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The Geology of the Severn Barrage Area

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The Geology of the Severn Barrage Area NATURAL ENVIRONMENT RESEARCH COUNCIL Institute of Geological Sciences The geology of the Severn barrage area A report prepared by the Institute of Geological Sciences 621.49 for the Department of Energy 157 NATURAL ENVIRONMENT RESEARCH COUNCIL Institute of Geological Sciences SUNDERLAND POLYTECHNIC POLYTECHNIC LIBRARY 800K No .................~ 1. I : ...........4-'i .......:C.... 5r... -p"5 O{~ ACCESS NO .... .......... ............... DUE ~ RETURN ON. THE LAST DATE STAMPED IELCW The geology of the Severn barrage area "'f ,, ,., . - 8• C.; • 1'}J,.1 by G. W. Green and B. N. 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' : Priestman Library,~ MSD 2211 Institute of Geological Sciences Ring Road Halton and 5 Princes Gate Leeds LSI5 8TQ London SW7 IQN A report prepared b y t he Institute of Geological Sciences for the Department of Energy May 1976 CONTENTS Page Onshore geology 1 Notes on succession 1 Notes on the structure 2 Offshore geology 2 Geological information along the line of the barrage 4 The Institute of Geological Sciences was formed by the Engineering geology notes 4 incorporation of the Geological Survey of Great Britain and the Museum of Practical Geology with Recommendations for additional offshore work 8 Overseas Geological Surveys and is a constituent body References 8 of the Natural Environment Research Council. ILL UST RATI ONS © NERC copyright 1977 Fig. 1. Bathymetry of the Severn Estuary between Lavernock Point and Weston- super-Mare 3 Fig. 2. Institute of Geological Sciences offshore sample stations 5 Fig. 3. Geological map of the Severn barrage area 6 Fig. 4. Suggested offshore geological survey by the Institute of Geological Sciences 7 Printed in England by Walter Jenn Ltd. IGS 1883 555 3/77 su~ 1 OLY Priestman Library SUMMARY An account is given of the onshore and offshore geology of the Severn Barrage area, largely compiled from unpublished Institute of Geological Sciences work in the estuary and on land. The most The Geology of the Severn Barrage area important preliminary result is that much of the area of the barrage has bedrock exposed at the seabed by and is not covered by drift, contrary to evidence available in the report of the Select Committee. Proposals are made for a more detailed survey. G. W. Green and B. N. Fletcher ACKNOWLEDGEMENT The offshore geology in the area of the Severn Thickness A number of units from the Institute of Geological Sciences have contributed information which has Barrage scheme is not known in detail, but the m been used to compile this report. rock types are the same as those that occur in the Westbury Beds The continuous reflection profiling records in the offshore area were taken by the Radioactive and adjacent land areas. The onshore geology, which Soft dark grey to nearly black fissile Metalliferous Minerals Unit on two cruises in the Bristol Channel, and the sampling was by the is well known, is very relevant to the barrage pyritous mudstones and shales with Continental Shelf Unit 1. Dr D. B. Smith and Dr R. A. Waters (South Wales and Welsh Marches Unit) scheme and is discussed first. subordinate impure limestones contributed to the geology of the Lavernock Point area, whilst the engineering references were supplied Much of the work included in this report is and bands of fibrous calcite 7 -13 by the Engineering Geology Unit. based upon recent work by the Institute of Keuper Geological Sciences (IGS) and has not yet been Tea Green and Grey Marls published. Alternating greenish grey, grey and pink mudstone and siltstones with Onshore Geology discrete layers and nodular The geological successions etc are essentially masses and veins of gypsum similar on each side of the Bristol Channel. They (CaS04) and more rarely celestite comprise, in descending order: (SrS04) 16-31 Red Marls Thickness Alternating red and some green m mudstones and siltstones with QUATERNARY discrete nodular layers and veins of gypsum and more rarely celestite Drift deposits of sand, gravel and in uppermost parts; monotonous estuarine clays and silts up to 35 red silty mudstones with green SU patches forming main part of MESOZOIC . T, ;y formation +200 - ·- Dolomitic Conglomerate Cr • , C. 2.1 . 4-"\ T. 51 JURASSIC Limestone conglomerates with beds C ~·' Lower Lias of red sandstone and mudstone {?s-'4. 2~] h.) Mudstones and shales with some ·-·- argillaceous limestones (semi­ - Major Unconformity - costatum Zone) about 75 Sinemurian PALAEOZOIC South side of Channel-Fissile shales -·-Order ~Jo. ------ and mudstones with a few lime­ CARBONIFEROUS stones in middle and upper parts. Carboniferous Limestone Alternating shales, mudstones Hard, massive to bedded grey lime­ and massive limestones in lower stones and dolomites with some part. North side of Channel­ thin shales in parts about 1000 Massive limestones with thin shales (bucklandi Zone) 40-50 Hettangian Notes on succession Alternating thin limestones and shaley mudstones, though Drift deposits Inland, on both sides of the Bristol limestone/ shale ratio variable Channel, up to 30 m of soft unconsolidated muds, (angulata Zone) 15-50 silts and sands with some peat levels, are wide­ Shales and marly mudstones with spread. Details of successions can be supplied if a few layers of limestone nodules necessary. Their very low bearing strengths (liasicus Zone) 12-25 give rise to considerable engineering problems. Alternating limestones and soft Lower Lias The limestones are hard to very shales (planorbis Zone) about 14 hard, fine-grained and rather argillaceous, with individual beds ranging in thickness from 1-2 cm TRIASSIC to as much as 0.8 m. The limestone nodules are Rhaetic similar to the limestone beds. The fossil content White Lias and Cotham Beds of the Lias is such as to enable even quite small Calcareous mudstones, shales samples to be related to the general succession. with subordinate hard fine­ Pyrite is not uncommon. All the beds are grained limestones especially calcareous to some extent. in top 1-1 ½m (White Lias) 3-4 Rhaetic The White Lias comprise relatively pure, 1 fine-grained, hard to very hard splintery lime­ change of dip occurs. The structure in the stones. The Cotham Beds are weakly calcareous. South-West corner and along the South side of The mudstones and shales of the Westbury Beds the island is complex, with numerous East­ are non- calcareous and pyritous. North-East trending folds and much associated Tea Green, Grey and Red Marls are slightly shattering. The movement has occurred calcitic and dolomitic. The siltstones are harder beneath a low-angle bedding-plane fault or slide than the intervening marls and locally may which cuts obliquely across the island and runs approach silty limestone in composition. Apart into the sea in the South-East corner of the from colour differences, all these formations are island. The structural continuity with the main­ type. land is uncertain and it appears probable that the essentially similar in rock 0 rocks mask an area of N Dolomitic Conglomerate This is a fossil scree and intervening Mesozoic o_ deposit composed of Carboniferous Limestone considerable structural complexity in the fan "' ~., debris and is only found adjacent to outcrops of Carboniferous rocks. ..> Cl) Carboniferous Limestone. It passes laterally into b. Flat Holm. The major feature is an East­ 0 g z< I (0 ... the Red Marls, generally within 100 m of the North-East aligned anticline pitching gently to M -= the South-West but there are also many small C'i E I Limestone outcrops. '!=! ll) ~ Unconformity below the Mesozoic rocks This folds and small faults associated with the banded ...., ~ ~ .... represents an old and very irregular land surface. limestone-shale sequence noted above. An ., ,,E 2 (J .."':, .. Cl) fissures up to 1-3 important thrust dipping 45° to the South-South­ 0 E Locally the Triassic rocks fill ... g 0 the area to the C: 0. m wide in the underlying Carboniferous Limestone. East repeats the latter sequence in 0 ,, ~ The plane of the unconformity is typically very south of the lighthouse. (.) ~, :::ii::: "ll in 3. c. Wolves. The limestone appears to have a a: u steep with gradients ranging from 1 in 1 to 1 1 0 Patchy dolomitisation of the Carboniferous Lime­ gentle northerly dip. 2 stone may be locally important especially near 0 a: 0 UJ the junction with the Triassic rocks - this results 1 M > in a rather variable degree of hardness of the Offshore Geology 0 et ...J limestone and may result in increased porosity 2 The Institute of Geological Sciences have 0 w (in the dolomitised parts). .: w y carried out some continuous reflection c:c Carboniferous Limestone (for structure see below) recentl w ct Steep Holm. The succession here ranges from profiling supplemented by gravity coring in part a. ~ ~ limestones. Trias-filled of the area of both the Wilson and Shaw schemes .... I massive to well- bedded w c:c joints associated with minor galena-baryte minera­ for a proposed Severn Barrage. al w lisation are particularly evident in the south-west The most important preliminary result of thi? > 0. island. survey is that much of the area of the barrge has c:c :::> corner of the et (./) b. Flat Holm. The succession is similar to that bedrock exposed at the seabed. This is contrary I :::> 2 on Steep Holm but includes in addition a unit some to para 15 page 10 of the Select Committee .... (./) 0 35-40 metres in total thickness, comprising report (1975), which states that 'the limited w .... CJ) massive limestone beds (2-7 m) alternating with information so far available on the Severn 2 w thinly bedded mudstone and dolomite bands (1 - 5 m).
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