Geological and Geophysical Investigations in Lyme Bay
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Report 79/ 10 INSTITUTE OF GEOLOGICAL SCIENCES Geological and geophysical investigations in Lyme Bay • • • • INSTITUTE OF GEOLOGICAL SCIENCES Natural Environment Research Council Report 79/10 Geological and geophysical investigations in Lyme Bay D. M. Darton, R. G. Dingwall and D. M. McCann © Crown copyright 1981 ISBN 011 884105 X London Her Majesty's Stationery Office 1981 CONTENTS PLATES Summary 1 1 Side scan lines 7 and 2 20 2 Sparker line 7 21 Introduction 1 3 Sparker line 17 22 4 Sparker line 63 23 Summary of field work and geophysical 5 Sparker lines 39 and 2 24 instrumentation 2 Surveys from vessels 2 TABLE Interpretation of geophysical data 3 1 Stratigraphy 5 Surveying techniques 3 Stratigraphy 4 Five seismostratigraphic groups 4 Structure 6 Offshore zones 7 Major faults 7 Discussion and conclusions 8 Acknowledgemen ts 9 References 9 FIGURES Bibliographical reference 1 Survey area 11 DARTON, D. M., DINGWALL, R. G. and 2 Track chart 12 McCANN, D. M. 1980. 3 Sample localitity map 13 Geological and geophysical investigations in 4 Bathymetric map 14 Lyme Bay. Rep. Inst. Geol. SeL, No. 79/10. 5 Side scan transit sonar and coastal outcrop pattern interpretation 15 Authors 6 Structural and stratigraphic interpreta D. M. Darton, BSc tion 16 formerly with Engineering Geology Unit, 7 Geological interpretation of side scan Institute of Geological Sciences and sparker record line 2 17 8 Geological interpretation of side scan R. G. Dingwall, BSc, PhD and sparker record line 7 17 formerly with Continental Shelf Southern Unit 9 Geological interpretation of sparker , Institute of Geological Sciences record line 17 18 now Exploration Dept, British National Oil 10 Geological interpretation of sparker Corporation, 150 Vincent Street, Glasgow 2, record line 39 18 Scotland 11 Geological interpretation of sparker record line 63 19 D. M. McCann, BSc, PhD 12 Regional geology (in pocket) Institute of Geological Sciences, London 111 Geological and geophysical investigations in Lyme Bay D. M. DARTON, R. C. DINGWALL and D. M. MCCANN SUMMARY As part of a general research programme Probleme, die mit der Extrapolation der into geological mapping of coastal areas for Kustenstratigraphie bis zu irgendeiner grossen engineering purposes, a geological and geo Weite von der Kuste weg verbunden sind. physical survey of the sea floor to the south east of the coast between Lyme Regis and INTRODUCTION West Bay, Dorset, was carried out. The In recent years there has been increasing survey covered 90 km2 and resulted in detail interest in the engineering properties of the ed mapping of two eastward plunging anticlines sea floor and underlying geological structures and their complementary synclines. The east in the nearshore environment. These coastal west and north-east-south-west trending faults, areas form an interface zone between land and which produce major structural changes, occur marine activities and are under intense within 1 mile of the coastline and demonstrate pressure from demands for oil and gas pipe the problems of extrapolating coastal strati line installations, deep water anchorages, graphy to any great distance offshore. sewage pipes, navigational installations and recreational facilities. Selection of a develop Un leve geologique et geophysique du fond de ment area is controlled both by fundamental la mer, qui a ere" effectue entre Lyme Regis factors, such as geological structure and geo et West Bay, Dorset, faisait partie d'un morphological history, and by social and programme de la recherche generale, pour ecological circumstances. des fins techniques, sur la cartographie The first essential part of an investigation geologique des zones cotieres. Le leve, qui of any area where civil engineering develop s' etendait jusqu'a 90 kilometres carres, a ments are proposed is a geological survey of abouti au dressage des cartes detaillees de the sediment and rock types likely to be deux anticlinaux qui plongent vers I' est, et de encountered during the subsequent site investi leurs synclinaux complementaires. Les failles, gation of their engineering properties. In the qui se dirigent de I' est I' ouest et du nord- a shallow water coastal zone the sub-bottom est au sud-ouest, produisent des changements geology can sometimes be determined by structuraux majeures en deca de 1.6 km du extrapolating information from the adjacent littoral, et demontrent nettement les problemes land; usually, however, marine geological associes l'extrapolation de la stratigraphie a surveys are carried out using geophysical au littoral une grande distance au large. a techniques such as continuous seismic profil Als Teil eines allgemeinen Forschungs ing and sideways looking sonar in conjunction programmes fur die geologische Kartierung with borehole and gravity core samples. der Kustengehiete fur technische Anwendungen, The work described here is part of a machte man eine geologische und geophysische general research programme into geological Untersuchung des Meeresbodens sudostlich mapping of coastal areas for engineering der Kust zwischen Lyme Regis und West Bay, purposes. Particular attention is paid to the Dorset. Der Untersuchung eines Gebietes Dorset Coast in southern England, known von 90 Quadratkilometern folgte die throughout the world as a classic area for ausfUhrliche Kartierung von zwei nach studying Jurassic rocks. The major part of Osten fUhrenden Antiklinen und ihren ent the Jurassic system is observed in a series sprechenden Synklinen. Die Ost-West und of coastal exposures stretching from Pinhay Nordost-Sindwest gerichteten Verwerfungs Bay in the west to Portland Bill in the south spalten, die grosse Strukturveranderungen east. Many important papers have been verursacken, geschehen innerhalb einer published on this area (Buckman, 1910, 1922; Meile der Kuste, und zeigen deutlich die Lang, 1914, 1917, 1924, 1936, Howarth, 1957; 1 Dean et aI, 1961). Recently, however, SUMMARY OF FIELD WORK AND research has continued offshore into Weymouth GEOPHYSICAL INSTRUMENTATION Bay (Donovan and Stride, 1961) and the eastern English Channel (Dingwall, 1970). The main geophysical survey work was In the area between Lyme Regis and West carried out between 1970 to 1971 and the Bay, particularly in the area of Black Ven interpreted results were used to site and Golden Cap, rapid recession of the sampling and drilling locations for the later coastal cliffs has occurred because of land cruises of 1972-1975. slide activity. Landslide activity on Black Ven has seriously affected property and SURVEYS FROM VESSELS recreational facilities in Lyme Regis and Charmouth, and research studies have been Ts 'Somerset', 29th July -17th August 1970 carried out to examine the factors controlling The initial geophysical survey was carried the stability of this landslide (Denness, 1972; out on a grid with a line separation of 500 m Denness and others, 1975; Conway, 1974). (Floyd and Hallam, 1970). A Decca Mark 12 Current activity is related to the coastal navigation system was employed throughout geomorphology, which is controlled to a and 5-minute fixes were plotted on photo great extent by the geological history of graphically enlarged Decca lattice sheets the area, particularly in the immediate (6 in to 1 mile). Decca reception was offshore zone. excellent throughout the survey and the The debris from these landslides has lattice cut off angles were apprOximately 90° • produced large mudtlows on the beach and A high standard of accuracy was therefore this material is continuously eroded away maintained. by the sea. Undoubtedly this process has Precautionary measures against Decca been in progress for many thousands of errors were, however, taken. Sixteen years, as boulder arcs from old mudtlows sounding markers were surveyed in along can be observed some distance from the the coastline and four Dan buoys with radar beach in the nearshore zone. The bulk of retlectors moored in pre-selected positions. the material is swept away eastwards by a Both systems were used as cross checks to strong inshore current but chert pebbles the Decca system. All traverse lines were and some gravel and coarse sand can be run along the Decca lattice enabling a further found on the beach. cross check for accuracy. The Lyme Regis-West Bay area is ideal In order to calibrate the various echo for a study of coastal engineering problems sounding surveys, a tide gauge was erected both on land and in the immediate nearshore in Lyme Regis harbour. Observations were zone, so the geological survey of the fore taken at half hourly intervals between 0800 shore area was extended offshore using and 2100 hrs during the period 4th-15th marine geophysical surveying techniques August 1970. This tide gauge was levelled (Figure 1). The sub-bottom structure and into the bench mark on Lyme Regis quay, sea tloor morphology were examined by and echo sounding readings could then be continuous seismic profiling, side scan reduced to the local OD. sonar and echo-sounding equipment. Continuous seismic profiling was carried Sampling was carried out by scuba divers, out with an I. F. p. multi-electrode sparkarray Shipek grab, gravity coring and drilling. operating at 500J. Signals were received by The survey, which covering apprOximately an E. G. and G. Type 263B hydrophone and 90 square kilometres, illustrates the high recorded on a Huntec recorder. density of traverse lines necessary to allow Sonar records were obtained with an E. G. reliable interpolation between adjacent data and G. towed side scan sonar fish operated points in a highly complex area (Figure 2). by a modified E. G. and G. Type 254 The results highlight the danger of extra recorder on which the results were recorded. polation of the land geology into the offshore Bathymetric records were taken by a K. H. zone and the problem of carrying out routine MS. 36 echo sounder mounted over the side site investigations based solely on sample of the ship. evidence. Thirty-one bottom sediment samples were collected by Shipek grab. 2 'Dorset Lass'. 20th September - 3rd October and 16A were at 3.43 m above OD and 3.39 m 1971 above OD respectively.