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C. BAETEMAN
W. DE GANS
~
II/I(
T~-
BELGIAN GEOLOGICAL
GEOLOGICAL
SURVEY
SURVEY
OF THE NETHERLANDS
EARTH TECHNOLOGY
INSTITUTE
THE
Cecile
WESTERN
COASTAL
OF
BELGIUM
Baeteman
General
Introduction
The lowlying
Holocene.
,coastal plain was formed and shaped
However,
the present
day surface
very last step of its development
events
that built
to be acquired
Lowland
are
PLAIN
is the record
the
of the
and evolution. The sequence
the plain, are found in the subsurface
of
and are
by means of boreholes.
excursions
therefore
very
single
borehole
complex
mosaic.
the coastal
during
seldom
often
shows
unappreciated
the record
That mosaic,
evolution
can offer
splendid
outcrops
or misjudged.
of just one single
bearing
the sequence
and
Indeed
spot
in a
of events
of the, area, is to be unravelled
a
and
by means
of establishing the geometry of the various facies in the
depositional body of the coastal plain, so that every single
borehole
approach
is put into a larger context. Such a 3 -dimensional
forms a firm basis for the interpretation of the
individual core which then can be rate at its true value. The
3-dimensional approach also yield the framework in which all
relevant factors and processes of coastal development can be
integrated.
The western
southward
(fig.l)
extension
This
Embayment,
little
crossed
landward
although
relevance
excursion
parts
part of the coastal
by
extension
stops.
the
Moreover
sole
used
the present-day
to it, as will
plain
is characterized
river
to be
position
be discussed
the river
in. the
called
the
by a
plain
IJzer
of the river
has
at the different
is canalized
over the major
of its course as the entire 'plain has been reclaimed
since
about the 11th century AD, resulting in a polder with a
completely
The
controlled
elevation
drainage.
of the plain
is ranging
between
+2m
to +5m.
..'
Ordnance
Dutch NAP
i
1
I
I~
datum
(T.A.W.)
refers
(Normaal Amsterdams
to LLWS what
explains
peil) is 2.33m higher.
that
the
- .'.~.~! f?":
I
2
ij
j
i
I
t
f
I
I
.
,..4
N
",.~.
,,' ';
'':1
!',
!
{"
c;.
..
?
,
,""
'.
0.
':\,
North
Sea
''''''\
Fig. 1 Belgian
Coastal
t
i
I
i
!
i-'
!j
Plain
i
01\
..
?
c:>
.
10 km
0
Setting
Geological
The western
part of the coastal
plain
differs
significantly
from the eastern part and this in several aspects.
The main difference,
sequence
of the
subsoil.
Quaternary
deposits
Ieper
consisting
Clay
between
sequence
lies
top of
stiff
upon
compact
it, an erosional
a significant
relief
valley
outside
the plain in that landward
characterized
in the central
(fig. 2) with
part
part, the Eocene
shows
a more
gentle
shape is not pronounced
and
regular
any longer.
slope.
area. The
area.
deep
Just
deposits
are
direction
and
a level of +30m. In the very west of the plain,
surface
the
ranges
a rather
of the landward
by a very steep slope in the eastern
Eocene
clay,
surface,
+lm in the south and -30m in the very seaward
incised
valley
unconformably
of a ca 100m thick
(Yc). The
exhibits
reach
Holocene
deposits
The
top
is the well developed
in the western part which is to be explained by the
topography
Eocene
however,
the Eocene
Seawards,
the
II
I
I
[
I
i
3
C\J
C7'I
.
Ii
I
!
i
i
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j
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j
j
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I
.....
0
~
.c.
c..
ns
Len
0
c..
0
.....
en
4)
.-0
~Q.
0
c:
0
z
~
1
~
-+-
'"
I'd
C
0
...J
4
deposits
Pleistocene
The
clay is
Eocene
maximum thickness
covered
by pleistocene
:.deposi ts
a
v;ith
of 12m.
Marine
Pleistocene
deposits
were found
restricted
in the
seaward
area
(fig.3).
They consist
mainly of a shellcrag
with a
thickness
up to nearly
10m, occurring
between
-lSm to -30m.
However,
slightly
more landward,
less
energetic
deposits
are
found
(between
belong
to a later phase of t~e Eemian
-11m to -16m) such as fine sand and clay with
occasionally" peat, reflecting a coastal
plain/tidal flat
environment.
According
to a pollenanalysis,
these deposi ts
The typical
the
facies of the shellcrag
boreholes
of
the
Seaward
borehole Leeuwenhof (L,
In a limited
deposits
were
sediments.
reworked
to the
N-S
area,
found
Mos~
well
(fig.4),
of the
the
tidal
flat
in
facies
weichselian
fine
grained
in the channellag,
marine
Crag"
developed
mainiy
time,
of Cerastoderma
"Izenberge
Area;
will be demonstrated
in
fig.9).
Holsteinian
presence
(E6).
deposits,
edule
shells,
(Herzeele
fluvial
channel
they
characteri zed
most
Formation,
probably
Middle
fill
contain
by
the
belonging
Pleistocene,
Somrne et al.,1978).
In the very southern part of
Holsteinian peatlayer was encountered
south
peat
of
the
is also
1962)
river
IJzer
occurring
overlying
been
found
a sequence
starting
covered by floodplain
deposits
covered by finally weichselian
From the available
influenced
but
the
by a marine
data
are still
palaeogeographical
1978).
data,
reconstruction.
That
interglacial
of Lo
(Vanhoorne,
with
coastal
and underlying
coastal
fluvial
it is evident
scarce
fe\-Jlocalities
(at -8.70m)
sediments,
transgresssion
too
(fig.S), a
(at -3m to -4m) in a core at
deposits
+lm).
plain
in only
in the neighbourhood
and has recently
Woumen,
(Baeteman,
the
sediments
to be
(until
that the plain was
in the Middle
conclusive
I ,I
,
Pleistocene,
about
any
I~
rt
i
1
---~
---
----
~'_'-_'''_O
,,
6
.
c-
-".WO
-....---..--.......-..
-"-"-",-,,,,,,'''6
--.-------....
.1
...
N
G
A
~
Nor th Sea
,
\\
,
\
,
i,
V\
I
I,
.on
..,
I
I
.. ,,
:I
~
\
,
,
I
('
0
0
0
L
Ilu"
0---1
I
~""',
Occurrence
of
Pleistocene
Occurrence
Marine
Pleistocene
I:::.:::':~~ sheller a 9
1:'::':.1
fi g. 3
tldaillal
of
Deposits
(clay/sand/peat)
fig.4
Fluvial
Deposits
6
In the very western part of the plain, the Pleistocene
deposits' consist
of a thin cover
Eocene
On the other
deposits.
very well developed
(ca O.Sm)
hand,
of mainly
weichselian
:' I
reworked
Coversands
are
if
in the east.
The topography of the Pleistocene subsoil
rather
,
good similarity
to that of the Eocene.
(fig.6) shows a
It is dominated
by
I
I
a deep narrow depression going far south where it is bifurcating
into small valleys outside the coastal pla~n. The present-day
location of the river IJzer is south and east of this
depression.
t
Ii
N
I
A
c::J:I
North
.
t
Sea
I
Ii
I
\
\
\
\
\
!
\
~.
;:
-
':
I
,
-n
"""\
~
::I
0
(0
I
I
I
I
\
'
I
II
0
~
Fig.5
2km
Occurrence of UiddJe Pleistocene
Uarine
Deposits
I
I
~.
!\
!
.
T.
7
N
A
NORTH
SEA
,
.
\
\
11
;
::I
""
~
~\
\)
~
~.
.
\
\
Fi g. 6
Depth of lhe Pleistocene Subsoil
\
0
"
2km
,II
8
Stratigraphy
of
the
.
Holocene
coastal
I
i
deposits
The Holocene coastal'deposits are mainly formed under tidal
and semi-terrestrial
conditions. The
Holocene
infill
is:'also
known as Flandrian deposits or as being accumulated by the
Flandrian Transgression. The deposits reach their greatest
thickness of ca 30m in the seaward region and wegde out toward
the
pleistocene
These
hinterland.
unconsolidated
coastal
deposits
are characterized
by
lateral zonation. In the seaward region, only marine and
brackish
some
clastic
places.
sediments
In the
are present
central
part
o~erlyi~g
a basal peat in
of the plain,
the
deposits
consist in general of an alternation of brackish-marine
sediments
and
peatlayers.
Toward
the
Pleistocene
hinterland,
the
deposits are formed by only a basal peat overlain by a cover of
clastic brackish-marine sediments, while at -:he border, of the
outcropping pleistocene area, the cover of brackish-marine
sediments
SeauJard
forms
the
entire
Transi
region
Holocene
tion
t
1
,
,
Ii
t
r
i
i
f
i
r
I
~
ii
It
t
!
sequence.
landu~rd region
zor.e
Clas::.ic ::~lcx
..
,.:".::..:"
.', ,,-;:'';'"
." ",'
:.:,','
.",..;.:.:/.:.;.,;::::"'''~
",#.,'.".,'.,.,'.,",
..
'.,".,,'
.
I'
;;..:;;..;;..t7:~.~:.;::t;~~:::;;:{~(~t~~£~'~~~i~:i:~;}t/;0~:~~'i~:I:~i~t~di;?::'?::::;:~~""
[J
KO : ~hKO : u~per clzstic seQuer.:e
A:
qhA:
s~litting up se~uence
KU :,QhKU : l~er
[]:
;:le:sto::ene
se(ji::\er.ts
K:
Cl,:st~:
~;ed l~e~
+5
~
IIIll
PEat
~h~: clastic sequance
08 : ~"':;=
: Q:~ar.ic basal
sE'quence
clastic sequerce
Fig.
7
Lithological
Classification
iI.
f
..........
9
Such
plains
lateral
of the southern
lithological
vertical
North
succession
and
Baeteman,
1981,
Strijdonck,
and
peat
and
is
Sea,
of
coastal
et
deposits
the
coastal
based
to the geological
Streif,1978,
& Van
of the Holocene
(transgressions?)
history,
the
clastic
Baeteman
subdivision
deposits
of a
on
of
al., 1977,
Baeteman,1987,
The traditional
definitely
for
interfingering
(Barckhausen
Dunkerque
typical
led to the development
lateral
Mostaert,1985,
1989).
Calais
belongs
which
classification
sediments
into
zonation,
indeed
at least
for this
part of the excursion.
The lithological classification consists of complexes
i
i
i
sequences
I
!
deposits
i!
Ii
J
I
j
!
I
I
belong
to the clastic
viz. the clastic
represented
the
I
(fig.7). In the seaward and very landward
as organic
plain,
grouped
The
by
into
organic
as
clastic
the
basal
possibly
basal
labelled
characterized
are
sequence,
complex
bearing
underlain
sequence.
sequence
the
sequence,
by the basal peat,
zone,
and
interfingering
one
In the central
transition
sediments
regions,
and
the
part
of
deposits,
intercalated
peat layers,
complex.
on the
one hand
and
the
splitting
up sequence, pearing one or more peatlayers, on the other hand
offer the possibility for the development of a geochronology,
link~ng the lithological classification. 14-C dates from the
peatlayers
.
given
are
-
in the description
Central
Area.
lithological classification also forms the basis for
The
relevant
of main
mapping
profile
units.
types;
the Holocene
The complexes
X-type
for the interfingering
where
of the
complex
:are represented
for the clastic
and z-type
using the main profile
complex,
for the peat
sequence mainly consists
types, a general
by means
Y-type
complex,
of peat.
profile
type map can
I
!
I
i.
be established
j
Holocene
i
(fig.
peat)
(fig.8)
geological
8) the
setting.
occurrence
subsoil
On
a very
the
the landward
are uncomplete
is beyond handauger
reach.
clear
general
of the organic
was only indicated'in
(X2) as many boreholes
i
..........
revealing
overview
profile
basal
regions
sequence
of the
type
map
(basal
for the X-type
as soon as the Pleistocene
10
N
A
North
I
f
f
1
i
Sea
Ii
1
I
i
!
,
Fig.
8
General Profile Type Map
D
0
~
X 1 typ e
X2 typ e
Y typ e
~!:::~: Z typ e
border
of the
0
coastal
plain
2km
11
During the excursion several boreholes of 3 selected areas
labelled as seaward, central and landward area (fig.9) will be
demonstrated.
The particular
facies of the Holocene" and
Pleistocene sediments will be discussed, as well as the coastal
evolution, which is the result of the geological mapping of the
plain based on sedimentological investigation, however, without
palynological nor micropalaeontological analyses (except for
diatomanalysis
for
few
selected
boreholes)
N
"1\
-"\
I»
:)
0
~
t</~ ~
dunes
1. seaward
2. central
area
area
3. landward
area
0 excursi°':1
stop
.
location
of demonstrated
boreholes
0
Fig.
I
.1
9
2km
T
i
12
I
;.
SEAWARD AREA
point:
Excursion
The
seaward
coastal
area
dune barrier,
sediments
Wulpen
(fig.10)
is characterized
(fig.l1, profile
are occurring
the profile
located landward from the broad
,
types
in restricted
by a dominance
#1,2,&
areas.
3). Peat intercalations
It should
type map is only representative
as handaugering
was hampered
In the clastic
be mentioned
different
that
until a depth of -2m
by the water-saturated
sequence,
of clastic
sand.
facies can be recognized.
cross-section parallel to the coast with boreholes
A SW-NE
until the Eocene deposits, reveals different events. and a
different
evolution
Boreholes
Fig.10
NORTH
in
the
inf ill
of
the
plain
M.V. and Groenendijk
will be demonstrated.
LOCATION
IN THE OOSTDUINKERKE
OF BOREHOLES
(fig .12) .
A~EA
SEA
907
906 0
9080
O::JSTDUINKERKE
a
OUINENI..9:)IJ
0
KOKSIJO[
a
948
0
947
0
330
0
946
0
901
0
902
0
903
0
976
0
91J
.
943
0
94~
145
0
939
0
LANGELEED
1
d
9,57
.
9H
975 0
0
963
0
958
9620
956
0
NOORO GASTHU!S
0
969
w
9~p
957
0
0
MIL/II..IR
0
KWINTEO
~
934
933 0 827
0 826 0
0 332
951
0
953
949
'952
0
955
936
0
830
0
828 0629926
0 905 0
924
928 0
0
0
960
VLIEGVELD
Q' 5
."0
954
0
0
0
965
0
967
0
927
0
923
0
0
972
0
916
0
.
DOORN~EED
0
9b3
917
0
988
0
820
825
0989
999
0
0
922
°91
0.0
818
960
0
99814~
82>9
0 0
0
918 SCHOUDERVLIET 997
915
0
9b9
9~7
0
996
0
981
0
0
151
0
980
0=500
i
I
I
f
~
°983!
0
984
0
921 0
0831
91900
0920
I!
!
9
0
334
0
971
9g~
932
0
333
0
968
I
913
°82~2~
0910 0
I
91~ 0
!
0
985,
909
0
986
931
995
994
0
930
8~3 0
0824
0
0 0992
8'2°82
993
57 0 6
i:
.
9;9i
92 9
0
0331
0
9~S
0
0
0905
911
942
0
iI
970
912/0
0
9610
977
0
90~
0
I. .
!
0
.
938
9400
i
I
m
13
AREA
BELGIAN COASTAL PLAIN-OOSTDUINKERKE
Profi Ie type map of the Holocene
deposits
N
,
r
I'll
NORTH
Ii i
:\
rlill
rll
SEA
I
'I
I
I
meTric
b
scale
500m
1000m
I
I
I
- -:.
. . :.:
..'
.'.
.'..
:
..
. .
. °0 : °0 ::'
.,°,: .,°,: .,°:
"
:
(
::: ':.
,°. '0:
." ".
.
::.
'..
. .
::: :'0: :.: ".
...
...
>
'"'.':. .
; :: :',:
',.
.. . .
'," .
...
. \
Fig.11
~'f[:"
r-""-.'
.
"'----
--.....-----.--.
''-
...
---,
~~-'-''''
~
~
~-"-_.""-~._-"'-'~..' .~--
~--_._.-
G,oen.nd'lk
NE
sw
H,Q,
M.Y.
Kwlnte
Clay ISIIFSIMSI
langeleed
),
m5
Clayls,!FsjMs(
m5
claylsllFS~sI
w ,
J.j.
Clay
m5
I SIIF slMsl
dune
""
mudfla t
JL
channel
,
"'"
W
0
channel
",,-,
0
0
channel
, -..
A. fJ
mudllat alll/cla, In'erbeddlng
0
Interbeddin9
mixed lIat clayfnnd
JI
#I' .., , ..,-
"' "'"
.ez>
clayfnnd
t
-
-5
nndllat
..
."
clayfsand
Interbeddtng
sillfclay
interbeddln9
-5
Inter
mudflat
mixed.sa ndlla t
storm deposit
bedding
-5
Slndfla t
nndllat
- "
III
mudllat
lag
,
-5
""
II
.. .~
G>
mudllat
cha nnel
0
-;-..
.5
mS
StlFS~~
;
.4-
V<tI
vv
Clayl
~
1
.."
vv
channel.
clayfsand
sand!!at
mudlla t
48110! 70
Inter bedding
.""..., J
'If
channel
Slndflat
II
0
~.
'"
.:!'J
. If,
-10
clay/nnd
nndllat
lower
-=>
mixed lIat
21 ~
clayfnnd
.",
11
e
iI
i
!
.~.
",/
sheller
I
clayfu.nd
~./
~,
inte~7~'
..-..
shellcrag
--to-:::
,
.."'::...
Yo
-20
clayfsand
Intf"
shellcrag
ddlng
~
-IS
PI
~,
~~/~
.."
/
naser bedding
",/
#/
x-bedding
plant roots
v
burrow
reworkedpeallragmenl
/I
(i)
bioturbation
pebbles
shell in lite position
0
x.lamination
shell (-deblis)
'"
""
<=>
channellag
shellcr a g
parallellamin:ttion
lag deposit
500m
-
ctay pebble
y.""
peat
peaty
erosional boundary
""'"
channel
lag
..." ..
'OJ ~
Q.-.
j,
/'
Yo
-15
Yo
~0./\:1
I
sandllat
channellag
subtidal
.I'
-20
°1
!5~
.
.JL
SI ndllat
' "
i
",
mixed lIa t
._-----
, '" ,
-IS
V"oI
" ""'"
-10
Slndllat
...,...". C!>
subtidal
. I ,
-IS
::!>
.,
Interbeddin9
""',
o
.
Interbeddln9
channel
-10
-10
Fig.
12
--,4
15
Only in the west (borehole N.G.), the Holocene begins with a
basal peat (7620f90 BP at -13.75m) which is covered by mudflat
clay. However, a sandflat eroded the mudflat and dominated the
environment most probably during the rest of the Holocene. In
borehole
M.V.
a
subtidal
evolving
to a well
borehole,
however,
base
environment
developed
clear
mixed
assumed
that several meters
A subtidal
environment
developed,
and mud flat. In the Kwinte
evidence
in the form of channellag
initially
of erosion
and erosional
of pleistocene
accumulated
sand
are
found
boundary.
deposits
and clay
at the
It can be
were eroded.
for about
Sm
and was replaced by a sandflat which, in view of its thick
accumulation,
a mixed
persists
and
both M.V.
mud
for a long time. It finally
flat,
and Kwinte
representing
boreholes
a regressive
evolved
into
sequence.
the top of the mudflat
In
is eroded
by a tidal channel.
Borehole
consists
of
completely
shows
from
by an erosional
the channel
revealing
sandflat
different
sediments
were
Groenendijk
to mixed
dated
peatlayer
picture
sequences
tidal channel.
that the uppermost
Borehole
a quite
two minor , regressive
developed:
truncated
from
Langeleed
as
(although
flat),
Reworked
each
it
not
time
peat fragments
at 4880f70
BP
(fig.12),
has been eroded.
is characterized
in its lower part by a
neariy complete regressive sequence, but the mixed flat is
truncated
that
by a tidal channel
this channel
situated
channel
at about -9m. It is quite probable
is to be correlated
with
the slightly
higher
in Langeleed.
It is very remarkable that a thin mudflat deposit is
occurring between the channel and the sandflat, but it is not
inconceivable that this half a meter of bioturbated clay is just
an erosive part of a mudflat redeposited in its entirety. The
lower and upper boundar ies are indeed very sharp. Such a
phenomenon will be .demonstrated in borehole Doornleed where
about 2 meter of clay and peat were redeposited as a whole in
the
channel
The
second
borehole
sorted
which
~
fill.
Groenendijk
sand
has
regressive.
and
been
clay
sequence
is interrupted
with
interpreted
numerous
?s
storm
(as
from
by half
shells
-9m, onward)
a meter
and peat
deposit.
Also
in
of badly
fragments
at this
and
t
!
16
J
E
f
II
N
z~
E
(:)
i:
0
(1)
>(1)
(1)
II,)
0
a:J
C"')
,...
I
t
i
!
:i
-
~
.-en
u.
I
I
I
~ex:
i
~Z
w
z
0
......
::;0
(:)
i
Iu
0
w
w
I
i
D
lV)
0
'II,)
--+0)Cll,)
V)
co
00.
.c(1)
0"0
V)
V)
0
0:::
U
->,
0,,.-:)
1-01
W
1111'r'I"
:1:':1:':1
I-
......
V)
0
0...
::2
0
u
w
(:)
""')
~o
~
0
~
+I/J
o-+'f-II,)
0
"0.
:)(1)
~"O
[J
-+-11,)
~-+'f- .-II,)
,,0
co.
0(1)
V>"0
a
"f0Q)
Q..
D
-+0
-II,)
'f--+-II,)
00
"00.0)
.1-"0
1
i
t
I
i
t
.,
-------
17
location
the
top of the regressive
tidal channel
sequence
which here is less developed.
is truncated
Moreover
by a
it consists
mainly of clay and reworked peat and shows a clear silting up
. .
sequence
evolving
The
borehole
demonstrate
sequence
that
which
erosional
area
with
is dated
of
overlap
the
Landward
second
Area)
M. V.,
there
Only
Most
in which
laterally
could
phase
in this particular
only
that
it
probably
a tidal
in a sandflat
beyond
erosional
which
area
BP and 3490t60
erosional
the reach
the
phase
is to be
is occurring
in
(fig.13) and which
BP. 14-C dates
of this
are shown on fig. 14. The
remains
questionable
area as shown
faint peat
was
continuous
a major
channel
initiations
tidal
under
inlet
(see
or several
tidal
in borehole
full
ones were
with mixed and mudflats
of the direct
on fig. 12,
tidal
is to be situated
migrating
at its borders.
influence,
peatbogs
lfp 2
Wo
m
2.0
1:1
- - 2970:t60
- - -
1.5
-.j
1.0
m
2710:!:60
355c:!:60
0.5
0.0
- - -
i
-0.5
- - -
1.0
0.5
3490:t60
i
i
erosion,
develop.
\1'1
,
!
of
distinct
mudflat.
overlap
from 3 locations
developed,
indicates
influence.
first
two
in the
.
The fact that
no peatbogs
and
-clearly
particularly
peatgrowth,
in this seaward
3580f60
region
the phases
mixed
the
more
last
the transgressive
between
seaward
In between
that
peatlayer
transgressive
the
as sand,
probable
the
Holocene,
after
happened.
developed
the uppermost
from
developed
It is quite
correlated
data
in the upper
phases
quietly
age
to mudflat.
- - -
4970:t70
0.0
-
- -
- - -
355o:t60
- - -
h220:!:65
f
t
~
---
!!
f
I
!
~
t
i
t
i
1
1---1
clay
[[[[III]
pea t
Fig. 14 14-C dates of transgressive overlap in y BP
~ . .';
~
18
The tidal
of
the
inlet most probably
Holocene
deposits,
The
infill
like
in borehole
typical
facies
as
existed
basa,l
N.G.
and
as from the beginning
peat
covered
Schoudervliet,
of the basal
peat
evolving
by
are
lacking.
to
freshwater
,marsh and finally mudflat,
will be shown in
'Schoudervliet (fig.15), ,located slightly landwards
SW-NE cross-section
borehole
mudflat
borehole
from the
i.. -
(fig.12). The mudflat in the lower part of
Schoudervliet
is truncated
by a channel lag. The channel
fill deposits evolve to a sandflat which in turn is replaced by
a tidal channel as from -8.30m with fragments of reworked peat
at its base. Tpe channel deposit evolves to a sandflat until
-1.4m and displays a very sharp upper boundary. The overlying
silty clay, sandy at the base and with numerous rounded
peatfragments, is characterized by a.o. cross-laminations and
unconformaties. The boundary with the overlying peat is formed
by a thin truncating
erosional
the peat
is occurring
is found at the level
here,
(which generally
the uppermost
is situated
+0.5 and +2m, fig. 13), and the 14-C date of 4540t65
base
is in accordance
with
The tidal
channel
occurring
as the one displayed
12) and from which the incision
That
from
between
BP for the
of it is problematic.
as from -8.30m is most
in the SW-NE
that the clay and p8at covering
as channel
probably
cross-section
is to be dated around
-1.40m is to be interpreted
channel
whole
means
peat
the age of the base of the uppermost
peat layer 'in that area, the presence
the same
sharp
upper and lower boundaries.
Although
layer
sand layer. Also the peat layer shows
(fig.
3500 BP.
the sandflat
fill from the tidal
formed during the second erosional phase. Also here,
peat layer was eroded and redeposited
as
in its entirety.
the
J:
.
-- --
- -
f:Schoudervliet
m 3.20 :7l:.'.~~C,:_~
I
I
U
:~~}li~
u
--
i-
0
I
channel
4540!65 BP
'~Y--l~
~~f!4
;:
!:'=:.J
,.~-:f;j
...
51.
1"'(
':s.
~I"
:~~~
~
~d:j:
$-
!.~;~
.-
~'.
,.;. -r
rt.".
~~.I"-L'"
1-
. '~~-r~
~
sandflat
.'.'C.."I;'",;
~
'1":
",'{
-5
.-
~
"V':.;:
:';.
~
-1...
F.,".
"'.'"
,-
't
t- "t
:,'7.~
, "" ;
.~~
,
"-
channel
:
~~~~
!
i
I!
.
i
;"',I::
;;-1'
I
r
II
':;'\
~II
'
1
sandflat
-10
.
t
(..-
I
.~~~
S~
i
is-
t
fi~~:';.1
:"~:1~
'
..,-
':/~
; - ::
.
:.I
1
~
.,
_:,
,_
~
. "f ,: t
,'
..,. ' '_.
,
. -,
.
.
"
.
channel
:.
~}J
i,-
~A::j
~~~
"'-':j,-";-;
oJ-
-15
~
mudflat
.1-
;
,,
t
'f.r~'
wp:~~~~~1
.t~;~:'~
"~~~
;.L~~;f.
~- -;=8440:t1 30
~';_-.;.:9940:t110
.!
BP
basal peat
BP
'J:"""
':0' t 'j
I
JI
I
i
I
J
f
~
I
I
l t
--..
Fig. 15
.
,
',,>
'~-.
",'.
:"':~;~'::>.~>~:j.>l~\Rf~:'{}&":f'
".,
"
',.::.'~':
"::",,,"'B.:~;\.~'.i?~6'..'~~-"
~.,
-;\:.::J_-'
"::;>3
\-'.
"
. !~
!
20
CENTRAL
Excursion
The
Holocene
point:
central
area
infilling
AREA
Zoutenaaie
(fig. 16) pre-eminently
of the plain is dominated
4 cross-sections
(fig. 17, 18,
area, demonstrate
that the topography
is very similar
19 & 20)
exhibits
that
the
by its subsoil.
The
covering
this
of the Pleistocene
entire
subsoil
to that of the Eocene Ieper clay.
N
A
North Sea
2
4
.. },
I,
~
-;
~
('I
It
I,
,
,
I
I,
I
I
Central
Location
Area
of cross-sections
0
Fig.
16
2km
.
~
;
21
The
which
Pleistocene
consists
reach a maximum
of this area,
evidently
and
with
clay
organic
( fig.
numerous
3)
extent
of
were
at the
bivalves
These
encountered
top,
of
deposits
an
fluvial
deposits
of 11m. Only in the very ~rw part
deposits
eroded
deposits.
landward
thickness
coastal
of -11m,
of Weichselian
consisting
Corbicula
most
Eemian
as from a depth
of fine sand
fluminales
and
represent
the
probably
coastal/estuarine
embayment
.
The central area also forms one of the particular regions
wh~re the typical cyclic formation of coastal deposits where
peat is alternating
The oldest
known
the pre-existing
developed
flat
at 8170t90
channel
clastic
valley
however
in this area was
(fig. 19) where
(at -15.60m).
quickly
filled
(Y-type on fig.8).
replaced
the valley
found
a basal peat
In the valley
by
in
a sand
for about
a mud
flat
8m with
and
only
sediments.
from
reached
that
the altitude
period
(basal peat)
the entire
area. Between
basal
transgressed
peat
sediments
periods
of
regularly
in
on the more
about 7000
the
lower
elevated
continue
peatgrowth.
however,
The
not
altitude
are
or with
The best developed
peat
BP.
parts,
and
dominated
5300
BP, the
subsoil, --while mud
to accumulate,
peats
7000
parts,
BP and about
over the pleistocene
at the same
thickness.
,
of -7.5m at about
on, _mud flats
peatgrowth
fla~
infill
BP years
which
Sea level
As
Holocene
pleistocene
developed,-
tidal
with tidal flat deposits
with
always
the same
is situated
several
occurring
extension
between
and
-2m and
-
-3m and dated
in the period between
As from 5300
marine
influence
developed
J
I
5000
-
5800
forms
BP.
BP on, the plain was silted up and the
was very much reduced.
which
BP and 6400
the uppermost
A very extensive
peatlayer
peatbog
of the series
of
I
i
intercalated
i
I
.
I
Pleistocene
peats.
subsoil
It merges
with
is at higher
the
basal
peat
where
the
elevations.
I
The uppermost
i
is
generally
intercalated
situated
peat is mostly
1m to 2m thick and
between
the
altitudes
literature
it
is usually
of ca
-0. 5m and
I
+lm.
In
surface
the
peat.
Belgian
Such a classical
alternation
referred, ,to as
of peat and mudflat
j
j
,
j
I
l
1
i
.......
clay
is well
displayed
will be demonstrated.
at borehole
Cayenne
(C, fig.
19) which
. ---~
Z
N
I
Sm
NAP
NAP
0
0
-5
-5
13
G
[S]
D
-'0
tlda I lIa t fa cles
mud
lIat
mixed
facies
channel
I ',J" 1 channel
entia
ted)
facies
flat
tidal
I undlffer
IIIIIllI
~
facies
p e it t
Pleistocene
L?~jx(1 Eocene
lag
-10
deposits
deposits
SOOm
0
L-L-.J
-15
-15
Cross-section
1
17
Fig-
s
E
m5
N
I I
-
N
N
Sm
--
0
0
-5
-'0
-10
2. -1800t80
3. 6275i55
SOOm
0
4. 7155t270
L-L-J
-15
Cross-section
-15
2
Fig.
~_..~..
-~,...,- .
-
-,
- ,-"""-''''.-.,
'
.- ._- ,.--"-'-
.
,~
,,,
,..~~.-
.,.~._~.,.."",
''''''-~,"::.~
,,
,
,
,... }-,.~..,
,..,., ~
v.
~
18
".'i'tJ'o"'II\o1..If""~""""""""""~I-jjIp~,,,,,,,~,,,,,,,~'I~~.~:-1
~'"4toII"
""'~'~",
I";,"'''''''.
""I\~'!""
"~''1IO:''''''''':~''''''''~''''''''.~''~'''''''''''.'''.''''r'\I~'
~JfIII""'~'1'f.:f~;"J".,~,
:-U-; '!t.-{ojo':w.~~:~'4r.~\."~'"
I!,
\
23
radiocarbon
The
intercalated
range
:>
peat
))
from
of 5360t70
1989). However,
dates
from
the
the
entire
BP to 4220t65
the greatest
base
coastal
of
this
plain
uppermos t
show
an age
BP (Baeteman & Van Strijdonck,
number of dates reveals an age in a
much smaller range, i.e. 4700 BP
5220 BP in which moreover a
-
concentration of dates is observed between resp. 4700 BP
4970
-
BP and 5130 BP - 5160 BP. The two youngest dates happen to corne
from locations in the seaward area. On the other hand, the
oldest dates were obtained from the peat occurring in the very
landward part of the plain, rather close to the outcropping
Pleistocene
deposits.
The radiocarbon
1610t55
(fig.
dates from the top show an age range of
BP to 3290t80
19)
forms
BP. The very young age of 1185t40
a striking
BP
exception.
Mudflats and peatbogs were not the only environment,
however. The area was crossed by tidal channels, some of them
were located where the pre-existing Pleistocene valleys are
situated. Most of these channels have been active during the
entire
Holocene
infill,
intensity.
Although
it is not
of renewed
activity
or incision
boreholes,
the evidence
seen in the sequence
where
the typical
and channel
Zoutenaaie
also
period
phase
is
channellags
BP
and
"
I
tidal
I
I'
I1
J
I
obvious
with
to recognize
~n the sequence
of their .adjacent areas
mud flat deposits
the
of
same
phases
channels
in
active,
is
(fig. 17 & fig. 19)
are replaced
by- 'mixed flat
is well displayed
in borehole
significant
after
not
the same location,
the
last
known.
of
of -5.70m
deep
During
reworked
respectively
the very
of the Holocene
peatgrowth..
Indeed,
at a depth
BP,
ero$ion
these tidal channels
peat
which
scouring
17 and
in
incision
fragments
and -3.40m were
(fig.
sequence
dated
fig.
from
3890t70
18).
of the channels
The
remains
.9uestionable too.
I
I
always
(Z, fig. 17) which will be demonstrated.
2680t60
explanation
always
deposit_s. The evidence
produced
the
not
that they were continuously
However, 'at excactly
j
I
I
however
On the other hand, it is quite probable that many other
channels
carne into
existence
only
in the period
after
the
end of the peatgrowth and are the result of but one erosional
phase. Most remarkable examples of peat eroslon and tidal
channel fill will be demonstrated in boreholes Eendekot and
Devisch (E & D, fig.
18 & fig.
20).
s
C
N
I
m5
NAP
..
Sm
. . . . , . . ., , . . . . . . . . . . .
. . . . . . . . ., . .
""'"
. .. .. .. .. .. .. .. .. "..
. . . . . . . . I. . . .,.
. .. .."""""""'.
...,
,
. .. . . ... .. . ... .. . ... ...
..,. . ... . .. ,.. ... ... ... . ...,.
"
0
I
. . . . .
. . ". . .
. . "'
. . .
. ."...,...
. . .
. . "'......
. . .
'
.
.
.
.
.
.
.
.
.
.
NAP
. . .
. . ,.. .
. . . , .. . .
. . .
""
-5
-10
-10
2. 6680!80
1.
2 4750;170
3. 5050!80
4. 6750!80
5. 7000i80
6.81701'90
3. 6870t80
-15
SOOm
0
L-L-.J
Cross-section
. 2690t45
2.5130t70
3.5810t75
4.6190t65
5.7110t90
6. 7230,!85
3
-15
1.
2.
3.
4.
1 1 ~';':~!.~\it%
4920~55
6375t60
6665t60
N
+--
Fig.
19
-.
s
0
N
I
m5
I I
Sm
NAP
NAP
0
-10
0
SOOm
L-L-.J
Cross-section
4
Fig.
_..
"'
'"'''-''-'''
"'~'-
.-
-
-.'_.'---"'-~-""
-
...
~...
".~.
...,
... ,/ ,
,
,~
,
,j
20
"..._..
",,,,,,"...,-,,.
r'
,,~.,~
~t
".1I' 'I"",."
,..,
,..".."."",
,""I~'I
~..~".,
I"
.
I"J.
"':"
---
T~
I
------
I
I
I
I
I,
I
i
55
II
!
References
BAETEMAN, C.,
Belgique.
1978. L'extension de la Formation d'Herzeele en
In: J. SOMME
et al.,
1978.
La Formation
d'Herzeele:
un nouveau
stratotype
du Pleistocene
moyen
marin
de la Mer du Nord. Bull. de l'AFEQ, 54, 55, 56, 141-144.
BAETEMAN, C., 1981. An alternative classification
and profile
type map applied to the Holocene deposits of the Belgian
coastal plain. Bull. Belg. Ver. Geol., 90, 4, 257-280.
BAETEMAN,
C.,
concerning
1987.
mapping
Mapping
units.
a
coastal
Geobound,
plain
Problems
1, 1, 3-9.
BAETEMAN,
C. and VAN STRIJDONCK,
M., 1989.
peat from the Holocene
coastal deposits
C. BAETEMAN
(ed.). Quaternary
Sea-Level
Belgium,
Prof. paper~ 6, 241, 59-91.
Radiocarbon
dates on
in West Belgium.
In:
Investigations
from
BARKHAUSEN,
J., PREUSS, H. & STREIF, H., 1977. Ein lithologische
Ordningsprinzip
fur das Kustenholozan
und seine Darstellung
.
in Form von Profiltypen.
Geol. Jb., A44, 45-77.
MOSTAERT,
F. ,
1985.
Bij drage
tot de kennis
van de
Kwartairgeologie
van de oostelijke kustvlakte op basis van
sedimentologisch
Proefschrift,
558p.
en
lithostratigrafisch
Universiteit
onderzoek.
Gent.
SOMME, J., PAEPE, R., BAETEMAN, C., BEYENS, L., CUNAT, N.,
GEERAERTS, R., HARDY, A.F., HUS, J., JUVIGNE, E., MATHIEU,
L., THOREZ, J . & VANHOORNE, R., 1978 . La Format ion
d'Herzeele: un nouveau stratotype du Pleistocene Moyen marin
.
de
la Mer
STREIF,
du
H.,
Nord.
Bull.
de
l'AFEQ,
1978. A new method
sedimentary
Engineering
sequences
in coastal
Conference,
Hamburg,
VANHOORNE,
R., 1962. Het interglaciale
Wet. Tijdschr.,
44, 58-64.
54,
55,
56,
81-149.
for the representation
regions.
Proc.
.1245-1256.
veen
te Lo
16th
of
Coastal
(Belgie).
Nat.
Acknowledgements
.
The assistance with field-work in the Landward Area provided
by Mr. Li Hao and Mr. Abdul Karim Fofana (IFAQ) is greatly
appreciated.
Mr. Vunddaraju Singaraju (IFAQ) drafted the figures 10, 11 &
13; Mr. Li Hao the figures 21 and 24 till 32 on the INTERGR~fH
computer
system
at the Earth Technology
Institute
(Vrij e
.
Universiteit
Brussel).
~"
83
motorway
point:
Excursion
at
Sint-Pieters-Kapelle
(Middelkerke)
In coastal
infrastructural
railroads,
always
unconsolidated
to the
the
lowlands
elements,
fact
problems
condition
that
rising
brings
sea
construction
of buildings
and
such as roads, canals, bridges,
of
the
the sediments
level
and
along.
Holocene
were
never
The
main
sediments.
deposited
had
under
the occasion
reason
is the
This
is due
influence
of
to be dewatered
in a natural way. The result is that the sediments are
characterized by a high moisture content, a low bearing
capacity,
and last but not least a high compressibility.
But the deposits
of a coastal
plain are also characterised
by frequent and singificant changes in lithology. Complex
alternations
highly
of peat,
irregular
these different
clay,
patterns
lithologic
silt and sand are moreover
of sand-filled
fossil
cut by
channels.
units bear their proper
All
geotechnical
characteristics.
'Consequently
of the
in a coastal
sediments
plain
are different
the
from place
geotechnical
to place.
properties
The
decisive
property wi th respect to these changes undeniably is the
compressibility,
leading to differential compaction when
dewatering
and/or loading the deposits.
Civil
engineers
characteritics;
sediments
are
technologies
however
allow
high
so
variability.
well
known.
of these
and
geotechnical
complexity
Nowadays
of
the
the. engineering
to solve most of the problems.
use of adequate
to our society
concerning
will
Oudenburg
geological
be demonstrated
The question
information
by means
can
of an
road building.
the end of the seventies
the western
To
not
the proper
contribute
At
the
aware
is, at what price.
How
example
are very well
a rnotorway was
constructed
coastal plain of Belgium, more particularly
in
between
and Nieuwpoort.
constru'ct the road,. about
1 to 2 m of the superficial
layers are removed and replaced by sand, forming a cunet in this
84
way.
Then
the
several
cunet
months
unconsolidated
in
The
systematically
only
to
more
sand
achieve
for
a
1 to
2 m for
pre-load
Such a method, which
of
the
is indispensable
of the superficial layers is done very
until 'a certain depth without taking into account
of the sediments.
the upper
occurred
order
by
sediments, requires a huge amount of infilled
removal
the nature
raised
sediments.
for unconsolidated
sand.
is
part
slightly
It already has been observed
of a 2 m thick peat
deeper, was removed.
layer, which
that
locally
In such a situation
the
cunet is not very functional since half of the peat layer, being
I
the most sensitive
I
to compaction, is still present.
Since some years now the road shows the well
"washboard"
phenomenon
due
to differential
compaction
known
(Hageman,
I
1984).
This can be observed
especially
in the central part of
I
the surveyed
route (at the exit Middelkerke) which ~ow will
become one of the money swallowing roads because of the
J.1-
recurrent
j
maintenance
This
section
costs.
of the motorway
the use of geolog~cal
techn;cal
and
problems
after
location
Moreover
the
of
information can contribute
as well as the financial
construction
the
forms a clear
road
just
to the
of how
to decrease
implication
by selecting
in relation
example
the
during
the appropriate
geological
setting.
a huge amount of the already scarce raw materials,
like
sand, could have been saved as well.
A simple
consultation of the geological profile type map
would have highlighted how unfortunate the location of this
motorway
wsw
.
section
t.. I I I I
.
. I I It'
has
I
been
, I .
planned.
I ,
I
. I
I
I I
I
.
I I ,
I
I
. I
EIIE
I
I
I
I
I
I
I
L.
AP
UP
-10
I
500
I .
D
lac4, .i1~,
[IT]:
tidal
cb~Dnel
ela1
tacie.
-2
-L
-6
-8
-10
UIIIIIp~.t.
D
Pleilt.ocene
lediatDtl
Fig 1
~
-'{
-,..
~
i
tj
tl
1
i
1
f"
85
'2
N
Fig. 3
r
'
Simplified pr ofile-type map
---
--/
;''' "
.1
,
'2'
.
0
Explana tion of profile
types
Holocene
.
. 1 .
:
: ITJ
.)~<
[E
Lid
~
deposits
>.>
'--'>--->
. 2
'
J~~~'
'
D
ElR
~
:::.::.:
::...:'..-.
'.~'':~~::
eo'".
-:.
..: :'.:.:::'
:::::::."
:.: ..:.eo"
:. .."°0"..
..:..:..~
..
.."
: .°.".
" :.:.
._.a..;,".
"..." :
""
The cross-section
clay
IIIIIII
peat
E2J
tidal
7777
Pleistocene
along
channel
relief.
because these
are known
much
the motorway
~
is an important
sediments,
to be relatively
more
bearing
capacity
----
motorway
-.-.-.-
alternative
location
subsoil
Pleistocene
greater
sediments.
This
Pleistocene
facies
in the first place that the Pleistocene
varia,ble
Outcropping
(fig. 1) demonstrates
subsurface has a highly
factor for road building,
consisting
consolidated,
than
the
of
coversands,-
hence
coastal
having
a
Holocene
86
In the Holocene
obs~rved
channel.
complex
tidal
sediments.
characterised
Besides,
crosses
of the central
the
can be
twice a"
is much
part
of peat and clastic
tidal
flat
sediments
are
properties.
geometry,
all
To the west
having
different
of the studied
section,
the
sequence
of mainly sandy sediments overlain by one peat layer in
is
section
obvious
will
compaction.
replaced
dewatering
unavoidably
lead
Even if the upper
layer
to
are regarded
loading
compaction
are removed
will
take
homogeneous,
i
and
within
underlying
a
place.
the upper
bearing
uniform
characteristics.
The unfortunate location of the motorway is clearlyillustrated
For
by a fragment
the sake
of the geological
of clearness
a very
simplified
profile
(fig. 3) was drawn. It consists of only four profile
however
type
give
sufficient
#1 represents
directly
a thin
the pleistocene
thickness
ranging
Pleistocene
information
between
surface.
cover
deposits.
type map.
profile
type map
sediments
In #2 a basal
Profile
overlying
peat,
with
few cm and about 1 m, is covering
In the areas where the Pleistocene
a
the
surface
is occurring at a greater depth, profile type #3 is found
consisting of an alternation of clay, silt and sand on the one
hand, and one or more peat layers on the other hand. Basal peat
may be absent or present. Finally #4 represents the tidal
channel
facies.
The simplified profile type map demonstrates that the
concerned section of the motorway crosses for a great part
profile
The
type
#3,
geological
suitable
In the
areas
east,
Pleistocene
the area which
is most
sensitive
to compaction.
setting of the region, however,
and
about
surface,
an alternative
2.5 km more
hence
location
the relatively
offers
is proposed
to the south
more
(fig 3).
of the motorway,
consolidated
I
~
1
!
i
j
J
I1
j
~
types which
for this purpose.
of clay
I
~
J
road-
differential
occurring
unit,
the sediments
as being
this
signifi'cant
for each lithologic
far too often
geotechnical
an/or
2 m of sediments
differential
timespan
It is true,
peat
that
by sand,
different
i
iI
geotechnical
turn covered by clay.
It
t
by frequent changes of sand, silt and clay layers
irregular
consists
The sequence
and consists of an alternation
flat
with
quite a' lot of variations
too. The eastern part of the motorway
sand-filled
more
deposits
the
i
II
,
I
~I
87
sediments,
is at a high elevation.
that alternative
to
a
cover
reaching
location
of
road-building,
are, in the eastern
tidal
a maximum
The Holocene
flat
clay
thickness
underlain
of 1 m
part,
by
(fig.2).
this part offers suitable
deposits
along
restricted
a basal
peat
With
respect
to
conditions,
because
it
has a good bearing capaci ty and very low compaction rate.
Besides, a cunet would remove all the compressible Holocene
layers. More to the west the fossil tidal channel could be used
as
the
central
simple
i
I
I
J
f
i
!
of
foundation
compaction
-
part
would
it is sand
engineering
occur
which
filled
and
provisions.
moreover
would
requires
very
happen
only
limited
very
quickly
(which is not the case for clay and peat). On the other hand
these sediments will not yield differential compaction. This
alternative
location
certainly would not result
in a
washboard-road, very important for the maintenance costs after
few
years.
II
2
.
.
.
.
.
.
.
.
.
.. .
.
.
.'.,""
. .
.. .
.
.
. .
.
.
.
.
-
. . .
.
. . . . . . . . .
.
.
.
.
.
. .
. . . . . . . .
.
. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
. . .
.
"..'.'
. . . . . . . .
. . . . . . . .
. . . . . . . . . . . . - . - . . . . . .. .. .. . . .. . . . .
,
,.
,
-,
, . ,
,
'
,
,
,
'
'
'
'
,",',',',',',',',',',',',',',','.~
,",',',',',',',',',',',',',','/.
-8
'
,
'
, . ,
'
. <. ~.~.~~~.~;-:.-:,'
","""""
,
,
'
-2
borehole
.
r-- :1 clay
"
'
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./
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,
'
',;/.
.
[ll]]]]]
.
.
p
e&t
.
-4
c=J
0
tid&l
ch&nnel
Pleistocene
!&cies
sediments
0
L
500
J
..
10
-10
fiE.
t
I
j
i
..
NAP
.. ...>..>:-,:..</
-6
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.
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.
. . . . . .
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. . . . . . . . . . . . . .
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. . . "'..
. .
. . . . . . . .
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. . . . . . . . . . . . . .
.
-4
I
---------- --------
NAP
-2
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I I
4
..
2
References
BAETEMAN,
C.
&
PAEPE,
R.,
1991.
Planning and Geology
i,n a
1
i
,
'1
,il
i
1
I
"4
-'
~
.~ \.
Coastal
HAGEMAN,
Plain. Bull. Belg. Ver. Geol., 100/1-2, 195-201.
B.P.,
1984. Geological
environmental
planning
Geol. Jb., A75, 93-123.
with
information,
emphasis
a vital element
on
coastal
in
plains.