Major Erosion Surfaces in the Basal Wealden Beds, Lower Cretaceous, South Dorset
Total Page：16
File Type：pdf, Size：1020Kb
Journal of the Geological Society, London, Vol. 148, 1991, pp. 105-113, 10 figs. Printed in Northern Ireland Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south Dorset STEPHENP. HESSELBO & PHILIP A. ALLEN Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK AbstrPd: The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconfor- mity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces aretreated as candidate sequenceboundaries (WBl-WB3), with the thin intervening ?lagoonal depositsrepresenting peaks of transgression.The pattern of oneminor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to theEarly Cretaceous sequence stratigraphy suggested in recent ‘global cycle charts’. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas. A long-standing problem in British stratigraphy has beenthe phases of deposition within these sequences. definition andunderstanding of the junctionbetween the The twomost importantexposures of the transitional Lower Cretaceous Purbeck Beds and the Wealden Beds in strataare at Mupe Bay (SY 844797) andBacon Hole the WessexBasin (sensu Whittaker 1985) of southern (SY 841797) on the Isle of Purbeck (Fig. 1). These are the England (Strahan 1898; Arkell 1947; Howitt 1964; Morter only localities in the WessexBasin where the transitional 1984; Lake & Shepherd-Thorn 1987). There is here a strata are well exposedand not extensively faulted.Both transition,from evaporitic and lagoonal to fluviatile localities have restricted access asthey lie within the sediments, which has long been interpreted as a regressive Lulworth firing range. The exposure at BaconHole has succession, modulated by minorfluctuations in salinity recently (1986) been enhanced by landslipping. (Arkell 1947; P. Allen & Keith 1965; P. Allen 1975, 1981; The sedimentary facies at these two localities represent a Morter 1984; Anderson 1985) but essentially conformable range of environments,from lagoonal through to fluvial (e.g. Arkell 1947; Hughes & Croxton 1973). In view of the (Fig. 2). Abasal fine sand(unit 1 in Fig. 2) provides a importantRyazanian and Valanginianerosive and con- definite link between the twoexposures, being traceable densedepisodes in themarine successions of eastern across the cliff-top. A massive green, red-mottled, mudstone England (Casey 1973; Rawson & Riley 1982), the apparent (unit 2) seen at Bacon Hole probably continues to the gully continuity in southern England is somewhat surprising and with cliff stairs at Mupe Bay. It is overlain at both localities seems to contradict the assertion of Haq et al. (1988) that by fine sandstones and mudstones(units 3-6). The lower this part of the Lower Cretaceous records two of the largest erosion surface at Mupe Bay (WB2 in Fig. 2) truncates the eustatic sea-level falls that occurred during the Mesozoic. stratification in the underlying sandstone and is overlain by In discussing the role of sea-level in influencing Wealden greymuds with laminationconcordant tothe erosion stratigraphy, P. Allen (1981, p. 378)modified his 1959 surface. This surface cannotbe recognized unambiguously at hypothesis onthe overwhelming dominance of eustatic Bacon Hole, possibly because of truncationbeneath the sea-level changeagainst other factors,partly on the basis overlying erosionsurface. At Mupe Bay theupper, that there was a lack of evidence for deep erosion during prominent, erosion surface (WB3) occurs at the horizon of supposed phases of eustatic sea-level fall. Asimilar point the well-known conglomeraticpalaeo-oilseep. WB3 prob- was made by Hallam (1984)with regard tothe onset of ably correlates with the erosion surface at Bacon Hole which Wealden deposition. In this paper we present evidence from separates unit 6 from 7. southDorset demonstrating the presence of twomajor Unit l (Fig. 2) has Seen chosenby most workers as being erosionsurfaces, whichwe interpretas unconformities. the first major sandstone in the Wealden Beds of this area, These are associated with the change to fluvially-dominated the base of the Wealden Beds being arbitrarily taken at a environments and we suggest that regressive continuity is in point 2.35m lowerwhere thin sandlaminae first appear fact illusory. In addition we present evidence, in the form of (Strahan 1898 p. 100; Arkell 1947). More recently P. Allen facies analysis, showing distinct transgressive and regressive (1975) andMorter (1984)have classified the uppermost 105 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/1/105/4891689/gsjgs.148.1.0105.pdf by guest on 01 October 2021 106 CRETACEOUSWEALDENSURFACES,EROSION 82 83 84 85 t)l GAULT1U.G.S ALBIAN 1 West”Lu1worth I 80 WEALDEN Cove Bacon HAUTERIVIANBEDS Hole l km INA 4AL ANGlNlAN U.RYAZANIAN BEDS - .. approximate positionof the strata under consideration. strata of the PurbeckBeds Durlston Formation with the interpretation,but other beds are also briefly described. Wealden Beds. In this paper we prefer to follow the purely The successions are summarized in Fig. 2. lithostratigraphic usage of past workers. Laminated purplelgrey mudstone with shelly limestone Geological setting The sedimentsunderlying unit 1 are grey topurple, The Mupe Bay and Bacon Holelocalities lie onthe moderately well laminated muds, with occasional fine sand southern(downthrow) side of majora E-W-striking intercalationstowards thetop. Limestones containing synsedimentary fault system, evident as the Purbeck-Isle of abundant Viviparus occur 7 m below unit 1 and lower in the Wight Disturbance (Fig. 1B; Colter & Havard 1981; sequence. Thetransition with the overlying sandstone is Stoneley 1982; Chadwick 1986; Karner et al. 1986; Penn et gradational. al. 1987; Selley & Stoneley 1987). The fault system forms the Thisfacies is interpreted as deposited in afresh to northern margin of the Central Channel Basin (Whittaker brackish lagoon onthe basis of bivalve associations, 1985). Subsidencehistories (Chadwick 1986; Karner et al. ostracode assemblages and palynofacies (Morter 1984; 1986) andstrata1 geometries (Stoneley 1982; Selley & Anderson 1985; Clements et al. 1987). Ostracodefaunas Stoneley 1987) show that the main extensional activity on have a progressively greater fresh-water aspect up through thefault system tookplace in earlyJurassic and late the top Purbeck Beds (Anderson 1985). Jurassic/earlyCretaceous times, and that the faults were probably active throughout deposition of the Purbeck and Cross-stratified and rippled fine white sandstone and Wealden Beds. brown mudstone Unit 1 MupeBay and Bacon Hole. The basalWealden Facies and environments sand,unit 1 (Figs 2 & 3), comprises coarsely and finely We concentrate on three key units (units 1, 6 & 7 at Mupe interlayered fine sand and mud with moderate amounts of Bay) whichallow theleast ambiguous environmental dark brown comminutedplant material. The sandsare Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/1/105/4891689/gsjgs.148.1.0105.pdf by guest on 01 October 2021 S. P. HESSELBO & P. A.ALLEN 107 Bacon Hole fluvianle 7 m3- L L 1 l l l lagmnal/lacusnne WE33 1 l I jl inner lagmnal shoreline '"a cm n lagmnal 5 basal Wedden, l -B! west side Mupe Bay Fig. 2. Sedimentary succession at Mupe Bay (SY 844797) and Bacon Hole (SY 841797). Arrows indicate palaeocurrent directions, north at top of page. Unit numbers do not necessarily imply correlation between the two localities. white, quartzose,moderately poorly to moderately well sorted,sub-angular to angularand of low sphericity. The muds are medium-brown and often interlaminated with silt or very fine sand. Three main primary sedimentary structures are present: tabular cross bedding, climbing-ripple cross lamination, and mud-draped symmetrical ripples (Figs 3 & 4). Tabular cross-strata vary in their complexity. Cross-strata1 planes are picked out by finely comminuted plant debris (bed la) or the concentration of fine sediment as drapes in toe-sets (Fig. Fig. 3. Detailed log of the Mupe Bay succession. Basal Wealden in 3, bed le; Fig. 4B). Locally, small-scale cross-laminae dip in the sense of Strahan (1898) & Arkell (1947). the opposite direction to the larger scale cross-stratification (bed lc). Tabular cross-sets in higher parts of unit l at Mupe Bay are apparentlysimple with nodrapes. Trochoidal highly undulating and discontinuous contact between sand wave-ripple profiles (wavelength 50 mm, height 8 mm) are and. mud layers. Followed