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Journal of the Geological Society, London, Vol. 143, 1986, pp. 107-115, 7 figs. Printed in Northern Ireland

Mid- tectonics and sedimentation in the Bristol Channel area

I. P. TUNBRIDGE Department of Geological Sciences, Plymouth Polytechnic, Drake Circus, Plymouth PL4 8AA, U.K.

Abstract: The Hangman Group (Eifelian) of North contains two continental ‘Old Red Sandstone’ formations. The Trentishoe Formation is some 1OOOm thick and consists oflithic arenites which petrographically match closely with those in the higher parts of the Lower Old Red Sandstone in South Wales. This formation was derived mainly from erosionof the highest Lower Old Red Sandstone in early mid-Devonian times as a result of end-Caledonian uplift and erosion in S Wales. The Rawns Formation which follows contains 120 m of conglomerates and pebbly with angular clasts of porphyry, tuff, quartzite and lithic arenite which do not match with rocks in possiblesource areas in Wales, but could have been derived from the Lower Palaeozoic and rocks believed to lie at shallow depth to the north of the Bristol Channel Fault Zone. Fault movements along this zone are believed to have generated relatively short-lived coarse clastic supply at various times and places around the Bristol Channel in Lowerto early mid-Devonian times, in response to end-Caledonian movements. The later production of late Namurian to Westphalian sands from the Bristol Channel area was unrelated to these Devonian events, and represents uplift in response to Variscan events in SW England.

Evidence is gradually growing for the existence in the late in S Wales tothe mainlymarine area of NorthDevon Palaeozoic of a positive area in the region of the present (House 1975). This wedge developed at a time of tectonic Bristol Channel in southern Britain. The exact nature and uplift in S Wales(Allen 1974a) and of rising sea levels extent of this ‘Bristol Channel Landmass’ is poorly throughout the Devonian oceans (House 1975). Tunbridge understood,but clues to its developmentcome from a (1983) hasargued that the progradation of theHangman number of areas. Williams (1964, and in Owen et al. 1971) clastic wedge at sucha time was a function of very high studied the ?Middle Devonian RidgewayConglomerate of sedimentation rates produced by rapid uplift and erosion in SW Dyfed and deduced that the formationwas derived from the S. Wales area,due toend-Caledonian orogenesis a nearby southerly source. This source included quartzites (Phillips et al. 1976). and phyllites, and lay in the region of the present Bristol The main continental phase of the Hangman Group is Channel. Allen (1975) examined the Llanishen Conglomer- represented by the fine-grained red and green sandstones of ate of Seigennianage in the district andinferred theTrentishoe Formation (Tunbridge 1984) (Fig. 1). It frompebble assemblages that this too had asoutherly follows the shoreline of the Hollowbrook Formation source of quartzite and acid porphyry rocks. He suggested (Tunbridge1983) and was deposited as a series of sandy thatthesources fortheLlanishen and Ridgeway sheetfloods,and ephemeral fills (Tunbridge1984). conglomeratescould bethe precursors tothe major After deposition of the mud dominated, pedogenic Yes Tor source for parts of the Pennant Sandstone of Member(Tunbridge 1980b) coarser, exotic pebbly sand- S Wales(Kelling 1964, 1974). Tunbridge (1980a, 1983, in stones of the Rawns Formation flooded into the area. Anderton et al. 1979)has hinted thata source for the angularpebbly material in the Rawns Formation of the The Trentishoe Formation HangmanSandstone Group (Middle Devonian) of North Devon could be derived from a nearby source to the north, The Trentishoe Formation is the main continental phase in in the region of the present Bristol Channel. He has refuted theHangman Group. It is about1000m thickand is the suggestion by Dewey(1982) thatthe whole of the characterizedby mainly fine grainedsandstones and a HangmanGroup wasderived from the ‘Bristol Channel generalabsence of pebblymaterial. Evidence for the Landmass’, but a full discussion of the development of the derivation of this formation from the S Wales area comes North Devon area in relation to aBristol Channel Landmass fromsandstone petrology, heavy mineral studies and is, until the present paper, lacking. This paper outlines in palaeocurrentdata. Palaeocurrents measured from cross detail the development of sources for the Middle Devonian beddingshow anortherly derivation for theformation of NorthDevon and proposes tectono-a (vectormean 175”; variance 60”; N = 53).This indicates sedimentary model for the evolution of the Bristol Channel derivation either from S Wales or a more proximal region. If region in mid-Devonian times. the Trentishoe Formation was derived from the erosion of LowerOld Red Sandstone in S Wales,then petrographic similarities should be evident, but if derivation was from a and petrography ‘Bristol Channel’ source then this might be determinable by The Hangman Sandstone Group (Tunbridge 1980b) (Fig. 1) dissimilarities. Someevidence fora dissimilarity in the representsan early Middle Devonian (Eifelian) clastic composition of theTrentishoe Formation to that of the wedge which prograded southwards from a continental area Lower Old Red Sandstone of S Walescomes from heavy 107

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l I E. WALES CENTRAL S.W. WALES S.W. WALES NORTH DEVON and AVON N. of Ritec Fault) (S. of RitecFault F Skrinkle MorteSlate - Sandstone -- i llfracombe Beds ( 1000 m.)

f Little Hgmn. Fm. (100rn 6 p SherrycombeFrn. (IXm &$ RawnsFm. (120m Trentishoe Fm. (1000m X HollowbmokFm. (8Om 1 A?- Lynton Beds (400m.l Brownstone Cosheston Ridgeway Conglomerate Group Group t --___ ( 1505 - 1800m ) (0- 420m ) ( 0 - 1220m) 7-J- ?.

MilfordHaven Group Group Milford Haven 1840 - 2637rn) Group ( 425 - 94% )

Raglan Mar1 Group (305 - 610111)

1 Downkto, Castle Fig. 1. Stratigraphic summary chartof the Lower to Middle Devonianin the Bristol Channel region. The stars indicate coarse grained intervals. EL, East Lyn Fault Conglomerate; LB,Linsway Bay Conglomerate; LFC, Llyn-y-fan Conglomerate;LL, Llanishen Conglomerate; WBC, Woodhill Bay Conglomerate. Data compiled from Tunbridge(1983), Tunbridge & Whittaker (1978), Allen (1974a) and Allen & Williams (1978).

mineralstudies made by Hallam (1934). He foundno tetrabromoethane.Garnet is thedominant heavy mineral, detrital garnet in the Hangman Sandstones of the Quantock especially in the lightly cemented samples collected from the Hills, in contrast to the abundance of that mineral in the WelshBorderlands. Samples from the more indurated LowerOld RedSandstone of S Wales(Allen 1974a). sandstones collected farther west contain a lower proportion Significantly, neitherthe Llanishen Conglomerate (Allen of garnetand those present are abraded and pitted by 1975), the RidgewayConglomerate nor the Pennant intrastratalsolution. Zircon and tourmaline are the other Sandstone(Upper Carboniferous) contain detrital garnet dominant components, together with minor rutile. From the (B. P. J. Williams & G. Kelling, pers. comm.). Could it be TrentishoeFormation, 12 sampleswere similarly treated. thenthat the Trentishoe Formation shares commona Zircon and tourmaline dominate, with minor rutile. Garnet ‘BristolChannel’ source with the garnet-free formations althoughpresent, is rare,and is usually highly etched mentioned above? suggesting considerable chemical attack (Fig. 2). Twopossible interpretations can be placed on these Heavy minerals. To testfor possibledifferences in findings. Either the variations in heavy minerals represent detrital heavy mineral composition between the Lower Old source area differences, or the proportion of garnet in the Red Sandstone in S Wales and the Trentishoe Formation, 12 TrentishoeFormation is reduceddue to post-depositional samples of fine sandstone collected across the Anglo-Welsh destruction.Garnet is known to beprone to diagenetic outcrop of theBrownstones (highestLower OldRed dissolution, often being one of the earliest minerals lost on Sandstones)were disaggregated andseparated using deep burial (Dryden & Dryden 1946; P. Allen 1949; Friis

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granophyricintergrowths of quartzfeldspar.and Metamorphic fragments areless common, consisting of highly sericitized phyllite and chloritic metaquartzite. In the sedimentaryrock fragments there is asubtle difference betweenthose from the Brownstones and from the TrentishoeFormation. Although both contain lithic and micaceous quartzites, only the Brownstones containfragments of calcite-cementedsandstones. Allen (1974a) attributed the latter to reworked lowest Old Red Sandstone. If the sands of the Trentishoe Formation were derived by reworkingthe Brownstones, then further transport mayhave destroyed these relatively weakly consolidated grains. The compositional similarities between the sandstonesof the highest Lower Old Red Sandstone and the Trentishoe Formationsuggest asimilar primary provenance. Allen (1962, 1974b) has concluded that the higher formations of the Lower Old Red Sandstone were derived from Central andnorth Welsh LowerPalaeozoic volcanic rocks and sediments, and from reworked lowest Old Red Sandstone. Themid-Devonian uplift which affected S Walesbrought depositionin that area to an end and the petrographic evidencesuggests that detritus derived from the Welsh uplands was deposited farther south in North Devon. The recently deposited Brownstones,which may have been loose orpoorly consolidated at that time, were also perhaps reworked.

Fig. 2. (a) Highly etched detrital garnet separated from a sampleof the Trentishoe Formation collectedat Porlock Weir (SS 858484), W . The garnets are found rarely, in contrast to their abundance and relatively unabraded state in the highest Lower Old Red Sandstoneof S Wales and theWelsh Borderland. (b) A typical example of the abundant, unabraded garnets from the Brownstones (highest Lower Old Red Sandstone) collectedat Ross-on-Wye, Hereford and (SO 595240). Field of view is approximately 500 microns in each case.

1974), as confirmed by experimentalstudies of garnet corrosion (Nickel 1973). The greater proportion of garnet preserved in the samples from the Welsh Borderlands may thus be a function of the relatively minor indurationof those rocks. Burial diagenesis increases from east to west for the Upper Palaeozoic rocks of S Wales (Gill et al. 1977; Parker et al. 1983), explaining the increased pitting of the garnets. In North Devon the rocks are strongly silica-cemented and haveundergone considerably more tectonic deformation. Thus detrital garnets are unlikely to bewell preserved in the North Devon area.

Sandstone petrology. Further evidence for similarities in composition between the Lower Old Red Sandstone of S Wales and the Trentishoe Formation comes from sandstone petrology.The sandstones of theBrownstones and the TrentishoeFormation are mostlymedium to fine grained lithic arenites of similarcomposition (Fig. 3). Quartz consistsmainly of singlegrains with undulose extinction, b together with some polycrystalline quartz. Feldspars make Fig. 3. (a) Photomicrograph (cross polars)of a medium-fine up only 5% of therock and are represented by altered grained lithic arenite collected from the Trentishoe Formationat orthoclase,microcline, braid perthite and plagioclase Porlock Weir (SS 858484). (b) Photomicrograph (cross polars)of a Rock fragments abound and areof similar typesin medium-fine grained lithic arenite collectedfrom the Brownstones theBrownstones and Trentishoe Formation. Igneous rock at Ross-on-Wye (SS 595240). Note the similarities in composition, fragments consist of dust tuffs, spherulitic grains, acid lava to the sandstone from the Trentishoe Formation, explained in detail fragments with feldsparmicrolites, and grains with in the text.Field of view is approximately2 mm in each case.

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The Rawns Formation grainedsediment deposition was terminated by marine transgression,marked by the base of theSherrycombe The Rawns Formation is of different petrographic character Formation. tothe Trentishoe Formation, consisting of c. 120m of The best exposures of the Rawns Formation lie on the coarse pebblysandstones with an assemblage of angular coast tothe west of CombeMartin (SS 587484-see exotic clasts. Thesecoarse clastic rocksfollow the finer Tunbridge & Whittaker 1978 for details). Here it consists of sandstones of theTrentishoe Formation after a mud- a series of sheet-like multi-storey beds of medium to coarse dominated,pedogenic interval (the Yes Tor Member of grained pebbly sandstone, interbedded with thinner beds of Tunbridge (1980b)) which represents a depositional pause. desiccated and fine sandstone (Fig. 4). Palaeocur- A new,nearby upland was thedominant source for the rents, given by largescale cross beds, show a northerly RawnsFormation. This short-lived episode of coarse- derivation(vector mean 190"; variance 75"; N = 43). The

5m Break

Metres

3.

2.

I

S FMC SFMC SFM C S FMC Fig. 4. Log of the top 60 m of the Rawns Formationat Little Hangman Point, westof Combe Martin, North Devon, showing multistorey pebbly sandstones (channelfills) alternating with thinly bedded floodplain sandstones and .

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sequence compares well with those of many low sinuosity Clast composition. Suites of clasts were collected at streams (Miall 1977). Farther east in the Quantock Hills, the randomfrom the Rawns Formation and theHodders stratigraphic equivalent of the Rawns Formation is found in Combe Beds. The dominant rock types are quartz-feldspar the Hodders Combe Beds (see Webby 1965) which crop out porphyry (ll%),aplite (3%), acidcrystal and dust tuffs in small quarries at Hodders Combe(ST 150408) and consist (36%), quartzite (ll%), lithicarenite (6%), and of sheets 5-30 cm thick of poorly sorted angular set metaquartzite (9%), together with vein quartz (24%). The in a fine sandy matrix. These ill-sorted sediments represent a main types are illustrated and briefly describedin Fig. 5. For proximal facies deposited from sheet flows carrying a very detailedpetrographic descriptions see Tunbridge (1978), high load (cf. Bull 1977). which alsoshows thatimportant petrographic differences

Fig. 5. Photomicrographs (cross polars) of main types from the Rawns Formation, collected from Combe Martin(SS 582477). (a) Pink to orange coloured quartz-feldspar porphyry showing quartz and feldspar phenocrysts in a fine quartzo-feldspathic groundmass. Reading University Department Archive No S25008. Field of view is 2.5 mm across. (b) A feldspathic acid lava. These brown to yellow andesitic lavasare non-porphyritic and consistof hyalopilitic massesof small, simply twinned feldspars. Archive No. S25020. Field of view is 2.5 mm across. (c) Crystal-lithic tuff with flow structures around broken and resorbed quartz crystals. Lithic fragments, including sandstones, polycrystalline quartz and dust tuffs are found together with broken feldspar phenocrysts. Archive No. S25009. Fieldof view is 6 mm across. (d) Aplite. This represents the coarsest material found. Pink in hand specimen, it consistsof 40% quartz, 33% perthite, 25% labradorite, and 2% biotite in a hypidiomorphic granitoid texture. Archive No. S25017. Fieldof view is 2.5 mm across.(e) Quartzite. Usually blackor buff, this type consists of over 90% quartz grains and4% plagioclase, with occasional heavy mineral bands in a quartz- and calcite-cemented fine sandstone. Archive No.S25019. Field of view is 1.3 mm across. (f) Lithic arenite. Fine to very fine grained buff, greenor brown in hand specimen, consistingof moderately well sorted quartz (50-70%), sericitized plagioclase(1-5%), muscovite (1-5%), and rock fragments (20-30%) including dust tuff, metaquartzite, and argillaceous fragments. Clay matrix constitutes to up 10%. Archive No. S25012. Field of view is 1.2 mm across.

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are highly durableand some clasts couldbe multicyclic. Sandstonesand acid extrusive rocks are common in the LowerPalaeozoic and Precambrian of Wales.Within this context four possible areas exist for consideration: (a) The Lower Palaeozoic and Precambrian rocks of W Dyfedinclude extensive suites of andesiticand rhyolitic lavas and tuffs, granites,granophyres, quartz porphyries, quartzitesand conglomerates (George 1970). These rocks couldpossibly provide a source but they lie over 110 km away and to the NW (125"-145" from North Devon). For this area to act as a source, exotic debris would have to be transported across the Old Red Sandstone terranes of SW Dyfed before reaching North Devon. There is no evidence for such a dispersal path. The W Dyfed sources are rejected ongrounds of distance,direction and the nature of intervening ground. (b) The Lower Palaeozoic rocks of central Wales include rhyolitic, spilitic andkeratophyric lavas and tuffs, and quartzites(Jones & Pugh1941). Although these rocks lie over 90 km N of Devon they are in a suitable position to act asasource consistent with palaeocurrent data. Aplites cannot be matched to this area, and the intervening ground shows no evidence for pebbles dispersed from the central Wales region in Middle Devonian times. The distance from this putativesource to NorthDevon is alsolarge, considering the angularity of clasts. (c) The of the Tortworth-Mendip area contains 50 mm quartzitesand acid volcanic rocks (Van de Kamp1969; Curtis 1972). These rocks match well with the pebbles found in the Llanishen Conglomerate, and Allen (1975) proposed Fig. 6. Pebble shapes of clasts collected fromthe Rawns that a westerly extension of the Silurian outcrop in the east Formation. Top row: Lithic and dust tuffs. 2nd row: porphyries, 3rd Mendips provided a source for the Llanishen pebbles. The row: sandstones. 4th row: porphyrites. Bottom row: aplite and vein marked easterly positionof the Tortworth and Mendiprocks quartz. makes these an unlikely source for the Rawns Formation, although an extension of these types of rock farther west could form a possible source. (d)Lower Palaeozoic and Precambrian rocks occur beneaththe present Bristol Channeland S. Carmarthen area.Brooks & Thompson (1973)mapped positivea occur between these pebbles and those of the Brownstones residual Bouguer anomaly lying southof Gower (Fig. 7) and andTrentishoe Formation. A reference collection of this interpretedit as an anticlinewith Lower Palaeozoic and material is housedin the Reading UniversityGeology Precambrian rocks in its core. Further evidence for rocks of Department Archive. this nature in the northern part of the Bristol Channel was A striking feature of the clasts is their size, which ranges presentedby Brooks & James (1975) whofound from up to 50 mm, andtheir angularity (Fig. 6).Clasts are seismicrefraction experiments ahigh velocity layer at subrounded to angular suggesting a short transport distance, shallow depthbeneath the Mesozoic andCarboniferous the most angular fragments being of tuff. These often have strata. They interpreted this layer as possible or pointed terminations or show rounded corners in prismatic Pebidianvolcanic rocks, similar to thosein W Dyfed. forms. Quartz-feldspar porphyries have rounded corners to Subsequent geological findings in SW Carmarthen by Cope triangularpyramids. The sandstonesare better rounded, (1979) and in Carmarthen Bay by Tappin & Downie (1978) andare often blade-like in shape, parallel to bedding. support the existence of Lower Palaeozoic and Precambrian Metaquartzitesare moderately rounded and vein quartz sediments and volcanic rocks inthe area. Similar rocks were ranges from angular to well rounded. inferred to lie at shallow depths farther east in the northern part of the Bristol Channel by Brooks, Bayerley & Llewellyn (1977) andBrooks & AI-Saadi (1977). These Location and composition of some source areas for rockscould not betraced south due to the intervening the Rawns Formation Bristol ChannelFault Zone. Brooks & Al-Saadi (1977) The vectormean of palaeocurrentsforthe Rawns suggested that movements on this fault could be traced to Formationindicates a northerly derivation for the clasts, Upper Palaeozoic times. andthe notably angular nature of many clasts suggests a These Lower Palaeozoic and Precambrian rocks lie some nearby source. 20-30 km north of the outcrops of the Rawns Formation The association of clasts suggests a source area consisting andthus fulfil1 the necessaryrequirements of the of high-levelcrustal rocks, mainly sandstones, and composition,direction and distance for the source of the pyroclasticextrusive rocks. Quartzitesand metaquartzites Rawns clasts.

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Fig. 7. Sketch map and cross sections of the Bristol Channel area showing inferred structural elements. Geophysical data summarized from Brooks & Thompson (1973), Brooks & James (1975), Brooks& AI-Saadi (1977) and Brooks, Bayerly& Llewellyn (1977). Devonian exotic conglomerates shown in solid tone.

Upper Palaeozoic tectonics and sedimentation in the porphyries,acid lavas, and tuffs, lithic sandstonesand Bristol Channel area quartzites (Allen 1975). In late Lower Devonian times the WoodhillBay Conglomerate was deposited in the Bristol There is now appreciable evidence for locally-derived coarse area (Fig. 7); it contains clasts of acid lavas, quartzites, lithic clastic rocks being shed both north and south of the present sandstones(Wallis 1927). Mid-Devonian fault activity was Bristol Channelfrom Lower to early MiddleDevonian probablyresponsible for supplying clasts of quartzite, times. These localized pulses were not synchronous, neither and phyllite to form the Ridgeway Conglomer- are all theresultant conglomerates of identicallithology, ate.The source area for the RidgewayConglomerate although they show a broad genetic similarity, being mainly probably lay south of, but near to, the present coast of SW of acid igneous material together with quartz or lithic-rich Dyfed.The Rawns Formation alsodeveloped in early sandstone clasts. It wouldseem that localizedshort-lived Middle Devonian times, but has little petrographic similarity pulses of sediment were produced sporadically around the with the RidgewayConglomerate. Late Lower Devonian Bristol Channel area, from sources whose remnants are at timesalso saw localized clastic rocksbeing shed from an present at shallow depths beneath the northern part of the uplift in the Llandefaelog-Swansea Valley area in S Wales Bristol Channel. This intermittent sedimentation may relate (Tunbridge 197813; Cope 1979) and from sources associated to periodicmovements along the Bristol Channel Fault with theLynmouth-Brendon Fault in NorthDevon Zone. (Tunbridge and Whittaker 1978; C. Pound, pers. comm.). Inearly Lower Old RedSandstone times, polymictic The various phases of uplift described above are all of conglomeratescontaining various igneous clasts accumu- Loweror early Middle Old Red Sandstoneage and their lated as a result of movements on the Ritec Fault (Allen & developmentcan be linked tothe final stages of the Williams 1978; Dunne 1983). Instability around the Bristol Caledonian orogeny in southern Britain. Dunne (1983) has ChannelFault Zone yielded theLlanishen Conglomerate discussed the pre-Variscan development of the Benton and (Fig. 1) whichcontains quartz-feldspar and feldspar Ritecfaults in SW Dyfed. These faults lie parallel to the

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BristolChannel Fault Zone. Kellaway & Hancock (1983) Conclusions haveextrapolated the Bristol Channel Fault Zone east Coarseangular clasts in the MiddleDevonian Rawns across southern Britain into theFaille de 1’Aube in N France Formationinclude porphyries, tuffs, quartzites, lithic andGardiner & Sherridan(1982) have speculated on a arenitesand aplite. They were derived from anearby westwardextension towards the Goban Faults onthe ‘Bristol Channel Landmass’. The main part of the Hangman westernmargin of theNW European continental shelf. Sandstone Group was not derived from the Bristol Channel Weaver(1975) hintedpre-Variscan aat phase of area,but came from the mid-Devonianerosion of Lower development of the SwanseaValley Fault,and late Old Red Sandstone in S Wales. The conglomerates of the Caledonian movements in the Swansea Valley region were Rawns Formations represent one of a series of short-lived suggested as a possible influence on the development of the and often localized lower to mid-Devonian phases of coarse LowerDevonian Llyn-y-Fanconglomerate by Tunbridge clastic input, all of whichwere derived from short-lived (1980b). sources in the region of the present Bristol Channel. The UntilCarboniferous times, the S Walesarea shows sourcecomprised Lower Palaeozoic and ?Pre- evidencefor continuing to develop under the influenceof sediments,volcanic rocks, and meta-sediments and can Caledoniantectonics (Matthews 1981; Cope 1979; Dunne probably be matched with rocks which lie at shallow depths 1983) which culminated in the mid-Devonian uplift, erosion beneath the floor of the Bristol Channel, north of the Bristol anddeformation of theLower Old Red Sandstone in S Channel Fault Zone. Wales (Allen 1965). It was this Caledonian uplift which shed Inthe Bristol Channel region, the localized‘pop-up’ clastic material south to yield the mid-Devonian Trentishoe topographichighs which were the sources of local coarse Formation in NorthDevon. These Devonian movements clastic sediment in the Devonian may have been generated coincide with the oblique closure of Iapetus in the British by strike-slip movements on the Bristol Channel Fault Zone Caledonidesand with thestart of alarge-scale strike-slip during end-Caledonian movements. These sources became offset of Britain and N America (Piper 1978). largely inactive after mid-Devonian times, but reactivation Giventhe nature of the large-scaleCaledonian rnove- in late Carboniferous times produced a large scale source on ments, localtectonic activity producing the conglomerates the northern margin of the Cornubian basin. aroundthe Bristol Channel area couldconceivably have resultedfrom strike-slip movements. Badham (1982) has argued that much of the development of the Cornubian area was under a strike-slip influence. This mechanism of fault References movement is particularlyattractive thesporadicas ALLEN, J. R. L.1962. Petrology, origin and deposition of the highest Lower movement of a network of branching strike-slip faults (cf. OldRed Sandstone of ,England. Journal of Sedimentary Crowell 1974) could generatetheshort-lived local Petrology, 32, 657-97. - 1965. Upper Old Red Sandstone (Farlovian) palaeogeography in South topographic highs which seem to have yielded the Devonian Wales and the Welsh Borderland. Journal of Sedimentary Petrology, 35, conglomerates in thearea. Many of thefeatures of 167-95. strike-slip-influencedsedimentary accumulations listed by - 1974a. The Devonian rocks of Wales andthe Welsh Borderland. In: Reading (1980) areseen in the Devon-BristolChannel OWEN,T. R. (ed.) The Upper Palaeozoic and Post-Palaeozoic Rocks of Wales, University of Wales Press, Cardiff, 47-84. region (short-lived, restricted sources, rapid facies changes, - 1974b. Sedimentology of the Old Red Sandstone (Siluro-Devonian) in lack of immediatelyassociated volcanic horizons), and in theClee Hills area, Shropshire,England. Sedimentary Geology, 12, sucha regime conglomerates of differentages could be 73-167. locally sourced and then covered by ‘regional’ deposits from - 1975.Source rocks of theLower OldRed Sandstone: the Llanishen Conglomerate of the Cardiff area,South Wales. Proceedings of the the mainmid-Welsh sourceregions. It is thusconsidered Geologists’ Association, London, 86, 63-76. unlikely thatthe ‘Bristol ChannelLandmass’ was a -& WILLIAMS, B.P. J. 1978. The sequence of the earlier Lower Old Red continuous inpenetrable ridge as envisaged by Gardiner & Sandstone (Siluro-Devonian), north of Mllford Haven, southwest Dyfed Sherridan(1981), butrather comprised series a of (Wales). Geological Journal, 13, 113-36. fault-bounded blockswhich appearedand disappeared at ALLEN, P.1949. Wealden petrology: the Top Ashdown Pebble and top AshdownSandstone. Quarterly Journal of the Geological Society of differenttimes along various parts of the Bristol Channel London, 104, 257-321. region. ANDERTON,R., BRIDGES, P.H., LEEDER,M. R. & SELWOOD,B. W. 1979. A Alengthy period of tectonicinactivity followed these Dynamic Stratigraphy of the British Isles. George Allen & Unwin, mid-Devonian, end-Caledonian events. It was not until late London. BADHAM,J. P. N. 1982.Strike-slip orogens-an explanation for the Namurianand Westphalian times that further uplift Hercynides. Journal of the Geological Society, London, 139, 493-504. occurredaround the Bristol Channel area, when clastic - & AL-SAADI,R. H. 1977.Seismic refraction studies of geological wedgeswere shed northand south from a larger ‘Bristol structure in theinner part of theBristol Channel. Journal of the Channel Landmass’.This source was a major ridge Geological Society, London, 133, 433-46. - & JAMES,D. G. 1975. The geologicalresults of seismicrefraction extending along much of the length of the present Bristol surveysin theBristol Channel 1970-1973. Journal of the Geological Channel, shedding large quantities of sand northwards into Society, London, 131, 163-83. S Wales and also southwards into the Bude Basin (Kelling - & THOMPSON,M. S. 1973. The geologicalinterpretation of agravity 1974; Matthews 1981). Such a landmass was very different survey in the Bristol Channel. Journal of the Geological Society, London, from the Devonian sources in the area and developed in a 129, 245-76. -, BAYERLEY,M. & LLEWELLYN,D. J. 1977. A new geological model to different manner, under a Variscan tectonic regime. A likely explain the gravity gradient across Exmoor, North Devon.Journal of the mechanismfor the development of thissource may be Geological Society, London,133,385-93. linked tothe fillingof theBude Basin, witha late BULL, W. B. 1977. The alluvianfan environment. Progress in Physical Namurian-Westphalian uplift thein Bristol Channel, Geography, 1, 222-70. COPE,J. 1979. The Swansea Valley Fault, Wales. Geological Magazine, 118, perhapsdeveloping along the earlier lines of weakness 309-10. developed during Devonian or earlier tectonism. CROWELL,J. C. 1974.Origin of late Cenozoicbasins in California. In:

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Received 30 January 1985; revised typescript accepted 1 November 1985

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