Eocene Inversion History of the Sandown Pericline, Isle of Wight, Southern England

Journal of the Geological Society, London, Vol. 156, 1999, pp. 327–339. Printed in Great Britain Eocene inversion history of the Sandown Pericline, Isle of Wight, southern England 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 Cretaceous 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 Chalk Group (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 Bembridge–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 Sandown Bay (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.

Load more