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
F0l.
interbedded with sandy clays. The following descriptions are based on field observations and petrographic
studies.
Poin te
~
RECENT
SAND DUNES
(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.
!hillJ
PORT L AND IAN
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PREPORTL ANDIA N
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CONGLOMERATE
LOCALITIES 8
DIST RIBUTION ~:N
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Boulonnais, France.
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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
PORTLANDIAN OF THE BAS BOULONNAIS
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83
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
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with common Ampullospira ceres (Table 1), the sedi•. f ;t'l. - C". s o n d . L.,d itr Qro n u l, s .
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place s is a mixture of loose sand with sandsto ne nodules
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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.
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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:
~ .~~.~. :- -6 ·
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FIG. 3. Facies relationships in the Gres des Oies and Calcaire
des Oies.
(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
87
88
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
90
W. G. TOWNSON AND W. A. WIMBLEDON
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.
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