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Earth Science Conservation Review
| Orlock Bridge – Lower Palaeozoic | Down |
| Site number: | 1353 | ||
| Locality Type: | Coastal Section, Foreshore | Status: | ASSI |
| Grid Reference: | J564829 | ||
| Google maps: | 54.67035,-5.57568 | ||
| County: | Down | Council area: | Ards & North Down Borough Council |
| Period: | Ordovician, Silurian | ||
| Site Description | |
|---|---|
| Highlights: | |
Coastal outcrop below and east of Orlock Bridge, on the Bangor to Donaghadee coast road, provides the most spectacular example of fault-associated deformation in Northern Ireland. The most intense deformation is seen on the floor of a steep-sided 20m wide gully at J56488301, where it is possible to examine and collect breccia and mylonite generated from the surrounding greywackes and shales by the faulting processes. North and south of the gully a coastal path allows easy access to a belt of deformation which is over 100 metres across. | |
| Introduction: | |
Orlock Bridge lies close to the north-western margin of a very large area of Ordovician and Silurian rocks, comprising virtually all of the Southern Uplands of Scotland and a large triangular continuation in Ireland with the counties of Down, Longford and Louth at its apices. This area, shaded black on the small inset map on Figure 1, is the Southern Uplands-Down-Longford Terrane, a highly distinctive suspect terrane in the collage of terranes assembled through the closure of the Iapetus Ocean at the end of the Silurian. The terrane is dominantly composed of well bedded greywacke sandstones and shales. At some localities these can be seen to rest on a pelagic succession of graptolite-bearing black mudstones, cherts and spilitic basalts. All of these are strongly folded and faulted, so that characteristically the bedding dips steeply, with sedimentary structures commonly demonstrating inversion. During deformation the rocks have been weakly metamorphosed, typically to prehnite-pumpellyite grade. | |
| Description: | |
Peach and Horne (1899), in their classic Memoir on the Lower Palaeozoic rocks of Scotland, divided the Southern Uplands into Northern, Central and Southern Belts and although their model of Southern Upland structure has since undergone radical revision (Craig and Walton 1959) and modern reinterpretation (Mitchell and McKerrow 1975, McKerrow, Leggett and Eales 1977), the original tripartite division remains intact and is in current use, not only in the Southern Uplands but also in its Irish continuation. The definitive account of the Northern and Central Belts occurs in the penultimate paragraph of page 37 of the Memoir (Peach and Horne 1899): "The Central Belt, from twenty to twenty-five miles in width, composed mainly of Llandovery and Tarannon Rocks (b4), stretches from the Mull of Galloway north-eastwards by Castle-Douglas and Moffat to Melrose and thence to St.Abb Head. The Northern Belt, occupied by Arenig, Llandeilo and Caradoc Rocks (b1, b2, b3), extends from the northern slope of the Lammermuir Hills south-westwards, through the Moorfoot and Leadhills by Sanquhar to Loch Ryan and Portpatrick." The Northern Belt is thus exclusively Ordovician, the Central Belt mainly Silurian. The location of the boundary between the two was not discussed in detail by Peach and Horne, save by reference to the Memoir's Index Map. Clearly the definitions above imply that the boundary is the northern limit of Silurian strata and, since Southern Upland turbidites are rarely fossiliferous, this simply means that the boundary must be drawn north of all outcrops of graptolite-bearing Silurian mudstone and shale. This obvious freedom has allowed the boundary to be variously portrayed in maps and sections over the last 80 years in forms consistent with the contemporary concept of Southern Upland structure. The boundary between the two belts crosses the Irish coast at Orlock Bridge where it was recognised as a fault and named the Orlock Bridge Fault by Anderson (1962), who saw it as a steeply inclined fracture separating the Ordovician sediments of the North Down coastal strip from the predominantly Silurian greywackes, with some Ordovician black shale inliers, which underlie most of the rest of County Down. Anderson and Oliver (1986) correlated the Orlock Bridge Fault with the Slieve Glah Shear Zone in County Cavan, the Cairngarrock Fault in Galloway and the Kingledores Fault in the central part of the Scottish Southern Uplands, so establishing a fault trace extending over 400km from Cavan to Berwickshire (Fig. 1), separating Northern and Central Belts along its entire length. It was the movements on this fault, taking place at the end of the Silurian Period, some 400 million years ago, which produced the intense deformation at Orlock Bridge. The description of the deformation which follows is based largely on Anderson and Oliver (1986). For the purpose of describing the fault fabric at several localities along the fault trace they found it practical and useful to recognise and map three zones of progressively more intense fault-associated deformation as Zones 1, 2 and 3. All three are represented at the Orlock bridge type locality (Fig. 2). Zone 1 Zone 1 is essentially characterised by inhomogeneous brittle deformation. Bedding is everywhere recognisable but variable in orientation. Sandstone beds, which away from the Fault are normally continuous and have constant thickness, are sliced into rhombs and lenses by numerous shear fractures at an acute angle to bedding (Plate 1). Shear is extensional with respect to bedding and the regional S1 cleavage, and in plan view is typically of sinistral sense. Where best developed, lenses or augen of sandstone float in a matrix of sheared argillite and smaller sandstone clasts. Strain is clearly very inhomogeneous, localised in the argillite. Small wrench faults are numerous and joints closely spaced throughout Zone 1. Segregations of quartz are rare. Zone 2 Within the outcrop of Zone 2 bedding and cleavage are typically transposed into a strongly developed foliation parallel or subparallel to the fault. The rock is essentially a phyllonite with numerous, diagnostic, quartz veins and segregations parallel to the fabric but without significant refolding. The segregations are extremely variable in shape and thickness and increase toward the boundary with Zone 3. Locally the phyllonitic foliation, or S-surface, is cut by and rotated into narrow sinistral shear ("cisaillement") zones or "C-surfaces" (Berthe et al. 1979), which are typically some 15 degrees anticlockwise of the foliation and spaced at 10 to 50mm in the style of a coarse, extensional, crenulation cleavage. The C-surfaces are clearly and macroscopically visible on many outcrops and in thin sections. Under the microscope the C-surfaces are seen as thin bands of cataclasis, marked by a coating of small opaque grains, mica flakes, finely comminuted quartz and, rarely, feldspar. The intersection, at an angle of 15 degrees, and relationship of S- and C-surfaces is essentially as described and figured by Berthe, Choukroune and Jegouzo (1979), by Simpson and Schmid (1983) and by Lister and Snoke (1984) in their account of Type I S-C mylonites. Late quartz segregations occur parallel to both S- and C-surfaces. Zone 3 Zone 3 is characterised by abundant discontinuous and tightly folded quartz segregations, comprising about 15 to 25% of the whole rock (Plate 2). The segregations have a modal thickness of 5mm, in a range from less than 1mm to 20mm, and are embedded in intensely foliated, micaceous and chloritic, grey-black phyllonite. Although the rock is not a schist the discontinuous, laminar, foliation-parallel, quartz segregations in the dark phyllonite give a characteristic, banded, 'schistose' appearance, enhanced by the intense small-scale folding (Plates 2). Within this intensely banded rock some beds and lenses of fine sandstone survive. Segregations and foliation display a variety of small folds (Plates 2 and 3). The outcrop of the Fault at Orlock Bridge The traces of the Orlock Bridge Fault shown on the 1968 Edition of Sheet 29 of the Geological Survey of Northern Ireland, the maps of Anderson (1962), of Anderson and Cameron (1979), and of Craig (1982 and 1984) all coincide on the gully at J56488301 and so leave all the known Silurian graptolite localities in County Down on the southern side. Craig (1984, Fig. 1) recognised five fault-defined slices or "blocks" in the Ordovician rocks north of the fault. As in the Northern Belt of the Southern Uplands, the base of the turbidites, resting on black shale, rises incrementally southward, from the top of the gracilis Zone in the most northerly (Ballygrot) block to the linearis Zone in the Ballymacormick Block, three blocks to the south. The Orlock Formation, composing the block immediately north of the Orlock Bridge Fault, has yielded no fossils but is regarded as Ordovician (Craig 1984, Fig. 1b and p.189). Immediately south of the fault the age of the turbidite formations is uncertain, although Griffith and Wilson (1982, p.15) recorded a "sparse" fauna from south of the fault on Copeland Island which "probably indicates the Lower Llandovery zone of Monograptus cyphus”. 4km south-east of the fault the base of the turbidites at Coalpit Bay rests on Birkhill Shale lithologies with graptolites extending up to the sedgwicki Zone. Thus, as in Scotland, though less tightly constrained by the palaeontology, the gradual diachronous southerly ascent of the base of the turbidites is foreshortened by some 5 zones at the outcrop of the fault. A map, showing particularly the distribution of deformation Zones 1, 2 and 3, and cross-section of the coastal outcrop at Orlock Bridge are presented in Figures 2 and 3. For 200m along the N-S trending coastline, between E-W Grid lines J8290 and J8310, the rocks have undergone fault-associated deformation. The formations on either side of this broad belt are strikingly different. South of J56608290 coarse sandstones, up to 1m thick, alternate with thin, 0.1m, shales in Tabe and Tace (Bouma 1962) units. The beds dip uniformly south at between 35o and 75o but all young north on excellent graded-bedding and sole-mark evidence. The regional S1 cleavage is parallel to bedding in the shales and non-penetrative of the sandstones. This weak and restricted development of the main regional cleavage is characteristic of many outcrops in the northern part of the Central Belt. North of the Fault the graded sandstones and laminated shales of the Orlock Formation show a strong, completely penetrative, slaty cleavage, associated with tight, east-plunging, F1 folds and containing a well-developed grain elongation or stretching direction plunging at c.45o west. F1 folds and S1 cleavage are refolded by both south- and north-verging fold pairs, of variable plunge and locally associated with crenulation cleavage. North of the fault there is also a slight (c.10o) but erratic clockwise or dextral rotation of cleavage and bedding toward the fault. South of the fault there is a similar but anticlockwise rotation of bedding. Cleavage and bedding steepen toward the fault trace from both sides. The fault-associated deformation is obviously very asymmetric at Orlock Bridge (Fig. 2). Zone 1 characteristics are distinguishable in a 110m-wide outcrop south of the fault trace, separated from the regionally deformed ?Silurian sediments at J56548290 by a fault trending 080o. North of the Fault trace, Zone 2 deformation is recognised in a number of bands, 1 to 10m wide, crossing the outcrop of the Orlock Formation in directions between 060o and 090o. These Zone 2 bands become more numerous and ultimately coalesce southward toward the trace. Regional (D1) deformation in the outcrop of the Orlock Formation is so intense as to make any recognition of Zone 1 difficult and arbitrary. Zone 3 is some 16m wide, deeply eroded and partially covered by shingle. Zone 1 is developed mainly in fine-grained sandstones, siltstones and shales and is superimposed on a pre-existing, well developed, regional fabric of tight F1 folds, facing up, verging south and plunging very gently east, with an axial-surface S1 cleavage. Thus, both lithologically and tectonically, there is a strong contrast across the fault marking the southern limit of Zone 1 [J56548290]. Immediately north of this bounding fault there is an open fold pair, refolding the S1 cleavage and rotating bedding to a flat, inverted attitude in its 12m-wide short limb (Figs. 2 and 3). The fold pair plunges at 10o west and verges north. There are no post-D1 or north-verging folds in the immediately adjacent Silurian rocks to the south so it is probable that this and other similar north-verging fold pairs in Zones 1 and 2 are related to fault movement. Northward, fractures and shear zones become increasingly common, bedding progressively more broken and lenticular. North-verging fold-pairs, of short-limbs 1m to 4m, deform the D1 fabric. Between the stream at J56508296 and the lamprophyre dyke at J56508301 shearing becomes intense in thickly bedded sandstones. Lenses of sandstone, up to 2m in diameter, float, augen-like, in sheared argillite. The southern edge of the Zone 3, at J56488301, is sharply defined and coincides almost exactly with the northern edge of a 2.4m wide, pinkish-weathering, pyrite-bearing, grey lamprophyre dyke, trending at 093o. South of the dyke is a narrow (0 to 1.5m) band of Zone 2 deformation, with numerous, foliation-parallel, segregation veins of quartz, which are generally not folded, and a sharp southern contact against the sheared, lenticular sandstones of Zone 1. Thus the transition from Zone 1 to Zone 3 takes place over less than 2 metres. Immediately north of the dyke there is a 3m wide outcrop of Zone 3 extending along the dyke from high water mark for some 30m toward low water mark. To the north again there are a few very small outcrops of Zone 3 visible beneath the shingle and a few decimetres of Zone 3 deformation at the base of the cliff on the north side of the bay where there is a narrow transition to Zone 2. In this limited Zone 3 area 183 fold hinges have been measured (Fig. 2c). The individual folds are close to isoclinal, with subvertical, E-W, axial surfaces, parallel to the modal orientation of the fault fabric (Fig. 2c). The larger examples, with a short-limb length up to 0.4m, have a well-defined chevron geometry (Plate 3) but most are rounded, with short-limb lengths of a few centimetres and irregular, non-rectilinear hinges. There is some refolding and interference but all clearly fold the phyllonitic fabric and a majority of the quartz segregations. A crenulation of the fabric and a fold-associated, post-phyllonite cleavage are evident at some hinges. Plunges vary widely in a well-defined, east-west, vertical, partial girdle (Fig. 2c). Within the girdle there is an obvious single mode, plunging 75o east, and a marked absence of fold hinges plunging gently west. Many of the smaller folds are essentially intrafolial, with discontinuous limbs, but fold pairs can be clearly recognised on both margins though typically not in the central part of Zone 3. 50 fold pairs show sinistral or oblique sinistral-north vergence, compared with 14 dextral pairs. The fold pairs in Zone 3 are cut by vertical, planar segregations of quartz, apparently generated in post-folding strike-slip. Although unfolded, these planar segregations are locally offset by renewed pressure solution along the composite fault foliation. C-surfaces are not so obvious at Orlock as at other outcrops of the Orlock Bridge Fault, possibly because S- and C-surfaces converge and merge in the relatively narrow Zone 3 outcrop at Orlock. At high water mark in the centre of Zone 3 [J56488301], vertical C-surfaces, striking 080o, dipping 86o north, make an angle of 13o anticlockwise from the local fault foliation, oriented at 093o/85o north. Some 8m north, at the transition between Zones 2 and 3 on the north side of the fault gully, sinistral S-C relations are clearly but locally displayed, S-surfaces striking at 090o and C-surfaces at between 069o and 081o. The southern margin of Zone 3 and the 2.4m lamprophyre dyke are displaced by several, late Caledonian, sinistral, strike-slip faults trending between 010o and 040o with slips of 1 to 5m. Angular breccia along the fault surfaces demonstrates the brittle nature of this late deformation. Towards low water mark the essentially undeformed lamprophyre encloses a lens or raft, 4m by 0.4m, elongate parallel to the dyke margin, of Zone 3 folded phyllonite. A 0.15m thick subsidiary dyke of the same lamprophyre extends for 7m along strike, 0.6m north of the northern contact of the main dyke and entirely within Zone 3. Very clearly in these outcrops, the intense fault fabric, the associated folding and many of the segregation veins predate the intrusion of the dyke. A few small sinistral shear zones, with slips less than 50mm, cut the margin of the main dyke and the subsidiary lamprophyre. Both dykes show joints and veins of quartz continuous from the enveloping phyllonite. At low water mark a very conspicuous, late, sinistral, shear zone, parallel to and 0.6m north of the main dyke, truncates folds, segregation veins and phyllonitic foliation. The intrusion of lamprophyre was thus clearly later than most of the deformation in Zone 3 but some sinistral movement did occur after the intrusion. The dyke is one of several hundred Caledonian lamprophyres exposed on the coast of County Down and petrogenetically related (Reynolds 1931) to the Newry Granodiorite which is dated at 399 My (Meighan and Neeson 1979). A similar dyke at Ballywalter, a few kilometres to the south, has recently been dated at 415 + 12 My (Anderson and Oliver, in press). The Orlock Bridge Fault crosses the Copeland Island, 2km offshore (Fig. 2b), with obvious topographic expression. The continuous coastal exposure on the island is broken only at Port Dandy [J58568365] and Port Rammon [J59458396] where sandy bays, a few tens of metres across, mark the trace of the fault. At both localities the polydeformed rocks of the Orlock Formation lie north of the exposure gap. More massive sandstones, with a simpler deformation history and a single, non-penetrative cleavage, lie to the south. Fault-associated deformation, of Zones 2 and 3, is present in outcrop for some 5 to 10m north of the beach at Port Rammon. The composite fault foliation, 090o/87o north, is intersected by C-surfaces oriented 071o /87o south, in an almost exact replication of the sinistral S-C geometry in the Zone 2/Zone 3 transition immediately north of the fault trace at Orlock Bridge. Both S- and C-surfaces are cut by late, planar, quartz segregations, whose orientations again indicate sinistral slip. The valley following the fault inland from Orlock Bridge and the three outcrops at Orlock Bridge, Port Dandy and Port Rammon all lie on a rectilinear trace at 073o (Fig. 2b). Yet the modal orientation of the vertical foliation in Zone 3 coincides with the well defined girdle of Zone 3 fold hinges at Orlock Bridge on a trend of 090o (Fig. 2c). Two explanations of this 17-degree divergence are apparently viable. The trend of the fault, and of the other block-defining, accretion-related faults and formation boundaries to which it is parallel, is 090o at outcrop but on the map scale this has been rotated to 073o by numerous late sinistral wrench faults, like those seen displacing the southern margin of Zone 3 at Orlock Bridge. This first hypothesis not only requires the rotational wrench-fault strain to be remarkably homogeneous at the map scale but is clearly at odds with the modal orientations of bedding and the S1 cleavage (Figs. 2d and 2e), and of the fold-hinge girdle at about 073o, in the rocks outside Zone 3 . Alternatively and more convincingly the fault foliation is essentially an S-surface, in the plane of flattening and folding, making an angle of some 17o with the plane of shearing (C-) and the fault itself. The geometrical configuration of C- and S-surfaces, of fault foliation and the fault itself is thus a mirror image of the model dextral shear zone figured by Platt (1984, Fig. 1). This argument, the relationship of the S- and C-surfaces at the Zone 2/Zone 3 boundary, the orientation of the ‘late’, cross-cutting segregations and the dominance of sinistrally-verging fold-pairs, leads to the conclusion that the geometrical features of the fault fabric at Orlock Bridge are a consequence of sinistral slip, very probably repeated, mainly pre-lamprophyre and thus Caledonian. The north-verging, post-cleavage and fault-related, fold pairs in Zone 1 may well be related to a phase of northerly downthrow, in obvious contrast with the south-easterly downthrow on the accretion-related strike faults, the general south-easterly vergence of the accretion-related D1 folds (Anderson and Cameron 1979) and the fact that relatively older rocks occur on the north side of the fault trace. Further discussion on the displacement and significance of the Orlock Bridge Fault in tectonic models of the Southern Uplands-Down-Longford Terrane, including evidence from other major outcrops of the Fault and its distinctive fabric, is presented in the paper by Anderson and Oliver (1986). | |
| Importance: | |
Since 1977 the sedimentology and structure of these rocks have attracted international interest because of their interpretation (McKerrow, Leggett and Eales 1977) as an accretionary prism developed by subduction on the north-western margin of the Lower Palaeozoic Iapetus Ocean. | |
| Interpretation: | |
The Orlock Bridge Fault is a steeply inclined fracture separating the Ordovician sediments of the North Down coastal strip from the predominantly Silurian greywackes, with some Ordovician black shale inliers, which underlie most of the rest of County Down. | |
| Conclusions: | |
Coastal outcrop below and east of Orlock Bridge, on the Bangor to Donaghadee coast road, provides the most spectacular example of fault-associated deformation in Northern Ireland. | |
| Notes: | |
TO BE SOURCED AND ENTERED Figure 1. Map showing the trace of the Northern Belt/Central Belt boundary in Scotland and Ireland. Figure numbers refer to illustrations in Anderson and Oliver (1986). The inset map shows, in black, the extent of the Southern Uplands-Down-Longford Terrane. Figure 2. (a) Geological Map of Coastal Outcrop at Orlock Bridge [J565830]. (b) Inset Map of the geology of the north-east coast of County Down (c) Stereogram of 22 poles to fault foliation, 7 poles to planar (i.e. unfolded) quartz segregations, and 183 fold hinges developed in fault foliation. All from the 25m wide outcrop of Zone 3 at Orlock Bridge. (d) Stereogram of 38 poles to S1, of 12 F1 fold hinges and of 17 post- F1 fold hinges, from the complete area shown on Fig. 2a. (e) Stereogram of 42 poles to bedding in the area shown on Fig. 2a. Legend for Figure 2(a) Figure 3. Vertical geological cross-sections of the coastal outcrop at Orlock Bridge, County Down, along the lines A-B and C-D as indicated on Fig. 2a but with the scale increased by 1.25 times to show structural detail. Plate 1. Lenses of sandstone in sheared argillite on sea stacks at the northern end of the Zone 1 outcrop, Orlock Bridge. Black and white scale squares on lens cap are of 10mm side Plate 2. Late, sinistrally verging, fold pair in Orlock Bridge Zone 3 outcrop immediately N of the lamprophyre dyke. Note folded, foliation parallel, quartz segregations. Plan view at J56488301 with top to N. 10mm scale squares. Plate 3. Late, sinistrally verging, chevron fold pair in Orlock Bridge Zone 3 outcrop immediately N of the lamprophyre dyke. Plan view at J56488301 with top to N. 10mm scale squares. The full range of ESCR site relating to the Lower Palaeozoic are as follows: Outer Ards Stratigraphic, Palaeontological and Structural sites Coalpit Bay – Lower Palaeozoic (record no. 1350) Helen’s Bay – Grey Point – Lower Palaeozoic (record no. 1351) Kearney Point – Knockinelder Bay – Lower Palaeozoic (record no. 1352) Orlock Bridge – Lower Palaeozoic (record no. 1353) Tieveshilly – Carrstown – Lower Palaeozoic (record no. 1354) Orlock Bridge - Structural (record no. 652) Whiskin Rocks - Structural (record no. 653) White House Port, Cloghy - Structural (record no. 654) Knockinelder Bay and Kearney Point - Structural (record no. 655) Millin Bay - Structural (record no. 656) Pomeroy area Craigbardahessiagh and Bardahessiagh River – Palaeontology (record no. 29) Lime Hill Farm – Palaeontology (record no. 33) Little River and Slate Quarry Bridge – Palaeontology (record no. 31) | |
| Keywords | |
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| Rocks: | Turbidite, Greywacke, Shale |
| Access | |
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| Approach: | Public access via National Trust coastal path at Orlock Point |
| Restrictions: | Public access |
| Management: | |
Mainly within National Trust ownership it is managed for recreational purposes. Adjoining lands are used for agriculture. | |
| Threats: | Coastal development, especially coastal defences, would pose a threat. |
| Record type: | ESCR report | ||
| Entered by: | I J Enlander | ||
| Last updated: | 1 April 2019 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Orlock Bridge – Lower Palaeozoic, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1353. Accessed on 2026-11-08. |
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