Geology of Kent and the Boulonnais
Historical geology of Kent and the Boulonnais
C. J. Wood, E. R. Shephard-Thorn and C. S. Harris
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(Links to: Channel tunnel facts and brief history; Detailed geology on each side of the Channel Tunnel; Chalk, the basic facts; Chalk, the White Cliffs; Landslips of East Kent; Channel tunnel, a detailed sequence stratigraphy)
The work was originally undertaken in order to assist geological understanding at both ends of the Channel Tunnel.
The work involved not only researching the surface and subsurface geology of Kent, but also that of the Boulonnais. It relied heavily on the experience of both Chris Wood, the main author and Roy Shephard-Thorn.
Rocks of the Pre-Carboniferous basement of the area, to those of Quaternary age are discussed in stratigraphic order.
This paper has been uploaded onto the www to allow anyone interested in the Geology of Kent and the Boulonnais to have free access to it. This page is currently in draft form and is still under construction. In particular a number of figures are in preparation.
Figure showing the regional geology of SE England and NW France
The Palaeozoic rocks of the Boulonnais belong to two separate tectonic units, the western extensions of the Dinant Synclinorium to the south and the Namur Synclinorium to the north, respectively. These two synclinoria are separated by a tight anticlinal structure (Condroz Zone), which formerly represented a barrier to northward transgression of the Devonian sea onto the Brabant Massif until mid-Devonian times. In the Dinant Synclinorium, Upper Silurian rocks are overlain conformably by Lower Devonian rocks. In contrast, the middle Devonian rocks of the Namur Synclinorium rest on folded Lower Palaeozoic rocks that were tectonised during the Caledonian Orogeny and overlie the Precambrian core of the Brabant Massif. In the course of the Variscan Orogeny the sediments of the two synclinoria were strongly folded and brought into juxtaposition along a major east-west low-angle thrust inclined to the south (Grande Faille du Midi). To the south of this thrust, the structural style is one of complex folds without any thrusts. Immediately adjacent to (and below) the Grande Faille du Midi, there is a zone of strongly folded and faulted rocks traversed by two major, closely spaced, low-angle thrusts, the Hydrequent Thrust and the Haut Banc Thrust. The thrust slice between the Grand Faille du Midi and the Hydrequent Thrust (Massif d'Hydrequent) comprises an overturned, tightly folded syncline of Devonian and Carboniferous rocks. The basal Hydrequent thrust brings Fammenian rocks into juxtaposition with folded Carboniferous rocks of the underlying thrust slice (Massif de Haut-Banc), which are in turn brought into juxtaposition with Coal Measures of the autochthonous Massif de Ferques by the Haut-Banc Thrust. The autochthonous succession (Middle Devonian to Westphalian Coal Measures) rests unconformably on folded Upper Silurian shales and is truncated to the north by the subvertical Landrethun Fault (Pruvost, 1992b) (**Fig.). The conventional view is that the Dinant Synclinorium has been thrust over the Namur Synclinorium and the underlying Brabant massif along the Condroz line, which is now expressed by the Grande Faille du Midi and its associated thrusts. The Grande Faille du Midi is a major seismic reflector and is commonly regarded as the Variscan (deformation) Front, i.e. the northern limit of transpressive movement of the mobile belt from the south against the foreland (cf. Lefort and Max, 1992, Fig. 2). This conventional interpretation has been challenged by Bouroz (1989), who considered that the fact that the Namur Synclinorium exhibited a greater degree of tectonism than the Dinant Synclinorium could only be interpreted by inferring a southward subduction of the Namur Synclinorium beneath the Dinant Synclinorium.
The extensive drilling programme in recent years has provided a detailed picture of the pre-Permian Palaeozoic floor, revealing an area of relatively simple anticlines and synclines to the north of the zone of low-angle thrusts traversing the Ferques inlier (Wallace, 1983, Fig. 11.3). Detailed palaeogeographical maps and analyses of the sedimentary history of northern France were produced by Colbeaux et al (1977). Figure ** (modified from insets to BGS 1:250 000 series maps) depicts the pre-Permian Palaeozoic floor for the entire area under review.
A recurring theme in this section of the Web Site will be the structural control exercised on the Mesozoic and Palaeogene rocks by the largely concealed Palaeozoic rocks of the London-Brabant Massif. Apart from the inlier of Palaeozoic rocks centred on Ferques in the Boulonnais, our knowledge of these rocks and some of the Mesozoic rocks that overlie them has come from boreholes and shafts sunk in connection with mining for coal and, more recently, from seismic surveys and boreholes in connection with the search for hydrocarbons offshore and the Channel Tunnel itself.
Geological section across the Ferques Palaeozoic Inlier
A contour map of depths in kilometres below sea level (Cameron et al, 1992, Fig. 11) to the top of the Lower Palaeozoic in southeast England reveals a small, relatively shallow, NW-SE basinal structure outlining the area occupied by the Carboniferous rocks of the Kent Coalfield. Here the base of the Carboniferous is as much as 2 km below the surface. The basin is bounded to the west and northwest by a topographic 'saddle' between 0.5 and 1 km down. Both these structures are aligned approximately parallel to a southeasterly extension of the conjectural edge of the Midlands microcraton (Chadwick, 1989, Fig. 9). To the west of the saddle, the top of the Lower Palaeozoic presents a gently inclined, irregular plane sloping down to depths of over 3.5 km. In this area, deep boreholes have proved Middle and Upper Devonian marine sediments overlying Lower Palaeozoic rocks. Within and immediately adjacent to the Kent Coalfield Basin, continental Middle Devonian rocks in Old Red Sandstone facies are present beneath the Carboniferous cover. On the crest of the saddle, Devonian rocks are apparently not preserved, and the top of the Palaeozoic floor is formed of Silurian or Ordovician rocks. To the north of the basin, undated Old Red Sandstone facies sediments as well as fossiliferous marine Devonian shales have been proved in deep boreholes. In east Kent, there is no borehole or seismic evidence for the existence of pre-Ordovician rocks. However, shallow marine and basinal Cambrian to Lower Ordovician sediments are more than 300 m thick on the southern flank of the London-Brabant Massif in Belgium (Walter, 1980) and may be similarly thick at the base of the Lower Palaeozoic sequence beneath the Dover Strait (Cameron, 1992). From the limited borehole evidence, it is inferred that east Kent and the Boulonnais occupied an area of shelf to intermediate depth seas situated to the east of a landmass of low relief during the Ordovician and Silurian. During the early Devonian (near the Pridoli-Lochkovian boundary), east Kent is assumed to have lain within a marine area on the basis of fossiliferous limestones, that are possibly of this age, in the Little Missenden borehole. No Lochkovian sediments are known from the Ferques inlier in the Boulonnais, but variegated red and green schists of this age have been proved in deep boreholes adjacent to the coast. Later in the Devonian, the position of east Kent oscillated between the littoral zone and shelf sea, with the North Downs-Thanet area representing the edge of the landmass to the north (Cope et al, 1992).
In the Bobbing borehole [TQ 874 652], Middle Jurassic (Great Oolite) rests directly on marine mudstones with a macrofauna of late Ordovician aspect and assigned a Caradocian age by acritarchs (Aster et al, 1969). Probable Wenlock neritic sediments containing an atrypoid and Eoplectodonta were proved in the old Cliffe borehole [TQ 719 785] (Cocks et al, 1971) and deeper water shales were found to the southeast, in the Chilham and Brabourne boreholes. In the Chilham borehole [TR 088 545], Upper Llandovery (Telychian) shales dipping at 80 degrees and yielding pyritised graptolites indicative of the Monograptus crispus Biozone were proved beneath Lias. In the Brabourne borehole [TR 077 423], red marls of possible earliest Jurassic age overlie an undated 9.7 m conglomerate that is of Triassic Dolomitic Conglomerate aspect, but probably not of that age. This conglomerate rests on steeply dipping shales and mudstones dated by acritarchs and chitinozoa to the late Llandovery or early Wenlock (Molyneux, 1991).
Evidence for the former existence of Upper Silurian sediments in east Kent is provided by a pebble of crinoidal limestone with a shelly fauna of probable Ludlow age in a conglomerate with a red sandstone matrix from a borehole near Chislet; the conglomerate is presumed to be of post-Ludlow age, possibly latest Silurian (Downtonian, i.e. Pridoli) or earliest Devonian (Dittonian, i.e. Lochkovian) in continental Old Red Sandstone facies (Holmes, 1981). In the Boulonnais, folded early Ludlow (Gorstian) graptolitic shales with Monograptus colanus have been proved beneath unconformable Middle Devonian (Givetian) sediments in the Ferques inlier, but at Caffiers these shales are directly overlain by the Gault (Pruvost, 1992b; Pruvost and Pringle, 1924). The Devonian sediments here belong to the Namur Synclinorium. To the south of the Grand Faille du Midi, i.e. on the northern flank of the Dinant Synclinorium, the Wirwignes borehole proved even younger Silurian rocks, of presumed Ludfordian age, with Dayia navicula. Farther south, the Samer Borehole was reported (Wallace, 1983) to prove early Devonian (Gedinnian, i.e. Lochkovian) marine strata overlying Upper Ludlow, but with no mention of intervening beds of Pridoli age.
Red and green mottled sandstones, conglomerates and siltstones of Old Red Sandstone facies were proved beneath Carboniferous Limestone in the Harmansole borehole [TR 142 529], just to the west of the coalfield and also in three boreholes near Chislet at the northern limit of the coalfield (Holmes, 1981). On palynological and plant macrofossil evidence, the rocks at Harmansole are of Givetian to possibly even earliest Frasnian age (Mortimer and Chaloner, 1972); they also yielded estherians and crossopterygian fish scales of either mid-or late Devonian age. An Old Red Sandstone facies conglomerate in one of the Chislet boreholes yielded the pebble of Ludlow limestone noted earlier. To the west of our area, fossiliferous shelf sea limestones with corals were cored in the Esso Tatsfield and Bletchingley boreholes and are actually exposed on the other side of the Channel in the Ferques inlier (Calcaire de Blacourt). Despite the proximity of the two areas, the Devonian succession in the Boulonnais at this time was predominantly marine, rather than continental as seen in the Harmansole borehole. In the late Devonian, with the extension of the sea to the north, east Kent is inferred (Cope ci at., 1992), to have lain partly in the littoral zone and partly in an area of shelf sea which extended eastwards to the Boulonnais, where shelf limestones with coral-brachiopod faunas (Calcaire de Ferques) were deposited. This interpretation is supported by the record of Frasnian marine shales with brachiopods including Cyrtospirifer verneuili from a borehole near Chislet (Holmes, 1981).
Although there is no direct evidence from boreholes, geophysical modelling (Shephard-Thorn et al., 1972; Shephard-Thorn, 1988) suggests that considerable thicknesses of Devonian rocks are probably present beneath the Carboniferous of the Kent Coalfield and its extension beneath the Dover Strait.
The Devonian succession in the Ferques inlier in the Boulonnais (circa 700 m) rests with marked discordance and a basal conglomerate on folded Silurian strata and belongs to the Namur Basin. The sequence here begins in the Middle Devonian (Givetian) with clastics containing plant debris, which are followed by shallow-water platform carbonates; earlier Devonian (Lochkovian) sediments belonging to the Dinant Basin are known from deep boreholes to the south of the Grande Faille du Midi, e.g. at Samer. The Ferques inlier succession is divided into six formations and is of international importance for it's abundant, diverse and well-preserved coral and brachiopod faunas. Brice (1985) gave a useful overview with a comprehensive bibliography and Wallace (1969, 1970) described the sedimentology and palaeoecology.
In earliest Carboniferous times, lagoonal conditions occurred in east Kent. With rising sea level, alternations of shales and thin crinoidal marine limestones were deposited. During the later Tournaisian and Visean, a persistent landmass extending from Wales eastwards into Belgium, the St. George's Land to London-Brabant High, dominated the palaeogeographical and depositional setting for the first time. To the south of this landmass, marginal shallow water platform carbonates with rich coral-brachiopod faunas were developed. For most of this period, the southern limit of the landmass oscillated backwards and forwards across east Kent, but during the late Visean (Brigantian) sea level rise it may have been situated temporarily far to the north in the area of East Anglia (Cope et al, 1992).
There is no evidence for Namurian sediments in east Kent: It is assumed that any marine deposits from the shelf seas fringing the landmass have subsequently been lost by erosion and that the greater part of east Kent formed part of the landmass itself at this time. Namurian rocks are also unlikely to occur beneath the Dover Strait.
In Westphalian times, east Kent and the Boulonnais occupied a narrow east-west paralic basin between the London-Brabant High to the north and the Variscan mountain range to the south. During the earlier part of the Westphalian, coal-swamp conditions prevailed in the area, with periodic short-lived marine incursions being registered by thin marine bands. The sediments deposited are predominantly argillaceous, with subordinate sandstones and numerous coal seams. Following uplift of the London-Brabant High, the former coal-swamp environment became increasingly dominated by the sedimentation of clastics resulting from the increased erosion of the uplands.
Within the Kent Coalfield basin, about 300 m of Tournaisian and Viséan Carboniferous Limestone are probably present beneath Westphalian Coal Measures, Namurian rocks being absent. Offshore seismic sections show that the limestones continue beneath the Dover Strait and southeast of Dover may be as much as 900 m thick (Cameron et al, 1992).
The Carboniferous Limestone of east Kent boreholes can he subdivided into a thin basal 'Lower Limestone Shale' division, comprising dark shales with thin limestones including crinoidal limestones; and an overlying 'Main Limestone', mostly massive crystalline limestones including some oolites. There are no boreholes penetrating the full thickness of the Carboniferous Limestone. The maximum known thickness is 135 m in the Trapham Borehole near the western margin of the coalfield, but it is probable that at least 300 m may be present beneath the central part of the coalfield. In 14 out of 31 boreholes, fossiliferous horizons in the higher part of the Limestone can be assigned on coral-brachiopod faunas (notably the occurrence of Linoprotonia corrugatohemisphaerica) to the Holkerian Stage of the Viséan (Mitchell, 1981), there being no evidence anywhere for the overlying Asbian and Brigantian stages. Rocks belonging to both these stages are assumed to have been deposited, but subsequently removed in the period of uplift and erosion during Namurian times. Tournaisian 'Lower Limestone Shale' marine sediments, rocks with Courceyan brachiopod-bivalve faunas have been proved some distance beneath the Holkerian in the Trapham Borehole (Mitchell, 1981) with no palaeontological evidence here or elsewhere in Kent for the intervening Chadian and Arundian stages, which may be thin or not represented (George et al, 1976). Even earlier Courceyan shales and limestones with bivalves indicative of a lagoonal environment are present in the 8 m of Dinantian 'Lower Limestone Shales' proved beneath Lias in the Harmansole Borehole.
In the Ferques inlier in the Boulonnais, the transition from the Devonian to the Carboniferous occurs in the tectonised Calcschistes de la Vallée Heureuse. The Carboniferous Limestone is represented by up to 200 m of Tournaisian-early Visean dolomites, rich in large crinoids at the base (Formation du Hure), overlain by at least 270 m of Viséan limestones divided into six formations (Colbeaux et al, 1985). Some of these limestones take a high polish and have been exploited for centuries in huge quarries as ornamental stone, incorrectly described as marble (Marbre), of which the most famous is perhaps the 'Marbre Napoleon'.
In the Boulonnais, the equivalent of the Carboniferous Limestone is followed by the Formation des Plaines, a succession comprising sandstones with brachiopods overlain by predominantly clastic sediments with thin coals that range in age (Colbeaux et al, 1985) from early Namurian to early Westphalian (Langsettian).
Following the prediction by Godwin Austen (1856) that coal would be found beneath the Mesozoic rocks of the Weald, Coal Measures rocks were finally proved in 1890 at -340 m OD in a borehole near Shakespeare Cliff (southwest of Dover, on the site of the abortive first Channel Tunnel workings) and at the Dover or Shakespeare Colliery (and the recent Channel Tunnel working site). Later exploration in connection with the development of the Kent Coalfield established that Westphalian Coal Measures non-sequentially overlying Carboniferous Limestone were preserved in an elongated WNW-ESE synclinal basin (Fig.**). This basin is clearly reflected in the Bouguer anomaly trends (Shephard-Thorn et al, 1972, Fig. 1).
The eroded upper surface of the Coal Measures slopes in a southwesterly direction from -244 m OD near Deal to depths greater than -396 m OD at Folkestone (Fig. **). The onshore limits of the basin are ill-defined and are possibly in part fault-controlled, notably on the western margin, where there is a rapid rise in the Carboniferous Limestone surface from about -915 m OD in the Bishopsbourne Borehole within the coalfield to about -320 m OD in the Harmansole Borehole, which is situated just outside the coalfield and where the contact between the Carboniferous Limestone and the Devonian is considered to be faulted. There is some evidence that the northern boundary of the coalfield is also fault-bounded (Shephard-Thorn, 1988).
Offshore, the Coal Measures are believed to extend at depth across the Strait of Dover. Borehole evidence suggests that the Kent Coalfield Basin may comprise two subsidiary troughs separated by an anticlinal structure to the southwest of the Snowdown and Tilmanstone collieries. In the northern trough, the St. Margaret's Bay Borehole penetrated 860 m of Coal Measures (Bisson et al., 1967), terminating an estimated 20 m above their base at about -1160 m OD. An even greater thickness (up to 968 m) of Coal Measures is inferred to be present in the southern trough.
The Coal Measures everywhere rest non-sequentially on eroded Carboniferous Limestone, Namurian rocks being absent. The contours on the upper surface of the Carboniferous Limestone broadly parallel those of one of the main coal seams (the Kent No. 6 seam). The surface of unconformity as seen in cores is sharp and there is little sign of deep weathering of the underlying limestones despite the major hiatus. However, in some cores the top of the limestone is deeply fissured with infills of arenaceous and pyritous Langsettian (Westphalian A) sediments extending several metres below the surface.
The Coal Measures of the Kent Coalfield are divided into two parts (for an outline coverage see Shephard-Thorn, 1988 and references therein). The Lower Westphalian Shale Division (213 m) at the base comprises mudstones with subordinate sandstones and includes 8 main coal seams and at least four marine horizons. It is overlain by the Upper Westphalian Sandstone Division (670 m), which comprises a lower coal-bearing succession with sandstones, thick mudstones and six main coal seams, of which the lowest (Kent No. 6) was formerly of considerable economic importance; and an upper succession with few workable coals in which thick sandstones form over 70 per cent of the lithologies. Faunal and floral evidence shows that the Shale Division belongs predominantly to the Langsettian and Duckmantian stages, although the earliest Langsettian is not represented; at the top there is a thin unit of Bolsovian measures. The Vanderbeckei and Aegiranum marine bands at the base of the Duckmantian and Bolsovian respectively contain some the richest marine faunas to be found at these levels anywhere in Britain. The Sandstone Division is entirely of Westphalian C and D age and there is no evidence for the existence of sediments of Stephanian or younger age.
Although a maximum of less than 900 m of Westphalian sediments has been proved in the Kent Coalfield, the geochemical maturity of the coals indicates that a considerable thickness of younger strata, probably of Carboniferous age, was deposited but removed prior to the Jurassic transgression (Hamblin et al., 1992). Seismic profiles suggest the existence of up to 1600 m of Westphalian sediments in the offshore extension of the coalfield beneath the Dover Strait, implying the possibility of greater thicknesses of coal-bearing strata than in the onshore part (Cameron et al., 1992).
In the Boulonnais, the coal basin is fragmented into several components (Becq-Giraudon et al., 1981) by normal faults trending N30 degrees and, in marked contrast to the Kent Coalfield, it is affected by thrusting associated with the Variscan deformation front. To the west, a small coalfield is preserved overlying Carboniferous Limestone in a shallow syncline at Strouanne on the coast northeast of Cap Gris-Nez (Wallace 1983, Fig. 11.3) and the presence of Coal Measures in a syncline beneath Calais was suggested by Godwin Austen (1856). The juxtaposition of Carboniferous Limestone on Carboniferous Limestone beneath the Haut-Banc Thrust locally preserves thin wedges of Coal Measures in the western part of the Ferques inlier and, farther to the east, thin coals of early Westphalian age were mined until 1936 in the Hardinghen Coalfield (Colbeaux et al., 1985; Leplat and Colbeaux, 1985).
At the close of the Carboniferous, the Variscan orogenic phase of folding, faulting and uplift resulted in the temporary retreat of the sea from the entire area of Britain, much of which became a desert landscape. Throughout the Permian, southern Britain remained land under continuing desert conditions and was unaffected by the later Permian (Zechstein) marine incursions to the north. No Permian or Triassic deposits are known from east Kent or the Boulonnais.
The Mesozoic sediments of east Kent and their continuation eastwards to the Boulonnais were deposited at the eastern end of an elongate, structurally controlled east-west Weald basin within the larger Wessex-Channel Basin. The Weald Basin was delimited from the remainder of the Wessex-Channel Basin by a WNW-ESE positive area, the Hampshire-Dieppe High (Hamblin et al., 1992, Fig. l4a), and was margined to the north by an intermittently emergent positive area of Palaeozoic rocks called the London Platform, which separated the Weald Basin from the North Sea Basin. The edge of the platform is marked by a zone of closely spaced normal faults of east-west (Variscan) trend throwing down progressively to the south (Chadwick, 1985, 1986). Mesozoic depositional history as known from the Kent Coalfield borings and the outcrops in the Boulonnais records repeated transgressions onto and regressions from this ancient massif (Lamplugh et al., 1923) as illustrated by Figs ***. These phases of coastal onlap and offlap were determined partly by tectonic uplift and subsidence, resulting from the periodic reactivation of deep-seated Variscan and preVariscan basement structures and partly by episodes of eustatic sea level change. The relationship in the Weald between faulting and folding in the Mesozoic cover and faults in the basement was discussed by Shephard-Thorn et al., (1972), Lake (1975) and Mortimore and Pomerol (1991), all of whom drew particular attention to the fact that the main structural lineations represent continuations of the main shear zones in northern France.
Within the depocentre of the subsiding Weald basin, situated well to the west of the area under review (for isopachytes of individual rock units see Sellwood et al, 1986), up to 3000 m of Jurassic and early Cretaceous sediments accumulated. The Wealden succeeds the Portland-Purbeck succession conformably in the Weald Basin but, towards the basin margin, it progressively onlaps the upturned underlying Jurassic strata to rest on the Palaeozoic floor in east Kent. The Jurassic-Lower Cretaceous basin fill was eroded and truncated in an intra-Aptian uplift phase (for cross-sections see Ruffell, 1992, Fig. 5), following which renewed transgression resulted in the deposition of the higher part of the Lower Greensand, followed by the Gault and Chalk. Minor basin inversion phases in the late Cretaceous, notably in Coniacian to early Campanian times (the so-called sub-Hercynian tectonic phases), controlled Chalk deposition to a not insignificant extent (Mortimore and Pomerol, 1991). A more important phase of inversion is seen in the folding, uplift and erosion that followed the end-Cretaceous period of regression and eustatic sea level fall. The highest beds of the Chalk were stripped off and the Chalk is everywhere overlain with considerable unconformity by early Tertiary sediments. However, the major inversion of the Weald Basin took place later in the Tertiary during the main (Miocene) Alpine orogenic phase.
Map showing the northern limits of sedimentation during the Jurassic and early Cretaceous
Mesozoic depositional history in the Boulonnais largely mirrors that in east Kent, but it was also strongly controlled by a mosaic of tectonic blocks delimited by N110 and N30-50 trending faults initiated at the close of or after the Variscan Orogeny. The longitudinal faults (N110) are dextral tear faults; movement of these faults in the Cretaceous resulted in anticlinal and synclinal folding with the same axial trend (Robaszynski and Amédro, 1986). Upper Cretaceous strata both onshore and offshore rest with slight angular discordance on the underlying strata in consequence of this folding phase. Some of these faults remained intermittently active until Quaternary times; differential synsedimentary movement of tectonic blocks had a profound effect on the sedimentation of the Chalk and correlative deposits (Mortimore and Pomerol, 1987). The most important of these tear faults is the arcuate Zone de Cisaillement (shear zone) Nord-Artois, which is situated approximately parallel to, and a little to the north of, the Grande Faille du Midi (Colbeaux et al, 1977, Fig. 1). This major shear zone forms a suture between the Ardennes and Brabant blocks and was probably initiated as early as the Cambrian over an inhomogeneity in the crust. It is marked by significant seismic activity and is expressed on both aeromagnetic and Bouguer anomaly maps. French geologists consider that this shear zone extends westwards beneath the Channel to Dungeness. Bergerat and Vandycke (1994) have documented palaeostress fields in the Cretaceous of east Kent and the Boulonnais based on an analysis of fracture patterns and fault-slip data. They have identified six successive tectonic events, ranging in age from Cretaceous to Recent. Strike-slip movement of early Cenomanian age may possibly be related to dextral movement of the Nord-Artois Shear Zone induced by an early sub-Hercynian tectonic phase.
Geological section across the former Kent Coalfield
During the Triassic, continued uplift of the Variscan mountain chains led to the re-establishment of the London-Brabant high, with its southern limit now permanently constrained by the final Variscan deformation front, which stretched from Belgium, via the Boulonnais and east Kent to the northern part of the Weald. Crustal extension in latest Triassic times followed by thermal relaxation led to the initiation of the Wessex-Channel structural/depositional basin and the beginning of a marine transgression from the west (Sellwood et al., 1986, Fig. 4). In the early Jurassic, the advancing Lias sea progressively inundated the western part of the Wessex-Channel Basin, but fully marine conditions were not established in the eastern part of the basin until later.
In the Brabourne Borehole [TR 077 423], (Pliensbachian) Lower Lias overlies undated red, purple and green marls above a 9.7 m thick conglomerate comprising pebbles of limestone, quartzite, calcareous sandstone and chert in a marl matrix. Although these much-discussed red-beds are of Triassic aspect (Lamplugh and Kitchin, 1911), the most recent view is that they are likely to be of earliest Jurassic age and comparable with the sub-Lias red-beds in the Ashdown No. 2 Borehole (Bristow and Bailey, 1972) to the west (Dr I. F. Penn: pers. comm., 1994). Similar undated red mudstones resting on Carboniferous Limestone were found beneath a conglomeratic calcareous sandstone of Rhaeto-Liassic age in the Warlingham Borehole (Worssam and Ivimey-Cook, 1971).
In the Boulonnais, relicts of Rhaeto-Lias continental clays and sands with plants are preserved in karst pockets in the Carboniferous limestones in the Ferques inlier (Corsin, 1951). Red marls, sandstones and conglomerates below Lias in the Framzelle Borehole (Pruvost, 1922a) near Cap Gris-Nez have been compared with the sub-Lias strata at Brabourne and are variously cited as Devonian, Triassic or Rhaeto-Lias. In the Boulogne Borehole, thin Pliensbachian marine sandstones rested discordantly on continental sediments, including sandstones and clays with lignite and plant debris, of inferred terminal Triassic or 'Infraliassic' age (Bonte, 1974).
In the depocentre of the Wealden Basin, the Ashdown No. 2 Borehole proved 380 m of Lias, with relatively nearshore facies at the base, indicating that open-shelf conditions had not yet been fully established (Bristow and Bazley, 1972; Hamblin et al, 1992). At Brabourne, the sandy base of the Lias is of Pliensbachian age. Farther to the east, the Lower Lias rests on Carboniferous Limestone at Harmansole, on Coal Measures within the area of the Kent Coalfield and, outside the coalfield, on the Lower Palaeozoic floor (Silurian) in the Chilham Borehole. The Lower Lias thins rapidly northeastwards onto the London Platform, and is overlapped by the Middle and Upper Lias, which themselves thin in the same direction: there is no evidence for overstep of Middle or even Upper Lias on to the Palaeozoic. In the northeast of the area, the Lias is absent due to erosion and is overstepped by younger Jurassic strata (Fig. **). The thickest development in the area under review is the 42.6 m recorded in the Brabourne Borehole, subdivided by Lamplugh and Kitchin (1911) into Lower Lias (24 m), Middle Lias (13.7 m) and Upper Lias (4.6 m). The Dover Colliery shaft proved 11.57 m of Lias, comprising Lower Lias (4.87 m). Middle Lias (4.87 m) and Upper Lias (1.83 m).
The Lower Lias is typically developed as grey and locally black fossiliferous clays with ammonites indicating the Prodactylioceras davoei Zone; lower beds at Brabourne have been tentatively attributed to the basal Pliensbachian Uptonia jamesoni Zone. The 1.2 m of Lower Lias in the Chilton Borehole are predominantly limestones with a basal bed containing compound corals (Montlivaltia). In several boreholes, e.g. those at Folkestone, Elham and Adisham, the base of the Lias contains small, well-rounded pebbles of Coal Measures sandstones and appears to be infilling minor irregularities in the underlying surface. In all the boreholes to the east of Brabourne, the Hettangian and Sinemurian stages of the Lower Lias are absent and the base of the Lias records the major early Pliensbachian transgression.
The Middle Lias (Upper Pliensbachian) in the east Kent boreholes can be broadly divided into a thin, relatively arenaceous or shaly unit overlain by a much thicker unit with persistent limestones. As the succession thins eastwards, limestones become predominant. In the Chilton shaft, the base of the Middle Lias is marked by a bed of pebbles of quartz grit, perhaps indicating short-lived uplift and erosion of the Palaeozoic landmass. Although Lamplugh et al (1923) inferred (in the absence of ammonites) that the two units corresponded to the Amaltheus margaritatus and Pleuroceras spinatum zones of the Upper Pliensbachian respectively, rhynchonellid brachiopod evidence (Ager, 1954) indicates that only the margaritatus Zone is present and that the top of the Middle Lias is missing due to erosion.
The Upper Lias (Toarcian) is represented in the east Kent boreholes by a succession comprising sandy limestones of the Dactylioceras tenuicostatum Zone overlain by clays and shales of the Harpoceras falciferum and Hildoceras bifrons zones. However, in the Brabourne Borehole, blue shales with ammonites indicative of the bifrons and Grammoceras thouarsense zones rest non-sequentially on green sandy Middle Lias limestones. At Dover Colliery, there is ammonite evidence for the tenuicostatum Zone and the lower part of the falciferum Zone; the overlying 9 m were not cored and cannot consequently be dated.
In the Boulonnais, the fully cored Boulogne Borehole proved, beneath the basal Bajocian conglomerate, a Lias succession comprising 13.10 m of Aalenian and Toarcian clays, marls and many limestones with Dactylioceras tenuicostatum at the base, resting on sandstones (0.8 m) with probable Pliensbachian bivalves (Bonte, 1974; Bonte et al, 1985). In other boreholes near the coast, the basal part of the Lias includes beds of ferruginous ooliths: in the Framzelle Borehole (Pruvost, 1922a), the lower of two such beds included rolled crinoids and rhynchonellid brachiopods. These horizons have been correlated with the Lotharingian (Upper Sinemurian) transgression in the Ardennes. Thin relicts of Lias with phosphates and conglomerates are also preserved in karst pockets in the surface of Carboniferous limestones in the Ferques inlier.
The top of the Upper Lias is missing in the east Kent boreholes, as are the Lower and Middle subdivisions of the Inferior Oolite. This is perhaps the result of the mid-Bajocian regression and erosive phase. The succeeding major late Bajocian transgression is recorded in the Dover Colliery shaft by 8 m of shelly sands with a basal conglomerate of small phosphatic nodules and pebbles of vein-quartz, which rest on eroded Upper Lias. No ammonites were found, but a diverse bivalve fauna including Trigonia suggests that the beds belong to the Upper Inferior Oolite, equating with the Stenoceras garantiana Zone, Upper Trigonia Grit and possibly extending up into the topmost Bajocian Parkinsonia parkinsoni Zone (Cope et al, 1980). Oolitic limestones resting on Upper Lias in the Brabourne Borehole have also been tentatively assigned to the Inferior Oolite on lithological grounds, but without any palaeontological support. To the northeast of Dover, the Inferior Oolite is overstepped by the Great Oolite.
Following the procedure adopted by the BGS for the English Channel (Hamblin et al, 1992), the Great Oolite Group in the east Kent boreholes is here broadly subdivided into the Fullers Earth, Great Oolite, Forest Marble and Cornbrash formations. However, the Kent succession has considerable palaeontological and lithological similarity to that developed to the west of Oxford, notably in the occurrence of rocks that may equate with the sandy facies of the Chipping Norton Formation, the Hampen Marly Formation and the White Limestone Formation (Lamplugh et al, 1923; Arkell, 1933; Cope et al, 1980).
The Great Oolite in our area exhibits what Arkell (1933) called 'the most important transgression of Jurassic times against the southern border of the London landmass.' The group overlaps the sandy Upper Inferior Oolite seen at Dover and (questionably) at Brabourne, overstepping progressively northeastwards on to Upper Lias, Middle Lias, Coal Measures and, finally, on to the pre-Carboniferous Palaeozoic floor (Ordovician) at Bobbing (Fig. **). There is also overlap of lower by higher beds of the Great Oolite Group. In some of the colliery shafts, the base was marked by a pebble bed composed of rocks eroded from the underlying strata.
In the Brabourne Borehole, the (inferred) Inferior Oolite is overlain by calcareous shales, with intercalated thin brecciated limestones towards the base. The only fossils were poorly preserved bivalves of no biostratigraphical value. These beds have been tentatively assigned broadly to the Fullers Earth (Lamplugh and Kitchin, 1911; Arkell, 1933). However, to the north and east of Brabourne in the direction of the shoreline, the base of the Great Oolite Group is marked by sands and sandy limestones (e.g. Dover Colliery shaft), or muddy limestones with thin beds of sandy limestone: these arenaceous beds were correlated by Arkell (1933) with the sandy facies of the Chipping Norton Limestone (Formation) of the Oxfordshire succession, i.e. (Cope et al, 1980) the basal Bathonian Zigzagiceras zigzag Zone.
The Great Oolite Formation comprises massive bedded, generally fine-grained, cream or grey oolitic limestones with sporadic intercalations of clay or marl. In the Dover Colliery shaft, 10.97 m were proved overlying 10.05 m of the basal sands and sandy limestones.
At Tilmanstone, shelly clays with abundant Praeexogyra hebridica beneath the main limestones were correlated by Arkell (1933) with the Hampen Marly Beds (Formation) of Oxfordshire.
The greatest thickness of the Forest Marble Formation in the area is the 5.48 m of bluish grey and pale green, poorly fossiliferous calcareous claystones with lignite overlain by yellowish oolitic limestone proved in the Dover Colliery shaft. In the thinner successions to the northeast, e.g. Tilmanstone (2 m), the Forest Marble is represented by greenish-black shelly and lignitiferous clays beneath limestones. Arkell (1933) suggested that, since there was no palaeontological evidence to show that the upper part of the Forest Marble was present in the Kent boreholes, there must be a significant non-sequence between the Forest Marble and the overlying Cornbrash. However, as the faunas at these levels are largely facies controlled, recognition of such a non-sequence on this basis should not be taken as conclusive (Dr. B. M. Cox: pers. comm.).
In the Boulonnais, Rhaetian continental sediments preserved in karst pockets in the Palaeozoic floor in the Ferques inlier are overlain locally by yellowish argillaceous sands, the Sables d'Hydrequent. These sands have been attributed by French authors to the Bajocian, which might suggest a correlation with the (Upper) Bajocian shelly grits in the Dover Colliery shaft. However, as noted by Ager and Wallace (1966), there is no palaeontological evidence for this interpretation. The only fossils known from this unit are poorly preserved moulds of bivalves near the top, which (by inference from Pringle, in Pruvost and Pringle, 1924) permit correlation with the (Lower Bathonian) sandy facies near the base of the Great Oolite Group.
These sands are overlain by marly limestones and marls, the Marnes d'Hydrequent, which contain oyster lumachelles composed of Praeexogyra hebridica and a diverse fauna including echinoids (Acrosalenia, Clypeus, Nucleolites) and brachiopods including Epithyris oxonica and Kallirhynchia concinna. These marls compare with the oyster-rich shelly clays at Tilmanstone. The marls pass up into limestones, the Calcaires de Rinxent, which equate with the lower part of the Great Oolite Limestone Formation. Any one of these three units may overstep the underlying Mesozoic strata to rest locally directly on the Palaeozoic floor. The Calcaires de Rinxent comprise finely bioclastic and slightly sandy limestones, passing up into cross-bedded and oolitic bioclastic limestones. The unit contains numerous hardgrounds and terminates in a hardground. The rich fauna is dominated by rhynchonellids (notably Kallirhynchia concinna), terebratulids and echinoids, with solitary and colonial corals occurring locally at the top. A pteridophyte flora (Lomatopteris, Otozamites and Pterophyllum) has also been documented. The record of Procerites (Gracilisphinctes) cf. laeviplex and the nautiloid Procymatoceras subcontractum places these limestones unequivocally in the Lower Bathonian (Bonte et al, 1985).
The overlying Oolithe de Marquise consist of bioclastic and oolitic limestones, commonly cross-bedded and terminating in a hardground. They have yielded no cephalopods, but the occurrence of common Burmirhynchia hopkinsi in the central portion indicates the Middle Bathonian and permits correlation with the White Limestone Formation in the Bath district.
The Forest Marble Formation is represented in the Boulonnais by the Marnes des Calhaudes (2.5 m), which comprise finely bioclastic limestones, bluish-grey to black marls and argillaceous limestones. The fauna is characterised by the rhynchonellids Burmirhynchia elegantula and Kallirhynchia yaxleyensis, associated with echinoids such as Acrosalenia lamarcki.
The Abbotsbury Cornbrash Formation in the east Kent boreholes comprises a variable impure limestone with a rich brachiopod, bivalve and echinoid fauna. The thickest and biostratigraphically most useful section is provided by the Tilmanstone Colliery Shaft in which the Cornbrash is a little over 7 m thick. Brachiopod records from Tilmanstone can be used to infer that both the Lower Cornbrash (uppermost Bathonian, Clydoniceras discus Zone) and the Upper Cornbrash (lowermost Callovian, Macrocephalites herveyi Zone) are represented (Callomon, 1955), although the admixture of species suggests possible reworking (Arkell, 1933). Macrocephalites cf. herveyi was recorded from the uppermost 2 in of the Cornbrash and some distance above Microthyridina lagenalis, the index of the youngest Cornbrash brachiopod Zone (Callomon, 1955). Macrocephalites was also recorded from the top of the Cornbrash in the Bobbing Borehole (Lamplugh et al, 1923) and from an unspecified level in the Cornbrash of the Guilford Colliery Shaft (Callomon, 1955). In the Dover Colliery shaft, 3.96 m of 'bluish grey streaky sandy clay, masses of indurated calcareous rock with soft brown nodules, and markings like borings' were tentatively assigned to the Cornbrash by Lamplugh and Kitchin (1911). The list of bivalves from these beds is more suggestive of Lower than Upper Cornbrash (Dr. B. M. Cox: pers. comm.), but a record by Callomon (1955) of a Macrocephalites from Dover in the Sedgwick Museum suggests that both subdivisions are likely to be present.
In the Boulonnais, the Cornbrash is represented by 1 m of irregularly bedded oolitic ragstone (Calcaires des Pichottes): both the uppermost Bathonian discus Zone and basal Callovian herveyi Zone have been recognised by the characteristic ammonites and brachiopods.
The Cornbrash in the east Kent boreholes is overlain by up to 2 m of clay with a distinctive bivalve fauna including Meleagrinella braamburiensis. This argillaceous unit has been traditionally referred to as Kellaways Clay but, following Page (1989), it should strictly be called the Cayton Clay (Dr B. M. Cox, pers. comm.).
The main mass of the Kellaways Formation comprises the Kellaways Sand Member, the Kellaways Rock of previous literature on the east Kent boreholes. Isopachytes for the formation (Sellwood et al, 1986, Fig. 11) appear to reflect the underlying Kent Coalfield basin. Cope et al. (1980) noted that this unit 'is very variable in thickness in Kent, ranging from c.6.5 m at Brabourne and Harmansole to c.13.2 m at Fredville. In addition, there is a marked lithological variation, the impure manly sandstones with ferruginous beds of the southern and eastern area (i.e. Dover) being replaced northwards by ferruginous marlstones which locally have the composition of very glauconitic 'millet-seed' ironstones.' The iron is mainly in the form of coffee-coloured polished limonite ooliths. The ironstones of the Kellaways Formation resemble those of the Corallian Group but the iron content is lower and there is a greater admixture of impurities.
The base of the Kellaways Sand is marked by the entry of abundant oysters such as Gryphaea (Bilobissa) dilobotes; these occur throughout the unit and become progressively larger upwards. Ammonites are very rare. Arkell (1933) noted that the record of a possible Proplanulites from the basal bed at Oxney (Lamplugh et al., 1923) suggested the existence of the Proplanulites koenigi Subzone [now Zone]. The only well authenticated record is a Kosmoceras medea from near the top at Guilford (Callomon, 1955), indicative of the K. medea Subzone of the Kosmoceras jason Zone; the record of the zonal index of the underlying Sigaloceras calloviense Zone from Tilmanstone (Lamplugh et al, 1923) was attributed questionably by Callomon also to K. medea. The Kellaways Formation thus extends biostratigraphically higher in Kent than in other areas and correlates with the lower part of the Lower Oxford Clay [Peterborough Member of Cox et al, 1992] of the Midlands (cf. Cope et al, 1980, Fig. 8).
In the Boulonnais, the Kellaways Formation is represented only by a thin unit (up to 5 m, but typically much less) of ferruginous oolitic marls, the Marnes ferrugineuses de Belle, with a rich bivalve and ammonite fauna including Sigaloceras calloviense.
This formation has been proved in several of the Kent boreholes. The total thickness varies from ca. 53 m at Brabourne to ca. 32 m at Dover. Lamplugh et al. (1923) subdivided the succession into three broad units. At the base, the 'Ornatus Beds' comprise brown and greenish brown marly clays containing bivalves and species of Kosmoceras. These beds are broadly equivalent to the Lower Oxford Clay or Peterborough Member of the Midlands. The zonal index of the Kosmoceras jason Zone has been recorded from the base of the Oxford Clay at Guildford and Tilmanstone (Callomon, 1955). Records of Erymnoceras sp. from Chilham and Fredville indicate the overlying Erymnoceras coronatum Zone. Lamplugh et al's record (1923) of Kosmoceras duncani suggests that at least part of the Peltoceras athleta Zone might also be represented.
The succeeding 'Renggeri Beds' of Lamplugh et al. (1923) are smooth bluish-grey clays [equivalent to the Stewartby and basal Weymouth Member of Cox et al, 1992] with rich faunas of Quenstedtoceras, including Q. lamberti and oppeliids including Creniceras and Hecticoceras. Cope et al. (1980) suggested that these beds might belong in part to the athleta Zone and the basal Oxfordian Q. mariae Zone, as well as the intervening Q. lamberti Zone. The recorded ammonites are most typical of the Q. lamberti Zone and the overlying Cardioceras scarburgense Subzone of the mariae Zone [B. M. Cox: pers. comm.]. The younger praecordatum Subzone of the latter zone is represented by Lamplugh et al 's 'Mariae Beds', smooth pale bluish-grey clays [Weymouth Member of Cox et al (1992)] with Quenstedtoceras mariae and perisphinctid ammonites. Elsewhere in southern England, the top of the Oxford Clay lies within the cordatum Zone. This is probably also the case in east Kent, although there is no recorded evidence of the latter zone within the 'Mariae Beds'.
In the Boulonnais, the Oxford Clay is represented by the Argiles de Montaubert (10-25 m), with rare ammonites and common serpulids; and the Argiles du Coquillot with basal argillaceous limestones (2 m) containing Quenstedtoceras lamberti (the equivalent of the Lamberti Limestone of British successions) overlain by up to 19 m of clays with small early Oxfordian ammonites in pyritic preservation. The former extensive exposures of these units in brickpits are no longer available.
Lamplugh et al (1923) recognised three subdivisions in east Kent, namely Lower Corallian, Corallian Limestones and Upper Corallian. The Lower and Upper divisions are lithologically similar, comprising clays and marls, marlstones and rubbly oolites, with minor beds of impure limestone. The lower part of the Lower Corallian includes up to 8 m of clays with limonitic oolitic grains and a concentration of crinoids ossicles (Millericrinus) near the top. The middle division comprises pale muddy coral-bearing limestones overlain by a development of Coral Rag comparable with that seen in the Corallian Group at outcrop. At Brabourne, corals occurred throughout the whole thickness of the limestones (40.84 m), this being the most extensive development of coral rag in England (Arkell, 1933). Near Dover, the admixture of polished limonitic grains in the sediment near the top of the Upper Corallian is sufficiently concentrated to form a significant thickness of 'millet seed' iron ore. The maximum thicknesses were found at Dover Colliery (4.8 m) and Farthingloe Borehole (4.5 m); while at Abbot's Cliff borehole, two beds of ore about 1 m and 2.4 m thick respectively were proved. The ore contains about 33 per cent iron and 15 per cent silica, with the reserves being estimated at 100 million tons (Lamplugh et al, 1923). This high Cr development of iron ore is approximately correlative with the Westbury Ironstone of Wiltshire.
A generalised succession for the Corallian Group of the area was given by Cope et al (1980), although not all the beds indicated in the lower part of the succession can be recognised at Dover. The ironshot marls yield abundant cardioceratids, probably largely indicative of the cordatum Zone. Above these, a thin unit (up to 4 m) of highly fossiliferous grey mudstones with black-coated fossils has been recognised in many of the boreholes. It contains a rich ammonite fauna of 'Aspidoceras', Peltoceras and perisphinctids, as well as Cardioceras and a diverse fauna of bivalves and brachiopods. The recorded ammonites are indicative of the C. cordatum Zone and/or the next youngest C. densiplicatum Zone; Cope et al (1980) assigned it to the latter. No ammonites are known from the succeeding 35 m of limestones, but up to 7 m of black clays with Amoeboceras overlie them in a few boreholes (e.g. Folkestone) and may represent the basal Ampthill Clay of eastern England. The succeeding sandy limestone, present in all the boreholes, has been equated with the Clavellata Beds of Dorset, but there is no palaeontological evidence to support this interpretation. This limestone is overlain by tough, silty clays without ammonites, presumed to correlate with the Sandsfoot Clay. The succeeding ironstones, discussed above, yield Ringsteadia, and are followed by pisolitic marls and clays, which approximately correspond to the Ringstead Clay and the complex condensed succession that includes the Ringstead Coral Bed.
From a maximum thickness in east Kent of about 104 m at Brabourne, the Corallian thins to about 88 in at Dover, with further reductions being recorded in the boreholes to the northeast. At Brabourne and Dover, the full succession is preserved beneath the Kimmeridge Clay, whereas in the localities to the northeast, pre-Cretaceous erosion has removed the Upper Corallian and higher part of the Corallian limestones.
In the Boulonnais, the greater part of the Corallian Group is represented by three thick units of clays, terminating upwards in coralliferous limestones, the lower two and the upper unit being interpreted (Bonte et al., 1985) as regressive and transgressive sequences respectively. The lowest sequence comprises the Argiles de Coquilot, the Marnes et Calcaires a Millericrinus and the Calcaires d'Houllefort, the latter having yielded a rich ammonite fauna. The middle sequence commences with the poorly fossiliferous Argiles de Selles, without ammonites, and ends in the Calcaires du Mont des Boucards, which have likewise yielded many ammonites. The top sequence comprises pyritic and sideritic black clays (Argiles du Mont des Boucards), rich in the oyster Deltoideum delta, with poorly bedded, lenticular coralliferous limestones (Calcaires de Brucquedal) near and at the top of the succession. These beds are followed by calcite-cemented, dark brown and red sandstones (Gres de Brunembert) overlain by a complex succession comprising oolitic limestones and marls (Oolithe d'Hesdin l'Abbe) surmounted by alternations of thin, bored limestones and thin marls (Caillasses d'Hesdigneul), which pass laterally into glauconitic sandstones (Grès de Questrecques). Bonte et al (1985) provided a graphic section of all these beds, which today are poorly exposed.
Despite their relative proximity, it is surprising that it is not possible to present detailed correlations of the successions in the east Kent boreholes with those of the Boulonnais. Without a modern reassessment of the fossils collected from Kent, it would be unsafe to attempt to update the Corallian part of the table given by Pruvost and Pringle (1924). The beds with Millericrinus are clearly equivalent on both sides of the Channel, as are the terminal pisolitic marls of Kent and the Oolithe d'Hesdin l'Abbe. Arkell (1935-1948; 1956) reviewed the ammonite evidence and concluded that, of the three coral limestones that might correlate with the Coral Rag of Kent, the Calcaires d'Houllefort equated with the Elsworth Rock of East Anglia (and were therefore older), while the fauna of the Calcaires du Mont des Boucards suggested a horizon younger than the top of the Clavellata Beds of Dorset, implying that both it and the succeeding Calcaires de Brucquedal represented coral rag developments within the equivalent of the Deltoideum-rich Sandsfoot Clay. There appears to be no evidence for the existence of an equivalent of the Clavellata Beds in the Boulonnais, suggesting that they may be missing at the basal Upper Oxfordian non-sequence. The Upper Corallian iron ore can be inferred to correlate broadly with both the Sandsfoot Grit of Dorset and the Grès de Brunembert in the Boulonnais. On ammonite evidence, the Oxfordian-Kimmeridgian boundary probably falls within the Caillasses d'Hesdigneul.
NE limits of Jurassic sediments in the Boulonnais
The CEGB onshore boreholes at Dungeness proved up to 25 m of Kimmeridge Clay comprising dark grey mudstones with thin calcareous beds and bands of phosphatic nodules (Lake and Shephard-Thorn, 1987). Approximately 6 km NE of Dungeness Point, offshore boreholes proved that Kimmeridge Clay formed the eroded core of a faulted anticline (Crosby and Fletcher, 1988) with a WNW-ESE trend.
Lamplugh and Kitchin (1911) and Lamplugh et al (1923) divided the Kimmeridge Clay in the east Kent boreholes into Lower Kimmeridge Clay, characterised by Nanogyra virgula and Upper Kimmeridge Clay, which was without E. virgula, but yielded Modiolus autissiodorensis; the Lower Kimmeridge Clay was more arenaceous and calcareous and of a more shallow water facies than the Upper Kimmeridge Clay (Smart et al., 1966).
The greatest preserved thickness of Kimmeridge Clay in the district was proved at Brabourne; here the succession was 79.85 m, subdivided into 19.5 m of Upper Kimmeridge Clay and 60.35 m of Lower Kimmeridge Clay. At both Brabourne and Ottinge, the Portland Group rests non-sequentially on the Kimmeridge Clay. Ammonite records quoted by Lamplugh et al (1923) suggest that the Upper Kimmeridge Clay may extend as high as the Pectinatites pectinatus Zone or even Pavlovia pallasioides Zone, as in theWarlingham Borehole (Callomon and Cope, 1971). Elsewhere, the Kimmeridge Clay, where present, is overlain by Cretaceous strata, which downcut progressively from west to east (Fig. **). By the Abbot's Cliff Borehole, the thickness of the formation has reduced to 46.63 m, and only 13.41 m of Kimmeridge Clay are present at Dover Colliery shaft, where the succession comprises glauconitic and ironshot impure clays, calcareous beds and limestones. Only the Lower Kimmeridge Clay is represented: the ammonite evidence reported by Lamplugh et al (1923) suggests that the youngest beds, beneath the Cretaceous unconformity, belong to the Aulacostephanus mutabilis Zone.
In the Boulonnais (Wignall, 1991), the argillaceous Kimmeridge Clay Formation of the east Kent boreholes is represented by a more marginal facies of silty mudstones and limestones alternating with thick sandstones and sands; towards the top, the facies becomes even more marginal and includes two horizons of phosphatised pebbles and reworked fossils (P1 and P2, or La Rochette Nodule Beds). The well-preserved trace fossils in the sandstones attest the shallow water environment (Ager and Wallace, 1970), while the argillaceous beds represent transgressive phases, with horizons yielding predominantly subserpenticone ammonites indicating maximum flooding surfaces (Herbin et al, 1995). Full details of the sequence stratigraphy of this succession were given by Roust et al (1993). Furthermore, Herbin et al (1995) have revised the ammonite biostratigraphy, based on accurately horizoned material. The basal beds are not exposed on the coast hut are seen inland: the Oolithe d'Hesdin label at the top of the correlative of the Corallian Group is overlain by some 4.87 m of hard marly limestones (Caillasses d'Hesdigneul) with the Kimmeridgian zonal index Rasenia cymodoce (Bonte et al, 1985).
The magnificent coast sections between Boulogne and Wissant (details in Ager and Wallace, 1970; Wignall, 1990) begin with the Argiles du Moulin Wibert of which the basal metre falls in the mutabilis Zone, while the remainder belongs in the succeeding emulous Zone. Much of the succession includes beds with significant Total Organic Carbon (TOC); the highest TOC values are found in a thick unit (up to 30 m) of laminated pyritic black shales (Argiles de Carillon), which correspond in part to the relatively organic-rich sediments of the Pectinatites elegans Zone in Dorset (Herbin and Geyssant, 1993). A higher organic-rich horizon is found in the Argiles de la Crèche (inferred P. wheatleyensis Zone), which is overlain by two thin limestones enclosed by the phosphatic nodule beds. The interval between these nodule beds may correspond to the highest of the organic-rich beds in the British sequence (huddlestoni-pectinatus zones), for which there is no ammonite evidence.
The equivalent of the higher part of the Kimmeridge Clay Formation is represented by a unit of sandy glauconitic clays alternating with sandy glauconitic limestones, the Argiles de Wimereux. These clays yield ammonites of the pallasioides Zone and terminate in a third horizon of phosphatised pebbles (P3 or Tour de Croi Nodule Bed). The nodule bed contains pebbles of Jurassic rocks (Pruvost and Pringle, 1924) and phosphatic Pectinatites and Pavlovia reworked from the pallasioides Zone, as well as indigenous iridescent ammonites comparable with those from the pallasioides Zone, Hartwell Silt and Swindon Clay of the English Midlands (Townson and Wimbledon, 1979). Although the Tour de Croi nodule bed was previously equated with the upper Lydite Bed of the latter area (i.e. the basal conglomerate of the Portland Group), the new ammonite evidence was interpreted by Towson and Wimbledon to show that the nodule bed represented an intra-Kimmeridgian (pallasioides Zone) event, perhaps partly equivalent to the Lower Lydite Bed. There is no evidence in the Boulonnais for the succeeding topmost Kimmeridgian zones (Pavlovia rotunda and Virgatopavlovia fittoni), which may be here extremely attenuated or absent (Townson and Wimbledon, 1979; Herbin et al, 1995).
About 13 m of Portland Beds were proved in the CEGB boreholes at Dungeness, the succession comprising mudstones and siltstones with cement stones, overlain by bioturbated shelly fine-grained sandstones (Lake and Shephard-Thorn, 1987). Offshore, the Portland Beds flank the anticline NE of Dungeness Point (Crosby and Fletcher, 1988). Evidence for the Portland Group in east Kent comes from the Brabourne, Hothfield and Ottinge Boreholes (Smart et al, 1966), where the successions are much thinner. The Portland Beds in the Brabourne Borehole are 9.44 m thick and comprise, from above, sandy limestones (including a bed of crystalline nodular ferruginous limestone, and a bed of oolitic limestone) with calcareous sandstone at the base; resting on sandy mudstone and bituminous calcareous sandstone with a basal bed of hard bituminous conglomeratic rock resting with a sharp junction on the Kimmeridge Clay. The much thinner succession in the Ottinge Borehole (5.25 m) is predominantly arenaceous and can he inferred to be largely glauconitic from the original description of beds with green grains.
In the absence of ammonites, the Portland Group succession at Brabourne must be interpreted on a lithological basis by means of the ammonite-dated successions in the Henfield and Warlingham boreholes outside the immediate area. In the Henfield Borehole, Glaucolithites occurred in the Portland Group immediately below the junction with the overlying Purbeck Limestone Group. At Warlingham, the ammonite records extend up to and including the kerberus Zone, indicating that the Portland Group there equates not only with the Portland Sand (as at Henfield), but also with the basal part of the Portland Stone of Dorset (Worssam and Ivimey-Cook, 1971). This may also be the situation in the Boulonnais. By analogy with Henfield and Warlingham, it is likely that the Portland Group in east Kent correlates with the Portland Sand and basal part of the Portland Stone of the type area, with a significant non-sequence at the contact with the overlying Purbeck Limestone Group (Lulworth Formation).
In the Boulonnais, the Portlandian (sensu anglico) is 20 m thick (Townson and Wimbledon, 1979). The Tour de Croi nodule bed is overlain by sandy glauconitic clays (Assises de Croi), which are followed by sandy nodular limestones. The higher part of the succession is developed as sands and calcareous sandstones (Gres des Oies), i.e. in Portland Sands facies. Towards the top, an intra-formational conglomerate of pebbles of Palaeozoic rocks, angular fragments of Portland limestones and rolled specimens of Laevitrigonia gibbosa (Poudingue de la Rochette) rests with strongly erosive contact on a bored surface of the underlying beds (Pruvost and Pringle, 1924).
The ammonite biostratigraphy has been reviewed by Townson and Wimbledon (1979), who concluded that the Kimmeridgian-Portlandian boundary probably lay within the lower part of the Assises de Croi. Epivirgatites spp. and Progalbanites albani from the middle Assises de Croi indicate the presence of the basal Portlandian albani Zone and the occurrence of fragmentary Glaucolithites body chambers in the upper Assises de Croi allow the recognition of the G. glaucolithus Zone. Ammonite records including Crendonites gorei suggest that the Galbanites okusensis Zone is likely to be present and there is good evidence, notably common Titanites giganteus, for the existence of the Galbanites kerberus Zone in the Lower and Middle Grès des Oies. No ammonites are known from the Upper Grès des Oies, but it is suggested that these may also belong in the kerberus Zone. The Boulonnais succession is essentially comparable with that found in the Warlingham Borehole and as developed at Swindon and Aylesbury.
In this account, the Cretaceous System is taken to begin with the Purbeck Limestone Group, which is here divided, following BGS practice and Clements (1992) into a lower Lulworth Formation and an upper Durlston Formation, with the base of the latter being drawn at the base of the distinctive Cinder Beds Member. Current research, reviewed by Allen and Wimbledon (1991) and by Feist et al (1995), places the base of the Cretaceous, as defined by the base of the Tethyan Berriasian Stage, at a low level within the Lulworth Formation.
About 55 in of Purbeck Limestone Group strata, including a thin group of evaporites and a possible representative of the Cinder Bed, were proved in the CEGB boreholes at Dungeness (Lake and Shephard-Thorn, 1987) and Purbeck rocks are also present on the flanks of the faulted anticline NE of Dungeness Point (Crosby and Fletcher, 1988). Strata belonging to the Purbeck Limestone Group were recognised by Lamplugh et al (1923) in the boreholes at Hothfield and Brabourne, where the succession was 20.72 m thick in each case. Purbeck strata are not present to the east of these localities. The junction with the Portland Group was not clearly seen at Brabourne. Details of the Brabourne succession and some fossil records were given by Lamplugh and Kitchin (1911). The occurrence of a 1 m thick breccia of fragments of mudstone and lignite in a matrix of greenish loam is noteworthy, as is the record of Protocardia purbeckensis indicating short-lived brackish or even quasi-marine conditions (Morter, 1984). About 6 m above the base of the succession at Brabourne, a layer of phosphatic nodules might possibly represent the Cinder Bed of expanded successions.
In the Boulonnais, the marine Portlandian is overlain by a thin (0.2-1.3 m), laterally variable succession (Calcaires des Oies) locally comprising tuffaceous concretionary algal limestones largely formed by the blue-green alga 'Spongiostromata' and closely comparable with the beds at the base of the Lulworth Formation in Dorset (Ager and Wallace, 1966). These limestones pass laterally into limestones composed of the bivalve Eocallista socialis, which is common in Dorset at the junction between the Portland and Purbeck groups (Townson and Wimbledon, 1979).
The Wealden Supergroup consists of two broad subdivisions, the Hastings Group below and the Weald Clay Group above. The former comprises an alternating succession of predominantly arenaceous formations (Ashdown Beds, Lower Tunbridge Wells Sands, Upper Tunbridge Wells Sands) and mudstones with lenticular sandstones (Wadhurst Clay, Grinstead Clay). The major sandstone units are typically overlain by pebble and/or bone beds marking transgressive events. The depositional environment of the Hastings Group has been reviewed in a series of papers by Allen (1959, 1975, 1989). The marked lithofacies change to mudstone sedimentation at the base of the Wadhurst Clay probably records the early Valanginian marine transgression at the end of the Continental 'Wealden' Formation period in Germany. This dating of the Wadhurst Clay as Valanginian is supported by charophyte evidence (Feist et al, 1995).
The change to mudstone deposition at the base of the Weald Clay Group is usually related to the early Hauterivian transgression in the marine realm. The Weald Clay Group comprises silty mudstones and muddy siltstones, typically with thin clay-ironstone beds near the base. In the middle part of the succession, there are groups of beds made up of low diversity or even monospecific gastropod associations, the so-called Small-'Paludina' beds (with Viviparus infracretacicus) and Large-'Paludina' Beds (with V. fluidram), in ascending order (Morter, 1978). A short distance above the Small-'Paludina' Beds, one or more horizons yielding quasi-marine faunas with the gastropod Paraglauconia [formerly cited as Cassiope] and bivalves including Cuneocorbula, Modiolus, Nemocardium and oysters (Praeexogyra) have been identified in the western Weald near Horsham, as well as in the Warlingham Borehole (Morter, 1978); charophytes from just above this level in the Warlingham Borehole indicate an early Barremian age (Feist et al, 1995). Above the (freshwater) Large-'Paludina' Beds, the depositional environment becomes increasingly saline, and the same quasi-marine molluscan assemblage reappears close to the top of the Weald Clay.
In the Hastings area, where the Hastings Group totals up to 385 m in thickness, the lower part of the Ashdown Beds is predominantly argillaceous (the Fairlight Clays of the old literature) and it is only the top 30 m that are dominantly sandy (Lake and Shephard-Thorn, 1987). The 'Fairlight Clays' have yielded a rich and well preserved flora, which includes pteridophytes, cycads and conifers. 20km to the east, the CEGB boreholes at Dungeness proved up to 165 m of Ashdown Beds, Wadhurst Clay and Tunbridge Wells Sands (Lake and Shephard-Thorn, 1987): although the boreholes started below the top of the Hastings Group, the total overall thickness can be inferred to be significantly less than at Hastings. The group crops out on the seabed to the north and south of the boundary faults of the faulted anticline NE of Dungeness Point (Crosby and Fletcher, 1988). The Hastings Group thins to the northeast and becomes progressively more arenaceous, so that the various subdivisions of the thicker successions, such as the Wadhurst Clay, can no longer be recognised. Near Dover, the sediments are particularly coarse and pebbly and the Hastings Group isopachytes record the position of a large valley trending in a southwest direction from the Palaeozoic uplands (Allen, 1967).
The succeeding Weald Clay Group is probably more or less complete in the western part of east Kent: Two 'Paludina' limestones 6 m apart were recorded 81 m from the top of the 122 m of Weald Clay in the Hothfield borehole near Ashford and an horizon with Praeexogyra is present 10 m below the top of the Weald Clay at Hythe. At Dover Colliery, the top of the Weald Clay at the contact with the Atherfield Clay is deeply burrowed. Here the occurrence of freshwater faunas with Viviparus and Unio in the top 1.5 m suggests that the highest part of the succession (with quasi-marine faunas) was removed by erosion at the time of the early Aptian marine transgression. In the St. Margaret's Bay Borehole, where the top of the Weald Clay is similarly burrowed, there is ostracod evidence for the higher part of the Weald Clay and a Large-'Paludina' limestone was found at the base above a downhole lithological change to arenaceous deposits of Hastings Group facies (Bisson et al, 1967; Worssam, 1978). Isopachytes for the Weald Clay (Worssam, 1978, Fig. 6) also show the valley identified in the Hastings Group near Dover and an additional narrow valley extending southwest from the basin margin in Thanet. In view of the lack of faunal evidence for the lower part of the Weald Clay in the east Kent boreholes, Worssam (1978) suggested that some of the supposed Hastings Group sands may actually be of Weald Clay age. This would fit with the picture, derived from seismic imaging, of the Weald Clay Group exhibiting a transgressive relationship to the underlying Hastings Group (Ruffell, 1992).
The Wealden thins from Ashford, where 217 m were recorded in a borehole (Smart et al, 1966), to 94.18 m at Brabourne and then to Dover Colliery where only 25.91 m remain. There is further thinning to the north, with the Weald Clay onlapping the feather-edge of the Hastings Group facies sands to rest directly on Coal Measures in a triangular area to the southwest of Ebbsfleet.
In the Boulonnais, the greatest known thickness of Wealden sediments is the 66.5 m of clays and sands recorded in a borehole at Wissant (Olry, 1904). Erosional relicts of grey lignitic clays of Wealden facies are found in pockets in Portland and Purbeck limestones near Boulogne and Wimereux. Sections of Wealden facies were formerly seen in quarries in the south of the Boulonnais near Nesles, but today the only available sections are in the eastern part of the area. Only the highest part of the succession is exposed, comprising up to 8 m of mottled reddish clays, overlain by black clays and white sands (Robaszynski and Amédro, 1986). Although Allen (1967) considered that the 'Wealden' sediments in the Boulonnais were comparable with the Hastings Group of southern England, palynomorph evidence (Herngreen, 1971) indicates a Late Barremian-Aptian age, pointing instead to a correlation with the higher part of the Weald Clay. There is thus an approximate parallel with the situation in east Kent, where sediments of Hastings Group facies may well be of Weald Clay age. The Wealden is overlain with erosive contact by either Lower Aptian or Lower Albian glauconitic sands, in each case with a basal concentration of phosphatic nodules.
The base of the Aptian in the Weald Basin is inferred to lie within the Weald Clay, by extrapolation from the Isle of Wight; where the early Aptian Vectis Magnetozone is situated within the correlative Vectis Formation (Kerth and Hailwood, 1988). There is a significant non-sequence in east Kent at the base of the Lower Greensand.
In Kent, the Lower Greensand Group at outcrop comprises, in ascending order, the Atherfield Clay Formation, the Hythe Beds Formation, the Sandgate Beds Formation and the Folkestone Beds Formation. The last three formations have their stratotypes in the vicinity of Folkestone. The Hythe Beds are overstepped from west to east by the transgressive Sandgate Beds and are already missing by Dover Colliery (Casey, 1961; Hesselbo et al, 1990; Ruffell and Wach, 1991). There are facies changes to the north-northeast as the Lower Greensand plunges undercover. The Atherfield Clay Formation in east Kent consists of bluish-grey, locally brown-mottled, sandy clay and pale grey, slightly glauconitic clay; typically with chocolate-brown clay in the lower part. It rests with sharp contact on the eroded, burrowed and bored top of the Weald Clay. Outcrop records and data from cored boreholes in the Kent Coalfield were reviewed by Casey (1961). There is a significant non-sequence at the basal contact: the Perna Bed and the overlying Chale Clay Member of the Isle of Wight succession are missing and the ammonite evidence (Casey, 1961) indicates that here the Atherfield Clay begins in the upper part of the Deshayesites forbesi Zone (D. callidiscus Subzone), the equivalent of the Crackers and Upper Lobster Beds. At Dover Colliery, the Atherfield Clay is 13.10 m thick and includes a bed of clay rich in Pinna robinaldina. There is a basal thin bed of grit with chert pebbles, bone fragments and Hybodus teeth. Simpson (1985) reinterpreted the ammonite evidence (Deshayesites spp., including the zonal index, as well as a single Roloboceras) to show that the succession could also include the equivalent of the Lower Lobster Bed (D. kiliani Subzone), albeit in the apparent absence of the subzonal index. However, Ruffell and Watch (1991) considered that the onlapping base of the Atherfield Clay in East Kent corresponded to the terminal forbesi Zone (callidiscus Subzone) Upper Lobster Beds transgression. The Atherfield Clay is overlain non-sequentially by the transgressive Sandgate Beds; the top of the clay exhibits greensand filled pholadid crypts, with the bivalve shells (Girardotia) still preserved (Lamplugh and Pringle, 1911, p1.1). In the St. Margaret's Bay Borehole (Bisson et al, 1967), the Atherfield Clay (5.13 m) yielded a rich fauna of molluscs, predominantly bivalves, as well as Deshayesites forbesi and Toxaster fittoni. Nearer to the margin of the basin, the Atherfield Clay at Harmansole is reduced to 0.6 m of unfossiliferous sandy clay with phosphatic pebbles.
The Hythe Beds Formation of the type area in east Kent comprises closely-spaced alternations of blue-grey limestones (locally known as rag) and poorly cemented, glauconitic, argillaceous sands (hassock). Fossils are preserved solid in the rag, but are typically crushed in the hassock. The Hythe Beds of the type area belong to the Lower Aptian Deshayesites deshayesi and Tropaeum bowerbanki zones. In the Maidstone area, the Hythe Beds succession includes strata of Late Aptian Cheloniceras (Epicheloniceras) martinioides Zone age, the Boughton Beds Member (Casey, 1961; Ruffell, 1993a,b; Worssam, 1993). The lower part of this unit in places comprises lagoonal deposits with plants and dinosaur remains, reflecting the martinioides Zone regression; towards the top, there are locally beds rich in sponges, with horizons of chert replacing the ragstones. Ammonite evidence (Casey, 1961) permits the recognition of the debile and gracile subzones, with the topmost subzone (buxtorfi) being possibly represented in the chert beds. Between Maidstone and Folkestone, erosion connected with the martinioides regression has resulted in the transgressive base of the Sandgate Beds resting on successively lower levels in the Hythe Beds. This major break in the Lower Greensand succession, recognised by Casey (1961) as the 'martinioides retrenchment' followed by the 'nutfieldiensis transgression', was identified (Hesselbo et al., 1990; Ruffell and Wach, l991) with the 109.5 Ma sequence boundary of the EXXON chart (Haq et al, 1988).
The original sections in Hythe are no longer available. The Otterpool Manor quarry [TR 112366], where 10.7 m of strata comprising the entire Lower Aptian Hythe Beds succession with the exception of the basal beds, were exposed beneath Sandgate Beds, is the standard reference section for the immediate area (Casey, 1961; Ruffell, 1993a), but is now backfilled. Most of this section was in rag and hassock facies; near the top of the Hythe Beds, a shell-rich calcareous sandstone with phosphatic nodules (Bed 30 of Casey, 1961) marked a transgression near the base of the meyendorffi Subzone of the Tropaeum bowerbanki Zone, above which the highest 1.5 m were developed in hassock facies with belemnites (Neohibolites ewaldi). At Mill Point, southwest of Folkestone, erosion has cut down to the transitoria Subzone of the bowerbanki Zone (Casey, 1961) and bored cobbles of rag limestone have been reworked into the basal Sandgate Beds. Ruffell (l993a) has recorded a 15 m Hythe Beds section at this locality by piecing together for the first time discontinuous exposures of the underlying beds.
The Sandgate Formation consists of glauconitic silty clays and silts, generally with a basal phosphatic nodule bed in glauconitic loam, which locally (e.g. Sellindge) yields the subzonal index of the highest (Cheloniceras (Epicheloniceras) buxtorfi) subzone of the martinioides Zone. Strata coeval with the highest Hythe Beds of the Maidstone area are thus represented northwest of Folkestone by a condensed sequence at the base of the Sandgate beds. At Mill Point, the base of the Sandgate Beds is marked by ferruginous boxstones impressed into the eroded top of the Hythe Beds. These boxstones include three components: debris from the erosion of Hythe Beds sediments; buff phosphatic nodules including C. (E.) buxtorfi; and an indigenous fauna of large brachiopods of nutfieldiensis Zone age. At Dover Colliery, the hiatus beneath the Sandgate Beds extends down to the top of the Atherfield Clay Formation (forbesi Zone). Farther to the north in the Ebbsfleet Borehole, the Sandgate Beds rest on Weald Clay and finally onlap the Palaeozoic floor (Kirkaldy, 1937). The lower part of the Sandgate Beds proper belongs to the Parahoplites nutfieldiensis Zone.
Nolaniceras spp. indicative of the (basal) N. nolani Subzone of the Hypacanthoplites jacobi Zone occur near the top of the formation west of Folkestone, together with a fauna dominated by Lamellaerhynchia caseyi (Casey, 1961).
The greater part of the Folkestone Beds Formation comprises typically white and/or yellow cross-bedded sands representing sandwave sedimentation under tidal conditions; the directions of foresets indicate movement to the southeast (Narayan, 1971; Bridges, 1982). Erosion surfaces marked by stratigraphical hiatuses identified by means of ammonites (Casey 1961) divide the Folkestone Beds as traditionally understood into three separate packages of sediment, which have been informally named the lower, middle and upper Folkestone Beds (Hesselbo et al, 1990) and studied in a sequence stratigraphical context. The upper Folkestone Beds of this classification, comprising the sediments of the topmost Lower Albian Douvilleiceras mammillatum Superzone overlain by the basal bed of the Gault, was treated by Owen (1992) as a separate unit of highly condensed beds, the Gault-Lower Greensand Junction Beds. Owen considered that the separation of this unit from the underlying and overlying beds was justified because horizons within it form major glide planes and are consequently of significance in civil engineering projects.
Spatial and stratigraphical relationships between the three units of the Folkestone Beds are highly complex: at no single locality is the entire succession preserved and the contact with the Sandgate Beds was only ever exposed in the coastal exposures on either side of Folkestone. Traced southeastwards from Brabourne to Folkestone (Smart et al, 1966, Fig. 1), successively higher units of the Folkestone Beds sands are preserved beneath the condensed mammillatum Superzone.
In the key, but now obliterated Sandling Junction sandpit section, 14 m of (lower) Folkestone Beds comprising uppermost Jacobi Zone Hypacanthoplites anglicus Subzone (terminal Upper Aptian) sands were formerly exposed. Soft rotted ironstone concretions in clay-rich sands near the base of the succession in both the pit and in the nearby railway cutting yielded Hypacanthoplites simmsi. A bed of partly phosphatised hollow nodules in the pit contained rare ammonites of the anglicus Subzone (H. simmsi and H. anglicus), together with a rich molluscan fauna, including a gastropod with the intestines exceptionally preserved in phosphate (Casey, 1960). The same ammonites were found in a bed of ferruginous sandstone (Red Bed) towards the top of the anglicus sands. There is no evidence in the exposed part of the Folkestone Beds here of the underlying Hypacanthoplites rubricosus Subzone, although this subzone is represented in the complex basal nodule bed east of Folkestone (see below). The recent discovery in a temporary motorway section near Lenham of indigenous H. simmsi in nacreous, rather than phosphatic preservation, in a bed of silty clay inferred to be at the top of the Sandgate Beds (Ruffell and Owen, 1995) is of significance in this context.
These lower Folkestone Beds sands were overlain with erosive contact (erosion surface LG3 of Hesselbo et al, 1990) by Lower Albian H. milletioides Subzone sands with sandstones containing the subzonal index. The LG3 hiatus includes the Aptian-Albian boundary: the phosphatic nodules (anglicus nodules) marking this hiatus include ammonites of the anglicus Subzone, but there is no evidence for the basal Albian farnhamensis Subzone.
In the two Folkestone sections, the lower Folkestone Beds anglicus sands seen to the west are no longer found and the milletioides Subzone sediments are overlain by similar beds of regularis Subzone age, the two subzonal packages constituting the middle Folkestone Beds of Hesselbo et al (1990). At Mill Point, west of Folkestone, the anglicus nodules rest directly on the Sandgate Beds and the milletioides Subzone is only 5.5 m thick; to the east at East Cliff, the bored surface of the Sandgate beds was formerly seen (Casey, 1961) to be overlain by a complex nodule bed in which the black anglicus nodules rested on spherical ferruginous nodules with a Hypacanthoplites rubricosus Subzone fauna. Here the milletioides Subzone sediments are reduced to 60 cm of sandy clays and the greater part of the (middle) Folkestone Beds belong to the Leymeriella regularis Subzone. Ruffell and Wach (1991) considered that the 107.5 Ma sequence boundary of the EXXON chart (Haq et al, 1988) was probably expressed by the end of sand deposition in regularis Subzone times. The Gault-Lower Greensand Junction Beds sensu Owen (1992) contain three important fossiliferous horizons. The base is marked by the kitchini nodule Bed, which yields phosphatised ammonites of the Sonneratia kitchini Subzone. The Main mammillatum Bed, some distance higher, includes fossils in two states of preservation incorporated in a locally cemented grit: phosphatic pebble fossils derived from the Cleoniceras floridum and Otohoplites raulinianus subzones; and partly phosphatised, indigenous fossils of Protohoplites (Hemisonneratia) puzosianus Subzone age. The Sulphur Bed at the top of the traditional Folkestone Beds and so named from the mineral jarosite (hydrated potassium iron sulphate: a decomposition product of pyrite) comprises greyish, pyritic phosphatic nodules set in a dark grey loam with pyrite (Ruffell, 1990). Owen (1992) has recorded Otohoplites crassus from the Sulphur Band at an inland section near Folkestone and has consequently assigned this bed to the Otohoplites ebulliences Subzone, but noted that there was also some admixture of derived puzosianus Subzone material. In the nodules immediately overlying the Sulphur Bed at Copt Point there are rare specimens of Hoplites (Isohoplites) steinmanni (?= eodentatus), indicating the eponymous subzone of the Hoplites dentatus Zone and the base of the Middle Albian as interpreted by Casey (in Smart et al, 1966) and by Destombes (1977).
In the Boulonnais, there is no record of earliest Aptian marine sediments equivalent to the Atherfield Clay Formation and the lower part of the Hythe Beds Formation. The succession begins with a glauconitic sandstone with Cheloniceras (Formation de Cat-Cornu), which is found in beach exposures near Wissant. Elsewhere in the Boulonnais, a period of intra-Aptian erosion has removed this sandstone, and it is represented at Menty in the south of the area only by a lag of sandy phosphatic nodules at the base of the Formation de Verlincthun, resting on the burrowed top of Wealden white sands. The nodules have yielded brachiopods and bivalves, as well as a rich ammonite fauna which indicates a concentration of the Deshayesites grandis Subzone of the D. deshayesi Zone and the Tropaeum bowerbanki Zone. The Formation de Cat-Cornu thus correlates with the higher part of the Hythe Beds Formation of the type area. The overlying Formation de Verlincthun comprises glauconitic sands with one or more horizons of large oysters (Ostrea leymerii, Aetostreon latissimum and Rastellum macropterum), over-lain by cross-bedded white sands. In the absence of ammonites, these beds have been correlated on general similarity of facies with the Sandgate Beds Formation, although there is no sign of the martinioides Zone nodule bed at the base.
The white sands at the top of the Formation de Verlincthun terminate in an irregular omission surface and are overlain by the Formation de Wissant, about I m of glauconitic sandy clays with black phosphatic nodules and sandy phosphatic concretions. The latter yield a rich ammonite assemblage comprising species of Hypacanthoplites including H. anglicus and H. rubricosus, indicating the eponymous subzones of the H. Jacobi Zone. This bed is the equivalent of the complex rubricosus/anglicus nodule bed at the base of the Folkestone Beds Formation in the Folkestone East Cliff section. The latter formation (including the Gault-Lower Greensand Junctions Beds of Owen) is represented at
Wissant by the Formation des Gardes, comprising only I m of dark green coarse glauconitic sands and sandstones intercalated between two beds of phosphatic nodules designated P1 and P2 by French stratigraphers. P1 yields reworked Hypacanthoplites together with indigenous Birostrina salomoni and Burrirhynchia leightonensis as well as ammonites in pebble preservation that are indicative of the Leymeriella regularis Subzone and also the Cleoniceras floridum Subzone of the mammillatum Superzone. In the context of the Folkestone Beds succession, the lithological equivalent of the main mammillatum bed rests here directly on the expanded correlative of the rubricosus/anglicus nodule bed, the regularis Subzone sands and sandstones of Folkestone being unrepresented by sediment. However, in the southeast Boulonnais near Samer, sediments of this age, comprising glauconitic sands with Leymeriella were reported beneath P1 by Robaszynski and Amedro (1986).
The higher part of the Formation des Gardes in the vicinity of Wissant is laterally variable and there are significant differences between the section normally visible (see Robaszynski and Amedro, 1986) and that occasionally exposed in the area of sand dunes 0.8 km northeast of Wissant (Owen, 1992). In the latter section, a terminal bed of large, flattened greyish brown nodules with an early Otohoplites bulliensis Subzone ammonite fauna is separated by only 5cm from a bed of glauconitic clay containing pyrite-encrusted phosphatic nodules with a pusozianus Subzone fauna (Owen, 1992, Fig. 16). In the normally exposed section, these two beds merge to produce the phosphate bed P2 with the subzonally mixed ammonite fauna listed by Robaszynski and Amédro (1986). The lithological and ammonite subzonal correlation between the two Wissant successions and the succession at Folkestone is highly complex (Owen, 1992, Fig. IS) and will not be discussed further here.
The Gault succession was originally described from the cliffs and foreshore section at Copt Point near Folkestone, which is the type locality for the formation (De Rance, 1868; Price, 1874, 1875, 1879) and one of d'Orbigny's original type sections for the Albian Stage. Here the Gault is 40 m thick, but the basal part is relatively condensed. The standard classification into thirteen beds designated by Roman numerals stems from Jukes-Browne (Jukes Browne and Hill, 1900). Detailed sections of the stratotype were given by Smart et al, (1966) and (with a more detailed and somewhat modified bed classification) by Owen (197la; 1975), who then took the base of the Gault, following Geological Survey practice (Casey, 1961; Smart et al, 1966), at the base of the Greensand Seam. Subsequently, Owen (1992) revised this classification, taking the lower limit of the Gault Formation at the base of the so-called dentatus nodule bed and retaining the Greensand seam in his Gault-Lower Greensand Junction Beds. The relationship between the bed notation and the ammonite biozones is shown in Fig. **.
The Gault comprises predominantly highly fossiliferous mudstones with several horizons of phosphatic nodules. It is divided into two lithologically dissimilar parts, with the boundary being taken at the strongly erosive base of the so-called Junction Bed (Bed VIII). The Lower Gault (circa 10 in) comprises generally dark, pyritic mudstones and is of mid-Albian age. The thin (0.3 m) Junction Bed, previously taken (Smart et al., 1966) as the top of the Lower Gault, but more logically regarded (Owen, l97la) as the base of the Upper Gault, consists of two beds of phosphatic nodules separated by clays with common Birostrina sulcata. This distinctive species also occurs in flood abundance in the overlying Bed IX. In marked contrast to the sediments of the Lower Gault, the relatively thicker and more rapidly deposited Late Albian Upper Gault mud-stones (ca 30 m) are much more calcareous and correspondingly lighter grey. Within the Upper Gault, an erosively based bed of highly glauconitic clays containing phosphatic pebbles (Bed XII), the 'Greensand Seam' of Price (1874), provides an important marker horizon. This bed is similar to the Glauconitic Marl at the base of the Chalk and in the past has been confused with it in slipped and disturbed sections.
The change in facies between the bulliensis Subzone Sulphur Band, marking the traditional top of the Folkestone Beds, and the sediments of the Greensand Seam may reflect the global 103 Ma Type II sequence boundary (Hesselbo et al, 1990, Fig. 8). Within the Gault itself, significant concentrations of phosphatic nodules are found in Bed I (dentatus nodule bed), Bed II, Bed IV, Bed VII, Bed X and Bed XII. Hesselbo et al (1990) interpreted the dentatus nodule bed, the Bed IV nodules and the Bed X nodules as reflecting condensed sections associated with maximum flooding surfaces. On the other hand, the complex Bed VIII and Bed XII nodule beds, both of which result from strongly erosive events, were interpreted as corresponding to the 99 Ma Type II and 98 Ma Type I sequence boundaries respectively. The only evidence for the predicted minor Type II 100.5 Ma sequence is provided by the Bed II nodules.
The outcrop sections of the Gault in the area under review, including the stratotype, lie on an anticlinal structure that periodically had a significant effect on deposition. At Folkestone, the basal Lower Gault Hoplites (Isohoplites) steinrnanni and Lyelliceras lyelli subzones are extremely condensed and represented by only 0.45 m of glauconitic sediments (Greensand Seam) between the Sulphur Band and the dentatus nodule bed (Owen, 1992). By contrast, Folkestone is the only Lower Gault succession in which thick Euhoplites lautus Zone sediments have been preserved from mid-cristatum Subzone erosion. This is one of the thickest known developments of the terminal Anahoplites daviesi Subzone in Europe, but even here the top of the subzone is absent, for remanié ammonites of this age are found in the cristatum Subzone nodule bed. In the Upper Gault, Folkestone is the only succession at outcrop where Callihoplites auritus Subzone clays (Bed XI) are preserved; elsewhere, the equivalent of Folkestone Bed XII rests with erosive contact on Hysteroceras varicose Subzone sediments (Owen, 1975, Fig. 5).
The borings in the area of the Kent Coalfield show that the Lower Gault succession expands eastwards from the outcrop between Folkestone and Maidstone. In this trough area, the basal Lower Gault steinmanni and lyelli Subzones are well developed and correspondingly thicker, while the remainder of the Lower Gault does not change significantly in thickness. In the Channel Tunnel No 1 (Aycliff) Borehole, 6.7km east-northeast of Copt Point, the equivalent of the dentatus nodule bed is underlain by 3.7 m of clay, gritty in the lowest 60cm and glauconitic with phosphatic nodules at the base: these clays contain Hoplites and Protanisoceras and were assigned to the lyelli Subzone by Owen (l971a). At Guilford Colliery, material collected from the dump included phosphatised Lyelliceras including the subzonal index as well as Protanisoceras (Owen, 1971 a). Even farther to the northeast at Chislet Colliery, Lower Gault with a basal conglomerate of Douvilleiceras mammillatum Superzone age rests directly on Coal Measures. Here a fragmentary phosphatised Hoplites (Isohoplites) was collected near the base, with pyritised Lyelliceras cf. lyelli a short distance above. Borehole and water well records show that the Gault succession as a whole on the east coast of Kent thins considerably northwards. In the St. Margaret's Bay Borehole (Bisson et al, 1967), where the top 800 ft. was fragmented and the thickness of the Gault is therefore not known, the occurrence of a mortoniceratid ammonite indicative of Upper Gault only 4.16 m above the base of the Gault led Owen (1971 a) to infer that the Lower Gault here was either very much reduced in thickness or had been partially or wholly cut out as a result of early Late Albian faulting. In the nearby Seagas Deal Gasworks borehole [TR 374 5331, the Gault is 26.21 m thick, i.e. approximately half the thickness of the stratotype, but there are no data for the thickness of the Lower Gault. However, the Ebbsfleet Borehole proved 8.23 m of Lower Gault. To the north of the coalfield, the Herne Borehole and water wells in the Isle of Thanet show that the Gault thins dramatically: at Margate, there are recorded thicknesses of 20.57 m in the Thanet Water Board Well [TR 365 701] (Owen, 1971a) and as little as 17.4 m in the Dane Pumping Station Borehole (Shephard-Thorn, 1988; Whitaker, 1908). These very thin Gault successions may possibly comprise Upper Gault only and lie on the north side of an easterly extension of the northern boundary fault identified at Cliffe by Owen (1971 b). It is noteworthy that the Chalk also thins significantly towards this area (Mortimore and Pomerol, 1987).
Compared with the standard succession at Folkestone, the Gault of the Boulonnais is much thinner (Fig. **), viz. about 11 m at Wissant as against about 40 m at Folkestone. The Wissant Gault succession is placed in the Formation de Saint-Pô, which comprises predominantly fossiliferous grey mudstones with 4 horizons of phosphatic nodules (P3-6). As shown by Robaszynski and Amédro (1986) and Amédro (1992), these nodule horizons broadly correspond to the Bed I, IV, VIII and X phosphates of the Folkestone succession respectively. Uniquely at Wissant, the top of the Albian succession, equivalent to Beds XII and XIII of the Gault at Folkestone, is missing and the Cenomanian rests with erosive contact on the equivalent of Bed XI. Elsewhere throughout the Boulonnais, these highest beds are represented by the Formation de Lotthingen: dark green, highly glauconitic clays with a basal phosphatic nodule bed which equates with the Bed XII phosphates at Folkestone. At Cap Blanc Nez, this formation is only 0.15 m thick. In the southern part of the Boulonnais, however, the equivalent of the Gault is much less condensed. In the fully cored Dannes Borehole (Amedro et al, 1990), the Saint-Po and Lotthingen successions are 17 m and 5 m thick respectively; here the Lower Albian (Formation des Gardes (1 m) rests directly on clays and sands of Wealden facies.
Due to local structural control, the Wissant equivalent of the Lower Gault is relatively condensed and is approximately half as thick as the Folkestone stratotype (Owen, 1971a). In the expanded succession in the Dannes Borehole, however, the corresponding thickness (ca. 12 m) is 2 m more than at Folkestone (Amédro et al, 1990). At Wissant, a conglomerate of remanié bulliensis Subzone (mammillatum Superzone) phosphatised nodules (P2) with ammonites at the top of the Formation des Gardes is overlain directly by sediments with spathi Subzone phosphatised ammonites at the base. No steinmanni Subzone or lyelli Subzone sediments are present and consequently the 103 Ma sequence boundary is readily apparent. Locally the contact is highly irregular, so that the spathi Subzone phosphatic nodules (P3 - the equivalent of the Folkestone dentatus nodule bed) occupy pockets between the bulliensis Subzone nodules. The daviesi Subzone is missing, but earliest cristatum Subzone sediments that are absent at Folkestone are preserved here beneath the cristatum nodule bed. This is the only section known where basal Upper Albian sediments are preserved in an uncondensed succession (Owen, 1971 a). The relationship between the Wissant succession and the Folkestone stratotype, with particular reference to the extent of biostratigraphical non-sequences and condensed sections is shown in Fig. **.
Inland, in the vicinity of the Landrethun Fault (Fig **), the equivalent of the base of the Upper Gault with Birostrina sulcata rests directly on folded Ludlow graptolitic shales (Pruvost, 1922b). In the offshore successions, a separate unit of one or more beds of glauconitic and micaceous sediments, informally designated Zone 6a on the basis of its microfaunal content (Carter and Hart, 1977; Hart, 1993), is intercalated between the Gault proper and the Glauconitic Marl. Stratigraphical details of Zone 6a on the French side of the Channel have been given by Amédro (1994). Zone 6a has so far yielded no recorded macrofossils (the poorly preserved ammonites collected from the UK Crossover supposedly from this bed now being held to be of uncertain provenance and identification) and is consequently of uncertain age. It is delimited above by the basal Glauconitic Marl erosion surface and below by an erosion surface which locally cuts through the underlying Bed XIII to the glauconitic Bed XII. This unit was of great significance in the Channel Tunnel project and is to be the subject of a separate paper on this Web Site, currently under construction.
The regional dip brings the Chalk Group to sea level to the east of Folkestone in East Wear Bay. Chalk occupies the high cliffs from there to Dover and thence as far as Kingsdown, where the cliffs are lower, obscured by vegetation and set back a short distance from the sea. To the north of Deal, the Chalk outcrop is interrupted by the Tertiary deposits of the Richborough Syncline. The Chalk then reappears in the Isle of Thanet, where the highest beds are exposed in relatively low cliffs. Excellent exposures at the base of the cliffs, supplemented by sections provided by cliff paths (Abbot's Cliff, Lydden, Aker's Steps) between Folkestone and Dover, and Langdon Stairs to the east of Dover, permit a composite succession of some 180 m to be logged in detail (Jenkyns et al, 1994, Figs 13a-c).
A useful summary of the Chalk of the Dover and Thanet areas, with a comprehensive bibliography, was given by Shephard-Thorn (1988); the earlier account of the coast sections between Folkestone and Dover (Smart et al, 1966), however, is now considerably out of date. A revision of Peake's (1967) field guide was published by Mortimore (1997). The biostratigraphy of the Upper Chalk was reviewed by Bailey et al (1983, 1984).
The Chalk Group is traditionally divided into Lower, Middle and Upper subdivisions, which are effectively of formation status. The top of the Lower Chalk is marked by the Plenus Marls, the base of the Middle Chalk being formed by the Melbourn Rock or by its lateral equivalent. The base of the Upper Chalk in condensed platform and marginal successions is taken at the base of the Chalk Rock, a complex sequence of glauconitised and phosphatised hardgrounds (Bromley and Gale, 1982). In the more basinal successions of Sussex and east Kent, where the Chalk Rock is not developed, this datum was drawn instead at the base of the traditional Sternotaxis plana Zone, which is there marked by a succession including conspicuous, laterally continuous marl seams and large flints, constituting the so-called Basal Complex (Mortimore and Wood, 1986). These basal beds are very conspicuous in the cliff sections east of Dover (e.g. Jukes-Browne and Hill, 1904, Fig. 45) and they consequently attracted the attention of many earlier workers, notably Hill (1886) and Rowe (1899, 1900), as well as that of visiting Chalk stratigraphers from the other side of the Channel such as Hebert (1874) and Barrois (1876)- see Gale and Cleevely (1989) for a useful historical review.
In recent years, the biostratigraphical classification of the Chalk in southern England into a succession of somewhat subjectively defined macrofossil assemblage biozones, which has dominated Chalk stratigraphical philosophy to its detriment for most of this century, has been supplemented by schemes based on lithostratigraphical units and marker horizons, such as marl seams and flint bands. Schemes of this type have been introduced for the Middle and Upper Chalk (the White Chalk of Rowe) by Robinson (1986) and by Mortimore (1983, 1986) for the successions in the North Downs (including east Kent) and Sussex respectively. As shown by Mortimore (1987), Mortimore and Pomerol (1987) and Gale et al (1987), the Mortimore scheme, which was established for the thicker and consequently more complete South Downs successions, is largely applicable to the North Downs as well, with most of the major marker marl seams being recognisable in both areas. In this account, the Mortimore scheme has been used in preference to that of Robinson, except in the case of the highest part of the succession, for which, following the recommendations of Gale et al (1987), it is necessary to employ the term Margate Chalk.
The traditional concept of the Upper Chalk in which the base falls at the base of the Basal Complex and within the Lewes Chalk Member of Mortimore (1986) has been revised by BGS in its current map and memoir programme (Bristow, Mortimore and Wood, in prep.). In this new classification, the base of the Upper Chalk is taken not at the (local) onset of conspicuous nodular flints as before, but at the onset of nodular chalk at the (redefined) base of the Lewes Chalk. The old and new classifications, together with the Mortimore subdivisions and the key marker horizons for all the preserved Middle and Upper Chalk of Kent are shown in Fig. **.
For a composite detailed graphic log from the base of the Chalk to the top of the preserved succession in Thanet see Jenkyns et al (1994, Figs 13a-c); another composite log was given by Robinson (1986). In east Kent, Tertiary erosion has removed the greater part of the Upper Chalk and the succession extends only up to the lowest part of the Offaster pilula Zone (Shephard-Thorn, 1988, Fig. 11). Wireline logs of boreholes for the east and north Kent coast (Mortimore and Pomerol, 1987) demonstrate significant thinning of the succession onto a structure in the Isle of Thanet, a thinning also seen in the underlying Gault.
The Chalk successions exposed in the cliffs of east Kent are essentially similar to those found in the cliffs of the French Channel coast and in inland exposures in the Boulonnais. Not withstanding the different stratigraphical nomenclature employed in the two countries (Robaszynski and Amédro, 1986, 1991; Amédro, 1993; Mortimore and Pomerol, 1987). Only the Lower Chalk and the (traditional) Middle Chalk are considered here.
Table: Stratigraphical summary (using the traditional concept of the M-U Chalk boundary)showing total thickness
Table showing the thickness of the Middle and Upper Chalk Zones
The Lower Chalk between Folkestone and Dover is 78 m thick (Jenkyns et al, 1994, Fig. l3a). The classification introduced by the Geological Survey at the turn of the century (Jukes-Browne and Hill, 1903) and still in use today divides the Lower Chalk into a basal Glauconitic Marl, succeeded upwards by the Chalk Marl, Grey Chalk and Plenus Marls. However, it is not generally appreciated that those authors established the classification in areas where the Chalk Marl and Grey Chalk were separated by the Totternhoe Stone (i.e. Chiltern Hills and areas to the north) and explicitly advised against its use in southern England (notably the Folkestone succession), where the upper limit of the Chalk Marl is difficult to define. At Dover, Jukes-Browne and Hill (1903) recognised two additional units within the broadly conceived Grey Chalk and below the Plenus Marls: their Bed 7 [usually referred to as Jukes-Browne Bed 71 and Bed 8 or the 'White Bed', in ascending order. Kennedy (1969, Fig. 2) published the first detailed measured sections of the Lower Chalk exposed in the cliffs between Folkestone and Dover and divided the succession beneath the terminal Plenus Marls into 14 numbered beds. Subsequently, better exposures than were available to Kennedy, particularly on a rotated block on the foreshore have allowed detailed measurements of the succession for the first time and have corrected previous misconceptions on the part of all observers regarding the stratigraphy and thickness of the lower part of the Chalk Marl (see Gale, 1989, 1990; Jenkyns et al, 1994).
The Cenomanian Chalk in the Anglo-Paris Basin can be divided into five disconformity-bounded units, which were identified by Robaszynski et al (in prep.) as sequences (see also Amédro, 1993, 1994). The Cenomanian Chalk of the basin (and farther afield) displays a more or less rhythmic succession of marl-limestone couplets, which has allowed the development of a refined cyclostratigraphy on a metre scale (Gale, 1990,1995). This cyclostratigraphy provided a higher resolution tool for correlation than any hitherto available. The Lewes Chalk successions on the two sides of the Channel, with the exception of the basal beds, are virtually identical (Amédro, 1994).
The base of the Lower Chalk is marked by the Glauconitic Marl, a unit up to 7 m or more in thickness, comprising dark grey marl with abundant glauconite grains and sporadic phosphatic clasts. The Glauconitic Marl rests non-sequentially and with erosive contact on the Upper Gault, typically on Bed XIII, but locally (e.g. Channel Tunnel Site Investigation, Craelius No. 2 Borehole) on Bed XI or even, in offshore boreholes (e.g. P000), on the undated Zone 6a (Hart, 1993, Fig. 2). The sediment of the Glauconitic Marl is piped down into the underlying Gault in a Thalassinoides burrow system. The unit is intensely bioturbated: the most conspicuous ichnofossils, generally but probably incorrectly referred to Spongeliomorpha, are cylindrical burrows with a central core of marl without conspicuous glauconite. The Glauconitic Marl exhibits poorly developed rhythmicity towards the top and is locally overlain by a thin, weakly glauconitic limestone (Marker horizon M2 of Gale, 1989), which has yielded the zonal index of the Neostlingoceras carcitanense Subzone.
Notwithstanding the recommendation of Jukes-Browne and Hill (1903), most workers on the Folkestone Lower Chalk succession (including TML) have recognised a broad and ill-defined subdivision of the interval from the Glauconitic Marl to the Plenus Marls into the Chalk Marl and Grey Chalk. The Chalk Marl is characterised by relatively dark, rhythmically bedded sediments and an overall CaCO3 content below 75%. Each rhythm or couplet typically comprises a basal dark bioclastic marl resting on the burrowed surface of the underlying couplet, above which there is an upward gradation with decreasing clay and increasing carbonate content to a pale cemented, spongiferous limestone (Destombes and Shephard-Thorn, 1971, Fig. 2). In the lowest and highest beds of the Chalk Marl (in the traditional sense, not that of TML), the sedimentary rhythmicity is very conspicuous, but in the middle part of the succession the rhythmicity is rather indistinct due to a higher overall clay content. The rhythmicity is commonly inferred to reflect orbital control of productivity in the precession cycle of the Milankovich Band (Gale, 1990; Hart, 1987; Paul, 1993; Gale, 1995). The detailed stratigraphy of the lower part of the Chalk Marl in the Channel Tunnel is in preparation and will appear on this Web Site shortly. Near the top of the Chalk Marl as traditionally understood, a closely spaced pair of conspicuous massive, prominent-weathering limestones provides an important marker horizon in the cliffs between Folkestone and Dover and on the opposite Channel coast. The lower of these limestones overlies a conspicuous dark bed characterised by a 'pulse fauna' including Oxytoma seminudum and 'Chlamys' arlesiensis (Paul et al, 1994). The higher limestone (the so-called Tenuis Limestone from the occurrence of Inoceramus tenuis) underlies the famous 'Cast Bed' of 19th Century fossil collectors, which was so named from the relative abundance of composite moulds of originally aragonite-shelled molluscs, notably gastropods. The Cast Bed yields very rare examples of the belemnite Actinocamax primus and abundant Entolium.
The Tenuis limestone marks the top of the Formation de Petit Blanc-Nez (Robaszynski and Amédro, 1986) in the Boulonnais cliff sections and also the boundary between the Craie Bleue and Craie Grise units used by TML in the Channel Tunnel investigations in the French sector (Amédro, 1994; Jouchoux, 1994; Amédro et al, 1994). In addition, it forms the boundary between the two mappable members (West Melbury and Zig Zag, in ascending order) into which BGS divides the traditional Lower Chalk above the Glauconitic Marl (Bristow et al, in prep.).
The Cast Bed is followed by several metres of conspicuously rhythmic marl-limestone alternations characterised by Orbirhynchia mantelliana and constituting the Orbirhynchia mantelliana Band as originally described by Kennedy (1969). However, it is now known that this is the topmost of three such Orbirhynchia bands developed in southern England. The (third) Orbirhynchia mantelliana Band terminates in a limestone with a flood abundance of Sciponoceras baculoide, above which there is a sudden shift in the ratio of planktonic to benthonic foraminifera, with an increase of the former over the latter. This shift is termed the PB break and also, because it is coincident with evidence of sedimentary discordance in mid-Channel boreholes (Carter and Destombes, 1972; Hart, 1993; Amédro, 1994), the mid-Cenomanian non-sequence (Carter and Hart, 1977 and references therein). Six further marl-limestone couplets are found above the PB break, which thus falls within, but not at the top of the Chalk Marl.
In contrast to the Chalk Marl, the Grey Chalk is characterised by paler coloured, less distinctly rhythmic sediments with an overall CaCO3 content exceeding 75%. A typical couplet begins with a thin flaser marl and terminates in a marly chalk rather than a hard, spongiferous limestone (Destombes and Shephard-Thorn, 1971, Fig. 2.). As in the case of the Chalk Marl, the rhythmicity is inferred to reflect orbital control of productivity. The Grey Chalk is generally much less fossiliferous, although the terebratulid brachiopod Concinnithyris subundata is relatively common near the base and 'Inoceramus atlanticus characterises the highest beds.
The Grey Chalk is followed by a group of beds of relatively coarse bioclastic, extensively bioturbated chalks, some 2 m thick, containing large specimens of the zonal index ammonite Acanthoceras jukesbrownei, as well as calcarenite-filled structures (the laminated structures of earlier literature), which tend to stand proud on weathered surfaces, and give a distinctive appearance to the bed.
Although these have been interpreted as scours, current opinion increasingly favours the idea that these structures represent truncated burrow-fills. This bed is traditionally known as Jukes-Browne Bed 7, although Robinson (1986) formally designated it as the Hay Cliff Member. The more many basal part is characterised by a concentration of small oysters (Pycnodonte sp.), which is also found in correlative developments elsewhere.
The White Bed is a unit restricted to the North Downs and the northeast margin of the Paris Basin (Mortimore et al, 1989) comprising extremely soft, poorly fossiliferous, homogeneous white chalk, with regular transverse and vertical joints. It is also present in northern France, where TML refer to it as the Craie Blanche (Amédro, 1994). The soft nature of this unit is emphasised by the quarry workers' name 'soapstone'. Few fossils are found apart from sporadic concentrations of the exogyrine oyster Amphidonte sp. and sparse localised occurrences of Inoceramus ictus.
The Plenus Mans comprise a thin, clearly defined unit of alternating, relatively fossiliferous, slightly green-coloured marls and marly limestones, which has commonly been given member or even formation status. The base of the Plenus Marls is a major erosion surface and sequence boundary, with Plenus Marls sediments piped down in burrows for up to 0.5 m into the underlying chalk of the White Bed. Based on the work of Jeffries (1963), the succession is divided into eight beds, which have been inferred to represent, together with the basal limestone of the overlying succession, five marl-limestone precession couplets (Gale, 1990; Lamolda et al, 1994). The Plenus Marls takes its name (and the earlier name of Belemnite Marls) from the common occurrence of the belemnite Actinocamax plenus in the higher part of the succession in Beds 4 to 6. The Plenus Marls of southern England and, in a broader context, of the Anglo-Paris Basin, must not be confused with the Plenus Marls Formation of North Sea offshore successions, which is probably slightly younger. The Plenus Marls mark the base of a major complex positive 13C excursion, which extends into the basal part of the overlying beds (Gale et al, 1993, Fig. 2) and is commonly referred to as the Oceanic Anoxic Event II. This excursion is accompanied by a significant stepwise extinction of the greater part of the microfauna and microflora, which has been generally, but not universally, attributed to increasing anoxic conditions followed by a gradual recovery as oxic conditions became reestablished. These faunal and geochemical changes have been the subject of intense multidisciplinary investigation in the relatively thin Plenus Marls successions at and near Dover (Jarvis et al, 1988; Leary et al, 1989; Jeans et al, 1991; Lamolda et al, 1994).
Middle Chalk Formation
Compared with the Sussex succession, the basal part of the Middle Chalk (uppermost Cenomanian and Lower Turning) in Kent is greatly condensed, forming the so-called Melbourn Rock Beds (Robinson, 1986), the Grit Bed of earlier literature. This part of the succession constitutes the Holywell Chalk of the standard lithostratigraphical classification used by BGS (Bristow et al, in prep.). The topmost Cenomanian (terminal Metoicoceras geslinianum Zone and Neocardioceras juddii Zone), including the two lower pairs of Meads Marls of the Eastbourne succession (Mortimore, 1986; Pomerol and Mortimore, 1993; Gale et al, 1993), is hence represented by about 1 m of intensely hard limestones with Sciponoceras, the marls themselves being recognisable only as thin marl wisps. The base of the Turonian Stage is taken at the base of the succeeding less indurated chalks by extrapolation from the Sussex succession (Gale et al, 1993; Pomerol and Mortimore, 1993). The greater part of the Holywell Chalk is characterised by shell-detrital (predominantly fragmented and comminuted Mytiloides spp.) and intraclastic chalks. Near the base, a bed of calcarenite largely composed of microcrinoid debris (Roveacrinus) yields Fagesia catinus. Higher in the succession, the content of shell detritus reaches a maximum (including a bed with serpulid-encrusted Mytiloides, the so-called Filograna avita bed, which can be traced throughout the Anglo-Paris Basin), above which there is a rhythmic alternation of shell-detrital chalks and non-shelly chalks. By extrapolation from N. American successions, the base of the Middle Turonian can be inferred to lie in the higher part of this rhythmic alternation, above an horizon which has yielded the highest Mammites nodosoides, together with Morrowites wingi and Metasigaloceras rusticum. However, the first unequivocal records of Middle Tunonian ammonites (Collignoniceras woollgari) are from the basal part of the overlying New Pit Chalk in Sussex, i.e. above the upper limit of the shell-detrital chalks of the Holywell Chalk.
The Holywell Chalk is succeeded abruptly by relatively poorly fossiliferous, smooth, inconspicuously rhythmically bedded white chalks without shell-detritus. The unit is flintless in east Kent, except at the top, where small flints are locally found within the Glynde Marls sequence. However, to the west, in the vicinity of the Medway, small inconspicuous flints (the Glyndebourne Flints of Sussex) are again developed at the base of the member. Three conspicuous clay-rich marl seams, several centimetres thick (the Round Down Marl, New Pit Marl 1 and New Pit Marl 2, in ascending order) are a feature of the New Pit Chalk in the cliff path sections west of Dover. Close to the top of the member, a closely spaced and laterally variable group of up to 6 marl seams (the Glynde Marls) provides another useful marker; Robinson (1986) introduced the name Maxton Marls for this group to emphasise the difficulty of achieving exact correlations between the individual marl seams in the North Downs succession at this level with those comprising the Glynde Marls of the South Downs.
The base of the Upper Chalk is now taken by BGS at the (revised) base of the Lewes Chalk, an horizon which is marked by the onset up-section of nodular chalk, a short distance above the highest of the Glynde/Maxton Marls and at approximately the level of the first flint in the succession (Lydden Spout Flint). It must be emphasised that the base of the Upper Chalk as currently mapped by BGS (Bristow et al, in prep.) is significantly lower than the traditional lower limit at the base of the so-called Basal Complex (**).
As with the underlying New Pit Chalk, there are several conspicuous marl seams, ranging from less than one, to several centimetres in thickness. All these marl seams produce distinctive spikes in downhole geophysical logs of wells and boreholes (e.g. Mortimore, 1986; Mortimore and Pomerol, 1987) and can thus be more or less readily correlated in the Chalk subcrop. In addition, Wray and Gale (1993) have demonstrated that each marl seam has its own characteristic trace element composition which provides a more or less unique geochemical 'fingerprint'. Rare earth element analysis of the marls by Wray (in prep.) has demonstrated that the New Pit Marls, the higher Glynde Marls, Southerham Marl 2 and Bridgewick Marl 2 have the signature characteristic of a detrital marl, whereas Glynde 1, Southerham 1, Caburn and Bridgewick 1 are distinguished by a significant negative europium anomaly and are thus probably of vulcanogenic origin and comparable to the approximately contemporaneous tuffs in the German Chalk (Wray, 1995).
Palaeogene rocks are preserved in two separate basins within the limits of the region. The southern North Sea Basin has onshore outcrops in northeast Kent and in northwest France, east of Calais, which are connected offshore. In the south, a part of the Hampshire-Dieppe Basin (Curry and Smith, 1975) is represented only by sea-bed outcrops (**). It is believed by some authors that these two basins formed a single depositional area prior to the mid-Tertiary (Alpine) folding. However, it is probable that the essential features of the Tertiary folds were established during the Laramide tectonic phase (Mortimore and Pomerol, 1991) and that the Weald-Artois axis was in existence as a positive feature from end-Cretaceous times (Cope et al, 1992). The marine and marginal marine sediments preserved in the two basins are mainly clastic clays and sands with sporadic limestones. Thin flint pebble beds usually mark non-sequences followed by transgressions. Contemporaneous vulcanicity in Western Scotland and elsewhere in northwest Europe, linked to sea floor spreading, gave rise to discrete thin, but widespread, ashfall layers (tephra) at some horizons (Knox and Morton, 1983, 1988) and a general presence of smectitic clay minerals in the sediments.
The earliest Palaeogene rocks in our area were deposited during transgressions over the eroded surface of the Chalk, in marginal marine environments for the most part. A major unconformity occurs between the earliest Palaeogene sediments and the Chalk in both basins. This break marks a phase of uplift (Laramide) and gentle folding accompanied by subaerial and marine erosion of the Chalk during the period when earliest Palaeogene Danian carbonates were laid down in the central North Sea and in the Paris Basin (Pomerol, 1989).
Correlation between the onshore and offshore developments of the Palaeogene is hampered by the absence or non-preservation of calcareous planktonic foraminifera and nannoplankton, which form the basis of international correlation of marine sequences.
Lithostratigraphy combined with magnetostratigraphy (Townsend and Hailwood, 1985; Aubry et al, 1986) and biostratigraphy based on organic-walled microplankton (dinoflagellates) (Costa and Downier, 1976; Costa and Manum, 1988) tends to be the most useful method of studying these rocks on a regional basis. For an integrated study see Neal et al, 1994.
The cliff sections of northeast Kent were the scene of much early research on the Palaeogene rocks of the London Basin. The cliff sections at Herne Bay, Reculver and Pegwell Bay were studied by Prestwich (1850, 1852, 1854), Whitaker (1866), Gardner (1883) and Burrows and Holland (1897). The term Thanet Sands was first used by Prestwich (1852), implicitly using Pegwell Bay as the type section.
More recently, Haynes (1955, 1956-58) has studied the foraminifera of the Thanet Formation, discovering many derived Cretaceous planktonic forms associated with the indigenous species. This is a reflection of the marginal marine environment and highlights the difficulties of correlation with more complete deeper water sequences. Ward (1977, 1978) has redescribed the sections at Pegwell Bay and Herne Bay respectively and has given revised macrofaunal lists. These sections are also covered in the relevant BGS Memoirs (Shephard-Thorn, 1988; Holmes, 1981). The palynofloral associations of the Thanet Beds were documented by Jolley (1992) and the foraminiferal biostratigraphy of the Palaeogene formations was reviewed by Murray et al (1981). The lithostratigraphy of these formations has been discussed by Hester (1965), Ellison (1983) and Ellison et al (1994). The latter authors attempted to revise and rationalise the classification of these rocks in the light of recent cored boreholes and field experience in the London Basin and East Anglia. Their proposed new terminology will be used in the following account: Table ** shows their scheme in relation to the nannofossil zones and former usage.
This formation is believed to have a maximum thickness of about 30 m in northeast Kent. Up to 24 m are exposed above the unconformity with the Chalk at Pegwell Bay, where the contact with the overlying Upnor Formation is not seen (Ward, 1977; Shephard-Thorn, 1988). The latter junction is exposed in the cliffs and foreshore at Herne Bay, but in the absence of a distinctive basal bed its precise position has been controversial in the past. Whitaker (1872) recognised five lithological groupings in the Thanet Sand Formation, which were subsequently named by Haynes (1956) as follows:
(a)Bullhead Flint Conglomerate
Of these, units a, b, d and e are present at Pegwell, while unit c, the Kentish Sands, replaces the Pegwell Marls and Reculver Silts in outcrops nearer London. The Stourmouth Clays, Pegwell Marls and the Reculver Silts were given formal member status by Ward (1978), but Ellison et al (1994) considered that these units appeared to be of significance only in east Kent and that informal nomenclature was more appropriate.
With the addition of the overlying Upnor Formation (see below), the two sections at Pegwell Bay and Herne Bay constitute the composite stratotype of the Thanetian Stage of the Paleocene. For details of the calcareous nannoplankton biozonation, magnetostratigraphy and inferred sequence stratigraphy of the stratotype in the context of data from a cored borehole through an unweathered Thanetian succession at Bradwell, Essex, see the recent review by Knox et al, (1994). It is noteworthy that those authors considered that the type Thanetian Formation could be assigned to two separate sequences: the lower sequence, comprising the beds up to and including the Stourmouth Clays was correlated with the Heersian of Belgium; the higher sequence comprising the Reculver Silts was equated with Lower Landenian. See also Neal et al, 1994, Fig. 18.
At Pegwell Bay, Upper Chalk of the Marsupites testudinarius Zone is overlain with sharp unconformity by the Bullhead Bed, comprising 0.15 m of dark green, marly glauconitic sand packed with rounded, dark-coated flint pebbles. This in turn is overlain by the Cliffsend Greensand Bed (Ward, 1977), which is a bed of compact, fine grained glauconitic sandy marl. The occurrence of igneous grains in association with zeolites in these basal beds (Knox, 1979) is attributed to ash falls belonging to an early phase (Phase I of Knox and Morton, 1983) of Tertiary volcanism. Above these basal beds of the Thanet Sand Formation, the Stourmouth Clays comprise thin alternations of clay and silty clay totalling 4.5 m and devoid of fossils. The Pegwell Marls above include 13 m of uniform dark greenish-grey marls, with a 0.9 m dark sandy, glauconitic marl (the 'Black Band') at the base. The latter yields the foraminifer Astacolus crepidula, while the marls are rich in bivalves including Arctica morrisi, Cyrtodaria rutupiensis and Thracia aff. oblata. Only the lowest 5 m of the Reculver Silts are preserved at the top of the Pegwell section and consist mainly of pale yellowish-grey, very fine silty sands speckled with glauconite and mica. Drifted bivalve shells are common and form a thin band at the base.
Large rounded calcareous 'doggers' of sandstone, up to 0.4-1.5 m, occur in two layers about 1.25 and 2.5 m above the base.
Up to 17.5 m of Reculver Silts are present at Herne Bay which probably overlap with the highest beds at Pegwell, giving a total thickness for the unit of about 20 m. The cliff section is largely decalcified, but fresh exposures can be found on the foreshore at low water. The fauna is very rich and well preserved (see Ward, 1978 for biostratigraphical details) and comprises predominantly molluscs and vertebrates. Both non-silicified (notably large specimens of Arctica spp.) and silicified (Corbula regulbiensis) molluscs are represented, the latter type of preservation being particularly characteristic of the higher beds. A silicified facies with a rich molluscan fauna, but not including Corbula, is found in inland exposures between Faversham and Canterbury: this facies is not represented at Herne Bay. The bivalve fauna at Herne Bay includes A. morrisi A. scutellaria, Corbula regulbiensis, Cucullaea decussata, Dosiniopsis orbicularis, Glycymeris terebratularis, Nucula fragilis and Panopaea remensis.
This formation, formerly known as the 'Bottom Bed' of the Woolwich and Reading Beds (= Lambeth Group, see Table **), is preserved, up to 10 m thick, below the erosion surface at the base of the Harwich Formation (formerly Oldhaven Beds) in northeast Kent. Although excluded from the type Thanet Formation, the Upnor Formation is included in the type Thanetian Stage (Knox et al, 1994) and is attributed to nannoplankton Zone 9 and to Chron 24R. All trace of the succeeding Woolwich Formation (sensu Ellison et al, 1994) was removed in the erosional episode preceding the deposition of the Harwich Formation.
Ward (1978) discussed the problem of recognising the junction with the Thanet Formation in the Herne Bay section and adjacent areas, owing to the absence of a distinctive basal pebble bed. He argued that the Beltinge Fish Bed, a 0.4 m bed of dark olive-grey silty clay, rich in teeth and hones of sharks and other fish, which rests on a burrowed erosion surface, be taken as the junction. The remainder of the Upnor Formation, assigned by Ward (1978) to the Woolwich and Reading Formation, is seen for up to 5.2 m at Herne Bay, where it comprises fine glauconite-speckled silty sands with few fossils, but heavily bioturbated.
This term has been introduced by Ellison ci at. (1994) to bring together various units of sandy and pebbly lithology which may he regarded as forming a basal bed of the London Clay, including the beds with tephra layers that are well seen in the Harwich district (Knox and Ellison, 1979)(Table **).
In northeast Kent, it was formerly known as the Oldhaven Beds or Formation, of which the thickness at Herne Bay (Ward, 1978) totals about 7 m. Above the erosional base, a 0.3 m basal pebble bed of rounded brown and black flint pebbles is usually present in a soft sandy or harder ferruginous matrix. A bed of glauconitic silty sand 0.75 m thick overlies the pebble bed and, where the latter is locally absent, rests on the Upnor Formation. Above these two thin units, the main part of the formation comprises 6 m of glauconitic, cross-bedded sands, with lenticles of drifted bivalve shells fairly common throughout. Tabular calcareous doggers of cemented sand occur within this part of the formation. Knox (1983) reported disseminated well-preserved volcanic grains in the Oldhaven Beds at Herne Bay, which he attributed to the main Balder Phase of volcanism (Phase 2b of Knox and Morton, 1983) in the North Sea.
Extensive outcrops of this formation occur in the Isle of Sheppey and the Blean areas of northeast Kent (Holmes, 1981), where a full thickness of about 145 m is preserved locally. King (1981) erected a fivefold lithostratigraphical subdivision based on widespread major sedimentary cycles. The Harwich Formation discussed above corresponds to King's lowest Division A.
The major part of the London Clay is made up of blue-grey or olive-grey clays and silty clays with silty sands, which usually weather to grey-brown shades near the surface. Bands of septarian nodules with calcite veining occur at certain levels, as do pebbly or sandy horizons. The formation has yielded a rich molluscan fauna, including bivalves, gastropods and nautiloids, as well as crustacean and avian remains. It is noted particularly for its plant fossils, which reflect the tropical climate of the period (Chandler, 1961; Collinson, 1983).
The Palaeogene rocks of northeast Kent pass beneath the sea at Herne Bay, where they strike E-W on the north flank of the Isle of Thanet for about 50 km, before veering to a N-S strike for a similar distance to cross the French coast west of Calais. The submarine outcrops have been detailed by geophysical profiling and sea-bed boreholes and sampling (BGS 1:250000 sheets Thames Estuary (1989) and Dungeness-Boulogne (1988). Isolated remnants of Palaeogene formations are preserved in the Boulonnais and to the south.
Beneath the North Sea, the strata equivalent to the London Clay and Harwich formations are grouped as the Balder Formation, the equivalents of the Woolwich and Upnor formations as the Sele Formation and those of the Thanet Formation as the Lista Formation.
Onshore in northern France near Calais and in the Boulonnais, the equivalents of the Thanet Formation are represented by the Argile de Louvil, a clay formation 15-30 m thick, overlain diachronously by the Sables et grès d'Ostricourt et de Saint Josse, comprising 18-36 m of sands and sandstones. The succeeding Argile de Saint Aubin, 18 m thick, is approximately equivalent to parts of the Upnor and Woolwich formations. The London Clay is represented by the Argile de Flandres, 30-90 m thick. Traces of early Palaeogene sands are preserved in karstic solution pockets in the Chalk. Several metres of these sands, overlain by Ypresian sands and clays with a basal flint pebble bed, were formally exposed at the base of the Noires Mottes beneath supposed Pliocene sands (Somme, 1985).
The entirely submarine outcrops of this basin were described by Curry and Smith (1975). They comprise rocks equivalent to the Woolwich and Reading formations of Hampshire, as well as to the London Clay Formation, Bracklesham Group and Barton Group, all in lithologies comparable to their onshore outcrops. Within the study area defined in Fig. 4.1, only the northern limb of the basin is represented.
Isolated remnants of Miocene-Pliocene deposits
At Lenham, near Maidstone in Kent, isolated pockets of ferruginous sands (the Lenham Beds), variously accorded a latest Miocene or earliest Pliocene age, occur at about 180 m OD on the Chalk escarpment of the North Downs, associated with residual deposits of Clay-with-flints. The shallow-water molluscan fauna of these sands has been correlated in the past with that of the Miocene Diest Sands of Belgium. However, it is not strictly age-diagnostic and, moreover, includes elements of distinct English Channel, as opposed to North Sea, affinities (Janssen in Balson, 1990). This faunal difference may imply that the Weald-Artois axis was extant at the time of their deposition. The Lenham Beds appear to be relicts of the sediments deposited by a marine transgression from the west through the English Channel region (Cameron et al, 1992). Similar deposits, but without fossils, occur on the French coast south of Sangatte at 100-150 m OD resting on Thanetian sands. These deposits are also generally correlated with the Miocene Diest Sands.
Marine deposits of similar age and facies occur at or about sea level in East Anglia and Belgium, so that the height differential between these and the Lenham Beds requires an explanation involving warping, subsidence or a mechanism related to the development of the North Sea Basin and the Weald-Artois axis.
The first significant structural inversion of the Weald Jurassic-Cretaceous basin may be considered to have begun in the interval represented by the late Cretaceous-Palaeogene unconformity. This is shown by the fact that the zonal position of the Chalk below the unconformity may range within the area from the Offaster pilula Zone in Thanet down to the Micraster coranguinum Zone around Faversham. The reworking of flints into the Palaeogene deposits also implies that the Chalk was being uplifted and eroded at this time.
The major phase of inversion is generally regarded as of mid-Miocene age, so that the Lenham Beds and related deposits seemingly postdate it.
The Quaternary Period is conventionally divided into the Pleistocene and the Holocene and extends back over the last 2.5 million years of earth history. The Pleistocene makes up by far the greater part and includes a number of major climatic oscillations from very cold to temperate regimes. Studies of the variation in oxygen isotope ratios in foraminifera preserved in continuous sedimentary sequences in deep oceans (Shackleton and Opdyke, 1973) have shown that over 20 climatic oscillations occurred. Northern Europe was covered by regional ice sheets during at least three of the more recent cold episodes, usually known as glacials. The growth of polar ice caps in these periods led to global sea level falls of 100 m or more. The intervening temperate periods, known as interglacials, saw the return of temperate vegetation and faunas and rises of sea level.
These changes of climate and base level had profound effects on the final shaping of the landscape of the study area. The most significant of these was the breaching of the Weald-Artois Chalk ridge to open the Strait of Dover, thereby connecting the waters of the English Channel and the North Sea and, incidentally, providing the requirement for the Channel Tunnel.
Much new information on the Quaternary geology of the study area has accrued in the last 30 years from researches carried out for the Channel Tunnel project, as well as English and French government-funded marine surveys and academic studies, all of which have been reviewed by Hamblin et al (1992) and Cameron et al (1992). One aspect that remains controversial is the extent of former glaciations and, in particular, the proposal that the English Channel was invaded by an ice sheet from the west (Kellaway et al, 1975).
Whether or not glaciers were ever active in the study area, it was surely exposed to the full rigours of periglacial weathering during some of the cold/low sea level periods of the late Quaternary. At such times, the Chalk and other rocks now forming the sea-bed would have been exposed at the surface and subject to subaerial weathering by streams and rivers, deep frost and chemical weathering and locally covered by sediments.
A simplified Quaternary history, partially based on Hamblin et al (1992) is given in Table **. The exact timing of the first breach of the Weald-Artois ridge to open the Strait of Dover remains uncertain, but most authors favour the view that it resulted from the overspill of an ice-dammed lake in the southern bight of the North Sea during the Anglian (Elsterian) glacial (Gibbard, 1988).
Apart from climatically induced changes of base level and possible isostatic effects, long-term eustatic trends operated simultaneously, partly linked to crustal warping and subsidence associated with the North Sea Basin. Thus, in general, the older river terraces and raised beaches occur at higher elevations than the younger examples.
It is likely that much of the eastern English Channel was land at the beginning of the Quaternary and that the Weald-Artois ridge formed a positive feature from which drainage flowed north and south.
The oldest Quaternary deposit is the Clay-with-flints (argile a silex of France), which is a residual red-brown stony clay resulting from the virtually in situ weathering of Palaeogene strata (Reading Formation) resting on an erosion surface cut on the dip slope of the Chalk (Catt, 1986). Solution piping commonly effects the Chalk beneath the Clay-with-flints, which is let down into the solution hollows. This phenomenon is an aspect of karstic weathering as developed on the chalklands of England and France (Rodet, 1991, 1993).
The early Pleistocene marine formations of East Anglia (Ludhamian to Beestonian) are not represented in the study area, probably because they were not deposited over the Weald-Artois ridge. It is possible, however, that the high-level Netley Heath and Headley Heath deposits of Surrey may represent remnants of once more extensive marine formations of this period.
No physical evidence of glaciation in the early Quaternary is preserved in the study area.
This long period of over 600,000 years includes the 'Cromerian Complex', the Anglian (Elsterian) glacial, the Hoxnian (Holsteinian) interglacial and the penultimate 'Wolstonian' (Saalian) glacial (Table **).
Little is known about the Cromerian interval in the study area, hut the Goodwood-Slindon raised beach of West Sussex has been attributed to it on faunal and aminostratigraphical evidence. This raised beach and the overlying deposits preserve sites of flint tool manufacture and have recently yielded a tibia of an early hominid ('Boxgrove Man'), which must represent one of the earliest human records (0.5 Ma) in Europe: it has been attributed to Homo cf. heidelbergensis (Roberts et al, 1994; Gamble, 1994). There is no faunal evidence of the opening of the Strait of Dover, but it is likely that the river system on the floor of the English Channel began to develop during glacial/low sea level periods.
The Anglian glacial period was long and extensive ice sheets developed over England (north of the Thames), the North Sea and northern Europe, which laid down vast areas of till and associated outwash sands and gravels. The ice sheet in the southern North Sea is believed to have dammed up a lake against the Weald-Artois ridge, which eventually overspilled to the south eroding a gap to the English Channel. Colbeaux et al (1980) and Robaszynski and Amédro (1986) have suggested that the initial breach in the ridge resulted from the reactivation of an offshore Cretaceous graben with a N30E orientation parallel to the regional trend of one of the two main sets of faults traversing the Boulonnais. A possible marginal overflow channel filled with gravels, sands and clays has been recognised near Wissant (Roep et al, 1975). The initial gap or gaps have been widened and deepened to form the present Strait of Dover in subsequent periods of oscillating sea level and climate.
The Fosse Dangeard (Destombes et al, 1975) is an anomalous, infilled, closed hollow extending for some 18 km W10N in the English half of the Strait of Dover.
Destombes et al (1975) proposed a subglacial origin for this feature, linked to the glaciation of the English Channel, which attracted little support at the time. Subsequently Wingfield (1989, 1990) has suggested that the Fosse Dangeard and other similar incisions are caused by high energy discharges of glacial lake waters, known as jokulhaups, at the tidewater margins of ice sheets. Smith (1985, 1989) proposed a mechanism involving a catastrophic breaching of the Chalk ridge, which excavated the 'fosse' and related hollows in the manner of plunge pools. For the moment the age and origin of the Fosse Dangeard remain an enigma, but part of the answer may lie in the unsampled sediments in its deeper parts.
The Ipswichian interglacial is now generally recognised as compound, with two temperate intervals separated by a cooler one. The raised beach at Sangatte near Calais is attributed to this period and is regarded as equivalent to the raised beaches at Brighton in Sussex and near Sandwich in east Kent (Shephard-Thorn, 1988). This high sea level seems to have marked the breaking of Palaeolithic cultural links between Britain and continental Europe, suggesting that the Strait of Dover had become a considerable physical barrier by this time.
Sea level fell again in the Devensian (Weichselian) glacial, exposing most of the Channel bed to severe periglacial weathering. Silty, sandy outwash deposits on the floor of the southern North Sea were probably the source of extensive loessic deposits (brickearth) laid down at this time in east Kent (Kerney, 1965; Pitcher et al, 1954) and in northern France (limons).
Periglacial weathering in the late-Glacial period that closed the Devensian was instrumental in the development of the coombes on the scarp face of the Chalk escarpment in Kent (Kerney, 1963; Kerney et al, 1964; Preece, 1994). Deposits laid down in the floor of the coombes and as aprons in front of them comprise chalky washes due to seasonal runoff interspersed with thin fossil soils marking warmer episodes. Land molluscan faunas, pollen studies and radiocarbon dating of these deposits yield valuable evidence of climatic change from the late-Glacial through the Holocene to the present day (Kerney et al, 1980). Periglacial weathering also affected rocks in the Strait of Dover exposed during the low sea level of the Devensian glacial episode, having some impact on the final routing of the Channel Tunnel.
The last 10,000 years, since the last retreat of European glaciers, is variously known as the Holocene, Flandrian or Recent period. It was marked by a rapid rise in sea level in the first 5000 years, with gentler oscillations since. The channels of the rivers and estuaries of England and northern France were 'silted up' with up to 30 m or so of alluvial/estuarine deposits laid down in pace with rising sea level. Peat beds and 'submerged forests' record regressions when the sea temporarily withdrew once more. Studies of such sequences have thrown much light on sea level changes during the Holocene (Dubois, 1924; Devoy, 1979, 1982; Somme et al., 1992). Offshore, the present system of sandbars and shoals has developed in response to tidal currents.
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