Journal of the Geological Society, London, Vol. 156, 1999, pp. 327–339. Printed in Great Britain

Eocene inversion history of the Pericline, , southern

A. S. GALE1,2, P. A. JEFFERY2, J. M. HUGGETT1 & P. CONNOLLY3 1School of Earth and Environmental Sciences, University of Greenwich, Medway Campus, Chatham Maritime, Chatham, Kent ME4 4AW, UK 2Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK 3Department of Geology, Imperial College, Prince Consort Road, London SW7 2BP, UK

Abstract: Study of the composition and distribution of derived lithoclast and fossil suites collected from the Selsey, Barton and Becton Formations in Whitecliff Bay, Isle of Wight has enabled a detailed reconstruction of Mid–Late Eocene uplift of 500 m+ on the northern limb of the Sandown Pericline. The stratigraphical distribution of clasts and fossils derived from older formations demonstrates the erosion of progressively older Eocene, Palaeocene and sediments during growth of the structure. The presence of delicate reworked fossils and clasts, together with limited palaeocurrent data support very local derivation from the south. The preservational state of the derived materials is used to identify the diverse processes of erosion and transport. Two phases of uplift (Lutetian, Bartonian), separated by a period of quiescence and peneplanation have been identified; rates of Eocene uplift of about 100 m Ma"1 are postulated to have taken place.

Keywords: Eocene, Structural inversion, southern England, derived fossils.

The Palaeogene succession in Whitecliff Bay in the east of the septarian concretions within the Selsey Formation, and Isle of Wight has attracted geologists since the nineteenth independently, P.A.J. found Bracklesham Group molluscs century (e.g. Prestwich 1847; Fisher 1862), on account of the high in the Barton Clay. These finds stimulated us to search spectacular, near-vertical exposures of variegated sediments, further, and led to the discovery of beds containing Reading many of which are fossiliferous. More recently, the sedimen- Formation clay clasts in the Becton Sand and other specimens tological work of Plint (1984, 1988) established Whitecliff Bay of fossils and clasts derived from older Palaeogene sedi- as a classical locality in which to study sequence stratigraphy ments in the Selsey, Barton, and lower part of the Becton in alternating estuarine and shallow-marine facies. The section Formations. The high-resolution biostratigraphical and is probably the most stratigraphically complete, and certainly chronostratigraphical framework which exists for the Eocene the best exposed Eocene succession in the Anglo-Paris Basin, in Whitecliff Bay (e.g. Aubry et al. 1988; timescale updated by and has been important in the calibration of Eocene onshore Berggren et al. 1995) enables precise dating of periods of uplift, magneto- and biostratigraphy with the deep ocean record and we have been able to calculate rates of uplift on the (Aubry et al. 1988). The locality has also been used as a Sandown Pericline during the Lutetian–Bartonian interval. reference section in the development of global chrono- The absolute amount of uplift during this interval (minimum stratigraphical and eustatic charts (e.g. Haq et al. 1988; J. of 500 m) is considerable, and may have wider implications for Hardenbol. pers. comm.). the tectonic history of southern England during the Early It has been suggested by various authors that uplift was Tertiary. taking place during the Palaeogene in the Isle of Wight. Daley & Edwards (1971) used evidence from thickness variation and erosional surfaces in the Solent Group (Late Eocene–Early The succession in Whitecliff Bay Oligocene) to identify fold axes in the northern Isle of Wight. This section provides a brief description of the succession Murray & Wright (1974) recorded Upper Cretaceous micro- exposed in the southern part of Whitecliff Bay, and provides a fossils in the Venus Bed (Headon Hill Formation, Late stratigraphical context to the sediments which were the source Eocene) of Headon Hill, and at the same level found lime- of clasts and fossils reworked into the Mid–Late Eocene. The stone clasts derived from the lower part of the Headon Hill succession is shown in outline in Fig. 1. Formation. Plint (1982, 1984) interpreted sand units in the Bracklesham Group (Wittering Formation, Mid-Eocene) of (Late Cretaceous). The White Chalk Formation Whitecliff Bay as ‘fans’ derived from earlier Eocene formations (Turonian–Campanian) in the Culver–Whitecliff section during inversion of the east–west basement faults which under- includes nearly 400 m of chalk containing numerous flints lie the Isle of Wight monocline. Plint’s (1984, fig. 12) palaeo- (Rowe 1908; White 1921). The succession is terminated by an geographic reconstruction of the Solent region during unconformity. Derived flints are a conspicuous component of deposition of the Bracklesham Group shows eroding Chalk the Palaeogene succession. uplands immediately to the south of the rising monocline, but to date there is no firm evidence of the timing and amount of Lambeth Group (Palaeocene–Eocene). This group is inversion which took place during the Eocene. represented in the Isle of Wight by the continental Reading During study of exceptional foreshore exposures in Formation (about 40 m thick) which is composed dominantly Whitecliff Bay between 1992 and 1996, A.S.G. and J.M.H. of variegated red, grey, purple and white silts and clays which discovered numerous reworked fragments of London Clay show strong pedogenic mottling (Buurman 1989).

327 328 A. S. GALE ET AL.

Clay includes a major hiatus 40 m above the base which is overlain by a lenticular conglomerate.

Structural history of the Sandown Pericline Whitecliff Bay is situated on the vertical northern limb of the Sandown Pericline (Fig. 2). The presence of east–west faults at depth underlying the steeply-dipping northern limbs of both the Sandown and Brixton Periclines has been suspected for a long time (White 1921). It is now known that the periclines are underlain by two en-echelon faults which are part of a major basement lineament called the Isle of Wight–Purbeck structure (Stoneley 1982; Underhill & Paterson 1998). The structure continues westwards into Dorset and eastwards from the Isle of Wight as the –St Valéry Line which extends across the Channel into the Pays de Bray in northern France (Smith & Curry 1975). During the Early Cretaceous (Barremian–Albian), regional extension caused normal movement on the Variscan basement faults defining the northern margin of the Channel Basin, including the structure beneath the Sandown Pericline (Chadwick 1986; Whittaker 1985; Ruffell 1992; Underhill & Paterson 1998). Clasts and fossils derived from the Jurassic rocks forming the footwall are common in the Barremian (Vectis Formation) and Aptian (Ferruginous Sands Forma- tion) sediments of (Radley et al. 1998), and testify to extensional movement of the structure at least into Early Albian time. Hardgrounds and condensed sections in the Late Cretaceous Fig. 1. Late Cretaceous and Palaeogene succession exposed in Chalk Group of Culver Cliff (immediately south of Whitecliff Whitecliff Bay. The scale on the left of the lithological column is in Bay) have been interpreted as reflecting growth of the hundreds of metres. Sandown Pericline (Mortimore 1986; Mortimore & Pomerol 1997). They identified two periods of inversion, one in the Thames Group (Eocene, Ypresian). The stratigraphy of the Early Coniacian, and another in the Late Santonian–Early Harwich and London Clay Formations in Whitecliff Bay has Campanian. We disagree with this interpretation for several been described by King (1981) and Huggett & Gale (1998). The reasons. Firstly, the White Chalk Formation thickens south- thin (1 m) Harwich Formation comprises glauconitic sands wards by over 100 m across the Island (especially marked in and silts deposited in a shallow marine setting. The London the Coniacian), which is more consistent with con- Clay is 135 m thick at Whitecliff Bay, and is made up of sands, tinued extensional movement on the Purbeck–Isle of Wight silts and clays deposited in marine and estuarine environments. structure than it is with inversion. Secondly, the phosphatic The lower part of the London Clay at Whitecliff Bay is clay chalks found in the Late Santonian–Early Campanian of dominated, the upper part sand dominated, but within this Culver (Whitecliff Ledge Chalk of Gale et al. 1987) are known overall trend there are four upward coarsening units (Divisions elsewhere in the Paris Basin only in erosional cuvettes A–D of King 1981). The London Clay contains ferroan (Jarvis 1980). We think that a local erosional basin unrelated carbonate-cemented septarian concretions (Huggett 1995), to the Sandown Pericline was responsible for this local phosphatic concretions and in the uppermost part sideritic facies. concretions. The unconformity between pelagic limestones of the Chalk Group and continental clays of the Reading Formation is a Bracklesham Group (Eocene, Ypresian–Lutetian). This litho- striking lithological and facies change and represents a major logically diverse group comprises four formations, deposited in hiatus with a duration of about 15 Ma. The uplift of the Chalk lagoonal, estuarine and shallow open shelf environments (Plint probably took place in the Danian, because Maastrichtian 1984, 1988; Huggett & Gale 1997). The Wittering Formation chalks are present offshore south of Dorset (Curry 1990) and (55 m) is a heterogeneous unit, including silts, glauconitic movement predates deposition of the Thanet Formation in the sands, laminated sands and clays and a rooted coal. The London Basin which is Thanetian in age. Gentle regional Earnley Formation comprises 30 m of highly glauconitic fos- folding caused a slight dip (a fraction of a degree) to the south siliferous sands (locally 40%+glaucony). The Marsh Farm west to develop in the Isle of Wight, as shown by westwards Formation (15 m) is dominated by dark interbedded sands and overstep of the Chalk (Curry 1965, 1990 fig. 13.2). Subsequent clays. The Selsey Formation (40 m) includes glauconitic sands, erosion removed about 150 m of Chalk of latest Campanian slits and silty clays. and Maastrichtian age. There is no evidence for inversion of the Isle of Wight–Purbeck structure itself during the latest Barton Group (Eocene, Bartonian). This group is made up of Cretaceous or Palaeocene. the marine Barton Clay (47 m; green clays and silty clays) and No previous evidence of Eocene movement on the Sandown the Becton Sand (66 m; grey and yellow fine sands). The Barton Pericline has been presented, but Reid & Strahan (1889, p. 114) EOCENE INVERSION, ISLE OF WIGHT 329

Fig. 2. Map to show the position of and the major structural features of the southeastern Hampshire Basin, and the position of localities mentioned in the text. The inset section XY illustrates the inferred structure of the Sandown Pericline; (a) Palaeozoic basement; (b) Permo-Trias; (c) Jurassic; (d) Lower Cretaceous; (e) Chalk Group; (f) Palaeogene. The letter ‘A’ denotes the position of the disconformity at Ashey Down described by Reid & Strahan (1889). described a railway cutting at Ashey, south of Ryde ([TQ (ii) Fractured but unworn Thalassinoides burrow flints, 574880], 5 km NNW of Whitecliff Bay) in which the basal coated with dark green glaucony and retaining traces of chalk London Clay was directly overlain by the Bracklesham Group. bioturbation fabric (Bromley & Ekdale 1987) are common in Although there was no sign of a tectonic contact, they inferred the middle Barton Clay conglomerate (Fig. 6, 89.8m; Fig. 3b,c) the presence of a strike fault. We suggest that this is an where over 25 have been collected. They probably developed intra-Eocene unconformity caused by uplift of the Sandown a glaucony coat on the eroded surface of the chalk Pericline. during marine transgression, like the ‘Bullhead Bed’ flints at It has been widely believed that the major development of the base of the Thanet Formation in the London Basin the Brixton and Sandown Periclines (and most other east– (Cooper 1976). west trending folds in southern England) was during the (iii) Angular shards of unweathered flints, 1–5 cm in length, Mid-Tertiary ‘Alpine storm’ (e.g. Wooldridge & Linton are common in the mid-Barton Clay conglomerate and occur 1955), because Oligocene strata are affected by the folding. in two horizons in the lower part of the Becton Sand (Fig. 6, However, the concept of a brief tectonic episode in the 105.4 & 112 m; Fig. 3a). The presence of type (ii) and (iii) flints Miocene is now replaced by a longer, more complex history of indicates local derivation directly from the chalk. uplift from the Palaeogene continuing into the Quaternary Some of the derived flints are soft and white, like those (Jones 1981). described from elsewhere in the Hampshire Basin (‘rotten flints’, Curry 1964, 1987). This type of silica dissolution is brought about by alkaline conditions and is only known to Lithoclasts and derived fossils occur in soils (Stank 1991; Luedtke 1992). Supporting evidence This section describes clasts and fossils derived from the Chalk is provided by the Late Eocene–Early Oligocene Bembridge and the older Palaeogene formations which have been Marls in Whitecliff Bay, where derived flints in palaeosols are collected in the Selsey, Barton and Becton Formations in all soft and white. The occurrence of white flints is therefore Whitecliff Bay. The material is illustrated in Figs 3–5. taken as tentative evidence of derivation from soils within older Palaeogene sediments.

Flints Different morphological types of derived flints reflect contrast- Red clay clasts ing processes of erosion from the chalk, and subsequent modes Small rounded fragments (0.5–2.0 cm) of brick-red and of transport, weathering and reworking. purple-red clay occur commonly in the lower part of the (i) The most common flints in the succession are spherical, Becton Sand (over 150 recorded) and are concentrated in two oval or flattened oval in shape, commonly 1–5 cm in maximum beds (Fig. 6, 105.4 & 112 m). The outside of some clasts is dimension but rarely much larger in size. These are mostly green on account of reduction of iron associated with the clay black, but can also be grey, mottled, or white. Surface chatter or perhaps by superficial replacement by glaucony. The vari- marks indicate rounding by wave action on pebble beaches egated red colours are characteristic of clays affected by situated adjacent to chalk cliffs. Rounded flint pebbles are pedogenic processes and are similar to those of Reading concentrated in thin (<30 cm) transgressive lags in the London Formation clays. This provenance is supported by XRD Clay Formation in Whitecliff Bay (base of Divisions C, D1, analysis of clasts which shows the presence of a distinctive D2, E; King 1981; Huggett & Gale 1998) and are also found in smectite with an intercalated organic component which is the Bracklesham and Barton Groups. Rounded flints can only found in the top few metres of the Reading Formation survive multiple cycles of reworking and do not therefore in Whitecliff Bay. In addition, several clasts contain the provide evidence of penecontemporaneous erosion from the burrow Scoyenia identical to a form present in the Reading White Chalk. Formation. 330 A. S. GALE ET AL.

Fig. 3. Derived clasts and fossils from the Barton and Becton Formations, Whitecliff Bay foreshore. Scale bar in millimetres. (a) angular shard of white flint (F) in sandy matrix, from sparse conglomerate dominated by clasts of Reading Formation Clay, Becton Sand Formation, 105.4 m on Fig. 6 (PE PEI 227). (b,c) Unworn whitened soft chalk flints, coated with glauconite, and encrusted with serpulid worm tubes (S), from middle Barton conglomerate, Barton Clay Formation, 89.8 m on Fig. 6 (PE PEI 228, 229). Both flints have a complex history of derivation; they were originally derived from the White Chalk Formation by gentle marine erosion, and were superficially glauconitized. They subsequently spent time in a soil where they were partially desilicified, and were then re-exhumed into a marine environment where encrustation with epibionts took place. (d, e, f) Worn valves of the bivalve Venericor planicosta planicosta (Lamarck), derived from the Earnley Formation. Note the secondary pyritization (P) which has occured after reburial. (d) and (e) are from the middle Barton conglomerate, Barton Clay Formation (89.8 m in Fig. 6; PE PEI 231), (f) is from 1.3 m higher in the Barton Clay (90.9 m; PE PEI 232).

Septarian ferroan carbonate concretions sparse conglomerates containing abundant molluscs and rounded flint pebbles (Fig. 6, 21.3 m, 25 m, 29.1 m, 32.2 m, Over 200 fragments of subangular to subrounded fragments 42.8 m). The concretions have formed in a matrix of silt of septarian ferroan calcite concretions (1–20 cm in diameter, and silty mudstone with a little fine sand, and contain and weighing up to 0.65 kg) have been collected from the clavate borings (Gastrochaenolites) attributed to boring middle part of the Selsey Formation, where they occur in bivalves. They are encrusted by bryozoans, serpulids and

Fig. 4. Septarian carbonate concretions derived from the London Clay, collected in the Selsey Formation, Whitecliff Bay foreshore. (a–c) and (e) come from the sparse conglomerate about 2 m above the Tellina Sand (42.8 m in Fig. 6); (d) comes from the sparse conglomerate at 25.0 m. Scale bars in millimetres. (a–c) Subangular-subrounded fragment of septarian concretion, showing fracture-filling calcite (C), boring by bivalves (Gastrochaenolites, G), and extensive superficial fractures (F) formed after reburial in Selsey times (PE PEI 225). (d) Fragment of London Clay septarian concretion which has broken along the calcite septarian fracture cement (PE PEI 226). (e) Extensively bored (Gastrochaenolites) fragment of London Clay concretion, heavily encrusted by the bryozoan Villcharixa navalis (PE PEI 224). (f) SEM of bryozoan Villicharixa navalis (Davies) encrusting concretion shown in (e); the scale bar is 286 ìm in length. (g, h) Photomicrographs of thin sections of derived concretions; (g) comes from a sparse conglomerate at 42.8 m (in Fig. 6); (h) is a thin section of the specimen figured in (d). In (g) the ferroan calcite fracture cement formed during original burial diagenesis (C) has been fractured and cemented by pyrite (black, P) during reburial in Selsey times. In (h), borings of bivalves (Gastrochaenolites) with darker alteration haloes (Ox) were broken on the sea-floor; the concretion was then buried and underwent fracturing, and the fractures filled with glauconitic Selsey sand. Pyrite (black, P) subsequently cemented both the fractures and the sand. Scale bar 1 mm. EOCENE INVERSION, ISLE OF WIGHT 331 332 A. S. GALE ET AL.

Fig. 5. Fossils from a sideritic concretion (partially phosphatized superficially) in the mid-Barton conglomerate (89.9 m in Fig. 6; PE PEI 230), Barton Clay Formation, Whitecliff Bay. Scale bars in millimetres. (a, b) Views of sideritic concretion; note external and internal moulds of molluscan fossils, and borings (?Gastrochaenolites). (c) Silicone rubber cast of stratigraphically diagnostic molluscs Haustator editus (H) and Elliptotellina ambigua (E) from this concretion; at the Barton stratotype, these species are characteristic of Bed G, and their presence in a clast in the mid-Barton conglomerate in Whitecliff Bay provides evidence of the extent of a major hiatus (A3-?H) at this level. (d, e)Rightvalvesof E. ambigua from Bed G (stone band) of the Barton Clay, Barton on Sea, Hampshire (d; PI TV 461; e; PI TB 462). (f) H. editus from a clay parting in Bed G of the Barton Clay, Barton on Sea (PI TG 4225). oysters (Fig. 4). The borings display alteration haloes Phosphatic concretions (Fig. 4h). Small (2–6 cm) phosphatic concretions with a softer buff The clasts are petrographically identical to septarian ferroan exterior, a hard dark core and Chondrites on the outer surface calcite concretions found in Divisions A–C of the London Clay are found in the Barton Clay (Fig. 6, 105.4 m). The surfaces in Whitecliff Bay (King 1981; Huggett 1995; Huggett & Gale show polygonal fractures and exfoliate from the cores. Con- 1998) and are not known from any other level in the Hamp- cretions of this type are known only from Division B of the shire Basin. In comparison to indigenous London Clay con- London Clay which is inferred to be the source of the derived cretions these derived specimens have undergone a further material. fracture event and a second phase of diagenetic cementation by pyrite. The fractures are conspicuous on the external surfaces of the concretions (e.g. Fig. 4a–c) and in thin section (Fig. 4g) are seen to cross-cut the radial fibrous calcite of the septaria. Sideritic concretions These fractures were then partially infilled with glauconitic (i) Small (<5 cm) subrounded fragments of siderite cemented sand of the Selsey Formation (Fig. 4h). Pyrite subsequently brown and yellow silty sand occur in the middle part of the formed in the fractures, replaced parts of the interior of the Selsey Formation (10) and in the middle part of the Barton concretions (Fig. 4g), and locally cemented Selsey Formation clay (6). Identical concretions are present in situ at five narrow sand onto the exteriors (Fig. 4h). levels in the uppermost part of the London Clay and Wittering EOCENE INVERSION, ISLE OF WIGHT 333

Fig. 6. Detailed sedimentological log of the Selsey, Barton and Becton Formations on the foreshore at Whitecliff Bay, to show levels yielding derived concretions and fossils (denoted with a black dot). The silty clay facies developed between 20 and 44 m is the ‘Brook Bed’. TL, Transgressive lag, associated with transgressive or wave ravinement surfaces. SC, Sparse conglomerate, probably storm deposited.

Formations in Whitecliff Bay (Huggett et al. in press), and it is ribs and wide intercostal spaces of the derived specimens agree possible to match derived clasts to these petrographically. best with morphotypes found in the Earnley Formation, (ii) A single flattened red-brown sideritic concretion, particularly towards the base of that unit (J. Todd pers. superficially phosphatized, was collected from the mid-Barton comm.). Clay conglomerate (Fig. 5a,b; Fig. 6, 89.8 m). The surface is (iii) Well-rounded fragments of the scallop Mimachlamys worn and pitted by the boring Gastrochaenolites. Broken trigintaradiatus (J. de C. Sowerby) are frequent in the basal surfaces display external casts of gastropods and bivalves, unit (Cerithium Bed, Fig. 6) of the Selsey Formation in including common Elliptotellina ambigua (J. de C. Sowerby) Whitecliff Bay. Identically coloured specimens of this species and Haustator editus (Solander) (Fig. 5c) which indicate are common in the Earnley Formation (S. Tracey pers. comm.) derivation from a level equivalent to the upper part of Burton’s from which the Selsey fragments were probably derived. Bed F or more probably Bed G at Barton, Hampshire (Burton (iv) The delicate crassatellid bivalve Bathytormus hemileius 1929, 1933). This horizon is uniquely represented by the (Wood) was collected from the glauconitic bed 1 above the concretion, and sedimentation in Whitecliff Bay probably mid-Barton Clay Conglomerate (Fig. 6, 90.9 m). This species recommenced in the equivalent of the overlying Bed H. is restricted to the middle and upper parts of the Selsey Formation (Tracey et al. 1996) from which the specimen was derived. Fossils derived from the Bracklesham Group (v) Abraded fragments of the large distinctive foraminiferan Nummulites laevigatus (i) A single steinkern of the bivalveVenericor planicosta are found rarely in the lower 10 m of the suessoniensis (d’Archiac) preserved in siderite cemented Selsey Formation. This species occurs in the Earnley Forma- glauconitic siltstone was collected from the upper part of the tion and a marine sand within in the Marsh Farm Formation. Barton Clay, 1.0 m above the middle Barton Clay Con- It is only abundant in the top few metres of the Earnley et al glomerate (Fig. 6, 90.9 m). Abundant examples of V. Formation (Curry . 1977, 1978). planicosta suessoniensis occur exclusively at a single level in the middle part of the Wittering Formation (Fisher Bed IV) in Whitecliff Bay. This level contains partially lithified, siderite Provenance of reworked material and its significance cemented glauconitic siltstone concretions (Huggett et al.in The distribution of derived clasts and fossils in the Whitecliff press) identical to the matrix of the derived steinkern. succession is summarized in Fig. 7. The first appearance of (ii) Fragments of large bivalve Venericor planicosta plani- material derived from an older unit may postdate the first costa (Lamarck) are common in the two thin glauconitic sands exhumation of that unit, but assemblages do provide an present in the upper Barton Clay (Fig. 6, 89.8 m & 90.9 m), accurate indication of which strata were undergoing active including both rolled and angular pieces (Fig. 3d–f; 25 erosion at any given time. The successive appearance of clasts specimens collected). This species is a characteristic fossil of and fossils from older formations occurs in the predicted order the Earnley Formation where it occurs abundantly. It occurs which progressively deeper erosion of the Palaeogene succes- less commonly in the Selsey Formation and disappears in the sion would produce. Thus, derived fossils from the Earnley lowest part of the Barton Clay. The high form, low number of Formation appears at the base of the Selsey Formation (1), 334 A. S. GALE ET AL.

Fig. 7. Distribution of derived fossils and clasts in the late Eocene of Whitecliff Bay and interpretation of uplift history. clasts from the London Clay appear within the Selsey Eocene tectonic and sedimentary history Formation (2), and Reading Formation clays (3) and fresh chalk flints (4) enter in the Barton Clay. Pre-inversion sedimentation Evidence of the geographical provenance of the reworked The even thickness of the Reading Formation (35–40 m) across lithoclasts and fossils is provided by palaeocurrent data and the Isle of Wight and the north to northwest thinning of the preservational state of derived material. Palaeocurrent data in London Clay (King 1981; Edwards & Freshney 1987, fig. 1) is the Bracklesham Group in Whitecliff Bay can be obtained taken as evidence that deposition of these units was not from only two horizons, one in the middle part of the affected by the Purbeck–Isle of Wight structure. The lower part Wittering Formation, the other in sands of the Selsey of the Bracklesham Group (Wittering, Earnley and Marsh Formation in the cliff section only. Although exposure is Farm Formations) thin in a north or north-westerly direction limited, both levels display unidirectional cross-bedding indi- (Edwards & Freshney 1987) and were also unaffected by cating a northerly current direction (Plint 1984). The presence east–west structures. of delicate molluscan fossils and soft clay clasts among the derived material is evidence of very local derivation, because such fragile items would not survive long transport. It is concluded that the main source of the derived fossils and Phase 1a. Lower Selsey Formation—breakdown of the lithoclasts was the rising northern limb of the Sandown barrier coastline and erosion of the Bracklesham Group Pericline which was situated a short distance (one to several The boundary between intertidal laminated clays and silts of hundred metres) south of the area where the Eocene succession the Marsh Farm Formation and the overlying glauconitic now exposed in Whitecliff Bay was deposited. We therefore use pebbly shelly sand of the basal Selsey Formation is abrupt, and the detailed stratigraphical distribution of reworked material was interpreted by Plint (1984, 1988) as one of a succession of to reconstruct the history of uplift of the Sandown Pericline similar transgressions. However, this transgressive event differs during the Eocene. The amount of uplift has been calculated significantly from those preceding it in the London Clay and from the thickness of sediment which it is necessary to remove Bracklesham Group in that it marks the last appearance of to expose the source of the oldest derived materials present at lagoonal, barrier coastline facies in the region for several any particular level. For this, we have used the thicknesses of million years. It also represents the first appearance in the formations as presently exposed at Whitecliff Bay. Because succession of clasts and fossils derived from older Palaeogene formations are likely to have thinned across the rising pericline sediments. The presence of abraded fossils derived from the in the Eocene, the amounts of uplift are maximum values. Earnley Formation, in the lower part of the Selsey Formation EOCENE INVERSION, ISLE OF WIGHT 335

Fig. 8. Cartoons to show the proposed history of the Sandown Pericline during the Late Eocene. (a) During deposition of the Earnley Formation the area was covered with a shallow sea. (b) During deposition of the middle part of the Selsey Formation (‘Brook Bed’) an actively eroding London Clay cliffline with mudflows was situated a short distance to the south of the present outcrops, which provided clay and silt grade sediment and fragments of septarian carbonate concretions, probably washed offshore during storms (see Fig. 4). This reconstruction is based on the north coast of the Isle of Sheppey, Kent at the present day where London Clay is undergoing erosion. (c) Quiescence and peneplanation during deposition of the highest part of the Selsey Formation (Sii), time of deposition of Nummulites variolarius Bed (Fig. 7). (d) Further uplift exposed the Reading Formation and the Chalk during deposition of the lower Becton Sand Formation, as evidenced by abundant pebbles of Reading clay and occasional unworn flints and angular flint shards (see Fig. 3). This reconstruction is based on Whitecliff Bay at the present day. R, Reading Formation; LC, London Clay; W, Wittering Formation; E, Earnley Formation; Si, lower part of Selsey Formation; Sii, uppermost part of Selsey Formation (N. variolarius Bed); B, Barton Clay Formation.

(Cerithium Bed, Lentipecten Bed; Fig. 6) is evidence that moved far from the shore; we suggest a maximum distance of erosion of older Bracklesham Group sediments was taking a few hundred metres by analogy with comparable modern place nearby. 20–30 m of uplift would have been necessary to environments in the Thames Estuary (Fig. 8b). bring about erosion of the Earnley Formation. The part of the Selsey Formation which contains derived clasts is approximately equivalent to nannofossil zone NP 15 which had a duration of about 3–3.5 Ma (Berggren et al. 1995). Phase 1b. Middle Selsey Formation—erosion of the A total uplift of 200–300 m must have taken place during this London Clay interval in order to expose most of the London Clay. Rates of "1 The clay-dominated middle part of the Selsey Formation in the uplift of about 100 m Ma can be calculated during deposi- thick foreshore succession (Huggett & Gale 1997) includes thin tion of the Selsey Formation, but there is no evidence as to (<10 cm) sparse conglomerates (SC in Fig. 7) which contain whether this occurred in short pulses or was sustained through- scattered fragments of London Clay septarian concretions and out the time period. dark well-rounded flint pebbles, together with abundant and diverse indigenous bivalves and gastropods. The clay-rich level is an expanded correlative of a silty clay facies found widely in the Hampshire Basin (e.g. Edwards & Freshney 1987), called Quiescence and peneplanation the ‘Brook Bed’. The highest 6 m of the Selsey Formation are a highly fossilif- Both the abundance and state of abrasion (subangular- erous very fine sand (the Nummulites variolarius Bed) which subrounded clasts) of the derived London Clay concretion does not contain any derived clasts or fossils. This unit is fragments is evidence that they were originally concentrated on widely developed in the eastern part of the Hampshire Basin a beach of moderate energy adjacent to an actively eroding (west Sussex, southeast Hampshire) and at Selsey and White- cliff-line, probably with mudflows as in the Isle of Sheppey, cliff Bay was characterized by Curry (1965) and Murray & Kent, at the present day. The presence on the clasts of Wright (1974) as a ‘seagrass meadow’ facies deposited in bryozoan encrusters and a suite of borers including Gastro- shallow warm water. Because of the widespread development chaenolites indicates that the clasts subsequently spent time of this distinctive, shallow-water biofacies, and the absence of exposed on the seafloor in an offshore subtidal habitat, into derived material, we suggest that this period was one of brief which they were probably washed by storms. The large size of tectonic quiescence (Fig. 8c). The base of this unit was taken as some clasts (up to 0.65 kg) makes it unlikely that they were a transgressive surface by Huggett & Gale (1997). 336 A. S. GALE ET AL.

Phase 2a. Selsey Formation–Barton Clay disconformity The material in the mid-Barton conglomerate is extra- In Whitecliff Bay the basal bed of the Barton Clay is a ordinarily diverse and is interpreted as a transgressive lag glauconitic sandy silt which rests abruptly upon well sorted formed during a sea-level rise at the level of Bed H of the fine sands and silts of the Nummulites variolarius Bed, into Barton Clay Formation; it represents the more durable sweep- which it is piped in Thalassinoides fills (Fig. 6, 50.0 m). Todd ings of a submarine erosion surface (and possibly a beach), cut (1990) demonstrated that a significant disconformity is present through the entire Palaeogene succession, together with flints at this surface, and that between 15 and 20 m of sediments derived directly from the Chalk, and from soils on its surface. present in the Ramnor Inclosure borehole (see also Edwards & This suite of clasts and their preservation state indicates that Freshney 1987) in the New Forest are absent in the southeast the entire Palaeogene succession and at least the higher part of of the Hampshire Basin. The magnitude of the hiatus increases the Chalk were actively eroding nearby on the northern limb of towards the eastern Isle of Wight, and we interpret it to have the Sandown structure in the Bartonian. formed as a consequence of renewed uplift during or shortly after deposition of the highest part of the Selsey Formation. A comparable disconformity is also found in south central Phase 2c. Becton Sand—continued uplift Hampshire at Lee, Fawley and Dibden (Kemp et al. 1979) and ff Totton (Jeffery 1996), which may be related to contem- The lower part of the Becton Sand in Whitecli Bay contains poraneous movement on the Portsdown Anticline (cf. Plint two beds in which small rounded clasts (mean diameter 1 cm) 1982). of red clay derived from the Reading Formation are abundant, which also contain rarer angular flint shards. These clay clasts are significant for a number of reasons. Such clasts are so fragile that they cannot have been transported far from the Phase 2b. Derived clasts in the Barton Clay and the eroding source—distances of 200 m maximum are likely by mid-Barton hiatus and conglomerate analogy with present-day seaside exposures of Reading clays. The lower part of the Barton Clay in Whitecliff Bay contains Secondly, clay clasts are only found in abundance where rare phosphatic concretions derived from the London Clay mudflows of clay are undergoing active erosion, as in White- (horizons indicated Fig. 7; five collected), and a few fragments cliff Bay at the present day. We therefore propose that an of Reading Formation clay and angular flints. These suggest analagous scenario existed during deposition of the Becton that uplift and erosion had continued and that the Chalk Sand conglomerates; a short distance to the south of Whitecliff and Reading Formation were exposed although they were Bay lay mudflows and low cliffsofin situ Reading clay, behind contributing very little material to the Whitecliff Bay which was a more resistant Chalk slope which contributed succession. angular shards of flint to the Reading clay mudflows (Fig. 8d). Blue-green shelly clays of the lower part of the Barton Clay Wave action eroded clasts of clay which together with the flint equivalent to Bed A3 at the Barton stratotype are overlain chips were moved offshore by storm action. The absence of abruptly by a thin (<5 cm) glauconitic coarse shelly sand chalk clasts is possibly a consequence of decalcification of the containing lenses of conglomerate comprising mostly flints, Becton Sand. herein called the mid-Barton conglomerate. The sandy matrix Because the Reading clays are so soft, it is likely that they and smaller clasts infill Thalassinoides piped into the top 10 cm would have been rapidly eroded during initial transgression, of the underlying clay. This surface is a major disconformity and would not be expected to contribute further to clast on which the entire middle part of the Barton Clay is missing. assemblages. Their abundance at levels far above that where Faunal evidence (see above) leads us to the conclusion that the first exhumation occurred affords tentative evidence of gap includes all units of the Barton Clay between the upper continued pulses of uplift during formation of the Becton part of A3 to the base of Bed H (see above and Fig. 3 caption), Sand. represented at (30 km to the east) by about 40 m of The period of uplift in the Barton Clay and Becton Sand clays and glauconitic sandy clays, and at Barton itself by 26 m. falls within nanoplankton zones NP16 to (?) NP18, represent- This non-sequence is present only in the Whitecliff Bay section ing a total period of about 5–6 Ma (Berggren et al. 1995). It is and can be attributed to contemporaneous uplift on the calculated from present thicknesses that cumulative uplift eastern Isle of Wight structure. through the Lutetian and Bartonian of a minimum of 500 m Both the lenticular sandy mid-Barton conglomerate and a was necessary to enable erosion of the highest 100 m of the thin sandy bed 1 m higher contains a diverse suite of clasts and White Chalk. However, two unknown factors make the fossils derived locally from both older Palaeogene deposits and absolute value rather uncertain. Firstly, we cannot ascertain directly from the Chalk. Most abundant are rounded flint exactly how much of the White Chalk Formation was exposed pebbles, either black, grey or white; rare large clasts weigh up at the time; this could perhaps be demonstrated by biostrati- to 5 kg. These pebbles were probably derived directly from graphical study of foraminiferan assemblages in soft flint the Chalk. Unworn glauconite-coated burrow flints and pebbles (cf. Curry 1964, 1987). Secondly, it is not known how angular flint shards formed by their fracture are also common. much sediment was deposited over the crest of the Sandown Numerous (30+ recorded) phosphatic concretions derived Pericline during the quiescent period between Phase 1 and 2. from Division B of the London Clay are also present. Molluscs The occurrence of several important local disconformities in and fragments of sideritic concretions derived from the the Barton Group, and the concentration of derived clasts in Bracklesham Group are common in the two glauconitic sand discrete thin beds may represent short, pulsed inversion events beds. Most common are worn valves (mostly the robust with a duration of less than 1 Ma each. The reactivation of hinges) of Venericor planicosta, derived either from the faults is known to be episodic and can occur on several scales. Wittering Formation (Fisher Bed IV; Fisher 1862; Plint 1984) Short duration stick–slip cycles are well known from contem- or the Earnley Formation. Fragile molluscs in the sand matrix porary earthquakes, but are probably not resolvable in the include taxa derived from the Selsey Formation. stratigraphical record. Longer term episodes (10 ka to 1 Ma) EOCENE INVERSION, ISLE OF WIGHT 337 of inversion related to the regional tectonics are more likely to be detected from the stratigraphical record. It is possible that these events are recorded in the episodic Phase 2 inversion during the Bartonian.

Continued uplift in the Late Eocene–Early Oligocene A small assemblage of derived fossils and lithoclasts was collected from 12.8 m above the base of the Bouldnor Formation (Fig. 1), Bembridge Marls Member (base bed xi of Daley 1973). This includes small unworn calcitic fossils from the White Chalk (echinoid radioles, foraminiferans, ostra- cods), Nummulites variolarius from the Selsey Formation, and rounded clasts of Bembridge Limestone. The fact that the Chalk and Bracklesham Group were undergoing erosion at this time is taken as evidence that uplift continued.

Discussion Consequences for Eocene palaeogeography Plint (1984) reconstucted the palaeogeography of the the Solent region during the time of deposition of the Bracklesham Group, and concluded that the region was occupied by a narrow east–west inlet of the sea, centred on the present Solent channel, and opening to the east. The southern margin of the inlet was formed by the rising Purbeck–Isle of Wight mono- cline, south of which were Chalk uplands. Rivers drained into the inlet from the north and west. The same general reconstruction was used by Murray (1992) in the Geological Society’s Atlas of Palaeogeography and Lithofacies. Whilst we admire Plint’s (1984) detailed palaeogeographical work in western Hampshire and Dorset, we do not think the evidence supports his wider regional reconstruction, for the following reasons. Firstly, if the Chalk was undergoing active uplift a short distance (a few hundred metres) to the south of the present Fig. 9. Palaeogeography of the Hampshire Basin during (a)the exposures in Whitecliff Bay during deposition of the lower part Early Lutetian (Earnley Formation), and (b) the Late of the Bracklesham Group, then fresh, unworn flints derived Lutetian-Bartonian (Selsey and Barton Formations). The shoreline from the footwall should occur in this part of the succession. in (a) is based on the known westernmost occurrences of Earnley Formation; the similarities of detailed succession in the London and They are only found much higher, in the Barton Clay and Hampshire Basins are taken as evidence that both basins were Becton Sand. Secondly, the succession in the Wittering connected at this time. In (b), inversion of the Purbeck–Isle of ff Formation in Whitecli Bay is the most marine and therefore Wight structure caused formation of anticlinal islands (Brixton and presumably the most distal to the shoreline in the entire Sandown) for which evidence is presented in this paper. The uplift Hampshire Basin. Additionally, the Earnley Formation in on the Weald and Portsdown axes is more conjectural. Whitecliff Bay was deposited in open marine conditions with no sign that a shoreline was situated a few hundred metres away. Thirdly, if the Isle of Wight monocline had controlled proto-Thames (Jones 1981), similar to the reconstruction sedimentation, formations of the Bracklesham Group would shown by Curry (1990, p. 395, fig. e). be expected to thin towards the southern margin of the basin, Uplift during the Late Lutetian on the Sandown Pericline which they do not do; rather, they thin north westwards. described above was probably accompanied by comparable We also have doubts about the northern margin of the inlet penecontemporaneous movement on other structures such as in Plint’s reconstruction. The succession in the ‘Bagshot Sands’ the Portsdown Anticline (Plint 1982). We therefore envisage a in the west of the London Basin is remarkably similar to that palaeogeography during the Selsey Formations of low east– developed in the Bracklesham Group of the eastern Hampshire west elongated islands formed of London Clay and Brackle- Basin, and representatives of the Wittering, Earnley and Selsey sham Group sediments, rapidly reduced by marine erosion Formations are present (C. King pers comm.; James & Ward (Fig. 9b). Continued uplift and erosion in the Bartonian lead 1976). In the Earnley Formation, formerly exposed at Yateley to exposure of the more durable chalk cores of the Brixton and (Berkshire) individual distinctive beds present in the Hamp- Sandown Periclines. The fault underlying the Brixton Pericline shire Basin can be identified. This close similarity of detailed extends westwards beneath Purbeck and it is likely that the stratigraphy makes it most unlikely that the two basins were exhumed chalk core had a comparable extent. The abundance separated, and the coastline probably ran approximately of very large (up to 10 kg), storm-beach rounded flint pebbles north–south (or NNE–SSW; Fig. 9a) through the region with in the Late Lutetian Boscombe Sand of Dorset and the basal embayments marking the estuaries of the proto-Solent and Barton Clay in Alum Bay may be evidence for erosional 338 A. S. GALE ET AL. breaching of the Brixton Pericline between Purbeck and the The often-quoted occurrence of clasts derived from the Isle of Wight. At the present day in the English Channel, Lower Cretaceous Hythe Beds in the basal Upper Bagshot formation of pebbles of this size and degree of roundness only Pebble Bed near Aldershot in west Surrey (Dines & Edmunds occurs on beaches exposed to Atlantic storms. 1929) is of considerable relevence to the Eocene tectonic history of southeast England because it affords the earliest evidence of unroofing of the Weald (e.g. Curry 1990). We have Eocene tectonics and eustasy in Whitecliff Bay examined an exposure of the pebble bed, and confirm that about 10% of the pebbles are Hythe Bed chert. The Upper Plint (1984, 1988) identified a series of transgressive events in Bagshot Beds are correlative with the Selsey Formation (C. the Eocene of the Hampshire Basin which he correlated with King pers. comm.), and it is therefore likely that the Phase 1 the global eustatic cycle chart of Haq et al. (1988). Plint’s T4 uplift also took place in the Weald. The abundance of Hythe lay at the base of the Selsey Formation, and his T5 at the base Bed clasts supports a closer provenance (perhaps the Hindhead of the Barton Clay (1988, fig. 2). Subsequently, Huggett area) than that in southeast Sussex proposed by Curry (1990). & Gale (1997) have identified two additional transgressive surfaces within the Selsey Formation, and two in the Barton Group. It is interesting to re-examine the sequence strati- graphy of the succession in the light of the tectonic history Conclusions described herein. The first major sedimentary consequence of (1) The stratigraphical sources of derived fossils and lithoclasts uplift was a facies change (base Selsey Formation), with collected in the Mid–Late Eocene Bracklesham and Barton long-term disappearance of lagoonal and estuarine facies from Groups of Whitecliff Bay are identified in detail using palae- the region. The abundance of derived clasts and fossils in ontological (taxonomic), petrographical and mineralogical transgressive deposits of the first two sequences of the Selsey criteria. Formation suggests that rising sea-level cut readily through the (2) The stratigraphical distribution of the reworked material Palaeogene succession on the rising Sandown Pericline, (Cretaceous and Eocene formations) has been used to recon- forming and eroding cliffs and wave-cut platforms in London struct the uplift history of the Sandown Pericline during the Clay. The absence of derived material in the third Selsey Mid- and Late Eocene. A total minimum uplift of 500 m took trangressive event is interpreted to be a consequence of place in two pulsed episodes with a mean uplift rate of about temporary tectonic quiescense accompanied by marine 100 m Ma"1. peneplanation. Accommodation space adjacent to the (3) It is likely that the Eocene inversion here described on northern limb of the Sandown Pericline was evidently very the Sandown Pericline also took place contemporaneously on limited throughout uplift and only thin representatives of three other structures in southern England. transgressive systems tracts of the Selsey Formation (Mid– (4) This work demonstrates that reworked lithoclasts and Late Lutetian) sequences were preserved in the Whitecliff Bay fossils are extremely valuable and under-employed tools in the succession. The relatively deep water facies of the lower Barton detailed reconstruction of structural uplift histories. Clay, deposited during Phase 2 uplift, is evidence for signifi- cant eustatic sea-level rise at this time. We therefore conclude "1 The work published here was initially stimulated by J. Cartwright that Eocene uplift at moderate rates (about 100 m Ma ) had during an Imperial College Petroleum Geology MSc field excursion a major affect on facies, reduced accommodation space, to the Isle of Wight. Cartwright’s assertions that the Palaeogene but did not affect the development of marine transgressive succession of Whitecliff Bay was associated with onlapping updip surfaces. unconformities so incensed A.S.G. that he returned to the locality in order to collect data to disprove the idea, and in so doing discovered that Cartwright was right. This paper developed from these Evidence of Eocene inversion elsewhere in southern discoveries. We would like to thank J. Cosgrove, C. King, J. Todd, England J. Hooker, R. Goldring and S. Tracey for their helpful comments. P. Ensom kindly let us describe material from the Bembridge Marls It is probable that uplift on the Sandown Pericline was which he had collected. Photographs were taken by the Photography accompanied by contemporaneous movement on other struc- Unit, NHM. P. D. Taylor identified and photographed the bryozoan tures in southeast England. Difficulties encountered in depicted in Fig. 4. C. Amphlett (P.C.U., NHM), prepared the silicon testing this are a consequence of poor exposure in both the rubber peel depicted in fig. 5c. inland Hampshire and London Basins and of precise dating in the unfossiliferous continental successions of the western part of the Hampshire Basin in particular. Plint (1982) inter- References preted 100 m of alluvial gravels containing huge flints at A, M-P., H,E.A.&T, H.A. 1988. Magnetic and Creechbarrow in Dorset to be a result of uplift on the Purbeck calcareous nannofossil stratigraphy of the lower Palaeogene formations of structure, but these cannot be dated accurately; they immedi- the Hampshire and London Basins. 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Received 7 November 1997; revised typescript accepted 28 July 1998. Scientific editing by Peter Haughton.