Journal of the Geological Society, London, Vol. 149, 1992, pp. 443454, 12 figs. Printed in Northern Ireland Early to mid-Cretaceous tectonics and unconformities of the Wessex Basin (southern England) A. H. RUFFELL Department of Geology, Queen’s University, Belfast, Northern Ireland BT7 INN, UK Abstract: Sediment distribution patterns, time-subsidence plots, seismic data and outcropanalysis of the Lower Cretaceous (?Ryazanian-Albian) of the Wessex Basin, southern England, suggest that a number of unconformablehorizons exist and that there is no single ‘late Cimmerian unconformity’ surface. A tectonic change, from areally restricted deposition in the Weald and Channel sub-basins to extensive sedimentation across the whole of southern England, began in the mid-Aptian. This change ended with transgression of basement ‘highs’ in the early Albian of the Dorset area,and later Albian of the Devon and London Platform basin margin areas: its onset can be linked to the Austrian tectonic phase of Europe. The onset of marine deposition in the Aptian is coincident with periodic transgression of the margins of the Wessex Basin throughout the Aptian-Albian. Mid-late Aptian and early/mid-Albian transgressions were preceded by periods of erosion. The Aptian-Albian Lower Greensand is characterized by transgressive- regressive phases, suggesting a short-term controlling process (third-order eustatic changes or intra-plate mechanisms). Tectonic changes in the mid-Aptian include uplift followed by widespread subsidence. This change coincides with a time of plate reorganization in the European and North Atlantic areas. The Mesozoic Wessex Basin of southern England is a structur- (Drummond 1970; Stoneley 1982). Thus structures such as the ally complexbasin (Whittaker 1985), where a predominant ‘mid-Dorset Swell’ (Drummond 1970, 1982) may also be re- east-west pattern of extensional faults is cut by a seriesof lated to basement faults. northwest-southeast oriented faults (Karner et al.1987). These Chadwick (1 985) described the mid-Cretaceous develop- structurescontrolled the accumulation and preservation of ment of the Wessex Basinwith referenceto the ‘late Cimmerian thickMesozoic sedimentary successions within the basin Unconformity’, and produced a map of the older Jurassic and (Chadwick 1986). Major faults commonly form the marginsof Cretaceous formations that subcrop beneath the unconformity the upstanding basement blocks that surround much of the surface. In certain places within the Weald sub-basin(see Figs 1 basin, as well as intra-basinal highs (e.g. the Isleof Wight & 3) no obvious mid-Cretaceous unconformityis apparent, yet Monocline; Fig. 1). Inversion of the Mesozoic basins occurred onthe basin margins a number of discreteunconformities in the latest Mesozoic (late Cretaceous) to mid-Cenozoic occur. Chadwick (1985) took the oldest Cretaceous deposit (?Oligocene) interval, when these formerly upstanding Meso- above the unconformity surface (the oldest commonly being zoic massifs and highs became the sites of accumulation of early Aptian, the youngest late Albian), as his datum. The thick successions of Cenozoic clastic detritus (Karner et al. unconformity was also represented as a widely variable time 1987). gap by Karner et al.(1987), this being explainedby differential Numerous authors have published analyses (based on out- erosion priorto transgression of the basin margins. However, a crop, borehole and seismic data) of the Mesozoic-Cenozoic number of Aptian-Albian unconformities are known to occur development of the Wessex Basin, andsurrounding areas. above the lowest horizon mapped by Chadwick (1985) as the Theseinclude Stoneley (1982), Chadwick (1985,1986), ‘late Cimmerian Unconformity’ (Hesselboet al. 1990a; Ruffell Sellwood et al. (1986) and Karner et al.(1987). Previous evalu- & Wach 1991), and further unconformable horizonsbelow are ations of thetectonic evolution of the WessexBasin have demonstrated here. placed great importance on the early to mid-Cretaceousinter- val, and the development of the ‘late Cimmerian Unconfor- Methodology mity’ (Chadwick 1985; Karner et al. 1987). This surface of un- conformity was linked by Karner etal. (1987) to the How best may the numberof unconformities, which may exist, post-Hercynian tectonic development of the northwest Euro- be definedwithin the basin, andhow may theybe characterized? pean region, and specifically to the ‘Austrian’ (Glennie 1986) The methods of study are outlined below as: (I) sediment dis- tectonic movements.The stratigraphyof the Lower Cretaceous tribution patterns from boreholes and isopach mapping; (ii) sediments is shown in Fig. 2, with a summaryof known tectonic time - subsidence plots and sediment accumulation (and pre- events from Glennie (1986). servation) patterns from outcrop and borehole sections; (iii) Stoneley (1982), Chadwick (1985) and Karner et al. (1987) seismic data; (iv) outcrop analysis. all give accounts of Mesozoic extensional faulting within the Wessex Basin, and subsequent compressionand basin inversion Sediment distribution patterns duringthe late Cretaceous-earlyCenozoic. Such compres- sional structures (e.g. the Purbeck-Isle of Wight monocline) The formal group-status divisionsof the Lower Cretaceous of are offset in an en echelon mannerwhich was relatedby Drum- southern England (sensu Rawson et al. 1978; Stewart 1981; mond (1970, 1982) to strike-slip motion of underlying base- Simpson 1985) are the smallest scale at which isopach maps ment faults. Mid-Cretaceous sediments in Dorset were sub- can be made, as borehole logs from hydrocarbon and water jected to minor inversion along northwest-southeast trends explorationrarely allow more detailed correlation. Gross 443 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/3/443/4891976/gsjgs.149.3.0443.pdf by guest on 03 October 2021 444 A. H. RUFFELL P I l 1O"W 0" O" 55"N rz, TROUGH SEA 50"N WESTERN APPROACHES TROUGH PARIS BAY OF Area of Map C l BISCAY ; I I I I C 2"W 1 "W 0" l"E LONDON PLATFORM HUMBLY GROVE D ,,O Fig. 1. Location maps. (a) Location of WEALD WINCHESTER 0 SUB-BASIN study area, with other basin successions mentioned in text. (b) Aptian-Albian MID-DORSET (Lower Greensand) outcrops in south SWELL eastern England and France with outcrops (1-15) utilized in sequence 0 BOREHOLE stratigraphic study of Aptian-Albian W WlLMlNGHAM A ARRETON sediments (Fig. 12). (c) Tectonic features V lNGN \ of the Cretaceous of the Wessex Basin. 0 25 50 km Lines of seismic sections: A, Fig. 9; B, Fig. 10; C, Fig. 11. Major boreholes l I I mentioned in text. facies changes occurwithin all the groups studied (Casey 1961; (Fig. 3). A single transgression cannot explain the complex Owen 1975; Allen 1989 and references therein),making distribution of mid-Cretaceous sediments on themargins of the attemptsat decompaction difficult, as different lithologies Wessex Basin, as erosion occurred prior to each transgressive follow different compaction trends. The spacing of boreholes phase (e.g. Hesselbo et al. 1990b). In boreholes sunk to the within the basin is not dense enoughto allow mapping of large- north of the Isle of Wight Monocline (Figs 1 & 4), which was an scale facies changes, so only present-day (compacted) thick- area of positive relief in the early to mid-Cretaceous, the base- nesses are discussed in this section. These maps demonstrate Gault Carstone Formationis the first deposit to be preserved. that the basin depocentre remained around the core of the This rests unconformably on Jurassic strata (Fig. S), and the present-day Weald anticline(Fig. 3) for much of the time base of this formation might be taken as the 'late Cimmerian' or represented by the Wealden (?RyazanianNalanginianto 'Austrian' unconformity. In contrast, on the crest of the Lon- BarremiadearlyAptian) and Lower Greensand (Atherfield don Platform the first transgressive deposit is the Upper Gault Clay and Hythe Beds Formations: early Aptian). Inthe mid- to Clay, whilst on the flanks of the platform in the northern mar- late Aptian, deposition commenced on formerly upstanding gins of the Wessex Basin (Fig. 1) the first transgressive deposit fault blocks within and on massifs surrounding the Wessex preserved varies from early Aptian to early Albian in age. The Basin. These deposits are patchily preserved as the Sandgate, thin sedimentarycover on such areas gives some indication (e.g. Folkestone and CarstoneBeds of the Lower Greensand and the horizons of phosphatic nodules containing remanie Lower Gault Clay (Owen 1975). The Upper Gault Clay is the ammonites) of previous, now eroded transgressive phases. By first deposit to cover all topographic highs around the basin determining the age of these remanie deposits, and tracing their Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/3/443/4891976/gsjgs.149.3.0443.pdf by guest on 03 October 2021 CRETACEOUSUNCONFORMITIES, WESSEX BASIN 445 REGIONAL EVENTS REGIONAL ERA GROUP FORMATION APTUN - EARLY Ma STAGE TETHYS ATLANTICTETHYS PERIOD CHANNELWEAL1 CHANNEL WEALD ALBIANAMMONITE ZONES UPPER GREENSAN UPPER GAULT. UPPER GREENSAND mammrllalum LOWER GAULT y CARSTONE ' 0 / MID- SANDRO~K FOLKESTONE ATLANTIC 2 BEDS lardeturcata 50 SPREADINGALPINE OROGENY ERRUGINOUSSANDGATE SANDS HYTHE SANDS PLATEAU ATHERFIELDCLAY lamb! EASALTS - W CLOSURE m+ UPPER VECTIS WEALD Wlkld,e"S#S loo SEA.FLOOR z5 yI CLAY SPREADING 2 ROTATION NEWFOUNDLAND c > OFlsERlA -IBERIA d manlomdes OQ AUSTRIAN 9 > LOWER ClMMERlAN - WEALDWESSEX 150 TECTONICS bowerbad! ~~~~$~~ CLAY Fig. 2. Tectonic history