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Earth Science Conservation Review
| Ballygawley Delta Complex - Fallaghearn Deltas | Tyrone |
| Site number: | 491 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H608628 Centroid | ||
| Google maps: | 54.51041,-7.0613 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Description: | |
The area around Fallaghearn, extending about 2km north from the southernmost sand and gravel limit, is characterised by 4 flat delta surfaces, dissected by 10-40m deep meltwater channels. The resulting isolated sand and gravel fragments are separated by steep-sided, deeply entrenched valleys and flat alluvial or peatland-dominated spreads. Delta surfaces occur between 233-235m (Dardis, 1982). Exposure of delta sediments is observed in a series of linked pits at Altagallagy (H608628). The Altagallagy pits, 6km northwest of Ballygawley, are a series of three large pits developed in three dissected delta fragments. Linked pits extend to 1.5km long and 400m wide, covering a total area of c. 0.6km2 (60 ha). There is major exposure in the west and east pits. The west pit has two main exposures. These are described separately but interpreted together. West Pit Exposure A - Facies description This exposure, 6m high and 42m long, shows poorly bedded (10-50cm thick) and sorted pebble to cobble-dominated (<20cm diameter) facies. Gravel facies are clast-supported with only a minor (10-40%) matrix component of granules and coarse sand. Beds are laterally discontinuous and are interbedded with discontinuous (to 6-8 m) beds of well-sorted and stratified medium- to fine-grained sand. Contacts between sand and gravel facies are sharp and erosional. Sand beds are often wedge-shaped with occasional arched bedding. Towards the flanks of the exposure, gravel foresets with sandy interbeds dip towards 020°. West Pit Exposure B - Facies description This exposure, consisting of two steep faces separated by a worked shelf, is c. 12m high and 30m long. This exposure shows laterally continuous interbedded facies of well-sorted sands and poorly sorted clast- and matrix-supported pebble gravel beds. Sand beds (10-50cm thick) are associated with erosional upper surfaces and laterally continuous interbedded stringers of sand and gravel facies. These form shallow-dipping (010°-015° to 020°-030°) foreset surfaces. Degree of interbedding increases upwards as facies become more laterally discontinuous. Texture of gravel-dominated facies also coarsens upwards. These upper gravel beds are matrix-supported (medium and coarse sand). Deep channelisation is not observed in these facies. West Pit - Facies interpretation These sediment sequences record periodic flow pulses into a subaqueous depocentre. Exposure B, which is approximately perpendicular to flow input direction, shows relatively flat bedding structures, vertically offset bed stacking, shallow channelling and reactivation surfaces. This is characteristic of a shallow water, mid-fan environment (Rust and Romanelli, 1975) dominated by processes regulated by the input of sediment pulses into the system. These processes include mass and grain flows which may have given rise to better-sorted facies downslope and shallow cross-bedding (cf. Exposure A). Sandy interbeds record periods of flow cessation. Wedge-shaped geometries are due to erosional accretion and reactivation. Charlesworth (1924, pp.273-275) interpreted these deposits as having accumulated in a series of glacial lakes with water flow from a retreating ice margin to the north, and ponded by end moraines to the south. Sedimentary and morphological evidence show that this interpretation is invalid. Dardis (1982, pp.312-324) interpreted the Altagallagy site as a Gilbert-type delta composed of an upper boulder gravel unit overlying a steeply dipping crossbedded (to 35m deep) unit dominated by avalanche face processes. This was deposited from an ice margin in the Todd's Leap channel (H603612) with water flow towards the northeast. The water body was held up to the north by an unidentified mechanism. However, no such sediment facies or a steep delta front are observed, and moraines form only a minor component of the system. Therefore, contrary to Dardis (1982), a Gilbert-type delta is not identified. The east pit has one main exposure. East Pit Exposure A - Facies description This exposure, about 4m high and 140m long, is characterised by laterally continuous (to 10m) and stacked beds (5-20cm thick) of well-sorted and stratified fine- to medium-grained sand with discontinuous pods and interbeds of matrix-supported granules and pebbles (<15cm diameter). Interbedding complexity, erosional bed contacts and the gravel fraction component increase to the right hand side of the exposure where degree of sediment sorting decreases. Stacked sand beds dip 010° to 055°. There is little vertical or lateral variation in sand bed texture or sorting. Individual beds are planar, very occasionally backset rippled, and associated with discontinuous granule stringers along bedding surfaces. Pebbles also occur as (1) isolated pods between sand units, (2) individual clasts entombed within sand units, and (3) wedge-shaped layers which increase in frequency towards the top and right hand side of the exposure. Wedges may be stratified or massive, inversely or normally graded, may have a basal lag surface and are mostly matrix-supported. East Pit - Facies interpretation This exposure records the subaqueous deposition of distal sediment sequences dominated by density and instability-driven processes, such as grainflow and massflow (Martini, 1990). The stacked facies sequence from right to left reflects a decreasing-energy environment, i.e. a more distal depositional setting. This is evidenced by an upsequence decrease in grain size and frequency of erosional surfaces, and an increase in sorting. However, this exposure is stratigraphically below those described from Altagallagy west pit. This exposure therefore describes the type of depositional environment that existed before major delta progradation. A likely erosional surface between these (toeset) beds and the overlying delta foresets is not observed at this site. Lack of channelisation and the low angle of sand beds infers that deposition was not in shallow water and that sand sedimentation and resedimentation by a moving grainflow carpet (Nemec, 1990) was not affected by seasonal changes in sediment input characteristics. This moving basal layer may also have led to the bimodal grain size distribution of sediment contained within inter-sand layer gravel pods. In more proximal locations, sedimentation processes were affected by periodic sediment pulses. Massflows resulted in graded gravel lenses, often with erosional or lagged basal surfaces. This exposure shows that a water body was in existence at this site prior to delta progradation, and that the delta front probably advanced between 1.0-1.5km from the likely ice margin to the south. Fine-grained 'toeset' beds have not been described from this area by either Charlesworth (1924) or Dardis (1982). | |
| Notes: | |
For site specific information on other sites within the Ballygawley Delta Complex see the following sites: Key Site 492 - Ballygawley Delta Complex - Lurganboy Delta and Outwash Key Site 493 - Ballygawley Delta Complex - Tiroony Ridges For overview information and references on the Ballygawley Delta Complex see the following site: Key Site 483 - Ballygawley Delta Complex - Overview For general information and references on the sand and gravel landscapes of N. Ireland, and for directions to other regional sand and gravel sites, see the following site: Key Site 439 - Introduction to the Sand and Gravel Landscapes of N. Ireland This site is also a Site of Local Nature Conservation Importance. See Key Site 727 - Fallaghearn. | |
| Keywords | |
|---|---|
| Relations: | Deltaic sediments, Glacial deposit |
| Geomorph: | deltas |
| Paleoenv: | delta |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 23 Dec 2003 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Ballygawley Delta Complex - Fallaghearn Deltas, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=491. Accessed on 2026-11-05. |
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