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| Carboniferous Subarea-3; Kesh-Omagh | Fermanagh, Tyrone |
| Site number: | 1176 | ||
| Locality Type: | Various | ||
| Grid Reference: | H180638,H450725 Centroid, Not Entered | ||
| Google maps: | 54.52272,-7.722 | ||
| County: | Fermanagh, Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Arundian, Chadian, Courcyean, Holkerian, Tournasian, Visean | ||
| Lithostrat: | Ballyshannon Limestone Formation, Benbulben Shale Formation, Bin Mountain Sandstone Formation, Bundoran Shale Formation, Claragh Sandstone Formation, Drumchorick Siltstone Formation, Keenaghan Shale Formation, Mullaghmore Sandstone, Omagh Sandstone Group, | ||
| Site Description | |
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| Description: | |
INTRODUCTION The geological mapping of the Carboniferous rocks in Subarea-3 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps: GSNI, 1994. 1:50,000 Solid Geology of Sheet 32 and 31 (Kesh) GSNI, 1995. 1:50,000 Solid Geology of Sheet 33 (Omagh). The Omagh and Tyrone groups are represented in this area. Although the GSNI have remapped the greater part of Subarea-3, a sliver of Carboniferous rocks, located at the northern edge of their outcrop, located within 1:50,000 Sheet 25 (Newtownstewart) have still to be mapped in detail. By far the bulk of this outcrop is occupied by rocks of the Omagh Sandstone Group and the Claragh Sandstone Formation, the basal unit of the Tyrone Group hereabouts and it is not believed that any new information of real consequence to this study, will be forthcoming. The area was first surveyed in detail by the Geological Survey of Ireland and the Kesh and Omagh One Inch maps were published in 1885. Their interpretation of the stratigraphy of these Carboniferous rocks conformed precisely to the standard Geological Survey nomenclature of the times. This being so and having failed to recognise the importance of major strike faults causing repeated repetition of the sequence, the geologists were forced to assign what appeared to them to be stratigraphically higher and therefore younger limestones and sandstones, to the Upper Limestone and Yoredale Sandstone and Shale divisions. In the Kesh area this raised slight problems in the southern part of the outcrop. However, in the Omagh area to the east, the problems created by this erroneous assignment were enormous and virtually all of their age determinations for the various limestone, sandstone or shale units are now known to be incorrect. This was unfortunate as their mapping of the outcrop of the various lithological units was extremely good and in some instances, particularly with the lowest horizon, now referred to the Omagh Sandstone Group, was invariably correct. The Carboniferous geology of the Kesh-Omagh area was subsequently revised by Simpson (1955) who subdivided the strata into four lithostratigraphical groups, with a total thickness of between 3500 feet (1060m) and 8200 feet (2500m). He recognised the impact of strike faults on the distribution of the various lithologies and correctly correlated similar lithologies of the same age that occurred on opposite sides of these major faults. Of these groups only the Omagh Sandstone Group is retained as an integral part of the Carboniferous with its upper and lower boundaries being represented by unconformities. Lying stratigraphically above the Omagh Group are the multitude of formations and members that constitute the Tyrone Group. At its base is the Claragh Sandstone Formation, reduced from the Group status conferred by Simpson. The Tyrone Group also includes both the Pettigoe Limestone and Clonelly Sandstone groups of Simpson. It is now apparent that both groups are composite units encompassing many of Oswald's (1955) standard lithostratigraphical formations such as the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations and new units recognised by the GSNI (1994, 1995). I - OMAGH SANDSTONE GROUP The oldest Carboniferous rocks in the Kesh-Omagh area are referred to the Omagh Sandstone Group (Simpson, 1955). Their outcrop is confined largely to the area north and NW of Omagh town and to the northern and western margins of the Lack Inlier (GSNI, 1995). The Omagh Group is the one remaining unit of the four groups originally erected by Simpson. There are several reasons for retaining this nomenclature. The first and most important is that the lower and upper contacts are unconformities. At its base the group overlies metamorphic rocks of the Southern Highland Group in the SW Sperrin Mountains and Lack Inlier and at its top it is succeeded by the Claragh Sandstone Formation, the basal clastic unit of the Tyrone Group. At two places in the Kesh-Omagh area, the unconformable relationship between the Omagh Sandstone Group and Claragh Sandstone Formation is evident. The second reason for retaining the Omagh Sandstone Group as a separate unit is due to the likelihood of formation and member units being recognised within its outcrop. Strong facies changes occur within the group outcrop and detailed study might identify particular lithologies or marker horizons, that can be traced not only within the Kesh- Omagh area but to the Newtownstewart Outlier (Subarea-8). For these reasons it was decided (GSNI, 1995) to retain Simpson's Omagh Sandstone Group as a unit separate from the Tyrone Group. The maximum thickness of the Omagh Sandstone Group is estimated to be about 250m. This is based on some very incomplete sections and the belief that gentle folding and faulting may have combined to amplify the outcrop width in some parts of the Omagh area. The age of the Omagh Sandstone Group can be ascertained from the occurrence of abundant, moderately to well preserved miospores. These belong to the CM Biozone and contain specimens of the index taxa Auroraspora macra and Schopfites claviger. Most significant however is the absence, in most samples, of the miospore Lycospora pusilla, the basal Vis‚an (Pu Zone) index taxon. The selection of one section that adequately reflects the many lithologies that comprise the Omagh Sandstone Group is difficult. Simpson (1955) did not nominate a stratotype but in choosing the group name clearly signalled the area within which lay its main outcrop. His lithological descriptions are based mainly on the long section in the Killyclogher Burn, located to the NE of Omagh. The basal unconformity with metamorphic basement is exposed (H4725 7437) in the bed of the river and the succeeding 100m of strata consist largely of red-beds, predominantly sandstones but with layers of coarse white quartz pebbles and pedogenic carbonate nodules. The only fine-grained sediments exposed in the Killyclogher Burn are exposed ~1.2km downstream of the basal unconformity, in the midst of a housing estate. The basal unconformity between the Omagh Sandstone Group and meta- morphic basement is also exposed (inter alia) at the SW end of the Lack Inlier. Simpson included these rocks in his Claragh Sandstone Group but they are lithologically quite dissimilar to that formation and contain abundant miospores of the CM Biozone and are undoubtedly Tournaisian. For site specific information see; Key Site 217 - Glendurragh River, Lack. II - TYRONE GROUP Detailed geological mapping of the Kesh-Omagh area by the GSNI indicates that all of Oswald's standard lithostratigraphical formations, between and including the Ballyshannon Limestone and Benbulben Shale formations occur in that area. Because the litho- logical definition of these formations, namely the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations conforms closely to their description from the type areas in Co. Sligo, and other sections in Co. Fermanagh (Subarea-1), sites selected for them are described only briefly. During Vis‚an times in N. Ireland, the Kesh-Omagh area lay close(r) to the northern shoreline of the Northwest Carboniferous Basin. In consequence it not only received a greater amount of clastic sediment than, for example, the Derrygonnelly area of Co. Fermanagh, but was also continually affected by episodic changes in the bathymetry of the basin. As a result the deposition of the 'normal' sediments of the standard formations was continually interrupted by the influx of clastic debris and by the sudden and temporary introduction of very shallow water conditions that significantly altered the nature of the sediments being deposited and of the sedimentary processes operating at the time. The resultant effects on the Carboniferous succession in the Kesh- Omagh area were thus quite dramatic and numerous new formations, and particularly members, are therefore recognised in that area and their designated stratotype sections are located there. The areal and stratigraphical distribution of all of these new litho- stratigraphical units is depicted on the 1:50,000 geological maps of the Kesh (Sheet 32) and Omagh (Sheet 33) areas (GSNI, 1994,5). (i) KEENAGHAN SHALE FORMATION In the SW of Subarea-3 the sub-Carboniferous basement consists either of the Upper Dalradian Southern Highland Group or of the Moinian Lough Derg Group (GSNI, 1994, 1:50,000 Sheet 32). The former metamorphic rocks outcrop along the northern margin of the Carboniferous outcrop between Lough Derg and the southwestern foothills of the Sperrin Mountains. In the east they are overlain unconformably by the Omagh Sandstone Group and in the west by the thick (up to 500m) coarse-grained and pebbly sandstones of the Claragh Formation. These clastic sediments only occur east of the Pettigoe Fault. West of the Pettigoe Fault, on the NW shore of Lower Lough Erne is the outcrop of the psammite-dominated sequence of the Lough Derg Group. The 'basal clastic' component, of the Lower Carboniferous succession, that rests unconformably on the Lough Derg Group consists of ~20m of fine-grained clastic and carbonate rocks of the Keenaghan Shale Formation, the Claragh Sandstone Formation not being represented. The former formation is overlain conformably by the Ballyshannon Limestone Formation. For site specific information see; Key Site 218 - Keenaghan Lough. (ii) CLARAGH SANDSTONE FORMATION The Claragh Sandstone started life as one of Simpson's (1955) four lithostratigraphical group divisions of the Carboniferous rocks in an area, which he termed, the "Omagh Syncline", but which is now simply referred to as the Kesh-Omagh area. He estimated its thickness at between 300m and 1060m and correlated the unit with Vaughan's C1 Zone, equivalent to the basal Vis‚an Chadian Stage of George et al. (1977). Unfortunately he did not select a stratotype section for the unit. Remapping by the GSNI of much of the outcrop of the Claragh Sandstone has identified slight variations in the nature of the component lithologies, but these are both thin and laterally impersistent and the unit consists mainly of very coarse to coarse-grained arkose. On this basis it was decided (GSNI, 1994) to retain the name Claragh Sandstone but to reduce its status to that of a formation and to place it within the Tyrone Group. The outcrop of the Claragh Sandstone Formation extends in an arcuate zone from Omagh, northwestwards to Bin and Bolaght Mountains and west to the line of the Pettigoe Fault, about 6km east of Lough Derg. Virtually all the rocks are unfossiliferous but occasionally, beds of dark grey mudstone do occur in the sandstone sequence and these contain abundant, diverse and well preserved miospore assemblages that include the Pu Biozone index taxon Lycospora pusilla and thus are of Vis‚an age. These mudstones occur in a stream section (H3036 7613) 3.3km NW of Drumquin. Cropping out in the same stream, some 350m to the SE (H3078 7589) ie. downstream, is a bed of limestone 0.5m thick. This consists of medium to dark grey, very coarse-grained pack- stone and oncolitic algal boundstone, slightly sandy in patches with bioclasts including brachiopods, gastropods, ostracods and serpulid debris. Abundant algae in the rock include the forms: Girvanella sp. Koninckopora sp. (monolaminar) Koninckopora sp. (bilaminar) Shartymophycus fusus Ortonella sp. Plurilocular foraminifera are restricted to two forms: Earlandia sp. Spinobrunsiina sp. The presence of the dasyclad alga Koninckopora, with both monolaminar and bilaminar examples is significant and favours a late Chadian to early Arundian age. In view of the late Chadian age for the beds overlying the Claragh Sandstone, a Chadian age is favoured for this formation. Elsewhere in the outcrop of the Claragh Sandstone the dominant lithology is a thin to massively bedded, very coarse-grained, gritty and often pebbly subarkose, arkose and lithicarenites. Sedimentary structures are very common and include parallel and cross laminations, normally graded beds and channelised bedforms with erosional conglomeratic bases. Colours are concentrated in the pale grey, white and fawn range although iron-rich carbonate cements, when present, give rise to rusty brown staining. These lithologies may be seen throughout the outcrop of this formation. The GSNI estimate the thickness of the Claragh Sandstone Formation to vary between 325-500m. For site specific information see; Key Site 220 - Binnawooda Spring. (iii) TERMON RIVER LIMESTONE FORMATION This is a new lithostratigraphical formation first recognised by the GSNI in the Kesh area. It is ~35m thick and consists mainly of carbonates of the peritidal facies and is laterally equivalent to the lower part of the Bin Mountain Sandstone Formation. The main outcrop of the formation occurs towards the northern edge of the Kesh-Omagh area (Subarea-3) along the course of the Termon River, NE of Pettigoe and on the east face of Dunnaree Hill, west of Drumquin. A sparse macrofauna and relatively abundant micro- biota indicate a late Chadian age for this formation. Both the upper and lower contacts and the top 18m of the formation are exposed in the stratotype section. For stratutype section site specific information see; Key Site 219 - Giant's Grave section. (iv) BIN MOUNTAIN SANDSTONE FORMATION This new lithostratigraphical formation first recognised by the GSNI in the Omagh area is known to extend southwestwards into the Kesh area. The existence of "calcareous horizons near the middle of the (Claragh Sandstone Group) succession" was known to Simpson (1955, p.396) who pointed out that they were well exposed on the scarp face of Bin Mountain in the form of arenaceous limestones, being both conglomeratic and oolitic. Sheridan (1972, p.48) demonstrated that this horizon was Vis‚an in age and that the youngest beds in the section, from an unknown horizon, contained archaediscids. His correlation however of the Bin Mountain horizon with the Lower Limestone (Ballyshannon Formation) and the Lower Calp Shale (Bundoran Formation) is incorrect. The formation is at least 53m thick at the stratotype section on Bin Mountain and is exposed at the emergence of Binnawooda spring and in the rough lane leading south from the spring. Some 6.8km SW of the stratotype, in the Giant's Grave section the member is reduced in thickness to ~6m and is now separated from the top of the Claragh Sandstone Formation by another new lithostrati- graphical unit, the Termon River Limestone Formation. At the Binnawooda spring section the latter formation is absent, this being explained entirely in terms of lateral facies variation. Palaeontological evidence indicates a late Chadian age. Towards the top of the formation there is a gradational transition to the Ballyshannon Limestone Formation and within this section are the Chadian-Arundian and early-late Arundian boundaries. For site specific information see; Key Site 220 - Binnawooda Spring. (1) TULLYARD CONGLOMERATE MEMBER The occurrence of white quartz pebbles in the Bin Mountain Sandstone Formation is concentrated in thin discontinuous layers in the lowest 12m of the stratotype section. Towards the eastern extremity of the formation outcrop the occurrence of white quartz pebbles is concentrated into a section of strata about 4m thick, although scattered pebbles do occur above and below this horizon in bioclastic sandstones. This horizon is referred to as the Tullyard Conglomerate Member and represents a new lithostrati- graphical unit within the Bin Mountain Sandstone Formation. For stratotype section site specific information see; Key Site 222 - Tullyard Townland stream. (2) RUSHINDOO OOLITE MEMBER During the remapping of the Kesh area (1:50,000 Sheet 32) by the GSNI the sandstone unit near the top of the Giant's Grave section was believed to represent a new member, the Gortnaree Sandstone Member, and to be the lowest part of the Ballyshannon Limestone Formation. However on completion of the regional remapping of the entire Carboniferous outcrop in the Kesh-Omagh area it became apparent that this sandstone is neither a new member, nor part of the Ballyshannon Limestone, but is simply a thin development of the Bin Mountain Sandstone Formation. Accordingly the use of the term Gortnaree Sandstone Member is now discontinued. Although the Gortnaree Sandstone Member does not exist, the Rushindoo Oolite does represent a separate and mappable lithology, not at the base of the Ballyshannon Limestone, but within the upper part of the Bin Mountain Formation. For site specific information see; Key Site 223 - Rushindoo Townland quarry. | |
| Notes: | |
(v) BALLYSHANNON LIMESTONE FORMATION This formation name, first used by Oswald (1955), replaced the term Lower Limestone that was coined by the Geological Survey of Ireland in the late 19th Century. In his revision of the geology of the Kesh-Omagh area Simpson (1955) erected the Pettigo Limestone Group to include the principal limestone formation in that area. It is now evident however that his Pettigo Group is a composite unit and includes new formations such as the Termon River Limestone and Bin Mountain Sandstone (in part) and Oswald's Ballyshannon Limestone and Bundoran Shale formations. In order to avoid further confusion the term Pettigo Limestone Group was not used on either the Kesh (Sheet 32) or Omagh (Sheet 33) geological maps and will not be utilised in this report. Because the Ballyshannon Limestone has been exploited for hundreds of years, and is continuing to be used as a source of agricultural lime, coarse aggregate and in cement production, quarry sections occur in virtually all parts of the formation outcrop. The designation of ASSI is therefore based largely on such sections, mostly in quarries that are abandoned but also in active quarries. The formation outcrop covers two main areas. In the north of the Kesh-Omagh area it forms an arcuate zone extending from Belleek in the west, along the north shore of Lower Lough Erne to Pettigoe and northeastwards on the south side of the Termon River. Exposure on Bin and Bolaght Mountains is sporadic, but the best outcrop west of Drumquin is continuous and well displayed in quarry sections. As a result of later faulting there is a large outcrop of the Ballyshannon Limestone in the Kesh-Ederny area, near the southern margin of the Carboniferous outcrop. The Ballyshannon Limestone Formation includes 4 new lithostrati- graphical members having stratotype sections in this area (GSNI, 1994, 1995). The new members are the Magherameena Limestone, Crockanaver Limestone, Drumowen Sandstone and Ederny Limestone. The Magherameena Member consists of Waulsortian mudmounds and is confined to the Belleek area of Co. Fermanagh while the Drumowen Member is confined to the Drumquin area in Co. Tyrone. The Crockanaver Member has a more widespread distribution, though with a patchy outcrop, while the Ederny Member, everywhere in the Kesh- Omagh area marking the top of the Ballyshannon Limestone, is developed across the entire formation outcrop. The new members are all relatively thin and when exposed by quarrying operations are invariably associated with strata of the Ballyshannon Formation. (1) MAGHERAMEENA LIMESTONE MEMBER The outcrop of this member is only found west of the Pettigo Fault (1:50,000 Sheet 32 - Kesh) and comprises three isolated mounds of Waulsortian limestone that occur just above the base of the Ballyshannon Limestone Formation. For stratotype section site specific information see; Key Site 224 - Magherameena Quarry, Belleek. (2) DRUMOWEN SANDSTONE MEMBER The outcrop of the Drumowen Sandstone Member is confined to the eastern part of the Carboniferous outcrop in Subarea-3, in Co. Tyrone, west of Drumquin. It is located stratigraphically at the top of the Ballyshannon Limestone Formation, directly beneath the Ederny Limestone Member. In the Omagh area (1:50,000 Sheet 33) it can be traced from the northern edge of the sheet (H28 76) for ~5km to the SE (H31 72) to the disused quarries WSW of Drumquin. At the stratotype the member is 2.7m thick and overlain directly by the Ederny Member. At the NW end (H307 731) of the line of disused quarries west of Drumquin and north of Dunnaree Hill, at the southeastern end of the members outcrop, thin sandstone beds and lenticles and sandy limestones occur in the top 7.3m of strata of the Ballyshannon Limestone beneath the Ederny Member. For stratotype section site specific information see; Key Site 225 - Carrickaness Bridge, The Black Water. (3) CROCKANAVER LIMESTONE MEMBER The outcrop of the Crockanaver Limestone Member is confined to the eastern part of the Carboniferous outcrop in the Kesh-Omagh area (1:50,000 Sheet 33 - Omagh) and within that area is lenticular and thus discontinuous. Stratigraphically the member is always located directly below the Ederny Limestone Member and does not exceed a thickness of 10m. The member is a very distinctive unit consisting of virtually unbedded to poorly bedded, shale-free, pale grey to fawn crinoidal grainstones and contrasts markedly, particularly in quarry sections, with the well-bedded limestones and shales below and the rubbly (palaeokarstified) Ederny Limestone above. The best sections in the Crockanaver Member are found in the many quarries, most of which are now closed, that are located on the north face of Carn ridge between Crockanaver (H222 635) in the west and Drumgivern (H242 631), 2km to the east, SE of Ederny. The strato- type quarry also exposes 12m of strata typical of the Ballyshannon Limestone, below the base of the member. For stratotype section site specific information see; Key Site 226 - Crockanaver Quarry, Ederny. (4) EDERNY LIMESTONE MEMBER Throughout the Kesh-Omagh area the top of the Ballyshannon Limestone Formation is marked by a thin development of very distinctive limestones that are referred to as the Ederny Limestone Member. These consist of pale grey micritised packstones and were subjected to the effects of karstification and pedo- genesis. Based on its highly distinctive appearance (Photographs 58 and 59), its unique occurrence at this level in the Lower Carboniferous of the Kesh-Omagh area, and its widespread geographical distribution (1:50,000 Sheets 32 and 33), the Ederny Member is by far the most important and mappable unit utilised in compiling the geological maps of this area. The Ederny Member is invariably overlain by the Bundoran Shale Formation. For stratotype section site specific information see; Key Site 227 - Ederny Quarry. (vi) BUNDORAN SHALE FORMATION In the Kesh-Omagh area the Ballyshannon Limestone Formation is always overlain by the Bundoran Shale Formation and their respective outcrops are thus contiguous on the 1:50,000 geological maps of the Kesh and Omagh (Sheets 32 and 33) areas. The formation varies in thickness from 40-100m and consists of fine-grained, calcareous, dark grey micaceous mudstones and siltstones with thin, often lenticular, blue-grey bioclastic limestones. Fossils are abundant throughout the formation and are dominated by brachiopods, bryozoa and solitary and colonial corals. At the base of the formation is the Skea Sandstone Member. This was deposited only in the central (North-South) part of the Kesh-Omagh area and does not occur either in the Drumquin area (Sheet 33), or as a sandstone in the Kesh-Ederny area. Exposures of the Bundoran Shale, whilst very distinctive and usually fossiliferous, are often of limited extent. The strato- type section in Moffatt's Quarry exposes almost completely, the basal 40m of the formation and the top 50m of the underlying Ballyshannon Limestone. For site specific information see; Key Site 228 - Moffatt's Quarry, Drumquin. (1) SKEA SANDSTONE MEMBER In parts of the outcrop of the Bundoran Shale Formation in the Kesh-Omagh area the basal beds consist of the sandstones and conglomerates of the Skea Sandstone Member. Although this member was first recognised in Skea Townland (H17 71) in the Kesh area (1:50,000 Sheet 32) and despite the excellent exposures there, the stratotype locality selected for the Skea Member occurs at Shanvin, 10km NNE of Kesh (1:50,000 Sheet 33 - Omagh). The origin of the detritus that forms the Skea Member is discussed in some detail in Key Site 1173. Briefly, it is believed that marine regression, coupled with localised uplift of the northern metamorphic land area and partial exposure of the top surface of the Ederny Limestone Member hereabouts, led to enhanced erosion and production of coarse detritus. This debris was not deposited universally across the Kesh-Omagh area but channelled southwards in a palaeovalley that incised into the top of the Ballyshannon Limestone; proximal parts of the deposit include blocks of the same fossiliferous limestone. The resultant sediment plume dispersed mainly southwards but was restricted in its distribution to the east so that the member is absent in the Drumquin area. There are indications that the palaeovalley may have had a N-S to NE-SW orientation. For site specific information see; Key Site 229 - Shanvin (stratotype section) Key Site 230 - Drummahon Lane. (vii) MULLAGHMORE SANDSTONE FORMATION In the Kesh-Omagh area the outcrop of the Mullaghmore Sandstone Formation occupies a large part of the Carboniferous outcrop north of the Cool Fault (1:50,000 Sheets 32 and 33). However many of the exposures are small and frequently consist only of sandstone which may seem reasonable, given the lithological qualifier attached to the formation name, but which, in reality, is somewhat misleading. Two distinct sandstone members occur within the Mullaghmore Formation. At the base is the Dromore Sandstone Member and in the middle is the Drumskinny Sandstone Member. The Dromore Member is recognised across all of the Carboniferous outcrop and, because of the relatively resistant nature of the sandstones, compared to the underlying Bundoran Shale, tends to form rock-cored scarps with long, gently inclined dip slope surfaces. The upper and lower contacts, in particular the upper surface, are very well defined and have been recognised in outcrop. The Drumskinny Member is conversely a much less well defined sandstone unit whose outcrop pattern is much more difficult to define accurately. Outside its stratotype area and section very little information is available and nowhere has it been possible to identify unequivocally the lower and upper contacts. For site specific information see; Key Site 231 - Bannagh River. (1) DROMORE SANDSTONE MEMBER The basal strata of the Mullaghmore Sandstone Formation in the Kesh-Omagh area consist mainly of pale-coloured sandstones, up to 30m thick that are assigned to a new lithostratigraphical unit, the Dromore Sandstone Member. This member is present across the whole area and from north to south in its outcrop becomes increasingly influenced by the marine environment, in respect of the occurrence of limestones, marine fossils and bioturbation structures, and sedimentary structures. For stratotype section site specific information see; Key Site 235 - Dromore Big Quarry. (2) DRUMSKINNY SANDSTONE MEMBER This new member (GSNI, 1994) of the Mullaghmore Sandstone Formation is ~40m thick and consists mainly of medium to coarse- grained, fawn-white feldspathic sandstones. It is located strati- graphically almost in the middle of the formation. Of all the lithostratigraphical units in the area this is the only one for which it has not been possible to identify the lower and upper contacts in outcrop with any confidence. The principal outcrop extends from Boa Island (north side of Lower Lough Erne) to Bannagh Bridge (H162 654) and from Clonelly (H148 663) to Drumskinny. For stratotype section site specific information see; Key Site 236 - Montiaghroe Townland Quarry. (viii) DRUMCHORICK SILTSTONE FORMATION In much of NW Ireland the top of the Mullaghmore Sandstone Formation is sharp reflecting an abrupt cessation in the transport of sand into the basin. Widespread mud deposition, represented by the Benbulben Shale Formation, recommenced, both within and to the NW of the basin (Philcox et al., 1992). This transition is well illustrated by Key Site 179 in Subarea-1. In the Kesh-Omagh area however this abrupt change is not evident, and being located closer to the continent margin vestigial amounts of sand, mixed with quantities of silt and mud-grade sediment continued to enter the basin. This interval between the top of the Mullaghmore Sandstone and the base of the Benbulben Shale, in this area, is represented by some 90-100m of sedimentary rocks that fitting the description of neither formation have been separated as a new formation, the Drumchorick Siltstone Formation. The lower contact, with the top of the Mullaghmore Sandstone, is exposed at the top of the falls in the Bannagh River and described in Key Site 231. The upper contact, with the Benbulben Shale, is exposed in a river (H20 66) ~4km NE of Kesh and described in Key Site 238. The Drumchorick Siltstone Formation represents a unit with an average grain size intermediate between the sandstones of the Mullaghmore Formation below and the shales of the Benbulben Formation above. Grey-brown micaceous, mainly interbedded silt- stones and mudstones dominate the succession but sandstones do occur and although bed thickness rarely exceeds 0.1m, and is often much thinner and lenticular, they are an important and typical part of the definition of this formation. Also characteristic of this succession are thin, lenticular beds of limestone turbidities (? tempestites) containing abundant graded bioclast debris and merging upwards into sparsely fossiliferous, parallel laminated, fine-grained, very calcareous sandstones. These beds commonly contain the remains of linguaphillipsid trilobites, normally associated with a shallow water, carbonate platform environment. The existence therefore of a contemporaneous carbonate platform though not evident in outcrop, is indicated by the presence of these trilobites and by the great quantities of debris of shallow water benthos. The late Arundian age of the Drumchorick Formation is indicated by the association of involutus stage Paraarchaediscus and compound corals that are intermediate in character between Siphonodendron and Lithostrotion and show a 'cerioid tendancy'. ------------------------------------------------------------------ For more information on the Carboniferous System in N. Ireland see; Key Site 1171 - The Carboniferous System in N. Ireland - Introduction. Key Site 1172 - Carboniferous Biostratigraphy - Overview. Key Site 1173 - Carboniferous Palaeogeography of N. Ireland. | |
| Keywords | |
|---|---|
| Rocks: | Limestone, Sand, Sandstone, Shale |
| Structures: | bedding, cross-bedding, massive, palaeokarst, ripple marks |
| Relations: | Marine sediments |
| References (4) | |||
|---|---|---|---|
| George, T.N., Johnston, G.A.L., Mitchell, M., Prentice, J.E., Ramsbottom, W.H.C., Sevastopulo, G.D. & Wilson, R.B., 1976. A correlation of Dinantian rocks in the British Isles. Geological Society of London Special Report, numb. 7, pp.1-87. | |||
| Oswald, D.H., 1955. The Carboniferous Rocks between the Ox Mountains and Donegal Bay. Journal of the Geological Society of London, vol. 111, pp.167-186. | |||
| Sheridan, D.J.R., 1972. Upper Old Red Sandstone and Lower Carboniferous of the Slieve Beagh Syncline and its setting in the northwest Carboniferous basin, Ireland. Special Papers of the Geological Survey of Ireland, vol. 2, pp.1-120. | |||
| Simpson, I.M., 1955. The Lower Carboniferous Stratigraphy of the Omagh Syncline, Northern Ireland. Quarterly Journal of the Geological Society of London, vol. 110, pp.391-408. | |||
| Map(s): | GSNI 1:50,000 Solid Geology Sheet 31 (part of); GSNI 1:50,000 Solid Geology Sheet 32 (Kesh); GSNI 1:50,000 Solid Geology Sheet 33 (Omagh). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Carboniferous Subarea-3; Kesh-Omagh, County Fermanagh, Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1176. Accessed on 2026-11-05. |
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