Along-Strike Variation in the Stratigraphical and Structural Profile of the Southern Uplands Central Belt in Galloway and Down

Journal of the Geological Society, London, Vol. 144, 1987, pp. 807-816, 6 figs. Printed in Northern Ireland

Along-strike variation in the stratigraphical and structural profile of the Southern Uplands Central Belt in Galloway and Down

R. P. BARNES’, T. B. ANDERSON2 & J. A. McCURRY3 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK, 2Department of Geology, The Queen’s University of Belfast, Belfast BT7 INN, UK 3 Department of Geology, The University of St. Andrews, St. Andrews, Fife KY16 9ST, UK

Abstract: The modern interpretation of the Southern Uplands depends on the recognition of laterally extensive, linear, fault-bounded tracts of northward-younging Lower Palaeozoic sedlments with rare volcanics.The tracts become progressively younger southwards and are thought to have been sequentially accreted by northward underthrusting above a subduction zone. Detailed mapping and tentative correlation of three, well-exposed, coastal profiles through the Central Belt in SW Scotlandand NE Ireland offers a newtest of the Southern Uplands model. Comparison of the northern parts of each section indicates the presence of strike-parallel tracts with model structure. MoffatShale outcrops associated with thetract-defining faults demonstrate a diachronous,incremental, southerly decrease in theage of the base of theoverlying turbidites, essentially as in the Northern Belt. Southward this becomes markedly less pronounced,particularly in Down where the Central Belt is much wider than in Galloway. Distinctive Hawick Group lithologies permit the correlation of the southern parts of the sections. Large areas of predominantly southward- younging occur in each section, especially in Galloway where the southward-younging area is 12 km across. Here northerly-verging D, fold pairs are consistent with observed fault movement opposite in sense to the northerly underthrusting of the model. Back thrusting, in the style of the Pleistocene Cascadia Basin, is invoked to account for the landward-verging structure of these areas and to explain the narrowing of the Central Belt in Galloway. The structure of the Central Belt thus differs significantly from that of the Northern Belt and the accretionary prism model.

TheSouthern Uplands were sub-divided intothree the basis of this assumption is weakened by the evidence of strike-parallel ‘belts’ (Fig. l), for the “sake of convenience major sinistral displacement onthe Orlock Bridge Fault of description” by Peach & Horne (1899). Simply termed separating the twobelts (Anderson & Oliver 1986). The theNorthern, Central and Southern Belts, they are relatively small amount of detail published from the Central composed of Ordovician, mainly Llandovery and Wenlock Belt pre-datesthe accretionary prism model, with the strata, respectively. These divisions have since proved to be exception of Cook & Weir (1979, 1980), Stringer & Treagus fundamental in theinterpretation of SouthernUplands (1980, 1981) andWebb (1983) in Scotland,and Anderson geology, each belt having distinctive characteristics. (1978), Anderson & Cameron (1979) and Cameron (1981) TheNorthern Belt is divided into a number of in NE Ireland. The Central Belt has two distinct parts. The strike-parallel tracts of greywacke by discontinuous narrow northernpart is characterized by the proximal turbidite outcrops of fossiliferous black mudstone and chert (Moffat facies of the Gala Group, in which the sandstone is usually Shale), rarely including basic lavas. The thin Moffat Shale quartzose in compositionalthough locally pyroxenous. successions are clearly the imbricated repetitions of a pelagic Moffat Shale inliers occur but are much less continuous than sequence which formsthe base of the stratigraphical those farther north, defining relatively indistinct tracts. The sequencein each tract.The overlying greywackes young southern part of theCentral Belt is composed of an very consistently NNW although a few southward-younging extremelyuniform, relatively distal turbidite facies (the short limbs of south-easterly-verging F1 folds do occur. The Hawick Group), in which the sandstone is compositionally base of the greywacke sequencebecomes progressively distinct from GalaGroup sandstone by virtue of its high younger southwards in successive tracts and the sandstone in content of primary carbonate detritus. each is compositionally distinct (e.g. Floyd 1982). The Recent work in three well-exposed coastal sections of published maps (e.g. Leggett et al. 1979, fig. 2, 1982, fig. 3) SW Scotland andNE Ireland (Fig. 1) allows detailed show tracts of remarkably consistent width, typically about cross-strike and along-strike analyses of the Central Belt 5 km, persistent forat least 30 km,and commonly over (see below). 100 km, along strike.These Northern Beltstratigraphical The Southern Belt is composed of Wenlock greywackes andstructural relationships arethe essential basis of the of varied facies, unique in the Southern Uplands in that they accretionary prism model forthe Southern Uplands as a can be precisely dated by the commonoccurrence of whole. The model was conceived in the Northern Belt and is fossiliferous, argillaceous siltstone beds. As in the Northern still largely substantiated by observations made in that area. Belt, a number of strike-parallelfault-bounded tracts The distinctive Northern Belt pattern is generally becomeyounger southwards (e.g. Kemp & White 1985), assumed to continue southwards into the Central Belt but although important areas of southward-younging strata (e.g. 807

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Ireland, overlain in the M. sedgwickii Zone by some 20 m of thickly bedded, coarse-grained greywacke (C). Massive coarse sandstone (C(B,)) in beds up to 4m thick with very well developed sole markings, dominates the Millisle Block, althougha distinctive 40m unit of siltstoneand red mudstone is also present. Thickly bedded, massive sandstone (C), with interbedded bentonite bands (Anderson and Cameron 1979), crops out at the base of the succession in the Ballywhiskin Block. The greywacke sequence fines upward into siltstone and shale (D) with some red mudstone bands. A few monograptids have been obtained from shales (G) nearthe top of the succession. The siltstone (D) composing the northern part of the Ballywalter Block has abundantcarbonate concretionsand some thin limestone beds. The succession again includes thin red mudstone beds. In the southern part of the block more normal, quartz-rich, thinly bedded, fine sandstone and siltstone (D)crop out. The Wallace’s Rocks Block comprises a variable sequence of coarse- to fine-grained greywacke (C(A,) and D). Two formations,predominantly of sandstone, crop out in the Portavogie Block. The Rowreagh Formation is medium- to coarse-grained (C) with interbedded black graptolitic shales demonstrating M. crispus Zone age. The succeeding Portavogie Formation is generally finer grained (C(D)) and Fig. 1. The locations of the three study areas with respect to the broad subdivisions of the Southern Uplands of Scotland and NE unfossiliferous. Thinred mudstonebands occur ininter- Ireland. bedded siltstone units. Both sandstones and siltstones show evidence of extensive intra-bed slumping and soft-sediment Warren 1964) suggest possible similarities of Central Belt deformation. Inthe Portaferry Block there is extensive structures (see below). coastal outcrop of a succession of well-defined stratigraphic- a1 units. Coarse greywackes of theTara Formation(C) include some thin but persistent bands of black shale with a The Central Belt in Down and Galloway M. crispus Zone fauna. These are overlain by some 100m A brief summary of the stratigraphical and structural of dark siltstone and mudstone (D(G)) with numerous thin characteristics of each area is given, supported by maps, redmudstone beds atthe base of the thick (>500m) including structural detail (Figs 2-4), and biostratigraphical Kearney Formation.The KearneyFormation consists detail(Fig. 5). Ineach of thethree areas strike-parallel largely of thinly bedded, carbonate-rich siltstone and shale faultsseparate a number of structural ‘blocks’ with (C) with a few beds of coarse pebbly greywacke (A4). Inland distinctive lithostratigraphies. In the northern parts of the drainage work has recently produced a temporary section in areas these faults are marked by outcrops of Moffat Shale. Moffat Shales atthe hinge of an anticline in theTara Otherwise the blocks comprise turbidite sequences de- Sandstones. The shales range in age from late Ordovician up scribed below with reference to the classification of turbidite to Rastrites maximus Subzone and are apparently sliced and facies (A to G) described by Walker & Mutti (1973). repeated by faulting. The Ballyquintin Formation (D), which composes the most southerlyfault block, closely resembles the carbonate-rich Kearney Formation described Ards Peninsula (Fig. 2) above. The Northern and Central Belts are separated by the major Throughout the peninsula the sandstone is monotonously Orlock Bridge Fault, described in detail by Anderson and quartz-rich. No petrographical equivalent of the pyroxene- Oliver (1986). Aspects of the stratigraphy and structure of bearing greywacke of the Central Belt in SW Scotland has the Central Belt in County Down have been recorded by been recognized. Thinbentonite bands,though rare and Anderson (1962, 1969, 1978), Anderson and Cameron difficult to recognize in the coarsesandstone formations, (1979), Cameron (1981) and Griffith and Wilson (1982). crop out in each of the nine blocks. Reynolds (1931) described the numerous minor intrusions. The structure of the northern blocks is characterized by The northernmost (Donaghadee) Block is a composite of strata dippingsteeply south but younging north (Fig. 2), severalstructural slices with three repetitions of Moffat although major and minor F, fold pairs obviously produce Shaleand an infaultedwedge of Ashgillian strata(the significant southerly-younging tracts, particularly in the PortavoeFormation). The Moffat Shalesrange upto the Millisle and Ballywalter Blocks. The Portavogie Block is Monograptus cyphus Zones and are conformably succeeded dominated by the vertical,1.7 km thick,southward- by turbidites, of variable but predominantly coarse clast size younging, northern limb of the Castle Hill-Portavogie (facies C, Walker & Mutti 1973), including rare Syncline. The siltstones of the two most southerly blocks are conglomerates of intrabasinal origin (AZ). Interstratified intensely folded with the sheet dip typically sub-horizonal or silty mudstonebeds are generally less than 0.2 mthick. inclined gently to the north. F, fold axes plunge gently or Laminated shale (G), in units up to 10m thick, crops out moderatelyeastwards throughout the Peninsula except in locally within the turbidites. The narrow Coalpit Bay Block the northern parts of the Ballywalter Block and in outcrops exposes the best-preserved Moffat Shalesequence in on the SW coast of the Portavogie Block, where steep to

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Fig. 2. Geological map of the Ards Penin- sula, County Down. BA, Ballywalter Anti- cline; CHPVS, Castle Hill-Portavogie Syn- cline; CLYF, Cloughy Fault; KCBF, Kircub- bin Fault; MBA, Millin Bay Anticline; MS, Millisle Syncline; OBF, Orlock Bridge Fault; PTV, Portavoe Formation; SCBF, Southern Coalpit Bay Fault; TVSHF,Tieveshilly Fault.

vertically plunging F1 folds are common. The S, cleavage is Rhinns of Galloway (Fig. 3) either parallel tothe axial surfaces of F, folds,as it is throughoutthe BallyquintinBlock, or transectsthem Despite the excellent coastal sections, the recent work is the clockwise at an angle of some 10-20”, as it does throughout first since this area was mapped by the Geological Survey the Portaferry Block (see Anderson this issue). Later (Irvine 1872). folds are only developed in the two most southerly blocks, The northernmost unit, the Money Head Block, consists where open, southward-verging F2 folds have gently inclined of massive, coarse-grained greywacke (facies A,B,(C)) in axial surfaces andare associated with a well-developed beds varying from 1 to 10m in thickness, with subordinate crenulation cleavage (Anderson and Cameron 1979). thinly bedded siltstone and fine-grained greywacke (F), Lamprophyre dykes occur throughout the peninsula but overlying an intensely imbricated Moffat Shale sequence. are common only in the three most southerly blocks. The The greywackes includea 400 m wide tract whichis ‘Older Series’ of Reynolds (1931) is typically composed of petrographically distinct, being markedly pyroxenous. The bed-parallel mica-lamprophyres carrying a tectonic foliation Float Bay block consists of 1 m thick greywacke beds (C) continuous with the S, cleavage in the intruded sediments of interdigitated with units of silty mudstone (G(E)) which the Portaferry and Ballyquintin Blocks. Unfoliated dykes of vary from 2 to 100 m in thickness. Thickly bedded, medium- the ‘Newer Series’ cropout in all nine blocks butare to coarse-grainedturbidites (C(A,B,)) dominatethe common only in the Portavogie Block. Ardwell Point Block, although interbedded silty mudstone

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Money NNW Head I Float Bay I Ardwell Polnt I Mull of Logan I Block Block Block Block Block Block 1 Block

Fig. 3. Geological map of the southern part of the Rhinns of Galloway. CFZ, Clanyard Bay Fault Zone; CGF, Cairngarroch Fault; DFZ, Drumbredan Bay Fault Zone; DM, Duniehinnie Member; NW,Nick of Kin- dram Fault; PGD, Portencorkrie Granodior- PG0 NKF TiF ite; PLF, Port Logan Bay Fault; SFZ, Strandfoot Fault Zone; SPF, Salt Pans Bay Fault; TBF, Tarbet Fault. Othersymbols as l pLF for Fig. 2. units with redmudstone beds occur nearthe base of the Thestructure of the Rhinns of Galloway broadly succession. The Mull of Logan Block hasa distinctive conforms to a major anticlinorium with its axial region lithostratigraphy.Medium- to coarse-grained greywacke in centred on the PortLogan Bay Fault.North of this fault beds less than 1 m thick (C(E)) is overlain by a sequence of strata are dominantly northward-younging, and bedding and coarse-grained massive sandstone (A3A4) and a 500 m thick S, cleavage dip steeply to the south. South of the fault strata conglomerateunit (F) (theDuniehinnie Member). The are dominantly southward-younging with beddingand lattermember consists of well-rounded,poorly organized cleavage inclined at a steep to moderate dip to the north. intrabasinal clasts with more angular blocks (up to tens of The sheet dip in these two areas descends steeply to the metres in diameter) of greywacke and siltstoneoccurring north and south, respectively, although locally becoming near the northern margin. In the Port Logan Block medium- more gently dipping. Major faults north of the Port Logan to coarse-grainedgreywacke (C), forming both fining and Bay Fault have a southerly downthrow whereas south of this coarseningupward sequences, includesunits of laminated fault their movementdirection is more variable although silty mudstone (E & G) up to 120m thick. The structurally most throw down to the north.F1 folds are typically close to repeated Moffat Shale inliers at the northern margin of the isoclinal with angular hinges plunging gently to moderately Clanyard Block are overlain by medium- to coarse-grained, NE or SW. In large areas adjacent to the faults fold hinges thickly bedded greywacke (C(BD)) with interbedded thin may plunge steeply, becoming locally downward-facing as a black shale bands and thick (upto 40 m) silty mudstone result of rotation by post-S, shearing. A penetrative slaty or units (G(E)). South-west of the Nick of Kindram Fault, pressure-solution cleavage developedthrough thearea fine-grained,calcareous greywacke beds less than 60 cm usually transects fold axial surfaces. thick are separated by green-grey silty mudstone with thin Post-F, folds form open to close and intermediate scale red mudstone beds (C & D). Coarser grained channelized structures. Southerly-verging F2 foldsbecome increasingly greywacke beds occur locally in units up to 5 m thick. common southwards and are associated with a crenulation

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cleavage (Sz). Later northerly-verging folds and associated fossiliferous silty mudstone (C);several slumped sequences thrusts occur mostcommonly in the southernmost two also occur. The contact with the Moffat Shale atthe blocks. A system of late conjugate faults affects the whole southern edge of this block (exposed in Gillespie Burn, NX area although N-S sinistral fractures generally 257539) is gradational. The first 50m of the greywacke predominate. sequence includes fossiliferous mudstone beds which young Lamprophyreand felsic dykes areabundant in the northwards from Alcdographis acuminatus to Coronographis Rhinns of Galloway(e.g. Read 1926). Hornblende cyphus zones. Graptolites representative of these zones also lamprophyresoccur throughoutthe peninsula but biotite occur in a few thin mudstone beds in the 5 km long coastal lamprophyres are restricted to its southern tip. section. However, here theybecome younger southwards, suggesting that a number of structural slices must be Wigtown Peninsula (Fig. 4) present. Moffat Shale exposed in the south of the Garheugh Block is intensely deformed and only provides a maximum In Wigtownshire a thick drape of boulder clay obscures age for the base of the turbidite sequence. In the Garheugh much of the coastline. Inland exposure compensates for this coastsection 500m of relatively thinly bedded (0.5-1 m) to some extent althougha broad, strike-parallelbelt greywacke (C) incorporates 10 m of laminated siltstone and betweenMochrum andMonreith is generally very poorly silty mudstone with redmudstone bands (D) and two exposed. Gordon (1962) described the northern part of this bentonite bands. Towards the northern edge of this block a area in detail and the southern part has been described by single pebbly arenite/ruditebed (30m thick) and a few Rust (1965) andBarnes in press. Detailor the minor metres of ‘blocky arenite’ are exposed. Blocky arenite (F) intrusions has recently been given by Barnes et al. (1986). (massive fine- to coarse-grainedsandstone packed with Thenorthernmost three blocks, separated by Moffat angularsiltstone and greywacke blocks upto 0.5 m Shale outcrops, are dominated by thickly bedded, massive diameter) is well developedin thenorthern part of the or poorly graded, fine- to coarse-grained greywacke (facies Corwall Block where it occurs towards the top and base of a Bz). In the Kilfillan Block, very thickly bedded, massive 700 m thick unit of medium- to very coarse-grained massive greywacke units (up to 100 m thick) are separated by units sandstone.This is overlain by fine-grained greywacke and of relatively thinly bedded greywacke and sparsely sparsely fossiliferous laminated siltstone (C). To the south,

=Areas of thick.unboken drift cover Ordovlclan strata I Northern Belt)

Gre wacke Madot Shale 3 Syncllnal 8 antlcllnal fold axlal troces ...... Geologicalboundary --_Fault *-*Thrust fault Strlke 8 dlp of beddmg =:$$md

Key to StereographtcProjections

15% Polestobedding (number of poles [ 11 contoured per 1% area 1.1.

’ Flrst fold (1x1s

Fig. 4. Geological map of the western coastal area of the Wigtown Peninsula. GBF, Gillespie Burn Fault; GF, Garheugh Fault. Other symbols as for Fig. 2.

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fine-grained,thin- to medium-bedded greywacke and Correlation interbedded silty mudstone (C) (locally yielding sparse M. Comparing the results of the three studies it is immediately turriculatus Zone faunas)include thick, massive sandstone evident that correlation is difficult because of the limited sequences ((AJB,). NearMochrum, a few exposures of lithostratigraphical variation and the lack of other distinctive fine-grained greywacke beds (upto 1 mthick), with characteristics. well-developed sole marks, in a relatively high proportion The successful use of greywacke composition as the (c. 40%) of laminated silty mudstone are tentatively principal means of correlation in the Northern Belt cannot separated as the Mochrum Block. be extended into the Central Belt because of the very much FromMonreith toBurrow Head a very uniform more uniform composition of the sandstones. However, cor- sequence comprisesinterdigitating units of fine-grained, relation in the Central Belt is possible in two ways: (1) by medium to thinly bedded (less than 0.6 m) greywacke, with comparing the lithostratigraphy of each block, constrained silty mudstonepartings thin or absent (C), and laminated by the limited biostratigraphical evidence,and (2) by silty mudstone with very thinbeds of fine-grained comparing the structure (principally DJ of the different greywacke or siltstone (D).Rare thickerbeds of poorly areas. These criteria are quite distinct and are considered gradedor massive greywacke (B,) rangefrom fine- to separately below. One important change along strike, coarse-grained and are often channelized. These strata are immediately apparent from Fig. 1, is a widening of the subdivided intothe Carghidown Formation, in which red Central Belt from 30 km in Wigtownshire to a minimum (no mudstone beds become thinnerand less common north- SouthernBelt Strata being exposed) of 45 kmin NE wards, grading into the Kirkmaiden Formation without red Ireland. Although theapparent width in Wigtownshire is mudstone. At Burrow Head fossiliferous (M. centrifugus to reduced by N-S sinistral wrench faulting, this cannot M. riccartonensis Zones), carbonaceous laminated siltstone entirely account for the increase in width to the west, which beds indicate that the greywacke sequence is part of the is reflected in the greater number of blocks recognized in the Riccarton Group(Southern Belt). A few thin black Ards Peninsula. mudstone laminae, which occur in the greywacke sequence atKirkmaiden, yield a M. griestoniensis Zonefauna. The only evidence for the age of the Carghidown Formation is Stratigraphical correlation thatit may be stratigraphicallyoverlain by the Riccarton Thethree areas are broadlycomparable in the pre- Group strata at Burrow Head (Barnes in press). dominance of proximal, medium- to coarse-glained turbidite The structure of the well-exposed parts of the Wigtown facies (GalaGroup) in the north as distinct from the peninsula is dominated by steeplydipping, northward- relatively distal, fine-grained, carbonate-rich greywacke and younging strata with the notableexception of the silty mudstone facies (Hawick Group) seen in the south. Kirkmaiden Formationoutcrop which is intensely folded. Strata attributable to the Hawick Group (Fig. 6) are of very In detail different sectors, corresponding in the north of the similar lithology in all three areas and are also of similar age area with individual blocks, are structurally distinct (see (Fig. 5). The northern boundary of the Hawick Group in the stereograms, Fig. 4). The limited outcropin the area Ards Peninsula is the strike-parallel Cloughy fault. between Mochrum and Monreith is dominantly southward- Southwards the succession in the Portaferry Block is unique younging. Farthersouth, faults defining structural blocks in thatthe KearneyFormation of the Hawick Group cannotbe identified. Minorreverse faulting is common stratigraphically overlies theTara Formation, of Gala locally and varies from D1 toD, in age. Major folding is rare Group lithology and age, and this in turn rests on Moffat south of Port Counan, the only largesouthward-younging Shale. fold limb being that which includes the Carghidown Three Gala Groupblocks recognized in the northern part Formation-Riccarton Group Boundary. The widely variable of the Wigtown areacorrelate readily with the northern plunge of F, folds in thissector is dueboth to strongly blocks in the Rhinns of Galloway on the basis of distinctive curved folded hinges, locally downward facing, and to the lithological elements. The ‘blocky arenite’ exposed in the common occurrence of steeply plunging sinistral fold pairs. Corwall Block is seen in morea proximal form (the The latter also occur in a narrow strike-parallel belt which Duniehinnie Member) in the Mull of Logan Block. Moffat crops out on the shore at Monreith. Shale is exposed at the base of the Garheugh and Ardwell Post-D1 structures are best developed in the area SE of Point blocks, with turbiditesedimentation from about C. Monreith where open to close, steeply inclined D, folds are gregarius Zone or lower M. convolutus Zone in both. Units locally well developed.These occur together with a of silty mudstone and siltstone with redmudstone beds conjugate set of recumbent, open D, folds at Kirkmaiden. occur in the lower part of a dominantly massive sandstone Rare, northward-verging, open folds occur in the Carg- sequence in both areas.The composite Kilfillan Block is hidown Formationand alsoin theGarheugh Block. probablyequivalent tothe Money Headand Float Bay Conjugate N-S and NW-SE wrench faults are locally well blocks of the Rhinns of Galloway; the turbidite successions developed. in the Float Bay and Kilfillan blocks are very similar, being Dykes occur through the Wigtown peninsula and include characterized by massive sandstone sequences separated by widespread hornblende lamprophyres and a range of felsic greywacke and silty mudstone. Poorly fossiliferous mud- types. Biotite lamprophyres are restricted to the Hawick and stone beds in the Float Bay Block are of similar age (C. Riccarton Groupoutcrops where they areabundant. All cyphus) to strata in the Kilfillan Block. The poorly exposed dykes post-date F1 and S, buttheir complex relationships Mochrum Block has some sedimentological characteristics in with D2 folds and the conjugate wrench faults indicate an common with the Port Logan Block. No equivalent of the alternation of extensional and compressional events (Barnes Clanyard Block is recognized in Wigtownshire, possibly et al. 1986). because of faulting and poor exposure in the vicinity of Port William.

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Graptolite zones GALA GROUP 1AWICKGROUP I I M crenulala AROSPENINSULA C .-0 M grieslontmsis ._-a M crispus II m M IurrIculaIus M scdgwlckli Llandovery M convolutus C gregarlus c cyphus C vtsiculosus A--__ ocurnlnatus G persculptus Ashglll D OncePS .-0 D cornplanatus .-U > P Ilncorls 0 D cllngani g Caradoc c w,lsOni C pclttler

OB CBB MB BWB BWLB WRBPVB

M crenulalo RHINNS of GAL.L OWAY C P M grlestonlensis -3 M crispus m M lurrlculalus M sedgwlckli LLandovery M convoIutus C gregarius c cyphus C ves1cuIosus A---_ acurntnatus I_ G persculptus I AshgIII I P D complanalus P llnearls > I 0 D cllnganl I g Caradoc C W,lSOn, F B B APB MLB PLB CLB PLB MLB1HB APB FBB L:OB MGB M rlccarlonensls WIGTOWN PENINSULA Wenlock C rnurchlsonl C centrllugus 11' M crenulata C Ill ._0 M grlestonlensks M crlspus 111 .--a l m M Iurrlculalus ?l M sedgwtckli lncludlnq RCN LIOndovery M convoIutus Rlccarton Group) C gregarlus C cyphus C vesIcuIosus KMN & CGD A- acufinats - G persculptus 'BLOCKY ARENITE Ashgill ancefJs FOSSILIFEROUS SILTY MUOSTONE cornplanatus .-0 D TURBIDITE SEQUENCES .-U linearls > P MOFFAT SHALE D cllnganl P X DIAGNOSTIC FOSSIL ASSEMBLAGE 0 lili& RANGEOF UNCERTAINASSEMBLAGE 4 TrTiqlz[ 1 AGEUNKNOWN Fig. 5. Time-stratigraphic diagrams of the three study areas. Tectonic! blocks are arranged according to the correlation detailed in the text and shown under the headings Gala Group or Hawick Group according to the predominant age and lithological affinities of the greywackes. Where no evidence is available about the age of the stratigraphic sequence in a tectonic block it is placed in relation to its neighbours assuming a progressive southerly decrease in age. Abbreviations of formation names are as on Figs 2,3 and 4.

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Fig. 6. Geological map of Galloway and east Down illustrating the correlations effectedin the text. The Lecale Peninsula is basedon Cameron (1981). AGB, Ardglass Block. Other abbreviations as for Figs 2, 3 and 4.

Correlation of the northern blocks in the Ards peninsula dominant in boththe Portavogie and Clanyard blocks, with SW Scotland is less clear. In the Donaghadee Block, which may be correlated on this basis. the Moffat Shaleranges up to C. cyphus Zone in age, The northern and southern parts of the three areas can suggesting that much of this block is equivalent tothe be correlated reasonably well using stratigraphical criteria. ArdwellPoint andGarheugh blocks. Thereforethe On this basis the widening of the Central Belt tothe northernmost tract in Scotland must either be represented south-west is due to thepresence of at least two extra blocks by the northern part of the Donaghadee Block or it fails to in the Ards Peninsula compared with SW Scotland (Fig. 6). reach NE Ireland. No equivalent of the narrow Coalpit Bay The time-stratigraphical diagrams (Fig. 5) show that the Block is seen in SW Scotland. Very thickly bedded, massive base of the greywacke sequence becomes progressively sandstone in the Millisle Block may be correlated with the younger southwards; abruptly so across the northern tracts massive sandstone and blocky arenite sequences exposed in in all areas,but markedly less so tothe south in SW the Mull of Logan and Corwall blocks. The blocky arenites, Scotland. In the Ards Peninsula and the graptolite evidence, representing aseries of amalgamated debris-flow deposits although limited to three of the most southerly seven blocks, which probably resulted from massive slumping within the suggests littlechange in the age of turbiditesediments basin, are unlikely to havea considerable lateral extent; exposed south of Coalpit Bay. hence theirabsence from the Millisle Block is not significant. None of the blocks from Ballywhiskin tothe Hawick Group boundary can clearly be correlated with the Structural correlation Galloway succession. The calcareous strata forming the The overalltectonic style of the northerntracts is northern part of the Ballywalter Block are not seen in SW comparable with the overall Northern Belt structure. A few Scotland butthe southern part of this block andthe southward-verging folds occur in dominantly steeply Wallace’s Rocks Block are both superficially similar to the dipping,often overturnedstrata, the sheet dip inclining PortLogan Block. Thickly bedded, massive sandstone is steeplynorthwards. These blocks become progressively

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younger southwards (Fig. 5), requiring major reverse faults continuity,albeit with some uncertainty, especially across between the blocks (cf. Craig & Walton 1959). Steeply the North Channel. However, aspects of the central parts plunging folds adjacent to these faults are common in the are unusual compared with the generalpicture of the Rhinns of Galloway but are not seen in the northern parts of Southern Uplands. The widening to the west is associated the other two areas. with an increase in the number of structural blocks and a Centraland southern parts of thethree areas differ flattening out of the trend of successive blocks to become markedly from this pattern. The sheet dip is very variable, younger southwards. often being gently inclined or horizontal in intensely folded Major tracts of southward-younging strata, including areasand dippingsteeply southwards in large areas of northward-verging structures, suggest overthrusting towards southward-younging strata with northward-vergingstruc- the north rather than the northerly directed under-thrusting tures. The latter style is particularly well developed in the required by theSouthern Uplandsaccretionary prism Rhinns of Galloway, south of the Port Logan Fault (Fig. 3) model. Inthe southern part of the Rhinns of Galloway where strata are dominantly overturned, dipping steeply to major faultsthrow down tothe south, and the overall thenorth. Extensivebelts of steeplydipping, southward- structural configuration is consistent with northerly directed younging strata occur in the Ards Peninsula, as a result of a over-thrusting. This is also the case in the Ards Peninsula major fold in the Portavogie Block (Fig. 2), and in the where thestructure of the Portavogie Block can be poorly exposed central part of the Wigtown area. interpreted in terms of anorthward-directed, landward The Kirkmaiden Formationoutcrop in the Wigtown thrust rising from and transporting material in the opposite Peninsula has a very similar structure to the Portaferry and direction tothe southerlydirected Cloughy Fault. It is Ballyquintin Blocks in the Ards Peninsula. In both areas the importantto recognize that whatever mechanism is Hawick Group strata are intensely folded such that the sheet proposed for development of the southerly young- dip is inclined gently northward. A more complete section ing/northward-verging areas,it mustbe an integral part through the Hawick Group in NE Ireland, exposed in the of theD, deformation since all of themajor structures Lecale area (Anderson & Cameron 1979, fig. 2; Cameron observed are of this age. Post-D,, northerlydirected 1981, fig. 3) shows a strikingly similar structureto the thrusting would not generate wide areas of southward- Wigtown peninsula section. younging strata,and any related, northward-verging The first folds in all three areas generally plunge gently structures would fold SI. The development of major back or moderately tothe NE or SW,but steep to vertically thrusts as an integral part of the main thrusting event (DJ, plunging, sometimesdownward-facing, folds areabundant effectively shortening the over-riding thrust sheet(s), meets locally (Fig. 6), commonly forming minor- to intermediate- this criterion and is proposedas an explanationfor the scale,sinistral pairs. Only onezone of steeply plunging Central Belt structures.These could nototherwise be structures, near the northern margin of the Hawick Group, accommodated in the acceptedtectonic model forthe occurs in all three areas: others are of only local extent. One Southern Uplands.Back-thrusting also explains the pervasive fabric (S,) occurs throughout the three areas and narrowing of theCentral Belt eastwards with the loss of is congruous with the D, structures. In the Ards Peninsula it certain blocks but would allow correlatives of these to varies from parallel to fold axial surfaces to transecting, reappear farther east. typically 20" clockwise. In SW Scotland cleavage is usually Landward fold vergence and overthrustinghave been clockwise transecting,although by variablea amount. described fromthe Pleistocenetectonic accretion of the Models to account for transecting cleavage (e.g. Borradaile Cascadia Basin on the continental slope off Washington and 1978; Sanderson et al. 1980, 1985; Stringer & Treagus 1980; north-western Oregon (Silver 1972). The possible reasons Murphy 1985) invokeshear, usually sinistral,asthe for this geometryhave received detailedtheoretical and principal cause. Evidence for syn-D, sinistral shear is also model analogue analyses by Seely (1977). In the Cascadia seen in the zones of steeply plunging folds. Since neither Basin,landward vergence developed on a lower-slope these folds nor transecting cleavage are always developed terrace, contemporaneously with seaward vergence on the continuouslyalong strike inindividual structural blocks, outer part of the continental shelf, in the early Pleistocene there must have been significant variations in strain within (Seely 1977, p. 191). The suppression of the normally each thrust sheet if the blocks are indeed such. dominant, landward-dipping, seaward-verging thrusts is Second phase(DJ structures, usually opento close, favoured by the presence of a basal layer of low shear southward-vergingminor- to intermediate-scalefolds, are strength, typically awater-retentive clay overpressured by only well developed in the Hawick Group outcrops in the the rapiddeposition of overlying turbidite sands (Roberts three areas. Minor intrusions in all three areas increase in 1972; Seely 1977). Seaward-dippinganisotropy in the abundance southwards, with anintense zone of biotite sediments, or in the underlyingoceanic floor, andgentle lamprophyres coincident with the Hawick Group outcrop in topographic slopes are other possible favourable factors. At Scotland but extending into the Portavogie Block in Ireland. least the first of these factorsprobably obtained in the These dykes have a range of ages, the earliest being pre-S, Southern Uplands Central Belt, with rapid deposition and (butpost-D, folding) in Ireland but post-S, although still growth of Upper Llandovery turbidite fans across the fine pre-D, in Wigtownshire, and were intrudedthroughout a silts,shales and mixed-layer clay bentonites (Cameron & series of compressional and extensionalevents (Anderson Anderson 1980) of the Moffat Shale sequence. South of the 1969; Barnes et al. 1986). northerly verging structures,the return to predominantly southerly,seaward vergence in the Hawick Group of the Ards and Wigtown Peninsula and throughout the Southern Belt may reflect different turbidite sedimentation rates, a Interpretation changein thenature of the underlying Moffat Shales, or As shown abovethe northern and southern parts of the simply that the main dkcollement horizon has climbed above three areas can be correlated, indicating some strike-parallel the Moffat Shales.

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Conclusions from Canada and Scotland. Geological Society of America, Bulletin, 89, 481-93. Althoughsome tracts can betentatively correlated across CAMERON,T. D. J. 1981. The history of Caledoniandeformation in East thethree study areas, it is evident that strike-parallel Lecale,County Down. Journal of the RoyalDublin Society, Earth continuity of thecharacter and width of the greywacke Sciences, 4, 53-74. tracts, so remarkably demonstrated in the Northern Belt, is - & ANDERSON,T. B. 1980. Silurian metahentonites in CountyDown, difficult to define in theCentral Belt of Galloway and Northern Ireland. Geological Journal, 15, 59-75. COOK, D.R. & WEIR,J. A. 1979. Stratigraphy of theaureole of the Down. Whilst some changes along strike, particularly across Cairnsmore of Fleetpluton, southwest Scotland. In: HARRIS,A. L., theNorth Channel, may be a reflection of original, HOLLAND, C. H. & LEAKE,B. E. (eds) The Caledonides of the British sedimentary variation in the profiles of the turbidite fans, Isles-Reviewed. The Geological Society, London, Special Publication 8, tectonic repetition and excision also appear to be important. 489-94. - & ~ 1980. The stratigraphicalsetting of theCairnsmore of Fleet The large areas of northward fold vergence and Pluton, Galloway. Scottish Journal of Geology, 16, 125-41. southward-younging strata described in this paper represent GRAIG, G.Y. & WALTON,E. K. 1959. Sequence and structure in the Silurian a significant deviation from the current model of Southern rocks of Kirkcudbrightshire. Geological Magazine, 96, 209-20. Uplandsstructure. We arenot aware of any proposed FLOW, J. D. 1982. Stratigraphy of a flysch succession: theOrdovician of WestNithsdale, SW Scotland. Transactions of the Royal Society of mechanism wherebypackets of strata severalkilometres Edinburgh, Earth Sciences, 13, 1-9. thick could beunderthrust and accreted in an essentially GORDON,A. J. 1962. The LowerPalaeozoic Rocks around Glenluce, invertedposition. Majorareas of dominantlysouthward- Wigtownshire. PhD thesis, University of Edinburgh. younging strata strongly suggest overthrusting to the north. GRIFFITM,A. E. & WILSON, H.E. 1982. Geology of the country around Carrickfergus and Bangor. Memoir of the Geological Survey of Northern We recommend that the existing model be modified, in at Ireland, Sheet 29. leastthis south-western part of theSouthern Uplands, to IRVINE, D. R. 1872. Explanation of l :50,0,000 Sheet 1. Memoir of the include such overthrusting or back thrusting, generating the Geological Survey of Scotland, HMSO, Edinburgh. northward-vergence we describe,penecontemporancously KEMP, A. E. S. & WHITE,D. E. 1985. Silurian trench sedimentation in the SouthernUolands. Scotland: imolications of new aeedata. Geoloaical I, ,L Y with sequential northward under-thrusting. Magazine, l22, 275-7. LEGGETT,J. K., MCKERROW,W. S. & EALES,M. H. 1979. The Southern

Uplands of Scotland: a Lower Palaeozoic accretionary~. prism. Journal of the Geological Society, London, l36, 755-70. The paper has been improved by comments from B. Lintern and --, & CASEY,D. M. 1982. Theanatomy of a Lower Palaeozoic anonymous referees. T.B.A. wishes to thank B.Rickards for accretionary forearc: the Southern Uplands of Scotland. In: LEGGETT,J. identification of graptolite faunas. R.P.B. thanks members of the K. (ed.) Trench-Forearc Geology. The Geological Society,London, Special Publication 10, 495-520. BGSSouthern Uplands Project team for valuable assistance and MURPHY,F. C. 1985. Non-axial planar cleavage andCaledonian sinsitral discussion, and publishes with permission of the Director, British transpression in Eastern Ireland. Geological Journal, 20, 257-79. GeologicalSurvey (NERC).J.A.McC. wishes to thank A. E. S. PEACH, B. N. & HORNE, J. 1899. The Silurian rocks of Britain, l, Scotland. Kemp for helpful discussion of landward vergence, I. Strachan, A. Memoir of the Geological Survey, Scotland. W.A. Rushton and D. E. Whitefor identification of graptolite READ, H. H. 1926. Mica lamprophyres of Wigtown. Geological Magazine, faunas and the Departmentof Education for Northern Ireland for a 63, 422-9. REYNOLDS,D. L. 1931. The dykes of theArds Peninsula, Co. Down. post-graduate research grant. Geological Magazine, 68,97-111, 145-65. ROBERTS,J. L. 1972. The mechanics of overthrust faulting: a critical review. 24th International Geological Congress, 3, 593-8. RUST,B. R. 1965. The stratigraphy and structure of theWhithorn area of References Wigtownshire, Scotland. Scottkh Journal of Geology, 1, 101-33. SANDERSON,D. J., ANDREWS,J. R., PHILLIPS,W. E. A. & HUTTON,D. H. W. ANDERSON,T. B. 1962. The stratigraphy, sedimentology and structure ofthe 1980. Deformationstudies in theIrish Caledonides. Journal ofthe Silurian rocks of the Ards Peninsula, CountyDown. PhD thesis, Geological Society, London, l37, 289-302. University of Liverpool. -, ANDERSON,T. B. & CAMERON,T. D. J. 1985. Strain history andthe -1969. The geometry of a natural orthorhombic system of kink bands. In: development of transectingcleavage, with examples from the BAER, A. J.& NORRIS,D. K. (eds) Proc. Conf. on Research in Tectonics Caledonides of theBritish Isles (Abstract). Journal of Structural (Kink bands and brittle deformation). GeologicalSurvey of Canada Geology, l, 498. Paper 68-52, 200-28. SEELY,D. R. 1977. The significance of landward vergence and oblique -1978. Day 2: The Ards Peninsula, County Down: A profile section of the structural trends on trench inner slopes. In: TALWANI,M. & PITMAN,W. SouthernUplands subduction complex. GeologicalSurvey of Ireland, C. (eds) Island Arcs, Deep Sea Trenches and Back Arc Basins. American Guide Series, 3, 19-30. Geophysical Union, 187-98. - 1987. The onsetand timing of Caledonian sinistral shearin County SILVER,E. A. 1972. Pleistocene tectonic accretion of the continental slope off Down. Journal of the Geological Society, London, 144, 817-25. Washington. Marine Geology, 13, 239-49. - & CAMERON,T. D. J. 1979. Astructural profile of Caledonian STRINGER,P. & TREAGUS,J. E. 1980. Non-axial planar S1 cleavage in the deformation in Down. In: HARRIS, A. L., HOLLAND, C.H. & LEAKE,B. Hawick Rocks of theGalloway area, Southern Uplands, Scotland. E. (eds) The Caledonides of the British Isles-Reviewed. The Geological Journal of Structural Geology, 2, 317-31. Society, London, Special Publication 8, 263-7. - & - 1981. Asymmetrical folding in the Hawick Rocksof the - & OLIVER,G. J. H. 1987. The OrlockBridge Fault: A major late Gallowayarea, Southern Uplands, Scotland. Scottish Journal of Caledonian sinistral fault in the Southern Uplands terrane, British Isles. Geology, 17, 129-48. Transactions of the Royal Society of Edinburgh, Earth Sciences, 77, WALKER,R. G. & MUTTI,E. 1973. Turbidite facies and facies associations. 203-22. In: MIDDLETON,G. V. & BOUMA, A. H. (eds)Turbidites and Deep-Water BARNES,R. P. in press. The geology of the Whithorn District. Explanation of Sedimentation. Society of Economic Palaeontologists and Mineralogists, l :50,000sheet, British Geological Survev, Sheet 2, Scotland. Pacific Section, Short Course, Anaheim, 119-57. -, ROCK,N.M. S. & GASKAR~,J. W. 1986. LateCaledonian WARREN,P. T. 1964. The stratigraphyand structure of the Silurian rocks dyke-swarms in SouthernScotland: new field, petrological and south-east of Hawick,Roxhurghshire. Quarterly Journal theof geochemicaldata for the Wigtown Peninsula, Galloway, Geological Geological Society of London, 120, 192-222. Journal, 21, 101-25. WEBB, B. 1983. Imbricatestructure in theEttrick area, Southern Uplands. BORRADAILE,G. J. 1978. Transected folds: A study illustrated with examples Scottiyh Journal of Geology, 19, 387-400.

Received 20 March 1986; revised typescript accepted 23 January 1987

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