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Earth Science Conservation Review
| Tempo Valley Delta Complex - Glengesh and Tattinweer | Tyrone |
| Site number: | 513 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure, Pit | Status: | TBC |
| Grid Reference: | H366494 End(s) of linear site | ||
| Google maps: | 54.39236,-7.43655 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Description: | |
This area, about 6km long and 1.5km wide, extends from the head of the valley at Glengesh to Tattinweer (H366494). The upper sand and gravel limit declines downvalley from c. 180m elevation to c. 90m elevation between these two locations. A number of flat surfaces occur, notably at Dooneenroe (H326526), Tonyglaskan Bridge (H377518) (both c. 150m) and Imeroo (H378511) (c. 120m). Downvalley (to the south) and away from the bedrock uplands (to the east), these surfaces become less distinct, more dissected and the limit of sand and gravel less easily defined in the landscape. These sediments have two morphological forms. (1) Isolated linear hills are 200-500m long, 100-300m wide and 3-12m high. These dominate in the central part of the valley such as at Tonyglaskan and Imeroo. (2) Flat outwash spreads which underlie postglacial peat. These sediments have ill-defined limits, probably onlap bedrock and dominate the eastern part of the valley, such as at Brockagh (H392522). Two exposures are recorded in isolated, dissected hill morphologies (Type 1) in pits at Dooneen and Tonyglaskan. A small exposure in Type 2 morphologies near Brockagh (H391524) shows massive, well-sorted medium-grained sand. No further sedimentary structures are identified in this exposure. DOONEEN PIT The Dooneen pit is a largely disused sand pit covering an area of c. 1ha. Two small exposures (<6m high, 16m long) occur and are described and interpreted together. Facies description Exposure A This exposure, 4m high and 16m long, is characterised by stacked beds of well-sorted massive and cross-bedded fine- and medium-grained sand. Beds are laterally continuous and separated by diffuse and sharp planar contacts. Erosional surfaces are generally absent. Two features are of particular interest. (1) Concretions are associated with one sand facies. They occur along bedding planes and between bedded units within the Sm facies. They are aligned bed-parallel, are nodular, have dimensions of up to 0.5 x 6 x 10cm and generally form a discontinuous three-dimensional network along bedding surfaces. Concretions are composed of cemented sand of the same texture, or slightly finer, than that of the surrounding sand facies (medium-grained sand). (2) Mud curls occur in cross-bedded medium-grained sand facies (Ss) immediately overlying the concretion bed. Curls are up to 3 x 6 x 10cm and are contorted about both the a and b axes. Thicker curls are composed of fine sand and thinner curls of silt and clay. Silt and clay curls show the greatest deformation. Within the Ss facies, curls occur at the boundaries between individual crossbeds, especially towards the base of the facies. Curl rolling direction is always transverse to the inferred direction of deposition. Curls are therefore usually, but not always, a axis transverse. Exposure B This exposure, up to 6m high and 25m long, is characterised by stacked beds of well-sorted massive, rippled and cross-bedded medium-grained sand. Beds are 0.4-1.0m thick, laterally continuous, and separated by sharp conformable contacts. Erosionally scoured surfaces are uncommon and are not characterised by gravel lags. Towards the top of the exposure, beds are thinner (0.2-0.4 m), directional indicators (i.e. ripples and cross-beds) more apparent and variable in morphology, and beds are dissected into broad channels 0.8m deep and 3m wide infilled by undifferentiated medium-grained sand. Flow indicators are generally unidirectional and show palaeoflow towards 040§. Facies interpretation These exposures show sediment sequences characteristic of proglacial deposition in an impounded water body, dominated by grainflow processes, and sediment derived and reworked from the ice margin in shallow subaqueous channels. These sequences record almost continuous aggradation of well-sorted sediments. Lack of sediment rhythmicity and lack of textural variation infers that deposition was largely unaffected by season cycles of sediment or meltwater input. No clay or silt drapes are recorded. This means that (1) the impounded water body was not thermally stratified during the winter, or (2) a seasonal ice cover or low density water cap was not present, or (3) active (perennial) water outflow ensured the mixing of deeper waters. Option (3) may be the most likely, given the dominance of (inferred) grainflow and mass flow/resedimentation processes, subaqueous channelization, and evidence elsewhere for outflow across the watershed. Particular characteristics of the two exposures are now discussed. Exposure A Concretion formation is associated with the migration of water along a pressure gradient. In a proximal glacial setting, groundwater can migrate ahead of the freezing front when it moves during autumn and spring between quasi-stable summer and winter positions (cf. McRoberts and Morgenstern, 1975). Further, subglacial groundwater can also be expelled along permeable subglacial aquifers (Boulton et al., 1995). Water flow through permeable sediments may be enhanced where conditions are glacioterrestrial, i.e. not associated with a water body, and where the sediments are unfrozen (McRoberts and Morgenstern, 1975). Drainage of an impounded marginal water body will create glacioterrestrial conditions, and may also indicate a decrease in ice activity and unfrozen ground. Reactivation of the freezing front, during a period of ice advance or ice marginal oscillation, would cause the reactivation of subglacial groundwater flow and the start of groundwater migration ahead of the ice margin (Boulton et al., 1995). This may lead to groundwater flow through pre-existing discontinuities, i.e. bedding planes, in the surrounding sediments (Carter et al., 1977). However, this scenario is hindered by four problems. (a) There is no equivocal evidence for the location of the ice margin. This can be only inferred from the limits of distribution of sand and gravel. (b) No other concretions are recorded elsewhere in the exposure, but occur in a similar facies type and palaeoenvironmental setting in the Pubble pit south of Tempo (see Key Site 514). (c) There is no evidence at this site for a readvance of the ice margin or the development of later cold-climate conditions. However, sand wedges are recorded elsewhere in the Tempo Valley complex (cf. McCabe, 1969) (at the Pubble exposure). (d) No distinctive marginal moraine is seen and there is no evidence for glaciotectonic shortening by ice readvance. Mud curls, or rip-up clasts have not often been recorded from glacigenic environments. Several problems are involved in the interpretation of mud curls in this exposure. These are (a) their origin, (b) mechanism of deformation, (c) mechanism of transport and (d) mechanism of deposition. (a) Origin Mud curls, or rip-up clasts are transported blocks of allochthonous fine-grained material deposited and preserved in the lithostratigraphic record. Because preservation potential is low, they have to be derived from an adjacent (upstream) sediment source. Disaggregation of these sediment clasts occurs mainly by water erosion during transit. (b) Deformation The mud curls show a complex history of deformation. They are rolled in a simple fashion, compressed and contorted. They also show multiple phases of redeformation in different styles and with different directional characteristics. (c) Transport Curl preservation has several implications. Curls may have been transported while frozen, which would increase the preservation potential, but this is incompatible with their soft sediment style of deformation. Also no dewatering structures or post-depositional deformation associated with water escape is seen. A short period of transport would decrease the time period available for erosion, but a proximal source for the mud curls has not been identified. (d) Deposition Mud curl association as an intraformational part of a cross-bedded medium-grained sand facies (Ss) assumes contemporaneity. These different sized sediments have to be accommodated by the velocity of the depositing water flow. The cross-beds identify a unidirectional flow component and curls are characterised by an a(t) (a axis transverse) curl fabric which may indicate traction current transport. However, they are not water-rounded, disaggregated or armoured, all of which might occur if they were transported in a 'soft sediment' state and rolled down a prograding cross-bedded foreslope. The mechanism of deformation and emplacement proposed for these mud curls is as follows. An active ice margin to the south of the Dooneen exposure built up a variety of sediment facies. These probably included interbedded sands, gravels and muds, perhaps associated with moraine emplacement. Active ice marginal processes (moraine building) and mass flow processes may have contributed to mud deformation. Fine-grained sediments, with generally low permeability, would have a higher preservation potential than surrounding coarser sediments. These finer sediment fragments were then reworked into proglacial outwash in which any marginal moraine was destroyed. Exposure B This exposure records stacked facies of massive, cross-bedded and channelized well-sorted medium-grained sand. There is no vertical or lateral grading into other sediment fractions, no changes in facies geometry or evidence for syn-sedimentary deformation, ice meltout or ice margin readvance. These facies are interpreted to result from continuous sedimentation (and resedimentation) by grain- and mass-flow processes. These sediment characteristics require a stable depositional environment and uniform rate of sediment input. An impounded water body supported by a static ice margin with seasonally-unvarying sediment (and meltwater) inputs may be characteristic of warm-based stagnant ice in the surrounding area. That no topography traditionally regarded as resulting from stagnant ice (cf. Kurimo, 1980) is seen in the Tempo Valley today might indicate that the contribution of supraglacial debris was not present and that sediment was mainly derived from the subglacial zone by meltwater reworking. This reworking is evidenced by the mature (unimodally sorted) sediment texture. A lack of supraglacial debris may also indicate that frost-shattered material was not transferred from upland source areas, such as the Fintona Hills, to the ice surface, and that shearing-up of basal material associated with ice-marginal activity was not important in this area. These inferences may also provide evidence for a warm (stagnating ice) climate with meltwater (possibly channelized) expelled from the subglacial zone into shallow marginal lakes. TONYGLASKAN PIT The Tonyglaskan pit, an active sand and gravel pit covering 0.5ha, shows well-exposed faces up to 12m high and 60m long. There are three main exposures (A, B and C). Exposure A Facies description This exposure, 50m long and 12m high, is characterised by interbedded sand and gravel facies in beds 0.2-1.5m thick. Individual beds are highly variable in texture and are composed of well-sorted and normally- to inversely-graded tabular and cross-bedded clast- and matrix-supported gravel units; heterogeneous channelized or lensate fills of chaotic clast- and matrix-supported cobbles and boulders (<60cm diameter); and laterally-discontinuous (<3m) well-sorted massive and stratified medium- and fine-grained sand interbeds. These facies form channelized laterally extensive tabular beds with sharp and graded conformable contacts. Overall geometry shows that individual gravel facies form a stacked interbedded sequence separated by laterally continuous erosionally scoured surfaces with occasional channelized scour and fill structures. Facies are conformable with no erosional lags present. Chaotic channelized boulder fills pinch and swell in geometry. Gravel foresets dip 030§ to 170-190§. There is a rapid west-east transition from gravel- to sand-dominated facies. Facies interpretation This exposure records the deposition in an impounded water body of slope deposits by mass- and gravity-flow processes, and high-energy marginal meltwater activity, forming a delta with a prograding foreslope. Facies show directional indicators recording a sediment source to the west and northwest associated with meltwater flow from the higher ground of the Fintona Hills. High-density chaotic and channelized boulders are characteristic of high energy meltwater flows into a subaqueous depocentre (cf. Powell, 1990). Variations in the location and magnitude of sediment input, and flow density and composition resulted in multiple phases of erosion, bed stacking and resedimentation. Finer-grained sandy interbeds between gravel units record periods of flow cessation and rainout of material through the water column. General depositional direction (from 350-010§) contrasts with that of the Dooneen exposure which is at a topographically lower level. If water levels fell over time by ice retreat, this indicates that the Tonyglaskan delta antedates sediment deposition at Dooneen. Further, it shows that sediment input direction changed, probably by the withdrawal of the ice margin (forming the Tonyglaskan delta) westward into the higher ground (Charlesworth, 1924), and the southward withdrawal of the Tempo/Clogher Valley ice from around Glengesh to south of Dooneen. Exposure B Facies description This exposure is characterised by laterally continuous and stacked interbeds of massive sand and gravel. Sediments are well-sorted and separated by sharp planar and erosional contacts. Sand facies are dominant. These facies consist of laminated beds 1.5 to 2m thick of massive medium- and fine-grained sand. These facies also contain dispersed pebbles (<15cm diameter) of limestone, ORS and schist in discontinuous planar intrasand beds or as isolated pods 0.3m deep by 1m wide supported by a sand matrix. Pebble frequency increases laterally towards the right hand side of the exposure, with an increase in interbedding complexity. Gravel facies are characterised by bedded sequences 0.5 to 1m thick of well-sorted, normally-graded and clast supported granules and pebbles (<6cm diameter). Occasionally these facies are interbedded with more chaotic matrix- and clast supported pebble beds which form discontinuous and undulatory pods 1.5m wide by 0.4m deep. Facies interpretation This exposure records a sequence of subaqueous deposition with variable energy and sediment inputs. Characteristics of the main sand facies (geometry, continuity, thickness, sorting), and no depositional cyclicity, require a low energy standing water body with little variation in energy input. Pebble sized clasts within this facies may have resulted from aperiodic flood events or clast rafting. Winnowing may also have resulted in clast concentration, crude layering and interbedding, especially at the right hand side of the exposure. Gravel facies overlie a sharp undulatory erosional contact with occasional shallow scours. Gravel beds record periodic discharge pulses into the sediment depocentre. Winnowing may also have occurred during sediment transport, with the matrix component settling through the water column during flow cessation. Exposure C Facies description This exposure, up to 4m high and 20m long, is characterised by rhythmically bedded very fine sand and silt beds (15-20cm thick) associated with discontinuous silt interbeds and occasional pebble lags. This sand facies is erosionally overlain by vaguely bedded units 30-40cm thick of matrix-supported pebbles and cobbles (<30cm diameter). Rhythmically bedded packages are slightly normally graded (very fine sand to silt) and are separated by diffuse and undulatory contacts. The rhythmic packages show no ripple or current structures and no lateral variations in thickness, grading or texture. The gravel facies contains both chaotic and sorted components. An area of scour and fill, up to 0.3m deep and 1m across is characterised by an infill of chaotically bedded pebbles and cobbles (<25cm diameter) supported in a granule and coarse sand matrix. This fill grades laterally over a distance of 3m into stacked sequences (0.25-0.3m thick) of better sorted, normally-graded granule and pebble (<8cm diameter) beds. These beds are separated by diffuse, amalgamated contacts. Facies interpretation This exposure records the deposition of well-sorted sediments into a standing water body. Rhythmically-bedded sands record subaqueous sediment pulses. These pulses may be associated with seasonal variations in meltwater discharge, and winter thermal stratification of the ponded depocentre. Lack of current structures (ripples, laminae) may indicate a dominance by subaqueous grainflow processes, or that there was little bottom current activity. Chaotic, discontinuous gravel beds record periodic high-energy sediment pulses. Flow events are characterised by a chaotic boulder facies which overlies an erosional, undulatory basal contact. Chaotic beds are succeeded laterally and vertically by smaller, better sorted clasts showing crude stratification. This is characteristic of subaqueous debris flow sequences (cf. Hampton, 1972). | |
| Notes: | |
For site specific information on other sites within the Tempo Valley Delta Complex see the following sites: Key Site 514 - Tempo Valley Delta Complex - Tempo and Pubble Key Site 515 - Tempo Valley Delta Complex - Lough Eyes Key Site 516 - Tempo Valley Delta Complex - Upland areas of the Fintona Hills For overview information and references on the Tempo Valley Delta Complex see the following site: Key Site 485 - Tempo Valley 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 | |
| Keywords | |
|---|---|
| Rocks: | Gravel, Sand |
| Geomorph: | outwash |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Tempo Valley Delta Complex - Glengesh and Tattinweer, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=513. Accessed on 2026-11-07. |
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