The Portlandian strata of the Bas Boulonnais, France

The Portlandian strata of the Bas Boulonnais, France W. G. Townson and W. A. Wimbledon TOWNSON, W. G. & W. A. WIMBLEDON. 1979 . The Portlandian strata of the Bas Boulonnais, France. Proc. Geol. Ass. 90 (2), 81-91. The Marine Portlandian strata in the Bas Boulonnais are c. 20 m thick, comprising the argillaceous glauconitic Assises de Croi overlain by the calcareous quartzose of the Gres des Oies. These are succeeded by a thin 'Purbeckien' algal limestone, the Calcaire des Oies (1 m), overlain by 'Wealdien' clays and sands (10-20 m?). The KimmeridgianlPortlandian boundary (sensu anglico) lies within the basal few metres of the Assises de Croi and not at the Tour de Croi Nodule Bed as previously thought. The lower four Portlandian ammonite zones (Albani to Kerberus) are now proven to be present up to the Middle Gres des Oies , enabling correlation with England. The 'Purbeckien' limestone and 'Wealdien' clays are interpreted as non-marine Portlandian deposits and the 'Wealdien' sands as Lower Cretaceous in age. The Portlandian strata comprise an overall regressive sequence deposited in environments ranging from middle neritic to lacustrine.

W. G. Townson, clo Exploration Department, Brunei Shell Petroleum Co. Ltd., Seria, State ofBrunei, Borneo. W. A. Wimbledon, Nature Conservancy Council, Foxhold Ho. , Thornford Rd., Crookham Common, Newbury, RG15 BEL.

Pointe de la Creche (see fig. 3 of Ager & Wallace 1966b). Just north of Wimereux , six sections have been measured in the cliffs between Pointe aux Oies and Pointe de la Rochette. The younger part of the succession was also studied on the south side of Boulogne, 8 km from Wimereux, at the top of the Cap d'Alprech. The Portlandian outcrop in the Bas Boulonnais covers an area of about 20 sq. km and the maximum thickness of the marine facies is approximately 20 m; this is thinner than the type section in Dorset (Townson 1975, Melville in press) or the sequence in the Vale of Wardour (Wimbledon 1976), but thicker than in the sections of the South Midlands (Wimbledon & Cope 1978).

I. INTRODUCTION

The best exposed outcrops of Portlandian strata in southern England and northern France are the coastal sections of Dorset (type area) and the Bas Boulonnais, some 250 km due east. In between these two areas only limited borehole data exists. The Bas Boulonnais outcrop has been studied for many years and since the pioneer work of Fitton (1836) and d'Orbigny (1842-51, 1849-52, 1850-52) at least thirty papers have been published on various aspects of the succession. This contribution provides modem detailed lithological descriptions, a compilation of recorded fauna, a discussion of the ammonite stratigraphy, approximate correlations with the Portlandian of Dorset and an interpretation of the depositional history. The English use of the terms Kimmeridgian and Portlandian unfortunately differs from that on the Continent. The 'Portlandian' of d'Orbigny (1850-52) included the Upper Kimmeridge Clay and the Portland Group of England, i.e. it commenced at the base of the late Kimmeridgian zone of Pectinatites elegans , whereas the Portlandian ofthe English (used herein) commences later, at the base of the zone of Progalbanites albani (for details see Cope 1967, Wimbledon 1974, and Wimbledon & Cope 1978). The Portlandian is regarded as being the youngest stage of the Jurassic, spanning the post-KimmeridgianJpre-eretaceous interval. The Purbeckian is considered to be merely a facies term which has no stage status. The only locality in the Bas Boulonnais where a complete section through the Portlandian succession can be studied is just south of the town of Wimereux (Fig. 1), in the cliffs between the Marine Biological Station and

2. LITHOSTRATIGRAPHY

The sequence which has long been regarded as equivalent to the 'Portland Beds' or Portland Group of Dorset (Townson 1971,1975) was divided for 100 years into a lower 'Marne (or Argiles) aPerna bouchardi et Ostrea expansa' and an upper 'Gres (or Couches) a Trigonia gibbosa', They were renamed by Ager & Wallace (1966a) as the Assises de Croi overlain by the Gres des Oies (see Table 1). The base of this sequence is conveniently put at a condensed horizon , the Tour de Croi Nodule Bed (Table 1) which has hitherto been regarded as equivalent to the Rotunda Nodule Bed of Dorset and the Upper Lydite Bed of the South Midlands (for references see Ager & Wallace 1966a and Table 1). The Tour de Croi Nodule Bed rests on what was known as the 'A rgiles a Exogyra dubiensis ' of the Kimmeridge Clay, renamed (Ager & Wallace 1966a) as the Argiles de Wimereux. The 81

82

W. G. TOWNSON AND W. A. WIMBLEDON

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interbedded with sandy clays. The following descriptions are based on field observations and petrographic studies.

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(a) Lower Assises de Croi (2·80-2·85 m; Fig. 2) The basal bed, the Tour de Croi Nodule Bed (Table 1), is exposed at the foot of the cliff just north of Pointe de la Creche. It consists of a phosphatised concentration (0'15 m) of vertebrate fragments, bivalves, ammonites, gastropods, brachiopods, echinoids, belemnites and indeterminate phosphatic pebbles up to 20 mm across, see Part 3b(i). The clay matrix contains lignite, indigenous bivalves (Part 3b(i» and ammonites, plus granules of quartz, quartzite and chert (lydite). Above this lies O· 2 m of slightly calcareous and glauconitic clay which contains Rhynchonella and lenses of Exogyra in its upper part. This is capped by a nodular limestone (0'15-0'2 m) composed of microsparite with <1 per cent quartz and glauconite, but with occasionallydite grains. The overlying O· 7 m consists of laminated calcareous silty sandy clay (vf-f quartz sand) with poorly preserved ammonites.

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1. Litho- and bio-stratigraphy of the Portlandian of the Bas Boulonnais.

Assises de Croi and the Gres des Dies are here each sub-divided into Lower, Middle and Upper parts, generally corresponding with previous French divisions (Table 1). The Gres des Dies is overlain by a thin bed of limestone traditionally called 'Purbeckien' and named the Calcaire des Dies by Ager & Wallace (1966a). Above this lie non-marine clays and sands known as 'Wealdien', see Part 2(h). The Assises de Croi and the Gres des Dies consist of mixtures of clay, very fine to coarse quartz sand, glauconite and calcium carbonate. The succession is dominantly siliciclastic and at one horizon includes a conglomerate containing Palaeozoic pebbles. Limestones occur only as thin nodular horizons of micrite

Above, there is O· 7 m of sand-free clay containing large Myophorella (up to 0·15 m); this is succeeded by two beds of fine-grained nodular limestone, each O· 2-0· 25 m thick «1 per cent qtz & glc), separated by 0·4 m of silty sandy clay (1-2 per cent medium quartz sand) containing common large Isognomon. Noteworthy features of the Lower Assises de Croi are: the heterogeneity ofthe sequence, the rapid vertical changes in the amount of sand and in its grain size and sorting, and the low degree of bioturbation. (b) Middle Assises de Croi (4·6-4·7 m; Fig. 2) This sequence consists of alternations of 0·2-0·5 m

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containing fine to med ium sand, < 1 pe r cent glau conite, common Exogyra, Astarte , echinoids and lime stone nodules which appear to be altered large Thalassinoides . Above is 0·8--0·9 m of hard nodular limestone with scatte red bivalve fragments and < 1 per cent f-rn quartz and glauconite. Th e overlying 1·5 m consists of three th in nodular limestones similar to that below, sepa rated by calcareous silty clay with some f-m q uartz and < 1 per cent glaucon ite. Th e seq uence is characte rised by the relatively high proportion of limest ones and the incoming o f large Thalassinoides. (d) Lower Gres des Oies (2'5-2·85 m ; Fig. 2)

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The separa tion of these beds from th e Middle Gres des Oies is arbitrary and made to correspond with the sx U '" French division between lower beds with common 'Car0 All ., nOl io n i 0 1 n od u l o r ' 0 1'1 0 ., dium pellati' ( = Protocardia dissimilis) and higher beds ~ I i... . "' uO\l O"' " . it h o r<;) . s ill y Lo . tn t r 'll" 0. '" 0 with common Ampullospira ceres (Table 1), the sedi•. f ;t'l. - C". s o n d . L.,d itr Qro n u l, s . ... , 4d t , 1"I,,,1IC. M ment, howe ver , is esse ntially the same throughout. The GIOu COI'I, I , u l) t o to·/•. P h O,gh o t ic . v . ' r , 10 _ 0"' '" H. O"y b ;Olu, b ol ;OI'l ( Rh" oc o. o ll iu"" l bed s consist of soft vf-f quartz sands with < 5 per cent '" 'oo, ... o d ic O'OOS, I,OI'l A... ... ol"I,tr ' , bi . OI,,' , . 'o' o ....n. I•• OI'I, . '" clay and carbonate, and < 1 per cent glauconite. T hin ? H o 'd Q, o u l'ld, . '" cemen ted horizons are present (0' 1--0-3 m) composed of i: ~ AlI .'I'I OI,on , .ot , ; lI y ClOy . c l o ,'" L, . ' 1"I "Qr ·? hI! ' C1d l, \1 01'1 . e ee .. .... \10 1'1 • • Lo ' q . t ell v. fine quartz sand (80 per cent) with shells and shell c a R OOl d n . " "c . L ~" .'" .,.0,.... A M O" ' ... . LO' fragm ents of bivalves and gastropods « 1 per cent _ 1. " .1 0 1 b io lu ' bot ,on . d. o o " I , o '" T. d. C ~ .: ~.:".,(~ .... glauconite) in a microsparite cement. Th e lower O·7 m in ....r',~"~ h: ft ~~·,· .~';''': . . "'~i 'l N 8 -.. E ArQillu dt E L' . . .... . Qy ... . dell . place s is a mixture of loose sand with sandsto ne nodules .;, Wim.reu ¥ ~ S h OI., . ,I h b ' vCh'" ond A...... O,.,' I. I Oul •• I'I' ' ' ' ' C. " Vert ic a l form ed by diagen etic modifi cation of large ThalasA c t u c I s ect ion at Pointe d. I a C r i c h e Scale in sino ides. ( Desc r ipt io n me tr e , an d i n te r p r t t a t i o n Ot n er o l ised I The d istinctive features of the Lower G res des Oies are the dom inence o f fine-grained well-sorted sand and FlG. 2. Summa ry description and inte rpretation of the Portlandian str ata of the Bas Boulonn ais. the presence of Protocardia. U

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beds of sa ndstone with 0·1--0·15 m beds of limestone. The latter are nodular lime mudstones which contain up to 20 per cent v. fine-medium quartz sand and 1 per cent glaucon ite. The sands are v. fine to coarse, argillaceous and silty, with lydite granules and up to 10 per cent fresh medium sand-grade glauconite. Sand-grade shell fragments and a few calcareous foraminifera are also present. The sediment is very heavily bioturbated (by ?Rhizo corallium) but in places original laminations of clay and sand are preserved. Large oysters are in places common at the top of the limestones, some of which may be encrusted surfac es (Ager & Wallace 1970). The distinctive features of the Middle Assises de Cro i are the relatively high amounts of glaucon ite and med ium to coarse quartz sand, plus the high degree of bioturb ation .

(e) Middle Gres des Oies (0-3-5 m; Fig. 2) Th ese beds are composed of up to 80 per ce nt wellsorted v. fine quartz sand with onl y a small amount of clay, < 1 per cent glauconite, micrite pellets (up to 1 mm) , she lls and fragments of bivalves and gastropods. A shell y cemented horizon (0,3 m) occurs in the middle , composed of microsparite with a var iable amount of v. fine sand and no glauconite. The soft sands are bioturbated but at three localities some cro ss-bedding is discernable with fore sets dipping northwards. The variation is thickness, and in places absence , of the se beds is due to down-cutting by the Upper Gres des Oies, The distinctive features of the Middle Gres des Oies are the very fine sand, the common Ampul/ospira and the pre servation of some cross -bedding. (f) Upper Gres des Oies (1·5-5+ m ; Figs. 2 & 3)

(c) Upper Assises de Croi (3'0-3,1 m; Fig. 2) These consist of alternations of no dular lime mudstones with argillaceous silts and fine sands. A basal limestone (0-45 m) with Camptonectes and oysters on its upper surface is succeeded by 0·7 m of calcareous clay 6

The base of thes e bed s is a hard shelly sand stone (O·1-Q· 2 m) composed almost e ntirely of v. fine qua rtz

sand with a ferroan microspar cem ent. T he bed is a 'hard-ground' with its upper surface bored by Lithophaga and encrusted with serpulids and oysters.

84

W. G. TOWNSON AND W. A. WIMBLEDON

Moulds of aragonitic bivalves are either filled with the overlying sediment or remain empty. Considerable variation occurs above this hard ground, described as follows:

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(i) North of Wimereux At Pointe aux Dies the hard-ground rests on 2·02·5m Middle Gres des Dies and can be traced southwards for about 1 km, maintaining approximately the same level. However, at Pointe de la Rochette in a distance of 100 m it cuts down through the Middle Gres des Dies to rest on the basal 1·5 m of the Lower Gres des Dies (Fig. 3). This bored horizon also occurs at the next exposure, about 1 km south of Wimereux, where it rests on 3·0 m of Middle Gres des Dies. The down cutting at Pointe de la Rochette is thus a local phenomenon (Fig. 3). At Pointe aux Dies the bored surface is covered with a layer (0'05~'1 m) of Laevitrigonia and shell fragments with pebbles and blocks (up to 0·4 x 0·1 m) eroded from the hard-ground. This is covered by a fine-grained quartz sandstone and the whole bed (0·25 m) has been burrowed by small Thalassinoides. Above is 0·25 m ofv. fine sand (almost 100 per cent quartz) which is horizontally laminated, shows primary current lineation and is burrowed in places by Thalassinoides . Fallen blocks show the bored horizon sometimes covered directly by this sand and in these cases the sediment fills borings and empty moulds of aragonitic bivalves . Diapiric intrusion and 'fissure eruption' of bioturbated shelly sand through the overlying fine sand took place at Pointe aux Dies. These structures have been discussed by Ager & Wallace (1966a & b, 1970) but the role of the hard-ground is not mentioned. It seems prob-

(ii) South of Wimereux Near the Marine Biological Station (Fig. 1) the Upper Gres des Dies consists of up to 4 m of soft sand with north-dipping cross-bedding visible at the base and with concretions in the upper half. At Cap d'Alprech, south of Boulogne, these beds are again sands and sandstones with cross-bedding dipping to the north. In contrast to the sections to the north, no pebble beds or diapirs are known.

PORTLANDIAN OF THE BAS BOULONNAIS

(iii) The Rochette Conglomerate (Table 1; Fig. 3.) At Pointe de la Rochette, where the channel-like erosion exists, there is a conglomerate up to 2 m thick resting on the hard-ground and containing great eroded and bored blocks (up to 1-5 m across) from the cemented bed (Fig. 3). The conglomerate also contains pehbles of earlier Jurassic rocks (including Kimmeridge Clay) and Palaeozoic rocks such as red, white and pink quartz, Lower Carboniferous limestone and chert, Devonian psammmites, Silurian slates and Cambrian quartzites. There are also rolled Portlandian bivalves and gastropods, reptile bones and 'Isastrea', apparently in growth position. Above the conglomerate is at least 3 m of inaccessible cross-bedded sands and sandstones. The Rochette Conglomerate is recorded in a quarry about 1 km south-east of Wimille where it is 2 m thick with the Lower Gres des Oies exposed below and sands above (Rigaux 1892). It is also visible in a road cutting on the Nl about 1 km south of Wimille where 1·5-1·6 m of pebbles are exposed, with current-deposited trigoniids in the upper part. The conglomerate rests on 1·5 + m of hard shelly sandy limestone with ammonites. The top of the conglomerate is covered irregularly by red clay with ironstone known as 'Wealdien'. The conglomerate is not recorded in other sections at this level elsewhere in the region. (g) The Calcaire des Oies (0·2-1·3 m; Fig. 3) The Gres des Oies is capped generally by algal limestones but in places there are beds with ostracods or beds composed almost entirely of the tiny bivalve Eocallista socialis (common in Dorset at the junction between the Portland and Purbeck Groups, Townson 1975). In the northern part of the outcrop, around Pointe aux Oies, the algal limestone is 0,9-1·3 m thick, resting on laminated or bioturbated calcareous sands of the Gres des Oies, although at one point (Fig. 3) it thins to only 0-2 m. The limestone is tufaceous, consisting of large circular concretionary masses identical to those of the basal beds of the Lulworth Formation in Dorset (Townson 1975, West 1975). The bulk ofthe Calcaire desOies has been formed by the blue-green alga 'Spongiostromata' (Pugh in Ager & Wallace 1966a) and consists of laterally-linked hemispheres (LLH) and concentric stacked hemispheres (SC-H) of Logan et al (1964), as described by Pugh (1968) in Dorset. The topmost 0·1-0·15 m of the Calcaire des Oies often contains minute bivalves and pisolitic algal debris which rapidly grades up into grey and red clays of the 'Wealdien'. South of Wimereux, near to the Marine Biological Station, the Calcaire des Oies consists of 0-5-1'0 m of limestone entirely composed of whole casts of Eocallista, forming a bivalve granule grainstone. A little further south, towards Pointe de la Creche, the facies

85

changes to lime mudstone (1,1 m) containing minute bivalves, with a 0·2 m clay bed in the middle (Fig. 3). The limestone contains 1 per cent quartz sand, bivalve fragments, indeterminate biochems, superficial ooids and possible shrinkage cracks. Above Pointe de la Creche, Rigaux (1865) recorded 1·8 m of limestone, 'concretionary' in the lower part, rich in Eocallista in the middle and fine-grained above. The only other locality lies just south of Cap d' Alprech where Rigaux (1865) recorded 1·4 m of fine-grained concretionary limestone with the base rich in ostracods and the top becoming laminated. (h) 'Wealdien' (? 20-30 m;fide Arke1l1956) Grey and red clays rest on the Calcaire des Oies, with an apparently transitional base. There is no marked break at this level despite the strange record of Bonte & Broquet (1962) of'Wealdien' with a basal conglomerate containing green Albian sandstone, Chalk and Chalk flints. (At Le Fart the 'Wealdien' of Bonte & Broquet, 1962, does contain Chalk flint pebbles, according to Ager & Wallace, 1966b). The clays immediately succeeding the Calcaire des Oies do not contain ostracods (fide Arkell 1956) but have a mixture of continental and marine macrofossils. See Part 3b(v). 'Wealdien' yellow sands with ironstone layers lie above and cut down through the grey and red clays as far as the Gres des Oies at Pointe aux Oies and near Cap d'Alprech (Ager & Wallace 1966a, Fig. 3) and even as far down as the Rochette Conglomerate near Wimille (pers. obsv.).

3. BIOSTRATIGRAPHY (a) Ammonites and correlation with England Ammonites are uncommon and often poorly preserved in the Boulonnais sections; museum collections are sparse and a lack of consistency exists in the literature when dealing with ammonite identification. Nevertheless, in the light of recent work on the Standard Portlandian Stage in England (Wimbledon 1974, Wimbledon & Cope 1978) it is appropriate to review the ammonite stratigraphy of the Boulonnais sections and to make certain broad correlations with England. (i) The age of the Tour de Croi Nodule Bed This horizon, with its fauna of rolled phosphatised ammonites and other molluscs (see Part 3b(i)), has been interpreted in the past as the basal conglomerate of the Portlandian succession (sensu anglico), equivalent to the Upper Lydite Bed of the English South Midlands but supposedly covering a greater non-sequence (Rotunda

86

W. G. TOWNSON AND W. A. WIMBLEDON

Zone to Albani Zone inclusive). Pruvost (1925) stated that some 130 ft. of the Dorset succession was not represented. This conclusion was followed by Arkell (1935) and Ager& Wallace (1966a& b). The accepted absence of the Albani Zone is, however, inconsistant with the ammonite records of Sauvage (1911), Dutertre (1925) or Pruvost & Pringle (1924), see Part 3a(iii). The Tour de Croi Nodule Bed just north of Pointe de la Creche (Fig. 1) contains rolled Pectinatites and Pavlovia. In addition, fresh irridescent Pavlovia are present, which in their preservation are close to those of the Hartwell-Swindon Clay of the English South Midlands (Neaverson 1925). This suggests that the break represented by the bed is intra-Kimmeridgian, falling within the Pavlovia Zones and perhaps in part equivalent to the Lower Lydite Bed of England (ArkellI933). (ii) The topmost Kimmeridgian Zone

This unnamed zone (soon to be described from the type locality in Dorset; Cope 1978) has yet to be identified in the Bas Boulonnais. If present it must lie in the Lower Assises de Croi. Assuming the Tour de Croi Nodule Bed is of Pavlovia pallasioides Zone age, then the Pavlovia rotunda and this new unnamed Zone must be extremely attenuated or absent. (iii) The Progalbanites albani Zone This earliest Portlandian zone is certainly present in the Bas Boulonnais section. Specimens of Epivirgatites, typical of the zone in Dorset, have recently been collected from the Middle Assises de Croi. Sauvage figured a similar individual in 1911 (pI. 9, Fig. 1); also, twenty four years before Arkell named the species, a specimen of Progalbanites albani (Arkell). Sauvage's assignment of these two species to the Lower Gres des Oies must, however, be incorrect (see Part v). The presence of Epivirgatites spp. and P. albani enables correlation of the Middle, and possibly some of the Lower Assises de Croi with at least part of the Massive Bed to Emmit Hill Marls interval of the Portland Sand Formation of East Dorset (Townson 1975, Table 1). Thus, the Kimmeridgian-Portlandian boundary must lie in the lowest few metres of the Assises de Croi. (iv) The Glaucolithites glaucolithus Zone

The hard micrite beds in the Upper Assise de Croi have recently (Wimbledon 1974) yielded fragments of body chambers readily assignable to the distinctive genus Glaucolithites and identical to material from Dorset and the Vale of Wardour in England. This does little more than prove the presence of the zone however, its precise limits are unknown. An unlikely mixture of forms has been recorded in the past, included 'Behemoth

lapideus', Titanites pseudogigas and Kerberites, but none of these species in fact occurs in the Glaucolithus Zone assemblage (Wimbledon & Cope 1978). This is the first record of Glaucolithites from the Boulonnais and it permits correlation with at least part of the White Cementstone-Black Sandstone section of East Dorset (Corton Hill to Gad Cliff Members, Portland Sand Formation, Townson 1975, Table 1) and with the Upper Lydite Bed of the South Midlands (Wimbledon 1974). (v) The Zone of Galbanites okusensis

No material has been collected during the recent study which can definitely be assigned to this zone. Numerous references in the literature to Crendonites gorei (Salfeld) and 'Behemoth lapideus' strongly suggest, however, that an Okusensis fauna is present. The name 'B. lapideus' seems to have been used for evolute forms within the Okusensis Zone fauna, including perhaps G. okusensis itself. The name C. gorei has been used indiscriminately in the past for any Crendonites species of the Okusensis or overlying Kerberus Zone. The type specimen of C. gorei came (fide de Loriol & Pellat 1874-5) from the long-defunct quarries at Wimille (Fig. 1) which caused Pruvost (1925) to believe that it must have come from the Upper Assises de Croi. Both Sauvage (1911) and Dutertre (1925), however, confined the Gorei fauna to the Lower Gres des Oies but Pruvost (1925) considered that it ranged from the Upper Assises de Croi up into the Lower Gres des Oies. A complication to an understanding of the older accounts is the fact that Crendonites species were often confused with the inner whorls of the much larger Glaucolithites. It was this factor which, until recently, prevented recognition and definition of separate Glaucolithus and Okusensis Zones. The range of the Okusensis Zone in the Bas Boulonnais is at present unknown. The loss of the Wimille quarries is particularly unfortunate, because in their heyday they were clearly a good source of ammonites, presumably from both the Okusensis and Kerberus Zones. (vi) The Zone of Galbanites kerberus The characteristic coarsely-ribbed species of the Kerberus Zone in England, including certain Crendonites, have neither been found in situ in this study nor are previously recorded. However, other Kerberus Zone species are amongst the most common ammonites in the Boulonnais sections. Pruvost & Pringle (1924) erroneously placed Kerberites stratigraphically below 'Behemoth'. They stated that the Middle Gres des Oies contains Perisphinctes bononiensis (de Loriol) and P. giganteus (Sowerby). Pruvost (1925) unfortunately later placed the occurrence of P. giganteus above that of P. bononiensis, following the then-accepted correlation of

POIlTLANDIAN OF TIlE BAS BOULONNAIS

the latter species with ammonites within the 'Cherty Beds' and the former with species in the overlying 'Freestones' of the Dorset 'Portland Stone '. In fact both these species, plus Kerberites and Crendonites spp., co-exist in the Basal Shell Bed on Portland and Arkell's Beds J-1' in the Isle of Purbeck (top of the Dungy Head Member, Portland Limestone Formation, Townson 1975). Titanites giganteus (Sowerb y) (recently redefined by Wimbledon & Cope 1978) and related species , persists in Dorset throughout most of overlying 'Cherty Beds' (Dancing Ledge Member, Townson 1975). It is these larger forms of the Kerberus Zone assemblage which are most common in the cliffs north and south of Wimereux. Titanites giganteus is present from approximately 2 m above the base of the Lower Gres des Oies to the top of the Middle Gres des Oies. Several specimens can be examined in situ on ledges formed by the hard cemented horizons south of the Marine Biological Station. (vii) The Titanites anguiformis Zone No ammonites of this, or younger, Portlandian zones have been found in the Bas Boulonnais. In fact, apart from a deri ved fragment mentioned by Dutertre (1925), no ammonites have been found at all above the top of the Middle Gres des Oies. The age of the Upper Gres des Oies, the Calcaires des Oies and the basal clays of the 'Wealdien' is unknown but it is likely that they were deposited during Kerberus and/or Anguiformis Zone times . Thi s would be consistant with the situation in Buckinghamshire (Wimbledon 1974) and in certain boreholes in S.E . England (Townson 1971), where the change from ammonite-bearing to non-ammonitebearing facies occurs immediately above beds with a Kerberus Zone fauna (W. A. Wimbledon; pers. obs.). (b) Non-ammonite macrofauna and flora

Because the previous records of workers on the Boulannais are so widely scattered it is considered useful to include the following compilation of macrofauna and flora, supplemented by personal observations. The term s 'common' and 'less common' are purely subjective, based on recurrence of observations. Unfortunately it has not been possible to assign records to the subdivisions of the Gres des Oies or Assises de Cro i. In terms of diversit y the biota is similar to that of the Portlandian in England (Townson 1975 and references therein), given the fact that the descriptions are mainly limited to rare horizions with lithologies suitable for preservation. None of fossils listed is considered to be of use for time-correlation within the Portlandian.

(i) Tour de Croi Nodule Bed (Dutertre 1927) Phosphatised: Anisocardia sp. Corbicella sp. Lithophaga sp. Lucina sp. Pleuromya tellina Agassiz Pleuromya sinuosa Roemer Corbula sp. 'Macrodon' Various trigoniids Protocardium morinicum de Loriol 'Goniomya' sp. ' Cyprina portlandica' ( = Eocallista sp?) Dicroloma sp. Ampullospira atheta d'Orbigny Pleurotomaria sp. Rhynchonella sp. •Terebratula' sp. ('Zeilleria') Unphosphatised: Exogyra nana Sowerby Plagiostoma boloniensis de Loriol 'Chlamys' sp. Ox ytoma octavia d'Orbigny Laevitrigonia sp. Protocardium morinicum de Loriol Ampullospira athleta d'Orbigny Rh yn chonella subvariabilis Davidson ' Belem nites' sp. Also variou s reptiles and fish bones , Crustacean remains, Cycads, 'Sequoia', Pinites, Pinus (ii) Assises de Croi Common : Myophorella sp. Laevitrigonia sp. Isognomon bouchardi Oppel Ostrea expansa Sowerby Ostrea dubiensis Contejean Camptonectes lamellosus Sowerby Plagiostoma rustica Sowerby Pleuromya te/lina Agassiz Musculus autissiodorensis Cotteau Glomerula gordialis Schlotheim Less common: Thracia sp. Oxytoma octavia d'Orbigny Buchia mo squensis Buch Protocardium morinicum de Loriol Plicatula boisdini de Loriol Panna constantini de Loriol Barba tia cavata de Loriol Pholadomya rustica Phillips Anomia suprajurensis Buvignier

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W . G. TOWNSON AND W. A. WIMBLEDON

Astarte saemanni de Loriol Eocal/ista implicata de Loriol Mactromya verioti Buvignier Corbula bayana de Loriol Natica athleta d'Orbigny Pleurotomaria sp. Nue/eolites sp. Pseudodiadema sp. Cyphosoma sp. Hemipedina bouchardi Wright Hemicidaris sp. Cidaris legayi Sauvage Acrosalina koenigii Wright Rhynchonel/a subvariabilis Davidson 'Terebratula' bononiensis Sauvage & Rigeaux ( = 'Zeilleria', ? = T. ovoides Sowerby? = Rouilleria Makridin?) Discina latissima Sowerby Lingula ovalis Sowerby (iii) Gres des Oies Common: Laevitrigonia gibbosa Sowerby Myophorel/a sp. Protocardia dissimilis Sowerby Plagiostoma rustica Sowerby Camptonectes lamel/osus Sowerby Less common : Eomiodon cuneatus Sowerby Ostrea blakei Cox Camptonectes suprajurensis Buvignier Corbula autissiodorensis Cotteau Isocardia lettesoni de Loriol Anomia suprajurensis Buvignier Eodonax pel/ati de Loriol Corbicel/a union ides de Loriol Eocal/ista pulchel/a de Loriol Cyrena pel/ati de Loriol Mactromya verioti Buvignier Musculus autissiodorensis Cotteau Nue/eolites sp. Pseudodiadema thirriai Etallon Cidaris florigemma Phillips 'Isastrea' (oblonga? Edwards & Haime) Chenopus beaugrandi de Loriol Procerithium leblanchi de Loriol Procerithium manseli de Loriol Procerithium pseudoexcavata de Loriol Ampul/ospira ceres de Loriol Natica elegans Sowerby Natica pel/ati de Loriol Nerita davidsoni de Loriol Nerita sinuo sa Morris Nododelphinula vivauxea Buvignier Trochus morierei de Loriol Littorina bononiensis de Loriol

Dentalium pel/ati de Loriol Also various reptiles and fish remain s (iv) Calcaire des Oies Eocal/ista socialis d'Orbign y ? Eocal/ista intermedia de Loriol (v) 'Wealdien' (Dutertre 1925) Ostrea, Mytilus, Corbula, Cyrena , Paludina , Cyclas, Unio. Goniophis, Megalosaurus, Strophodus, Hybodus, Lepidotus, Pyenodus, lschyodus, Iguanodon. 4. DEPOSITIONAL HISTORY The late Kimmeridgian to end-Jurassic period is considered to have been a time of worldwide fall in sea level (Hallam 1969). In Dorset this is manifested by the vertical change from marine shales to silts, to carbonate mudstones and grainstones, to evaporitic non-marine deposits (Townson 1975 , West 1975). Periods of relatively slow depos ition have been recognised in Dorset but subsidence was generally continuous. In the marginal areas however, such as the South Midlands and the Bas Boulonnais, the succession is thinner, with evidence that sedimentation varied from rapid to slow or even ceased at times . Deposition was dominated by terrigenous supply and periods of erosion occurred. The sequence in the Bas Boulonnais (Fig. 2) shows an overall shallowing with variation in rates of accumulation following the intra-late Kimmeridgian break at the base of the Tour de Croi Nodule Bed. Gritty pho sphatic horizons occur above and below this level (Pruvost 1921) and , in view of the ammonite evidence, (Part 3a) the break in time represented by this particular horizon may not be particularly significant. The Lower Assises de Croi were relatively rapidly deposited with little time for biological homogenisation of three silt-clay-lime mud cycles (similar to those of the Corton Hill Member at Gad Cliff, Dorset; Townson 1971, 1975). Subsidence and/or supply rate evidently slowed during deposition of the Middle Assises de Croi, witnessed by the high concentration of glauconite, presence of phosphate, heavy bioturbation and possible encrusted surfaces. The presence of coarse quartz sand and chert granules ('lydites') indicates proximity of supply but glauconite and Rhizocorallium suggest generally low energy middle neritic conditions (Porrenga 1967, Ager & Wallace 1970). Shallowing continued during deposition of the Upper Assises de Croi , as indicated by the presence of large Thalassinoides (Ager & Wallace 1970) and the low glauconite content. The supply of coarse clastics was reduced, however. Deposition of lime mud was common and a diverse echinoid-bivalve fauna flourished prob-

89

PORTLANDIAN OF nlE BAS BOULONNAIS

ably in generally medium to low energy inner to middle neritic conditions. The Lower and Middle Gres des Oies are medium and high energy inner neritic sands, as indicated by their grain size, sorting, cross-bedding, diverse bivalvegastropod-ammonite fauna and bioturbation. The contact with the Upper Gres des Oies is an erosion surface locally cutting deep into the Middle and Lower Gres des Oies (Fig. 3). The Upper Gres des Oies was deposited by the following sequence of events: (i) Downcutting of a relatively firm substrate in an estuarine/tidal channel at Pointe de la Rochette and inland (Figs. 1 & 3, angle with coastline unknown). Concordant lateral erosion of the Middle Gres des Oies probably also occurred, possibly to a considerable extent. The gap in time at this level is unknown and it is possible that a substantial thickness of strata was deposited and eroded. (ii) Deposition of fine shelly sand in sub- to intertidal (beach?) conditions with a marine/euryhaline fauna. (iii) Continued regression resulted in exposure to vadose conditions. This is indicated by solution of aragonitic skeletons and possible cementation of the hard-ground (deposition of ferrous calcite under phreatic reducing conditions was probably a much later unrelated phenomenon). (iv) Subsequent transgression resulted in sub- to intertidal boring (or renewed boring) and erosion of blocks and pebbles of the hard-ground. Incorporation of a marine fauna resulted in a conglomeratic shelly sand. Moulds of aragonitic skeletons were filled with fine sand. It is suggested that at this time a subtidal to subaerial pebble beach (ct. Chesil Beach, Dorset; Carr & Blackley 1974) migrated southwards and filled the tidal channel previously cut into the Middle Gres des Oies. This would account for: (a) the presence of a layer of exotic pebbles at Pointe aux Oies which pass southwards into the thick Rochette conglomerate; (b) the apparent absence of pebbles south of Wimereux (Fig. 1); (c) the presence of the marine fauna in the Rochette conglomerate. The diapirism and 'fissure eruption' at Pointe aux Oies to the north accompanied this increase in water depth. (v) Shallowing recommenced with the deposition of cross-bedded, calcareous, fine to coarse quartz sands and lignitic clays. The rapidly laterallyvarying facies, the euryhaline to marine fauna, the presence of only small Thalassinoides and the absence of ammonites suggests deposition in coastal lagoons/tidal swamp/intertidal sand beach conditions in close association. The thin Calcaire des Oies was deposited under

hyper- to hyposaline coastal lagoonal conditions during a brief absence of terrigenous influx. The southward change in facies (Part 2g, Fig. 3) may indicate a shallowing in that direction. The transitional change to continental conditions was completed by deposition of 'Wealdien' red and grey clays in a lacustrine/hyposaline lagoonal environment.

s. DISCUSSION The abandonment of the Purbeckien 'stage' and the division of the post-Kimmeridgian Jurassic into nine Portlandian ammonite zones (Wimbledon & Cope 1978) means that the Portlandian of S.E. England and northern France now includes the non-marine strata. In Dorset the lowest six zones are present in marine facies, whereas in the Boulonnais only the lowest four are proven. The 'Wealdien' of the Bas Boulonnais was found to be anomalous by Allen (1959) when compared with the Wealden of England and later Casey & Bristow (1964) proposed a 'Middle Purbeck' age. It is, however, reasonable to suggest that the 'Wealdien' clays correlate with the lower part of the Lulworth Formation of Dorset (Townson 1975). It is probable that it is the discordant ferruginous sands (Table 1, Fig. 3) which are 'Middle Purbeck' in age. They probably equate with the Whitchurch Sands-Cinder Bed 'event' in England, regarded as being basal Cretaceous (Casey 1963). With regard to the better known marine Portlandian, it is over 165 years since the ammonite distribution in a measured section of Dorset was first described (Middleton 1812) but it has only recently been possible to make a credible, albeit imprecise, correlation with the strata in the Bas Boulonnais. Thus, the Tour de Croi Nodule Bed is now considered to be intra-Kimmeridgian and hence older than the Upper Lydite Bed of the South Midlands. The latter Bed is of Glaucolithus Zone age and thus approximately correlates with the Upper Assises de Croi. The change in deposition in Dorset from siliciclastics to marine dolomites (Townson 1974, 1975) also takes place during this Zone. The lithological boundary between the Assises de Croi and Gres des Oies appears to fall within the Okusensis Zone, as does the boundary between the Portland Sand Formation and the Portland Limestone Formation of Dorset. The break in deposition between the Middle and Upper Gres des Oies perhaps lies in the Kerberus Zone and thus may equate with the end of the middle regressive cycle in Dorset (Townson 1975). The range of environments suggested for the deposition of the Upper Gres des Oies was recently examined by one of the authors in the State of Sabah (Malaysian Borneo). Clean beach sand, muddy tidal swamps, pebbly coastal streams, pebble beaches and tidal channels with basal lags of corals, marine bivalves, vertebrate

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bones and wood were seen to exist with in hundreds of metres of each other, bearing witness to the variety of conditions co-existing along a tropical coast backed by rapidl y eroding hills of clastic sediments. Such must have been the conditions in the Bas Boulonnais during the mid-Portlandian, with fluctuations in rainfall and drainage pattern giving variations in supply of locally derived coarse clastics, probably under sub-tropical to tropical conditions. Deposition of the marine Portlandian of the Vale of Wardour (Wimbledon 1976) and the South Midlands was also marginal to the basin and influenced by supply of terrigenous material but, in contrast with the Bas Boulonnais, these areas were bordered by land of lower

relief. This in turn contrasts with the Dorset area, where supply of terringeous material was virtually cut off during deposition of the Portland Limestone Formation on a swell away from the basin margin (Townson 1975 , 1976). ACKNOWLEDGEMENTS W. G. Townson acknowledges discussion in the field with Dr. A. Childs, N.E .R.C. for financing the research and Prof. A . Hallam for supervision at the University of Oxford. W. A . Wimbledon acknowledges field discussions with Drs. A. Adams, R. Lowman and A. Thorne and for financial assistance from N.E.R.C.

REFERENCES AGER, D . V. & WALLACE, P. 1966a. The environmental history of the Boulonnais, France. Proc. Geol . Ass., 77, 385-418. - - 1966b. Easter field meeting in the Boulonnais, France. Proc. Ceol. A ss., 77, 419-35 . - - 1970. The distribution and significance of trace fossils in the uppermost Jurassic rocks of the Boulonnais, Northern France. In T. P. Crimes & J . C. Harper (eds .), Trace Fossils, Geol. Jour. Sp . Issue 4, 1-18, Seel House Press , Liverpool. ALLEN, P. 1959 . The Wealden Environment: Anglo-Paris Basin. Phil. Trans. Roy. soc. B, 242, 283-346. ARKELL, W. J . 1933. The Jurassic System in Great Britain . Clarendon Press, Oxford. - - 1935. The Portland Beds of the Dorset Mainland. Proc. Ceol. Ass., 46, 301-47. - - 1956 . Jurassic Geology of the Wo rld. Oliver & Boyd, Edinburgh. BONTE, A. & BROQUET, P. 1962 . L'Aptien du Boulonnais. Bull. Cane Geol. Fr. No. 269 , 59. CARR, A. P. & BLACKLEY, H. W. L. 1974 . Ideas on the origin and development of Chesil Beach, Dorset. Proc. Dors et nat. Hist. archaeol. Soc., 95, 9-17 . CASEY, R. 1963 . The dawn of the Cretaceous period in Britain. Bull. S-East Un. scient. Socs ., 117, 1-15 - - & BRISTOW, C. R. 1964. Notes on some ferruginous strata in Bucks and Wilts. Geol. Mag., 101, 116-27. COPE, J . C. W. 1967. The palaeontology and stratigraphy of the lower part of the Upper Kimmeridge Clay of Dorset. Bull. Brit. Mus. (Nat. Hist .) Geol., 15, 1-79. COPE, J . C. W. 1978. The ammonite faunas and stratigraphy of the upper part of the Upper Kimmeridge Clay of Dorset. Palaeontology, 21, 469-533. D 'ORBIGNY, A. 1842-1851. Palaeontologie Francaise. Terrains jurassiques Cephalopodes. Paris Masson. 2 vols. - - 1849-52. Cours elementaire de palaeontologie et de la geologie Stratigraphiques. Paris Masson. 3 vols. - - 1850-1852. Prodrome de palaeontologies stratigraphique universelle des animaux mollusques et rayonnes. Paris Masson. 3 vols. DUTERTRE, A. P. 1925 . Notice geol sur laPointe auxOieset les abords de la station zool. de Wimereux. Glamures Biologiques Trav. Stat. Zoo/. Wimereu.x, 9, 66.

---1927. Contribution a l'etude palaeontologique du Portlandian du Boulonnais. Ann. Soc. geol. Nord ., 51, 240. FITTON, W. H. 1836. Additional notes on part ofthe opposite coast s of France and England, including some account of the Lower Boulonnais. Proc. Geol . Soc ., 51, 9-27. HALLAM, A. 1969 . Tectonism and eustasy in the Jurassic. Eartn Sci. Re v., 5, 45-{)8. LEMOINE, P. 1911. Geologie du Bassin de Paris. Paris . LOGAN, B. W., REZAK, R. & GINSBURG. R. N. 1964 . Class ification and environmental significance of algal stromatolites. J. Geol., 72, 68-83. LORIOL, P. DE & E. PELLAT. 1874-5 . Monographie des etages superieures de la formation Jurassique des en virons de Boulogne-sur-Mer. Mem. Soc . Phys. Hist. nat. Geneve, 23-4 , 1-326. MELVILLE, R. V. In Press. British Regional Geology: The Hampshire Basin and adjoining Areas (4th Edition). H.M.S .O. London. MIDDLETON, J . 1812 . Continuation of the account of the British Mineral Strata: 10th. The Purbeck Strata. The Monthly Mag., 34, (No 234) 395-{). London. NEAVERSON, E . 1925 . Ammonites from the Upper Kimmeridge Clay. Pap. geol. Dept. Univ. L'pool., 1, 1-5, pIs 1-4. PELLAT, E . 1878. Terrain Jurassique superieur du Bas Boulonnais et itineraires d'excursion dans ce terrain. Ann. Soc. geol. Nord. Vol 5, 1. PORRENGA, D . H. 1967. Glauconite and chamosite as depth indicators in the marine environment. Mar. Geol., 5, 495-501 . PRUVOST, R. 1921. Observations sur Ie terrain Portlandian du Boulonnais. Bull. Carte geol. Fr: No . 143. Vol 25, 75. - - 1925 . Les subdivision du Portlandian boulonnais d'apres les Ammonites. Annis. Soc . geo/. Nord.• 49, 187-215. - - & PRINGLE, J . 1924 . A synopsis of the geology of the Boulonnais, including a correlation of the Mesozoi c rocks with those of England. Proc. Geol. Ass., 35, 29-56. PUGH. M. E . 1968. Algae from the Lower Purbeck limestones of Dorset. Proc. Geol. Ass., 79, 513-33 . RIGAUX, E. 1865 . Notice stratigraphique sur Ie BasBoulonnais. Bull . Soc . Acad. Boul. 4.

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- - 1892. Notice geol du Bas Boulonnais. Mem. Soc. Acad. Boui., 14, 1-108. SAUVAGE, H. E. 1911. Sur quelques Ammonites du Jurassique superieur du Boulonnais. Bull. Soc. geol. Fr., 11, 455-63. TOWNSON, W. G. 1971. Faciesanalysis ofthe Portland Beds. Univ. Oxford D. Phil. thesis (unpubl.), 284 pp. - - 1974. Geology of Coombe Valley. Proc. Dorset nat. Hist. archaeol. Soc., 95, 7--8. - - 1975. Lithostratigraphy and deposition of the type Portlandian. Jl. geol. Soc. Lond., 131,619-38. - - 1976. Discussion of Portlandian faunas (reply to discusgeol. Soc. Lond., 132, 335-6. sion).

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WEST, I. M. 1975. Evaporites and associated sediments of the basal Purbeck Formation (Upper Jurassic) of Dorset. Proc. Geol. Ass., 86, 205-25. WIMBLEDON, W. A. 1974. The Stratigraphy and Ammonite Faunas of the Portland Stone of England and Northern France. Univ. Wales Ph. D. thesis (unpubl.) 199 pp. - - 1976. The Portland Beds (Upper Jurassic) of Wiltshire. Wilts. Nat. Hist. Mag., 71, 3-11. - - & COPE, J. C. W. 1978. The ammonite faunas of the English Portland Beds and the zones of the Portlandian Stage. Il. geol. Soc. Lond., 135, 183-190.