Report 79/ 10 INSTITUTE OF GEOLOGICAL SCIENCES

Geological and geophysical investigations in

• • • •

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 and INTRODUCTION West Bay, , 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 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 interpreted results were used to site and , 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 , 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. A shallow nearshore survey to obtain sonar coverage was carried out using the fishing Sample identification and analysis vessel Dorset Lass. This completed the The positions of all sample and borehole previous year's work and covered areas locations are shown on Figure 3. Gravity where ts Somerset was unable to operate. core samples were divided into three portions, Sonar records were obtained from a KH MS. one for analysis, one for lithological studies 47 transit sonar. Position fixing was and one for storage. Offshore cored boreholes achieved by operating the boat along fixed were analysed in detail for both micro and transit lines in a south-west - north-east macro-fauna. Samples collected by scuba direction, parallel to the Decca Lattice, and divers were analysed for microfauna. taking fixes with a sextant on ranging posts Velocity, density, and porosity determinations offset at right angles to the survey lines on were carried out on samples from boreholes the beach. 15, 16 and 16A (Culshaw, 1973). borehole A preliminary interpretation of the records 74/48 and gravity core samples 330 and 331 was carried out during the survey and these (Darton, 1975) and additional samples results used to determine sampling points collected on the foreshore. for the scuba divers.

RRS John Murray and mv 'Whitethorn', 1974 INTERPRET ATION OF GEOPHYSICAL DATA Results of gravity coring carried out during a geochemical research cruise on the RRS The survey used three main geophysical John Murray were incorporated into this surveying techniques: echo-sounding, sideways survey. Both continuous seismic profiling looking sonar, and continuous seismic profiling. and transit sonar coverage were obtained All three sets of instrumentation were on this cruise. The records, which covered operated simultaneously so that recorded fix a larger area than that in this paper, were positions were identical for each. It is used to cross-check the previous interpreta­ therefore possible to use the records to obtain tion. a simultaneous interpretation of both the seabed Core drilling, gravity coring and Shipek morphology and sub-bottom geology. This grab sampling were carried out by mv has resulted in an improved geological inter­ Whitethorn during routine surveying pretation (for example, possible faults on operations. The positions of the boreholes continuous seismic profiling records can be were fixed with the Alpine Precision Ranging checked against outcrop patterns on the system. sonar records). Both mv Whitethorn and RRS John Murray were fitted with Decca Mark 21 receivers SURVEYING TECHNIQUES operated in conjunction with an automatic track plotter. Echo sounding Continuous echo soundings were obtained and Mv 'Whitethorn'. 1975 tidal corrections. based on readings from the A further 3 boreholes were drilled during tide gauge at Lyme Regis, were applied. routine surveying operations. Again their As this gauge had been levelled into the positions were fixed by the Alpine Precision bench mark on Lyme Regis quay all readings Ranging system. were reduced to the local OD and contoured at 2.0 m intervals (Figure 4). Onshore boreholes at Charmouth. 1973 Three boreholes, numbers 15, 16 and 16A, Side scan sonar were drilled at Charmouth. Continuous These records were of variable quality due core was obtained for geological logging mainly to the difficulty of ensuring that the and selected samples were retained for sonar fish was positioned at its optimum geophysical laboratory measurements. operating position of 10 m above the sea floor. BH 15 was at 166.99 m above OD while 16 However, it did prove possible to incorporate

3 the results of the more widely spaced 1974 Table 1 and a full description of them can be John Murray survey. found in Wilson and others (1958) and Cope No quantitative work was attempted on and others (1969). these records which were used, in The units mapped offshore, also shown in conjunction with the echo-sounding data, to Table 1, are more broadly based than those define outcrop patterns and cross check recognised on land and the mapped boundaries the structural styles shown on the continuous do not always coincide with the accepted on­ seismic profiling records. All the available shore stage boundaries (e. g. the Callovian­ sonar data are plotted on Figure 5. Bathonian boundary). This lack of refinement can be attributed to the limitations of the Continuous seismic profiling offshore survey method, since the continuous These records provide the most important seismic profiling equipment did not have structural information, since the major sufficient resolution to differentiate among all geological features are clearly recognisable the lithological units present, and the and can be interpolated between survey lines. sampling programme supplied information It is important to realise that the geological on only the lithology and fauna of the strata at information is portrayed as a series of fixed localities. Therefore, the map shown seismostratigraphic groups, whose in Figure 6 depicts only the seismostratigraphic lithologies and depths can be determined only groups which can be resolved by the equiplnent by sampling of the sea bed outcrops or the used in the continuous seismic profiling collection of cores from boreholes. survey. Five main seismostratigraphic Geological interpretations were traced groups have been recognised offshore; each from the seismic records with basic depth has its own range of physical characteristics measurements expressed as two-way travel and may be described in engineering terms time in milliseconds (Sargent, 1966). following further, more detailed investigations. Conversion of their value to true depths Each of these five groups is described below requires a knowledge of the compressional and a summary of their lithological properties wave velocity in the various geological units is given in Table 1. and can be determined either by laboratory measurements on core samples or by sonic FIVE SEISMOSTRATIGRAPHIC GROUPS logging methods. Considerable information is available on Triassic (Rhaetic) (Group 1) the compressional wave velocity in all the Although Rhaetic strata outcrop at Pinhay geological units offshore but for convenience Bay, no samples of this unit have been the millisecond calibration is retained on the obtained offshore. At Charmouth, borehole cross-section. Depth conversions can be 16A passes into the White Lias at made but it is found that, in particular, the approximately 75 m below OD and terminates alternating clay/limestone sequence within in the Westbury Beds at 87.87 m. the Blue Lias and the different limestone types within the Inferior Oolite result in a Lower Jurassic (Base of Blue Lias to Top wide range of values for the compressional Bridport Sands) wave velocity measured in these rocks. Within this sequence two distinct seismo­ In general the multiple reflections were stratigraphic groups can be recognised, the clearly defined on the seismic records, Middle-Upper Lias sands and clays (Group 3) although some masking of the sub-bottom and the clay-limestone sequence of the reflectors did occur in the shallow nearshore Lower Lias below (Group 2). area. Environmental noise did not prove The sparker records of the lower group a problem because of the calm sea and the (Group 2) (Figures 9 and 11) show fact that ts Somerset had a wooden hull. characteristically closely spaced, strong reflecting horizons. These are probably associated with the clay-limestone sequence STRA TIGRAPHY and the upper boundary of the group is therefore taken to be the base of the Three The units occurring onshore are shown in Tiers Bed (Table 1). The overlying Middle

4 Table I Stratigraphy.

Chronostratigraphy Lithostratigraphic Chronostrati- Lithological Description Map Boundaries and Unit graphic Unit li identifying nomenclature Upper Oxford Clay Grey sandy shaly clay (Refer to Fig. 6) ju Callovian Kellaway Beds jc-jo Jurassic Grey sandy clay (more sandy than BOUNDARY Oxford Clay) 6 Upper Cream coloured marl with sandy limestones Cornbrash Lower Rubbly yellow limestone with marly parting, in part massive blue centred limestone Generally massive central limestone between two clay divisions. In places central lime- Forest Marble Bathonian 5 jn stone thins out and limestone may develop at the top of the formation Middle jm Jurassic Conchoidally fracturing marl weathering to Fullers Earth stiff clay, some limestones developed in the Clays middle of the formation MAP BOUNDARY Massive and rubbly limestone. Pale grey to yellow with large pale ooliths Inferior Oolite Bluish or brownish limestone, coarsely (with several Bajocian ironshot. Generally hard and crys talline 4 jb non-sequences) Light grey brown limestone with fine pale green or yellow oolitic grains MAP BOUNDARY Yellow micaceous sand with lenticular ) Upper Bridport Sands ) masses or bands of blue centred calcareous ) Lias ) sandstone ) ) Downcliffe Clays Grey sandy clay Toarcian (Southern Area) jt Junction Bed Conglomeratic containing pebbles ofmarlstone and Marlstone and sandstone sometimes limonite coated. Rockbed Marlstone matrix 3 Middle Thorncombe Sands Grey silty sand becoming brown and sandy Lias at top Downcliffe Sands Yellow brown sand (often indurated to massive sandstone) and sandy marl c-.. Clay Upper Blue micaceous marls and silt with je Pliensbachian impersistent calcareous sandstone bands Three Tiers Bed Hard sandstone separated by marl. The MAP boundary is not sharp i BOUNDARY Lower Green Ammonite Clay with irregular limestones at three \ North of 50· Jurassic ) Beds horizons. Clay becomes sandy towards the ) 42'N only ** \ jl top becoming a sandy loam Belemnite Marls Pale grey marl je Lower Dark marls and shales with occasional bands Lias Black Ven Marls of thin limestone or limestone nodules 2 Sinemurian Brownish paper shales with numerous seams js of beef Shales with Beef Bluish concoidal marls with impure lime- stone bands Blue Lias Hettangian Blue argillaceous limestone with subordinate jn MAP clay and shale i BOUNDARY Upper Rhaetic Rhaetian Grey limestone and black shales 1 tr Triassic tu

* Seismostratigraphic Group ** Only to the north of 50· 42'N are the lithostratigraphic units definable where they are approximately equivalent to the chronostratigraphic units

5 Lias - Bridport Sands sequence has been sparker records and can be best seen in the recognised from the sparker records by southern and eastern portion of the map thick uniform layers divided by persistent (Figure 11 (Fixes 13-15) and 12). but widely spaced stronger reflecting Twelve gravity core samples of Bathonian horizons. It is suggested that the strong age were collected. Along the southern edge reflecting horizons are probably the harder of the area three cored boreholes penetrated layers of cemented silty calcareous a Bathonian age clay succession. Detailed sands and limestone bands as seen in studies of these logs are in progress and borehole 74/40. At outcrop the harder bands they will be included in a future publication. are very resistant to tidal scour and form prominent sea floor features. These Upper Jurassic (Kellaways - Oxford Clay) features can be seen on the bathymetric (Group 6) map (Figure 4) and the sonar interpretation No samples of a Callovian age have been map (Figure 5). Sparker evidence also obtained from the area under consideration suggests that some of the horizons within but samples of this age have been found to the Middle Lias - Bridport Sands sequence the south and east (Dingwall, 1970). In the thin southerwards (Figure 9, Line 17, Fixes south-east corner of this area, a lithological 6-9). change can be detected from sparker evidence. Twenty-four gravity core samples and It is suggested that the clays of Callovian samples from boreholes 15, 16, 16A and age found further east continue into the newly 74/40 have been identified as coming from surveyed area here. various horizons within these sequences. To the north of the westward continuation of the Eype Mouth Fault, sampling has been STRUCTURE close enough to allow the sub-division of the Lias into lithological units approximating The geological structure in this area is to stages. South of this fault line it is described in detail by Arkell (1947), Wilson possible from sparker data to distinguish and others (1958), and Cope and others only the two major groups of the Middle­ (1969). The major faults and structures Upper Lias sands and clays and the clay­ are depicted on Figure 12, which shows a limestone sequence of the Lower Lias below. series of east-west trending folds plunging These divisions are shown on Figure 6. eastwards and cut by dip and strike faults. The earliest major period of folding that can Middle Jurassic (Inferior Oolite) (Group 4) be dated with certainty is the Pre Albian. The Inferior Oolite gives rise to It is suggested, however, that at least the characteristic reflections on the sparker initial structures may be as early as Toarcian records. This oolitic, occasionally (Buckman, 1922). The later Tertiary pisolitic, iron-shot argillaceous limestone (Miocene) folding and faulting has affected also forms such distinctive seabed features the whole area, resulting in both the that it has been possible to map it with reactivation of previously formed fault lines some confidence throughout the area and also the formation of new faults. It is (Figures 4 and 5). Five gravity core difficult to distinguish the effects of these samples have been identified as Bajocian. two periods of movement, especially in One cored borehole, 74/48, passes through areas where no Cretaceous rocks are a greatly expanded Inferior Oolite sequence involved. which suggests that the unit thickens in a In Figure 12 the onshore regional structure south-easterly direction (Dr I. Penn, is contoured into the newly surveyed area pers. com.). in which complex dip and strike faulted anticlines with their complementary Middle Jurassic, Lower Fuller's Earth - synclines plunge eastwards (Figures 8-11). Upper Cornbrash (Group 5) Some of the major onshore faults can be The lithological boundary between the seen to extend offshore for considerable Inferior Oolite limestones and the Lower distances and play a major part in the Fuller's Earth clays is distinctive on the structure of the area. These faults are

6 named and shown on Figures 6 and 12 and the -Ridgeway Fault Zone will be discussed later. (Figure 12). The dominant feature is the north-east - south-west trending anticline OFFSHORE ZONES with a core of Bajocian age strata. This area is characterised by a series of east­ For descriptive purposes the off-shore area west and north-east - south-west trending can be divided into three zones: strike faults which affect strata of Bajocian i The coastal area from to and Bathonian ages (Figures 10 and 11). Eype Mouth and southwards to the Dip faults appear to be rare. This fault westward continuation of the Eype Mouth system is similar in style to that found Fault. immediately to the south in the Weymouth ii The wedge-shaped area between the Anticline (House, 1961). Eype Mouth Fault and the westward continuation of the Bride Fault. MAJOR FAULTS iii The area to the south and east of the Bride Fault extension. The major faults and the evidence for them are discussed briefly below, commencing Zone 1 in the west and proceeding from north to The strata within this zone are characterised south. by a series of tightly folded north-south and east-west trending anticlines and Coastal faults synclines which are cut by north-south A series of small faults mapped on the fore­ trending faults. The complexity of the shore between Pinhay Bay and Eype Mouth folding is shown on Figures 5, 7 and 9 and can be traced offshore. Included in these is especially noticeable between Pinhay Bay are the Char Fault, the Ridge Faults and and Charmouth (Figure 6). the Fault, the largest of which is the Seatown Fault with a throw of 30 m Zone 2 (Wilson and others, 1958). It is impossible The structural outcrop pattern in this area to determine the throw of any of these faults is simpler than in Zone 1 and is well offshore. The sonar pattern map shows that illustrated by Figures 6 and 10. A broad intense folding of the sediments accompanied east-west trending anticline with the Middle­ the faulting (Figures 5 and 7), but despite Upper Lias sands and clays flanking a core this is is felt that the throws offshore are of the Lower Lias clay-limestone sequence not markedly greater than onshore. occurs in the central part of this area. On the northern limb of the anticline the The westward extension of the Eype Mouth rocks dip steeply to the north bringing Fault Bathonian strata to outcrop in West Cliff and This is a major fault which extends westwards Bridport Sands to outcrop at East Cliff from Eype Mouth for approximately 12 500 m. (Wilson and others, 1958). The southern Onshore it is the largest Pre Albian fault limb of the anticline dips gently towards the near Bridport. On the coast where Bathonian synclinal zone at about 50° 39'N where strata crop out at sea level it has a throw Bajocian strata (Inferior Oolite) crop out. of approximately 180 m down to the south A complex and intense fault zone, probably and it probably has a varying throw along its the south-westward extension of the length though further details are not known. Mangerton Tear Fault, separates this area Onshore its easterly continuation is displaced from the next zone and displaces the outcrops approximately 1000 m northwards by the on its southern side towards the north. Mangerton Tear Fault.

Zone 3 The southwards extension of the Mangerton This area contains many anticlines and Tear Fault synclines and is bounded to the north by the This fault zone can be traced in a south­ westward continuation of the Bridge Fault and westwards direction for approximately 6000 m, in the south by the westward continuation of the intensely folded and faulted zone being

7 clearly visible on the sonar outcrop pattern DISCUSSION AND CONCLUSIONS map (Figure 5) and the bathymetric map (Figure 4). It is a dextral tear with a The combination of sparker, transit sonar displacement of about 1000 m affecting the and sample evidence has re suI ted in the clay-limestone sequence of the anticlinal mapping of several plunging anticlines with area in Zone 2 (at 500 41' 30 'N) the adj acen t their complementary synclines (Figure 12). Bridport Sands outcrop (cropping out at East These structures are cut by major east-west Cliff) and, onshore, the Eype Mouth, trending faults some of which can be shown Symondsburyand Bridport Faults. The fault to be extensions of onshore fault systems. appears to have less effect on the strata North-south and north-east - south-west farther to the south-west and eventually dies trending faults bisect and often displace out at about 50° 40'30''N ~ 50'W. Onshore these structures, these again being traceable evidence shows that this fault can be onshore. assigned to the Miocene (Wilson and others, Offshore the age of these faults can only be 1958). inferred indirectly since the overlying Cretaceous rocks have been eroded. House The seaward extension of the Bride Fault (1961), however, suggests that Pre Albian This fault can be traced to the south-west faults downthrow to the south and are normal, for 10 000 m where it appears to terminate while Tertiary faults throw down to the north against an east-west trending strike fault and are reversed. In the absence of other (at 500 39'OO''N ~ 51'45''W). Onshore the evidence it is possible to assume that the fault has a throw of 50 m to the south, and same rule may be applied to this area in offshore an approximate throw of 30 m to the which case the Eype Mouth, Abbotsbury and south is predicted. other, smaller, southerly throwing faults are of Pre Albian age, while the smaller The faults between the Bride Fault and the northerly throwing faults are of Tertiary age. Abbotsbury Fault Belt As the Mangerton Tear Fault displaces all To the south of the Bride Fault lies an area previously formed structures it is obviously of Bajocian and Bathonian strata which is cut Tertiary in age. Although this rule may by many east-west and north-east - south­ therefore be applied in general, it is probable west trending strike faults. No estimate of that the later Tertiary disturbances reactivated throws is possible on the present information. earlier fault lines and may even have A similar zone occurs onshore where blocks reversed the original throw direction. of strata are separated by east-west step The directions of the various folds and faults faults throwing down to the north, repeating are probably determined by buried Caledonoid successions over a wide area. No age and Armoricanoid trends. Vertical movements determination of these onshore faults is and rotation of buried blocks resulted in the possible. development of intense compressional stresses. Where the overlying sequences consisted of The westward extension of the Abbotsbury clays, a considerable amount of these stresses Fault Belt would be absorbed in the formation of tight The fault belt limiting the Weymouth region anticlines and synclines but where limestones on the north has been described in detail and more brittle strata occurred considerable by Arkell (1947) with further details in faulting would take place. Such features could Wilson and others (1958). It is a Pre Albian have resulted from a series of Armorican fault with a downthrow to the south of horst and graben structures which probably approximately 180 m onshore. Together influenced not only the structure but also the with its complementary Abbotsbury Syncline sedimentary depositional history (Gattral and it can be traced for approximately 7000 m others, 1971). along the southern edge of this survey. The geological map produced by detailed No estimate of throw is possible. geophysical surveying shows that, in this area, the basic seismostratigraphic groups can be related to lithostratigraphic and chrono­ stratigraphic units or stages; hence a

8 structure and stratigraphic history can be IGS, Leeds and London Palaeontological Units outlined. This survey defined five basic respectively carried out the identification of lithological groups in the offshore area; the microfauna. We would like to extend our and following more detailed examination, it grateful thanks for the valuable assistance of will be possible to describe each in engineer­ the officers and crews of the various vessels ing terms. It is envisaged that this work mentioned in the text. will take the form of a detailed borehole This paper is dedicated to the memory of investigation coupled with a further Dr W. Bullerwell, who was Chief continuous seismic profiling survey using Geophysicist at IGS (1967-1977) and a constant a system capable of a higher resolution in source of encouragement to the authors. order to provide greater lithological detail. Sampling of the rock outcrops on the sea floor has revealed lithologies similar to REFERENCES those found on land and it is, therefore, not unreasonable to produce engineering ARKELL, W. J. 1947. The geology of the data on the offshore lithological units by country around Weymouth, , Corfe taking samples for analysis from appropriate and Lulworth. Mem. Geol. Surv. U. K. boreholes on land. BUCKMAN, S. S. 1910. Certain Jurassic Finally, it is immediately apparent from (Lias-Oolite) strata of South Dorset. both the sonar and the continuous seismic Q. J. Geol. Soc. London, Vol. 66, profiling records that there is little super­ pp. 52-89. ficial cover overlying the geological strata 1922. Jurassic chronology II - exposed on the sea floor. A thin layer of Preliminary studies. Certain Jurassic fine to medium sand appears to cover most strata near Eypemouth (Dorset). The of the area but this very rarely exceeds Junction Bed of Watton Cliff and associated 0.5 m in thickness. As previously mentioned, rocks. Q. J. Geol. Soc. London, Vol. 78, evidence from regional sampling surveys pp. 378-436. suggests that material eroded from the CONWAY, B. 1971. Geological structure of coastal landslips is swept away eastwards the Dorset coast foreshore from Pinhay Bay by the strong nearshore current and very to West Bay, Bridport. Intern. Rep. IGS li ttle of this material is actually retained or Eng. Geol. Unit. April 1971. deposi ted in the survey area. 1974. The Black Ven landslip, Charmouth, Dorset. Rep. Inst. Geol. Sci., No. 74/3. ACKNOWLEDGEMENTS COPE, J. C. W., HALLAM, A. and TORRENS, H. S. 1969. Guide for Dorset and South The authors wish to express their thanks to Somerset Excursion No. 1 International the large number of people both at sea and Symposium on the British Jurassic. Geology in the office who contributed to this paper. Dept. Keele University, April 1969. They are particularly grateful to Mr R. A. CULSHAW, M. G. and WHITTAKER, A. 1973. Floyd, who organised the 1970 and 1971 Logging, density, porosity and sonic velocity marine surveys, assisted by various members determinations on rock types from boreholes of the IGS Marine Geophysics Unit and at Charmouth, Dorset. Intern. Rep. IGS Engineering Geology Unit (EGU). Additional Eng. Geol. Unit No. 39. geophysical information was supplied by DARTON, D. M. 1975. Sonic velocity, Mr J. R. Miller who organised the 1974 density and porosity determinations on rock geochemical cruise. Mr B. W. Conway, samples from a borehole and gravity core Mr J. R. Hall am , Mr M. G. Culshawand samples from Lyme Bay, Dorset. Intern. Mr A. Forster of EGU provided valuable Rep. IGS Eng. Geol. Unit No. 71. assistance in organising the land operations. DEAN, W. T., DONOVAN, D. T. and The macrofauna were identified by HOWARTH, M. K. 1961. The Liassic Dr 1. E. Penn and Miss B. M. Cox of the IGS Ammonite Zones and subzones of the N. W. (London) Palaeontological Unit, while European province. Bull. Br. Mus. Nat. Dr A. W. Medd and Mrs B. E. Colman of the Hist. (Geol.) Vol. 4, No. 10, pp. 437-505.

9 DENNESS, B. 1972. The Reservoir WILSON, V., WELCH. F. B.A., ROBBIE, Principle of Mass Movement. Rep. Inst. J. A. and GREEN, G. W. 1958. Geology Geol. Sci., No. 72/7. of the country around Bridport and Yeovil. CONWAY, B. W., McCANN, Mem. Geol. Surv. U. K., (1" sheets 327 D. M. and GRAINGER, p. 1975. and 312). Investigation of a coastal landslip at Charmouth, Dorset. Q. J. Eng. Geol., Vol. 8, pp. 119-140. DINGWALL, R. G. 1970. The structural. and stratigraphic geology of a portion of the eastern English Channel. PhD thesis, University of London. DONOVAN, D. T. and STRIDE, A. H. 1961. An acoustic survey of the sea floor south of Dorset and its geological interpretation. Phil. Trans. R. Soc. 244B, pp. 299-330. FLOYD, R. A. and HALLAM, J. 1970. Cruise report for ts Somerset, 29th July - 17th August. Intern. Rep. IGS Mar. Geophys. Unit, No. 12. GATTRALL, M., JENKYNS, H. C. and PARSONS, C. F. 1971. Limonite concretions from the European Jurassic with particular reference to the 'snuff boxes' of S. England. Sedimentology, Vol. 18, pp. 79-103. HALLAM, A. 1958. The concept of the Jurassic axes of uplift. Sci. Prog. London, Vol. 46, pp. 441-488. HOUSE, M. R. 1961. The structure of the Weymouth anticline. Proc. Geol. Assoc. London, Vol. 72, pp. 221-238. HOWARTH, M. K. 1957. The Middle Lias of the Dorset coast. Q. J. Geol. Soc. London, Vol. 113, pp. 185-203. LANG, W. D. 1914. The geology of the Charmouth cliffs, beach and foreshore. Proc. Geol. Assoc. London, Vol. 25, pp. 293-330. 1917. The ibex Zone at Charmouth and its relation to the zones near it. Proc. Geol. Assoc. London, Vol. 28, pp. 31-36. 1924. The Blue Lias of the and Dorset Coasts. Proc. Geol. Assoc. London. Vol. 35, pp. 169-185. 1936. The Green Ammonite Beds of the Dorset Lias. Q. J. Geol. Soc. London, Vol. 92, pp. 423-437, 485-487. SARGENT, G. E. G. 1966. A review of acoustic equipments for studying sub­ marine sediments. Trans. Inst. Min. Met., Ser. B, pp. 113-118.

10 50 0 50'N

N AXMINSTER• t ~;;;R~o,~~~...... ~!!!li~~RTON BRADSTOCK DORCHESTER• • BEER HEAD 50 0 40'N

I-' I-'

2 0 2 4 6 8 10km I I I I I I

PORTLAND BILL 500 30'N

3°10'W 3°00'W 2°50'W 2°40'W 2°30'W 2°20'W

Figure I Survey Area. 1000 0 1000 2000 3000 4000 metres I I I 500 44'N LINE 7

Track Chart TS Somerset 1970 Dorset Lass 1971 Line of Cross Section

50 0 42'N

I-' ~

500 38'N

LINE 7

2°57'W 2°55'W 2°53'W 2°51'W 2°49'W 2°47'W 2°45'W

Figure 2 Track Chart. 015

1000 0 1000 2000 3000 4000 metres I I

50 0 44'N

Key x Diving sample o Cored bore hole -288 Successful Gravity xLB24 - Core Station

_62 -330 50 0 42'N

r -39 -38 ~ c:" -316 -332

0 _317 _331 50 40'N -315 -63 -40 -323 -324

-314 -64 -36 -319 _41 -313

-320

-42 -35 500 38'N -65 074/48

-325 2°57'W 2°55'W 2°53'W 2°51 'W 2°49'W 2°47'W 2°45'W 074/35

Figure 3 Sample locality Map. 1000 0 1000 2000 3000 4000 metres , 1 I I I I

500 44'N Contour interval 2 metres below OD

50 0 42'N

~ ~

500 40'N

28~ 500 38'N 28~""" ~

2°57'W

Figure 4 Bathymetric Map. Legend Rock Outcrop and Adjacent Shadow Zone ~ 1000 0 1000 2000 3000 4000 metres Coastal outcrop pattern taken to low water mark I 500 44'N

' ... -..

I ... ~ t.,--]:-.., "

-- ...... ,

50 0 42'N

./'

..... ( '­ 01 '\

50 0 40'N ! ///~ - ...... "" ", ~..,; ~ , / ..... - ..-~ ~ 4 /..­ , ..-,. -~ ~ '~" ~ ~ !r / .&.. 41' 500 38'N ---£- .- ,.-- 4IfU8t r --- """ ~ -----­ , -----4r

2°45'W

Figure 5 Side scan transit sonar and coastal outcrop pattern interpretation. Chronostrati - Seismostrati - KEY graphic Units graphic Mappable Groups(see table1) Geological boundary Callovi~n - Group 6 1000 0 1000 2000 metres Fault Oxfordlan Pliensbachian } I I I I --"'- I Anticline .:.:.: Bathonian Group 5 Sinemurian Syncline ..:: .. :'::" . Hettang ian _ Group 2 Outcrop trend pattern :::";::;-::': Bajoclan. Group 4 Pliensbach ian 50 0 44'N Line of section Pliensbachlan- Group 3 Hettangian General dip direction , Toarcian SEATOWN \ ...... Rhaetian Group 1 FAULT \ ...... __ ....

----~ I ' .... I----~ "'-~~-_J/ ' ......

500 42'N

.... Ol

500 40'N ~\.fy-

500 38'N

Figure 6 Structural and stratigraphic interpretation. Cl Cl ::0 ::0 N 0 0 S ""tl ""tl 0 0 -is:::0 -is:::0 »0(J)- (J)-»0 r zC::: r zC::: r r Fix » j;4 j; j; no. , 2 (J) 3 (J). o(J)(J) » 1 5 (J) 6 7 (J) 8 I 'I'~ &;1 F 25 ms 25 ms

50 ms 50ms I I 75 ms T5ms 100ms + 100ms 0 2000 metres I I I

~ -:J (J) N z S s:m c ::0 r Fix j> » no., 2 z (J) 3 4

25 ms

50ms

75 ms -·75 ms

100ms 100ms

o 2000 metres

Figures 7 and 8 Geological interpretation of side scan and sparker record lines 2 and 7. to to to N ""0 :JJ :JJ :JJ 5 C 0 0 a m ""0 ""0 ""0 Z 0 0 to 0 CJ) :JJ :JJ to l> to :JJ to to -13: -13: -I -13: l>r CJ)- CJ)- ~ ::I: ~ CJ)- ~ (")0 0 0 0 o l>0 l>0 (") l>0 (") ~~ zC r zC (") Z zC Fix 2l>m ol> j> j> j> nO.1 Z:JJ 3 1 4 ~ 5 6 CJ)CJ) 7 81 z 1 9 z 1°1 ~ 1 ~ l;11 z 12 ge; ,.:EI 130 I I 74/48 25 ms + r ,~-r~~ ___ t '" t. tl, Flt. t~ J +~t< ~ J25ms 50ms 50ms

75 ms 75ms

lOOms lOOms to MAJOR FAULT ZONE o 2000 metres N 5 I ""0 o to :JJ l> to ~ 5 -13: -I -I CJ)­ ::I: ~ ::I: l>0 0 o o . zC r Z (") Z ~ FIX ol> j> j> 9 00 no. 1 CJ) CJ) 2 CJ) 3 4 ~ 5 6 7 8 z 10 ~ 11

F //i -----t .\cjf-T 7 7 ~tt> ~ F~ ! t __ t-L 4r , t I > t f, 1<= ~ 25ms 50 ms 50ms

75mSr 75 ms

100 ms~ lOOms o 2000 metres

Figures 9 and 10 Geological interpretation of sparker record lines 17 and 39. CJ CJ CJ CJ CJ w :D :D :D :D :D E 0 0 0 0 0 a"'tl a"'tl a"'tl a"'tl a"'tl :D :D :D -is: ~ s: -i -is: ~s: (J)- s: »0 (J) 0 (J)o (J)o (J)o Fix C z C r z» r _ C »rz_ r »rz_ r »rz _ 1 2 j> 3 o » o » 5 l> 6 0» 7 l> 8 o » 9 no . (J)» 0»(J) (J) (J) (J)(J) (J)(J) (J) (J)(J) (J) (J)(J)

25 ms 25 ms

50ms 50ms 75 ms ------75 ms 100ms 100ms

0 2000 metres CJ I I I :D 0 "'tl W a CJ CJ E :D » CJ » ,.... -is: :::c-i » :::c-i CD (J)-»0 a L-a a zc z (") z 9 0» 10 l> 11 12 l> 13 l> 15 (J)(J) z z , 11~ Z

25 ms + _t-, ....j 25 ms

50 ms

75 ms r--~50m'I 75 ms MAJOR r~ MAJOR FAULT ZONE 100 ms I- FAULT 100ms ZONE 0 2000 metres ~._J I I

Figure 11 Geological interpretation of sparker record line 63. 2

Q) ...... Q) E o ~

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SPARKER LINE 7 Plate 2 Sparker line 7.

21 MID LlAS-BRIDPORT SANDS MID LlAS- 1------LlAS I I I LlAS BRIDPORTSANDS ____

1 0 2 kilometres I J I t>:) Horizontal scale t>:) MID LlAS - BRIDPORT SANDS r:sms ------IIf- BAJOCIAN -t- BATHONIAN -+BAJOCIAN I I I BAJOCIAN -1 [50ms Vertical scale

SEA BED

SPARKER LINE 17

Plate 3 Sparker line 17. 0 1LJ CI) ('t) <{ CD 1LJ Cf) 1LJ T 2 2 ::J ~ a: Cf) 2 1LJ ~ <{ 0 I a: ::J t- <{ <{ a.. CI) Cf)

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Dd. 696787 KIO Printed in England for Her Majesty's Stationery Office by Commercial Colour Press, London E7

23 LlAS ----t--- BATHONIAN BAJOCIAN ------t-I SPARKER LINE 39-1

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Plate 5 Sparker lines 39 and 2.

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