and local basin topography did not control deposition. The two upper mudstone-dominated units are different in terms of both sandstone content and their genetic significance within the overall basin-fill, and their potential relevance as reservoir analogues. The lower part of the Gull Island Formation contains three principal facies associations: (a) shallow turbidite channels and sheets representing channel margin and levee deposits, (b) mud-rich slumps, and (c) less than 1 m thick, rare, hemipelagic shales. More than 75% is deformed by soft-sediment deformation, but only to a smaller degree affecting sandstone units. The turbidites record transport to the ENE, along the axis of the basin, while the slumps were derived from an unstable northern slope and transported transversely into the basin towards the southeast. The distribution of turbidite sandstone and slumps is inversely proportional. Sandstones decrease in importance away from the basin axis as slumps increase in number and thickness. The lower part of the Gull Island Formation is interpreted to record progressive fill of a deep basin controlled by local, healed slope accommodation with onlap/sidelap of the basin margins. The instability resulted from a combination of fault-controlled differential subsidence between basin margin and basin axis, and high rates of sedimentation. The upper part of the Gull Island Formation is entirely dominated by mudstones, which grade upwards into siltstones. It contains rare, up to 15 m thick, isolated channels filled by turbidites, showing transport towards the east. The upper part records easterly progradation of a deep-water slope genetically tied to overlying deltaic deposits, and controlled by regional accommodation. The contrasts between the lower and upper parts of the Gull Island Formation show that onlapping/sidelapping turbidite successions have reservoir potential near basin axes, but that prograding deep-water slopes are less likely to have reservoir potential of significance. A suggested regional downlap surface between the two parts is a significant break and marker in terms of reservoir potential.

Turbidites in the Upper Carboniferous Ross Formation, western Ireland: reconstruction of a channel and spillover system

ABSTRACT The Upper Carboniferous deep‐water rocks of the Shannon Group were deposited in the extensional Shannon Basin of County Clare in western Ireland and are superbly exposed in sea cliffs along the Shannon estuary. Carboniferous limestone floors the basin, and the basin‐fill succession begins with the deep‐water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m thick). The type of turbidite system, scale of turbidite sandstone bodies and the overall character of the stratigraphic succession make the Ross Formation well suited as an analogue for sand‐rich turbidite plays in passive margin basins around the world. The lower 170 m of the Ross Formation contains tabular turbidites with no channels, with an overall tendency to become sandier upwards, although there are no small‐scale thickening‐ or thinning‐upward successions. The upper 290 m of the Ross Formation consists of turbidites, commonly arranged in thickening‐upward packages, and amalgamated turbidites that form channel fills that are individually up to 10 m thick. A few of the upper Ross channels have an initial lateral accretion phase with interbedded sandstone and mudstone deposits and a subsequent vertical aggradation phase with thick‐bedded amalgamated turbidites. This paper proposes that, as the channels filled, more and more turbidites spilled further and further overbank. Superb outcrops show that thickening‐upward packages developed when channels initially spilled muds and thin‐bedded turbidites up to 1 km overbank, followed by thick‐bedded amalgamated turbidites that spilled close to the channel margins. The palaeocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity. Stacks of channels and spillover packages 25–40 m thick may show significant palaeocurrent variability at the same stratigraphic interval but at different locations. This suggests that individual channels and spillover packages were stacked into channel‐spillover belts, and that the belts also followed a sinuous pattern. Reservoir elements of the Ross system include tabular turbidites, channel‐fill deposits, thickening‐upward packages that formed as spillover lobes and, on a larger scale, sinuous channel belts 2·5–5 km wide. The edges of the belts can be roughly defined where well‐packaged spillover deposits pass laterally into muddier, poorly packaged tabular turbidites. The low‐sinuosity channel belts are interpreted to pass downstream into unchannellized tabular turbidites, equivalent to lower Ross Formation facies.

Sir Richard Bingham and the Mapping of Western Ireland

Richard Bingham, a naval and military commander in Kerry in 1580 and later governor of Connacht (1584-96), was a prime mover in the production of nine Irish regional maps, all based on original observations and at least three achieving an unusually high standard of accuracy. Six of these maps survive, depicting, respectively, the Dingle peninsula, most of County Mayo, the routes leading from Ulster to the battlefield of Ardnaree, the town of Galway, the county of Sligo, and finally the whole province of Connacht. The maps no longer extant showed the upper Shannon basin, the county of Mayo, and the action at Ardnaree. One highlight of Bingham's administration was the employment of two talented cartographers, John Browne of the Neale and his nephew of the same name. Another notable aspect was the problem of choosing between the county and the province as the most convenient unit of Irish regional mapping.

ABSTRACT: Deep-Water Deposits of the Upper Carboniferous Shannon Basin, Western Ireland

ABSTRACT: Deep-Water Deposits of the Upper Carboniferous Shannon Basin, Western Ireland

ABSTRACT: Mid to Late Mississippian (Visean) Rocks Around the Shannon and Fergus Estuaries, Southwest Ireland, and Their Bearing on the Evolution of the Shannon Basin

ABSTRACT: Mid to Late Mississippian (Visean) Rocks Around the Shannon and Fergus Estuaries, Southwest Ireland, and Their Bearing on the Evolution of the Shannon Basin

The Clare Basin: a geological and geophysical outline

The Clare Basin (Fig. 1) is a poorly documented Carboniferous basin remnant, situated offshore west of Ireland, which is considered to be an extension of the onshore West Clare Namurian Basin (WCNB). It has in the past been variously described as, or as part of, the Clare Shelf and Shannon Basin (e.g. Cope et al. 1992), the Shannon Trough (e.g. Sevastopulo 1981), and the Dingle-Shannon Basin (e.g. Gardiner & MacCarthy 1981). It lies immediately north of the Iapetus Suture which is assumed to have closed during the Devonian. Figure 1 shows the general location of the Clare Basin, the outline of the younger Porcupine Basin and Slyne Trough, and the onshore geological outcrop map. Also shown are the locations of boreholes and the line of cross-section figured in the paper.

The lower carboniferous (dinantian) of the Swords area: Sedimentation and tectonics in the Dublin Basin, Ireland

AbstractThe litho‐ and biostratigraphy of the Lower Dinantian succession in a deeper part of the Dublin Basin is described. The sub‐Waulsortian Malahide Limestone Formation (emended) is described fully for the first time, and has proved to be very much thicker than was previously suspected, in excess of 1200 m. Succeeding the ‘Lower Limestone Shale’ unit, which is transitional from the underlying Old Red Sandstone facies, the following six new members are recognized: Turvey Micrite Member, Swords Argillaceous Bioclastic Member, St. Margaret's Banded Member, Huntstown Laminated Member, Dunsoghly Massive Crinoidal Member and Barberstown Nodular Member (top).The Malahide Limestone Formation is overlain by ‘Waulsortian’ limestones of the Feltrim Limestone Formation (new name) which form overlapping and isolated mudmounds with complex relationships with their enclosing non‐mound facies. Though very much thicker, the Courceyan succession is comparable with that elsewhere on the south side of the Basin, and is part of the Kildare Province (Strogen and Somerville 1984). Isopach maps for the region show that this province and the North Midlands are separated by the deepest part of the Dublin Basin, named the ‘East Midlands Depocentre’, in which a shale‐dominant facies is present. The top of the ‘Waulsortian’ is of early Chadian age. Formations younger than this are dominated by basinal calcareous shales (Tober Colleen Formation) and by storm deposits and calciturbidites with appreciable terrigenous input from the east (Rush Formation).The Courceyan main shelf conodont biozones are also greatly thickened in this area. The Pseudopolygnathus multistriatus Biozone (&gt; 300 m thick) is succeeded by a very thick (&gt; 900 m) Polygnathus mehli Biozone. The base of the Chadian is considered to occur below the top of the Feltrim Limestone Formation and, although equivocal, may be diagnosed in the Dublin Basin by the first appearance of the problematic microfossil Sphaerinvia piai and a primitive form of the calcareous alga Koninckopora.In the late Courceyan, the Swords area was part of a gently sloping shelf extending northwards into the basin. During deposition of the Feltrim Limestone Formation there was major deepening and there is evidence of initial break up of the Dublin Basin by faulting into separate blocks. By Chadian time the Basin was definitely subsiding by fault displacements and basinal limestones contain shallow water faunas and littoral sand and pebbles derived by turbidite flows from the margins of the higher blocks. The early subsidence was apparently by pure flexure, but in the Viséan the Dublin Basin was fault‐controlled, differing from the adjacent Shannon Basin in having both margins strongly faulted.