ESCR - Devonian - Fintona Block south of Tempo-Sixmilecross Fault

Earth Science Conservation Review

Summary Full report
Devonian - Fintona Block south of Tempo-Sixmilecross Fault	Fermanagh, Tyrone

Site number:	1152
Locality Type:	Various
Grid Reference:	H3548,H5768 End(s) of linear site, Not Entered
Google maps:	54.3799,-7.46134
County:	Fermanagh, Tyrone	Council area:	Fermanagh & Omagh District Council
Period:	Devonian
Stages:	Middle Devonian, Upper Devonian
Lithostrat:	Barrack Hill Andesite Member, Fintona Group, Gortfinbar Conglomerate Formation, Greenhill Andesite Member, Raveagh Sandstone Formation, Shanmaghery Sandstone Formation

Site Description
Highlights:
The Fintona Group outcrop south of the Tempo-Sixmilecross Fault covers an area of about 250km2 and consists entirely of red-beds and contemporaneous lavas. Although no fossils have been recovered from these rocks radiometric dating (K-Ar) of the lavas gives an age of 375 million years (Ma), placing their age of extrusion at about the Middle-Upper Devonian boundary.
Introduction:
In the Devonian outcrop of the Fintona Block south of the Tempo- Sixmilecross Fault the Fintona Group is divided into three apparently conformable, but as yet biostratigraphically undated formations with an overall thickness in excess of 3500m (GSNI, 1979). In ascending order these are the Shanmaghery Sandstone, Gortfinbar Conglomerate and Raveagh Sandstone formations. Within the Gortfinbar Formation are the Barrack Hill and Greenhill Andesite members which consist of flow-banded trachy-andesite or trachyte lavas. Large areas of sandstone have also been included in the Gortfinbar Conglomerate Formation but because the exposures are so poor, the relationship between the two lithologies is not clear. No stratotype sections were selected for any of the lithostratigraphical units in the Fintona Group south of the Tempo-Sixmilecross Fault. Location: The southern sector of the Devonian outcrop in the Fintona Block is located between the Tempo-Sixmilecross Fault and the faulted northern margin of the Carboniferous outcrop in the Clogher Valley. Bibliographic Review: The first detailed lithostratigraphy of the Fintona Group south of the Tempo-Sixmilecross Fault was established by the GSNI and published in 1979 on the 1:50,000 geological map of the Pomeroy area. The three formations recognised on that map were accepted by Simon (1984) in his review of Devonian rocks throughout the Fintona Block. Unfortunately his lack of biostratigraphical control also led him to erroneously use those names for similar lithologies cropping out north of the Tempo-Sixmilecross Fault and to correlate rocks that are now known to be of quite different ages. Rundle (in Mitchell and Owens, 1990, p.410) provided K-Ar wholerock radiometric dates for contemporaneous lavas in the Gortfinbar Formation, and these are still the only dates from this thick Devonian sequence. General Geology: Although the total thickness of the Fintona Group south of the Tempo-Sixmilecross Fault is estimated at 3500m the effects of faulting within the outcrop are not known and the very poor exposure of the red-beds, in particular the top two formations, adds a note of caution before this figure is accepted unequivocally. Indeed throughout the whole of this part of the Devonian outcrop it has not been possible to select any localities, from within the outcrop of each of the three formations, that are considered worthy of designation as Areas of Special Scientific Interest. At the northeast end of the Devonian outcrop the base of the Fintona Group rests unconformably on Lower Silurian graptolitic mudstones in the Pomeroy inlier. The actual plane of unconformity is concealed. Elsewhere the base of the sequence is faulted against surrounding Carboniferous rocks and the Tyrone Igneous Complex. According to Simon (1984) the basal conglomerate is fine-grained and the matrix supports small pebbles up to 13cm diamter of vein quartz, quartzite, biotite granite, tuff, jasper and mica schist. Fearnsides et al. (1907) also observed pebbles of chert, hornblende granite and fossiliferous Lower Palaeozoic sedimentary rocks. The thickness of the Shanmaghery Sandstone Formation is estimated at 600m and consists of interbedded purple-brown and red and green stained calcareous sandstones, siltstones and mudstones. Numerous small exposures occur on the steep slopes forming the northeast faces of Cornamaddy (H6968) and Shanmaghry (H7168) southwest and south of the southern tip of the Pomeroy inlier. The sandstones are coarse-grained, micaceous and parallel laminated and become finer-grained towards the top of the sequence. Thin beds of cross-laminated and ripple laminated sandstones with occasional ripple marked and mudcracked mudstones also occur. The Shanmaghery Formation is abruptly overlain by coarse conglomerates and contemporaneous flow-banded lavas of the Gortfinbar Conglomerate Formation. The conglomerates contain predominantly volcanic clasts up to 40cm in diameter and rare pebbles of vein quartz, quartzite and greywacke. The sequence becomes coarser grained when traced towards the lavas which probably were the source of the volcanic clasts. It is likely that this coarse-grained predominantly volcanic debris accumulated on alluvial fans, perhaps comparable to those that flank the early Devonian lavas in the Ochil-Sidlaw Hills, in the Midland Valley of Scotland. Palaeocurrent data from cross- stratified sandstones interbedded with the conglomerates indicate that both the coarse and fine sediments of the Gortfinbar Formation were largely derived from the east (Simon, 1984). The conglomerates, sandstones and lavas are estimated to be about 2500m thick. At the top of the Fintona Group, south of the Tempo-Sixmilecross Fault, the Gortfinbar Formation fines upwards to fine-grained sandstones of the Raveagh Sandstone Formation. These consist mainly of red-brown very silty and muddy fine-grained sandstones with abundant ripple laminations and desiccation cracks. Because of the excellent exposures of the Barrack Hill Andesite Member of the Gortfinbar Conglomerate Formation in Cappagh Quarry and the importance of the lavas in providing the only date for any part of the Fintona Group, south of the Tempo-Sixmilecross Fault, it is important that the quarry should be designated as an Area of Special Scientific Interest.
Description:
For site specific information see; Key Site 320 - Cappagh Quarry.
Notes:
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:	Andesite, Conglomerate, Mudstone, Sand, Sandstone, Siltstone
Structures:	cross-bedding, dessication cracks, fault, ripple marks
Relations:	Extrusion
Paleoenv:	alluvial fan

References (5)
Fearnsides, W.G., Elles, G.L. & Smith, B., 1907. The Lower Palaeozoic rocks of Pomeroy. Proceedings of the Royal Irish Academy, vol. 26(B), pp.97-128.
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.
Rundle, C.C., 1978. Preliminary report on dating of samples from the Tyrone Igneous Complex, Northern Ireland. Institute of Geological Sciences Isotope Geology Unit Internal Report 78/3 (Unpublished Report).
Rundle, C.C., 1988. Potassium-Argon ages for andesitic lavas from the Cappagh Quarry, Co. Tyrone, Northern Ireland. Natural Environment Research Council Scientific Services Isotope Geology Centre, Radiogenic Isotopes Section Internal Report RI/88/2 (Unpublished Report).
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 (1979) 1:50,000 Sheet 34 (Pomeroy); O.S. 1:50,000 Sheet 19 (Armagh).
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Devonian - Fintona Block south of Tempo-Sixmilecross Fault, County Fermanagh, Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1152. Accessed on 2026-11-07.