The rock floor of the English Channel and its significance for the interpretation of marine unconformities

The rock floor of the English Channel and its significance for the interpretation of marine unconformities Dennis Curry CURRY, D. 1989. The rock floor of the English Channel and its significance for the interpretation of marine unconformities. Proc. Geol. Ass., 100(3), 339-52. The English Channel is for the most part an area of erosion, not of permanent deposition. In an area extending from beyond the Scillies in the west as far as Ostend in the east there is solid rock at or near the seafloor almost everywhere. The overlying superficial sediment is coarse-grained and contains abundant derived fossils-a typical basement bed. The rock floor is suffering continuing erosion, but the materials so released are not accumulating locally, but are being carried towards the continental slope or into the North Sea. The situation appears to be a stable one, with a life expectancy of perhaps I m.y. The implications of this large area of marine non-deposition are discussed, using the examples of other unconformities in the earlier history of the Channel region. Mallard Creek, Spinney Lane, Itchenor, Chichester, West Sussex, P020 7DJ

1. INTRODUCTION Conventional sources of information give no hint that there is anything unusual about the floor of the English Channel and its western approaches (an area here referred to as the Channel). Admiralty charts (e.g. 2649, 2675) show a generally flat floor, dipping gently oceanwards towards the shelf edge, which lies 120-250 km from the nearest coastline. Islands and any shoals which stand abruptly above this floor are known to be of very hard rock, or could reasonably be presumed to be so. Reports of bottom indications are predominantly of sand with shells, but gravel is recorded widely within the Channel proper. Records of fine sand are in a minority, and there are almost none of mud or clay except in sheltered localities near the coast. Gradients on land near to the Channel coast are for the most part very much steeper than those at sea and are seen to be associated with the contrast between solid rock on land and sand offshore, a contrast particularly well seen in the western Channel. These observations could reasonably have led to the conclusion that the rock floor of the Channel has a dissected surface like that of the nearby land, and that its thalweg (or thalwegs) is perhaps 300 m down south of Cornwall; the whole being buried by a wedge of Recent or near-Recent sediments following a period of denudation and excavation associated with much lowered sea levels in Pleistocene (and perhaps earlier) times. The first general geological research at sea in Western Europe was carried out by Dangeard (1929) in the period between 1921 and 1928, using a dredge designed to prise solid rock off submarine outcrops. His results immediately cast doubt on any such sedimentary wedge hypothesis as he found widespread evidence of rock in situ, sometimes at a considerable distance from land. From nearly 1000 stations, mostly

in the Channel, he succeeded in collecting from bedrock at only a small minority of sites. However he was able to demonstrate its nearby existence in many cases. He was, for instance, able to identify probable submarine outcrops of Chalk by the local abundance in his dredgings of flints with their outer skin intact. He dredged Liassic limestone pebbles in restricted areas south of Plymouth and north of the Cherbourg peninsula and he found much evidence for the existence of Eocene limestones off the northern coast of Brittany. He also recognized the significance of the widespread presence of reworked fossils in the superficial sediments which he dredged, and of the destruction of bedrock by boring organisms. In all these cases it was more than 30 years before his observations began to be confirmed by later workers. W. B. R. King (1949, 1954) discussed the then state of geological knowledge of the Channel. He was particularly interested in a record by Dangeard of Triassic clay in mid-Channel south of the Isle of Wight. He thought this identification to be unlikely so in a succession of cruises he explored that area with a free-fall rockcorer, recovering 70 rock cores in all (Curry, 1962). Forty-three of these cores were collected in a period of four days. The number of stations prospected in that time could hardly have been more than 80; to imply a success rate of 50% or better. Dangeard's record was found to be either of Wealden or Reading beds, and the work established that drop-coring, in some areas at least, was an efficient method of rock sampling. However of a set of 10 stations manned by King in 1955 to the south of Beachy Head only one yielded rock, suggesting that drop-coring might not be so successful everywhere. Following King's retirement, W. F. Whittard took over his line of marine research, and over a period of years ran a coring programme over the whole area of the western Channel. Similar operations in the eastern

339

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DENNIS CURRY

half of the Channel were carried out under the guidance of D. T. Donovan, whilst teams of French geologists prospected the whole of the southern half of the Channel (see Smith, Hamilton, Williams & Hommeril, 1972 and Hamilton, Hommeril, Larsonneur & Smith, 1975 for references). The total number of core stations manned as a result was about 2000, from about half of which rock in situ was collected. This high density of successful sampling has been a main factor underlying the creation of the relatively detailed geological maps now available for the Channel area. At the same time this sampling has revealed areas where drop-coring has never succeeded in recovering rock, as for instance within 80 km of the edge of the continental shelf; or has a low success rate (some areas of the eastern Channel). The standard length of a tube used for rock-coring is 1 m and that length determines its maximum penetration. In its descent the corer cores any overlying sediment and so when a rock sample is recovered the thickness of the overlying superficial sediment can be measured. It follows, of course, that in any area with a high coring success rate much of the bottom must be covered by no more than a veneer «1 m) of superficials. Continuous marine seismic profiles (C.S.P.) give a picture of the thickness, attitude, structure and, to some extent, the composition of the submarine rock floor traversed. The information which they provide thus complements that relating to lithology and biostratigraphy which is furnished by rock cores and greatly improves the precision of any resultant geological interpretation. Superficial deposits are also recorded if they are sufficiently thick. However the sound frequencies most effective in providing good rock penetration are low (20-400 hertz) and, because of their long wave length, cannot resolve a thin cover (less than ! wave-length, sayj.v'This means that a survey aimed at rock structures may not reveal the presence of superficials if their thickness is less than 5 m or so. With this reservation, however, it may be said that over by far the greater part of the Channel, conventional C.S.P. records display no superficials, implying that, if present, these are less than 5 m thick. Thicker superficials are present locally; for instance in the form of elongated sandbanks (Varne, Bassurelle, etc.) radiating from the Dover Straits. They also fill (or partly fill) channels cut in the rock floor when it was exposed to erosion by retreat of the sea during recent cold episodes (e.g., Hurd Deep (Hamilton & Smith, 1972), eastern Channel area (Auffret, Alduc, Larsonneur & Smith, 1980)). Such local areas of thick superficials have however only marginal relevance to the ideas discussed in the present paper. Thus coring and seismic profiling, both aimed at determining geological structure beneath the Channel floor, have independently revealed as a by-product that the floor itself is constructed of rock at or near

the sea bottom, with only restricted sediment cover. Much has been published on the solid geology of the Channel based on these two means of observation. However little, so far as I am aware, has been published about the geological significance of the rock floor itself: its creation, maintenance and possible future history. This paper is designed to rectify that omission. 2. THE ROCK FLOOR OF THE ENGLISH CHANNEL AND ITS SEDIMENTARY COVER (a) Detailed character of the rock floor-evidence from cores The upper surface of cores taken by drop-coring is typically flat or slightly undulating, but in a small minority of cases (?5%) shows Recent borings, the largest being due to bivalves. These may be complete, but are mostly more or less truncated by erosion. Only rarely do the borings still hold the original valves, and the writer, amongst some tens of cases, has never retrieved a bored core with its contained boring mollusc intact. Very rarely indeed (<<1% of cases) does the surface carry attached living bios-bryozoans, serpulids for example-indicating that the core was collected from a bare rock surface. The rock itself is typically very fresh; evidence of oxidation is absent except in some cores taken in shallower water «40 m). Superficial induration is however not uncommon in cores from Chalk.

(b) Evidence from superficial sediments Superficial deposits are typically of shelly sand with a variable content of gravel which is almost without exception well-rounded, non-calcareous and very hard (flint, chert, sandstone, vein-quartz, and rare schist, granite, etc.) Pebbles of calcareous rocks (when present) are conspicuous because they are bored and clearly in course of destruction. Associated Recent mollusc shells are suffering the same fate, complete specimens are in a minority and the presence of large quantities of rounded shell fragments of all sizes is reminiscent of the English Crags. Detailed charts (Fig. 1) are available of the sediment distribution in the Channel (e.g. Larsonneur, Vaslet & Auffret, 1979) and Hamilton, Sommerville & Stanford (1980) have discussed this in relation to bottom currents. Extensive studies (e.g., Stride, 1963, 1982) have been made of the bedforms and internal structure of sediments around the British Isles, and these have been related to tidal and wave regimes and patterns of transport. However little systematic work has been published on the non-biogenic components of individual samples, and there is no comprehensive qualitative comparative study, so far as I am aware. A pilot study by the writer of 15 samples taken in the central Channel south of the Isle of Wight to Beachy Head showed median grain sizes between 150 f-l and

341

THE ROCK FLOOR OF THE ENGLISH CHANNEL

hard-rock coasts, coarse or medium sand is the rule offshore, fine sand and (especially) silty clay being present only in sheltered areas or regions of entrapment (see for example Larsonneur 1972b). Derived fossils are widespread. (c) Prolongations of the rock floor of the Channel

Fig. 1. Superficial sediments on the floor of the English Channel. 1. sandy muds; 2. sands; 3. gravelly sands; 4. sandy gravels.

2000 u, the median for all samples being about 500 j1 (the boundary between medium and coarse sand). This is a grain size of a coarseness almost unknown in the Mesozoic and Tertiary sediments exposed in the Channel region, only a few levels in the Permo-Trias, Corallian and Wealden yielding it in any quantity. The relevance of that contrast will be pointed out later. II of the 15 samples yielded derived fossils, which included belemnites, Purbeckian ostracods, Chalk foraminiferids, fish teeth and (common) nummulites. Some of the latter could be demonstrated to have travelled at least 10 km from the parent rock. Although this analysis relates to a very few samples from a single small area, my own observations confirm that these are typical of the Channel as a whole. Mean grain size is related to the local hydrodynamic regime, pebble banks are more common near exposed

In the Western Approaches (Curry, Hamilton & Smith, 1971) the rock floor disappears southwestwards under a thickening wedge of Recent sediments which may be up to 200 m thick at the shelf edge (Little Sole Formation, Evans & Hughes, 1984). This area of floor has been interpreted as an erosion platform cut at a time of low sea level; the platform having subsequently been covered by material which had accumulated to the east in Pleistocene times, mixed with sediment derived from downcutting of the floor of the Channel and the erosion of nearby land. East of the Channel the rock floor extends at surface at least as far as Ostend, beyond which it becomes buried under the Recent sediments of the North Sea Basin. (d) A nascent unconfonnity It will be seen that the rock floor of the Channel, with

its overlying sediment cover, presents the features associated with a typical geological unconformity underlying a marine succession. There is a widespread and extremely flat rock platform of variable age, indurated and bored in places, which is overlain abruptly by a thin series of relatively coarse sediments which include pebbles and derived fossils, some far-travelled. These units of course represent the surface of the unconformity and its overlying

Land's End

Devonion

Plymouth

(to Jurassic

Wealden

?Jurassic (to s!uth)

Ostend

-

Unconformity

Fig. 2. Simplified geological section approximately along the centre line of the English Channel. JI-Lower Jurassic; Jm-Middle Jurassic; Ju-Upper Jurassic; Wid-Wealden; Apt-Aptian; Ce-Cenomanian; Co---Coniacian; CaCampanian.

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DENNIS CURRY

basement bed respectively. Finally, to east and west , there is a continuing upward marine succession (see Fig. 2). Between these areas continuous deposition has not yet been established and the development of the unconformity is incomplete; hence the use of the word 'nascent' in the paragraph heading. A small section of the future unconformity is of course already in being, that nearest to the edge of the continental shelf. (e) Erosion of the rock floor of the Channel This has been documented off the Dorset coast (Donovan & Stride, 1961), where current erosion has revealed structural patterns in the Jurassic rocks , and is implied by reports of bare floor elsewhere (e.g. Stride, 1982, p. 66). Further evidence is found in the presence of borings and the generally fresh nature of the upper surface of rock cores. It is also provided by the abundance of derived fossils in the superficial sediment and the local presence in quantity of undamaged flints and lumps of silicified chalk; also of reworked glauconite and ooliths of limonite (the latter in areas of Lower Greensand) . In addition the lowest layer of sediment trapped above rock-cores frequently contains a slurry of the underlying rock; especially noticeable when that rock is Chalk or Trias. The writer believes that bottom erosion is, more or less intermittently , in progress in all areas of the Channel where there is only a thin sediment cover, and that this process is merely the seaward continuation of the (mostly lateral) erosion so obvious on receding coastlines generally . (f) The extreme flatness of the Channel floor

That flatness is not obvious from an inspection of Admiralty charts, with their scattered data points, but is seen very impressively on continuous depth recording profiles where, especially in the more central parts of the western Channel, the records may show only minimal depth variation for kilometres at a time. That situation would not be surprising if the Channel were floored by a deep sedimentary succession in process of accumulation. In that case the flatness could reasonably be explained as the expression of a near-equilibrium situation in which a surface layer of well-sorted sediments was in slow transit oceanwards under the influence of gravity and wave- and tidal-forces. However we know that there is no such succession and that the sediments present are thin and poorly sorted. The rock floor is known to have a geological composition of great variety . Why then is there no general development of scarps and valleys such as those on land in S.E . England in similar rock? Extreme peneplanation of a"former land surface followed by drowning seems out of the question as an explanation because of the general absence in the deeper areas of the Channel of any

fossil drainage pattern. Such a pattern is widespread in the eastern Channel, but even there it is incomplete (Auffret et al., 1980). Individual channels normally possess well-marked shoulders and may be discontinuous . Their interfluves are not domed , as would be the case on land, but flat, the whole pointing to some process of truncation . The writer believes that the explanation of the extraordinary flatness of the floor of the greater part of the Channel is to be found in processes seen at work in the Selsey peninsula, itself very flat and with a surface only a few metres above sea level. In Bracklesham Bay the shore line is marked by a storm beach of flint pebbles which mayor may not be backed by a low cliff. The whole is in rapid retreat (2 m/year) . The pebble slope gives way abruptly seawards a little above low water neaps to an expanse of sand up to 100 m wide at low water springs. At a mean depth of about 40 cm this Recent sand is underlain by the sands and clays of the Bracklesham Beds (Curry, King, King & Stinton, 1977), which dip at about 1° along an 8 km front, and are exposed only sporad ically and in small patches, especially following offshore winds . Individual beds vary in compactness from barely consolidated sand to calcareous sandstone and stiff clay, and are seen less or more frequently as a result. But even the harder units do not remain exposed for more than short periods because they are invaded by boring molluscs, which penetrate up to 10em and greatly weaken the rock sur face . In due course the colony is covered by,sand deeply enough to kill the molluscs and the bored rock ultimately collapses. Abrasion by shifting sand complements the destruction by boring. This comb ination of boring and abrasion ensures that the whole rock surface is almost always covered, its level being rather precisely controlled by the surface of the overlying sand , which is itself controlled by local hydrodynamic factors, mainly wave-induced currents. Marine boring organisms (molluscs , echinoids, worms, sponges and some others) use both mechanical and chemical means of attack and in combination are capable of destroying all but very hard rocks, and all calcareous rocks and except in a few areas, the rocks flooring the Channel all fall into one or another of these categories. The model provided by the situation at Bracklesham seems quite adequate to account for the flatness of the Channel floor in general. The main source of energy in the two cases is different-wave energy near the shore line and tidal currents and storm-generated oscillations in deeper water. But in each case the basic control is the hydrod ynamic regime and its local variations. This controls the shape of the surface of the superficial veneer of sediment which, by selective blanketing, moulds the shape of underlying rock. By this means a bland topography has been produced , broken only by marked projections where areas of rock with

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THE ROCK FLOOR OF THE ENGLISH CHANN EL

long-term resistance to boring are present. Sucn are , for instance, Seven Stones (granite) , Wolf Rock (phonolite) and Bane des Langoustiers (slate), etc. (g) Sediment transport in and through the Channel

Sedimentary input from rivers may be typified by that reported for the Seine (Avoine , 1987), whose catchment area comprises half of the total area draining into the Channel. Avoine reports that this input is wholly of silt and clay, with no net transport of bedload, and that situation is probably typical for most of the Channel's other rivers . The other continuous source of sediment is by erosion from the rock floor and as will be suggested later the supply from that source may be much larger than that from the rivers. Fig. 3 shows that some 90% of the area of the Channel is floored by rocks of Mesozoic and Tertiary age. Older rocks (basement, Devonian/Carboniferous) are concentrated west of Cornwall and in and near the Bay of St. Malo and, because of their hardness, probably contribute little sedimentary input to the Channel. About 30% of the floor is in calcareous rocks (Chalk , Eocene limestones) which on breakdown will leave little residue. The remainder (Permo-Triassic marls and sandstones, Jurassic clays, Lower Cretaceous and Tertiary clays and sands) are for the most part little consolidated and may be expected to break down into their original components. These last will thus comprise almost the whole of the material released to the Channel by bottom erosion. The grain size of this last group is very predominantly in the grades fine sand and finer;

medium and in particular coarse sand and gravels being a minor constituent except at some levels in the Permo-Trias. Very considerable quantities of silt and clay are suspended by tidal currents in the lowest few metres of seawater in the Channel; sufficient to make bottom photography difficult or impossible except at periods of slack water. Thus fine sediment is entering the Channel from rivers, it is being created by erosion from the rock floor and it is in transport there. By contrast, however , it is not present in quantity in bottom sediments except locally. As stated earlier the sedimentary cover of the Channel is typically a gravelly coarse to medium sand, except in sheltered areas where fine sands and silty clays may predominate. The conclusion is clear. Apart from what is held , more or less temporarily, in sheltered sites such as estuaries, or interstitially in coarser sediments, this fine material is not accumulating and so must be travelling towards the continental slope or into the North Sea. The coarser fractions of the sediments now found on the Channel floor have no obvious presently continuing source . They are essentially lag deposits, whose components have residence times which in some cases may be as long as the rock platform has been in existence. Studies (Stride , 1982, p. 76) of the directional bedforms (sand waves, ribbons, active banks) of the lag deposits suggest that net tidal transport in the western Channel is westwards, but with subordinate eastward movement locally in the bays of Plymouth and St Malo. In the eastern Channel net tidal transport is almost entirely to the east. A bed-load

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Ku

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Fig, 3. Simplified geological map of the floor of the English Channel and nearby sea area s. Based mainly on Sub-Pleistocene Geology of the British Isles and the adjacent Continental Shelf, 2nd edition, 1979. British Geological Survey. Areas in black-granites and granul ites; B-pre-Permian; Pt-Permo-Trias; J-Jurassic; KI-Lower Cretaceous; Ku-Upper Cretaceous; Pg-Palaeogene ; Ng-Neogene.

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DENNIS CURRY

parting therefore occurs along the Isle of WightCherbourg line. A bed-load convergence is forecast for the Straits of Dover, but this proposal is inconsistent with the presence there of areas of bare rock floor or with evidence for substantial sediment transport from the Channel into the North Sea (see later). (b) Rate of erosion of tbe rock floor This may be estimated by assuming that the floor profile is being maintained but is advancing eastwards to match the rate of coastal erosion (c. 1 m/yr; Smith, 1984, p. 260). Slope on the centre line of the western Channel (depth about 100 m) is about 60 m in 300 km (1: 5000). Rate of downcutting in that area would then be 1 m in 5000 years (the corresponding figure for the shore platform near Selsey is about 1 m in 50 years). Neotectonic movements in S.E. Britain and northern France (Rossiter, 1972; Fourniguet, 1987) are of the order of 1 m/1Ooo years, a figure comparable with the estimated rate of bottom erosion of the Channel in 20 m of water. This implies that neotectonic uplift on the present scale would be incapable of raising a land barrier across the Channel because marine erosion would keep pace with it. Similar considerations apply to a eustatic fall of the sea. The rate of lowering of nearby land by erosion can be estimated from the annual sedimentary discharge of the Seine (1.5 X 106 rrr' per annum maximum (Avoine, 1987, p. 146» and the area of its catchment (5.4 x 104 krrr'), and yields a figure of 1 m in 36,000 years. Some addition must of course be made to allow for the loss of Chalk by solution, but it is clear that any plausible total is of an order of magnitude less than that estimated for the erosion of the Channel floor. Thus this floor is apparently being lowered far faster at present than neighbouring land, with the corollaries that topographical relief as between land and sea floor is steadily increasing and that most of the sediment travelling across that floor is derived from the floor itself. The catchment area of the Seine comprises 50% of that draining into the Channel so total sediment input from rivers into the Channel may lie between 2 x 106 and 3 x 106 rrr' per annum. Using the hypothesis proposed herein of strictly lateral erosion at the rate of 1 m per annum it is estimated that (east of a line from Land's End to Ushant) erosion in the western and eastern halves of the Channel is at a rate of 13 x 106 and 7 x 106 rrr' per annum respectively. Using seawater samplings Eisma & Kalf (1987) have concluded that about 107 tons (c. 5 x 106 nr') of suspended sediment flow annually from the Channel into the North Sea. Thus about 17 x 106 rrr' are available for transport towards the shelf edge and beyond (all the estimates in this paragraph are subject to large margins of error, of course).

(i) Some other areas witb a rock platform at or near tbe sea floor

There are few published data for assessing the prevalence or otherwise of such platforms. However it appears that conditions similar to those in the greater part of the Channel exist along at least a substantial part of the Atlantic coast of the Iberian peninsula. Drop-coring in the area between 39°N and 42°N was successful in collecting rock in 95 cases out of the 169 sites prospected (Boillot, Berthou, Dupeuble & Musellec, 1972). On the north Spanish coast, coring was carried out between 6°50'W and 1°30'W and a total of 270 rock cores (Boillot, Dupeuble, Hennequin-Marchand, Lamboy & Lepretre, 1973) was recovered. Published data are incomplete but one area recorded (Boillot, Dupeuble, Le Lann & d'Ozouville, 1970) the collection of 41 samples from 89 stations. These success rates are as high as those recorded in the more fruitful areas of the western Channel and testify to the general presence of only minimal sedimentary cover. Core samples taken proved to be dominantly Late Cretaceous to Miocene where the rock landwards is mostly Palaeozoic. Associated C.S.P. Profiles give few indications of the presence on the shelf of superficial sediment and so confirm the indications from cores. Around the Iberian peninsula the edge of the continental shelf is much closer to land (c. 40 km) than off the British Isles and its slope (c. 1: 200) is correspondingly steeper. Perhaps for this reason it has proved possible to take rock 'cores right out to the edge of the shelf and at a depth of 200 m. By comparison, the greatest depth at which rock has been recovered by dropcoring in the Channel area is 130 m (49°N, 6°43'W, Miocene, about 100 km from the shelf edge). (j) Estimated life span of tbe nascent unconformity

The unconformable surface and the rocks underlying it along the central line of the Channel are shown in section in Fig. 2 and include strata ranging in age through much of the Phanerozoic. It will be seen that the sequences include several unconformities (Curry et al., 1971) and that fact will be discussed later. Fig. 4 shows the surface in plan, with an estimate of its extensions into the Celtic Sea and the Mer d'Iroise. Few data are available for these latter areas (Colin, Lehmann & Morgan, 1981; Curry, unpublished) but these confirm the existence of a regime similar to that of the western Channel and its approaches. The enlarged surface is larger than Scotland including the Hebrides, and might be held to rank in importance with the unconformities above and below the Upper Cretaceous in England and northern France. The unconformable surface continues to evolve as it is lowered by erosion. In addition, as hydrodynamic conditions change, its overlying sediments will

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THE ROCK FLOOR OF THE ENGLISH CHANNEL

HO Hurd Deep

o Fig. 4. From the Western Approaches to the North Sea-areas where solid rock is at or near the sea floor. For a map of the area with buried channels, see Smith, 1989, Fig. 9.

respond by encroachment or recession and the shape and size of the exposed area will change. Sooner or later this surface of erosion will become completely buried under an increasing pile of sediments, and only then will it attain its full status as an unconformity. When, in the Channel, might that happen? The present situation seems to be one of stability. Changes in sea level would merely move the area of exposed rock up or down the Channel, perhaps altering its size. Severe recession under glacial conditions would introduce additional factors such as the immobilisation of the North Sea area, the arrival of large quantities of sediment from the European heartlands and a reduction in tidal activity. The eastern part of the Channel, already reduced in area by recession, might as a result fill up, but bare floor would be gained near the continental margin where recently-deposited strata would be flushed away. Nevertheless the exposed surface could be expected to return towards its original state when the temperature rose, just as it has done recently . The only scenario leading to a rapid sealing of the unconformity would appear to be substantial subsidence (?ZOO m+) and associated infilling. On that basis the period to the sealing of the unconformity might prove to be measured in millions rather than tens of thousands of years.

subaerially (typically highly irregular) and under the sea (mostly uniformly flat), and it is with these latter that we are now concerned . Such marine erosion surfaces may be bored , or colonised by an attached fauna, whilst the immediately overlying sediments ('basement bed') are typically coarse-grained and pebbly. Fossils, if present, may be expected to include both indigenous and derived forms. This basement bed commonly passes upwards into a uniform succession of fine-grained deposits which may be many metres thick. An obvious modern analogue to such an assembly of phenomena is a sea beach with its associated shore platform, in course of transgression over a land surface. This provides the necessary ingredients-pebbles, the hard parts of marine organisms, contemporary and perhaps fossil-plus a supply of terrigenous fines to feed an area of deposition immediately offshore which has buried areas of beach and platform produced at an earlier stage. The above model (see Fig. SA) is one which has commonly been used in discussions of transgression and unconformity (e.g. Pomerol, Babin, Lancelot, Le Pichon, Rat & Renard, 1987) but its use implies a number of important consequences for interpretations based on it. 1. The basement bed must be diachronous and must young towards an area of land which is probably in course of uplift; without uplift the unconformity would peter out (see Fig. SA). 2. Any small area of a basement bed must represent only a short period of time as it will rapidly

A. Conventional model. S EDI"

ENTATION

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B. English Channel model. SEfil'2,~NT. > <

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R

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S

ION SEA lEVEL CONSTANT

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OR CHANGING SLOWLY

~y

(a) The interpretation of unconfonnities-guidance from the Channel area 'A break in sedimentation provable by an erosion surface is called an unconformity . .. ' (Bennison & Wright, 1969, p. 8). These authors go on to point out the contrast between those surfaces produced

HIATUS AND BASEMENT BED

x---x

EARLIER EROSION SURFACE

Fig. 5. Models of transgression and unconformity. Model A has been adapted from Pomerol et al., 1987, Figs. 42 & 103.

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DENNIS CURRY

have been sealed by the advance of the transgression and its offshore deposits. 3. The basement bed must be a beach or have been formed near to shore. It cannot have been formed in deep water. 4. Its content of derived fossils is unlikely to include far-travelled forms: unless very durable they must have been derived from the shore platform or nearby cliff. An alternative and much more accommodating model is provided by the present floor of the Channel as described earlier and schematised in Fig. 5B. In that model the area under erosion is very much larger, the erosion is submarine rather than marginal/ subaerial and the ultimate depositional area has been moved well offshore. Consideration shows that none of the constraints identified as associated with model A apply to model B. In the case of the Channel the basement bed is mostly not diachronous, and it may young in more than one direction. It may have been formed in almost any depth of water on the shelf and will rarely have been formed very close to land. Reworked fossils may be far-travelled. At any point the basement bed may have been in existence 105 years or even much more . During that period its biotic content will have been changing continuously and the associated hard parts will have been updated by destruction and recruitment, so that when the bed is finally sealed they will date the unconformity at that date . There is no need to postulate either uplift or a fall in sea level. As already seen, the evolution of the present Channel is proceeding without the intervention of uplift as a driving force and is not particularly sensitive to change in sea level. It is of course not contended here that the conventional 'beach' model is never appropriate. Some basement beds are fossil beaches , at least locally. But, in the writer's opinion, this is rarely the case as is demonstrated by the general absence from the fauna of basement beds of forms (e.g. limpets, littorinids) characteristic of rocky shores. The majority of marine basement beds are better interpreted as offshore phenomena as typified by the present day Channel. Finally, because erosion is generally associated with land, it has commonly been assumed that when beds of a certain age are cut out over a large area beneath an unconformity it may be concluded that that area was probably land at the time. The present situation in the channel demonstrates clearly that that premise may be false. The geological record of the future will contain no Channel sedimentation dated to the present day, except in a few favoured and untypical situations. As a result our successors in the far future may be misled into supposing that this important biogeographical linkway was in fact a 'Recent land barrier'. The writer believes that the example of today 's Channel provides a sharp warning for all

palaeogeographers. The presence of land at a particular time and place should never be deduced from the absence of deposits . Collateral evidence should be sought from contemporary beds nearby which have survived. This evidence might include indications of the presence of biotic barriers or bridges, and data in relation to sedimentation and tidal patterns amongst other factors, all of which should be evaluated before any conclusion is reached. (b) The unconformfties of the western Channel--Lower Cretaceous The western half of the English Channel displays a rather complete post-Palaeozoic depositional sequence which is almost entirely marine, except for continental episodes in the Triassic and early Cretaceous. The sequence displays several wellmarked unconformities, which may display both overlap and onlap, as schematised in Fig. 2. How does this historical record relate to the unconformity which is evolving in the Channel today? To discuss that question it is necessary first of all to place the western Channel area in a regional geological context and to discuss some of its history. Its more central parts form a trough filled with Mesozoic and Cenozoic rocks which in general dip gently inwards to the centre of the trough and oceanwards. Observed faulting and folding are essentially parallel to its length , although the orientation of the rather close-set fault patterns displayed by the Triassic and Jurassic sequences is mostly unknown. The trough is flanked to north and south by the ancient massifs of Cornubia and Brittany and their submarine extensions , which are built of more or less strongly metamorphosed and folded Palaeozoic and pre-Palaeozoic rocks. To its north-west are two similar, and similarly sited, troughs which extend from the Celtic Sea into the Bristol Channel and St George's Channel respectively. However these troughs close to the north and east, whereas the Channel trough has had links with the North Sea depositional basin since mid-Cretaceous times at least . Brittany and Cornubia are heavily intruded by granites, and granites also occur in the north of the Cherbourg peninsula. An area of granite is known 35 km N. by W. of Roscoff and another is suspected from gravimetric data (Bacon, 1975) 40 km N.W. of Guernsey. It has been suggested (Smith & Curry , 1975) that the presence of these granites has imparted persistent buoyancy (Bott, 1956, p. 59) to Brittany and Cornubia, and that it may also explain the attenuation or actual absence of sedimentary sequences in the area between S. Devon and Cherbourg (the Start-Cotentin line of Smith & Curry) , where metamorphic or igneous basement may be within 650 m of the surface (Day, Hill, Laughton & Swallow, 1956, station Dy 2). The Brittany-Cotentin massif is

THE ROCK FLOOR OF THE ENGLISH C H A N NEL

almost free of Mesozoic and later deposits, but such as occur do so close to present sea level. By contrast Albian beds are present at least up to 240 m in Devon (Haldon Hills). Recent authors (Dewey, 1982; Stoneley, 1982; Smith, 1984) interpret the West Channel trough as one of the many basins (Viking Graben, Wessex Basin , Cardigan Bay, etc. ,) initiated by rifting, stretching and subsidence of Triassic age which preceded the first opening (eastern USA-west Africa section) of the Atlantic Ocean. These basins became depocentres first for continental (Triassic) and later for marine (Jurassic) sequences. In early Cretaceous times the continental Wealden facies was developed in the southern basins as they filled up . At the same time a new episode of Atlantic spreading had been initiated and, probably in the Hauterivian , the Iberian peninsula started to split away from Newfoundland and the Grand Banks. That event ultimately set in train a dramatic reorganisation of the sedimentary regime across the southeastern half of the British Isles as it created for the first time a palaeogeography with many parallels to that of today. The new geographical situation involved the creation of new lengths of continental margin along the Iberia-Grand Banks split and its extension towards the LabradorGreenland suture, and also, of special interest to the present discussion, between offshore Brittany and western Ireland. The general presence of a sloping submarine shelf around the edges of the world's oceans, terminating at a depth of about 200 m, is due to the erosive power of the ocean itself and in particular to that of its storm waves, whose energy depends mainly on the available deep water fetch (distance of open water within which waves can build up). An associated factor in the production and maintenance of such shelves is that the material eroded from them or transported from land on to them can easily be disposed of by deposition in nearby deep water. Marine excavation of a marginal platform along the new coastlines was thus initially slow, but gathered pace in step with the widening of the newly created oceanic gulf. This gulf, by Aptian times (see Owen, 1983, maps 18-20 and Fig. 6 herewith) , would have been comparable in dimensions with the Gulf of Aden (800 km long, 300 km wide, 2km deep) . At the head of the gulf lay the tectonised and intruded ancient massifs of western Ireland, Cornubia and Brittany and their modern submarine extensions. Within these massifs and running landwards at right angles to the coast lay the rift valleys of the North Celtic Sea and western Channel with their infill of relatively soft Mesozoic rocks . Marine erosion along this coastline would have been strongly differential in character with at least an order of magnitude difference between its effects on the massifs and their rift valley infillings respectively . That difference may

347

APTIAN

Fig. 6. Palaeogeography in the Aptian .

be exemplified by the contrast between the present mean coastal retreat in the eastern Channel (c. 1 m/yr) and that implied by the situation to the west of Guernsey and Jersey, where the mid-Eocene coastline is, in places, within 10 km of the modern cliffs (Andreieff, Bouysse, Curry, Fletcher, Hamilton , Monciardini & Smith, 1975, Fig. 1). There are very few records of the early advances of the sea across the old rifted areas . Off southern Ireland the first marine influences (Colin et at. , 1981) are seen in the Barremian (at site 56/14 .1, for example) and a full marine fauna is present in the Aptian. That fauna, as would be expected, includes many Tethyan elements amongst the foraminifera (Orbitolina, Choffatella, Daxia) and ostracods, but, very significantly, ostracod species characteristic of mainland England and the Paris Basin are also present. To the east, marine beds of definite Barremian or Aptian age are not seen until the Wessex basin is reached, the small Lower Cretaceous inliers in the western Channel, apart from one occurrence of undated glauconitic clays, appearing all to be in Wealden facies (Barthe, Boillot & Deloffre, 1967; Curry, Hamilton & Smith, 1970). Deposits of Aptian age in central and southern England are widespread and well documented (e.g. Casey , 1961). They are exclusively marine although brackish influences are present in Dorset (Pun field beds). Their palaeogeography was interpreted by Middlemiss (1962), who envisaged an Early Aptian seaway from the S.W. to the established depositional basins of Wessex and the Weald and a Late Aptian connection with the long standing Early Cretaceous

348

DENNIS CURRY

marine successions of the North Sea depocentre. The analysis of Middlemiss was based mainly on faunal patterns, but Anderton, Bridges, Leeder & Sellwood (1979) stressed in addition the evidence provided by the important change in sedimentary style which accompanied the Aptian transgression (local abundance of glauconite, coarse sands, cross-stratification) and postulated the presence of a 'Dover Straits-type situation' over the intervening platform area when the southern and northern basins linked up. All of the above is entirely consistent with what is suggested here; which is that the original trigger for the evolving Aptian story was coastal erosion on the new Atlantic margin which cut gulfs along the rift areas of the west Channel and Celtic Sea. Excavation sufficient to carry seawater into the subsiding Wessex and Weald basins might have taken a million years, with a further million or so to link up with the North Sea. Shallow-water links between the Atlantic Ocean and the marine depocentres to the northeast would then explain the observed faunal, sedimentary and tidal phenomena. The area between Melksham and Leighton Buzzard, with its pattern of interrupted occurrence, high energy units (e.g. Faringdon gravels) and reworked fossils is particularly reminiscent of the present Channel floor and, with the example of the modern Channel in mind, one need not be surprised that Aptian deposits have not been recorded along the proposed line of advance of the sea in the western Channel. It is a truism that mechanical (as distinct from chemical) erosion can only continue so long as its products can be permanently removed from the eroded surface. Long-term mechanical erosion at a particular site therefore requires the presence nearby of a long-term sump, together with a continuing supply of energy to move the products of erosion from the eroding area to the sump. The edges of the oceans clearly provide the necessary conditions in quasipermanence; on the continents, however, only in areas of persistent uplift and persistent subsidence can erosion continue in the long term. A particularly favourable erosional situation is that of a shallow marine area lying between two sumps, when tidal energy is added to that of waves in keeping the waterway open. In all situations isostasy multiplies and prolongs erosional activity by its compensating effects of raising eroding areas and depressing the floors of sumps in course of filling. In the above context the marine link established in Aptian times between the proto-Atlantic on the one hand and the Wessex-Weald basins and, even more significantly, the North Sea depocentre on the other represented a major palaeogeographical event which has influenced the geological development of the Channel area ever since. The history of the western English Channel, a geographical unit first created by that event, is chronicled by the alternations of marine

deposition and erosion recorded there, and that history has been especially influenced by the (then narrower) Start-Cotentin strait and the buoyancy of the floor between in relation to global rises and falls of sea level. (c) The Cenomanian transgression in the western Channel In the light of the scenario of the modern Channel presented here the first episode of marine transgression, the late Barremian-Aptian advance, will have left no traces in the western Channel because any deposition which may have accompanied it was removed either prior to its close, or in the course of the development of the second, the so-called Cenomanian transgression. That second episode is well seen in the cliffs between Swanage and Sidmouth, and westwards to the edge of Dartmoor, where beds of late Albian age overstep progressively westwards from the Aptian to the Palaeozoic. The total thickness of beds overstepped is about 2500 m, although the amount removed at anyone point was almost certainly very much less, as will be discussed later. The band of overstep continues across the Channel (Fig. 7) and is seen (with gaps) along the French coast west of Le Havre, where late Jurassic beds are present beneath the Albian (and ? Aptian) in the area between Le Havre and Caen, and Cenomanian glauconitic sandstone rests on Lias and earlier beds 26 km south of Cherbourg (Vieillard & Dollfus, 1875). The overstep continues southwards via Argentan and Le Mans to beyond Saumur, following the western edge of the Paris tectonic basin. In this area

Fig. 7. Surface beneath the Upper Cretaceous unconformity. Modified from Curry et al. (1971). For key see Figs. 2 & 3.

THE R O CK FLOOR OF THE ENGLI SH C H A N N E L

Cenomanian beds rest on Jurassic beds down to the Lias and, locally, on Basement ; Lower Cretaceous units being absent. To the S.E . of the Paris basin, between Bourges and Verdun, Lower Cretaceous beds reappear and Cenomanian overlap is no longer apparent. To the west of the Paris basin the thickness of beds overstepped «500 m) is much less than on the Dorset and Devon coasts because the successions overlapped include condensed sequences and gaps. The Albian/Cenomanian transgression in the area under discussion no doubt relates to a world-wide rise in sea levels which is believed to have commenced in the mid-Early Cretaceous and terminated in the Maastrichtian (Vail, Mitchum & Thompson, 1977; Hancock & Kauffman, 1979). But the bevelling and relative tilting (about 1°, see Donovan. 1972) beneath must signal some tectonic event, and one of regional importance in view of the distance involved in and south of the Channel (450 km from Exeter to Saumur). A possible mechanism might be 'steer's head ' type uplift and erosion along the western edge of the Wessex and Paris basins (d. Dewey, 1982, pp. 389, 395) and such uplift may well be responsible in part. But the apparent absence of corresponding overlap along the S.E. flank of the Paris basin and the indications of exceptionally severe pre-Middle Albian erosion on the Devon coast suggest the existence of some other cause, either alone or in addition . The possible depth and timing of that erosion will now be discussed. Chadwick (1986, p. 479), in an analysis of the tectonics of the Wessex basin , proposed that the Kimmeridge Clay may have 'overlapped all earlier Mesozoic formations , to onlap against and eventually cover the remaining Palaeozoic massifs of southern England' . In that way he removed the problem provided by the perceived thickness of erosion of pre-Kimmeridgian units west of Weymouth . Nevertheless a problem still remains . Applying the proposal to Dartmoor we note that the sub-Albian erosion surface nearby in the Haldon Hills rises to about 250 m, which is some 350 m below the present maximum height of the Moor. Since Kimmeridgian times that height must have been reduced substantially by erosion-say by 250 malthough differential uplift of the Dartmoor granite might also have occurred. Also other pre-Albian rocks might have covered the Kimmeridgian. It is thus implied that perhaps some 600 m of strata were removed from the area by erosion before the Albian and that (subject to any general sea-level change) local uplift occurred of a similar amount. Chadwick's palaeogeological map of postBarremian times (1986, Fig. 27) shows the boundaries of the Lias in substantially their present-day positions, thus implying his belief that little or no submarine erosion occurred as a result of the (Aptian and) Albian transgression(s) . It follows that he believed that the indicated erosion occurred essentially during

349

the Wealden episode and therefore took place above sea level. In the analysis of erosion in around the modern Channel it has been suggested that submarine erosion may be many times more rapid than that on land in an area of little topographical relief. The present author therefore much prefers a model in which the erosion was carried out below sea level in one or more stages in Apto-Albian times. Such a model avoids the need to explain the postulated deep pre-Kimmeridgian (Corallian-Permian?) overlap in Devon when no such event is visible around the Paris basin, where , however , Apto-Albian erosion prior to the Cenom anian transgression was widespread (the local absence of Aptian beds does not of course preclude the possibility of Aptian erosion) . What thickness of beds might plausibly have been removed in Devon by postulated Apto-Albian erosion? All divisions of the Trias taper to zero against Dartmoor from their total thickness of 750 m at Lyme Regis (Audley- Charles, 1970, plates 7-13), and the thickness (300 m) of the Lias at Lyme Regis is less than half that seen in the Wessex basin, with the implication that it might have been less again further west. Thus the total of Lias and earl ier beds eroded near Exeter might have been only 300 m or so. Later units are more difficult to assess. Th e Bathonian to Oxfordian also appears to thin notably westwards, as is seen near the Mendips. In addition Chadwick (pp . 478, 481) has pointed to evidence of severe erosion in some platform areas (London platform, Cranborne-Fordingbridge high) contemporaneously with deposition of thick Ryazanian-Barremian units in the nearby Weald and Channel basins. Thus as little as 600 m in all may have been removed near Exeter in the Apto-Albian erosion episode before the resumption of deposition marked by the Cenomanian transgression . It is seen therefore that both hypotheses lead to the same requirement, namely pre-Cenomanian uplift in the Exeter area of about 600 m, although the timing of uplift in the two cases is different. Some insight into the nature and extent of intra-Cretaceous uplift can be gained from a study of the submarine successions around and westwards of the Cherbourg peninsula (Curry et al. , 1970, 1971; Andreieff et al., 1975; Larsonneur, Horn & Auffret, 1975 and Figs. 7 & 8). Evidence from that area is provided almost entirely by seismic profiling, spotdated by drop-coring; and little informatio n is available from continuous cores . As a result condensed successions may have been overlooked because of collection failure . With that proviso it may be stated that a full succession from the Aptian upwards is present south of the Isle of Wight and from the Cenomanian upwards in a region south and west of the Scilly Isles, the facies in both cases being those of southern England. Between these areas Cenomanian beds are mostly in condensed facies and are

350

DENNIS CURRY

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49

Fig. 8. Age of beds immediately above the Upper Cretaceous unconformity. At the three Senonian sites marked with a cross the sample collected was of glauconitic Chalk. Ce-Cenomanian; Sa-Santonian; Ca-Campanian; Ma-Maastrichtian.

(d) Possible causes for the NW-SE line of uplift between Exeter and Saumur Steer 's head type erosion along the margin of the Wessex and Paris basins , as already mentioned, is hardly consistent with the apparent absence of overlap along the S.E. margin of the Paris basin, and so is provisionally discounted. Two other uplift mechanisms may be considered. The first notes that the Start-Alderney line is axial to a series of graniteintruded basement massifs which extend from the Massif Central through Brittany to Cornubia and thence (with a gap) to the Republic of Ireland. These areas would be expected to display persistent buoyancy. For the second mechanism it is observed that this line of massifs is approximately parallel to the continental slope and marginal tumescence is invoked (cf. Allen, 1981, p. 4(0) . An explanation based on mass deficiency as a result of granite intrusion has the disadvantage that that mechanism should be a continuing one, and is inconsistent with the evidence suggesting a pulse of uplift confined well within the Cretaceous. The marginal tumescence hypothesis is linked to plate activity along the Newfoundland-Iberia split and the (probably) contemporaneous opening of the Bay of Biscay. That provides an appropriate timing (rifting in the earliest Cretaceous, plate separation and rotation in the Hauterivian?) . However the summit line of the proposed uplifted area is some 300 km from the continental edge and that distance may be considered too large to make a causative link plausible. Thus of the three proposed mechanisms none seems satisfactory by itself . The long history of unconformity and overlap in the western Channel (Figs 2 & 8) speaks in favour of the buoyancy hypothesis, and the effects of Cretaceous marginal tumescence may be invoked to account for the notable uplift and erosion associated with the Cenomanian transgression between Exeter and Saumur. What appears to be an exceptional depth of erosion in the region of Exeter remains unexplained.

absent locally. Turonian and Coniacian have not been proved. Higher beds are in Chalk facies. Santonian units occur widely, but appear to be absent near to the buried edge of the Brittany massif, and both Santonian and Campanian are locally glauconitic a little further north. Immediately Nand W of the Cherbourg peninsula Maastrichtian lies directly on Basement, and on the pen insula itself Maastrichtian calcarenites rest on glauconitic Cenomanian. Beneath the Upper Cretaceous successions the westward pattern of overstep seen on land reverses beyond the Start-Alderney line. Although detailed evidence of the nature of the reversal is incomplete , Lower Cretaceous units of Wealden facies reappear in a large area about midway between the Lizard and Ushant (Allen, 1981). The pattern of intra-Cretaceous unconformity and subsequent deposition displayed in the western Channel is thus demonstrated to be a symmetrical one and clearly signals a situation in which regional post-Albian deposition of chalks was inhibited by uplift along a general NW-SE Line. There is no hint anywhere in the post-Albian successions of deposition in very shallow water «30 m) ; even the Maastrichtian calcarenites of the Cotentin indicating a depth of 50-100 m. With the present-day Channel as a model therefore it is suggested that submarine erosion had (e) The Channel in the Cenozoic no difficulty in keeping pace with this uplift along the trough of soft rocks flooring the central part of the The Cenozoic history of the Channel as a marine western palaeo-Channel, though it was ineffective waterway intermittently connecting the Atlantic against the indurated Basement on either side . The Ocean through to the North Sea and beyond has been writer believes that there was a continuing marine discussed elsewhere, notably by Larsonneur (1972a) connection along the line of the western Channel at and Pomerol (1973). Those authors take a generally least from Aptian to Danian times. The tentative conservative line in their interpretations, linking suggestion of Larsonneur (1972a, p. 207) that the absence of deposition from time to time with the Channel seaway might have been closed during presence of some land barrier or shoal. With the Cenomanian and Turonian times is therefore rejected, example of the modern Channel and its rock floor in and the classical idea of Jukes-Browne (1900) that the mind, the present author would favour a more liberal

THE ROCK FLOOR OF THE ENGLISH CHANNEL

stance in which a marine environment was assumed in any area of the present Channel unless there was good evidence to the contrary. That difference in standpoint leads to notably different palaeogeographical interpretations of the Neogene development of southern England and northern France. 4. SOME GENERAL CONCLUSIONS 1. Basement beds, unless very irregular, are very probably marine and may have been laid down far from land. 2. They, and their immediately overlying units, mark the end of an era. The period (or many periods) of erosion which preceded them may have extended through much, though not all, of the time-span of the hiatuses preceding their deposition. In this context, expressions such as 'Cenomanian transgression' may

351

mislead. Such an expression is dating the succeeding beds, it does not date the erosive event which preceded new deposition, nor the phenomenon which caused that event. 3. Hiatuses, even over a wide area, are not incompatible with a marine environment. 4. Submarine erosion may be much more rapid than that on land. Note especially that submarine erosion is fastest on beds near to sea level; the converse is the case for continental erosion. Finally it may be observed that much interpretation of ancient marine sequences has in the past been carried out largely ex hypothesi, without adequate search for and comparison with possible modern analogues. In spite of the great body of research on present day sedimentology over the last few decades, more comparisons are still needed to make reliable reconstructions.

References ALLEN, P. 1981. Pursuit of Wealden models. fl. geol. Soc. Lond., 138,375-405. ANDERTON, R., P. H. BRIDGES, M. R. LEEDER & B. W. SELLWOOD. 1979. A Dynamic Stratigraphy of the British Isles. George Allen and Unwin, London. ANDREIEFF, P., P. BOUYSSE, D. CURRY, B. N. FLETCHER, D. HAMILTON, C. MONCIARDINI & A. J. SMITH. 1975. The stratigraphy of the post-Palaeozoic sequences in part of the western Channel. Phil. Trans. R. Soc. Lond., A279,79-97. AUDLEY-CHARLES, M. G. 1970. Triassic palaeogeography of the British Isles. Q. f. geol. Soc. Lond., 126, 49-89. AUFFRET, J. P., D. ALDUC, C. LARSONNEUR & A. J. SMITH. 1980. Cartographie du reseau des paleovallees et de l'epaisseur des formations superficielles meubles de la Manche orientale. Ann. Inst. Oceanogr., 56. 21-35. AVOINE, J. 1987. Sediment exchanges between the Seine estuary and its adjacent shelf. fl. geol. Soc. Lond., 144, 135-48. BACON, M. 1975. A gravity survey of the western English Channel between Lyme Bay and St. Brieuc Bay. Phil. Trans. R. Soc. Lond., A279, 69-78. BARTHE, A., G. BOILLOT & R. DELOFFRE. 1967. Anticlinaux affectant Ie Cretace a l'entree de la Manche occidentale. C. R. Ac. Sc. Paris, 264, 2725-28. BENNISON, G. M. & A. E. WRIGHT. 1969. The geological history of the British Isles. Edward Arnold, London. BOILLOT, G., P. Y. BERTHOU, P. A. DUPEUBLE & P. MUSELLEC 1972. Geologie du plateau continental portugais au nord du Cap Carvoeiro. La serie stratigraphique. C. R. Ac. Sc. Paris, 274, 2748-51.. - , P. A. DUPEUBLE, 1. HENNEQUIN-MARCHAND, M. LAMBOY & J. P. LEPRETRE. 1973. Carte geologique du plateau continental nord-espagnol entre Ie canyon de Capbreton et Ie canyon d'Aviles. Bull. Soc. geol. France, (7), 15, 367-91. , P. A. DUPEUBLE, F. LE LANN & L. D'OZOUVILLE. 1970. Etude stratigraphique des terrains affleurant sur Ie plateau continental nord-espagnol entre

Aviles et Llanes. C. R. somm. Soc. geol. France, 1970, 78-79. BOTT, M. H. P. 1956. A geophysical study of the granite problem. Q. f. geol. Soc. Lond., Ill, 45-67. CASEY, R. 1961. The stratigraphical palaeontology of the Lower Greensand. Palaeontology, 3, 487-621. CHADWICK, R. A. 1986. Extension tectonics in the Wessex Basin, southern England. fl. geol. Soc. Lond., 143, 465-88. COLIN, J. P., R. A. LEHMANN & B. E. MORGAN. 1981. Cretaceous and Late Jurassic biostratigraphy of the North Celtic Sea Basin, offshore Southern Ireland. In (Neale, J. W. & M. D. Braiser; eds.) Microfossils from Recent and fossil shelf seas. Ellis Horwood, Chichester. CURRY, D. 1962. A Lower Tertiary outlier in the central English Channel with notes on the beds surrounding it. Q. f. geol. Soc. Lond., 118, 177-205. - - , D. HAMILTON & A. J. SMITH. 1970. Geological and shallow subsurface geophysical investigations in the Western Approaches to the English Channel. Inst. geol. Sci. Rep., 70/3, 12 pp. - - , - - & - - 1971. Geological evolution of the western English Channel basin and its relation to the nearby continental margin. In (Delany, F. M.; ed.) The geology of the east Atlantic continental margin, Part 2: Europe. S.C.O.R. Symposium, Cambridge 1970. Inst. geol. Sci. Rep., 70/14, 129-42. - , A. D. KING, C. KING & F. C. STINTON. 1977. The Bracklesham Beds (Eocene) of Bracklesham Bay and Selsey, Sussex. Proc. Geol. Ass., 88,243-54. DANGEARD, L. 1929. Observations de geologie marine et d'oceanographie relatives a la Manche. Ann. Inst. Oceanogr., (N.S.), 6, 1-295. DAY, A. A., M.N. HILL, A. S. LAUGHTON & J. C. SWALLOW. 1956. Seismic prospecting in the western approaches of the English Channel. Q. f. geol. Soc. Lond., Ill, 15-44. DEWEY, J. F. 1982. Plate tectonics and the evolution of the British Isles. fl. geol. Soc. Lond., 139,371-412. DONOVAN, D. T. 1972. Geology of the central English Channel. Mem. B.R.G.M., 79,215-20.

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