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Field Meeting at Folkestone Warren, 29th November, 1987
Field Meeting at Folkestone Warren, 29th November, 1987 A. S. Gale GALE, A. S. 1989. Field meeting at the Folkestone Warren, 29th November, 1987. Proc. Geol. Ass., 100(1),73-82. In the autumn of 1987, heavy seas scoured weed and shingle from the foreshore of East Wear Bay, producing exceptional exposures of large landslipped blocks of Gault Clay (Albian), Glauconitic Marl and Chalk Marl (Cenomanian). These sections allowed the detailed stratigraphy of the Chalk Marl to be studied for the first time, and a correlation established between Abbot's Cliff and the foreshore sections in East Wear Bay. Important new fossil records from the section include numerous specimens of the early Cenomanian ammonite Sharpeiceras from near the base of the Chalk Marl.
School of Geology, Thames Polytechnic, Walburgh House, Bigland Street, London £1 2NG
1. INTRODUCTION The first Field Meeting of the Geologists' Association was held at Folkstone on 9th April, 1860 and a brief report was published in Volume 1 of the Proceedings (see Green 1989). Here, in Volume 100, it is appropriate to provide an account of the most recent visit of the Association to Folkstone in 1987. Seventeen members met at Folkestone station, and were taken to the British Rail car park in the Warren by car and minibus. The Director outlined the geology of the Warren from a vantage point which afforded good general views of both the exposed Cretaceous Chalk succession and the landslipped area. The Folkestone Warren is an area of landslipped undercliff, which extends along the coast eastwards from Folkestone to Abbot's Cliff, a distance of nearly 2 km. Its maximum width is approximately 500 m, and it is bounded on the landward side by high cliffs of Lower and Middle Chalk, which rise to 150 m above O.D. Slipping is initiated in the underlying Gault Clay. The Gault Clay (Albian) is at present exposed only intermittently on the foreshore in the Warren, near the base of the landslipped part of the succession; the extent of outcrop depends on sand and shingle movement. In the past, before the construction of a concrete toe weighting in East Wear Bay, successive movements of the landslip provided vast foreshore outcrops of the highly fossiliferous Gault. These sections, continually washed by high tides, yielded many of the rich and diverse Gault fossil collections seen in museums throughout the country. The cliff section of the Gault Clay, at Copt Point, is fortunately in good condition still, for all but the highest part of the succession. The descriptions of De Rance (1868) and Price (1874) were based on Copt Point. Price's bed numbering system (Roman numerals I-XIII) is still employed. The ammonite biostratigraphy is known in considerable detail, from the work of Spath (1923a, 1923b, 1923-1943) and Owen (1972, 1975). The Chalk succession of the cliffs between Dover
and Folkestone was first described by William Phillips (1818, 1821), who recognised a lower division, approximately 200 feet (60 m) thick, which he called Grey Chalk. This he distinguished by its darker colour and greater softness from the overlying, whiter, 'Chalk without flints and with few fossils'. The latter division spanned what is now considered to be the top of the Lower Chalk and the lower part of the Middle Chalk, and includes the Pienus Marl. The high cliffs at the back of the Warren display spectacular, but rather inaccessible sections through the Lower and Middle Chalk. The Plenus Marl is conspicuous on account of its slightly recessive weathering, and pale greenish-yellow coloration. The very white, nodular, Melbourn Rock immediately above weathers out prominently. The top of the Lower Chalk, and the Middle Chalk have recently been the subject of a detailed lithostratigraphical study by Robinson (1986) who demonstrated his work to the Association in the field (Robinson, 1985). However, Robinson did not study in detail the lowest of his units, the East Wear Bay Chalk Formation, which includes what has been widely known as the Glauconitic Marl, the Chalk Marl, and the lower part of the Grey Chalk. The biostratigraphy of this part of the succession was last described by Kennedy (1969), who provided a lithological log through the entire Lower Chalk. The Director, C. J. Wood and C. V. Jeans, are currently undertaking an investigation of the lithological succession and faunas of the Chalk Marl at Folkestone. Unlike the Gault, of which the bed-bybed stratigraphy has been known since the work of Price, no accurate lithological log of the Chalk Marl has ever been published. Price (1877) underestimated the thickness of the Chalk Marl by about 15 m, and Kennedy (1969) did not correlate the base of the Abbot's Cliff section with the foundered chalk blocks on the foreshore, and his published log for the Chalk Marl includes a total overlap of approximately 10 metres. Although many museum fossil collections contain 73
74
A. S. GALE
material from the Chalk Marl of Folkestone, From the car park the party set off eastwards into the commonly including ammonites, bivalves, echinoids, Warren, and reached the shore of East Wear Bay brachiopods, and vertebrates, the precise horizons 300 m to the west of Horse's Head (Fig. la). The top from which even the common species originate are of the Gault is presently well exposed over a distance generally unknown. Since Folkestone has yielded of some 50 m at the foot of the concrete platform. Bed numerous type and figured specimens, (e.g. see list in XII is a useful marker horizon on the foreshore, Smart, Bisson & Worssam, 1966) this is a matter of consisting of 1 m of intensely bioturbated, glauconitic, importance. During the present study, new collections dark grey-green silty clay, containing numerous of stratigraphy localised fossils have been made, and it phosphatic intraclasts. The base is piped down into has proved possible to infer the sources of specimens the underlying light grey clays of Bed XI by large idiomorphic Thalassinoides. Bed XI has a total in older collections. Both the past excellence of the Warren shore as an thickness of 15 m at Folkestone (Owen, 1975), and is area for fossil collecting, and confusion over the sparsely fossiliferous; it contains many light buff stratigraphy of the Chalk Marl, are attributable to the coloured phosphatic nodules, which have diffuse nature of the landslip. The competent, permeable boundaries. The base of the bed is not visible on the Chalk rests on incompetent Gault Clay; slip surfaces shore at present. The Gault at this locality dips were initiated in a sheet of plastic clay immediately landwards at approximately 10°, and the sinuous above the base of the Gault, and propagated up outcrop of Bed XII on the shore indicates that folding through the succession as listric faults. Large has occurred during formation of the slump. landward-dipping rotated slabs of Gault and Chalk The phosphatic intraclasts in Bed XII attracted thus make up the foreshore in East Wear Bay. The interest from members. The nodules formed originally Gault was thrown into ridges on the foreshore from early diagenetic concentration of calcium immediately after major landslides (Osman, 1917), phosphate, probably replacing burrows, and initially which were gradually bevelled by wave action. The resembled those described in Bed XI. Current scour whole area was previously a continually replenished exhumed nodules, and the harder, dark brown-grey source of fossil material, from the Gault and Lower cores were left as pebbles on the sea bed. These acted as substrates for attachment of oyster spat, and more Chalk. Landslipping resulted in the absence, or great rarely, serpulid worms. In some cases, a second burial scarcity, of in situ exposures of the highest Gault Clay, resulted in the addition of more phosphatic material, Glauconitic Marl, and much of the Chalk Marl. These of a lighter colour. Complex intraclasts of this type are units are only accessible (as continuous sections of any called hiatus concretions (Voigt, 1968), and resemble size) as rotated masses on the foreshore of the those described from the Glauconitic Marl of the Isle Warren. Accurate measurement of stratigraphical of Wight by Kennedy & Garrison (1975). The sections is complicated by numerous small faults, and darkness of the concretions is an indication of their irregular seaweed growth and shingle cover. relative age. Members who had in the past visited the Gault at Movement of the Warren landslip disrupted the Folkestone to Dover railway line in 1877, and most Copt Point, where Beds I-X are exposed, were dramatically, in 1914, when the line was partially disappointed by the scarcity of fossils in Beds covered, displaced by a maximum of 50 m and closed XI-XIII. Several phosphatic steinkerns of ammonites for 4 years (see Hutchinson, Bromhead & Lupini, (Stomohamites sp., Mortoniceras sp.), and an example 1980 for detailed account). Since further interruption of the enigmatic spherical organism Parkeria were seemed inevitable, the Southern Railway Company collected in Bed XII. The other fauna of this unit instigated research into the causes of landslip (Wood, includes frequent examples of Aucellina gryphaeoides 1955). It was discovered that slipping took place (J. de C. Sowerby), and the small brachiopod immediately above the base of the Gault, and that the Terebratulina sp. water in the underlying Lower Greensand was under artesian pressure. Locality 2. The Middle Chalk at Horse's Head The solution suggested was twofold; firstly, to drain the slips by driving headings back into them, thus (TR 256382) lowering the water table, and secondly by weighting As the tide was still ebbing, the party visited the the toe of the landslipped area, and protecting it from Middle Chalk exposed above the toe-weighting at erosion by the construction of sea walls and groynes. Horse's Head-a large rotated block from the lower These measures were successful, and the landslip has part of the Middle Chalk (Shakespeare Cliff Member not undergone significant movement for over 35 years. of Robinson, 1986) which dips northwards at about 30°. The air and spray-weathered surfaces are rough 2. LOCALITIES VISITED with fossil debris, mostly chips of inoceramid bivalves. Bedding is clearly picked out by alternately softer Locality 1. The Gault Clay west of Horse's Head marly and more nodular units, and scour structures, (TR 254380) infilled with white chalk intraclasts, are present.
FIELD MEETING AT FOLKESTONE WARREN
a
2.
25
2'
75
38
Loc.2
East Wear Bay 37
!
1 km !
!
!
!
'
I
Fig. 1. (a) Map of the Warren to show localities visited by the G.A. party. (b) (inset) is a geological map of the foreshore (at low tide) at locality 3, which displays the highest Gault, Glauconitic Marl and basal Chalk Marl in a large rotated slab. Markers MI-M6 refer to Fig. 2. Mapped by P. J. Haselock and A. S. Gale, Jan. 1988.
Fossils found include the small rhynchonellid Orbirhynchia cuvieri (d'Orbigny), formerly used as a zonal index for this part of the Chalk (e.g. Rowe, 1900), marginal ossicles of the asteroid Crateraster quinqueloba (Goldfuss), and columnals of the crinoid Bourgueticrinus sp.
of phosphatic nodules at the base. The base of Bed XIII is piped down into the top of Bed XII by Thalassinoides. Bed XIII is apparently 7 m thick at this locality, and all but the basal part consists of light grey marly clay; ammonites collected in the past indicate that it is of Stoliczkaia dispar Zone age (Kennedy, 1969; Owen, 1975). Locality 3. The foreshore slab in East Wear Bay The section through the Glauconitic Marl and (TR 261383) Chalk Marl displayed on the slab is shown in Fig. 2. The tide was now sufficiently low, and the party The base of the Glauconitic Marl rests with sharp moved onto the foreshore 200 m east of Horse's Head contact on Bed XIII of the Gault, and glauconitic to examine the best available section through the sediment is piped down into the underlying clay by highest Gault Clay, Glauconitic Marl and Chalk Marl Chondrites and Thalassinoides. The Glauconitic Marl (Fig. Ib). A continuous section of 30 m is exposed on comprises 7 m of dark grey-green glauconitic calcareous sand, strongly bioturbated throughout. a single rotated slab which dips landwards at 30-40°. At the eastern limit, the slab is truncated by a fault Prominently weathering, more lightly coloured beds which downthrows 25-30 m to the east, and brings the alternate with dark, softer units, reflecting the upper part of the Chalk Marl and the overlying Grey distribution of carbonate cement. Burrows which Chalk against the Glauconitic Marl and lower Chalk possess a dark, glauconite-rich core and a light marly Marl. The line of the fault is marked by 5 m of wall (several cm in diameter) are very conspicuous. slurried and brecciated chalk. The hurricane of The upper part contains frequent sponges October 16th had stripped the section completely (Plocoscyphia sp, Stauronema carteri Sollas), hinges of clear of seaweed and shingle, and it was possible to Inoceramus and a few small dark phosphatic intraclasts. The highest bed is a thin (0.1 m), laterally trace beds along strike for over 150 m. The Gault section at this locality is little disturbed persistent limestone which has yielded a specimen of by folding or listric movement, and Bed XII has an Neostlingoceras carcitanense (Matheron) to the outcrop parallel with the overlying 25 m of sediment. Director. There is significant lateral variation in the thickness Bed XII is 2 m thick, and is surmounted by a sharply defined omission surface, this is overlain by darker of the Glauconitic Marl in the area of Dover and clay at the base of Bed XIII, which contains pockets Folkestone. In the Aycliff borehole (Carter & Hart,
76
A. S. GALE
J
I LltholO'llcal bond 4a of
Markers
~ennedv
I
I Po_I
_ ..
I
(1969)
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~ llimltlIa abundllnt. fblticlorc11A .a:s:t.1.f.D:ma
occur.
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thin (1_) prondnent liJDestone bed
Iy
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SbarpeicrCJS occurs frequently
thin ncdJlar limestones and dilIrk muls. containing infrequent chosphatic intraclasts: succession confused by .... 11 silos
"'1- thin (12aDl soarsely glauconitic Umestone-sinqle Nrrg:stlingoc;:eCAS collected
nUcl-er port; noWI.. surroundl"'l ~ and .... 11 phospIotlc intraclasts oocur infrequently
orominently weathering 4b bed of lighter r1 containing several poorly defined
sponq.. St4lWlOlml ~ and ocesent, hinges of
~
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emission surfaces
rk grey-green glauconitic sand, intensely bioturbated by'Sponge) iqnga:bo;
D
(rn.rated ""rly chalk
R: :I
""rly chalk
I~ I'~'~;J rose of Glauconitic Marl into too of Qlul t Bed XIll of Cllul
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8
glauconitic
omission
: J
intraclasts
;. '.'
llhosohat. .
rty 1
sur(a~
spc.nqes in nt:dJles
Fig. 2. Section in Gault, Glauconitic Marl and Chalk Marl shown on the rotated slab on the foreshore (depicted in Fig. Ib), 200 m east of Horse's Head, East Wear Bay (TR 2613R3). MI-M6 are marker beds used in mapping (Fig. Ib). Scale on left of column in metres.
77
FIELD MEETING AT FOLKESTONE WARREN
1977) it is only 1 m in thickness, and Kennedy (1969) recorded 4.8 m at Folkestone (locality unspecified). In the Aycliff borehole, the upper part of the Glauconitic Marl is represented by several metres of silty, marly grey chalk containing numerous Aucellina sp. (C. J. Wood, pers. comm.). Resting on the top bed of the Glauconitic Marl are 1.2 m of marly chalk, in the lower part of which glauconite virtually disappears. Above this are several metres of dark marls containing many omission surfaces and sponge-rich nodular limestones, the precise succession of which is confused here by numerous small slips. The same beds are exposed in situ at the foot of Abbot's Cliff, where a detailed succession has been identified (see Appendix). Small brown phosphate nodules occur frequently at this level, and the toothed oyster Rastellum sp. occurs commonly. A prominent nodular limestone (Fig. 2, M3) which contains abundant and well preserved ammonites of the late Neostlingoceras carcitanense Subzone, notably Schloenbachia, inflated species of Mantelliceras and Hypoturrilites. The top of the ammonite bed displays complex bioturbation by large (0.1 m) idiomorphic Thalassinoides,' Chondrites and Planolites, filled with sediment from the dark, silty marl above. The overlying succession comprises rhythmic alternations of marl and marly chalk, separated by bioturbated omission surfaces. M3 is a useful marker horizon, consisting of a dark, prominently weathering silty marl containing oysters and phosphates; it is piped down into the underlying light grey marl in 0.5 m deep Thalassinoides burrow systems. 1 m above the base of M3, the distinctive, inflated Inoceramus virgatus appears in the succession, and remains common for several metres. M4 is a thin (0.15 m) prominent limestone, which has yielded ammonites including Mantelliceras dixoni Spath (10), M. saxbii (1), Scaphites obliquus J. Sowerby (7), Mariella sp. (1) and Hyphoplites sp. (1). This is the lowest record of the Mantelliceras dixoni Zone fauna at Folkestone. Since M3 contains a late N. carcitanensis Subzone fauna (dominated by inflated Mantelliceras) , the M. saxbii Subzone can be represented by a possible maximum of only 4.75 m of chalk at Folkestone. This interval has yielded few ammonites of stratigraphical value. It therefore appears likely that the entire M. mantelli Zone is condensed at this locality, as can be inferred from the many levels with phosphatic intraclasts. Near the top of the slab are two closely spaced thin, nodular, spongiferous limestones M5, strongly piped by dark-filled Planolites, and overlain by a thick (1 m +) mid-grey marl. The base of this marl is Kennedy's band 4a (1969, Fig. 2), which yields a rich M. dixoni Zone fauna. The marl contains frequent specimens (28 collected in total) of the rhynchonellid Orbirhynchia mantelliana (d'Orbigny). The same
beds, yielding identical faunas, also outcrop at the foot of Abbot's Cliff; thus, correlation between the shore sections in East Wear Bay and the cliffs to the east can be made with confidence. Members spent a considerable time examining the shore sections and collecting fossils, and subjected the Director's measured section and proposed correlation to detailed scrutiny. Important fossil records were made, and many specimens generously donated to the Director for inclusion in a research collection. In particular, Dr. S. K. Donovan (who travelled from Jamaica to attend the meeting) obtained a fine example of the echinoid Hemiaster morrisi (Forbes), and several regular echinoids. After taking lunch, the rising tide forced the party to press eastwards along the shore towards Abbot's Cliff, and mount the concrete platform. Locality 4. The Plenus Marl in East Wear Bay (TR 263384)
Near the easternmost point of access to the concrete platform, at the back of the sea-wall, is an airweathered section of the Plenus Marl which shows clearly the 8 beds described by Jefferies (1963). Dr. Ian Jarvis has been coordinating a detailed geochemical and micropalaeontological study of this interval, and he kindly outlined the results and conclusions for the party. This work shows the coincidence of the Late Cenomanian oceanic anoxic event (OAE), identifiable from an anomalous increase in (j 13C, with stepwise extinction of microplankton and benthonic microfossils (see Jarvis, Carson, Cooper, Hart, Leary, Tocher, Horne & Rosenfeld, 1988). A protruding specimen of the belemnite Actinocamax plenus (Blainville) in Bed 4 of the Plenus Marl was left in place for the benefit of future visitors. Localities 5 & 6. The Chalk Marl of Abbot's Cliff (TR 271385, 279386)
The short walk to Abbot's Cliff along the sea-wall crossed the back wall fault of the Warren landslip, and the party was able to examine the middle part of the Chalk Marl succession, here with the gentle regional dip to the north-east of 1 Unfortunately, all but the base of the Abbot's Cliff section is poorly exposed, and somewhat inaccessible, except on recent, dangerous falls of chalk. The stratigraphical section given herein (Fig. 3) for the middle part of the Chalk Marl was compiled on different occasions over several years, as parts of the cliff became well exposed. As the daylight was beginning to fail, we continued round beneath Abbot's Cliff, crossed the large cliff-fall, and reached a sea-washed cliff in the higher part of the Chalk Marl which has only been exposed for a few years. This section has proved to be of great value in working out the detailed stratigraphy of the Lower-Middle Cenomanian boundary at Folkestone. 0
•
78
A. S. GALE
IFa_I Notes I
I Lithological Markers I
Qrbirbytrllio IIMte1liAOil CCIlIDOr1
•
~
'cast Bed' of price: 1JI of tim sUty mel ]bCachiopods ge,1nitz.ll" Plotytbycis
ElltlllilD, and oraqonitlc CJlIotropocls
2 prcninent limestooes, separated by rly chalk which _thers proud: upper IlJnestone full of ~ and ""'14831001,,=: bose of lower llJoostone piped Into dork marl
below by large 11S'1tIsS'oolde:s, reworked by ~ :
·
dark IMct containing tfWly pyrite nodules
J.chlAmYJl ocle:sleosis, ~ ~ and
~
aaIl1nulllIR
OJnnlngtonlcrrol
• •
0QIlIll0[l
~
pCeRnt
Orbi rbyrrlliA mottlllOM carmon
• beds at:ove duker and more clay-rich
long burr""
~
present
10 __.!"""'oiII
bose of bed piped down by ccnspicuous, lIghtcontrasting blt!Sdooldes bed full of dork-<:ootro.tlng
~
Qrbjrbynchio lMotrll!4OA [
pyrite noWles
5 dark, recessing macl, fran which water seeps
···
Orbi rhm;bia motelliono frequent
2 thin, strongly bloturblt;ted, spongiferous limestonn:, muted· in fiq.2
MAntelliceros ~ oama'l
MootfclorellA
~
and
~~0QIlIll0[l
Fig. 3. Section in the Chalk Marlon Abbot's Cliff and at Lydden Spout, between Dover and Folkestone (TR 271385-279386). Key as for Fig. 2, with additions. Scale on left of column in metres. Note overlap with slipped foreshore slab depicted in Figs. la & 2.
FIELD MEETING AT FOLKESTONE WARREN
The 6 m of Chalk Marl visible at this locality include two conspicuous beds of limestone, the "prominent limestones" at the top of band 6 of Kennedy (1969, Fig. 2). The lower of these rests on a very dark marl which contains many pyrite nodules; the junction is burrowed by light-contrasting Thalassinoides which are reworked by Chondrites. The fauna of the dark marl is very diverse; it is characterised by the small scallop Chlamys arlesiensis (Woods) and the serpulid Glandifera rustica (J. Sowerby), both restricted to this bed. Oxytoma seminudum Dames is common, and Cunningtoniceras inerme (Pervinquiere) occurs both in this bed and the one below. The underlying 2 m of sediment contain abundant specimens of Orbirhynchia mantelliana, the second horizon of abundance of this species in the succession. Just beneath this is a level penetrated by the deep vertical burrow Bathichnus (Bromley, Schultz & Peake, 1975). These are visible as 0.01 m diameter dark burrows, surrounded by a 0.1 m light halo of oxidised sediment. The higher of the two prominent limestones is full of dark contrasting Thalassinoides, Planolites and Chondrites, which descend from the dark, silty marl above. This highly fossiliferous marl, 1 m in thickness, is the Cast Bed of Price (1877), from which the
79
professional collector John Griffiths obtained numerous pyrite-coated composite moulds of gastropods in the last century. In the lower part of the bed Entolium orbiculare (J. Sowerby) occurs in hundreds, together with the brachiopods Platythyris squamosa (Mantell), Modestella geinitzi (Scholenbach), and Kingena concinna (Owen). The small coral Micrabacia coronula (Goldfuss) is common, as is the serpulid Rotularia. The lowest record of Acanthoceras rhotomagensis (Brongniart) is from this bed. These beds are readily identifiable in slumped masses on the foreshore of East Wear Bay. As it was nearly dark, no time was available to examine the overlying limestone-marl rhythms, which contain the third and highest O. mantelliana horizon (Band 9 of Kennedy 1969), and abundant ammonites of the Turri/ites costatus Subzone fauna. The party commenced the walk back along the sea-wall in the twilight, just as the lights of Folkestone harbour were coming on. During the drive to Folkestone Central station, a vote of thanks to the Director was proposed. He in turn would like to thank David Wray for driving the minibus, Chris Wood for help, and Ian Jarvis for talking about the Plenus Marl.
References BROMLEY, R. G., M.-G. SCHULTZ & N. B. PEAKE. 1975. Paramoudras: giant flints, early burrows, and the early diagenesis of chalks. Kgl. danske Vidensk. Selsk. bioI. Skr., 20(10), 1-31, 5 pis. CARTER, D. J. & M. B. HART. 1977. Aspects of mid-Cretaceous stratigraphical micropalaeontology. Bull. Br. Mus. nat. Hist. (Geol.), 29, 1-135,4 pis. DE RANCE, C. E. 1lW!. On the Albian or Gault of Folkestone. Geol. Mag., 5, 163-71. JARVIS, I., G. A. CARSON, M. E. COOPER, M. B. HART, P. LEARY, B. A. TOCHER, D. J. HORNE & A. ROSENFELD. (1988). Microfossil assemblages and the Cenomanian-Turonian (late Cretaceous) Oceanic Anoxic Event. eret. Res., 9,3-103. GREEN, C. P. 1989. Excursions in the past: a review of the Field Meeting Reports in the first one hundred volumes of the Proceedings. Proc. Geol. Ass., 100, 17-29. HUTCHINSON, J. N., E. N. BROMHEAD & J. F. LUPIN I. 1980. Additional observations on the Folkestone Warren. Q. fl. eng. geol., 13, 1-31. JEFFERIES, R. P. S. 1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the AngloParis Basin. Proc. Geol. Ass., 74, 1-34. KENNEDY, W. J. 1%9. The correlation of the Lower Chalk of south-east England. Proc. Geol. Ass., 80, 459560. - - & GARRISON, R. E. 1975. Morphology and genesis of nodular phosphates in the Cenomanian Glauconitic Marl of south-east England. Lethaia, 8, 339-60. OSMAN, C. W. 1917. The landslips of Folkestone Warren and thickness of the Lower Chalk and Gault near Dover. Proc. Geol. Ass., 28, 59-82. OWEN, H. G. 1971. Middle Albian stratigraphy in the
Anglo-Paris Basin. Bull. Br. Mus. nat. Hist. (Geol.) Suppl., 8, 1-164. - - 1975. The stratigraphy of the Gault and Upper Greensand of the Weald. Proc. Geol. Ass., 86, 475-98. PHILLIPS, W. 1818. A selection of facts from the best authorities arranged so far as to form an outline of the geology of England and Wales. London. - - 1821. Remarks on the chalk cliffs in the neighborhood of Dover, and on the Blue Marie covering the Green Sand, near Folkestone. Appendix: Containing some account of the Chalk Cliffs, etc. on the coast of France opposite to Dover. Trans. geol. Soc. Land., 5, 16-51. PRICE, F. G. H. 1874. On the Gault of Folkestone. Q. Jl. geol. Soc. London., 30, 342-66. - - 1877. On the beds between the Gault and Upper Chalk near Folkestone. Q. fl. geol. Soc. Lond., 33,431-48. ROBINSON, N. D. 1985. Field meeting: the Chalk of the Kent coast, 21st. August 1983. Proc. Geol. Ass., 96, 93-%. - - 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proc. Geol. Ass., 97, 141-70. ROWE, A. W. 1900. The Zones of the White Chalk of the English coast. 1. Kent and Sussex. Proc. Geol. Ass., 16, 289-368. SMART, J. G. 0., G. BISSON & B. C. WORSSAM. 1%6. Geology of the country around Canterbury and Folkestone. Mem. geol. Surv. U.K. 337pp. SPATH, L. F. 1923a. Excursion to Folkestone, with notes on the zones of the Gault. Proc. Geol. Ass., 34, 70-76. - - 1923b. On the ammonite horizons of the Gault and contiguous deposits. Appendix II. Summ. Prog. Geol. Surv. G. B. (for 1922), 139-49.
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A. S. GALE
1923-1943. A monograph of the ammonoidea of the Gault. Palaeonogr. Soc./Monogr. /, 2 vols., 1-787, pis. 1-72. VOIGT, E. 1968. Uber Hiatus-Konkretionen (dargesteUt an
Beispielen aus dem Lias). Ceol. Rdsch., 58,281-96. WOOD, A. M. M. 1955. Folkestone Warren landslips: investigations 1948-50. Proc. Instn. dv. Engrs., Railway Paper, 56, 410-28.
APPENDIX Occurrence of the ammonite genus Sharpeiceras in tbe Lower Cenomanian Chalk Marl of Folkestone A. S. GALE and S. FRIEDRICH" The recently published Part II of the Palaeontographical Society memoir on 'The Ammonoidea of the Lower Chalk' (Wright & Kennedy, 1987) includes an account of the English material of the Lower Cenomanian acanthoceratid genus Sharpeiceras. Although geographically widespread, occurring throughout Europe, Africa (including Madagascar), the Middle East, southern India, Japan, Texas and Mexico, the genus is usually rare, and many species are known from only a few specimens (Wright & Kennedy 1984, 1987). The English material of Sharpeiceras was placed in three species by Wright and Kennedy: S. schlueteri (Hyatt), S. laticlavium (Sharpe) and S. sp., of uncertain affinities. Accurately horizoned specimens of S. schlueteri are known from the unphosphatised fauna of the Glauconitic Marl (Neostlingoceras cardtanense Subzone) on the Isle of Wight, and at Eastbourne, Sussex. At the former locality the species ranges up into the overlying Mantelliceras saxbii Subzone. The horizon of occurrence of the rare species S. laticlavium was unknown. From the lower part of the Chalk Marl exposed on the foreshore beneath Abbot's Cliff, east of Folkestone, we have collected 15 specimens of Sharpeiceras, in situ. These fall into both S. schlueteri (5 specimens) and S. laticlavium (6), in addition to a number of specifically indeterminate fragments (4). This material is associated with a diverse M. mantelli Zone fauna, which belongs to the upper part of the Neostlingoceras cardtanense Subzone. The section at this locality has not been previously described, and is now partly covered by a major cliff-fall (January, 1988). We therefore present a brief account of the stratigraphy and fauna of the locality. A total of 4 m of the Chalk Marl is intermittently exposed on the shore platform at low tide, at the foot of the easternmost 180 m of sea defences on the west of Abbot's Cliff (TR 269384; locality 7 of Fig. la). The succession is shown in Fig. 4. A low «1 m) cliff is present at the base of the sea wall, from which a shore platform extended seawards for 30-30 m. The 9 nodular, spongiferous limestones form low benches on the shore, which run out to the sea to the east as the dip brings in higher beds. To the west the section is abruptly terminated by the back wall fault of the Warren; to the east, the Chalk Marl disappears beneath rock falls and shingle. At the top of the sea wall, the middle part of the Chalk Marl is exposed. The horizon identified as M5 on Figs 2 & 3 (top of band 2 of Kennedy, 1969) is clearly visible over a distance of 50 m, and is measured here as 8 m vertically above the most prominent limestone (M3) on the shore • 17 Mallon House, Carr Street, London E14 7SE.
below. This information, and comparison with the slipped foreshore section shown in Fig. 2, places the Chalk Marl depicted in Fig. 4 at the base of that unit, immediately overlying the Glauconitic Marl. The section was noted by Kennedy (1969), who assigned it to the top of his band 2 and the base of band 3, and suggested that it fell within the Mantelliceras saxbii Subzone. Furthermore, he identified this as the level which yielded the material of the large oyster Rastellum carinatum (d'Orbigny) present in old collections from Folkestone, and redescribed by Carter (1968). The lower beds (based to limestone 7) seen at this locality correspond to Price's (1877) Bed II of the Chalk Marl, which he recorded as 10 feet (3 m) thick. The list of fossils which Price gives from this unit (1877, p. 435) includes 'Ostrea frons' and '0. frons, vaT. carinata' (=Rastellum carinatum), and 'Terebratula biplicata', which probably refers to Tropeothyris carteri (Davidson). Both species are restricted to this unit. The section seen in December, 1987 is shown in Fig. 4. At the base is 1 m of mid to dark grey marl which contains several burrowed omission surfaces, and scattered limestone nodules. The lowest level seen is weakly glauconitic, and cannot fall far above the top limestone of the Glauconitic Marl (compare Fig. 2). Above, are 5 thin, nodular limestones, full of sponges, separated by dark grey marls. The top of each limestone is piped with marl infilling Thalassinoides and (1) Planolites. The marls are silty, and strongly bioturbated, and contain occasional brown phosphate pebbles, and abundant comminuted inoceramid debris, especially above the third limestone from the base. Concave-up scour structures are present in the marls. Above a thick (0.4 m) conspicuously dark marl, are two nodular limestones, of which the upper (M3) is thicker and more laterally continuous. These two beds contain abundant ammonites. The upper limestone is surmounted by an omission surface burrowed by large idiomorphic Thalassinoides. The dark, very silty marl above contains light chalk intraclasts. The highest 1.5 m of the section includes two impersistent nodular limestones, in a sequence of marls containing burrowed omission surfaces. The calcitic macrofauna of the lowest 7 limestones and intervening marls is extremely distinctive, and includes species restricted to this level. The large terebratulid Tropeothyris carteri, generally considered to be a rare species, is frequent (22 specimens collected). Other brachiopods present are the small rhynchonellids Monticlarella rectifrons (Pictet & Campiche) and Crasirhynchia grasiana (d'Orbigny). Rastellum carinaturn is common, most notably in the dark marl beneath the level of ammonite abundance. The fauna of echinoids is diverse, including Holaster sp., Hemiaster morrisi, Epiaster sp.,
81
FIELD MEETING AT FOLKESTONE WARREN
.
+J
4
Q)
'0
c:
.,-i
.
0.. fI)
... ...
50
... ...
...... ·... ·.. ·... ...
o~~~~ Fig. 4. Section in the basal Chalk Marlon the foreshore at the western end of Abbot's Cliff, east of Folkestone (TR 269384), and selected macrofossil occurrences: each specimen is indicated by a dot. Key as for Fig. 2. Scale on left of column in metres. Position of section indicated in Fig. 2.
82
A. S. GALE
Tetragramma variolare (Brongniart) and Stereocidaris sp. Large specimens of Inoceramus crippsi Mantell are abundant. The nautiloid Cymtoceras is occasionally found. Ammonites are preserved as composite moulds coated with dark grey pyrite. In the lowest five limestones, ammonites are frequent, but never abundant. Schloenbachia varians is common, Mantelliceras occasional, and Hypoturrilites rare in these beds. Sharpeiceras is present in limestones I, 3, and 5, and in the marl overlying 3. Most of the specimens of Sharpeiceras are large (last septum at diameters of 200-260 mm, total diameters probably 350400 mm). Specimens of this genus in old collections from Folkestone (e.g. BMNH C3677) almost certainly originate from limestones 3 and 5. Limestones 6, 7 and the intervening marl have yielded several hundred ammonites, belonging mostly to the genera Schloenbachia, Mantelliceras, and Hypoturrilites. Mariella and Hyphoplites occur more rarely. The Mantelliceras belong dominantly to the infiated species M. mantelli (J. Sowerby) and M. cantianum Spath. This level is undoubtedly the main source of well preserved Hypoturrilites and Mantelliceras in 19th century collections from Folkestone. Only the top M. dixoni horizon (M6) contains comparably well preserved specimens, but these are characteristically M. dixoni and Schloenbachia sp. The highest beds are relatively poorly fossiliferous, and we have collected only Mariella sp., Schloenbachia sp., oysters, and I. crippsi. On the basis of recent collecting, the limits of ammonite Zones and Subzones in the Lower Cenomanian of
Folkestone can be revised. The scheme follows Wright, Kennedy & Hancock (1984). The earliest Cenomanian N. carcitanense Subzone in southern England appears to contain two successive ammonite assemblages, the lower characterised by the subzonal fossil and diverse heteromorphs, including Idiohamites, Anisoceras and Sciponoceras. This is best exemplified by the phosphatised N. carcitanense fauna of the Glauconitic Marlon the Isle of Wight. In the higher assemblage, N. carcitanense and Idiohamites sp. are rare or absent, but Sharpeiceras occurs, albeit rarely, in association with a Mantelliceras fauna dominated by inflated forms. The higher assemblage is found in unphosphatised preservation in the Glauconitic Marl of the Isle of Wight (e.g. Compton Bay) and Eastbourne, and in limestones 1-7 (of Fig. 4) at Folkestone. The top bed of the Glauconitic Marl at Folkestone (M2 on Fig. 2) has yielded a single Neostlingoceras sp, and the Glauconitic Marl thus probably belongs entirely to the lower N. carcitanense assemblage at this locality. At Petit Blanc Nez, in the Boulonnais, 40 km away, N. carcitanense is present in phosphatised perservation in the thin lateral equivalent of the Glauconitic Marl (Amedro, 1986). The Mantelliceras saxbii Subzone fauna, characterised by compressed species of Mantelliceras, is poorly represented at Folkestone. Pebble fossils in the dark, silty bed M4 (Fig. 2) include poorly preserved M. cf. saxbii. M5 yields an ammonite fauna characteristic of the M. dixoni Zone, which ranges up to bed M6. We would like to thank J. M. Hancock for commenting on the manuscript.
Additional References AMEDRO, F. 1986. Biostratigraphie des craies cenomaniennes du Boulonnais par les ammonites. Ann. Soc. geol. Nord, 105, 159-67. CARTER, R. M. 1968. Functional studies on the Cretaceous oyster Arctostrea. Palaeontology, 11, 458-85. KENNEDY, W. J. & C. W. WRIGHT. 1984. The Ammonoidea of the Lower Chalk. Part I. Palaeontogr.
Soc. (Monogr.), 1-126, pis. 1-40. - - & - - 1987. The Ammonoidea of the Lower Chalk.
Part II. Palaeontogr. Soc. (Monogr.), 127-218, pis. 41-55. - - , - - & J. M. HANCOCK. 1984. The Ammonoidea of
the Lower Chalk. Part I. Introduction. Palaeontogr. Soc. (Monogr.), pp. 1-:)7.
School of Geology, Thames Polytechnic, Walburgh House, Bigland Street, London £1 2NG
1. INTRODUCTION The first Field Meeting of the Geologists' Association was held at Folkstone on 9th April, 1860 and a brief report was published in Volume 1 of the Proceedings (see Green 1989). Here, in Volume 100, it is appropriate to provide an account of the most recent visit of the Association to Folkstone in 1987. Seventeen members met at Folkestone station, and were taken to the British Rail car park in the Warren by car and minibus. The Director outlined the geology of the Warren from a vantage point which afforded good general views of both the exposed Cretaceous Chalk succession and the landslipped area. The Folkestone Warren is an area of landslipped undercliff, which extends along the coast eastwards from Folkestone to Abbot's Cliff, a distance of nearly 2 km. Its maximum width is approximately 500 m, and it is bounded on the landward side by high cliffs of Lower and Middle Chalk, which rise to 150 m above O.D. Slipping is initiated in the underlying Gault Clay. The Gault Clay (Albian) is at present exposed only intermittently on the foreshore in the Warren, near the base of the landslipped part of the succession; the extent of outcrop depends on sand and shingle movement. In the past, before the construction of a concrete toe weighting in East Wear Bay, successive movements of the landslip provided vast foreshore outcrops of the highly fossiliferous Gault. These sections, continually washed by high tides, yielded many of the rich and diverse Gault fossil collections seen in museums throughout the country. The cliff section of the Gault Clay, at Copt Point, is fortunately in good condition still, for all but the highest part of the succession. The descriptions of De Rance (1868) and Price (1874) were based on Copt Point. Price's bed numbering system (Roman numerals I-XIII) is still employed. The ammonite biostratigraphy is known in considerable detail, from the work of Spath (1923a, 1923b, 1923-1943) and Owen (1972, 1975). The Chalk succession of the cliffs between Dover
and Folkestone was first described by William Phillips (1818, 1821), who recognised a lower division, approximately 200 feet (60 m) thick, which he called Grey Chalk. This he distinguished by its darker colour and greater softness from the overlying, whiter, 'Chalk without flints and with few fossils'. The latter division spanned what is now considered to be the top of the Lower Chalk and the lower part of the Middle Chalk, and includes the Pienus Marl. The high cliffs at the back of the Warren display spectacular, but rather inaccessible sections through the Lower and Middle Chalk. The Plenus Marl is conspicuous on account of its slightly recessive weathering, and pale greenish-yellow coloration. The very white, nodular, Melbourn Rock immediately above weathers out prominently. The top of the Lower Chalk, and the Middle Chalk have recently been the subject of a detailed lithostratigraphical study by Robinson (1986) who demonstrated his work to the Association in the field (Robinson, 1985). However, Robinson did not study in detail the lowest of his units, the East Wear Bay Chalk Formation, which includes what has been widely known as the Glauconitic Marl, the Chalk Marl, and the lower part of the Grey Chalk. The biostratigraphy of this part of the succession was last described by Kennedy (1969), who provided a lithological log through the entire Lower Chalk. The Director, C. J. Wood and C. V. Jeans, are currently undertaking an investigation of the lithological succession and faunas of the Chalk Marl at Folkestone. Unlike the Gault, of which the bed-bybed stratigraphy has been known since the work of Price, no accurate lithological log of the Chalk Marl has ever been published. Price (1877) underestimated the thickness of the Chalk Marl by about 15 m, and Kennedy (1969) did not correlate the base of the Abbot's Cliff section with the foundered chalk blocks on the foreshore, and his published log for the Chalk Marl includes a total overlap of approximately 10 metres. Although many museum fossil collections contain 73
74
A. S. GALE
material from the Chalk Marl of Folkestone, From the car park the party set off eastwards into the commonly including ammonites, bivalves, echinoids, Warren, and reached the shore of East Wear Bay brachiopods, and vertebrates, the precise horizons 300 m to the west of Horse's Head (Fig. la). The top from which even the common species originate are of the Gault is presently well exposed over a distance generally unknown. Since Folkestone has yielded of some 50 m at the foot of the concrete platform. Bed numerous type and figured specimens, (e.g. see list in XII is a useful marker horizon on the foreshore, Smart, Bisson & Worssam, 1966) this is a matter of consisting of 1 m of intensely bioturbated, glauconitic, importance. During the present study, new collections dark grey-green silty clay, containing numerous of stratigraphy localised fossils have been made, and it phosphatic intraclasts. The base is piped down into has proved possible to infer the sources of specimens the underlying light grey clays of Bed XI by large idiomorphic Thalassinoides. Bed XI has a total in older collections. Both the past excellence of the Warren shore as an thickness of 15 m at Folkestone (Owen, 1975), and is area for fossil collecting, and confusion over the sparsely fossiliferous; it contains many light buff stratigraphy of the Chalk Marl, are attributable to the coloured phosphatic nodules, which have diffuse nature of the landslip. The competent, permeable boundaries. The base of the bed is not visible on the Chalk rests on incompetent Gault Clay; slip surfaces shore at present. The Gault at this locality dips were initiated in a sheet of plastic clay immediately landwards at approximately 10°, and the sinuous above the base of the Gault, and propagated up outcrop of Bed XII on the shore indicates that folding through the succession as listric faults. Large has occurred during formation of the slump. landward-dipping rotated slabs of Gault and Chalk The phosphatic intraclasts in Bed XII attracted thus make up the foreshore in East Wear Bay. The interest from members. The nodules formed originally Gault was thrown into ridges on the foreshore from early diagenetic concentration of calcium immediately after major landslides (Osman, 1917), phosphate, probably replacing burrows, and initially which were gradually bevelled by wave action. The resembled those described in Bed XI. Current scour whole area was previously a continually replenished exhumed nodules, and the harder, dark brown-grey source of fossil material, from the Gault and Lower cores were left as pebbles on the sea bed. These acted as substrates for attachment of oyster spat, and more Chalk. Landslipping resulted in the absence, or great rarely, serpulid worms. In some cases, a second burial scarcity, of in situ exposures of the highest Gault Clay, resulted in the addition of more phosphatic material, Glauconitic Marl, and much of the Chalk Marl. These of a lighter colour. Complex intraclasts of this type are units are only accessible (as continuous sections of any called hiatus concretions (Voigt, 1968), and resemble size) as rotated masses on the foreshore of the those described from the Glauconitic Marl of the Isle Warren. Accurate measurement of stratigraphical of Wight by Kennedy & Garrison (1975). The sections is complicated by numerous small faults, and darkness of the concretions is an indication of their irregular seaweed growth and shingle cover. relative age. Members who had in the past visited the Gault at Movement of the Warren landslip disrupted the Folkestone to Dover railway line in 1877, and most Copt Point, where Beds I-X are exposed, were dramatically, in 1914, when the line was partially disappointed by the scarcity of fossils in Beds covered, displaced by a maximum of 50 m and closed XI-XIII. Several phosphatic steinkerns of ammonites for 4 years (see Hutchinson, Bromhead & Lupini, (Stomohamites sp., Mortoniceras sp.), and an example 1980 for detailed account). Since further interruption of the enigmatic spherical organism Parkeria were seemed inevitable, the Southern Railway Company collected in Bed XII. The other fauna of this unit instigated research into the causes of landslip (Wood, includes frequent examples of Aucellina gryphaeoides 1955). It was discovered that slipping took place (J. de C. Sowerby), and the small brachiopod immediately above the base of the Gault, and that the Terebratulina sp. water in the underlying Lower Greensand was under artesian pressure. Locality 2. The Middle Chalk at Horse's Head The solution suggested was twofold; firstly, to drain the slips by driving headings back into them, thus (TR 256382) lowering the water table, and secondly by weighting As the tide was still ebbing, the party visited the the toe of the landslipped area, and protecting it from Middle Chalk exposed above the toe-weighting at erosion by the construction of sea walls and groynes. Horse's Head-a large rotated block from the lower These measures were successful, and the landslip has part of the Middle Chalk (Shakespeare Cliff Member not undergone significant movement for over 35 years. of Robinson, 1986) which dips northwards at about 30°. The air and spray-weathered surfaces are rough 2. LOCALITIES VISITED with fossil debris, mostly chips of inoceramid bivalves. Bedding is clearly picked out by alternately softer Locality 1. The Gault Clay west of Horse's Head marly and more nodular units, and scour structures, (TR 254380) infilled with white chalk intraclasts, are present.
FIELD MEETING AT FOLKESTONE WARREN
a
2.
25
2'
75
38
Loc.2
East Wear Bay 37
!
1 km !
!
!
!
'
I
Fig. 1. (a) Map of the Warren to show localities visited by the G.A. party. (b) (inset) is a geological map of the foreshore (at low tide) at locality 3, which displays the highest Gault, Glauconitic Marl and basal Chalk Marl in a large rotated slab. Markers MI-M6 refer to Fig. 2. Mapped by P. J. Haselock and A. S. Gale, Jan. 1988.
Fossils found include the small rhynchonellid Orbirhynchia cuvieri (d'Orbigny), formerly used as a zonal index for this part of the Chalk (e.g. Rowe, 1900), marginal ossicles of the asteroid Crateraster quinqueloba (Goldfuss), and columnals of the crinoid Bourgueticrinus sp.
of phosphatic nodules at the base. The base of Bed XIII is piped down into the top of Bed XII by Thalassinoides. Bed XIII is apparently 7 m thick at this locality, and all but the basal part consists of light grey marly clay; ammonites collected in the past indicate that it is of Stoliczkaia dispar Zone age (Kennedy, 1969; Owen, 1975). Locality 3. The foreshore slab in East Wear Bay The section through the Glauconitic Marl and (TR 261383) Chalk Marl displayed on the slab is shown in Fig. 2. The tide was now sufficiently low, and the party The base of the Glauconitic Marl rests with sharp moved onto the foreshore 200 m east of Horse's Head contact on Bed XIII of the Gault, and glauconitic to examine the best available section through the sediment is piped down into the underlying clay by highest Gault Clay, Glauconitic Marl and Chalk Marl Chondrites and Thalassinoides. The Glauconitic Marl (Fig. Ib). A continuous section of 30 m is exposed on comprises 7 m of dark grey-green glauconitic calcareous sand, strongly bioturbated throughout. a single rotated slab which dips landwards at 30-40°. At the eastern limit, the slab is truncated by a fault Prominently weathering, more lightly coloured beds which downthrows 25-30 m to the east, and brings the alternate with dark, softer units, reflecting the upper part of the Chalk Marl and the overlying Grey distribution of carbonate cement. Burrows which Chalk against the Glauconitic Marl and lower Chalk possess a dark, glauconite-rich core and a light marly Marl. The line of the fault is marked by 5 m of wall (several cm in diameter) are very conspicuous. slurried and brecciated chalk. The hurricane of The upper part contains frequent sponges October 16th had stripped the section completely (Plocoscyphia sp, Stauronema carteri Sollas), hinges of clear of seaweed and shingle, and it was possible to Inoceramus and a few small dark phosphatic intraclasts. The highest bed is a thin (0.1 m), laterally trace beds along strike for over 150 m. The Gault section at this locality is little disturbed persistent limestone which has yielded a specimen of by folding or listric movement, and Bed XII has an Neostlingoceras carcitanense (Matheron) to the outcrop parallel with the overlying 25 m of sediment. Director. There is significant lateral variation in the thickness Bed XII is 2 m thick, and is surmounted by a sharply defined omission surface, this is overlain by darker of the Glauconitic Marl in the area of Dover and clay at the base of Bed XIII, which contains pockets Folkestone. In the Aycliff borehole (Carter & Hart,
76
A. S. GALE
J
I LltholO'llcal bond 4a of
Markers
~ennedv
I
I Po_I
_ ..
I
(1969)
!:!:!:~~ -+- ~i~nbv~~(~iTI~~:s~~.
H.10te
111cr
CAS
i11.ml1
CXlIrI'lQl
on _ ' s Cliff eectlon (Pia,])
~ llimltlIa abundllnt. fblticlorc11A .a:s:t.1.f.D:ma
occur.
-/'I..,.,'"'..... ~
~
thin (1_) prondnent liJDestone bed
Iy
"",otelllerrAs
J1Us:n1
aooeara
",otel) terrAs • !1.. and 'fypotyr r f 1 i It:S carmon
~
'"o
SbarpeicrCJS occurs frequently
thin ncdJlar limestones and dilIrk muls. containing infrequent chosphatic intraclasts: succession confused by .... 11 silos
"'1- thin (12aDl soarsely glauconitic Umestone-sinqle Nrrg:stlingoc;:eCAS collected
nUcl-
orominently weathering 4b bed of lighter r1 containing several poorly defined
sponq.. St4lWlOlml ~ and ocesent, hinges of
~
~camoo
emission surfaces
rk grey-green glauconitic sand, intensely bioturbated by'Sponge) iqnga:bo;
D
(rn.rated ""rly chalk
R: :I
""rly chalk
I~ I'~'~;J rose of Glauconitic Marl into too of Qlul t Bed XIll of Cllul
Ul
- IIqh qrey
in burr
rly clay
8
glauconitic
omission
: J
intraclasts
;. '.'
llhosohat. .
rty 1
sur(a~
spc.nqes in nt:dJles
Fig. 2. Section in Gault, Glauconitic Marl and Chalk Marl shown on the rotated slab on the foreshore (depicted in Fig. Ib), 200 m east of Horse's Head, East Wear Bay (TR 2613R3). MI-M6 are marker beds used in mapping (Fig. Ib). Scale on left of column in metres.
77
FIELD MEETING AT FOLKESTONE WARREN
1977) it is only 1 m in thickness, and Kennedy (1969) recorded 4.8 m at Folkestone (locality unspecified). In the Aycliff borehole, the upper part of the Glauconitic Marl is represented by several metres of silty, marly grey chalk containing numerous Aucellina sp. (C. J. Wood, pers. comm.). Resting on the top bed of the Glauconitic Marl are 1.2 m of marly chalk, in the lower part of which glauconite virtually disappears. Above this are several metres of dark marls containing many omission surfaces and sponge-rich nodular limestones, the precise succession of which is confused here by numerous small slips. The same beds are exposed in situ at the foot of Abbot's Cliff, where a detailed succession has been identified (see Appendix). Small brown phosphate nodules occur frequently at this level, and the toothed oyster Rastellum sp. occurs commonly. A prominent nodular limestone (Fig. 2, M3) which contains abundant and well preserved ammonites of the late Neostlingoceras carcitanense Subzone, notably Schloenbachia, inflated species of Mantelliceras and Hypoturrilites. The top of the ammonite bed displays complex bioturbation by large (0.1 m) idiomorphic Thalassinoides,' Chondrites and Planolites, filled with sediment from the dark, silty marl above. The overlying succession comprises rhythmic alternations of marl and marly chalk, separated by bioturbated omission surfaces. M3 is a useful marker horizon, consisting of a dark, prominently weathering silty marl containing oysters and phosphates; it is piped down into the underlying light grey marl in 0.5 m deep Thalassinoides burrow systems. 1 m above the base of M3, the distinctive, inflated Inoceramus virgatus appears in the succession, and remains common for several metres. M4 is a thin (0.15 m) prominent limestone, which has yielded ammonites including Mantelliceras dixoni Spath (10), M. saxbii (1), Scaphites obliquus J. Sowerby (7), Mariella sp. (1) and Hyphoplites sp. (1). This is the lowest record of the Mantelliceras dixoni Zone fauna at Folkestone. Since M3 contains a late N. carcitanensis Subzone fauna (dominated by inflated Mantelliceras) , the M. saxbii Subzone can be represented by a possible maximum of only 4.75 m of chalk at Folkestone. This interval has yielded few ammonites of stratigraphical value. It therefore appears likely that the entire M. mantelli Zone is condensed at this locality, as can be inferred from the many levels with phosphatic intraclasts. Near the top of the slab are two closely spaced thin, nodular, spongiferous limestones M5, strongly piped by dark-filled Planolites, and overlain by a thick (1 m +) mid-grey marl. The base of this marl is Kennedy's band 4a (1969, Fig. 2), which yields a rich M. dixoni Zone fauna. The marl contains frequent specimens (28 collected in total) of the rhynchonellid Orbirhynchia mantelliana (d'Orbigny). The same
beds, yielding identical faunas, also outcrop at the foot of Abbot's Cliff; thus, correlation between the shore sections in East Wear Bay and the cliffs to the east can be made with confidence. Members spent a considerable time examining the shore sections and collecting fossils, and subjected the Director's measured section and proposed correlation to detailed scrutiny. Important fossil records were made, and many specimens generously donated to the Director for inclusion in a research collection. In particular, Dr. S. K. Donovan (who travelled from Jamaica to attend the meeting) obtained a fine example of the echinoid Hemiaster morrisi (Forbes), and several regular echinoids. After taking lunch, the rising tide forced the party to press eastwards along the shore towards Abbot's Cliff, and mount the concrete platform. Locality 4. The Plenus Marl in East Wear Bay (TR 263384)
Near the easternmost point of access to the concrete platform, at the back of the sea-wall, is an airweathered section of the Plenus Marl which shows clearly the 8 beds described by Jefferies (1963). Dr. Ian Jarvis has been coordinating a detailed geochemical and micropalaeontological study of this interval, and he kindly outlined the results and conclusions for the party. This work shows the coincidence of the Late Cenomanian oceanic anoxic event (OAE), identifiable from an anomalous increase in (j 13C, with stepwise extinction of microplankton and benthonic microfossils (see Jarvis, Carson, Cooper, Hart, Leary, Tocher, Horne & Rosenfeld, 1988). A protruding specimen of the belemnite Actinocamax plenus (Blainville) in Bed 4 of the Plenus Marl was left in place for the benefit of future visitors. Localities 5 & 6. The Chalk Marl of Abbot's Cliff (TR 271385, 279386)
The short walk to Abbot's Cliff along the sea-wall crossed the back wall fault of the Warren landslip, and the party was able to examine the middle part of the Chalk Marl succession, here with the gentle regional dip to the north-east of 1 Unfortunately, all but the base of the Abbot's Cliff section is poorly exposed, and somewhat inaccessible, except on recent, dangerous falls of chalk. The stratigraphical section given herein (Fig. 3) for the middle part of the Chalk Marl was compiled on different occasions over several years, as parts of the cliff became well exposed. As the daylight was beginning to fail, we continued round beneath Abbot's Cliff, crossed the large cliff-fall, and reached a sea-washed cliff in the higher part of the Chalk Marl which has only been exposed for a few years. This section has proved to be of great value in working out the detailed stratigraphy of the Lower-Middle Cenomanian boundary at Folkestone. 0
•
78
A. S. GALE
IFa_I Notes I
I Lithological Markers I
Qrbirbytrllio IIMte1liAOil CCIlIDOr1
•
~
'cast Bed' of price: 1JI of tim sUty mel ]bCachiopods ge,1nitz.ll" Plotytbycis
ElltlllilD, and oraqonitlc CJlIotropocls
2 prcninent limestooes, separated by rly chalk which _thers proud: upper IlJnestone full of ~ and ""'14831001,,=: bose of lower llJoostone piped Into dork marl
below by large 11S'1tIsS'oolde:s, reworked by ~ :
·
dark IMct containing tfWly pyrite nodules
J.chlAmYJl ocle:sleosis, ~ ~ and
~
aaIl1nulllIR
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• •
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~
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long burr""
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bose of bed piped down by ccnspicuous, lIghtcontrasting blt!Sdooldes bed full of dork-<:ootro.tlng
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pyrite noWles
5 dark, recessing macl, fran which water seeps
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Fig. 3. Section in the Chalk Marlon Abbot's Cliff and at Lydden Spout, between Dover and Folkestone (TR 271385-279386). Key as for Fig. 2, with additions. Scale on left of column in metres. Note overlap with slipped foreshore slab depicted in Figs. la & 2.
FIELD MEETING AT FOLKESTONE WARREN
The 6 m of Chalk Marl visible at this locality include two conspicuous beds of limestone, the "prominent limestones" at the top of band 6 of Kennedy (1969, Fig. 2). The lower of these rests on a very dark marl which contains many pyrite nodules; the junction is burrowed by light-contrasting Thalassinoides which are reworked by Chondrites. The fauna of the dark marl is very diverse; it is characterised by the small scallop Chlamys arlesiensis (Woods) and the serpulid Glandifera rustica (J. Sowerby), both restricted to this bed. Oxytoma seminudum Dames is common, and Cunningtoniceras inerme (Pervinquiere) occurs both in this bed and the one below. The underlying 2 m of sediment contain abundant specimens of Orbirhynchia mantelliana, the second horizon of abundance of this species in the succession. Just beneath this is a level penetrated by the deep vertical burrow Bathichnus (Bromley, Schultz & Peake, 1975). These are visible as 0.01 m diameter dark burrows, surrounded by a 0.1 m light halo of oxidised sediment. The higher of the two prominent limestones is full of dark contrasting Thalassinoides, Planolites and Chondrites, which descend from the dark, silty marl above. This highly fossiliferous marl, 1 m in thickness, is the Cast Bed of Price (1877), from which the
79
professional collector John Griffiths obtained numerous pyrite-coated composite moulds of gastropods in the last century. In the lower part of the bed Entolium orbiculare (J. Sowerby) occurs in hundreds, together with the brachiopods Platythyris squamosa (Mantell), Modestella geinitzi (Scholenbach), and Kingena concinna (Owen). The small coral Micrabacia coronula (Goldfuss) is common, as is the serpulid Rotularia. The lowest record of Acanthoceras rhotomagensis (Brongniart) is from this bed. These beds are readily identifiable in slumped masses on the foreshore of East Wear Bay. As it was nearly dark, no time was available to examine the overlying limestone-marl rhythms, which contain the third and highest O. mantelliana horizon (Band 9 of Kennedy 1969), and abundant ammonites of the Turri/ites costatus Subzone fauna. The party commenced the walk back along the sea-wall in the twilight, just as the lights of Folkestone harbour were coming on. During the drive to Folkestone Central station, a vote of thanks to the Director was proposed. He in turn would like to thank David Wray for driving the minibus, Chris Wood for help, and Ian Jarvis for talking about the Plenus Marl.
References BROMLEY, R. G., M.-G. SCHULTZ & N. B. PEAKE. 1975. Paramoudras: giant flints, early burrows, and the early diagenesis of chalks. Kgl. danske Vidensk. Selsk. bioI. Skr., 20(10), 1-31, 5 pis. CARTER, D. J. & M. B. HART. 1977. Aspects of mid-Cretaceous stratigraphical micropalaeontology. Bull. Br. Mus. nat. Hist. (Geol.), 29, 1-135,4 pis. DE RANCE, C. E. 1lW!. On the Albian or Gault of Folkestone. Geol. Mag., 5, 163-71. JARVIS, I., G. A. CARSON, M. E. COOPER, M. B. HART, P. LEARY, B. A. TOCHER, D. J. HORNE & A. ROSENFELD. (1988). Microfossil assemblages and the Cenomanian-Turonian (late Cretaceous) Oceanic Anoxic Event. eret. Res., 9,3-103. GREEN, C. P. 1989. Excursions in the past: a review of the Field Meeting Reports in the first one hundred volumes of the Proceedings. Proc. Geol. Ass., 100, 17-29. HUTCHINSON, J. N., E. N. BROMHEAD & J. F. LUPIN I. 1980. Additional observations on the Folkestone Warren. Q. fl. eng. geol., 13, 1-31. JEFFERIES, R. P. S. 1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the AngloParis Basin. Proc. Geol. Ass., 74, 1-34. KENNEDY, W. J. 1%9. The correlation of the Lower Chalk of south-east England. Proc. Geol. Ass., 80, 459560. - - & GARRISON, R. E. 1975. Morphology and genesis of nodular phosphates in the Cenomanian Glauconitic Marl of south-east England. Lethaia, 8, 339-60. OSMAN, C. W. 1917. The landslips of Folkestone Warren and thickness of the Lower Chalk and Gault near Dover. Proc. Geol. Ass., 28, 59-82. OWEN, H. G. 1971. Middle Albian stratigraphy in the
Anglo-Paris Basin. Bull. Br. Mus. nat. Hist. (Geol.) Suppl., 8, 1-164. - - 1975. The stratigraphy of the Gault and Upper Greensand of the Weald. Proc. Geol. Ass., 86, 475-98. PHILLIPS, W. 1818. A selection of facts from the best authorities arranged so far as to form an outline of the geology of England and Wales. London. - - 1821. Remarks on the chalk cliffs in the neighborhood of Dover, and on the Blue Marie covering the Green Sand, near Folkestone. Appendix: Containing some account of the Chalk Cliffs, etc. on the coast of France opposite to Dover. Trans. geol. Soc. Land., 5, 16-51. PRICE, F. G. H. 1874. On the Gault of Folkestone. Q. Jl. geol. Soc. London., 30, 342-66. - - 1877. On the beds between the Gault and Upper Chalk near Folkestone. Q. fl. geol. Soc. Lond., 33,431-48. ROBINSON, N. D. 1985. Field meeting: the Chalk of the Kent coast, 21st. August 1983. Proc. Geol. Ass., 96, 93-%. - - 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proc. Geol. Ass., 97, 141-70. ROWE, A. W. 1900. The Zones of the White Chalk of the English coast. 1. Kent and Sussex. Proc. Geol. Ass., 16, 289-368. SMART, J. G. 0., G. BISSON & B. C. WORSSAM. 1%6. Geology of the country around Canterbury and Folkestone. Mem. geol. Surv. U.K. 337pp. SPATH, L. F. 1923a. Excursion to Folkestone, with notes on the zones of the Gault. Proc. Geol. Ass., 34, 70-76. - - 1923b. On the ammonite horizons of the Gault and contiguous deposits. Appendix II. Summ. Prog. Geol. Surv. G. B. (for 1922), 139-49.
80
A. S. GALE
1923-1943. A monograph of the ammonoidea of the Gault. Palaeonogr. Soc./Monogr. /, 2 vols., 1-787, pis. 1-72. VOIGT, E. 1968. Uber Hiatus-Konkretionen (dargesteUt an
Beispielen aus dem Lias). Ceol. Rdsch., 58,281-96. WOOD, A. M. M. 1955. Folkestone Warren landslips: investigations 1948-50. Proc. Instn. dv. Engrs., Railway Paper, 56, 410-28.
APPENDIX Occurrence of the ammonite genus Sharpeiceras in tbe Lower Cenomanian Chalk Marl of Folkestone A. S. GALE and S. FRIEDRICH" The recently published Part II of the Palaeontographical Society memoir on 'The Ammonoidea of the Lower Chalk' (Wright & Kennedy, 1987) includes an account of the English material of the Lower Cenomanian acanthoceratid genus Sharpeiceras. Although geographically widespread, occurring throughout Europe, Africa (including Madagascar), the Middle East, southern India, Japan, Texas and Mexico, the genus is usually rare, and many species are known from only a few specimens (Wright & Kennedy 1984, 1987). The English material of Sharpeiceras was placed in three species by Wright and Kennedy: S. schlueteri (Hyatt), S. laticlavium (Sharpe) and S. sp., of uncertain affinities. Accurately horizoned specimens of S. schlueteri are known from the unphosphatised fauna of the Glauconitic Marl (Neostlingoceras cardtanense Subzone) on the Isle of Wight, and at Eastbourne, Sussex. At the former locality the species ranges up into the overlying Mantelliceras saxbii Subzone. The horizon of occurrence of the rare species S. laticlavium was unknown. From the lower part of the Chalk Marl exposed on the foreshore beneath Abbot's Cliff, east of Folkestone, we have collected 15 specimens of Sharpeiceras, in situ. These fall into both S. schlueteri (5 specimens) and S. laticlavium (6), in addition to a number of specifically indeterminate fragments (4). This material is associated with a diverse M. mantelli Zone fauna, which belongs to the upper part of the Neostlingoceras cardtanense Subzone. The section at this locality has not been previously described, and is now partly covered by a major cliff-fall (January, 1988). We therefore present a brief account of the stratigraphy and fauna of the locality. A total of 4 m of the Chalk Marl is intermittently exposed on the shore platform at low tide, at the foot of the easternmost 180 m of sea defences on the west of Abbot's Cliff (TR 269384; locality 7 of Fig. la). The succession is shown in Fig. 4. A low «1 m) cliff is present at the base of the sea wall, from which a shore platform extended seawards for 30-30 m. The 9 nodular, spongiferous limestones form low benches on the shore, which run out to the sea to the east as the dip brings in higher beds. To the west the section is abruptly terminated by the back wall fault of the Warren; to the east, the Chalk Marl disappears beneath rock falls and shingle. At the top of the sea wall, the middle part of the Chalk Marl is exposed. The horizon identified as M5 on Figs 2 & 3 (top of band 2 of Kennedy, 1969) is clearly visible over a distance of 50 m, and is measured here as 8 m vertically above the most prominent limestone (M3) on the shore • 17 Mallon House, Carr Street, London E14 7SE.
below. This information, and comparison with the slipped foreshore section shown in Fig. 2, places the Chalk Marl depicted in Fig. 4 at the base of that unit, immediately overlying the Glauconitic Marl. The section was noted by Kennedy (1969), who assigned it to the top of his band 2 and the base of band 3, and suggested that it fell within the Mantelliceras saxbii Subzone. Furthermore, he identified this as the level which yielded the material of the large oyster Rastellum carinatum (d'Orbigny) present in old collections from Folkestone, and redescribed by Carter (1968). The lower beds (based to limestone 7) seen at this locality correspond to Price's (1877) Bed II of the Chalk Marl, which he recorded as 10 feet (3 m) thick. The list of fossils which Price gives from this unit (1877, p. 435) includes 'Ostrea frons' and '0. frons, vaT. carinata' (=Rastellum carinatum), and 'Terebratula biplicata', which probably refers to Tropeothyris carteri (Davidson). Both species are restricted to this unit. The section seen in December, 1987 is shown in Fig. 4. At the base is 1 m of mid to dark grey marl which contains several burrowed omission surfaces, and scattered limestone nodules. The lowest level seen is weakly glauconitic, and cannot fall far above the top limestone of the Glauconitic Marl (compare Fig. 2). Above, are 5 thin, nodular limestones, full of sponges, separated by dark grey marls. The top of each limestone is piped with marl infilling Thalassinoides and (1) Planolites. The marls are silty, and strongly bioturbated, and contain occasional brown phosphate pebbles, and abundant comminuted inoceramid debris, especially above the third limestone from the base. Concave-up scour structures are present in the marls. Above a thick (0.4 m) conspicuously dark marl, are two nodular limestones, of which the upper (M3) is thicker and more laterally continuous. These two beds contain abundant ammonites. The upper limestone is surmounted by an omission surface burrowed by large idiomorphic Thalassinoides. The dark, very silty marl above contains light chalk intraclasts. The highest 1.5 m of the section includes two impersistent nodular limestones, in a sequence of marls containing burrowed omission surfaces. The calcitic macrofauna of the lowest 7 limestones and intervening marls is extremely distinctive, and includes species restricted to this level. The large terebratulid Tropeothyris carteri, generally considered to be a rare species, is frequent (22 specimens collected). Other brachiopods present are the small rhynchonellids Monticlarella rectifrons (Pictet & Campiche) and Crasirhynchia grasiana (d'Orbigny). Rastellum carinaturn is common, most notably in the dark marl beneath the level of ammonite abundance. The fauna of echinoids is diverse, including Holaster sp., Hemiaster morrisi, Epiaster sp.,
81
FIELD MEETING AT FOLKESTONE WARREN
.
+J
4
Q)
'0
c:
.,-i
.
0.. fI)
... ...
50
... ...
...... ·... ·.. ·... ...
o~~~~ Fig. 4. Section in the basal Chalk Marlon the foreshore at the western end of Abbot's Cliff, east of Folkestone (TR 269384), and selected macrofossil occurrences: each specimen is indicated by a dot. Key as for Fig. 2. Scale on left of column in metres. Position of section indicated in Fig. 2.
82
A. S. GALE
Tetragramma variolare (Brongniart) and Stereocidaris sp. Large specimens of Inoceramus crippsi Mantell are abundant. The nautiloid Cymtoceras is occasionally found. Ammonites are preserved as composite moulds coated with dark grey pyrite. In the lowest five limestones, ammonites are frequent, but never abundant. Schloenbachia varians is common, Mantelliceras occasional, and Hypoturrilites rare in these beds. Sharpeiceras is present in limestones I, 3, and 5, and in the marl overlying 3. Most of the specimens of Sharpeiceras are large (last septum at diameters of 200-260 mm, total diameters probably 350400 mm). Specimens of this genus in old collections from Folkestone (e.g. BMNH C3677) almost certainly originate from limestones 3 and 5. Limestones 6, 7 and the intervening marl have yielded several hundred ammonites, belonging mostly to the genera Schloenbachia, Mantelliceras, and Hypoturrilites. Mariella and Hyphoplites occur more rarely. The Mantelliceras belong dominantly to the infiated species M. mantelli (J. Sowerby) and M. cantianum Spath. This level is undoubtedly the main source of well preserved Hypoturrilites and Mantelliceras in 19th century collections from Folkestone. Only the top M. dixoni horizon (M6) contains comparably well preserved specimens, but these are characteristically M. dixoni and Schloenbachia sp. The highest beds are relatively poorly fossiliferous, and we have collected only Mariella sp., Schloenbachia sp., oysters, and I. crippsi. On the basis of recent collecting, the limits of ammonite Zones and Subzones in the Lower Cenomanian of
Folkestone can be revised. The scheme follows Wright, Kennedy & Hancock (1984). The earliest Cenomanian N. carcitanense Subzone in southern England appears to contain two successive ammonite assemblages, the lower characterised by the subzonal fossil and diverse heteromorphs, including Idiohamites, Anisoceras and Sciponoceras. This is best exemplified by the phosphatised N. carcitanense fauna of the Glauconitic Marlon the Isle of Wight. In the higher assemblage, N. carcitanense and Idiohamites sp. are rare or absent, but Sharpeiceras occurs, albeit rarely, in association with a Mantelliceras fauna dominated by inflated forms. The higher assemblage is found in unphosphatised preservation in the Glauconitic Marl of the Isle of Wight (e.g. Compton Bay) and Eastbourne, and in limestones 1-7 (of Fig. 4) at Folkestone. The top bed of the Glauconitic Marl at Folkestone (M2 on Fig. 2) has yielded a single Neostlingoceras sp, and the Glauconitic Marl thus probably belongs entirely to the lower N. carcitanense assemblage at this locality. At Petit Blanc Nez, in the Boulonnais, 40 km away, N. carcitanense is present in phosphatised perservation in the thin lateral equivalent of the Glauconitic Marl (Amedro, 1986). The Mantelliceras saxbii Subzone fauna, characterised by compressed species of Mantelliceras, is poorly represented at Folkestone. Pebble fossils in the dark, silty bed M4 (Fig. 2) include poorly preserved M. cf. saxbii. M5 yields an ammonite fauna characteristic of the M. dixoni Zone, which ranges up to bed M6. We would like to thank J. M. Hancock for commenting on the manuscript.
Additional References AMEDRO, F. 1986. Biostratigraphie des craies cenomaniennes du Boulonnais par les ammonites. Ann. Soc. geol. Nord, 105, 159-67. CARTER, R. M. 1968. Functional studies on the Cretaceous oyster Arctostrea. Palaeontology, 11, 458-85. KENNEDY, W. J. & C. W. WRIGHT. 1984. The Ammonoidea of the Lower Chalk. Part I. Palaeontogr.
Soc. (Monogr.), 1-126, pis. 1-40. - - & - - 1987. The Ammonoidea of the Lower Chalk.
Part II. Palaeontogr. Soc. (Monogr.), 127-218, pis. 41-55. - - , - - & J. M. HANCOCK. 1984. The Ammonoidea of
the Lower Chalk. Part I. Introduction. Palaeontogr. Soc. (Monogr.), pp. 1-:)7.