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 faults are documented from some areas, and boreholes reveal con- siderable 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, an obviously coarse-grained matrix is 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 attempted to 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 material and 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/2/245/4891687/gsjgs.148.2.0245.pdf by guest on 29 September 2021 246 H. C. JENKYNS & J. R. SENIOR 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).