Along-Strike Variation in the Stratigraphical and Structural Profile of the Southern Uplands Central Belt in Galloway and Down
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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 Gala Group 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/5/807/4889554/gsjgs.144.5.0807.pdf by guest on 26 September 2021 808 R. P. BARNES, T.ANDERSON B. & McCURRYJ. A. 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