Geological Evidence for Intra-Jurassic Faulting in the Wessex Basin and Its Margins

Journal of the Geological Society, London, Vol. 148, 1991, pp. 245-260, 16 figs. Printed in Northern Ireland

Geological evidence for intra-Jurassic faulting in the Wessex Basin and its margins

H. C. JENKYNS’ & J. R. SENIOR’ ‘Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK ’Department of Adult and Continuing Education, University of Durham, 32 Old Elvet, Durham DH1 3HN, UK

Abstrad Geological observations on Jurassic outcrops close to major faults in the Wessex Basin- Mendip area reveal the local presence of ammonite- and brachiopod-bearing sediments penetrating underlying strata. Toarcian and Bajocian neptunian dykes and particularly sills are associated with the EypemouthFault and Bajocian sills with the BrideFault and MereFault. In the Mendiparea numerous neptunian dykes of Hettangian, Sinemurian, Pliensbachianand Bajocian ages, cross-cutting Carboniferous Limestone, are recorded, typically also associated with major basement faults (e.g. Cranmore and Leighton Faults). These periods of assumed sediment injection are taken as indicating times of displacement along the faults in question. Variations in facies(Hettangian-Sinemurian, Toarcian, Bajocian, uppermost Oxfordian, Kim- meridgian) spatially linked to faultsare documented from some areas, and boreholes reveal considerable fault-controlled thickness changes in Hettangian-Sinemurian, Bajocian and Kimmeridgian sediments. The timing of Jurassic faulting in the Wessex Basin-Mendip area thus polarizes into two intervals: Hettangian-Bajocian and latest Oxfordian onwards, correlating withthe early rifting phases of the Central and North Atlantic respectively.

A number of authorshave recently sought to clarify the Evidence from neptunian sills: Wessex Basin evolution of the Wessex Basin in terms of current tectonic models (e.g. Stoneley 1982; Whittaker 1985;Chadwick 1986; Karner et al. 1987; Lake & Karner 1987;Selley & Eypemouth Fault, Watton Cl#, West Bay Stoneley1987). In all theseworks the concept of Evidencefor syn-sedimentary faulting affecting the Marl- syn-sedimentary faulting is integral to models of basin stoneand Junction Bed during the Early Jurassic was subsidence andsediment accumulation. In this paper an presented by Jenkyns & Senior (1977), theinterpretation essentially field-based approach is adopted to spotlight the being based on thickness changesin these units and the geological evidence, much of it available at outcrop, for presence of sedimentary fissures. New(1987) rock falls Jurassic syn-sedimentaryfault movements in theDorset laying bare large sections of Watton Cliff (SY 448909) close sector of the WessexBasin andits marginal regions to so-called ‘Fault Corner’, allow more detailed interpreta- represented by the Mendip Swell. Thelatter region is tions of the section. Here Junction Bed is faulted against important in that it may offer a window into the nature of Fuller’s Earth and Forest Marble, but the feature of major the Hercynian basement that underlies the Mesozoic fill of interest is the dramatic increase in thickness of the former the Wessex Basin. unit as it is traced eastwards towards the fault plane (Figs 3, The evidencepresented includes documentation of 4), being overlain by horizontal shales of the Downcliff Clay phases of probablesediment injection along fault zones; (levesquei Zone, levesquei Subzone).Close tothe fault changes in stratigraphic thickness, in some cases calibrated plane itself, anobviously coarse-grained matrixis invaded to thezonal level, across and around a number of faults; and by numerous horizontally oriented sediment-filled fissures changes in facies associated with certain faults. The evidence that cause the local thickening of the Junction Bed (Fig. 5); assembled below points to the Hettangian to Bajocian and theseneptunian sills wereseen by Buckman (1922) and thelate Oxfordian-Portlandian intervals astimes of Jackson(1922, 1926) who attemptedto interpret themas particularly significant faulting and facies variation, with less normal members of a Marlstone/Junction Bed succession. pronounceddifferential subsidence during the intervening Buckman (1922) termed this unit the ‘Watton Bed’ and his period.A map of thearea, showing major faults, is and Jackson’s papers reveal their struggles to understand the presentedin Fig. 1, and a Hettangian to Bajocianzonal anomalous stratigraphy of the ammonites. scheme for Wessex Basin rocks in Fig. 2. The matrix is typically developedas agrey-green to Some forty years ago Arkell (1942) had already realized reddish-brown calcareous/ferruginous quartzarenite, locally the significance of changesin thickness and facies of conglomeratic and rich in shell materialand brachiopods, Oxfordian and Kimmeridgianstrata across east-west- whereas the fissures are usually filled with multiple fills of trendingfaults close to Oxford, a themeapparent in the fine-grained parallel- and cross-laminated buff to pale rose seminalwork of Bailey & Weir (1931,1932) onthe calcilutites andcoarser milky limestones,both of which Brora-Helmsdale Fault in East Sutherland, Scotland. contain sparse quartz grains. These distinctive sediments are 245

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V 0 10 20km

Fig. 1. An outline map of principal faults in the Wessex Basin-Mendip area. Jurassic sense of downthrow indicated where known. Fissure facies occur in sediments where both the Eypemouth and Bride Faults reachthe cqast. Data from relevant geological survey maps, House in Cope et al. (1969), Darton et al. (1981), Chadwick & Kirby (1982) and Selley & Stoneley (1987).

subsequently referred to as fissure facies (Fig. 6). Contacts organic-rich clays; the ageand geological context of this between fill and matrix are generally sharp but locally are cement, however, remain problematic. Some of these fluids gradational. Multiple fills may be represented by differently maywell haveentrained fine-grained sediment asthe coloured limestones but the various layers also differ in their isotopic composition of the equivalent to layer C in Fig. 7, density of included fossil fragments, indicating hydraulic also ferroan, is similarly negativewith respect to carbon: sorting during filling of the cavities. Some degree of grading 6I8O = -1.60%; 613C = -7.42%. It isalso possible that is locally apparent (Fig. 7), suggesting that filling was rapid. within these cavities specialmicro-environments existed, The micritic fabric is commonly peloidal, and is suggestive where isotopic chemistry was locally modified. of formation as high-magnesiuma calcite precipitate These neptunian sills typically have thicknesses of tens of (Macintyre 1985); coccoliths or similar nannoflora have not centimetres and may be traceable over a horizontal distance been identified. The lamination exists on various scales (Fig. of a metre or more; they generally have smooth undulating 8) and is constituted by peloids packed together in different bases,more irregular tops, and may end bluntly. densities and surrounded by differing volumes of micrite and Additionallytiny sediment-and calcite-filled cracks sparite matrix. (diameter 0.05-0.5 mm) occur locally as do somewhat larger The isotopiccomposition of thismaterial is typically geopetal cavities. Rare subvertical neptunian dykes cut both marine (e.g. 6"O = -0.48%; 6I3C= 0.24% PDB for one horizontal fissures and host rock: their contained sediment is sample) which compares not unfavourably with a previously coarser than the typical calcilutite of the sills and, in the established value for the Junction Bed of 6180= -3.75Oj; more horizontally orientedportions of the dykes, fibrous 6I3C =2.86% (Campos & Hallam 1979) andother and equant sparry calcite is patchily developed. unpublishedanalyses. Fibrousand equant spar, typically Some fissures containfauna; many are completely ferroan, fill theupper levels of some fissures. One barren. The vertically oriented dykes have yielded particularly well-developed sample of fibrous calcite from a gastropods,brachiopods, crinoid and echinoid fragments, sample adjacent to that illustrated in Fig. 7, when analysed thin-shelledbivalves (Bositra buchi) andforaminifera. In isotopically, gave 6"O = -5.5%; 6I3C = -8.8%, suggesting addition tothe abovesome sedimentary sills contain precipitation from waters of modified meteoric origin or medium-sized nautiloids, but their most abundant faunas are fluids perhaps derived from the underlying Lower Jurassic ammonitesand their fragments whichgenerally occur in

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/2/245/4891687/gsjgs.148.2.0245.pdf by guest on 29 September 2021 DORSET DORSET ZONES SUBZONES ZONES SUBZONES COAST COAST I 1 1

P. hawskerense MARLSTONE spinaturn P. apyrenum I I P. anis BEDASTARTE I I

oistoceras /igulinum GREEN Prcdactytioceras AMMONITE davoei Aegoceras BEDS

BELEMNITE STONE Beaniceras luridurn ~ ------Tragophybceras RED

TIOpidoceras CONGLOMERATE masseanum U.iamesoni BELEMNITE Lpl Lpl MARLS Upronia jamesoni RED BEDS

rarimstatum

Astemceras

BLACKVEN L. haugi IIIW

Gsu natum ~ni-ras Agassi&s BEEF sem'wstatum scipionianum

Anetites bucklad rotiforme Vermiceras conybeari

Fig. 2. Stratigraphic scheme for the Hettangian-Bajocian interval as represented by the sediments of the Dorset Coast, based on Cope et al. (1980u,6), Howarth (1980) and Ivimey-Cook & Donovan (1983). Boundary between the Blue Lias and Shales withBeef after Hallam (1960). The stratigraphic context of zones not given in this figure can be found in Cope et al. (19806).

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limestones indeterminate burrows orientedclusters and, if partially filledwith sparite, constitutegeopetal structures. The following forms have limestones with Thalassinoides beenidentified in this study: Grammoceras sp. (most quartz grains burrows common), Hammatocerasinsigne (tolerably common), Pseudogrammoceras subquadratum? and Haugia sp. indica- ea@, a echinoderm tive of Toarcian,the variabilis and thouarsense Zones.

clay-rich Q debris limestones Additionally, HildocerasHarpoceras sp., and I CL);),; a, undifferentiated Dactylioceras sp. havebeen observed, indicating the bijrons Gyfl;!R.b bioclastic material sands and falcijerum Zones.Jackson (1922,1926) alsorecorded belemnites what appear from his descriptions to be fissures containing: Alocolytoceras cf. germani,Elegantuliceras elegantum, clays/marls $3 ammonites Frechiella cf. subcarinata hildoceratidsand Thesesp.are similarly indicative of the Toarcian, bifrons and falciferum limestone planed-off large pebbles ammonites, Zonesand, in casethe of the Alocolytoceras, probably - locally inverted thouarsense to levesquei Zones. laminated Close to the Eypemouth Fault itself the matrix of the fissure-fills fissures becomesmore calcareous towards the top of the concretionary calcareous Marlstone/Junction Bed complex. A prominent hardground levels or rubbly in thesehigher, more calcareous levelsis marked by an limestones horizon of centimetre-scalecalcareous limonitic nodules Fe-P ooliths:berthierine/ (Fig. 4) similar to but smaller than the ‘snuff-boxes’ of the goethitelphosphate Bajocian Inferior Oolite (cf. Gatrall et al. 1972). Abraded ammonites found during this studyin the basal, goethitic/berthierine/phosphate concretlons of cm scale quartzarenite-rich part of this normal stratigraphic unit are Protogrammoceras paltum (rare)and Amaltheusgibbosus laminated cyanobacterial domes (rare).The firstis indicative of theToarcian, lower tenuicostatum Zone, the second of the Pliensbachian, upper Fig. 3. Codes used in stratigraphical sections, illustrated in Figs 4, margaritatus Zone and is interpreted as derivedfrom the 15 and 16. Thorncombiensis Bed. The matrix is not typical Marlstone

1 2 3 4 5 6 7 8 9

Bf-fissure

.1 km Fig. 4. Stratigraphy, calibrated to thezonal level, of the Marlstone, Junction Bed and sub- and suprajacent strata and their relationship with the Eypemouth Fault, Wessex Basin. Sections are based on personal observations with additional data from Buckman (1922) and Jackson (1922, 1926). Ma, margaritatus Zone; Sp, spinatum Zone; Tn,tenuicostatum Zone; Fl, falciferum Zone; Bf, bijrons Zone; Vr, variabilis Zone; Th, thouarsense Zone; Lv, levesquei Zone. Sections run from Ridge Cliff (l),through Doghouse Cliff (2), Thorncombe Beacon (3, in situ; 4, ex situ), east of Eypemouth (5) to Watton Cliff (6-9).

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Marlstone-Junction Bedfissure

I Fig. 7. Detail of Toarcian neptuniansill and matrix, showing Fig. 5. Sketch, from beach level, to illustrate the geometry of the grading of ammonites and their fragments (chieflyGrarnrnocerus), Marlstone-Junction Bed complex when traced eastward towards multiple filling with buff(A and B) cappedby white (C) micrite and the Eypemouth Faultat Watton Cliff the thicknessof the limestone subsequent fillingof fibrous calcite. Note that the graded unit immediately adjacent to the fault3.65 is m. Stratigraphically ammonite-bearing fill (A) is divided by a faunally sterile layer (B), below and above are the Pliensbachian Thorncombe Sands and which may represent a subsequent intrusion. Fallen block, Watton Toarcian Downcliff Clay. The Thorncombiensis Bed, which Cliff. elsewhere underlies the Marlstone, is not recognizableas a distinct unit in this locality. On the downthrown side of the normal faultthe Bathonian Fuller’sEarth and Forest Marble outcrop. thouarsense-Zone age are concentrated in the upper, more calcareous matrixof the Marlstone/Junction Bed complex of falciferum- and bifrons-Zone age, although they also occur, andseems to derivefrom mixing of that level (spinatum albeit rarely, in the more quartzarenitic portion below. The Zone, Upper Pliensbachian and tenuicostatum Zone, Lower data of Jackson (1926) also suggest the presence of some Toarcian:Howarth 1980) and the underlying Thomcomb- fissures of falciferum and bifrons age at this lower level. iensis Bed(margaritatus Zone); there is notrace of. the The maximum thickness of the Marlstone/Junction Bed intervening mar1 as seen at Thomcombe Beacon (Fig. 4). In complex of 3.65 m is developed immediately adjacent to the the upper, calcareous part of the normal stratigraphic unit Eypemouth Fault; it thins westwards (Figs 4 & 5). At the the following formshave been found: Hildoceras bifrons, feather edgeof this dramatically thinning unit no fissures are Harpoceras sp. and Dactylioceras sp.indicative of the apparent,although the outcrop is difficult of access and Toarcian, bifrons and falciferum Zones. detailed study hazardous. Our initial interpretation (Jenkyns The fact that in the most easterly section adjacent to the & Senior 1977) was of a series of syn-sedimentary faults that EypemouthFault, where theMarlstone/Junction Bed caused the abrupt changes in thickness. The new exposures complex is thickest, ammonites pertaining to the falciferum rather suggest that differential intrusion of the fissure facies, and bifrons Zone occur in fissures some 3 m below their in with maximum density in the fault zone, was the proximate situ stratigraphic level mustaccount forthe difficulties cause. experienced by Buckman (1910) and Jackson (1926) in The following sequential events inthe exposed fault zone interpreting the succession (Howarth, in Cope et al. 1980~). are envisaged. Detailed collectingsuggests that fissures with a fauna of

Fig. 8. Microfacies of Toarcian neptunian sill, showing variably Fig. 6. Toarcian neptunian sillof pale-pink micrite capped by packed peloids and their relationship to sedimentary lamination. matrix of coarser-grained calcareous-ferruginous quartzarenite. Note the geopetal structurein the ammonite, which aids orientation Diameter of coin is 2.5 cm. Fallen block, Watton Cliff. of ex situ samples. Fallen block, Watton Cliff.

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(1) Deposition of a ferruginous calcareous quartzarenite (Marlstone) during ?spinaturn Zone to early tenuicostatum Zone times. Much erosional reworking and incorporation of older fauna1 elements(derived from theThorncombiensis Bed of theupper margaritatus Zone); modestsubmarine lithification under conditions of minimal net sedimentation. (2) Deposition and precipitation of lime mud locally rich in ammonites,thin-shelled bivalves, gastropods,brachiopods, belemnites, echinoids and foraminifera (Junction Bed sensu stricto), similarly in anenvironment of veryslow sedimentaryrate. Development of non-sequencesmarked by formation of concentrically laminatedcalcareous limonitic nodules. (3) Variousphases of submarinefaulting leading to multiple intrusion of the Junction Bed,Marlstone and uppermost Thorncombiensis Bed by lime-mud variably rich in ammonites and gastropods. Injection of rapidly flowing sediment-charged liquids by vacuum suctioninto partially lithified sediment issuggested by grading, cross- and parallel lamination of the peloidal lime mud, the hydraulic sorting of fossils and the local gradational contacts between matrix and fill. Thatthese fissures ever gaped partially open on the sea-floor with the cavitiesinhabited by ammonites (cf. Wendt 1971) seems unlikely. However, small voids filled with fibrous and equant calcite suggest that at least some cavities were incompletely filled, unless theserepresent a dewatering phenomenon; and the uppermost levels of some fissures contain sediment that may have entered by passive filtration (e.g. layer C in Fig. 7). Demonstrable times of intrusion, assuming that the ammonites accurately represent this, were: falciferurn, bifrons, variabilisand thouarsense Zones, spanning an estimated tim of 4 Ma (e.g. Harland et al. 1982). (4) A further series of undated but pre-levesquei Zone movementsresulting in intrusion of vertically oriented neptunian dykes cutting matrix and some fissures (Jenkyns & Senior 1977). Presumed deepening of water,due to Fig. 9. Microfacies of Toarcian neptuniansill, showing graded regional tectonic and/oreustatic effects,leading to peloids and problematic radially structuredspherulites. Loose deposition of clay (Downcliff Clay) in levesquei-Subzone block, Bothenhampton. time (Sellwood & Jenkyns 1975).

Theseammonites indicate the levesquei Zone. Eypemouth Fault at Long Lane, Bothenhampton Alocolytoceras was alsorecorded from Watton Cliff in a Laminated pink and white fissure facies may be found in situ ‘finely laminated lithographic limestone’ by Jackson (1922). and abundantly in field rubble in this area, next to Long Lane (SY 478916 and 483916), where the Eypemouth Fault (Fig. 4) locally juxtaposes JunctionBed against Forest Bride Fault at Burton Beach Marble,Cornbrash and Oxford Clay. Referenceto the On the coast below the houses formerly known as Burton occurrence atBothenhampton of facies similar tothe Villas, atthe eastend of Burton Cliff (SY 489887), the ‘Watton Bed’ was made by Buckman (1922). The BrideFault (Fig. 1) reaches the coast,juxtaposing the fissure-facies are typically peloidal, with peloidspartly Bridport Sands against Fuller’s Earth and Forest Marble: a arranged in graded laminae, and locally contain problematic sliver of InferiorOolite (dated as subfurcatum Zone, radial spherulites in both micrite and sparite matrices (Fig. baculata Subzone) occurs in the fault zone where rare loose 9); other facies includemicrite with sparseechinoderm blocks are in anomalous facies containing bored boulders of fragments, foraminifera, phosphatic fish remains, shell hash, limestone andlaminated ferruginous concretions (snuff- silt-grade quartz and possible siderite-replacedberthierine boxes) set in a highly fossiliferousferruginous limestone ooids. matrix. Here the Bridport Sands, with their characteristic A sectiondescribed by Walker (1892) fromBothen- carbonate-cementedrhythms, contain bedding-subparallel hampton mentions a ‘white stone’with a lower thouursense- fissures full of laminatedpeloidal white calcilutites Zone faunawhich may be interpreted as fissurea facies. The containing variable quantities of angular quartz grains (Fig. abundance of such deposits in this area is here taken as 10). Large blocks displaying the geometry of the fissures, evidence for Toarcian movement along this sector of the both horizontal and vertical (neptunian dykes),are fault. Ammonites collected from ex-situ fissure facies in this occasionally uncovered on the beach along the line of the locality are Pleydellia sp.and Almolytoceras coarctaturn. fault;geopetal fills of calcite sparare a feature of the

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(Fig. 11). The uppermost levels of the cavity fills commonly contain fibrous calcite cements andfor equant spar. These fissure facies are, in terms of theirgeometry and petrography, identical with those exposed at Burton Beach and indicateinjection of Inferior Oolite sediment into Bridport Sands. Within these fissure facies the following fauna has been found: Parkinsonia sp.,probably P. acris, Astarte (Neocrassina)modiolaris, Eopecten sp. andindeterminate gastropods. The ammoniteindicates Bajocian,the garantiana Zone, acris Subzone (cf. Parsons in Cope et al. 1980b). The age of these fissuresis thussimilar to and possibly overlaps with those atBurton Beach. The importantpoint to note is thatepisodes of sediment intrusion along theEypemouth Fault took place in both Fig. 10. Sub-horizontal Bajocian limestone sills, penetrating Toarcian and Bajocian time. Interestingly, theacris Subzone Toarcian Bridport Sands. East end of Burton Cliff. Hammeris marks the time of the ‘Vesulian Transgression’ (Arkell 1933, 33 cm long. p.91, 230-1, 333-6) in the Wessex and Cotswold areas where it post-dates a substantial unconformity. sediment-filled cavities in theseblocks. The lamination of the fills is imparted by differing densities of micrite, peloids and microsparite; the peloids are graded in some laminae; Mere Fault at Cadbury Castle glauconitegrains, carbonate-replaced sponge spicules, The Mere Fault is a major east-west lineament that runs thin-shelled bivalves, bryozoanfragments and phosphatic close to the northern margin of the Wessex Basin (Fig. 1). intraclasts are not uncommon. Echinoderm debris is locally In certain sectors it can be mapped with some confidence abundant.These fissure-fills weredescribed by Buckman andthe downthrow varies from north to south along its (1910,1922) andRichardson (1928) who variously length (e.g. Mottram 1961; Chadwick & Kirby 1982). consideredthem as parts of thenormal sedimentary Towards its westernextremity, where the downthrow is sequence in exotic lithology or asasecondary deposit southerly,it becomes lesswell definedand a number of caused by downward percolation of water. Buckman splays are developed, one of which crosses Cadbury Castle referred to them as the ‘White Bed’ or ‘Nautilus Bed’, and and throws Toarcian Yeovil Sands against Inferior Oolite. recognized their similarity to the ‘Watton Bed’ adjacent to In this area(ST 626252),fissure facies occur locally at the Eypemouth Fault. The Geological Survey (Wilson et al. outcropintruding the upper levels of theToarcian sands 1958) came close to recognizing them as neptunian sills, the close to their contact with the overlying Inferior Oolite and interpretation favoured here. In the courseof this study, the abundantly infield rubble on the top of the hill fort and following fauna has been found in the fissures: Garantiana around the wooded slope to the west. The fissure facies are quenstedti,Garantiana sp.,brachiopods and echinoids developed as buff to paleorange laminated lithographic (?Hemicidaris). In addition Buckman (1910, 1922) and the limestonesand are found singleas blocks and as Survey recordednautiloids, bivalves, gastropodsand a finger-shaped pods of material in asandstone matrix; perisphinctid, as well as a Garantiana. This fauna is Upper spar-filled geopetalcavities are common. Petrographically Bajocian, garantiana Zone, probably tetragona Subzone. the laminated calcilutites are identical with those exposed at The matrix of the fissures (Upper Bridport Sands) contains Burton Beach. Pleydelliacomata andis thus of Toarcianage, levesquei Within these fissure-facies the following fauna has been Zone, aalensis Subzone. Thepenetration of Toarcian by found: Cymatorhynchia, nautiloidsand bivalves. Bajociansediments is interpreted as relatedto tectonic Cymatorhynchia typically occurs in mid-Jurassic rocks; and movements along theBride Fault during garantiana-Zone times. This is another example of a palaeofault active during the Jurassic which, like the Eypemouth Fault, down-throws to the south. The boulder-bed faciesof the Inferior Oolitein the fault zone at Burton Beach is presumably also related to syn-sedimentary movement during the Bajocian.

Eypemouth Fault at Markets Lane, Shipton Gorge From Bothenhampton the Eypemouth Fault runs eastwards through Shipton Gorge where it locally juxtaposes Bridport Sandsagainst Oxford Clay. In Markets Lane, Shipton Gorge (SY 501913) a trench dug for a water meter in the early 1980s encountereda series of limestonesassociated with theBridport Sands. These are now presentas field rubbleand are readilyrecognizable as fissure facies:they Fig. 11. Finger-shaped fissure-faciesof Bajocian age (light) in comprise finely laminatedlithographic cream limestones, Toarcian Bridport Sands (dark). Photocopy of polished loose block, partlyas single blocks, whose originaldimension is Shipton Gorge. Identical structures maybe found in boulders unknown,and partly as centimetre-scalepod-shaped occasionally visible, after major storms,on the foreshore at the east fissure-fillsin acalcareous micaceous quartzarenite matrix end of Burton Cliff.

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the relative abundance of nautiloids recalls once more the Moore (1867) from nearby localities and attributed to a level fissure facies atBurton Beach which werenamed the ‘about that of the passage of the Lower into the Middle ‘Nautihs Bed‘ by Buckman (1910, 1922). There seems little Lias’, which suggests he thought they were somewhat older doubt therefore that the Cadbury examples are equally of than spinatum Zone.Moore furthermore compared these Bajocianage. Penetration of Bajocian lime mud into pink crinoidal limestones with the Hierlatzkalk of Austria Toarcian sands is again indicated and points to movement and suggested similarages for both deposits. Although along the MereFault during deposition of theInferior typically of early Liassic age the Hierlatzkalk is occasionally Oolite, something for which there is independent evidence dated as mid-Liassic and rarely even late Liassic (Tollmann in terms of thickness changes of the formation in question 1976); a similar age range is by no means excluded for the (see below). Mendip examples, particularly as Copp in Duff et al. (1985) suggests that some crinoidal neptunian dykes may contain faunasas old asthe jamesoni and raricostatum Zones. Evidence from neptunian dykes: Mendip fissures Differences in facies are recognizable tothe extent that TheCarboniferous Limestone of theMendips is cut by some of theselimestones are richin belemniteswhereas numerous Mesozoic sediment-filled fissuresof varying trend, others,apparently, are not. The palaeogeographic sig- whose ages range from Triassic to Middle Jurassic (Moore nificance of Alpine-Mediterraneancrinoidal limestones is 1867; Kiihne 1946; Robinson 1957; Copp in Duff et al. such that they typically developed as submarine calcareous 1985). Observations made in a cutting formerly exposed on dunes in turbulent-waterenvironments floored byslowly the main roadimmediately tothe west of Leighton (ST founderingplatform carbonates (Jenkyns 1971), and a 703438) haverevealed the presence, within theCar- similar model would seem to be applicable to the Mendips. boniferousLimestone, of numerousneptunian dykes and Many of Moore’s (1867) pioneer observations were made sills of pink crinoidal limestones,locally discoloured to grey, in thequarry at Holwell andobservations have been runningclose andto roughly parallel with the concentrated in thisregion. Numerous neptunian dykes Cranmore/LeightonFaults (Fig. 1). In this area pink and containingcrinoidal limestone are exposed here and in grey crinoidal limestones are locally present in stratigraphic Coleman’s Quarry (ST 727453) whereadyke of pink sequencebetween the Carboniferous Limestone and the crinoidallimestone has yielded onejuvenile specimen of Inferior Oolite, and are themselves cut by neptunian dykes Cirpa sp. Thisform is typical of thePliensbachian. The and bed-parallel fissures containing fine-grained laminated matrix richis in echinodermfragments, locallywith carbonates.Oyster-encrusted planar hardground surfaces syntaxial overgrowths, accompanied by comminuted occur on boththe crinoidal limestones and on the brachiopod material, quartz grains and siltstone fragments. CarboniferousLimestone where the succession islocally An adjacent dyke has yielded, in situ, numerous specimens morecomplete (Fig. 12). The following faunahas been of thebrachiopod Acanthothyrisspinosa. Thisform is extractedfrom pink crinoidallimestones in dykes that characteristic of the Upper Bajocian,garantiana Zone, acris penetratethe Carboniferous Limestone: Quadratirhynchia Subzone in the Cotswolds (Arkell 1933, p. 243). The matrix sp., Prionorhynchia sp.and Cirpa sp.(juveniles). This ischiefly sparite with somemicrospar and micrite richin fauna is indicative of the Pliensbachian, possibly spinatum macerated brachiopod material and echinoderm fragments Zone(Ager 1956-67). We assume thisequally represents largely obliterated by diagenetic overgrowths. the age of the pink crinoidal limestones, which also contain These age assignments match those recorded by earlier Cirpa, locally present in the normal stratigraphic sequence workers but are somewhat more limited. Fauna collected by betweenthe Carboniferous Limestone and the Inferior Kuhne (1946) from fissure-fills at Holwell included material Oolite:they would thusequate with thecrinoid-rich of Hettangian,Sinemurian, Pliensbachian and Bajocian Marlstone of the Wessex Basin area.These local ages, although some of the material may have been derived intercalations of pink crinoidal limestone were recorded by andnot represent a true intrusion date. The microfaunal

Fig. 12. Sketch of the outcrop formerly (1977) visible on the main road to the west of Leighton. Note the hardgrounds between the irregular upper surface of the Carboniferous Limestone and the overlying Pliensbachian andBajocian sediments. Note also the numerous neptunian dykes of Pliensbachian pink crinoidal limestone that penetrate the older rocks. Crinoidal limestone in the stratigraphic succession is 0-75 cm thick; Inferior Oolite seen to 1 m.

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data of Copestake (1982) confirm an early Sinemurian age environments in this region, consistent with fault control on (bucklandi-semicostatum Zones) for siltstone in fissures at submarine topography. That the Toarcian sands would have Holwell and Cloford (ST 717445). Copp in Duff et al. (1985) provided a tolerably competent substratum is suggested by recordsmaterial of earlyBathonian age from fissures in the local presence of Bajocian fissure-fills and geopetal Holwell, butthis material may represent passive fill of a cavities therein;early lithification of thecarbonate- pre-existingcavity rather than an intrusion formed during cemented layers in the sands is indicated by their containing syn-sedimentary fracturing. Nonetheless sedimentary dykes, uncrushedmica flakes and inter-particle carbonate cement typically full of largeamounts of calcite andpods of fringes (Davies 1967; Kantorowicz et al. 1987). subhorizontally laminatedsediment, do locally cutthe The Ham Hill Stone passes eastand west intocoarse Inferior Oolite, and these are associated with offsets of the well-sorted quartzose sands, but northwards and southwards unconformity surface on the Carboniferous Limestone, into into finer grained,more bioturbated clastics. Similar which the fissures typically penetrate: small-scale Bathonian intercalations of bioclastic carbonates in the Bridport/Yeovil movement is thus possible. sands are known fromthe Winterborne Kingston and A systematic study of the directionality of the various Marchwood boreholes (Knox et al. 1982; Bryant et al. 1988), dykes has not been undertaken but the dominant trend for but their relationship, if any, with syn-sedimentary faults is thosedated as Jurassicis essentially east-west. It seems obscure. likely that injection of these sedimentary dykes was related The Inferior Oolite, of Aalenian-Bajocian age, is locally to movement along the Cranmore-Leighton Faults during characterized by the presence of large (tens of centimetres) Early to mid-Jurassic time.These faults aremapped as laminated limonitic-calcareous concretions known as down-throwing tothe south. The observation of Moore ‘snuff-boxes’ (Gatrall et al. 1972; Palmer & Wilson, 1990). (1867) that at Holwell onefourth ‘of thissupposed The best developed of these are of discites-Zone age and Carboniferous-limestone quarry’ is of Liassic age, although their distribution, relative to coeval facies, is such that they probably an overestimate judging by present-day exposures, were developed in a north-south belt, limited to the west by nevertheless indicates substantial Jurassic extension in this the Mangerton Fault and to the north by the Hooke Fault region. (Fig. 1). Without knowing the original sense of movement Other examples show that these extensional phenomena of the Mangerton Fault itis difficult to assign the snuff-boxes are by nomeans limited to the Holwell area but are, as to a particular part of any notional tilted block. However, argued by Moore, of regional extent. At Gurney Slade (ST somesort of initial faultcontrol seems likely with the 623500), forexample, a largesedimentary dyke, oriented snuff-boxes developingin highly turbulentwater on a east-west, is exposed in positive relief as the surrounding particularly shallow sector of the sea floor with water depths CarboniferousLimestone has been quarried away: it perhapsdeepening gently towards theeast and more contains both Triassic and Liassic material with brecciated abruptly towards the west. This would imply post-Bajocian clasts derivedfrom theCarboniferous, including both inversion, as the present downthrow of the Mangerton Fault limestoneand chert (Moore 1867; Robinson 1957). The is to the east. Lower Jurassicsediment is a grey to cream crinoidal- The Abbotsbury Fault (Fig. 1) is dated as pre-Albian as brachiopod biomicrite with numerous quartz grains that has it runs beneath the Upper Greensand (Arkell 1947; Wilson yieldeda fauna including Calcirhynchiacalcaria, bivalves et al. 1958). In Abbotsbury Village, to the southof the fault, andgastropods whichsuggest a lateHettangian age. the Abbotsbury Ironstone of Kimmeridgian age (cymodoce Interestingly, this dyke is not obviously spatially linked to a Zone) crops out. This ferruginous oolitic facies passes into majorfault. Green & Welch(1965) record a number of clays (Kimmeridge Clay) about 1 km east of the village fissures, from the Central Mendip area, generally dated as (Brookfield1973). Asimilar facies, of similar age, is early Pliensbachian (Donovan & Kellaway 1984). exposed inLitton Cheney (Cope 1971) south of a fault (Litton Cheney Fault) that could be a continuation of the Bride Fault (Fig. 1). The local presence of these ironstones Abrupt lateral facies changes in Jurassic strata is interpretedlinkedas to short-lived fault-induced To the west and WSW of Yeovil there is a local facies submarine topography and/or realms of reduced differential equivalent to part of the Yeovil Sands known as the Ham subsidence (Hallam 1966, 1975; Sellwood & Jenkyns 1975), Hill Stone: this isa grey trough cross-bedded bivalve-rich with the berthierine ooids generated on the upthrown side limestone indicating transport from the southwest and west of the fault-block andthen dispersed southward to (Cope et al. 1969). It is dated as latest Toarcian, levesquei accumulate in a perched basin (Fig. 13). The Abbotsbury Zone, probably moorei Subzone (Wilson et al. 1958; Fault down-throws tothe south, whereas the nearby Howarth in Cope et al. 1980~). Themajor outcrops, some Ridgeway Fault, to which it may be genetically related, 30 m thick or less, occur to the north of the Coker fault (Fig. down-throws to the north. TheRidgeway Fault is thought to l), an east-west structure, downthrowing to the south, in have had a southerly down-throw during its Jurassic history, which direction theHam Hill Stonethins toabout 6m but suffered tectonic inversion duringLate Cretaceous- before disappearing. The development of this facies implies Early Tertiary time (Stoneley 1982; Lake & Karner 1987). If depositionin areas shielded from clastics, presumablyan the model in Fig. 13 is correct,the local presence of area of relativetopographic high, where carbonate base-Kimmeridgian ooliticironstones would be predicted sand-bodies could accumulate. It is tempting to relate this to immediately to the southof the Ridgeway Fault. fault-induced submarine topography, an interpretation very The Westbury Ironstone, part of the Corallian Group of different to that of Davies (1969) who viewed it as a tidally lateOxfordian age (pseudocordata Zone), isalso of very influenced channel incised through barrier deposits. How- localized occurrence (Talbot 1974). In the type locality, in ever, Davies’ facies mapfor the moorei Subzone clearly northwest Wiltshire, it crops out just south of a southerly shows an east-west orientation of clastic sedimentary downthrowing fault, the Heywood Fault (Fig. 1). Facies and

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faulted boundary of a half-graben, possibly now represented by theBeacon Hill Fault (Fig. l), for which there is evidence of Early Jurassic mobility. In the Beacon Hill area, Downside Stone facies was depositedas late as early Pliensbachian, jamesoni-Zone time (Green & Welch 1965). Lateral facies changes of the typical BathonianGreat Oolite to the Frome Clay of the Mendip region (Penn & Wyatt 1979) may possibly also be related to fault-controlled palaeobathymetry.

Thickness changes in Jurassic strata Much information on general thickness variations of Jurassic stratain the WessexBasin is nowavailable fromthe isopachytes of Whittaker (1985) and Sellwood et al. (1986). Here the focus is on a selection of stratigraphic units where thicknesses of coeval strataeither side of faultscan be determined, namely Hettangian-Sinemurian, the Pliensbachian-Toarcian, Bajocian and Kimmeridgian stages andrelevant data, derivedfrom seismic andborehole v KIMMERIDGIAN sources, are displayed in Fig. 14. Asection from Bruton to Kimmeridge is instructive; Site of deposition of Fe-ioids north of the Mere fault the Blue Lias, Shales with Beef and of calcareous Site of oeneration mud8ailt61ones and \ / of Fe-ooids Black Ven Mark attain a humble 45.8m; at Winterborne Kingston, onthe downthrownside of a major fault bounding the Winterborne Kingston Trough, they achieve some 240 m. They thin southward and are represented by a h?Fe-encruated mere 61m at Wytch Farm onthe upthrownside of a sandstones northerly tilted block (Colter & Havard 1981); tothe southwest, on the downthrown side, these strata are locally estimated as (in the Kimmeridge 5 borehole) some 641 m thick. This represents a cumulative thickness increase across Perched basin more than one plane of displacement including a probable continuation of the Abbotsbury-Ridgeway Fault, and may also involve some tectonic replication of strata. Nonetheless thesethickness data clearly bring outthegeneral half-graben geometry of the various sub-basins in this area (e.g. Chadwick 1986; Karner et al. 1987). In the Pliensbachian-Toarcian the picture is a trifle more Fig. W. Map of faults in part of the Wessex Basin and their enigmatic (Fig. 14). The JunctionBed and Marlstone relationship with thickness and facies in the Kimmeridgian. (Pliensbachian,spinaturn Zone andthe bulk of the Thickness patterns are consistent with half-graben geometry. Toarcian) are typically only a few metres thick, except at the Cartoon shows possiblesites of generation and deposition of northernboundary of theWinterborne Kingston Trough. berthierine ooids (Abbotsbury Ironstone) in relation to temporary FromWinterborne Kingstonitself theseunits thin fault-induced submarine topography and is consistent with the southward,although the relatively expandedsection at known stratigraphy of the area. Various fault geometries producing Wytch Farm, particularlywhen compared with the a perched basin are possible. The separation of the locus of succession in Kimmeridge 5, is curious,as it is the exact generation from the locus of deposition of ferruginous ooids is opposite of the thickness patternsfor the Hettangian- implicit in the models of Cayeux (1922) and Knox (1970). Dispersal of small quantities of ferruginous ooids away from the immediate Sinemurian: this may be moreapparent than real and vicinity of the faults may help explain minor occurrences of these relatedtolateral facies changes into less calcareous sediments elsewhere in the Upper Jurassic of the Wessex Basin. sediments. Abrupt southward thickening across a small fault Thickness data for the Kimmeridge Clay from Arkell (1947), (the Manor House Fault) is, however, well displayed by two Wilson et al. (1958) and Cox & Gallois (1981). boreholes close to LangtonHerring (Fig. 14). Herethe Junction Bed and Marlstone change in thickness from 2.7 to 9.7 m; the overlying Downcliff Clay (levesquei Zone) shows thickness changes affecting Corallian strata (Oxfordian) in a similar but less dramatic relative change: from 50 to 71 m. the WessexBasin have also been related to fault-induced TheBridport Sands (Toarcian-Aalenian boundary) are topography by de Wet (1987). essentially of constant thickness(66-67m) in both Inthe Mendip area, Liassic sediments are locally boreholes. developedas the DownsideStone, a cream-coloured Thickness changesreturn to a morepredictable bioclastic limestone, chiefly Hettangian-Sinemurian in age half-graben pattern with the Inferior Oolite (Fig. 14). From and stratigraphically equivalent tothe Blue Lias of the Winterborne Kingston throughBere Regis, Wareham, Wessex Basin. Cornford (1986) has linked the nature and Stoborough to Wytch Farm there is a systematic thinning, distribution of this so-called 'littoral' facies to the northern followedby a jumpin thickness at Kimmeridge asthe

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BLACK VEN MARLS JUNCTIONBED SHALES WITH BEEF MARLSTONE V BLUE LlAS V

V INFERIOR OOLITE 0 10 ?Ohm F%. 14. Thickness data for the Blue Lias, Shales with Beef and Black Ven Marls and their equivalents (Hettangian-Sinemurian); Marlstone-Junction Bed (top Pliensbachian-Toarcian); and Inferior Oolite (Aalenian-Bajocian) and location of fissure faciesplotted on a fault-block mosaic. Data from Edwards & Pringle (1926), Richardson (1928), Arkell (1933), Gatrall et al. (1972), Penn et al. (1980), Colter & Havard (1981), Ivimey-Cook (1982), Holloway& Chadwick (1984),Whittaker et al. (1985), Sellwoodet al. (1986) and borehole records. Attribution of specific formations to a characteristic log signature may present difficulties where changes in facies occur, particularly in areas north of the Dorset coast; thicknesses may thus be approximate in some areas. Thus the demonstrable difference in facies of the Junction Bed (limestone) in the Dorset Coast as compared with the stratigraphically equivalent interval in the Winterborne Kingston borehole (silty mudstones), may account for the apparent anomalously thin representation of this unit in some boreholes. The considerable thickness of Hettangian-Sinemurian in Kimmeridge 5 may in part be related to fault repetition in the Blue Lias, as interpreted from log records. It should be noted that in the isopachytes of Gatrall et al. (1972), reproduced in Penn et al. (1980) and Penn (1982), the thickness of Inferior Oolite in the Wincanton Borehole would appear to be incorrect. The dips approach the vertical at depths close to the plane of the Mere Fault making estimates difficult, but 65-75 m would seem to be a realistic figure.

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Purbeck-Isle-of-WightDisturbance is crossed.Sections study. In the Frome region Penn & Wyatt (1979) record a from Langton Hemng (North and South) and Radipole to reduction in thickness of theUpper Fuller’s Earth when SeabarnFarm show the same trend, as does the transect traced southwards towards the Mendips, whereas the Lower from Sandhills to Arreton on the Isle of Wight. A section Fuller’s Earth and Fuller’s Earth Rock show little variation. from Bruton to Wincanton across the Mere Fault shows an Thesethickness patterns are related by theauthors to approximate tenfold thickening (Edwards & Pringle 1926; tilted-block geometry and interpreted as indicating limited Holloway & Chadwick1984). The overlyingBathonian Bathonian tectonic activity. Fuller’s Earth Claydoes not exhibit such marked Impressivethickness differences areapparent in the differences: 30.6m at Brutonversus 36.6m or less at KimmeridgeClay (Fig. 13). For example in the Ringstead Wincanton. It shouldalso be noted that fault-controlled Bay area of Dorset the formation (c. 180 m) is considerably thicknesschanges in Bathonian Fuller’s Earth at Herbury reduced in thickness relative tothe section atthe type (SY 612811: Lake 1986) couldnot be confirmed in this locality (c. 500m: Arkell 1947; Cope et al. 19806; Cox &

m -< -U m K 0 C --I I n 9 C +l-

Fig. 15.Section, calibrated to the zonal I.... I...... I level, through Aalenian and Bajocian sediments (Inferior Oolite) to the north of the Eypemouth Fault, Wessex Basin. Sediments of the various zones show no systematic thicknesschange, although there is an overall thinningtowards the fault. Bajocian intruding Toarcian (Lv) sediment is a feature of the fault zone 1 itself. Stratigraphic data from Senior et al. (1970), Parsons (1975) and personal observations. A, aalensis Subzone (Toarcian); 0,opalinum Subzone; Sc, scissum Subzone; D, discites Zone; La, .‘ laeviuscula Zone; Sa,sauzei Zone; H, 0 humphriesianum Zone; G, garantiana Zone; T, truellei Subzone; B, bomfordi (Subzone (Bajocian); Z, zigzag Zone; P, progracilis Zone (Bathonian). Note the l thickness changesof strata between beds BRIOPORT // l of the scissum Subzone andthe i \ ---I--’ l garuntiana Zone, suggesting intra- I \ Bajocian non-deposition and/or erosion. Sections run fromStonyhead Cutting (1) through Bonscombe Hill(2) to Peas Hill Quarry, Shipton Gorge (3) along Markets Lane to the Eypemouth Fault (4,5).

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Gallois 1981). The two sections, although some 15 km apart, Unfortunately,there is apaucity of dataon thickness are separated by the Purbeck-Isle of WightDisturbance. changes in uppermost Jurassic strata in many other areas of Similarthickness changes are also recognizable inthe the Wessex Basin as intra-Cretaceous erosion has cut deep Portlandian, reinforcing the evidence for the overall Jurassic into the succession. mobility of this fault which formed the northern margin of Ina furtherattempt to document the significance of the CentralChannel Basin(Holloway 1985; Selley & Jurassic tectonism and its effects on sediment geometry on a Stoneley 1987). At DurlstonHead near Swanage (SZ smaller scale, a number of Lower-mid-Jurassic sections, 036773), Lake (1986) has interpreted changes in sedimen- calibrated to the zonal level, have been plotted across some tary thickness across a faultin terms of intra-Portlandian of the faults in question. In Fig. 15 a series of sections from movement, although admittedly other interpretations of the top Toarcian to basal Bathonianillustrate fine-scale outcrop are possible, involving solution of evaporites and variation in a north-south direction across a block bordered post-Jurassic tectonics (e.g. Arkell 1947; West 1960). by the Mangerton and Eypemouth Faults, terminating with , the Bajocian fissure-facies of ShiptonGorge. The general I 1 km southward thinning is again notable, although there is by no means a consistent pattern. In Fig. 16 three sections close to / ; I: theCranmore Fault, Mendips are illustrated, and the l’l I,’ l control on preservation of Lower Jurassic sediment is clear: noToarcian is preservednorth of thefault, suggesting emergence or submarine erosion at this time. South of the fault the Pliensbachian-Toarcian section thins in a relatively consistent manner, in agreement with the notion of tilted-block geometry.

Discussion and conclusions 2 0 Nepiunian dykes and CZ sills rh -10 Apartfrom the examples documented above, syn- n m sedimentary fissures are apparently uncommon in the British 3 Jurassic. Sandstonesdykes and sills havebeen described fromthe KimmeridgeClay inEast Sutherland, Scotland, associatedwith the Brora-HelmsdaleFault and, similarly, from close to the Great Glen Fault (Bailey & Weir 1932). Intrusion of the sands wasclearly relatedto submarine faulting in Kimmeridgian time. Recently Alexander (1987) described sandstonedykes inthe Bajocian-Bathonian non-marineScalby Formation of Yorkshire,and more specifically Martill & Hudson (1989) havedocumented a similar structureinjected into the Lower Oxford Clay of Cambridgeshire close to andparallel to a major east-west-trending fault. Sediment-filled fissures are widely developedin the Mesozoic of theAlpine-Mediterranean region, with the intrusive material commonly of Triassic andJurassic ages (Castellarin 1965, 1970; Wiedenmayer 1963; Schlager 1969; Wendt 1971). Wendt’s detailed analysis of Jurassic fissures in western Sicily emphasized the geometrical variety of sills and dykes, with laminated pelagic micrites constituting the most common fills and with country rock of shallow-water carbonate or stratigraphically condensed pelagic limestone. Multiple intrusions, of differing ages, are common in the fissures of western Sicily. Castellarin’s(1970) work in the SouthernAlps indicated how thedensity of fissures increases towardsmajor tectonic lineaments which are Fig. 16. Section, calibrated to the zonal level, of Pliensbachian- interpretedas having formerlyconstituted the faulted Toarcian sediments (Marlstone and Junction Bed) across the margins betweenlarge tilted blocks of the Tethyan Cranmore-Leighton Fault System, Mendip Swell. Note the absence continental margin (Bernoulli & Jenkyns 1974). These of Toarcian sediments on the upthrown side of the fault. models are relevant to understanding the significance of the Pliensbachian fracturing is indicated by the presence of neptunian neptunian sills and dykes in the Wessex Basin-Mendip area. dykes of that age in the Carboniferous Limestone (Fig. 12). Zonal Although it is not always possible to separate the time of code as in Fig. 3. Sections runs from 0.2 km northwest of Doulting fissuring fromthe times of filling, which is commonly Church (1) through a point 0.9 km southwest of Dean (2) to the polyphase,principal times of sedimentinjection in the road section at Leighton illustrated in Fig. 12 (3). Southern Alps include the Hettangian-Sinemurian, Pliensb-

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achian,Toarcian, Bajocian and Tithonian (Wiedenmayer stressed by Drummond (1970) and Lake & Karner (1987). 1963; Castellarin 1965; Sturani 1971), and in Sicily the Alternatively, the evidence of sedimentinjection in the Toarcian and late-Oxfordian and early Kimmeridgian stages Wessex Basin may cause one to place undue significance on (Wendt 1971). In Hungary principal phases of extensional the Toarcianand Bajocian as particularly significant faulting, recognized from the dating of neptunian dykes and episodes of Jurassic movement. syn-sedimentarybreccias, similarly tookplace during the Sinemurian,Pliensbachian, Toarcian, Bajocian and Titho- The tectonic motif nian stages (Galficz & Voros 1972; Fiilop 1976; Haas et al. Takentogether, the evidence fromsediment-injection, 1985). These ages, Late Jurassic excepted, show remarkable facies and thickness changes, all closely associated with agreement with those of fissures fromthe Wessex east-west trending faults in the Wessex Basin area, suggests Basin-Mendips area,and Kimmeridgian-Tithonianmove- that significant syn-sedimentary movement was taking place ment is also indicated as a regional phenomenon over much during theHettangian to Bajocian interval: the late of Europe. Principal times of rifting, with delineation of Oxfordian through Portlandian interval was also important major tilted faulted blocks were, on both the northern and and presumably representsa renewed extensional phase southern margins of theTethys, Hettangian and earliest (Brown 1984; Chadwick 1986). Evidence from the Mendip Sinemurianand late Pliensbachian-Toarcian (Bernoulli & fissures confirms the Pliensbachian andBajocian as Jenkyns 1974; Lemoine et al. 1986). important times of extensional faulting, but also points to the Hettangian and Sinemurian as significant intervals too. No evidence of sediment injection at this earlier time has Thickness changes in strata been uncovered in the Wessex Basin, although the thickness The significance of thickness changes of coeval strata across changes illustrated in Fig.14suggest fault-controlled faultsneeds further discussion as such changes can be deposition during this interval. generated both during and after syn-sedimentary movement, The detailed analysis of stratigraphicthickness of in the latter case infilling pre-existing submarine topography JurassicWessex Basin sediments by Hallam & Sellwood (Bertram & Milton1988). If the Jurassic sea-floor of the (1976) indicates deposition of a post-Callovian sedimentary Dorsetsector of the WessexBasin was at all times blanket of more uniformthickness, implying lessening of topographicallyvery subdued,as assumed bySellwood & differential movement: the Kimmeridgian would appear to Jenkyns (1975), thenthe absolute thickness differences of be an exception to this, showing as it does both local facies decompactedstrata should closely approximate the syn- changes and considerable thickness variations. In summary, sedimentarythrow onthe fault.However, if relief was available data for the WessexBasin-Mendip area suggest relatively greaterduring periods of regional stratigraphic contemporaneous faulting throughoutHettangian to Bajo- condensation when units like the Junction Bed and Inferior cian time, with some local Bathonian movement, followed Oolitewere being laid down, thenan absolute thickness by period a of relative quiescence until thelate difference of a few metres could indicate significant Oxfordian-earlyKimmeridgian when renewed activity movement.Given thatthese condensed units were began. The Kimmeridgian faulting, also known from central apparently lithified on the sea-floor, maintenance of modest eastern England and eastern Scotland (Bailey & Weir 1932; sea-floor relief should have been possible, conceivably with Kirby & Swallow1987), presumably correlates with the development of fault-scarps. The fact remains that there extensional activity in the North Sea region (e.g. Bertram & is negligibleis documentedevidence forslope-induced Milton 1988). redeposition(i.e. slumps and turbidites) in the exposed Viewed overallthe Early Jurassic to Bajocianexten- Jurassic of the Wessex Basin, although such phenomena are sional activity in the WessexBasin-Mendip area must perhaps unlikely to have taken place duringperiods of reflect rifting in the central Atlantic-Tethyan domain, regional condensation when little sediment was being followed by a more passive period of thermal relaxation as deposited.Outcrop failure on the downthrownside of Africa separatedfrom North America and Europe. The palaeofaults also hinders collection of critical data, Bathonian-Callovian regional deepening phase, named the particularly in those zones along which maximum movement ‘Atlantic Transgression’by Lancelot & Winterer (1980) is was accommodated. assumed to mark the maturation of the Atlantic as a major Although the evidencefor sediment injection in the oceanfor the first timein its history (Winterer & Him WessexBasin indicatesmovement along faults during 1984). In the WessexBasin area and elsewhere in Britain Toarcianand Bajocian, the absolutethickness changes this transgression is marked by the Cornbrash, although to across various zones of displacement particularly during the what extent this deposit relates to a eustatic effect and to former interval is relatively modest. Indeed, Toarcian strata what degreeit reflects thermalrelaxation of the apparently thin onthe downthrownside of somemajor pen-Atlantic regionis notclear (Watts 1982; Haq et al. faults if true thicknesses are recorded (i.e. Marlstone and 1988). JunctionBed in Kimmeridge 5 Borehole).Hence the Onset of the latestOxfordian, Kimmeridgian- amount of dip slip was perhaps relatively trivial during these Portlandian extensional phase must indicate the initiation of intervals, and/or sea-floor relief greater than duringtimes of a different plate-kinematicregime whichZiegler (1982) mud andsand deposition and/or movement was pre- interprets as dominated by the Arctic-North Atlantic rift dominantly lateral rather than vertical. In this context it is system: indeed Kimmeridgian rifting is now established as a tempting to find some clue to the sense of movement along regional North Atlantic phenomenon (Hiscott et al. 1990). WessexBasin-Mendip faults in theorientation of the Themajor periods of fault movement and facies fissures. Does vertical fissuring indicate normal faulting and differentiation in the WessexBasin-Mendip region hence horizontal fissuring strike-slipmotion? The significance of correlate wellwith the rifting phasesthat predated the wrench faulting in the evolution of thearea hasbeen opening of the Central and North Atlantic Oceans.

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Received 12 October 1989; revised typescript accepted 26 July 1990

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