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Earth Science Conservation Review
| Altagoan River | Londonderry |
| Site number: | 303 | File Number: | H79 |
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H79929456,H80339300 End(s) of linear site, Not Entered | ||
| Google maps: | 54.79246,-6.75784 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Carboniferous | ||
| Stages: | Tournasian | ||
| Lithostrat: | Altagoan Formation, Drumard Member, Mormeal Member, Tyrone Group | ||
| Site Description | |
|---|---|
| Highlights: | |
The Altagoan River is the stratotype section for the Altagoan Formation and in particular the upper part which is referred to the Mormeal Member. Miospores recovered from this member date the top of the formation. | |
| Description: | |
Exposures of the Altagoan Formation in the Altagoan River commence, in the north, about 200m south (upstream) of Altagoan Bridge at H7994 9440 and extend southwards, more or less continuously for about 1400m to H8033 9300. Throughout the outcrop strata dip at ~15 deg. to the east and north. The section is heavily faulted and two faults in particular, in mid-section isolate the Mormeal Member of the formation within outcrops of the Drumard Member. Lithologies of the Drumard Member, at the north and south ends of the section consist of red-grey silty mudstones and siltstones with red micaceous sandstones, in varying proportions. The outcrop of the Mormeal Member located between the two major faults consists exclusively of grey beds. Sections occurring at the north end of their outcrop are repeated several times to the south and a relatively small thickness of strata is represented here. Grey shales and mudstones are the dominant lithologies though paler grey siltstones and greenish-grey, often rippled and channellised sandstones occur at intervals. Evaporites occur at 5 horizons with the thickest being evaporites B (0.23m) and E (0.43m). They always consist of interbedded and usually thin beds of grey shale, laminated (algal) and non-laminated micrite and evaporite replacement beds. The latter beds are particularly distinctive consisting of fawn to pale brown, soft clay with a network cellular texture representing pseudomorphs of calcite after halite, gypsum or anhydrite. Evaporite D consists of several thin evaporite beds and micritic algal stromatolites in 0.52m of grey shale. Miospore assemblages were recovered from samples of dark grey mudstone collected from the north end of the outcrop of the Mormeal Member, 320m south from Altagoan Bridge (Owens et al., 1977). One assemblage of well preserved miospores contained 68 taxa of which the following species have been recorded only from Tournaisian deposits: Anaplanisporites delicatus, Convolutispora circumvallata, Crassispora balteola, Cyclogranisporites palaeophytus, ?Hymenozonotriletes hastulus, Knoxisporites pristinus, Lophozonotriletes dentatus, Punctatisporites irrassus, Raistrickia corynoges, Rugospora polyptycha, and Stenozonotriletes perforatus. The occurrence of these forms plus the CM Zone index taxa Schopfites claviger and Auroraspora macra persuaded Owens et al. to assign the assemblage a late Tournaisian (CM Zone) age. However the occurrence of rare specimens (3 or 4 in 5000+ miospores) of the species Lycospora pusilla was considered to be insignificant by them and the assemblage was accordingly assigned to the CM Biozone. This conclusion was subsequently challenged by Higgs et al. (1988) who rejected this argument, believing that the paucity of L. pusilla is significant and is now known to be characteristic of basal Pu Biozonal assemblages. Based on the correlations of Riley (1993) the assemblage could be still late Tournaisian in age but is post-Courceyan and thus correlates with at least the early part of the Chadian Stage. | |
| Interpretation: | |
The sedimentary rocks of the Altagoan Formation were deposited at a time, in the late Tournaisian and possibly earliest Visian, when the influence of the first, northward migrating, Carboniferous marine transgression reached the north of Ireland and is recorded from such widespread localities as Donegal Bay, the Sperrin Mountains and Cultra-Craigavad in Co. Down. Coarse- grained alluvial fan sandstones and conglomerates of the basal Iniscarn Formation, and the steep fault-line scarps against which they were banked, were gradually submerged and ultimately concealed by fine red-beds of the Drumard Member of the Altagoan Formation. Throughout the latter member there is no evidence of a marine influence but there is abundant evidence of deposition subaerially on an alluvial plain that was prone to long periods of evaporation, and oxidation of the sediments, in the process producing pedogenic layers (soils) with calcrete nodules, and the red colouration so typical of these rocks. However towards the top of the Drumard Member there are signs of marine conditions encroaching from the south and grey sediments, with occasional bivalve-ostracod faunas, appear for the first time. The fauna of the Mormeal Member is believed to have flourished in a highly saline intertidal environment. This interpretation is supported by its association firstly, with evaporites and secondly, and in particular, with finely laminated stromatolitic (cyanobacterial) limestones. The latter are identical to carbonates that are forming at the present time in sabkha which is the supratidal environment that develops in high latitudes, for example around the Red Sea coast and is subjected to transitory marine incursions and intense evaporation. Highly saline brines bathe the sediments and persistent evaporation leads ultimately to the crystallisation of minerals such as halite, gypsum and anhydrite in pore spaces between the detrital grains. | |
| Conclusions: | |
The rocks of the Altagoan Formation represent the end member of an evolving environmental continuum. This began with desert cliff lines and coarse sediment of the Iniscarn Formation, became progressively finer upwards with time, and ended ultimately on a hot, brine-soaked plain adjacent to the sea which still lay somewhere to the south, possibly in the Omagh area. On the basis of miospore assemblages contained in the Altagoan Formation the rocks span the boundary between the CM-Pu Biozones but may still all be late-Tournaisian in age. | |
| Notes: | |
For information on other sites in Subarea-12 see Key Site 1185 - Subarea-12; Draperstown-Glenshane Pass-Limavady. | |
| Keywords | |
|---|---|
| Minerals: | Calcite |
| Rocks: | Algal limestone, Evaporite, Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone |
| Fossil Groups: | Bivalve, Crustacea, Miospore |
| Fossil List: | Lycospora pusilla |
| Relations: | Fluvial sediments, Inter-tidal feature, Marine sediments |
| Paleoenv: | alluvial fan, alluvial plain, desert, sabkha |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | ~1400 m | Width: | Height: | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Approach: | Exposures commence ~200m south of Altagoan Bridge, ~1.7km east of Draperstown. |
| References (3) | |||
|---|---|---|---|
| Higgs, K., Clayton, G. & Keegan, J.B., 1988. Stratigraphic and systematic palynology of the Tournaisian rocks of Ireland. Special Papers of the Geological Survey of Ireland, numb. 7, pp.1-93. | |||
| Owens, B., Gueinn, K.J. & Cameron, I.B., 1977. A Tournaisian miospore assemblage from the Altagoan Formation (Upper Calciferous Sandstone), Draperstown, Northern Ireland. Pollen et Spores, vol. 19, pp.313-324. | |||
| 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 (1995) 1:50,000 Solid Geology of Sheet 26 (Draperstown). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Altagoan River, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=303. Accessed on 2026-11-03. |
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