Journal ofthe Geological Society, London, Vol. 144, 1987, pp. 787-805, 13 figs, 3 tables, Printed in Northern Ireland Style, scale and significance of sand bodies in the Northern and Central Belts, southwest Southern Uplands G.KELLINGl, P. DAVIES' & J. HOLROYD2 1 Geology Department, University of Keele, Staffs. ST5 5BG, UK 21 Blythe Road, Forsbrook, Blythe Bridge, Staffs., UK Abstract: Sedimentological and biostratigraphical data fromthe Rhinns of Galloway andadjacent areas in SW Scotland confirm that deep-water depositional systems consistently operated along, and were sourced from, the northwestern margin of an asymmetrical basin during the late Ordovician and earlySilurian, while pelagic facies accumulated simultaneously tothe SE, providing ascenario analogous to many modern trench systems. Most of the observedsedimentological anomalies, with regard to thisgeneral model, can be explained within the context of the varied styles of trench-filling depositional systems, briefly reviewed here, and the major stratigraphic and sedimentologic features can be best explained in terms of a geotectonically evolving fore-arcregion. Two main phases of development are recognized: (a) Llandeilo-late Ashgill: during this time interval the fore-arc trench region was tectonically juxtaposed against an active continental margin arc. Small- to medium-scale, SE-prograding sand-rich fans were formed within a relatively narrow trench, leading to axial diversion of the fans, initially to NE but later mainly to SW. Simultaneously a coarse volcanilithic sediment apron, flanking the arc, migrated gradually northeastwards, probably in response to relative fault displacement of the arc and trench; (b) Llandovery: during this time interval the fore-arc trench region was dominated by a variety of mainly fan-typedepositional systems which were exclusively sourced(at least until theuppermost Llandovery) from the northwestern margin. The juxtaposition of coeval depositional systems differing significantly in sedimentological style and composition strongly suggests that the Llandovery sedimen- tation in this region occurred in severalstructurally separated sub-basins within the main trench system. In the plethora of stratigraphic, structural and petrographic regional synthesis of sand-body character and geometry as data and speculations accompanying the revival of interest determined primarily from the well-exposed coastal sections in theSouthern Uplands since its re-interpretation as an of the Rhinns of Galloway with supplementary information accretionaryprism, relatively few contributions have been from the shores of Luce Bay and Loch Ryan (Figs 1 & 7), concerned with description andinterpretation of the whereboth across- andalong-strike lithological variations externalgeometry and internal arrangement of the can bestudied. The evolution of sedimentary systems component sand-bodies, which constitute more than 90% of developed within tracts 1-6 of theNorthern and Central the sedimentthickness included in the imbricatedand Belts (McKerrow et al. 1977), representing the late fault-bounded tectonic elements. Llandeilo-late Llandovery time interval (Fig. 2), thus may Kelling (1964) and Walton (1965) speculated onthe beelucidated and assessed in terms of variations in the submarine canyon or fan-channel origin of rudite bodies in controlling geotectonic regime. theRhinns of Galloway-Glen App Ordovician sequence, Datafor the Ordoviciansections onthe Rhinns of while Kelling andHolroyd (1978) and Holroyd (1978) Galloway and adjacent sectors of the Northern Belt derive assembled further evidence for the fan-channel environment of similar ruditesin theGlen Afton, Carsphairnand Scaur-Shinnel areas of the Northern Belt (Fig. 1). Leggett (1980) describeda prograding outer-mid-inner submarine fan sequence fromthe Stobo area, and speculated that sedimentationin thelate Ordovician of theSouthern Uplands may have been analogous to the Quaternary of the Eastern Aleutians, with small fansextending from a Dumlrles (northwestern) lower slope across a mud-rich trench wedge. Hepworth et al. (1982) ascribed late Ordovician sequences fromthe Abington-Sanquhar area to proximal and distal sectors of axially deflected fans,entering the Southern Uplands trench from widely separated point-sources on a ,P northwestern-bounding slope. The examples cited above suffer, as Leggett (1980, p. 405) hasrecognized, fromthe problemsassociated with poor biostratigraphiccontrol and limited inlandexposure. The Fig. 1. General location map for SW Scotland. Ornamental purpose of the present paper is primarily to presenta segments denote regions described in more detail in the text. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/5/787/4889532/gsjgs.144.5.0787.pdf by guest on 27 September 2021 788 G. KELLING, P. DAVIES & J. HOLROYD l TIME-STRATIGRAPHICDIAGRAM FOR THE NORTHERNAND CENTRAL BELTSNorthern Belt I Central Belt Fault Fault Fault FaultI 6b Kmgledores Fault 5a 5b Leadhlls L~ne Glen App Fault 3a 3b -I I I Bav-ClanvardCalrnaarroch Bay 2 l2 2 I Rudltes J Thlckly-bedded greywackes X Thmly-bedded greywackes Slltstone LLANDEILO tzi Black shale / Major current dlrectlon! Carsphalrn Pwtpatrlck Shinnel Chert X Graptolltecontrol Water J Fig. 2. Time-stratigraphic diagram for the Northern and Central Belts in SW Scotland, showing generalized lithological successions and mean palaeocurrents. Numbers above columns relate to fault-bounded tracts of Leggett et al. (1979), as modified herein. mainly from studies by Kelling (1961, 1962 and unpublished rest upon a number of assumptions, the most important of observations),Holroyd (1978),Walton (1956), Welsh which are enumerated below: (1964), Floyd (1982) and recent work by P. Davies. (1) The depositionalenvironments in which these Informationrelating tothe Silurian sequences onthe predominantly deep-water clastics wereformed can be Rhinns and the eastside of Luce Bay is derived mainly from determined (in gross and in detail) from a study of the the following recent studies. vertical ordering of lithofacies, supplemented by knowledge The rocks of theRhinns of Galloway between of lateralrelationships and palaeoflow patterns(where Cairngarroch Bay [NX046492] and Clanyard Bay available),in terms of the submarine fan model (as [NX 1073801 haverecently been studied in detail by P. developed by Mutti & Ricci-Lucchi 1972; Walker & Mutti Davies and have been subdivided into six tec- 1973; Mutti 1979 Nilsen 1979), other deep-water deposi- tonostratigraphicunits, each containing between oneand tional regimes (Nelson & Nilsen 1984; Stow 1985) and the fourlithostratigraphic sub-divisions (Fig. 7). Biostrati- trench-fill modelsadvanced by Schweller & Kulm (1978) graphic control both here andin the Glenluce area is based on (see below). the collections of Peach & Horne (1899) and additional material (2) The mid-Ordovician-early Silurian rock succession collectedrecently by Davies and officers of the British considered here was deposited on or close to a continental Geological Survey. margin representing the northwestern limit of the Iapetus Onthe east side of Luce Bay, betweenGlenluce oceanic realm. [NX 1965741 andGarheugh [NX 2745041, Gordon (1962) (3) Themajor fault-boundedtracts enumerated by originally assigned the Silurianrocks to twostratigraphic Leggett et al. (1979) areadopted as tectonostratigraphic divisions, namely theGarheugh and Kilfillan Formations units in the following description, although it is recognized (Fig. 7). Recent investigations by the B.G.S. have identified that (a) several lithostratigraphic and tectonic elements may a further tectonostratigraphic unit (the Corwall Formation), occur in a single tract, (b) the same depositional system may lying immediately south of the Garheugh Block (see Barnes be represented in two or more tracts and (c) the nature and et al. this issue). Additional work by P. Davies has led to tectonic significance (including amount and direction of splitting of theGarheugh Formation into two tec- displacement) of the boundingfractures may be highly tonostratigraphic sub-divisions, here termed the Garheugh- variable, so that geographical matching across the tract A and Garheugh-B Formations. boundaries is likely to be unreliable and suspect. Correlation between the Rhinns of Galloway andthe east side of Luce Bay has been possible by comparing the Trench-filling depositional systems relative structural positions of the individual formations within their respective profiles and by using distinctive Within the accretionary prism model for the Southern petrographic and lithologic associations as marker horizons, Uplands (Leggett et al. 1979) it is likely that a variety of supplemented by biostratigraphic markers to identify coeval depositional systems operated within the marginal slope, tectonostratigraphic blocks. trench and ocean-floor environmentsand these may have The observations and conclusions offered in this paper included fan, axial wedge and debris (slope-base) wedge Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/5/787/4889532/gsjgs.144.5.0787.pdf by guest on 27 September 2021 NORTHERNANDCENTRAL BELTSAND BODIES. S. UPLANDS 789 from basin floor to fan-type facies (Nilsen 1979). Modern mud-rich systems range from small to large, with predominant channel and inter-channel areas and poorly to undevelopedlobes. In ancientsequences this type of fan system may be mud-dominated with scattered sandy packets exhibiting thinning- and fining-upward cycles representing channeldeposits (Nilsen