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Pdf/143/1/107/4893446/Gsjgs.143.1.0107.Pdf by Guest on 02 October 2021 108 I Journal of the Geological Society, London, Vol. 143, 1986, pp. 107-115, 7 figs. Printed in Northern Ireland Mid-Devonian 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 Sandstone Group (Eifelian) of North Devon 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 sandstones 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 Precambrian 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 Ridgeway Conglomerate 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 Cardiff district andinferred theTrentishoe Formation (Tunbridge 1984) (Fig. 1). It frompebble assemblages that this too had asoutherly follows the shoreline facies 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 lake fills (Tunbridge1984). conglomeratescould bethe precursors tothe major After deposition of the mud dominated, pedogenic Yes Tor Carboniferous 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 sediments 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 Lower Old Red Sandstone in S Wales,then petrographic similarities should be evident, but if derivation was from a Stratigraphy 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 Wales comes from heavy 107 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/107/4893446/gsjgs.143.1.0107.pdf by guest on 02 October 2021 108 I. P. TUNBRIDGE l I E. WALES CENTRAL S.W. WALES S.W. WALES NORTH DEVON and AVON N. of Ritec Fault) (S. of RitecFault Quartz Conglomerate 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/107/4893446/gsjgs.143.1.0107.pdf by guest on 02 October 2021 MID-DEVONIANCHANNELTECTONICS,BRISTOL 109 granophyricintergrowths of quartzfeldspar.and Metamorphic rock 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 greywackes 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
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