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Lower Chalk
Middle Chalk
Upper Chalk
Total Thickness of Chalk
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78 m
72 m
106 m
256 m
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Lower Chalk Formation
The Lower Chalk between Folkestone
and Dover is 78 m thick (Jenkyns et al, 1994, Fig. l3a). The classification
introduced by the Geological Survey at the turn of the century
(Jukes-Browne and Hill, 1903) and still in use today divides the
Lower Chalk into a basal Glauconitic Marl, succeeded upwards by
the Chalk Marl, Grey Chalk and Plenus Marls. At Dover, Jukes-Browne
and Hill (1903) recognised two additional units within the broadly
conceived Grey Chalk and below the Plenus Marls: their Bed 7 [usually
referred to as Jukes-Browne Bed 71 and Bed 8 or the 'White Bed',
in ascending order. Kennedy (1969, Fig. 2) published the first
detailed measured sections of the Lower Chalk exposed in the cliffs
between Folkestone and Dover and divided the succession beneath
the terminal Plenus Marls into 14 numbered beds. Subsequently,
better exposures than were available to Kennedy, particularly
on a rotated block on the foreshore have allowed detailed measurements
of the succession for the first time and have corrected previous
misconceptions on the part of all observers regarding the stratigraphy
and thickness of the lower part of the Chalk Marl (see Gale, 1989,
1990; Jenkyns et al, 1994).
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Glauconitic Marl
The base of the Lower
Chalk is marked by the Glauconitic Marl, a unit up to 7 m or more
in thickness, comprising dark grey marl with abundant glauconite
grains and sporadic phosphatic clasts. The Glauconitic Marl rests
non-sequentially and with erosive contact on the Upper Gault,
typically on Bed XIII, but locally (e.g. Channel Tunnel Site Investigation,
Craelius No. 2 Borehole) on Bed XI or even, in offshore boreholes
(e.g. P000), on the undated Zone 6a (Hart, 1993, Fig. 2). The
sediment of the Glauconitic Marl is piped down into the underlying
Gault in a Thalassinoides burrow system. The unit is intensely
bioturbated: the most conspicuous ichnofossils, generally but
probably incorrectly referred to Spongeliomorpha, are cylindrical
burrows with a central core of marl without conspicuous glauconite.
The Glauconitic Marl exhibits poorly developed rhythmicity towards
the top and is locally overlain by a thin, weakly glauconitic
limestone (Marker horizon M2 of Gale, 1989), which has yielded
the zonal index of the Neostlingoceras carcitanense Subzone.
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Chalk Marl
Notwithstanding the
recommendation of Jukes-Browne and Hill (1903), most workers on
the Folkestone Lower Chalk succession (including TML) have recognised
a broad and ill-defined subdivision of the interval from the Glauconitic
Marl to the Plenus Marls into the Chalk Marl and Grey Chalk. The
Chalk Marl is characterised by relatively dark, rhythmically bedded
sediments and an overall CaCO3 content below 75%. Each rhythm
or couplet typically comprises a basal dark bioclastic marl resting
on the burrowed surface of the underlying couplet, above which
there is an upward gradation with decreasing clay and increasing
carbonate content to a pale cemented, spongiferous limestone (Destombes
and Shephard-Thorn, 1971, Fig. 2). In the lowest and highest beds
of the Chalk Marl (in the traditional sense, not that of TML),
the sedimentary rhythmicity is very conspicuous, but in the middle
part of the succession the rhythmicity is rather indistinct due
to a higher overall clay content. Near the top of the Chalk Marl
as traditionally understood, a closely spaced pair of conspicuous
massive, prominent-weathering limestones provides an important
marker horizon in the cliffs between Folkestone and Dover. The
lower of these limestones overlies a conspicuous dark bed characterised
by a 'pulse fauna' including Oxytoma seminudum and 'Chlamys' arlesiensis
(Paul et al, 1994). The higher limestone (the so-called Tenuis
Limestone from the occurrence of Inoceramus tenuis) underlies
the famous 'Cast Bed' of 19th Century fossil collectors, which
was so named from the relative abundance of composite moulds of
originally aragonite-shelled molluscs, notably gastropods. The
Cast Bed yields very rare examples of the belemnite Actinocamax
primus and abundant Entolium.
The Cast Bed is followed
by several metres of conspicuously rhythmic marl-limestone alternations
characterised by Orbirhynchia mantelliana and constituting the
Orbirhynchia mantelliana Band as originally described by Kennedy
(1969). However, it is now known that this is the topmost of three
such Orbirhynchia bands developed in southern England. The (third)
Orbirhynchia mantelliana Band terminates in a limestone with a
flood abundance of Sciponoceras baculoide, above which there is
a sudden shift in the ratio of planktonic to benthonic foraminifera,
with an increase of the former over the latter. This shift is
termed the PB break and also, because it is coincident with evidence
of sedimentary discordance in mid-Channel boreholes (Carter and
Destombes, 1972; Hart, 1993; Amédro, 1994), the mid-Cenomanian
non-sequence (Carter and Hart, 1977 and references therein). Six
further marl-limestone couplets are found above the PB break,
which thus falls within, but not at the top of the Chalk Marl.
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Grey Chalk
In contrast to the
Chalk Marl, the Grey Chalk is characterised by paler coloured,
less distinctly rhythmic sediments with an overall CaCO3 content
exceeding 75%. A typical couplet begins with a thin flaser marl
and terminates in a marly chalk rather than a hard, spongiferous
limestone (Destombes and Shephard-Thorn, 1971, Fig. 2.). As in
the case of the Chalk Marl, the rhythmicity is inferred to reflect
orbital control of productivity. The Grey Chalk is generally much
less fossiliferous, although the terebratulid brachiopod Concinnithyris
subundata is relatively common near the base and 'Inoceramus atlanticus
characterises the highest beds.
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Jukes-Browne Bed
7
The Grey Chalk is followed
by a group of beds of relatively coarse bioclastic, extensively
bioturbated chalks, some 2 m thick, containing large specimens
of the zonal index ammonite Acanthoceras jukesbrownei, as well
as calcarenite-filled structures (the laminated structures of
earlier literature), which tend to stand proud on weathered surfaces,
and give a distinctive appearance to the bed.
Although these have
been interpreted as scours, current opinion increasingly favours
the idea that these structures represent truncated burrow-fills.
This bed is traditionally known as Jukes-Browne Bed 7, although
Robinson (1986) formally designated it as the Hay Cliff Member.
The more many basal part is characterised by a concentration of
small oysters (Pycnodonte sp.), which is also found in correlative
developments elsewhere.
White Bed (Capel-le-Ferne
Member of Robinson, 1986)
The White Bed is a
unit restricted to the North Downs and the northeast margin of
the Paris Basin (Mortimore et al, 1989) comprising extremely soft,
poorly fossiliferous, homogeneous white chalk, with regular transverse
and vertical joints. The soft nature of this unit is emphasised
by the quarry workers' name 'soapstone'. Few fossils are found
apart from sporadic concentrations of the exogyrine oyster Amphidonte
sp. and sparse localised occurrences of Inoceramus ictus.
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Plenus Marls
The Plenus Mans comprise
a thin, clearly defined unit of alternating, relatively fossiliferous,
slightly green-coloured marls and marly limestones, which has
commonly been given member or even formation status. The base
of the Plenus Marls is a major erosion surface and sequence boundary,
with Plenus Marls sediments piped down in burrows for up to 0.5
m into the underlying chalk of the White Bed. Based on the work
of Jeffries (1963), the succession is divided into eight beds,
which have been inferred to represent, together with the basal
limestone of the overlying succession, five marl-limestone precession
couplets (Gale, 1990; Lamolda et al, 1994). The Plenus Marls takes
its name (and the earlier name of Belemnite Marls) from the common
occurrence of the belemnite Actinocamax plenus in the higher part
of the succession. The Plenus Marls mark the base of a major complex
positive 13C excursion, which extends into the basal part of the
overlying beds (Gale et al, 1993, Fig. 2) and is commonly referred
to as the Oceanic Anoxic Event II. This excursion is accompanied
by a significant stepwise extinction of the greater part of the
microfauna and microflora, which has been generally, but not universally,
attributed to increasing anoxic conditions followed by a gradual
recovery as oxic conditions became reestablished. These faunal
and geochemical changes have been the subject of intense multidisciplinary
investigation in the relatively thin Plenus Marls successions
at and near Dover (Jarvis et al, 1988; Leary et al, 1989; Jeans
et al, 1991; Lamolda et al, 1994).
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Middle Chalk Formation
Holywell Chalk Member
Compared with the Sussex
succession, the basal part of the Middle Chalk (uppermost Cenomanian
and Lower Turning) in Kent is greatly condensed, forming the so-called
Melbourn Rock Beds (Robinson, 1986), the Grit Bed of earlier literature.
This part of the succession constitutes the Holywell Chalk of
the standard lithostratigraphical classification used by BGS (Bristow
et al, in prep.). The topmost Cenomanian (terminal Metoicoceras
geslinianum Zone and Neocardioceras juddii Zone), including the
two lower pairs of Meads Marls of the Eastbourne succession (Mortimore,
1986; Pomerol and Mortimore, 1993; Gale et al, 1993), is hence
represented by about 1 m of intensely hard limestones with Sciponoceras,
the marls themselves being recognisable only as thin marl wisps.
The base of the Turonian Stage is taken at the base of the succeeding
less indurated chalks by extrapolation from the Sussex succession
(Gale et al, 1993; Pomerol and Mortimore, 1993). The greater part
of the Holywell Chalk is characterised by shell-detrital (predominantly
fragmented and comminuted Mytiloides spp.) and intraclastic chalks.
Near the base, a bed of calcarenite largely composed of microcrinoid
debris (Roveacrinus) yields Fagesia catinus. Higher in the succession,
the content of shell detritus reaches a maximum (including a bed
with serpulid-encrusted Mytiloides, the so-called Filograna avita
bed, which can be traced throughout the Anglo-Paris Basin), above
which there is a rhythmic alternation of shell-detrital chalks
and non-shelly chalks. By extrapolation from N. American successions,
the base of the Middle Turonian can be inferred to lie in the
higher part of this rhythmic alternation, above an horizon which
has yielded the highest Mammites nodosoides, together with Morrowites
wingi and Metasigaloceras rusticum. However, the first unequivocal
records of Middle Tunonian ammonites (Collignoniceras woollgari)
are from the basal part of the overlying New Pit Chalk in Sussex,
i.e. above the upper limit of the shell-detrital chalks of the
Holywell Chalk.
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New Pit Chalk Member
The Holywell Chalk
is succeeded abruptly by relatively poorly fossiliferous, smooth,
inconspicuously rhythmically bedded white chalks without shell-detritus.
The unit is flintless in east Kent, except at the top, where small
flints are locally found within the Glynde Marls sequence. However,
to the west, in the vicinity of the Medway, small inconspicuous
flints (the Glyndebourne Flints of Sussex) are again developed
at the base of the member. Three conspicuous clay-rich marl seams,
several centimetres thick (the Round Down Marl, New Pit Marl 1
and New Pit Marl 2, in ascending order) are a feature of the New
Pit Chalk in the cliff path sections west of Dover. Close to the
top of the member, a closely spaced and laterally variable group
of up to 6 marl seams (the Glynde Marls) provides another useful
marker; Robinson (1986) introduced the name Maxton Marls for this
group to emphasise the difficulty of achieving exact correlations
between the individual marl seams in the North Downs succession
at this level with those comprising the Glynde Marls of the South
Downs.
Upper Chalk Formation
The base of the Upper
Chalk is now taken by BGS at the (revised) base of the Lewes Chalk,
an horizon which is marked by the onset up-section of nodular
chalk, a short distance above the highest of the Glynde/Maxton
Marls and at approximately the level of the first flint in the
succession (Lydden Spout Flint). It must be emphasised that the
base of the Upper Chalk as currently mapped by BGS (Bristow et
al, in prep.) is significantly lower than the traditional lower
limit at the base of the so-called Basal Complex.
As with the underlying
New Pit Chalk, there are several conspicuous marl seams, ranging
from less than one, to several centimetres in thickness. All these
marl seams produce distinctive spikes in downhole geophysical
logs of wells and boreholes (e.g. Mortimore, 1986; Mortimore and
Pomerol, 1987) and can thus be more or less readily correlated
in the Chalk subcrop. In addition, Wray and Gale (1993) have demonstrated
that each marl seam has its own characteristic trace element composition
which provides a more or less unique geochemical 'fingerprint'.
Rare earth element analysis of the marls by Wray (in prep.) has
demonstrated that the New Pit Marls, the higher Glynde Marls,
Southerham Marl 2 and Bridgewick Marl 2 have the signature characteristic
of a detrital marl, whereas Glynde 1, Southerham 1, Caburn and
Bridgewick 1 are distinguished by a significant negative europium
anomaly and are thus probably of vulcanogenic origin and comparable
to the approximately contemporaneous tuffs in the German Chalk
(Wray, 1995).
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Part 2. In the Field
The Gault Clay is visible
at Copt Point above the Folkestone Beds. The Gault is often slipped
and is currently exhibiting numerous mudslides due to the heavy
winter rainfall. The basal bed of the Cenomanian is currently
freshly exposed directly on the seaward side of the coastguard
lookout at the edge of the pitch and putt course, this being the
result of recent landslip movement.
The view from the cliff
top car park on the Dover side of Copt Point, Folkestone. Showing
the landslipped undercliff (Folkestone Warren), the sea defenses
and toeweights to the landslips, the 'White Cliffs' between Folkestone
and Dover, and Samphire Hoe/Dover Harbour (far right). Horses
Head and Abbot's Cliff (far right cliff) are visible.
The Warren is accessible
either by walking along the seafront (the favoured route, tide
permitting) or down the path adjacent to the cliff top cafe at
Capel le Ferne. This latter path is steep and when recently visited
showed signs of failure at the top, making it important to take
care in the descent. The path leads to a railway bridge from where
the cliffs can be observed.
The Horses Head tilted
block of Holywell Nodular Chalk. Looking towards Folkestone. Further
to the east, at the eastern end of the Groynes, the wavecut platform
when exposed yields sections ranging in age from the topmost gault
and Glauconitic Marl and basal West Melbury marly Chalk. This
section proved important in the interpretation of the Channel
Tunnel tunnelling horizon. Here the work of Gale (1989) allowed
a correlation across the channel to be established.
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The cliffs above Samphire
Hoe, the area of reclaimed land created with the spoil from the
UK side of the Channel Tunnel. The Folkestone to Dover railway
runs along the base of the cliffs. Flinty Chalk is present at
the cliff top. Here, although the Aker's Steps are essentially
inaccessible now, but the Plenus Marls and the Holywell nodular
Chalk are clearly visible in the cliff section. As it is now possible
to drive down to this location it is a good starting point for
the Chalk of the Folkestone to Dover section.
The cliffs above Samphire
Hoe, the area of reclaimed land created with the spoil from the
UK side of the Channel Tunnel, looking towards Dover showing the
access adit form the cliff top for the Channel Tunnel works and
Akers Steps directly to the right and above. The buildings are
part of the permanent facilities for the Channel Tunnel.
The view of the White
Cliffs from the eastern end of Dover seafront. The nodular chalk
of the Lewes Chalk is clearly visible. It is this section eastwards
to St. Margaret's Bay that is often considered to be the classic
white chalk section.
The view of the White
Cliffs from the eastern end of Dover seafront. The western end
of this section of cliffline is below Dover Castle.
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Fig 2. Section showing
the apparent dip of the Chalk in the White Cliffs between Dover
Castle and St. Margaret's Bay.
Recent minor rockfall
in the White cliffs at the eastern end of St. Margaret's Bay.
At least 6 rockfalls, some major (eg. below the lighthouses),
occurred during the winter of 2000/01 in the cliffs on either
side of St. Margaret's Bay.
The Chalk section viewed
from the car park in St. Margaret's Bay.
References:
Engineering Geology
of the Channel Tunnel (Harris et. al.)
The Chalk of Sussex
and Kent (Mortimore, 1997; G.A. Guide No. 57))
