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Eocene Sedimentation and Tectonics in the Hampshire Basin

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Eocene Sedimentation and Tectonics in the Hampshire Basin J. geol. Soc. London, Vol. 139, 1982, pp. 249-254, 3 figs. Printed in Northern Ireland. Eocene sedimentation and tectonics in the Hampshire Basin A. G. Plint SUMMARY: Sedimentary evidence in the Eocene (Cuisian-Lutetian) of the Hampshire Basin indicates important intra-Eocene movementon the Isle of Wight and Purbeck Monoclines, and on the Ridgeway Fault. Evidence for syn-depositional movement includes Jurassic and Cretace- ous chert and flint pebbles in the Eocene, the distribution of which suggests derivation from both E and W. At Whitecliff Bay, two units of cross-bedded sand have northward-directed palaeocurrents and may have been deposited as localized fans of debris reworked from earlier Tertiary sediments on the upwarped side of the monocline. Three units of laminated muds are intercalated with marine sediments in the eastern part of the basin and suggest periods of low salinity,tideless conditions. These episodes are attributed to the periodicisolation of the Hampshire Basin from the fully marine Dieppe Basin to the SE. This may have been the result of intermittent movement on a tectonic barrier to the SE of Selsey. The onset of uplift and subsequent exposure of the Chalk along the southern margin of the basin proceeded unevenly from ?early Cuisian to late Lutetian times. Movement on the Portsdown Anticline probably occurred over a similar period. The Isle of Wight and Purbeck Monoclines are the superficial expressions of faults in the Palaeozoic basement, and not theresult of lateral ‘Alpine’ pressures. A minor series of syn-depositional folds trend NW-SEacross the basinand may reflect a component of sinistral strike-slip on basement faults. Inthe Hampshire Basin,sediments of theEocene beds which record marine transgressions overthe east- Bracklesham Group, of Cuisian to Lutetian age (Curry ern part of the basin. The pebbles vary from moderate et al. 1977)and laterally-equivalent fluviatile sedi- to verywell-rounded and range up to300 mm in ments are exposed in coastal sections in the Isle of diameter. The pebbles comprise 95-99.5% flint with a Wight and on themainland (Fig. 1). Severalinland minorcomponent of rounded vein quartz and chert sections are available in working and abandoned sand pebbles, up to 15mm in diameter. The textural matur- and ball-clay pits in the Wareham area. ity of Suite A and their association with transgressive The structure of the Hampshire Basin is that of an marine sediments is strongly suggestive of rounding on E-W trending,asymmetrical basin with gently-a a beach. dippingnorthern limb and a near-vertical southern Suite B is, at present, only known from Creechbar- limbcomprising the Isle of WightMonocline. This ii. row Hill (NGR SY922824; Fig. 1) where it is found structure extends south-eastward beneath the Channel through about 30 m of fluviatile sediments underlying asthe Bembridge-St. ValCry line (Smith & Curry theCreechbarrow Limestone which caps the hill 1975); to the W, from Studland to Weymouth Bay, it (Hudleston1903; Q. G. Palmer,pers. comm.). The continues as the Purbeck Monocline (Phillips 1964). pebbles consist entirely of unabraded or sub-rounded W of Dorchester, the Hampshire Basin is bounded to flint nodules, weighing up to 30 Ib (13.6 kg) (Hudles- the S by the E-W trending Ridgeway-Abbotsbury ton 1903). Fault. TheEocene sediments of theeastern Hampshire iii. Suite C is confined to the Wareham area, making Basin aredominantly fine, silty clays andsands of mostits easterly appearance Tatchellsat Pit marineand brackish-water origin. Westwards, these (SY905885).From here, there is aprogressive wes- become progressively more sandy, passing into domin- terly increase in both abundance and clast size through antlynearshore sediments at Alum Bay and fluvial the pits at Povington(SY892824), Warmwell sediments in the Wareham area. (SY752881)and Bincombe Down (SY677856), cul- The evidencefor contemporary Eocene tectonism minating in thecoarse cobble gravels of Blackdown comesfrom three main sources: pebble suites; ?re- (SY612875) (Fig. 1). The pebblescomprise both worked Tertiary sands, and lagoonal sediments. The ‘local’ and ‘exotic’ components. Theformer include detailed sedimentology is given in Plint (1980). poorly-rounded flints (70%) uo to 350mm in diam- eter,Albian chert (13%) and Purbeck chert (2%). Sediments Exotic components include well-rounded vein quartz (14%)seldom exceeding 40mm diameter, black cherts, silificied limestonesand laminated siltstones Pebblesuites. Three distinct pebble suites are recog- (l”/o), probably derived from Palaeozoic sources, al- nized in the Eocene of the Hampshire Basin. thoughsome reworking from the Wealden is also i. Suite A comprises a number of widespread pebble likely. 0016-7649/82/0500-0249$02.00 @ 1982 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/3/249/4887383/gsjgs.139.3.0249.pdf by guest on 29 September 2021 250 A. G. Plint FIG. 1. Simplified geological map of the Hampshire Basin, showing the location of principal sections. Interpretation of the onset of movement on the Ridgeway Fault is difficult pebble suites todate precisely, butthe presence of SuiteC at Povington, about 60 m above the London Clay (Q. G. SuiteC was described in detail by Reid(1896) and Palmer,pers. comm.)suggests ?late Cuisian move- Arkell (1947). Both concluded, from the abundanceof ment. locally-derived Mesozoic material, that the Ridgeway The interpretation of SuiteB is relativelysimple. Fault,which had downthrown tothe S duringthe Upper Chalk is exposed 250 m to the S of Creechbar- Cretaceous,had reversed in theEocene, to expose row Hill. The full Eocene and Palaeocene succession Cretaceous and Upper Jurassic rocks on the S side to does not outcrop between the summit of the hill and subaerial erosion (Fig. 2). The very large size of the theChalk (Q. G. Palmer,pers. comm.),and the clasts derived from these formations imply high gra- Chalk-Tertiary contact must therefore be faulted. The dients and torrential run-off, suggesting deposition on petrographicsimplicity and textural immaturity of fault-scarp alluvial fans. Suite B is compelling evidence for derivation from an The explanation of exotic clasts is problematical. up-faultedChalk terrain, immediately tothe S of They mayhave been reworked from older Tertiary Creechbarrow. The Creechbarrow Limestone has, on formations capping the Chalk on the upthrown block the basis of its mammalian fauna, been dated as early to the S, or they may have been introduced by rivers Marinesian (Lower Bartonian) in age. The presence of draining eastwards along the axisof the basin (Fig. 2). flints to a depth of about 30 m below the limestone To the W, Eocene gravels of very similar composi- suggests exposure of the Chalk in this area in the late tionoccur on the Haldon Hills (the Bullers Hill Lutetian. Gravel,Hamblin 1973) and beneath the Oligocene The absence of pre-Chalk clasts larger than 15 mm clays of the Bovey Basin (the Aller Gravels, Edwards in SuiteA supports the inference, drawn from the 1973). The abundance of Chalk flints and Albian chert distribution of Suite C, that the easterly-flowing rivers in these gravels may reflect local contemporary fault- wereincapable of transportinganything but small ing, perhapsa precursor to that whichled tothe pebbles to the sea. An alternative source for the flint subsidence of the BoveyBasin in lateEocene- cobbles andpebbles of SuiteA must therefore be Oligocene times. sought.Two sources are likely. Verywell-rounded Uplift of the Weymouth Anticlineas a whole occur- flint pebblesare the main component of the basal redduring the late Cretaceous (Robbie 1950). The pebble bed of the Bracklesham Group of Whitecliff Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/3/249/4887383/gsjgs.139.3.0249.pdf by guest on 29 September 2021 Eocenesedimentation and tectonics in the HampshireBasin 25 1 FIG.2. Speculative reconstruction of sediment sources, based upon the composition of pebble suites in Eocene (Bracklesharn Group and equivalent) sediments in the Hampshire Basin. Bay, Selsey and Rookesbury (SW596123). These are currents in these two units are directed towards the N very similar in size and shape to pebbles in the upper- andNE. The BrackleshamBeds at this locality are most part (the Whitecliff Member, King 1981) of the dominated by highlybioturbated glauconitic sandy London Clay Formation, from which they were doubt- silts and laminated silty clays. The cross-bedded sands less derived. Much larger and often less well-rounded are incongruous, both in terms of lithology and their flint pebbles become common in the upper part of the position in facies sequences. It seems likely that these BrackleshamGroup. These mayhave been derived sands were derived from earlier Tertiary sedimentson directly from the Chalk, perhaps exposed locally on the upwarped side of the proto-Isle of Wight Mono- the flank of the ‘proto-monocline’.Erosion of the cline, and deposited in the basin as localized ‘fans’. Chalk may have been effectedby streams draining into the sea, or directly through marine erosion. It seems Lagoonal sediments likely that, as a result of their size, the pebbles were confined to a narrow coastal belt where wave action The Bracklesham Group of theeastern Hampshire and longshore drift could abrade and transport them. Basin as a whole is dominated by highly bioturbated, As a result of a series of marine transgressions, these glauconite-rich,sandy clayey silts with anabundant pebble beach sediments were carried across the basin, and diverse marine molluscan fauna. At Whitecliff Bay leavinga remani6 veneer of pebblesas evidence of (SZ640862), these are intercalated with three units of theirpassage. The numberand size of flint pebbles laminated,weakly bioturbated silty clays 2.5-15 m appearing in theUpper BrackleshamBeds suggest thick (Beds II+Lr. W,V & VI11 of Fisher 1862). Each extensive exposure and vigorous erosion of the Upper unit caps a marine sequence. The base of beds I1 and Chalk in the vicinity of Alum Bay. LrIV is gradational over afew dm while that of beds V and VI11 is sharp, burrowed and may have a shellylag.
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