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Earth Science Conservation Review
| Murrins Complex - Cashel Outwash | Tyrone |
| Site number: | 509 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H562788,H581821 End(s) of linear site, Not Entered | ||
| Google maps: | 54.65465,-7.12925 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Description: | |
This area extends from the northeastern-most frontal moraine system at Murrinmaguiggan (H562788) to the major point of dissection near Cashel Bridge (H581821). This area is 4km long and up to 3.5km wide. It is constrained laterally by the uplands of Mulderg and Loughmacrory Hill (H590773) to the west and east respectively, and Binnafreaghan and Cashel Rock (H600809) to the north. These bedrock areas acted as points against which sand and gravel surfaces could accumulate. The outwash surface is situated at 216-220m a.s.l. (Dardis, 1982, 1986, McCabe et al., 1992). There are several exposures at different ends of the outwash system: two mainly disused sand and gravel pits near Cashel Bridge, and one large pit at Murrinmaguiggan. These are now examined. The Cashel Bridge pits offer panoramas of well-sorted and bedded sand and gravel facies in the north and south pits. These pits are described separately but interpreted together. North pit. Facies description This exposure (<16m high and 200m long) is characterised by thick sequences of interbedded well-sorted and laterally continuous massive and stratified sand, and clast and matrix-supported pebble gravels. Facies are generally arranged in tabular or cross-bedded sets separated by diffuse and conformable bed contacts. Gravel beds dominate the lower part of the section. These are generally composed of massive and laterally continuous interbedded units (0.4-1.0m thick) of (1) clast supported, occasionally openwork, sorted pebbles (<10cm diameter), and (2) granule and pebble (<4cm diameter) beds supported by a coarse sandy matrix (20%). These units often grade laterally and are separated vertically by diffuse and undulatory bedding contacts. These units are also interbedded with discontinuous (to 6m) massive and stratified well-sorted sandy pods or large stringers. These sand units show sharp and undulatory upper and basal contacts. The upper part of the section comprises a cap of massive and stratified medium and coarse-grained sand beds (0.4-0.6m thick). Stratification generally increases upwards. Units are interbedded with thinner (dm scale) and discontinuous (3-4m) granule layers. The exposure also shows broad cross-bedding (<6m high) composed of alternating gravel and sand beds. Cross-beds dip 30§ to about 350§. South pit. Facies description This exposure (<12m high and 170m long) is characterised by massive, stratified and channelized beds (0.3-1.5m thick) of clast and matrix-supported pebbles and cobbles (<40cm diameter), interbedded with and capped by well-sorted medium and fine-grained sand. Sand and gravel facies are separated by sharply erosional and conformable contacts. The exposure can be divided into three parts on the basis of facies characteristics such as interbedding. (1) The left side of the exposure is composed of laterally discontinuous (to 4m) interbeds (30-50cm thick) of massive, matrix-supported granules and pebbles (<4cm diameter). Interbeds are inversely-graded, occasionally planar stratified and cross-bedded. Dip direction is generally between 015-030§ but shows high variability. Gravel units are interbedded with discontinuous pods of massive, well-sorted sand. Towards the top of the exposure sand facies are more extensive and dominant. These facies are massive to vaguely-stratified and alternate with thin (few cm) silty interbeds. (2) The central part of the exposure is composed of a dominant lower gravel component (95% of the exposure) and an upper sand component. Gravel facies are composed of discontinuous (to 4m) clast and matrix-supported lenses of granule to cobble-sized (<40cm) clasts. Facies generally fine-up and are interbedded with well-sorted and stratified coarse sand stringers and lenses. The sand component is separated from the gravel by a sharp conformable contact. Sand facies are medium to fine-grained and well-sorted. Beds are massive, occasionally stratified and normally graded, and form tabular, stacked sets, sometimes with a drape (cm scale) of very fine sand and silt. Facies generally dip towards 060§. (3) The right hand side of the exposure shows more complex interbedding relationships associated with a deep (10m) and wide (20m) channel (axis alignment 340-160§). Channelized facies are stacked and do not show lateral variations in bed geometry. These channels are therefore aggradational rather than erosional, and the channel infill is conformable to channel geometry. The channel is incised into well-sorted gravel units (1.5-2.0m thick) interbedded with discontinuous (to 5m) sand stringers, as in the central part of the exposure. The channel infill is dominated by alternating beds (0.4-0.6m thick) of well-sorted massive granules (<2cm diameter) and massive and stratified well-sorted medium-grained sand beds. Facies interpretation These exposures record the deposition of distal sediments in a subaquatic environment subject to little variability in energy and sediment input. Inferred processes of deposition include mass and debris flows and suspension settling through the water column (Rust and Romanelli, 1975). This depositional setting is evidenced by (1) lateral continuity and tabular geometry of individual facies, (2) facies stacking, (3) shallow erosional scours, and (4) the dominance of conformable bed contacts. Local shallow scour and fill promote facies interbedding. Scours may have been formed by high-density turbid mass and debris flows across the subaqueous sediment apron (Rust and Romanelli, 1975). These flows may also have promoted sediment size-sorting, inverse grading and outsized clasts (Hampton, 1975). Proglacial slopes may have encouraged sediment instability, and massflow generation downslope with sediment reworking, the lifting of fine material into suspension and basal scouring (Rust and Romanelli, 1975). Sand beds and fine sand and silt interbeds are laterally continuous and show no deformation, dropped clasts or ripples. These beds were deposited by low density grainflow processes and the settling out of fine-grained material through the water column during periods of flow cessation. Variations in cross-bed height in these exposures record changes in water depth related to the position of the southward ice margin. The depth of the overall sediment succession also reflects the likely depth of the water body. The Cashel Bridge exposures lie within the Owenreagh River valley which served as a major sediment depocentre (Charlesworth, 1924). The marked change upsequence in both exposures from gravel to sand facies may be related to a decrease in sediment and energy input to the system, resulting from a fall in water levels and local ponding. However, there is no evidence in these exposures for cold-climate conditions, i.e. sand wedges. The Murrinmaguiggan pit is a large, partly worked sand and gravel pit. The pit is extensive (800m long and covering 25ha) and presently contains a cement plant. There are four exposures of interest. These are described separately but interpreted together. Exposure A. Facies description This exposure (<7m high and 50m long) is characterised by clast and matrix-supported 'dirty' pebble to boulder-sized clasts (<45cm diameter) organised in vaguely stratified beds (1.5-2.0m thick). These are interbedded at the base of the exposure with discontinuous (to 5m) medium to coarse sand beds (30-50cm thick). The exposure is cut by a large channel (35-40m wide and 6m deep) infilled conformably with chaotic, stratified and cross-bedded facies, and cut by a number of other minor channels. Gravel beds are heterogeneous, occasionally inversely-graded and dominated by large, subangular clasts, often forming vague lines. These lines demarcate bed boundaries and generally overlie the more massive granule beds. Gravel beds are associated with small-scale sigmoidally cross-bedded (cm scale) granules, and have a minor matrix (20%) component of medium-grained sand. Exposure B. Facies description This exposure (<36m long and 4m high) is characterised by contorted and interbedded sand, gravel and silt facies. Four lithofacies associations (LFAs) are recognised. These are separated by localised erosional unconformities. LFA 1 (10% of the exposure) is composed of thin (cm scale), discontinuous (to 1m) alternating beds of sorted granules and medium-grained sand. This association has a sharp and contorted upper contact. LFA 2 (30% of the exposure) is composed of a conformable sequence of laterally continuous (to 6-8m) massive beds (30-50cm thick) of well-sorted medium-grained sand to silt, and has a sharp erosional upper contact. Facies are occasionally separated by sorted granule interbeds (<10cm thick). LFA 3 (30% of the exposure) is composed of chaotic and vaguely-bedded units (0.2-1.5m thick) of massive, clast and matrix-supported cobbles (<20cm diameter) interbedded with massive and continuous (to 3m) medium-grained sand and silt beds (dm scale), and contorted stringers and pods of sorted sand and granules (cm scale). This association is channelized, corresponding to a flow direction towards 280§, and has a diffuse and conformable upper contact. LFA 4 (30% of the exposure) is composed of vaguely-bedded (cm scale) and laterally continuous (to 10m) well-sorted granule layers. These are occasionally interbedded with massive, discontinuous lenses (dm scale) of well-sorted medium-grained sand to silt. Exposure C. Facies description This exposure (<24m long and 4m high) is composed of well-sorted and stratified medium to fine-grained sand beds (20-40cm thick) interbedded with discontinuous (dm scale) sorted granule pods. The base of the exposure comprises a massive, monospecific unit (60cm thick) of sorted granules and occasional outsized cobbles and boulders (<40cm diameter). These clasts are subangular TIC volcanics of local derivation (and are the same lithology as rock rafts found elsewhere in this pit; discussed later). Sand beds are continuous across the exposure, are stratified and occasionally capped by silt beds (cm scale). Low angle cross-bedding records palaeoflow to between 340-000§. An outsized TIC clast (<30cm diameter) occurs within the cross-bedded unit. Bed contacts are conformable or undulatory; a shallow scour (5m wide, 1m deep) occurs within the sand facies in the middle of the exposure. Exposure D. Facies description This exposure (42m long and 6m high) comprises a stacked sequence of well-sorted sand and gravel sediments which lie above and below an included rock raft which extends across most of the section. Vertically this exposure is comprised of three main units: a lower sand unit, a middle rock rafted unit, and an upper gravel unit. (1) The lower unit (40% of the exposure) is composed of well-sorted massive and stratified medium-grained sand in discontinuous (to 3m) pods and contorted stringers or shallow channels (<30cm thick). These facies are interbedded with clast and matrix-supported granules and pebbles (<6cm diameter) which form local chaotic or crudely-bedded infills (20cm thick). The right side of this exposure contains a large lens (2m deep and 7m across) of well-sorted medium to coarse-grained sand beds (0.5-1cm thick) with occasional granules. Beds are continuous across the lens and dip 035§ to 130§. The upper surface of this unit is sharp and undulatory and subjacent beds are undeformed. (2) The middle unit (20-30% of the exposure) is composed of two rock rafted bedrock slabs (<1.7m thick and 22m long, and 1.5m thick and 7m long) overlying an erosional contact that extends across the exposure. These rafts are composed of local TIC volcanics, and are heavily jointed. Rafts are generally tabular in geometry, with a sharply-erosional planar upper surface and sharply-erosional undulatory basal surface, and are associated with a thin (cm scale) aureole of well-sorted medium and fine-grained sand. Elsewhere in this pit, rafts of varying sizes (0.2-1.0m across) are entombed within a massive, clay matrix-dominant (85%) diamict with subrounded and striated pebbles and cobbles (<20cm diameter). (3) The upper unit (30-40% of the exposure) is composed of crudely-channelized and inversely-graded clast-supported granules and pebbles (<8cm diameter). Channels are 3m across and 1m deep. Gravel units are interbedded with discontinuous (to 1m) conformable lenses of well-sorted medium-grained sand and massive granule beds (<20cm thick). Facies interpretation These exposures demonstrate the range of different depositional processes associated with a changing ice-marginal glaciolacustrine environment (cf. Rust and Romanelli, 1975). In the Murrinmaguiggan pit, type of depositional environment is strongly controlled by the relationship of (1) sediments and (2) sediment geometry and structures to the inferred location of the ice margin, in particular changing energy and water levels. Exposures are characterised by interbedded, discontinuous and eroded gravel, sand and silt units. In addition, individual facies are well-sorted, clasts are subrounded and facies do not display significant lateral variability in sorting or texture. These characteristics indicate that sedimentation was episodic and was punctuated by short-lived erosional events in which the proglacial or ice-marginal zone changed morphology in response to changes in the energy and sediment input system. Changes in these characteristics may be related to ice-marginal oscillations, i.e. that the sediment record can be used as a proxy of ice activity. Variations in energy and sediment input also changed depositional processes. This site does not show evidence for extensive braided outwash environments (cf. Boothroyd and Ashley, 1975), instead favouring deposition into an impounded water body. Rapid changes in energy promote facies variability and interbedding (Rust and Romanelli, 1975). Processes of sediment emplacement include high density massflows, traction bedload and suspensional fallout. The importance of erosional events mean that reworking and resedimentation were probably the dominant processes, at least in the early stages of sedimentation after each erosional event. Sediment size-sorting infers (1) a relatively distal depositional setting, and (2) that material may have been sorted in water elsewhere in the sediment transport system. This may support Dardis's (1982) hypothesis that material was conveyed to the ice margin by subglacial eskers from the southwest (cf. Rust and Romanelli, 1975). Rock rafts preserved within the Murrinmaguiggan pit are evidence for subglacial deposition during ice readvance from the north. Lack of evidence for subjacent sediment deposition supports the concept of rock raft emplacement by low angle glaciotectonic shearing at the ice margin, i.e. by 'thin skin' tectonics (Croot, 1987). A sediment aureole around the rock raft may have acted as a shear plane for raft decollement from the subglacial zone (Croot, 1987). Lack of other sediments associated with the subglacial transport zone, i.e. melt-out till (cf. Dardis, 1982; Paul and Eyles, 1990), may indicate relatively clean ice, and decollement by thin skin tectonics (Croot, 1987). The stratigraphic position of the rock raft location directly above a sand and gravel zone with south to north directional indicators infers that two ice margins interacted in this area. These margins may either relate to different ice masses, or reflect individual lobes of the same ice mass. The location and significance of other rock rafts in this area are discussed later. | |
| Notes: | |
For site specific information on other sites within the Murrins Complex see the following sites: Key Site 508 - Murrins Complex - Glensawisk Burn Delta Key Site 510 - Murrins Complex - Lough Fingrean Frontal Moraines Key Site 511 - Murrins Complex - Laght Hill Retreat Moraines Key Site 512 - Murrins Complex - Coolesker/Beragh Eskers For overview information and references on the Murrins Complex see the following site: Key Site 479 - Murrins 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 710 - Cashel. | |
| Keywords | |
|---|---|
| Rocks: | Gravel, Sand |
| Geomorph: | outwash |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Murrins Complex - Cashel Outwash, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=509. Accessed on 2026-11-06. |
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