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Earth Science Conservation Review
| Ballynahone River | Armagh |
| Site number: | 265 | File Number: | H84 |
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H882463,H88494596 End(s) of linear site, Not Entered | ||
| Google maps: | 54.3582,-6.6433 | ||
| County: | Armagh | Council area: | Armagh City, Banbridge & Craigavon Borough Council |
| Period: | Carboniferous | ||
| Stages: | Tournasian | ||
| Lithostrat: | Annaclare Group, Armagh Group, Ballynahone Micrite Formation, Drumman More Sandstone Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
The highest strata of the Annaclare Group consist of interbedded peritidal limestones and shales up to 100m thick. The stratotype and best exposure is in the Ballynahone River. | |
| Description: | |
The Ballynahone River is a minor tributary of the Callan River that rises on the northern flanks of the Down-Longford Lower Palaeozoic massif. The section of river here described is located on the east side of Dean's Hill forming the southwestern boundary of Mullynure Townland. Exposures commence ~50m north of Dean's Bridge (H885 458) and consist of alternating purple-red silty mudstones, fine-grained sandstones and grey silty micritic limestones. These beds are exposed together with pink-grey fine conglomerates and grey calcareous sandstones to a point (H8849 4596) 150m downstream (north) of the Bridge. Between these beds and the lowest strata of the Ballynahone Formation is an exposure gap of ~50m. Limestones are exposed north of that gap, in the river bed, and are inclined at 24 deg. to the NW. Only the limestone beds are exposed, the intervening shales either having been eroded by the river or concealed by alluvium. The limestones consist of pale fawn-grey, often orange-brown stained oncolitic, fenestral micrites in beds up to 0.25m thick. They contain a sparse macrofauna of bivalves, gastropods, ostracods, very occasional brachiopods and algal growths either as cyanobacterial coats (oncolites) or very rarely as laminations resembling stromatolites. No diagnostic foraminifera are recorded however from this formation. Towards the top of this section (H8841 4611), 300m NNW of Dean's Bridge, a measured section of 3.7m of strata in the upper part of the formation is composed primarily of mudstones which constitute about 70% of the strata. They consist of pale grey-green to fawn- grey shaley mudstones and are usually finely laminated and unfossiliferous; the thickest bed in this section occurs at the top of the sequence and is 1.4m thick. Limestones occur in beds up to 0.45m thick and are typically unlaminated, very sparsely fossiliferous dolomicrites. Algal laminations tend to occur in beds <0.1m thick, or as 1 or 2 laminae in the massive limestones. The most northerly occurrence of micrites of the Ballynahone Formation is at H884 462, some 410m NNW of Dean's Bridge. Fossiliferous marine limestones of Arundian age are exposed some 25m downstream from this last micrite locality. It is believed that this gap in the section marks the position of a fault line that has excised some of the strata which occur at the base of the Armagh Group, and are only recorded in the borehole core at Wilson's Bridge (McPhilemy, 1988). The extent of the section in the Ballynahone River is extended to the NW for a further 200m to the sharp bend in the river at H8823 4633, so as to include these limestones of the Armagh Group and the outcrop of the Drumman More Sandstone. These strata are described in Parts B and C. | |
| Interpretation: | |
The 3 formations of the Annaclare Group represent the sedimentary deposits of a single depositional cycle. In the classic model, these represent a consistent and gradual evolution from a terrestrial environment, to brackish and fluvial conditions culminating at the top, in the youngest strata, in a peritidal (quasi-marine) environment. The gradual upward change in the colour of the rocks from red-purple to grey-green, in their increasing fineness and degree of sorting and in the later dominance of limestones over clastic sediments reflects a decrease in the influence of the terrestrial environment and heralds the increasing influence of the sea. Marine transgression in Ireland moved gradually northwards in Tournaisian times and from south to north the effects of diachronism are evident in the progressively younger age of the rocks, in particular of the timing of the arrival of the sea and the age of the first undoubted marine sediments. It seems likely that upstanding areas of land existed initially in late Tournaisian times in the north of Ireland. These included the Down-Longford Massif in south Co. Armagh and Co. Down and areas north of the Sperrin Mountains. Whether or not these land areas still existed by the time the marine transgression reached, for example, the Armagh area, is not known. Red-beds of the Killuney Conglomerate Formation were deposited subaerially, on a land surface. Coarse debris in this formation indicates either rapid erosion of higher ground or reworking of pre-existing coarse-grained alluvium. Because the formation is relatively thin and fines-upwards rapidly the author suspects that the early Carboniferous topography on the Down-Longford Massif was subdued and topographic variation may be the cause of variations in the thickness of the formation. The bulk of the sediments of the Retreat Siltstone Formation reflect deposition by water, either from rivers in the case of the coarse sandstones and gritty beds or in lakes as the finer fraction of mudstones and siltstones. Periodic emergence of fluvial and lacustrine surfaces permitted their colonisation by plants resulting in the formation of root beds and providing the miospore assemblages that are common in the argillaceous strata. Transitory marine incursions led to the formation of thin limestone beds but these conditions only began to dominate this area sometime later. Limestones and shales of the Ballynahone Micrite Formation are marine sediments. They were deposited in shallow water, peritidal conditions when higher concentrations of salts created an environment in which only a specialised benthos could survive. Truly marine animals such as corals, bryozoa and most brachiopods could not tolerate these conditions and never colonised that sea- bed at that time. Algae, in contrast, flourished in the hot brines and their former existence may be seen in the cyano- bacterial coating of oncolites and as flat-laminations in some limestones. Whether or not open marine conditions existed in the Armagh area in Tournaisian times is not known as the sedimentary record of these times ends with the Ballynahone formation and is succeeded disconformably (?) by the Arundian Armagh Group. | |
| Conclusions: | |
The sedimentary rocks that are typical of the third formation of the Annaclare Group are admirably demonstrated at the Ballynahone River section which is the type section of the Ballynahone Micrite Formation. * * Part B HIGHLIGHTS This site covers a 600m long portion of the Ballynahone River, north of Dean's Bridge. In addition to the stratotype of the Ballynahone Micrite Formation of the Annaclare Group, it includes fossiliferous marine limestones of the basal part of the Armagh Group. DESCRIPTION Between the top bed of the Ballynahone Micrite Formation, exposed at H884 462, and the lowest outcrop of the Armagh Group is a 30- 40m exposure gap in the river. Limestones exposed within that upper section extend downstream for ~30m from just upstream of the sharp bend in the river. They consist of medium to dark grey, fine-grained packstones and packstone/grainstones. The lowest limestone contains a diverse assemblage of foraminifera of mid- to late Arundian age including the following species: Brunsia spirillinoides Globoendothyra sp. Dainella elegantula Latiendothyranopsis menneri Earlandia sp. Mediocris sp. Endothyra sp. Paraarchaediscus sp. stage involutus Eoparastaffella simplex Pseudolituotuba multicamerata Eostaffella sp. Spinobrunsiina lexhyi Gigasbia sp. Tetrataxis sp. Identical limestones, interbedded with dull grey-brown shaley siltstones, are exposed sporadically in the river for a further 130m to the NW. The contact with the sandstones of the Drumman More Formation that are exposed in the river NW of the limestone outcrops is faulted. INTERPRETATION The discrepancies between the lithologies recorded in the Wilson's Bridge Borehole (WB3) below the Drumman More Sandstone, and in the Arundian sections of the Armagh Group at outcrop are difficult to reconcile. Both suites of rock are of the same age and it is unlikely that the absence in the borehole, of similar limestones, such as occur at outcrop, is due to facies changes or non-deposition. It is possible, given the softer nature of the sandstones and conglomerates, that they are concealed in all Arundian sections and that only the hard limestones are therefore exposed. The age of the limestones that represent the basal beds of the Armagh Group, at this site and the sections in the Callan River at Milford and near Oulart Villa, is not in doubt. Macro- and microfaunas of undoubted early Arundian age occur at the base of two of the sections, and all three localities are succeeded by younger strata that contain foraminifera, including involutus stage Paraarchaediscus, that indicate a mid- to late-Arundian age. These results indicate that the late Chadian age defined by the GSNI (1984) for these limestones is too old and that the main Vis‚an marine transgression occurred in earliest Arundian times in the Armagh area. ----------------------------------------------------------------- * * Part C DESCRIPTION At the very NW end of the section in the Ballynahone River is an exposure of soft, micaceous, carbonaceous, pale brown-grey sandstone, that dips eastwards at ~35 deg. At present this represents one of only two known exposures of the Drumman More Sandstone in the Carboniferous outcrop in Subarea-6. The other exposure occurs in a shallow ditch (H8879 4680) in Drumman More Townland, 1.8km NNW of Armagh Mall and comprises purple-brown, fine-grained quartz sandstone. The sandstone was formerly quarried (H533 486) in Drumman Beg Townland, 3.7km NNE of Armagh Mall in the mid-19th Century and the stone reportedly was used in the construction of the Protestant Cathedral in Armagh. Despite the very restricted exposures of the Drumman More Sandstone geologists of the Geological Survey of Ireland (Egan, 1873) astutely identified the existence of the Calp Sandstone in the Armagh area and positioned it stratigraphically between the Lower and Upper limestones. | |
| Notes: | |
For information on other sites in Subarea-6 see Key Site 1179 - Subarea-6; Armagh-Benburb. | |
| Keywords | |
|---|---|
| Rocks: | Limestone, Sand, Sandstone |
| Fossil Groups: | Bivalve, Brachiopod, Crustacea, Foraminifera, Gastropod |
| Fossil List: | Earlandia sp. , Endothyra sp. |
| Relations: | Marine sediments |
| Measurements | |||||
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| Length: | Width: | Height: | A: 100 m | ||
| Depth: | Area: | ||||
| Access | |
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| Approach: | North of Dean's Bridge on the NE outskirts of Armagh City. |
| References (2) | |||
|---|---|---|---|
| McPhilemy, B., 1988. The value of fluorescence microscopy in routine palynofacies analysis: Lower Carboniferous successions from Counties Armagh and Roscommon, Ireland. Reviews of Palaeobotany and Palynology, vol. 56, pp.345-359. | |||
| Riley, N.J., 1993. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles. Journal of the Geological Society of London, vol. 150, pp.427-446. | |||
| Map(s): | GSNI (1984) 1:50,000 Solid Geology of Sheet 47 (Armagh). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Ballynahone River, County Armagh, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=265. Accessed on 2026-11-05. |
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