Style, Scale and Significance of Sand Bodies in the Northern and Central Belts, Southwest Southern Uplands

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 sedimentation 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 trenchfrom 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.

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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

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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 1979). Modern mixed-sediment fans (Nilsen 1979) contrast in thatthe fan surface is dominated by acombination of channelsand lobes. An ancientsequence resulting from the progradation of this type of depositional system might be dominated in its lower part by thickening- and coarsening-upward cycles (representing lobe deposition) with a gradual transition from the basin plain facies into the fan facies. The upper part of this sequence may be dominated by thinning- and fining-upward cycles representing channel deposition. The area1 geometry of any type of system need not be radial and in modern environments itappears to bedictated primarily by the basin shape. Modern axial wedge systems have beenexamined by Schweller & Kulm (1978). These most commonly occur within trenches along plate margins, although similar depositional systems and related facies are developed in other narrow, elongate basins (see Hsu et al. 1980). Axial trench wedges, as described by Schweller & Kulm (1978), / are dominated by long linearchannels, flanked by thinly /A bedded turbiditic deposits. Lobes may be developed at the distal ends of the channels.Although the internal arrangement of this depositional system remains obscure, it seems that in ancient sequences axial trench wedge deposits may producesequences which are essentially similar to those of elongatefan systems which have beendiverted axially. In such circumstances differentiation would have to TSB: Trench Slope Break 1/1..... Trench-fill Facies be based largely on regional palaeocurrent studies. Modern debris wedges and slopeaprons consist of 4 SedimentTransport AP: Accretionary Prism turbidites, hemipelagic anddebris flow depositsand PelagicFacies slumped material accumulating in base-of-slope environ- OTH: OuterTrench High /\/\, OceanicPlate ments which may be locally incised by channels. In ancient sequences the degree of internal organization is minimal and Fig. 3. Conceptual models of depositional systems associatedwith trenches on active convergent margins:(A) lateral submarine fans; sections are dominated by slumps and debris flow deposits (B) axial wedge; (C)slope-derived debris wedge;(D) filling (Surlyk 1978; Nelson & Nilsen 1984) but may also include achieved by a combinationof systems (A)-(C). significant thicknesses of ‘proximal’ turbidites (Stow 1985). It is likely that several types of depositional systems were operative simultaneously within a basin on the scale of the systems (Fig. 3). Present opinions vary as to the true nature Southern Uplands/Longford-Down zone. of theSouthern Uplands ‘basin’, fore-arctrench and back-arc basin being just two of the options, but whatever the geotectonic scenario, a similar array of depositional Character, geometry and interpretation of sand- systems might be expected to develop. bodies In modern deep-water settings three main depositional systems are recognized (Reading 1986): (a) slope apron, (b) Tract 1 (late Llandeilo-early Caradoc) basin plain and (c) submarinefan. Of these systems the facies associations forthe submarinefan model are most Thistract is represented by the Corsewalt Group in the widely accepted and have been adopted here (see above), Rhinns and the Finnarts Group north of Glenn App. The supplemented by referenceto the modelsfor theother lower part of the Corsewall Group (Flaggy ‘Division’) major systems, where appropriate. A spectrum of fan types comprises at least 750 m of parallel-bedded, fine-grained exists, ranging from sand-rich (Mutti 1979) to mud-rich sediments(Table 1) with a few coarseturbidites. These (Nilsen 1979) systems, with variousintervening mixed- acyclic sequences display predominantly axial (to SW) sediment fan types (Mutti 1979). Modern sand-rich systems palaeocurrents with subordinate SE-directedvectors. The tend to be relatively small; channel environments predomin- thickness and uniformly fine-grained, even-beddedand ate and lobes are poorlydeveloped. In ancientsequences ‘distal’ character suggest a basin plain environmental setting. this type of system may manifest itself asaseries of The Flaggy Division rocks pass up rapidly intothe sand-dominated thinning- and fining-upwards cycles (repre- Conglomeratic ‘Division’, characterized by thick packets of senting the deposits of channels), with abrupt transition polymict rudites,and coarse arenites (Table 1) with

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E v! N 2

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m C -0 V m v) LL D m m >- m W a 0 Y D 1

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subordinate thin-bedded successions resembling the Flaggy flow, while cross-strata and structures in finer arenites and Division lithofacies. Most rudites are internally disorganized siltstones yield W-SW-directed vectors. (facies Al.l and A1.4 of Pickering et al. 1986), but about The lithologic and sediment transport attributes revealed 35% of the rudite units are organized (facies A2.1,A2.2, by this along-strike transect are consistent with the braided A2.3, A2.5 and A2.7 of Pickering et al. 1986). 'Bipartite' submarinechannel model advanced by Hein and Walker units (Kelling & Holroyd 1978) are locally important. (1982). On this interpretationthe sequences described Detailed mapping and logging of well-exposed rudites on above represent a lateral transition from an axial position in thenortheastern extremity of theRhinns peninsula, near a major NW-SE-trending channel-complex, with multiple Milleur Point, provide data on the lateral relationships and active thalwegs (Milleur Point-Boak Port Area), through a extent of the channelized and stratiform rudite bodies lateral terrace succession (Dove Cavesequence), passing encountered at intervals throughout the Northern Belt (Fig. westwards into ahigher terraceor leveed sequence at 4). Here seriesa thick,a lenticular units of boulder- Portleen (with overbank flow to W and SW). Farther SW, a and-cobble rudite and coarse arenite with erosive bases are thin-bedded siltier succession appears, which is interpreted stacked to athickness in excess of 300 m. The maximum as an inter-channel facies. The slightly younger Corsewell observed development of thick and coarse units occurs west Point channel-complex is excavated into these inter-channel of Boak Port (Fig. 4A) and a logged sectionthrough the deposits. lower part of this stack (Fig. 4B) reveals a predominance of These laterallysupplied channelized complexes are debris-flow and high-concentrationturbidity flow deposits, interpreted as proximal or inner fansediments and the arranged in cycles thatappear to fine and thin upwards minimum width of the laterally shifting active channel (5-18 m thick). Both erosional anddepositional sedimentary forming the Corsewell-Finnart Groups rudite bodies is structures indicatevery uniform palaeoflow towards the estimated at 2-3 km,comparable in scale tothe apical ESE-SE sectors (Fig. 4B). channels of modern fans such as the Rhone, San Lucas and About 700 m along strike to the west, a logged section Rosetta (Nile) (Fig. 5). Overall vertical and lateral near Dove Cave through roughly coeval sediments (Fig. 4C) relationships of the Conglomeratic Division with the Flaggy reveals a reduced proportion of rudites and a predominance Division are thus consistent with rapid progradation to SE of inverse-graded and organized units of high-concentration or medium-scale fans over an older axial wedge. turbidity flow origin. Cyclicity here is more complex, with NW of the Rhinns, around Glen Afton, the Marchburn alternations of relatively thin (4-8 m) fining- and Formation is broadly correlative with the Corsewall Group coarsening-up sequences. More variable palaeoflow vectors (Floyd 1982). Polymict pebble and small cobble-rudites are again dominantly towards the E-SE octant. Some 350 m within the Marchburn Formation are finer in grade than the farther west, a logged section on the west side of Portleen, Corsewall Formationequivalents, butare similar to their near Dulslouch (Fig. 4A) lacks true rudites but is dominated texture and sequential arrangement, and appear to be more by pebbly andcoarse arenites, with minorsiltstones and distal representatives of another small fan-system, again mudstones (Fig. 4D).Tractional structures (including prograding SE (Holyroyd 1978). current-ripple lamination and medium-scale cross- stratification) arecommon and the sequence is generally Tract 2 (lute Curudoc) acyclic, although poorly defined thinning-up andrare thickening-upsequences (15-20 m thick) are recognized. This tract is represented on the Rhinns by the Kirkcolm and Thepalaeocurrent pattern is crudelybimodal; vectors Galdenoch Groups. derivedfrom coarser units indicate predominantly E-SE The Kirkcolm Group (Kelling 1961) is characterized by

PROXIMAL FAN-CHANNEL DIMENSIONS

MODERNFANS i' f

Fig. 5. Graph indicating relationship between radius (maximum area1 dimension) of modern submarine fans and the average width of the active channel in the apical portion (between base of slope and first bifurcation). Data derived mainly from /X10 / Barnes and Normark (1983) and references <4 SOUTHERNUPLANDS therein, supplementedby Nelson and / / NORTHERNBELT Nilsen (1984) and Kelling et al. (1979). (A)-(D) indicate the range of estimated widths of proximal fan-channels within the Corsewall-Glen App, Afton, Carsphairn and Shinnel-Scar complexes (late Ordovi- cian of Northern Belt, Southern Uplands). See text for further discussion.

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relatively siliceous arenite compositions, lacking the fresh deposition in stackeda series of lobes built byflows ferromagnesianminerals andabundant mafic/ultramafic travelling axially along the trench both to SW and NE (see lithic clasts of the Corsewell Group. Regularly bedded palaeocurrent vectors in Fig. 6A & B). medium and fine arenites (facies b in Table 1) dominate the Onthe Rhinns the main indications of lateral supply Kirkcolm succession. Thick packets of silt-dominated (from NW) are within the stratiform and poorly channelized lithology are also common (Table 1, facies c), while the rudite sequencein the basal Kirkcolm south of Cave rudite/coarse arenite facies association (Table 1, facies a) is OchtreePoint, near Portslogan (Fig. 6C). Herethe rare in the Rhinns exposures. The sedimentological features well-rounded cobbles and pebbles are almostentirely of the Kirkcolm sequencesreveal the prevalence of composed of greywacke (petrographically comparable with non-channelized or open-sheet turbidity flows of high and adjacent Kirkcolm arenites, exceptfor carbonate cement) low concentrations, with occasional minor, poorly channel- and the rudites are incised into lobe-type sequences forming ized,high-concentration turbidity flows and debris flows. a thin cover (no more than 30 m) to a pelagic ocean-floor Most of the Kirkcolm succession is thusattributed to facies association of middle Caradoc age (including spilites, A CAIRN'DONALD B

CRAILLOCH - C F CAVE OCHTREE POINT B 2.1 D 2.1= pp+F.... Max. ... I Clast D 2.3= c 1.1 f D 2.1- - B 2.1 c 1.1 3 c 2.3= / - A 2.5 C 2.3- c=

c 2.1 - c 2.1 A 2.3 + A 2.1 B 2.2 c 2.2

B 2.1 A 2.1 c 1.1 2 D2.3 B 1.1 E 1.1: c 1.1 A 2.6 B 1.1 D1.1 A 2.1 c 1.1 f B 2.1 ..... + E 1.1= c 1.1 c 2.4 10 ... A 2.8 D2.3 p-...... + A 2.5 - B 1.1 D=

c 2.2 D 2.3= A 2.1 B 1.1 - A 2.5 c 1.1 D 2.3= - B 1.1 - C2.2 A 2.8 1.1 c SAPID c 2.3 I c 1.1

SAND Fig. 6. Logged sections within the Kirkcolm Group, Rhinns of Galloway (tract 2), illustrating general sedimentological characteristics of the Group. (A) and (B) exemplify faciestype (b) and (C) exemplifies facies (a)of Table 1. General lithological ornament as in legend for Fig. 4. Notations to left of lithological columnsrefer to facies typesof Pickering et al. (1986). (A) Caimdonald quarry [N.G.R. NW 9856761. (B) Crailloch quarry [NX0425881. (C) Coast 400 m south of Cave Ochtree Point, Portslogan [NW 9775861.

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cherts and black shales). This rudite body is interpreted as a Tract 3 (mid-Caradoc-? basal Llandovery) proximal mid-fan channel-fill, although the possibility cannot be excluded that it is an allochthonous block derived On the Rhinns this tract includes the Portpatrick Group and from a more proximal location in the canyon-fan system by the Portayew Rocks. gravitational sliding (Holroyd 1978). The Portpatrick Group, originally described by Kelling Stratigraphically succeeding the Kirkcolm Group,the (1961), is a fault-bounded stratigraphic unit of Ordovician Galdenoch Group is characterized by thick-bedded, age (Fig. 2) and is characterized by a volcanilithic relatively coarse turbidites, cyclically arrangedand with a petrography and a thickly bedded, coarse-grained nature. It ferromagnesian-rich petrography. Sedimentologic attributes comprises two petrographically distinctive stratigraphic (especially the small-scale and thickening-up nature of sub-units, the Acid- and Basic-clast Divisions (Fig. 7). component cycles; see Table 1) indicate that these sediments In terms of theircomponent facies the Acid- and were deposited in series a of relatively small and Basic-clast Divisions are very similar (see Table 1). The overlappingdepositional lobes, constructed by high- sedimentologic characteristics, exemplified in Fig. 8, suggest concentration turbidity currents probably flowing towards that much of the deposition was from high-density, open the SW-NW quadrant. sheet-flows and subordinate, broadly channelized flows, In the Glen Afton area the lower part of the probable giving rise to variousdepositional morphologies including GaldenochGroup equivalent, the BlackcraigFormation non-channelized slope,channel, interchannel and minor (Floyd 1982), contains thicklenticular bodies of boulder- lobes. Therandom vertical arrangement of these facies and cobble-rudites and coarse arenites, mainly belonging to suggests environmental instability and indicates thatthe the A2 and B2 facies classes of Pickering et al. (1986) and Portpatrick Group may not be the result of ‘fan’ deposition, carrying both extrabasinal andabundant intrabasinal which would normally produce more regularly ordered and (greywacke,shale andchert) large clasts. These bodies ‘cyclic’ sequences. Thusthe Portpatrick Group probably result from theoperation of high-concentrationturbidity represents a sandy apron depositional system, composed of flows andsubordinate debris flows, mainly flowing to SE several small coalescing depositional cones or very and S within a poorly defined channel system. The laterally ‘immature’ sandy fans. Palaeocurrents are rather variable in and vertically interdigitating medium-coarse arenites are of direction but indicate that these depositional systems were mid-fan aspect, with axial palaeocurrents (flow to SW or prograding from asource tothe S and SW, while the NE). Overall, the lower part of the Blackcraig appears to petrography demonstratesthe predominantly volcanic represent aproximal mid-fan association, laterally derived nature of this source. (from NW), but axially diverted within the basin. The width Within the upper part of the Basic-clast Division, there of the active channels in the basal Blackcraig is estimated at are several more siliceous units, rich in quartz andmetalithic 1.2-2.5 km,commensurate with afan system of medium fragments (lithofacies c of Table l), which are interbedded scale. with units exhibiting amore typical Basic-clast Division Throughout much of Tract 2, therefore, a trench-fill petrography (Fig. 8). The siliceous units range in thickness wedge largely constructed from axially diverted mid-fan from less than10cm to morethan 1.0m, and most are lobes is succeeded by more proximal,laterally prograding moderately well graded, exhibiting good Bouma sequences. fan elements of contrastingpetrographic composition. Thesefeatures indicate that they arethe depositsof Overall,a combination trench-filling scenario is indicated turbidity currentsrather than the products of tractional (Fig. 3D). winnowing andreworking. These graded siliceous units

Klllanlrmgan Bay

LUCEBAY

Local slratlgraphtc name

0 km 5 Baslc-clasl Dlv L Acld-clast DIV Porlayew Rocks Money Head Fm Floa! Bay Fm Kllilllan Fm 5a Sllnklng Blghl beds Garheugh-A Fm 5b GrennanPoml Fm Garheugh-E Fm ___ Malor strike Clanyardfault Ea+>= 4b6a Mull 01 Losan Fm Corwall Fm Late wrench iaull__.-- wrench Late The Chair Mbr Drurnblalr Mbr Dunlehlnnle Alflcry Mbr Slrallgraphlclransltlon 1 Mbr Daw PO Daw In1 Mbr Fig. 7. Map and table showing the distribution Sllurlan Cairnlem POSI Sllurlan 1 1 Flnnarl Chlppermore Mbr Mbr and nomenclature of tectonostratigraphicunits OrdovlclanISolurlanshales ILogan lPor1 6b Fm I in the central Rhinns of Galloway and the adjacent area on the east side of Luce Bay.

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Facies(m)Lith. Structure Facies (m) Lith.Struct ture __--

20 c2.1 Llthologtes c2.1

- Granularsandstone 82.2 D2.1 82.2 -82.2 Slltstone/shalelntraclasts c2.1 Sandstonetntraclasts c2.1 - Sandstone F2.1 Interbeddedsandstone/slltstcne c2.2 D2.t Slltstone Bl.l 14 c2.2 - I - Slltstone/shale F2.1 # l - I Shale Bl.l I 12 I 82.2 l C2.3 # I 02.1 I c2.2 # l Structures I D2.1 to I __ - __-_ Parallellamlnattcn C l - I A Rlpplecross lamlnatton c2.1 l c2.2 - 0 l EZZ Planarbedding F2.2 I - # I Crossbeddlng I I - Sharpcontact C2.3 6 I D2.1 I Erosional contact I D2.1 - l c2.1 - Grooves c1.t - ‘.” D2.1 Flutes c2.1 l* - LongltUdlnalrlppleS B c2.1 .. . m Slumphorlzon c2.2 2; 2 - B C2.3- c2.1 f Thlckenlng/coarsenlngup c2.1 I 0 __--___ 3. Thlnnlng/finingup

Sand Sand /sense) (knownTransport Vector units #-siliceous f-+ TransportVector (unknown sense) Fe. 8. Logged sequence from the upper part of the Portpatrick Group (Basic-clast Division), illustrating the general sedimentological characteristics of the Group. Notations to left of lithologic columns denote facies types of Pickering er al. (1986). Note relative abundance in this sequence of siliceous units, corresponding to facies (c) of Table 1. From coastal section, north of Mill Bawn, Portavaddie, near Killantringan [NW98215600].

therefore are interpreted as the open sheet-flow deposits of The Portayew ‘Rocks’ form a fault-bounded stratigraphic depositional systems derived from a different source to the unit of late Ordovician age (Fig. 7) whichwas originally Portpatrick Groupproper, and may relatedbe in included within the Acid-clast Division of the Portpatrick provenance to the Kirkcolm Group. Acomparison of the Group (Kelling 1961). Floyd (1982) noted the petrographic Portpatrick Group with along-strikeits petrographic differences between the rocks of the Acid-clast Division equivalents suggests thatthe onset of Portpatrick Group proper occurring to the north of Morroch Bay [NX016525] type sedimentation becomes progressively younger from the and those occurring to the south, which in general terms are SW in Irelandto the NE (Fig. 9). Available information much more siliceous. The latter were subsequently named from thesesequences indicates that they all share similar the PortayewRocks. Recent detailed mapping and sedimentological features andall may therefore represent the petrographic work suggests thethat Portpatrick same sandy apron depositionalstyle. This interpretation Group/Portayew ‘Rocks’ boundary is a majorfault, (together with the consistently immature petrography of the observed on the west coast of the Rhinns immediately south Portpatrick lithofacies) suggests that depositionoccurred of Port of Spittal Bay, at NX020519 (Fig. 7). within a proximal apron flanking the volcanic source and the The strongly sheared Portayew Rocks appear to include younging observed from SW to NE thus reflects relative twomain lithofacies (Table l),the sedimentological migration of the source rather thansimple progradation of a characteristics of which suggest deposition mainly from low- depositional system. to high-density open sheet-flows, with subordinate channel-

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STRATIGRAPHIC COMPARISON OF THE PORTPATRICK GROUP AND ITS ALONG-STRIKE PETROGRAPHIC EOUIVALENTS

CountyLongford County Down Rhmns of GallowayWest Nlthsdale (1) (21 (3) (4)

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~ ~ GL ,,,,*’ gracllls LLANDEILO

LLANVIRN Relatlve pos111ons AREWIG 01 secttons 1-4

Ct Carrlckateane Ct Forrnatlon RI IslandRedForrnatlon greywacke GL Glen LodgeGlenForrnatlonGL Rpt ROCkpOrt Forrnatlon <- blackshale ForrnatlonPolntForrnatlonGreyScar GP Sc lava PA PortpatrickAcld Divtslon ‘C l 1 POrtpatrlCk Basic Division i t OOkrn

Fig. 9. Diagram illustrating the along-strike diachronism exhibited by the Portpatrick Group and its petrographic equivalents. Compiled from data in Morris (1983), Craig (1984), Kelling (1961) and Floyd (1982).

ized flows. The sheet-flow sequences probably formed on a occurring on the Rhinns of Galloway and on the east side of N- or NW-facing ‘clastic slope’ cut by ephemeral channels, Luce Bay, respectively (Fig. 7). creatinga sandy apron analogous to that inferred for the A comparison of the sedimentological characteristics of Portpatrick Group, but rather more ‘distal’ in aspect. both Formations (Tables 2 & 3) reveals that the Float Bay Formation includes three basic lithofacies which suggest Tract 4a (early Llandovery) deposition mainly from unconfined sheet-flows, as lobes and more continuous sheets. In contrast the Kilfillan Formation This is represented only on the Rhinns, by the Money Head comprises three lithofacies, the characteristics of which Formation (Fig. 7), which is characterized by thickly suggest deposition mainly within channeland channel- bedded, coarse-grained arenitesand a relatively siliceous related environments. The change in sedimentological style composition with significant accessory volcanilithic detritus, between the Float Bay and Kilfillan Formations suggests a detrital pyroxenes and amphiboles. The Money Head distal-to-proximal relationship,a suggestion reinforced by Formation comprises three main lithofacies associations palaeocurrent evidence which indicates dominant palaeoflow (Table 2), the sedimentologicalcharacteristics of which to the SW. indicatedeposition largely within channel and channel- The presence within the Float Bay and Kilfillan margin environments, with minornon-channelized, open Formations of the deposits of various ‘fan-type’ environ- sheet-flows. ments, including a possible lateral transition from channel to The channel-dominated and uniformly coarse-grained lobe- and ‘fringe’-dominatedenvironments, together with nature of this Formation suggests that these sediments were the palaeocurrent evidence, indicates that these formations formed within a sand-rich depositionalsystem(s), cor- were deposited within an elongate ‘mixed’ sediment fan or responding to the ‘low-efficiency’ type of submarine fan of axial wedge depositional system (cf. Fig. 3B). Mutti (1979). Palaeocurrent evidence demonstrates that this depositional system was prograding from a northern margin. Tract 5a (mid-Llandovery)

Tract 4b (early Llandovery) The Stinking Bight beds and the Garheugh-AFormation are local representatives of the same tectonostratigraphic unit The Float Bay and Kilfillan Formations are local occurring on the Rhinns of Galloway and on the east side of representatives of thesame tectonostratigraphicunit Luce Bay, respectively (Figs 1 & 7).

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The Stinking Bight beds and the Garheugh-A Formation is the local representative on the eastside of Luce Bay (Fig. each comprise two basic lithofacies indicative of channel and 2). channel-related environments. Detailed comparison of these The Grennan Point Formation comprises a lithofacies two stratigraphic units (see Tables 2 & 3) reveals that the (Table 2) suggesting deposition mainly from sheet-flows but Garheugh-A Formation is more thickly bedded and coarser with someminor channelling. The Garheugh-B Formation thanthe Stinking Bight beds andpalaeocurrent evidence comprises two basic lithofacies (Table 3) representing lobe, from bothformations demonstrates NE-SW sediment slope-apron and minor channel environments: Fig. 10 shows transport.The change in sedimentary style from the the main characteristics exhibited by this stratigraphic unit. Garheugh-A Formation to the Stinking Bight beds thus may Palaeoflow in both formations was predominantlytowards reflect modification in channel-form from NE to SW, with the SW. In view of this palaeocurrent evidence the development of smaller channel forms in the SW. Garheugh-B and Grennan Point Formations probably were Palaeocurrent evidence and lateralsedimentological formed within the same elongate depositional system. This relationships suggest thatthe Stinking Bight beds and may have been a sand-rich fan as the coarse-grained nature Garheugh-AFormation wereformed within an elongate of the sediment suggests, but an interpretation as the ‘distal’ depositional system with the geometry appropriateto an part of an axial wedge system is not excluded. axial wedge (Schweller & Kulm 1978). Tract 6a (mid-?late Llandovery ) Tract 56 (mid-Llandovery) The Mull of Loganand Corwall Formationsprobably Onthe Rhinns of Galloway this is represented by the represent local developments of the samefault-bounded Grennan Point Formation while the Garheugh-B Formation stratigraphic unit on the Rhinns of Galloway and on the east

Facles (m1 Loth. Structure Facias (m) Lith. Struc Facies (m) Llth Structure

c2.1 c2.2 I - 1- c2.1 ; c2- 2 - I El.1 c22 I- c2.1 l (?l l c2.1 36 I ID2.1 I CA2

81.1 I - 1 61.1 I 81.1 1-

02.1 I 02.1 34 c22 l c2.2 I- I c2.1 82.1 1- l - c2.2 - I l - 82.2 81.1 I I 82.1 I c2.1 I - l c2.2 l I - l CA3

c2.1 c2.1 I i i 4m break D2.1

El.1 -

81.1 c2. l

Sand~~ ~~ ~ Sand Fii. 10. Logged sequences from the Garheugh-B Formation,illustrating the general sedimentological characteristics of this formation. Legend asin Fig. 8. From coastal sectionat the Rocks of Garheugh [NX 267950121.

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side of Luce Bay, respectively. The Mull of Logan rudite associations (lithofacies aand b of Table 2). The Formation hasbeen sub-divided into four members,the three lithofacies subdivisions of the Alticry Member (facies along-strikeequivalents of three of which are within the a, b and c of Table 3) are considered to bedistal equivalents CorwallFormation (Fig. 7). The CairnieFinnart and of the Duniehinnie rudites. Chippermore Members are thebasal divisions of the Mull of The irregularly bedded, rather than channelized, nature, Logan and Corwall Formations, respectively, and are local the sheet-like basal contacts and the simple proximal-distal variants of the same stratigraphic unit (Fig. 2). relationshipsexhibited by the Duniehinnieand Alticry The CairnieFinnart Member comprises one broad Membersindicate deposition as debris flows and high- lithofacies type, the sedimentological characteristics of which density sheet-flows on a large debris cone. Palaeocurrents imply depositionas suprafan lobes(Table 2). The and lateral grade changes concur in suggesting northeasterly Chippermore Member includes two lithofacies types (Table extension of this feature. 3) thedominant one (facies a) being very similar tothe The Chair Member overlies the Duniehinnie Member on characteristic lithology of the CairnieFinnart Member. the Rhinns and is correlated with the DrumblairMember However, differences areapparent: theChippermore which follows the Alticry Member in the Glenluce area. The Formation sandyturbidites usually exhibit less well Chair Member exhibits two main lithofacies types while the developed Bouma sequences, are more thickly bedded and Drumblair Member contains three lithofacies types, two of display more inter-uniterosional surfaces and slump which (facies b and c in Table 3) are very similar to those horizons. All thesefeatures are consistent with a simple within the Chair Member (Table 2) and reflect deposition, proximal-distal relationship(Tables 2 & 3). Palaeocurrent respectively, within fan-lobes and from open sheet-flows and informationindicates a more complex situation,the pelagic fallout. However, the lenticularity and coarse grade sediments of the CairnieFinnart Member exhibiting of the third Drumblair facies (a) of Table 3 suggest an origin palaeoflow to SE, while the Chippermore Member facies (a) in shallow channels.Palaeocurrent indicators suggest axial (Table 3) was transported towards SE and SW.It is thus transport from NE to SW in both the Chair and Drumblair possible that the Cairnie Finnart and Chippermore Members Members,the sediments of which probablyrepresent the were formed by morethan one depositionalsystem, outer-mid-fan to middle-mid-fan sectors of newly develop- probablymixed-sediment fans. The varied palaeoflow ing fans of this depositional area. pattern within theChippermore Member may then result from overlapping and inter-digitation of these systems. The Tract 6b (late Llandovery) subordinaterudite lithofacies (b) of theChippermore Member may represent the fill of shallow channels or, more This is represented by the Port Logan Formation, best probably, the product of high-density sheet-flow on developed on the west coast of the Rhinns of Galloway, and relatively small lobes. is afault-bounded stratigraphic unit characterized by very The Daw Point Member stratigraphicallysucceeds the thinly to thickly bedded, fine-to-coarse grained sediments CairnieFinnart Member within the Mull of Logan (Table 2) Formation, but there is no lithostratigraphicequivalent The Formationexhibits three well-differentiated litho- within the Corwall Formation of the Glenluce area. A thin facies (Table 2). Facies types (a) and (b) show many intra-formational conglomerate occurs at the deeply scoured features indicative of channel-fill and channel-levee origin, contact between the Cairnie Finnart and the overlying Daw respectively (Fig. 11). Lithofacies (c) may represent Point Members. Thiscontact probably reflects major fan-fringe deposition. The sandy parts of this formation are channel incision into the lobe-dominated morphology of the thusdominated by channel-relatedsequences intimately Cairnie Finnart Member. The Daw Point Member may be associated with thick fan-margin silts, an association which assigned to two divisions, the lower of which is dominated suggests that thePort Logan Formationsediments were by lithofacies (c) (Table 2), i.e. very thickly bedded and formed within a mixed-sediment to mud-rich fan system very coarse-grainedunits representing the fillsof stacked (Nilsen 1979). channels. The upper division is characterized by lithofacies (a) and (b) (Table 2), i.e. alternations of thickly and thinly beddedsediments, representative of channeland inter- Discussion and conclusions channel depositional environments. Inthe following section the sedimentologic evidence The Duniehinnie and Alticry Members are correlated by summarizedabove is utilized todetermine the general means of theirvery distinctive lithologic characteristics, both sequence of depositionalenvironments represented in the being dominated by intraclast rudites (Tables 2 & 3). On the later Ordovician and early Silurian succession of this part of Rhinns of Galloway the Duniehinnie Member succeeds the theSouthern Uplands. Where appropriate evidence is Daw Point Member, while on the east side of Luce Bay, the available fromadjacent parts of the SouthernUplands or Alticry Member directly overlies the Chippermore Member. NorthernIreland, this hasbeen incorporated into The basal contact of the Duniehinnie Member is marked palaeoenvironmentaland palaeogeographic interpretations. by soft sediment disruption within the uppermost part of the In combination with stratigraphic,structural and pet- Daw Point Member but does not appear to be particularly rographic data, this palaeoenvironmental analysis also erosional. Along strike the rarely exposed basal contact of illuminates aspects of the geotectonic evolution of this the Alticry Member also shows little sign of major erosion. region. It therefore seems that the basal contact of the Alticry and Duniehinnie Members is broadly planar.Both the Duniehinnie and Alticry Members are internally divisible Palaeoenvironmentallpalaeogeographicevolution intoseveral stratigraphic units onthe basis of lithofacies During the late Llandeilo-Caradoc interal (gracilis-clingani variation. The Duniehinnie Member displays two major Zones) pelagic facies accumulated in the oceanward areas

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A Facies [m)Lith.Structure B Facies(rn)Lith. Structure

Y -- 1

h--

c2.2

F2.2 c2.22 02.1 - Fig. 11. Logged sequences from the Port __ Logan Formation, illustrating general sedimen- D2.1 tological characteristics and measured palaeo- __ currents from (A) lithofacies (b) and (B) c2.2 Y lithofacies (a) of Table 2. Legend as in Fig. 8. C2.3 - Sequence A is from coastal section at Needles 0 0 and Pins, south of Port Logan Bay INX 091440201; sequence B is from Scrangie,- Sand Sand Sand south of Port Logan [NX O91240161.

represented in more southerly tracts. However, tracts 1 and highly volcanilithic Portpatrick Group of tract 3. Figure 9 2 yield evidence of an early stage in which medium-scale, illustrates the diachronous first appearance of the distinctive gravel- andsand-rich fans (Corsewall-early Carsphain- Portpatrickpetrography (andesitic detritus and minor Marchburn)prograded rapidly southeastwards across a blueschist content) and ‘proximal’ lithology. Althoughthe relatively narrowtrench-floor occupied by a poorly palaeocurrentevidence is equivocal, the bulk of this developed, silty axial wedge (Fig. 12A). sediment appears to have been conveyed by flows travelling Inthe Longford-Down area of Northern Irelanda NE and NW from this arc-type terrain. Stratigraphic and comparable scenario is indicated for the Llandeilo-Caradoc structural evidence (see Morris 1983, 1985) clearly indicates sequences in thenorthernmost tracts (represented by the that this arc was separated from thenorthwestern Finnalaghta-CoroneaFormations of Strokestown;see continental margin which supplied most of the quartz-rich Morris 1983). However, proximal fans oraprons, derived sediment to the late Ordovician trench, and that it probably froma nearby calc-alkaline volcanic arcand subduction lay on the oceanward side of (to the south of) the trench. complex, are recognized in moresoutherly tracts (Gowna The nature and location of this volcanic terrain is clearly Group of Morris 1983; Ballygrot Block of Craig 1984). The of great importance in determining the precise geotectonic significance of these elements is discussed below. status of this sector of theSouthern Uplands-Longford- In the succeeding stage, the diachronous southeastwards Down active margin during the late Ordovician. There seem spread of sandsedimentation was accomplished by lateral to be three main possibilities: encroachment of axially diverted, small- to medium-scale mixed-sediment fans(Kirkcolm-Afton systems). Diversion (1) Seamount chain. Volcanic edifices originally sited on of these northwesterly-derived systems may be attributed to oceanic crust have been identified elsewhere in the Southern a topographic feature marking the oceanward boundary of Uplands (Tweedale lavas; Bail Hill Volcanic Complex). The the trench-floor (the Outer TrenchHigh of Leggett 1980; cf. Bail Hill occurrence has an associated sedimentaryapron axial rise of Kelling 1964 and Walton 1965). (Kiln Formation, McMurtry 1980; Hepworth et al. 1982). In the latest Ordovician (late Caradoc and Ashgill) the However, such an origin is rejected for the Gowna- palaeogeographic pattern outlinedabove appearsto have Portpatrick Arc in view of (i) its calc-alkaline character, (ii) persisted (Fig. 12B) butthe apron of coarse volcanilithic thetotal volume of sedimentproduced and relative sedimentsderived from a calc-alkaline andesitic volcanic longevity of the volcanic source (? late Llanvirn-early source migrated progressively farther NE with time. In the Llandovery) and (iii) the occurrence of arc-derived detritus Rhinns this influx is first observed in the Galdenoch Group of ‘continental’ and subduction-complex origin (e.g. (Tract 2), here interpreted as a distal representative of the quartz-mica schists and blueschists).

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continental margin arc by tectonicmeans, most probably involving major oblique- or strike-slip displacements. This mechanism retains the essential elements of a Northern Belt accretionary margin and explains the diachronous appearance of the arc-apron by relative migration of the source with respect tothe trench. It accounts for the relatively short-lived nature of the volcaniclastic supply at any one site along the active margin and also explains the interbedding of volcanilithic and quartz-rich sediments seen in the Galdenoch and Portpatrick Groups of the Rhinns and in the Scar Formation of Nithsdale (Floyd 1982, p. 7). Sinistral relativedisplacement of arcand trench is B clmgant-anceps required along a fracture system (? transform faults) oblique to the trend of the margin. A broadly comparable situation hasbeen described from the Miocene of the Taupo- Hikurangi system of New Zealand(Cole & Lewis 1981), where it hasbeen attributedto increasingly oblique convergence. Moreover, McKerrow (1983) has postulated

L ~~ V late Caradoc-Silurian major transform motions of appropri- atetrend and displacementbetween the British Isles and Clasllc sedlmenls - - - Major wrench fault m North America. Within the Southern Uplands the original m ,”;p,;,;;:;,;e;;;sl and a Volcanlc edlllce tectonic elements responsible for juxtaposition of the

\lbChannels AP Accrellonary prism volcanic terrainand trench arcare now presumably concealed within or below later-formed structures. + Sediment transport dbrecfton OTH outer trench hlgh For the reasonsoutlined above this is the model Fig. U.Schematic diagram depicting the palaeogeographic and preferredhere (see Figs 12A & 12B).However, it is geotectonic evolution of the southwest Southern Uplands- important to recognize that if this interpretation is valid it Longford-Down zone during the later Ordovician (see text). AfL, means that any of the Portpatrick and equivalent facies Afton ‘Lobe’ (Blackcraig Formation); BB, Ballygrot Block; CL, subsequently incorporated into the accretionary prism must Corsewall Group ‘Lobe’; CnL, Carsphairn ‘Lobe’; FF, Finnalaghta have originated on the northern flank of the arc-bounding Formation; GG, Gowna Group; GaG, Galdenoch Group; LF, fracture system. Moreover, it is implicit in the interpretation Lackan Formation; ML, Marchburn Formation ‘Lobe’; PG, of the Portpatrick arcadopted here that the tectonic Portpatrick Group; PL, Portslogan ‘Lobe’ (Kirkcolm Group); PR, removal of this terrain must also have resulted in Portayew ‘Rocks’; ScF,Scar Formation; ShF, Shinnel Formation. deformation of earlier-formed Ordovician sediments, particularly since the earliestLlandovery sediments again yield evidence of ultimatederivation from northwesterly (2) The back-arclmarginal basin hypothesis. This ‘continental’ sources. envisages that during the late Ordovician (? end Silurian) active subductionoccurred south of thepresent Southern Throughout most of the Llandovery (pre-turriculatus), Uplands, generating thearc and openinga deep marine pelagic sedimentation (represented by the Moffat Shales) basin behind it, into which both south-derived volcanilithic seems to have persisted to the southeast of tracts 4 and 5, and north-derived quartzose sedimentswere conveyed providing an oceanward limit (now tectonically much (Morris this issue; Stone et al. this issue). shortened)to coarse sedimentation within the Southern Thismodel plausibly accounts for most of the Uplands trough. sedimentological and compositional data, but byitself it During the early Llandovery (vesiculosus-cyphus Zones) does notexplain the consistentdiachronous pattern of two broadly contemporaneous depositional systems have arc-apron development described earlier.Moreover, it been identified, represented by the Float Bay-Kilfillan and ignores the compelling stratigraphic/structural evidence for Money Head Formations. Theformer system is more accretion,atleast in theNorthern Belt of southern quartzose and is characterized by a channelized fan sector in Scotland,and the uninterrupted record of contemporary the Glenluce areasand mid-fan lobes in the Rhinns, pelagic/ocean-floor sediments on both flanks of the arc. The constituent elements of an elongate, SW-advancing mixed- presence of accessory amounts of ‘continental’ and sediment fan or axial wedge (Fig. 13A). The Money Head blueschist detritus in arc-derivedsediments favours a Formation was formed by a locally developed, sand-rich fan continental margin arc rather than an oceanic island arc. In or fans, probablyemanating from a northern margin,but this case it is surprising thatthere is no evidence of axially divertedtowards the SW. The presence of sediments produced during the early rifting stage. volcanilithic detritus and accessory ferromagnesian minerals Thus, while the back-arcmodel cannot be completely in the Money Head sandstones may reflect input from a discounted, it poses almost as many problems as it solves, if remnant of the strongly dissected Gowna-Portpatrick regarded as a unique solution. arc-terrain or may result from reworking of already accreted arc-apron sediments. (3) Thetectonically displaced arc hypothesis. In this The differing depositional styles and petrography, but model thelate Ordovician of theSouthern Uplands and parallel palaeoflow patterns, of these two contemporaneous Longford-Down is envisaged as an accretionaryfore-arc systems are difficult to reconcile with simultaneous region which is briefly juxtaposed against an active deposition within a single, unconstrained basin and an

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fans of diversesource (Cairnie Finnart and Chippermore Members: Fig. 13B) being succeeded progradationally on the Rhinns by amore proximal mid-fan assemblage (Daw Point Member). Thick intraclast conglomerates in the Chippermore Member of the Glenluce area probably reflect local reworking and may be associated with the extensive structural barrier to deformation zone postulated to separate the very different, but coeval, northern andsouthern systems. E gregar~uS-convOIutuS Slightly later in the Llandovery (turriculutus Zone) the coarse and extensive Duniehinnie-Alticry debris-cone resulted from major and repeated slope-failure events on an adjacent slope (presumably to the W or NW). Many of the volcanilithic andferromagnesian-rich greywacke clasts in these debrites resemble the Portpatrick Group sediments andindicate further reworking either of earlier-accreted Portpatrick-type sediments or of remnants of the opposite (southern)sedimentary apron flanking the Gowna- Portpatrickarc terrain.The succeeding Chair and DrumblairMembers mark the continuationand SE- progradation of theChippermore mixed-sediment, lobe- dominatedfan system (Fig. 13C). The structuralbarrier postulatedfor earlier intervals does not appearto have operated at this time. However, if the top of the Garheugh Formationextends intothe sedgwickii Zone itmay be D IurrIculalus-c necessary to infer persistence of this feature (at least locally) for the reasons stated earlier. During the late Llandovery (turricula~uus-crispusZones) asoutherly-prograding mixed-sediment or muddy fan of moderate scale was established in the Rhinns (Port Logan Formation) (Fig. 13D), while limited evidence from sequences onthe east side of Luce Bay suggests thata southwest-advancing sandy fan system was initiated during this interval. Data collected by J. A. McCurry (pers. comm. 1985) from broadly coeval sediments in the southern tracts LLLU] oelaglcsedlmenls ~p Accretionary prlsm + Sedlmenltransport dlreclton on the Rhinns indicates the arrival in this sector of a major OTH Outertrench hlgh northwards-prograding (? sand-rich) fan (Fig. 13D). cc?/ Channels and InhPS Fig. 13. Schematic diagram depicting the palaeogeographic and geotectonic evolutionof the SW Southern Uplands during the early Silurian (as illustrated by the Rhinns of Galloway and Luce Bay General geotectonic considerations sections: see text). ABb, Ardwell Bay beds; AM, Alticry Member; The Llandeilo to Llandovery rocks of theSouthern CM, Chippermore Member; ChM, Chair Member; CFM, Cairnie Uplands-Longford-Down zone areinterpreted in accord- Finnart Member; DM, Duniehinnie Member; DPM,Daw Point ance with Leggett (1980) as the deformedand accreted Member; DrM, Drumblair Member; FBF, Float Bay Member; GF, remnants of sedimentarysequences developed within a Garheugh Formation (with A and B Formations); GPF, Grennan fore-arcplate-tectonic setting. Temporal variations in the Point Formation; KF, Kilfillan Formation; MHF, Money Head tectonicconfiguration of the fore-arc region are invoked Formation; PLF, Port Logan Formation; SBb, Stinking Bight beds. hereto account for certain unusual sedimentological 1,2, a, b, etc. represent earlier accreted tectono-stratigraphic tracts. characteristics, which appear anomalous in terms of a simple fore-arc accretionary prism model. intervening structuralbarrier is therefore postulated. An analysis of the Llandeilo to Llandovery sedimentary However,it is possible thatthe two systems were also sequences from SW Scotland testifies to the persistence of bathymetrically separated,the alreadyrestricted Money deep-water and, specifically, submarine fan depositional Head system perhaps beingdeveloped in a lower slope styles throughout this time interval. Analysis of palaeocur- basin, rather than the open trench or trough floor. rents and facies associations demonstrate that fan systems CO-existing depositional systems are recognized also in werealmost entirely sourced from a NW margin, at least the succeeding gregarius-convolutw interval (represented in until the uppermostLlandovery. In addition, gross successive fault-bounded slices, tracts 5a and5b). The biostratigraphiccorrelations indicate that whilstfan northern system (Stinking Bight beds,Grennan Point depositionpersisted along the NW margin,contem- Formation and Garheugh Formation) is a relatively coarse poraneous pelagic sediments were accumulating further SE. axial wedge or highly elongate sandy fan, with southwesterly Thesefeatures are indicative of a long-lived, deep-water, palaeoflow. Thesouthern systems (lowermembers of the asymmetrical basin similar to thoseformed in modern Mull of Logan and Corwall Formations) are smallerand fore-arc regions. A back-arc basin model is rejected on the more complex, with earlier lobe-dominated mixed-sediment basis of this asymmetry of basin-fill and the apparent

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absence of terrestrial or shallow-water sediments related to some important constraints on current geotectonic models, an early rifting stage. it remains necessary to counsel caution in applying too During the Llandeilo to Llandovery interval the tectonic rigorously the conclusions offered above to distant sectors of configuration and palaeogeographic pattern of the Southern this zone. Uplands-Longford-Downfore-arc region probably under- went considerable modification. However, two main phases The authors wish to acknowledge the help and stimulation provided of developmentare recognized, each of which generated by discussions with many individuals but especially R. P. Barnes, J. distinctive sedimentological styles and patterns: P. Floyd, P. Stoneand J. A. McCurry. The help of A. W. A. Rushton, D. E. White and S. P. Tunnicliff in identifying faunas and Phase (i) (Llundeilo-Ashgill). During this time interval making available data from their graptolite-collecting programme is theSouthern Uplands-Longford-Downaccretionary fore- also gratefully acknowledged. We also thank H. M. Tomkinson, M. arc was juxtaposed against an active continental margin arc. Buchan, D. Kelsall and R. Burgess. P. Davies and J. Holroyd This situation explains the derivation from the W and SW of acknowledge financial support from N.E.R.C. while G. Kelling thick aprons of volcanilithic-rich sequences(anomalous in thanks the University of KeeleResearch Fund for material and the overallcontext of theSouthern Uplands), and the logistic contributions. interbedding of these with quartz-rich sediments of ultimate NW derivation. It alsopermits the development of thick References pelagic and ocean-floor sequencesbeyond the limits of active clastic sedimentation on the subduction oceanic plate. BARNES,R. P,, ANDERSON, T. B. & MCCURRY,J. A. 1987. Along-strike variation in the stratigraphicand structural profiles of theSouthern Gradual sinistral tectonicmigration of this active arc, Uplands Central Belt in Galloway and Down. Journal of the Geological relative tothe fore-arcregion, explains the diachronous Society, London, 144, 807-16. introduction of volcanilithic sedimentation from the SW to BARNES,N. E. & NORMARK,W. R. 1983. Parameters for comparing modern the NE. submarine fans and ancientturbidite systems. Go-Marine Letter.r, 3, end-Maps. COLE, J. W. & LEWIS,K. B. 1981. Evolution of theTaupo-Hikurangi Phase (ii) (Llundovery). During this timeinterval subduction system. Tectonophysics, 72, 1-21. sedimentation was dominated by fan systems prograding CRAIG,L. E. 1984. Stratigraphy in anaccretionary prism: the Ordovician from a NW margin while pelagic depositsaccumulated to rocksin NorthDown, Ireland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 74, 183-91. theSE. Deformation and uplift (dueto accretion) of the FLOYD,J. D. 1982. Stratigraphy of a flyschsuccession: theOrdovician of Ordovician sediments,and the tectonic removal of the West Nithsdale, SW Scotland. Transactions of the Royal Society of displaced arc probablywere largely accomplished by the Edinburgh: Earth Sciences, 73, 1-9. beginning of the Llandovery. Such a scenario would allow GORDON,A. J. 1962. TheLower Palaeozoic rocks around Glenluce, Wigtownshire. PhD thesis, University of Edinburgh. the incorporation within some of the earliestLlandovery HEIN,F. J. &. WALKER,R. G. 1982. The Camhro-Ordovician Cap Enrage sequences of lithic clasts petrographically comparable to the Formation,Quebec, Canada: conglomeraticdeposits of braideda Portpatrick Grouparc-apron, together with detrital submarine channel with terraces. Sedimentology, 29, 309-29. pyroxenes and amphiboles.Deposition of the sedimentary HEPWORTH,B. C.,OLIVER, G. J. H. & McMumw, M. 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Received 4 September 1986; revised typescript accepted 10 April 1987.

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