ESCR - Devonian - Shanmullagh Formation; Stratotype localities

Earth Science Conservation Review

Summary Full report
Devonian - Shanmullagh Formation; Stratotype localities	Tyrone

Site number:	1150
Locality Type:	Cliff, Quarry (disused)
Grid Reference:	H255598 Approximate
Google maps:	54.48646,-7.60652
County:	Tyrone	Council area:	Fermanagh & Omagh District Council
Period:	Devonian
Stages:	Famennian, Frasnian
Lithostrat:	Fintona Group, Shanmullagh Formation (Form Blm)

Site Description
Highlights:
North of the Tempo-Sixmilecross Fault the largest outcrop of Devonian rocks in the Fintona Block belongs to the Shanmullagh Formation and is confined to the Irvinestown Segment. They consist exclusively of fine-grained red-beds and although their outcrop covers an area of almost 250km2, and despite an abundance of exposures, it has not been possible either to recognise distinctive lithostratigraphical units or to calculate the overall thickness of the formation. The presence of miospore assemblages in green mudstones is known at many localities in the segment and allows these rocks of Upper Old Red Sandstone facies to be assigned specifically to the Frasnian-Lower Famennian Stages of the Upper Devonian. Location: The stratotype quarry in Drumharvey Townland (I) is located 2.4km north-northeast of Irvinestown. Dromore, and (II), is located in Co. Tyrone some 10km east-northeast of the stratotype quarry and the cliff exposures, beneath the ruined church, are on the north side of the Irvinestown Road. The local council have recently reinstated the roadside grass verge and constructed a concrete walkway between the town and the west end of the cliff exposure of (II), in effect creating a small park that also includes the ruined church sitting on top of the rocky knoll of Devonian rocks. Bibliographic Review: The Devonian rocks of the Shanmullagh Formation and the definition of the Irvinestown Segment were included in Mitchell and Owens (1990). General Geology: Although the Shanmullagh Formation consists entirely of red-beds of Upper Old Red Sandstone facies, there is a wide variety of lithofacies within the formation. Sandstones, siltstones and mudstones are interbedded in varying proportions and pedogenic horizons, frequently with small carbonate (calcrete) nodules, are common. There are no conglomerates or coarse-grained sandstones in the formation. Throughout the Irvinestown Segment the regional orientation of strata is a few degrees to the east of north except at its northern margin where anticlockwise rotation of bedding was caused by sinistral strike slip on the Cloghfin Fault.
Description:
For sites I and II specific information see; Key Site 316 - Drumharvey Townland quarry Key Site 317 - Dromore (cliff west of).
Interpretation:
All the available evidence from the Shanmullagh Formation indicates that the sediments accumulated on a low-lying alluvial plain that was prone to long periods of pedogenesis (soil formation) and desiccation. There was nevertheless a steady influx of sand and silt grade detritus reaching all parts of the plain as non-erosive sheetflood deposits and in some instances as shallow channels in a distal location on a ephemeral braided stream complex. The frequent association of ripple marks in sandstones, with desiccation cracks in succeeding thin mudstone laminae, indicates the intermittent nature of sheetflood sedimentation in a hot climate when evaporation was a virtually continuous process and mud-flakes thus produced were incorporated into the next sheetflood detritus. Evidence of the periodic influx of large volumes of surface water onto the alluvial plain are rare but when they occur are indicative of violent and deeply erosive episodes. In some examples the channels thus formed are up to 2m deep and 7m wide with steep, relatively smooth contacts with the enclosing laminated mudstones and siltstones. The sediment infill of these channels still consists however of fine-grained sandstone and the coarse lag deposit at its base usually comprises a few millimetres of reworked mudstone clasts and calcrete nodules with no coarse grains of extrabasinal quartz. The example of the channel complex at (II) illustrates the erosive effects of flashfloods on a desert landscape and on pre-existing stratified sandstones thus producing vertical and undercut "river" banks to a depth of at least 1m. Subsequent infill of the channel occurred at a slower rate and as the result of a succession of sediment pulses until the whole complex is concealed by the next sheetflood deposit represented by the top sandstone bed in the section. In neither of the examples quoted is the channel infilled by sediment coarser than a medium-grained sandstone. Conglomerates are absent. It seems likely that sediments of the Shanmullagh Formation accumulated on an alluvial plain far from the basin margins. Conditions of flashflood clearly occurred throughout the alluvial plain but were either a local event, on the plain, and thus only reworked the very fine-grained sediments thereabouts or, if originating on higher land at the basin margins, deposited any coarse sediment close to the margins, beyond the present outcrop limits, thus carrying only fine sand etc. into deep channels on the alluvial plain. Ephemeral sheet flood siltstones and thin very fine-grained sandstones and thick playa lake mudstones dominate many parts of the succession and indicate the remote location in which these sediments accumulated, near the axial region of the Upper Devonian basin.
Notes:
For general information and reference list on the Shanmullagh Formation with respect to miospore localities see; Key Site 1151 - Shanmullagh Formation; Miospore localities. For general information and reference list on the Devonian System in N. Ireland see; Key Site 1147 - The Devonian System in N. Ireland - Introduction.

Keywords
Rocks:	Mudstone, Sand, Sandstone, Siltstone
Fossil Groups:	Miospore, Miospores
Structures:	dessication cracks, ripple marks
Paleoenv:	alluvial plain, braided stream, desert

References (6)
Blieck, A., 1991. Reappraisal of the heterostracans (agnathan vertebrates ) of Northern Ireland. Irish Journal of Earth Sciences, vol. 11 pt. 1, pp.65-69.
Clayton, G., Higgs, K. & Keegan, J.B., 1974. Late Devonian and Early Carboniferous occurrences of the spore genus Emphanisporites McGregor in Southern Ireland. Pollen et Spores, vol. 19 pt. 3, pp.415-425.
Harper, J.C. & Hartley, J.J., 1938. The Silurian inlier of Lisbellaw, Co Fermanagh, with a note on the age of the Fintona Beds. Proceedings of the Royal Irish Academy, vol. B45, pp.73-87.
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.
Mitchell, W.I. & Owens, B., 1990. The geology of the western part of the Fintona Block: evolution of Carboniferous basins. Geological Magazine, vol. 127 pt. 5, pp.407-426.
Simon, J.B., 1984. Sedimentation and tectonic setting of the lower Old Red Sandstone of the Fintona and Curlew Mountain districts. Irish Journal of Earth Sciences, vol. 6, pp.213-228.

Map(s):	GSNI (1982a) 1:50,000 Sheet 45 (Enniskillen); GSNI (1982b) 1:50,000 Sheet 46 (Clogher); O.S. 1:50,000 Sheet 18 (Enniskillen).
Record type:	ESCR report
Entered by:	E M Porter
Last updated:	8 Jan 2005

Enlander, I., Dempster, M. & Doughty, P., 2026. Devonian - Shanmullagh Formation; Stratotype localities, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1150. Accessed on 2026-11-07.