Journal of the Geological Society, London, Vol. 145, 1988, pp. 887-890, 2 figs. Printed in Northern Ireland

Short Paper: The subaerial setting of the Volcanic Group, English

M. J. BRANNEY Department of Geology, University of Shefield, Mappin Street, Sheffield S1 3D, UK

Most Ordovician volcanic rocks of southern Britain were produced the onset of BVG volcanism in that area. The contained involcanic island settings and theBorrowdale Volcanic Group fossils could beinherited. This idea is supported by the (BVG) of the Lake District has traditionally been interpreted in this occurrence of closely similar mudstones with volcanic clasts way.However, a reviewof lithofacies associations in the BVG intruded into Group rocks at Greenscoe [SD 220 indicates a subaerial environment. Pyroclastic surge deposits, thin 7601 (Soper 1970). welded tuff beds, lava flowmorphologies, abundant erosional unconformities, and fluviatile and laharic facies indicate widespread subaerial emplacement. Volcano-tectonic subsidence aided preser- A subaerial origin for the BVG. A subaerial origin for vationof subaerial facies, although a preservation bias favoured parts of the BVG has been proposed on the basis of welding lacustrinedeposits. Erosional unconformities above subaerial in ignimbrites, and reddened autobrecciatedtops of lava aggradationalvolcanic sequences are to be expected, so the sub-WindermereGroup unconformity does not necessarily imply flows (Oliver 1955; Mitchell 1956). However,evidence in tectonic uplift. Wales (Howells & Leveridge 1980; Kokelaar et al. 1985; Reedman et al. 1987) suggests that tuffs can weld subaqueously,and this is supported by theoretical Whether the Llandeilo-Caradocian Borrowdale Volcanic considerations (Sparks et al. 1980). With regard to subaerial Group (BVG) of the English Lake District was emplaced weathering interpreted from red colouration of some of the under the sea or subaerially has been debated for over a BVG andesites, it is notable that most haveundergone century(see Mitchell 1956 andreferences therein). A intensehydrothermal alteration (from Ordovician to volcanic island environment has been proposed by several Mesozoic times) and low-grade metamorphism, and at least workers (Mitchell 1956; Fitton & Hughes 1970; Millward et some of the sheets with reddening are actually sills (Branney al. 1978; Turner & Wadge 1979), but volcanic lithofacies & Suthren 1988) and unlikely to have been exposed associations within the BVG contrast with those of similar subaerially. age elsewhere in , Wales and Ireland where small The best indicators of depositionalenvironment of volcanic islands areinterpreted from associations with ancient volcanic successions are lithofacies associations (Cas marine sedimentary strata (Stillman & Francis 1979; & Wright 1987). Those within the BVG are revealing; for Kokelaar et al. 1984). This note briefly reviews recently example,at Side Pike [NY 2900521 a facies association derived evidence which, on balance,favours a subaerial occurs which is best interpreted as having beenemplaced continental setting, similar to large caldera-forming volcanic subaerially (Fig. 1).An ignimbrite cooling unit is fields of western USA and central America. ‘Immobile’ and conformably overlain by a thin co-ignimbrite ash-fall deposit traceelement chemistry favoursa destructive continental (Sparks & Walker 1977) composed of extremely fine-grained plate margin setting for the BVG (Mathieson 1986). crystal-poor tuffwith accretionary lapilli. This is in turn Criteria previously used toargue subaerial versus overlain by a pyroclastic surgedeposit, which hasbeen submarineemplacement of the BVG require reconsidera- deeply incised by the erosive base of a coarse debris-flow tion. A submarine origin was proposed for lower parts of breccia. The latter formed as a result of phreatic explosions the BVG on the basis of apparent interbedding of volcanic in the underlying hot welded ignimbrite. Such sequences, rocks with mudstone horizons (Mitchell 1956; Millward et al. whose preservation within Lower Palaeozoic rocks is 1978). However, the volcanic succession interbedded with unusual, are typical of subaerial pyroclastic facies. marine strata in the north of the Lake District is now known The association of five facies, widespread in the BVG, to belong to an earlier (Llanvirn) phase of volcanism (the similarly indicates subaerial emplacement. Eycott Volcanic Group of Downie & Soper 1972), and the (1) Pyroclastic surge deposits (Fig. 2) are diagnostic of BVG overlies eroded marinesediments unconformably both subaerial eruptionand subaerialemplacement. They (Wadge 1978; Branney & Soper 1988). Immediately above occur within the Lingcove, Blisco, Airy’s Bridge, Seathwaite this unconformity, unita composed of mixed volcanic Fells Formations, andthe SidePike Complex (Branney fragments in a black mudstone matrix (the ‘Mottled Tuffs’ of 1988a), where they are characterized by a combination of Green 1913; Mathieson 1986) at Whinnybank Quarry [SD features, including low-angle sandwave cross-stratification, 1695 84451 has yielded marine acritarchs (Turner & Wadge poorsorting, abundant accretionary lapilli, and by their 1979). This led Soper (in Johnson et al. 1979) to infer a association with immediately enclosing pyroclastic facies. conformable relationship between the BVG and the marine Their common occurrence alone proves that asubstantial Skiddaw Group in the SW of the Lake District. However, part of the BVG was emplaced subaerially. close examination shows much of the mudstone to be in the (2) Parts of theBVG containnumerous thin welded form of accretionary lapilli. This suggests aphreatic or, lupilli-tu# beds, which are laterally extensive but commonly possibly, phreatomagmatic origin for this unit, involving less than 20cm thick (Suthren & Furnes 1980; Branney subaerial explosive reworking of older marine strata during 1988~).Whilst subaqueous welding of large ignimbrites may 887

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be possible, it seems highly unlikely that beds of such e I .. .e. limited thickness could have welded under water. )yroclastic surQe deDosit (3) Fluvialfacies have been reported from , co-ignimbrite- Borrowdale, and Pike 0' Blisco (Suthren 1977; Branney N .. 40 jeposit t 1988~).They have been interpreted asdeposits of small, ../ short-lived streams, on land-surfaces of low topography. Locally they are associated with laharic, hyperconcentrated stream and sheetwash deposits. Tops of several ignimbrites 1 Side Pike lgnimbrite within the BVG have suffered fluvial erosion, for example k(eutaxitic at Pike Blisco [NY 2700421. Rills and steep-sided I lapilli-tuff) 0' I box-shaped fluvial gullies typical of rapid subaerial erosion of unlithified tephra and sediment (Segerstrom 1950; Taylor 1958) occur abundantly within many bedded volcaniclastic sequences in the BVG. In the Blisco Formation (Branney & Soper 1988) for example, box-shaped channels over 10 m deep, with vertical and even slightly overhanging sides occur bedded tuffs [NY 271 0411. In Borrowdale,Langdale andDunnerdale, (pyroclastic surge small fluvial gullies occur within the Whorneyside Bedded Tuff which has been interpreted as a subaerially-emplaced Fig. 1. A section through typical subaerial pyroclastic faciesin the BVG at Side Pike (NY290 052) (after Branney 19886). Right: phreatoplinian tuff (Branney 1988a). This tuff has been location of the Borrowdale Volcanic Group (BVG) and SidePike traced laterally forover 20 km within theBVG, from a (asterisk) in the English Lake District. widespread facies reflecting deposition onto a dry-land surface (Branney 1988~)into restricted local facies reflecting shallow andephemeral lakes (Suthren 1977). Elsewhere, thin sedimentary andpyroclastic units interstratifiedwith lava flows and welded ignimbrites contain abundant evidence of erosion and internal unconformity. (4) Subaerial ehsion is also indicatedin the BVG by piles of highly vesicular pahoehoe, aa, andblucky lava flows, for example within the Lingcove Formation (Branney 1988~).Pockets of cross-beddedsand and silt commonly found occupying originally open interstices of block lava autobreccias are closely similar to occurrences on modern subaerial volcanoes (e.g.Cole et al. 1986; Branney & Suthren 1988). Typical submarine effusive sequences, such as pillow-hyaloclastite piles interstratified with fossiliferous volcanogenic and non-volcanogenic sediments are absent in the BVG. Sea cliffs and sea stacks such as occur in volcanic island sequences (e.g. in North Wales,Kokelaar pers. comm.)have not been identified within theBVG (cf. Moseley & Millward 1982). (5) Subaqueous volcaniclastic deposits occur abundantly within the BVG, such as in the Seathwaite Fells Formation, and the Honister, Kirkstone, and Tilberthwaite , which contain hyaloclastites and turbidites.However marine sedimentary facies and marine body fossils have not been recorded from these localities. After 150 years of research on the BVG, only two trace fossil specimens (Mitchell 1956; Suthrenpers. comm.) have ever beenfound. Apart from these, and despite the wide variety of sediment types and possible environments,bioturbation is absent.This is atypical of marine facies. The simplest explanation to account for thelack of both body and trace fossils is that the subaqeously deposited volcaniclastic sediments accumulated in freshwater lakes. This interpretation is consistent with sedimentological studies of Suthren (1977), who interpreted ephemeral lakes but found no positive evidence of marine sedimentary facies within the BVG. Desiccation cracks occur in lacustrine deposits at Sour Milk Gill, Borrowdale [NY 231 1231 (Suthren 1977) and BleaberryKnotts, Langdale [NY 285 0411 (Branney 1988a). The abundance of Fig. 2. A pyroclastic surge deposit in the Side Pike Complex subaqueous facies in theBVG probably reflects a bias (Branney 1988~)[NY 284 0411. Note the accretionary lapilli (top) favouringpreservation of subsidence-controlled lacustrine and typical low-angle cross-stratification(bottom). The scale is 1 m. accumulations. Contemporaneous subaerial pyroclastic de-

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posits and sedimentswould, without vegetation, have might be considered. undergone swift denudation to be under-represented in the If proven, the presence of a marine incursion within the geological record (Cas & Wright 1987). Interplay between BVG several kilometres above its base would support the erosion and deposition was controlled locally by volcanotec- model currently being developed (Branney 1988a) that the tonic activity. For example in theCentral Lake District BVG has been preserved by successive subsidence events. Fells, volcano-tectonic subsidence associated with volumi- However, in view of the puzzling geometry of surrounding nous ignimbrite eruptions caused a dramatic change from a rock units (Numan 1974), poor exposure, high strain, and subaerial and commonly erosive environment with low net poor knowledge of theBVG facies andenvironments at rates of sedimentation(characterized by thin ephemeral HolehouseGill, it would seem prudent,at present, to fluvial and sheetwash deposits with abundant erosiqn regard the Holehouse Gill mudstone/BVG relationshipas surfaces), to a lacustrine environment (the Seathwaite Fells ambiguous. Formation, Branney 1988a; Branney & Soper 1988). High and uneven erosion rates in an active tectonic and volcanic Discussion. The preservationpotential of submarine subaerialenvironment explain the rapid facies variations volcaniclastic accumulations around volcanic islands might within theBVG. These are the primarycause of the be expected to exceed that even of lacustrine deposits within enormousproblems of lithostratigraphy that have beset emergent parts of islands. By considering the dimensions of interpretations of the BVG. a typical intermediate to felsic volcanic island growing from A marine intercalation ? The occurrence of a thin unit 100111 below sea-level to 100 m above sea-level, Ayres containing black mudstone with acritarchs(Numan 1974), (1982) demonstrated that even without considering erosion, apparentlyinterbedded with subaerially-emplaced BVG by far the greater volume of a typical volcanic island would facies at Holehouse Gill [SD 182 9261 is intriguing. Marine be emplaced underwater.Even most of the pyroclastic or littoral facies are absentfrom penecontemporaneous material which was erupted subaerially would be deposited successions to the north () and south, around in the sea. The remaining,minor, subaerially-emplaced Millom Park(Mathieson 1986). The Central Lake District portion of the island would then commonly be removed by Fells record continued subaerial explosive eruptions, severe erosion, to beredeposited underwater. This explains why volcano-tectonic subsidence and block-faulting which prod- volcanic islands are oftenrecorded entirely as submarine uced faultscarps, and rapidemplacement of coarse sequences (e.g. Ayres 1982; Kokelaar et al. 1984, 1985). Yet pyroclastic and debris-flow breccias (N. Davis pers. comm.; in . theLake District, thick submarinesequences of Branney 1988~).Therefore, it seems that if the Holehouse pyroclastic and volcaniclastic sediments which are lateral Gill area formeda marked local topographicdepression, equivalents of the subaerialfacies of theBVG have not permitting amarine incursion, it would be prone to swift been recorded, so there is no evidence thatthe BVG inundation by volcaniclastic material derived from adjacent representsdevelopment of volcanic islands. Although unvegetated subaerial slopes. products of erosion of BVG volcanoes may have reached A combined volcanological, palaeoecological, and marine basins, it seems that these were beyond the bounds sedimentological facies investigation is still required to of the present Lake Districtinlier. However, further substantiate the relationshipbetween the Holehouse Gill modern sedimentological interpretations of the subaqueous mudstones and theBVG. Strata below and above the clastic units within the BVG are required to confirm this mudstone might record transgression and regression, view. respectively. Absence of transgressive and regressive Since theBVG was subaerial and aggradational, the sequences could be explained by erosion,rapid eustatic unconformity which oversteps the top of the BVG is to be changes, or volcano-tectonic subsidence and uplift. Howe- expected. It is geometrically similar to many unconformities ver, if the intensely block-faulted subaerial volcano did which occur within the BVG (some of which overstep earlier suffer local marine inundation, it is rather surprising that volcano-tectonically rotated sequences, N. Davis pers. there areno reports fromanywhere in theBVG of comm. 1988), and tectonic uplift is not necessarily implied unambiguous shallow marine and littoral facies and faunas, by its presence. Branney (1988~)and Davis (in prep.) such as developed repeatedly in volcanic rocks of the same interpret lithofacies associations in the BVG of the Central age in Wales (Kokelaar et al. 1984). Lake District Fells as proximal or ‘core’ volcanic facies. Thestructure of theBVG is more complex than has They also consider thatabundant minorintrusions, been recently envisaged. Both complex volcano-tectonic closely spaced volcano-tectonic faults, subvolcanic granitoid disruption (Branney & Soper 1988) andlate Caledonian plutons (mostly subsurface;Firman & Lee 1986),and faulting and thrusting (Soper & Moseley 1978) occur. The intensehydrothermal alteration (Millward et al. 1978 and BVG is tectonically intercalated with Skiddaw Group rocks references therein) in the Central Fells are additional at Ullswater (Moseley 1964) and with the proximal components of a large volcanic system. Whilst Groupnear Millom (Mathieson 1986), not far from evidence in theCentral Fells (Branney1988a; Davis in Holehouse Gill. Exposurearound Holehouse Gill is very prep.) specifically indicatesseveral local volcano-tectonic poorand the rocks are highly strained.Thus it seems subsidence events, the BVG generally thins away from the possible thatthe mudstones and associated volcaniclastic proximal areas, andhas been removed completely from sediments at Holehouse Gill are tectonically intercalated more distal areas such as Greenscoe, which lies south of the with theBVG. The lithostratigraphicunit to which they main BVG inlier andsouth of the subvolcanic plutons. belong may, like the fault-bounded ashy shalesat Drygill Volcano-tectonic subsidence, therefore,appears to have (Ingham & McNamara 1978), be post-Eycott Groupand been greatest aroundthe volcanic centres (i.e. the main pre-Coniston Limestone Formation in age, but not exposed BVG outcrop), and it accounts for the preservation of the elsewhere. Alternatively, an origin similar tothat of the subaerial volcanics. basal ‘Mottled Tuffs’ of WhinnybankQuarry (see above) In summary, the lithofacies associations exhibited by the

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BVG have many characteristics commonly associated with volcanism and associated tectonic processes, Ramsay Island, SW Wales. large, subaerial, caldera-forming volcanoes. They are closely Journal of the Geological Society, London, 142, 591-613. similar to facies models of subaerial ensialic volcanoes, -, HOWELLS,M. F.,BEVINS, R. E., ROACH, R. A. & DUNKLEY,P. N. 1984. The Ordovician marginal basin of Wales. In: KOKELAAR, B. P.& summarized by Cas & Wright (1987, figs 14.8 & 14.9), and HOWELLS,M. F. (eds) Marginal basin geology. Special Publication of the have few similarities with the dominantly marine ancient Geological Society, London, 259-73. facies commonly associated with volcanic islands and marine MATHIESON,N. A. 1986. Geology, structure and geochemistry of the volcanic successions (e.g. Mitchell 1970; Cas & Wright 1987; Ordovician volcanic succession in SW . PhD thesis, University of Sheffield. Busby-Spera 1988). MILLWARD,D., MOSELEY, F. & SOPER, N. J. 1978. 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Received 22 April 1988; revised typescript accepted 13 June 1988.

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