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Chapter 3
43
Chapter 3
Reconstruction of the Late Palaeogene tectonic activity
of the southern Dutch North Sea, based on
a sequence stratigraphic interpretation of log correlations.
Abstract
The existing stratigraphic framework of the Palaeogene in the Netherlands shows limited temporal resolution. Using this framework, it is difficult to unravel the influence of local tectonic move-ments and eustatic sea level changes on sedimentation. The precise dating of the onset of the Late Eocene - Early Oligocene Pyrenean orogenic phase, which caused inversion and uplift in the southern Dutch offshore, is also dubious as a result of the limited resolution.To improve the correlations and dating of the Palaeocene to Oligocene successions in the south-ern Dutch North Sea, a new sequence stratigraphic interpretation, based on correlated wireline logs, is presented. The method is based on the reconstruction of local sea level cycles, interpreted from differences in grain size distribution, as derived from gamma ray log response. The sea level signal in the study area is then calibrated with biostratigraphically defined sea level cycles from onshore Belgium. This reconstruction method enables a high-resolution correlation between the Late Palaeocene to Eocene tectono-stratigraphic evolution of the southern Dutch North Sea and the standard eustatic cycle chart.The correlation demonstrates that the Pyrenean phase, which started during the Early Priabonian (circa 37 Ma), was preceded by an earlier period of tectonic activity during the Middle to Late Eocene. The tectonic overprinting of eustatic sequences in the southern Dutch North Sea started in the beginning of the Lutetian (circa 48 Ma). The tectonic events in the study area can be correlated to time-equivalent tectonic uplift in the Brabant and Artois Blocks in Belgium, which suggests that the processes causing the tectonic activity are of regional importance
1. Introduction
1.1 Lithostratigraphic framework
The stratigraphic framework of the Palaeogene of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) shows low temporal resolution (Figs. 2.1 and 3.1), as a result of the limited data available for this interval. The data from the Dutch offshore are seismic surveys, wireline logs and lithologic cuttings from industrial boreholes. Detailed outcrop information is only available onshore, in more proximal depositional settings. The Palaeogene succession has been biostrati-graphically dated in a limited number of wells (Table 2.1, Fig. 2.2d). Most datings are based on foraminifers, derived from cutting samples, providing a low biostratigraphic resolution (Fig. 3.2). As a result of the limited stratigraphic resolution, it is difficult to unravel the relative influence of local tectonics and eustatic sea level variations on the Palaeogene sedimentation in the southern Dutch North Sea.The aim of this study is to introduce a new sequence stratigraphic interpretation of existing well
44
and seismic data. The interpretation provides a higher-resolution reconstruction of the Late Pal-aeogene tectono-stratigraphic evolution of the southern part of the Dutch North Sea than currently available. The detailed reconstruction of local relative sea level movements, correlated to the glo-bal eustatic sea level chart, assists in the recognition and dating of local tectonic activity.
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gen
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Middle � North � Sea
Lower North Sea
Gro
up
Age(Ma) 60504030
Epoc
h
Pala
eoce
ne
Eoce
ne
Olig
ocen
e
Ypre
sian
Palaeogene
Dan
ian
Sela
ndia
n �
Berggren et al., 1995Perio
d
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cene
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ta-�
nia
n
N
ethe
rland
s�O
ffsho
re to
SW
Gro
upFm
.N
ethe
rland
s�
S
E
Fig. 3.1 Simplified lithologic correlation between the Netherlands offshore, Netherlands southern onshore and
Roer Valley Graben (RVG).
Modified after Marechal (1993), Van Adrichem Boogaert and Kouwe (1997) and Vandenberghe et al. (1998).
Sequence stratigraphic interpretation of log correlations
Chapter 3
45
1.2 Geological setting and previous studies
The geometry of the Late Palaeocene to Oligocene marine succession of the southern Dutch North Sea area (Fig. 1.1) was significantly influenced by relative sea level movements, Oligocene inver-sion tectonics, salt tectonics and post-depositional erosion (Letsch and Sissingh, 1983; Remmelts, 1995). Nevertheless, the lithologic composition of the southern Dutch North Sea succession is relatively uniform, as the largest part of the area occupied a distal position with respect to the
biostratigraphic zonesKing (1989),North SeaBenthics
King (1989),North Sea Plankton
Doppert and Neele (1983),Dutch North
Sea
MiddleEocene
LateEocene
EarlyEocene
EarlyOligocene
LateOligocene
EarlyMiocene
LatePalaeocene
EarlyPalaeocene
Pal
aeo
gen
e
NSB 1a
NSB 1b
c
NSB 3a
b
NSB 5a
NSB 5b
NSB 5c
NSB 6a
NSB 6b
NSB 7a
NSB 7b
NSB 8a
NSB 8b
NSB 8c
NSB 9
NSB 4
NSB 2
NSP 9a
NSP 8
NSP 7
NSP 6
NSP 5
NSP 4
FE2
FE2
FE3
FF
FF
FHa
FHb
FHc
FI
FI
FI
FJ
FK
Neo
gene
Fig. 3.2
Correlation scheme of different
biostratigraphic zonations.
46
paleocoastline during most of the Palaeogene (cf. Huuse, 2000). Most sedimentary units consist of silty clays deposited in an open marine shelf setting. Only a few units are sandy, indicating coastal settings (Fig. 2.1).No Middle Eocene tectonic activity has been reported in the study area. Furthermore, accurate dating of the onset of the Pyrenean orogenic phase, which caused inversion and uplift in the study area during the Late Eocene to Early Oligocene, is not possible without additional data. Until now, it was assumed that tectonic movements in the area started close to the end of the Eocene (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). This broad age range is partly due to the wide-spread erosion of Late Eocene sediments in the study area (Fig. 2.5). As a consequence, it has been difficult to correlate the Late Palaeogene tectonic development of the Netherlands offshore to the other parts of the North Sea. Previously, there has been little effort to construct a sequence stratigraphic framework for the Dutch part of the Palaeogene North Sea Basin. The stratigraphic nomenclature of the Palaeogene
N
7o
6o
4o
3o
5o
51o
52o
53o
54o
55o
N
7o
6o
4o
3o
5o
51o
52o
53o
54o
55o
Location of wells in correlation panels of Figs. 3.6 and 3.8Outline study area
Location of wells in correlation panels of Fig. 3.7Outline study area
b)a)
K06-01
Knokke
K12-01
L13-01
P05-03Q01-01
P15-02
S05-01
L02-04 L02-04
K06-01
K12-01
L13-01
S05-01
S02-02S02-01
K07-02
Fig. 3.3
a) The locations of the wells in the correlation panels of Figs. 3.6a, b and 3.8.
b) The locations of the wells in the correlation panels of Figs. 3.7a and b.
Sequence stratigraphic interpretation of log correlations
Chapter 3
47
(Van Adrichem Boogaert and Kouwe, 1997) only mentions sequence stratigraphic interpretations when the transgressive or regressive nature of deposits could be unambiguously linked with the
‘Vail curve’ (Haq et al., 1988). Wong et al. (2001) presented a sequence stratigraphic reconstruction based on the analysis of a well in the Broad Fourteens Basin area. This well lacks a significant part of the Eocene interval due to post-depositional erosion during the Pyrenean phase.In contrast to the limited studies in the Netherlands, various high-resolution sequence stratigraphic studies of the on- and offshore Palaeogene in the areas surrounding the Dutch North Sea have been published (e.g. Jacobs and Sevens, 1993; Michelsen, 1994; De Batist and Henriet, 1995; Laursen et al., 1995; Jacobs and De Batist, 1996; Neal, 1996; Michelsen et al., 1998; Vandenberghe et al., 1998; 2004). An attempt can be made to correlate the relative sea level changes in the Neth-erlands offshore to these surrounding areas. Assuming that sequence stratigraphic boundaries are isochronous over long distances, such a correlation can be used for age dating.From the Late Palaeocene to the Late Oligocene, the southern North Sea Basin was a ramp-type continental shelf (Jacobs and De Batist, 1996). On the shelf, depositional angles were general-ly less than one degree. Distinct seismic clinoforms or coastal onlaps, which would facilitate a
‘classic’ sequence stratigraphic interpretation (sensu Vail et al., 1977; Posamentier and Vail, 1988; Posamentier et al., 1988), are rare or absent within the succession. However, Vandenberghe et al. (1998) have shown conclusively that eustatic sea level fluctuations, based on grain size variations, can be recognized in an open marine ramp-type margin setting.
2. Data and methods
To provide a sequence stratigraphic interpretation of the limited data, the available wells are first interpreted based on their log signature. The succession is subsequently correlated between wells and divided into sedimentary sequences. A local sea level curve is constructed. Subsequently, the Dutch succession is correlated with the Belgian lithostratigraphic succession, which is biostrati-graphically and sequence stratigraphically well calibrated, and correlated with the global eustatic cycle chart (Haq et al., 1988). This correlation enables an indirect age assessment of the Dutch suc-cession. Local deviations in sea level from the eustatic sea level chart in distinct parts of the Dutch territory are related to local tectonic activity.
2.1 Data
The correlations in this study are based on the interpretation of gamma ray (GR) and sonic (DT) logs from 74 wells (Fig. 2.2a, Appendix A). Of these 74 wells, 11 are presented in log-correla-tion panels in this chapter (Figs. 3.3a and b). The other wells were discarded for various reasons, such as poor log quality (e.g. logs run through casing), the availability of only a gamma ray log, the absence of lithology descriptions, or their position on the flank of salt domes. Lithology de-scriptions based on cutting samples were available for all 11 presented wells. The descriptions provide information on sediment composition and grain size, sediment colour and the occurrence of constituents such as glauconite, pyrite and fossils. Industrial biostratigraphy reports, based on foraminifers from cutting samples, were available for ten of the 74 wells (Fig. 2.2d, Table 2.1). Several 2D-seismic lines aided the reconstruction. The seismic panels illustrate the large-scale sedimentary geometry of the Palaeogene (Fig. 3.4).
48
1000
2200
800
2000
600
1800
1600
200
1400
400
1200
TW
T (
ms)
10 km
2400
0
Palaeogene
Salt dome
Salt dome
Neogene
Cretaceous
SNSTI-NL-87-11A A'
1000
800
2000
600
1800
1600
200
1400
400
1200
TW
T (
ms)
10 km0
Palaeogene
Neogene
Cretaceous
London-BrabantMassif
SNST-NL-83-01B B'
Fig. 3.4
a) Seismic section A-A’, showing post-depositional deformation of the Palaeogene sedimentary succession due
to salt-tectonics. In the inset, the Palaeogene succession is indicated in grey.
b) Seismic section B-B’, showing the depositional setting of the Palaeogene. Note the parallel internal layering
and the absence of clinoforms.
Sequence stratigraphic interpretation of log correlations
Chapter 3
49
N
7o
6o
4o
3o
5o
51o
52o
53o
54o
55o
A'
A
BB'
C
C'D
D'
SNSTI-NL-87-25
10 km0
SNSTI-NL-87-14
1000
800
600
200
400
1200
TW
T (
ms)
1000
800
600
200
1400
400
1200
TW
T (
ms)
10 km0
Palaeogene
Neogene
Broad Fourteens Basin
Broad Fourteens Basin
Neogene
Palaeogene
BFB Margin
D D'
C C'
BFB Margin
c) Seismic section C-C’, illustrating post-depositional erosion of Upper Eocene (Palaeogene) sediments within
the inverted Broad Fourteens Basin (BFB) and the geometry of the basin margin.
d) Seismic section D-D’, illustrating post-depositional erosion of Upper Eocene (Palaeogene) sediments within
the inverted Broad Fourteens Basin (BFB) and the geometry of the basin margin.
50
Data available:- Log response- Lithology- Biostratigraphy
Formations andmembersindicated on logs
Correlation panelsconstructed, linking wells in the study area
Higher-order sequencesdefined, using grainsize trends and distinctlog markers
Panels correlated to Belgianlithostratigraphic framework,using grain size trends and biostratigraphic data.
Belgian succession:- High-resolution biostratigraphy- Correlated to eus- tatic cycle chart
boreholewith
well-logs
litho-stratigraphic
boundary
sequenceboundary
Fig 3.5
Fm.Mb.Mb.
Fm.
Fig. 3.5
Flow diagram of the method of interpretation of the wells used in this study.
Sequence stratigraphic interpretation of log correlations
Chapter 3
51
2.2 Wireline log correlations and sedimentary sequence interpretation
For each well, the Palaeogene interval is subdivided using the log response and lithology descrip-tions. The lithostratigraphic interpretation is based on grain size variations and coarsening upward or fining upward trends (see Fig. 3.5 for a flow chart). The lithostratigraphic framework of the Netherlands (Van Adrichem Boogaert and Kouwe, 1997) is applied. Based on the subdivision, a low-resolution correlation between the boreholes is constructed (Fig. 3.5). Within the coarser correlation framework of formations and members, a higher-order correlation is made, which is based on trends in the gamma ray and sonic logs. Changes between fining-upward and coarsening-upward trends and distinct log markers are noted and form the boundaries between high-resolution sedimentary sequences within the limits of the coarse lithostratigraphic framework. Between the different wells, similar trends are correlated. This enables a detailed correlation of sedimentary sequences between boreholes in the area (Fig. 3.5). Although most trends correlate well, there are exceptions within some members. The results of the correlation are presented in four correlation panels (Figs. 3.6 and 3.7).
2.3 Age dating of the sequences
The sedimentary sequences in the study area cannot be correlated directly to the global eustatic sea level curve, due to the poor biostratigraphic control on the Dutch Palaeogene succession. There-fore, the sedimentary sequences are correlated first to the biostratigraphically calibrated sequences in Belgium (Fig. 3.8). The lithostratigraphic framework of Belgium (Marechal, 1993; Vanden-berghe et al., 1998; 2001) shows more detail than the Dutch Palaeogene lithostratigraphy (Van Adrichem Boogaert and Kouwe, 1997), as is illustrated in Fig. 3.1. The Belgian succession was deposited closer to the Palaeogene coastline of the North Sea Basin, compared to the more distal deposits of the Dutch offshore. Hence, in the Belgian succession, the sea level fluctuations are more clearly visible. Compared to the Dutch succession, it shows more evidence of erosion, trun-cation, channel incision and coastal onlaps (Jacobs and De Batist, 1996), as well as an alternation of marine and continental strata (e.g. Gullentops et al., 1988; Marechal, 1993). Sea level variations have been interpreted in detail from outcrops. A detailed sequence stratigraphic framework, with good biostratigraphic control, was constructed by Marechal (1993) and Vandenberghe et al. (1998, 2001, 2004). The recognition of eustasy-controlled sequences (based on well logs) in the study area, correlated to the lithostratigraphic framework of Belgium and the detailed sequence stratigraphic framework of Vandenberghe et al. (1998, 2001), allows an indirect correlation of the Palaeogene succession of the southern Dutch North Sea to the standard eustatic cycle chart of Haq et al. (1988). The Haq eustatic cycle chart was recalibrated by Hardenbol et al. (1998). A detailed age model for the sequence stratigraphic correlation is shown in Fig. 3.8. This Figure shows the Ypresian interval of well K06-01 and well Knokke in the Belgian offshore. The sediments in the wells were not influenced by the Oligocene tectonic movements and contain a continuous Late Palaeocene to Late Eocene succession. Ypresian sea level variations at both well locations, which can be recon-structed from the gamma ray log (K06-01) and grain size log (well Knokke, Fig. 3.8) respectively, are therefore likely to be of eustatic nature. Although the wells are situated at positions about 280 km from each other (Fig. 3.3a), their sedimentary successions can be correlated successfully. Through the correlation with well K06-01, other wells in the study area can be tied to the sequence
52
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ay
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.6a
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Fig
. 3.6
a) S
eque
nce
stra
tigra
phic
inte
rpre
tatio
n of
wel
l cor
rela
tions
bet
wee
n fiv
e w
ells
in th
e D
utch
Nor
th S
ea, l
ocat
ed o
utsi
de th
e M
esoz
oic
Bro
ad F
ourt
eens
Bas
in.
The
sedi
men
tary
com
posi
tion
of th
e se
quen
ces
refle
cts
the
loca
l rel
ativ
e se
a le
vel.
Arr
ows
indi
cate
sea
leve
l tre
nds.
The
pos
ition
s of
the
wel
ls a
re in
dica
ted
in F
ig. 3
.3a.
Sequence stratigraphic interpretation of log correlations
A la
rger
ver
sion
of t
hid
figur
e is
ava
ilabl
e as
a s
epar
ate
encl
osur
e
Chapter 3
53
stratigraphic framework of Vandenberghe et al. (1998, 2001). This, in turn, allows the recogni-tion of local tectonic activity, erosion or non-deposition in the study area. Sequence stratigraphic interpretation of the correlated wells in the study area generally yields good results. However, ex-ceptions occur within individual cycles in some wells, which is illustrated by deviating grain size trends.
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FY
NM
RFC
NU
Yp4
Yp4b
Yp3
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)
Q01
-01
Gam
ma
Ray
Soni
c
FJFI?
FF
Nor
thSo
uth
HST
HST
HSTTS
T
TST
TST
TST
TST
TST
040
8012
050
4030
1250
1200
1150
1100
1050
100095
0
900
850
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100500
depth (m)
NU
NLF
FY
NLL
FCN
LFFT
Yp4
Yp3
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)G
amm
a ra
y (A
PI-U
nits
)Tr
ansi
t tim
e (u
s/ft)
P05-
03G
amm
a R
ay
So
nic
040
8012
016
020
018
016
014
012
010
012
50
1200
1150
1100
1050
100095
0
900
850
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100500
depth (m)
NU
NLF
FY
NLF
FT/D
NLL
FC
Yp4
Yp3
P15-
02G
amm
a R
ay
So
nic
Fig
. 3.6
(co
ntin
ued)
b) S
eque
nce
stra
tigra
phic
inte
rpre
tatio
n of
wel
l cor
rela
tions
bet
wee
n th
ree
wel
ls in
the
Dut
ch N
orth
Sea
, loc
ated
with
in th
e M
esoz
oic
Bro
ad F
ourt
eens
Bas
in. T
he p
ositi
ons
of th
e w
ells
are
indi
cate
d in
Fig
. 3.3
a.
A la
rger
ver
sion
of t
hid
figur
e is
ava
ilabl
e as
a s
epar
ate
encl
osur
e
54
3. Sequence stratigraphic development
3.1 Large scale geometry
Seismic data show that the original geometry of the siliciclastic Palaeogene succession was af-fected by post-depositional tectonic movements and erosion (Fig. 3.4). Oligocene uplift in the centre of the study area resulted in severe erosion of Late Eocene sediments (Letsch and Sissingh,
040
80120
160
1250
1200
1150
1100
1050
1000950
180
160
140
120
100
020
4060
80100
1100
1050
1000950
900
850
800
240
200
160
120
80
2030
4050
60
850
800
750
700
650
600
550
200
180
160
140
120
NLFFB
NLFFB
NLFFB
NLFFM
NLFFM
NLFFM
NMRFC
NMRFC
NMRFC
2040
6080
850
800
750
700
650
600
550
170
160
150
140
130
120
NLF
FMN
MR
FC
2040
6080
100
900
850
800
750
700
650
600
180
160
140
120
NLF
FM
NM
RFC
K06-
01G
amm
a R
ay
Soni
c
K07-
02G
amm
a R
ay
Soni
cK1
2-01
Gam
ma
Ray
So
nic
L13-
01G
amm
a R
ay
Soni
c
L02-
04G
amm
a R
ay
Soni
c
NLL
FY
NLL
FY
NLL
FYN
LLFY
NLL
FY
NU
NU
500
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)G
amm
a ra
y (A
PI-U
nits
)Tr
ansi
t tim
e (u
s/ft)
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)G
amm
a ra
y (A
PI-U
nits
)Tr
ansi
t tim
e (u
s/ft)
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)
Nor
th
Sout
hW
est
A-R
T
A-T
TR
TA-T
R
T
TRT
TA-R
T
T
200
150
100
50
Eust
atic
cur
ves
Har
denb
ol e
t al.,
199
8
Yp 7
Yp 1
0
Lu 1
Lu 2
Lu 3
Lu 4
Bart
1
Pr 2
Pr 1
Pr 3
Pr 4
/Ru1
Ru3
Yp 8TR
?
T
T
R
Lege
ndRT
Lith
ostra
tigra
phy
NU
U
pper
Nor
th S
ea G
roup
NM
RFC
Rup
el C
lay
Mb.
NLF
FB
Asse
Mb.
NLF
FM/S
B
russ
els
Mar
l/San
d M
b.N
LFFY
Ie
per M
b
Yp3
seq
uenc
e bo
unda
ryT
t
rans
gres
sive
inte
rval
R
re
gres
sive
inte
rval
A
agg
radi
ng in
terv
al
T
Fig
. 3.7
a) S
eque
nce
stra
tigra
phic
inte
rpre
tatio
n of
wel
l cor
rela
tions
of t
he B
russ
els
and
Ass
e m
embe
rs (D
onge
n F
m.)
bet
wee
n fiv
e w
ells
, lo
cate
d N
orth
of t
he M
esoz
oic
Bro
ad F
ourt
eens
Bas
in. T
he p
ositi
on o
f the
wel
ls is
indi
cate
d in
Fig
. 3.3
b.
Sequence stratigraphic interpretation of log correlations
A la
rger
ver
sion
of t
hid
figur
e is
ava
ilabl
e as
a s
epar
ate
encl
osur
e
Chapter 3
55
1983; Van Wijhe, 1987a; 1987b). The uplift was accommodated by large fault zones bordering the inversion zone (Figs. 3.4c and 3.4d). In the North of the study area, the Palaeogene sediments were deformed by salt tectonics (Fig. 3.4a), which resulted in piercing of the succession and the forma-tion of associated rim synclines (Remmelts, 1995).
3.2 Detailed sequence correlation
3.2.1 Landen Formation (Thanetian) and Basal Dongen members (Early Ypresian)
The wells located outside the inverted Broad Fourteens Basin (Fig. 3.3a) comprise a relatively complete Palaeogene succession (Fig. 3.6a). The correlation of the Thanetian Landen Formation and the Early Ypresian Basal Dongen Sand and Tuffite Members is based on wireline log signature. The main body of the Landen Formation is a highstand systems tract, which is suggested by ag-grading to prograding (coarsening upward) log trends. The Basal Dongen Sand and Tuffite mem-bers are separated from the Landen Formation by a sharp gamma ray log boundary (Fig. 3.6a). The Dongen Sand and Tuffite members show a distinct low gamma ray response and ‘erratic’ sonic velocity with spikes (clearly visible in L13-01 and S05-01). The sonic spikes result from intermix-ing of tuffaceous ashes, derived from volcanic events further to the North (Jacque and Thouvenin, 1975; Knox and Morton, 1988). According to the log signature, the Dongen members were depos-ited during a short regressive phase, which was followed by the deposition of transgressive sandy clays (Letsch and Sissingh, 1983). The Landen Formation and Basal Dongen members (Fig. 2.1) cover sequences Se2-Yp2 (Van Adrichem Boogaert and Kouwe, 1997; recalibrated to Hardenbol et al., 1998).
3.2.2 Ieper Member (Ypresian)
Outside the Broad Fourteens Basin area, the Ieper Member has not been affected by post-deposi-tional erosion. This is indicated by the occurrence of the Brussels Sand or Brussels Marl Member on top of the Ieper Member (Fig. 2.5a, b). Within the Broad Fourteens Basin area, post-deposition-al erosion linked to the Pyrenean inversion phase resulted in the removal of most of the Ypresian sedimentary succession (Figs. 2.5a, b, 3.4). Only the lowermost Ypresian sequences are preserved (Fig. 3.6b). The sequence correlation of the Ypresian succession is based on wells in which the Ieper Member is complete.The sediments of the Ieper member were deposited in an open marine environment with water depths of tens to hundreds of meters (van Adrichem Boogaert and Kouwe, 1997). The lower half of the Ypresian succession is an aggrading interval of silty clays (Fig. 3.6a), in which small-scale (~50 m) transgressive and highstand sequences occur. The transgressive and highstand sequences are recognized by coarsening upward and fining upward grain size trends. Abrupt changes in log response typically indicate sequence boundaries, more pronounced in the gamma ray than in the sonic logs. The grain sizes inferred from the gamma ray logs (Figs. 3.6a, 3.8) indicate that the fluctuations in local sea level during the deposition of this part of the Ieper Member were relatively minor. Biostratigraphic results from well K12-01 (Fig. 3.6a) indicate that this section of the Ypre-sian succession spans the Dutch North Sea Plankton Zone FI (Early Eocene, Fig 3.2). The local sea level trend, inferred from the grain sizes in the succession, correlates well to the Belgian sequence stratigraphic framework (Fig. 3.8) and indicates that this part of the Ieper Member covers the inter-
56
020
4060
80
750
700
650
600
550
500
450
240
200
160
120
800
2040
6080
650
600
550
500
450
400
350
240
200
160
120
8040
040
8012
016
0
650
600
550
500
450
400
350
280
240
200
160
120
8040
sand
y in
terv
al
sand
y in
terv
alsa
ndy
inte
rval
S02-
02G
amm
a R
ay
Soni
c
NM
RFC
S05-
01G
amm
a R
ay
Soni
cS0
2-01
Gam
ma
Ray
So
nic
NM
RFC
NLF
FB
NLF
FB
NLF
FB
NLF
FS
NLF
FSN
LFFS
NLF
FYN
LFFY
NLF
FY
300
NM
RFC
NU
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)G
amm
a ra
y (A
PI-U
nits
)Tr
ansi
t tim
e (u
s/ft)
Gam
ma
ray
(API
-Uni
ts)
Tran
sit t
ime
(us/
ft)
Sout
hN
orth
A-R
A
R
T
200
150
100
50
Eust
atic
cur
ves
Har
denb
ol e
t al.,
199
8
Yp 7
Yp 1
0
Lu 1
Lu 2
Lu 3
Lu 4
Bart
1
Pr 2
Pr 1
Pr 3
Pr 4
/Ru1
Ru3
Yp 8TRT
R
T T
T
TR
TTR RR
?
Fig
. 3.7
b) S
eque
nce
stra
tigr
aphi
c in
terp
reta
tion
of w
ell c
orre
lati
ons
of th
e B
russ
els
and
Ass
e m
embe
rs (
Don
gen
Fm
.) in
five
wel
ls, l
ocat
ed
Sout
h of
the
Mes
ozoi
c B
road
Fou
rtee
ns B
asin
. The
pos
itio
n of
the
wel
ls is
indi
cate
d in
Fig
. 3.3
b.
Sequence stratigraphic interpretation of log correlations
A la
rger
ver
sion
of t
hid
figur
e is
ava
ilabl
e as
a s
epar
ate
encl
osur
e
Chapter 3
57
val Yp3 to Yp7 (54.6-51.6 Ma) of the sequence chart of Hardenbol et al. (1998).The upper part of the Ypresian succession (Fig. 3.6a) shows a gradual increase in mean grain size, which changes from slightly silty to very silty clay (wells K06-01 and K12-01 in the North). In well S05-01 in the South, a distinct sand layer is deposited at the base of the silty interval. The grain size increase in this part of the succession reflects a lowering of the local sea level. In well L02-04, relative fine grain sizes during the interval point to a higher local sea level, although the interval in this well also shows a gradual increase in mean grain size.Only in well L13-01, the gamma ray log trend indicates a continuous local sea level rise in the up-per part of the Ypresian succession (Fig. 3.6a). The local transgression indicates the formation of a local depression, as a result of tectonic activity during the Late Ypresian. This Ypresian tectonic pulse is the first of a series of short tectonic pulses, which finally culminate in the main Pyrenean tectonic pulse. The Ypresian tectonic pulse will be discussed in detail in Chapter 4. The subsequent pulses have a different character, and are discussed below.The sequence stratigraphic interpretation of the Upper Ypresian succession is not straightforward. Although generally the construction of a sequence stratigraphic framework works very well, with-in individual sedimentary cycles local deviations in grain size trends can be observed. In wells K06-01 and K12-01, the interval is interpreted as a Lowstand Systems Tract, which is suggested by gamma ray values indicating relatively coarse sediments. In well L02-04, the occurrence of much finer sediments, accompanied by a coarsening upward response of the gamma ray log, sug-gests that the interval comprises one or two Highstand Systems Tracts (Fig. 3.6a). This suggests that these sedimentary cycles are not exactly time equivalent.According to the biostratigraphic report of well K12-01 (Figs. 3.6a, 3.8), the upper interval of the Ypresian succession spans Dutch North Sea Plankton Zones FHc and Fhb, which would indicate a Middle to Late Eocene age (Fig. 3.2). However, the report mentions that the boundary between FH and the FI zones has been drawn rather arbitrarily in this well. A few benthic foraminiferal species considered characteristic for the lower part of the FH zone and the FI zone are found higher in the well, in samples interpreted to belong to the FHb subzone. Therefore, these samples might be of Early Eocene age, too. These ambiguous results make these biostratigraphic data inadequate for our research goals. Correlation of well K06-01 to the Belgian sequence stratigraphic framework (Fig. 3.8), based on local sea level trends inferred from grain size analysis, indicates that the upper part of the Ieper Member covers the interval Yp7 and Yp8, which is a period of long-term eustatic sea level fall.
3.2.3 Brussels members (Lutetian)
The Lutetian Brussels Sand Member was deposited in the South of the study area. It is a non-cal-careous, silty to sandy clay with local intercalations of siltstone or very fine sandstone beds. To the North, the member grades into calcareous clays of its more distal equivalent, the Brussels Marl Member. The Brussels members are coarser grained than the silty top of the Ieper Member (Fig. 3.6a). In a wide area, the Brussels members were partly or completely eroded in response to the Pyrenean inversion (Fig. 2.5b). A complete succession of the Brussels Members is found only in small parts of the study area. This is indicated by the occurrence of the Asse Member, which was deposited concordantly on top of the Brussels members (Fig. 2.5c).The log signature of the Brussels Members is less homogenous than the log signature of the Landen Formation and the Ieper Member. This reflects a more dynamic environment during deposition of
58
TS
T
TS
T
TS
T
HS
T
HS
T
HS
T
HS
T
HS
T
TS
T
TS
T
HS
T
HS
T
TS
T
TS
T
HS
T
LST
HS
T
HS
T
HS
T
LST
LSW
LSW
HS
TT
ST
TS
T
TS TS
020
4060
8020
016
012
080
40
GR
(A
PI)
DT
(m
icro
sec/
ft)
1000
1350
1300
1150
1250
1200
1100
1050
1500
1450
1400
950
Ieper Mb.
Land
en
Fm
.
BrusselMarl Mb.
Ass
e M
b.
K06
-01
(1)
Depth (m)K
nokk
e
150
200
250
100
0
2 13456 100
GR
(A
PI)
50
2.4
2.5
2.6
2.7
2.8
2.9
TA 2
Depth (m)
Haq
et a
l. (1
987)
Eus
tatic
sea
leve
l and
seq
uenc
esH
arde
nbol
et a
l. (1
998)
HS
T
FJFI
FH
c
FH
b
FH
a ?
Bio
stra
tigra
phy
wel
l K12
-01
Top W.Asteria Zone*NP13 NP11
0
8um
2um
100%
Van
denb
ergh
e et
al.
(199
8)
grain size
Biostr.Y
p 1
(54.
9)
Yp
3 (5
4.6)
Yp
4 (5
3.6)
Yp
5 (5
3.1)
Yp
6 (5
2.1)
Yp
7 (5
1.6)
Yp
10 (
50.0
)
Yp
2 (5
4.8)
200
150
100
Yp
8 (5
1.0)
sequ
ence
boun
darie
s(m
a)
rela
tive
sea
leve
l (m
)
rise
fall
sequ
en-
ces
Sequence stratigraphic interpretation of log correlations
Chapter 3
59
these shallow marine sediments.Wells K06-01 and L02-04 are located far away from faults that were active during the Palaeocene to Oligocene (cf. Fig 2.4, 3.3). These wells contain a tectonically undisturbed Middle to Late Eocene sequence, which reflects the eustatic sea level history (Fig. 3.7a). Wells K07-02, K12-01 and L13-01 are located close to the margin of the Mesozoic Broad Fourteens Basin. The Brussels Marl Member sediments in wells K12-01 and L13-01 were affected by post-depositional erosion, associated with the Pyrenean tectonic phase. Correlation of the sonic logs of wells K12-01 and L13-01 with wells K06-01 and L02-04 suggests that only a small part of the Brussels Marl Mem-ber was eroded (Fig. 3.7a).The Brussels Marl Member North of the Mesozoic Broad Fourteens Basin can be divided into three sedimentary sequences (Fig. 3.7a). The Brussels Marl Member contains a transgressive low-er unit, which is characterized by a fining upward trend. The upper boundary of the sequence is a maximum flooding surface, indicated by a gamma ray maximum, often followed by a change in the sonic log signature. The middle sequence shows an aggrading to slightly fining upward trend in wells K06-01 and L02-04 (possibly a Transgressive Systems tract), and an aggrading to slightly coarsening upward trend in well K07-02 (possibly a Highstand Systems Tract). In wells K12-01 and L13-01, closer to the Broad Fourteens Basin, the unit shows a coarsening upward trend (High-stand Systems Tract). The uppermost sequence of the Brussels Marl Member in wells K06-01 and L02-04 displays lower gamma ray values than the underlying sequence. This indicates relatively coarse grain sizes. In wells K07-02, K12-01 and L13-01, the gamma ray values indicate that the uppermost sequence starts with a grain size maximum. The gamma ray response indicates increas-ing grain sizes in the northernmost wells (K06-01 and L02-04), and fining upward grain size trends in the wells closer to the Mesozoic Broad Fourteens Basin. South of the Broad Fourteens Basin (S02-01, S02-02, S05-01), the Brussels Sand Member shows a coarsening upward trend (Fig. 3.7b). These opposing trends are examples of the deviations, which occur within individual cycles.Biostratigraphic results for well K12-01 (Fig. 3.6a) indicate that the Brussels Marl Member spans Dutch North Sea Plankton Zone FH (Eocene, Fig. 3.2). The Brussels Sand Member spans nan-noplankton zones NP13-15 (van Adrichem Boogaert and Kouwe, 1997), and therefore eustatic sequences Yp9-Lu2 (Vandenberghe et al., 2004). The sea level curve of Hardenbol et al. (1998) indicates that the fluctuations in eustatic sea level were much larger during deposition of the Brus-sels members, than during deposition of the Ieper Member (Fig. 2.1). The base of sequence Yp10 is marked by an abrupt fall in eustatic sea level, which possibly resulted in the end of open marine deposition of the Ieper Member clays. This sea level drop was followed by two transgressive in-tervals, Yp10 and Lu1. During deposition of sequence Lu2, eustatic sea level was close to the high
Fig. 3.8 (opposite page)
Sequence stratigraphic correlation between the gamma ray and sonic logs of well K06-01 in the Dutch North
Sea and the gamma ray log of well Knokke in Belgium (redrawn after Vandenberghe et al. (1998), their fig. 7).
The position of the wells is indicated in Fig. 3.3a. Vandenberghe et al. compared the sequence boundaries 1-6
(encircled) of the Ypresian deposits of Belgium to the standard sequence chart of Haq et al. (1988), which was
recalibrated in the same volume (Hardenbol et al., 1998). The interval numbered (1) in well K06-01 is the Basal
Dongen Tuffite Member. Biostratigraphic results of well Knokke are shown. Correlation to other Belgian wells
increased the accuracy of these results (Vandenberghe et al., 1998).
The * in the biostratigraphic results from well Knokke indicates the first major planktonic influx of foraminifers
and nannofossils into the basin, base biochron C24BN.
60
level of Lu1, although it slowly decreased. The eustatic sea level chart correlates well with the sequences which were interpreted in wells K06-01 and L02-04 (Fig. 3.7a), which are thought to be unaffected by tectonic activity and to reflect the eustatic sea level.
3.2.4 Asse Member (Bartonian)
In the central part of the study area, the Asse Member was completely removed by Eocene-Oli-gocene erosion. The unit is only present in the far North and South of the study area (Fig. 2.5c). The top of the member is a regional erosional unconformity. As the available data is limited, a detailed correlation of the Asse Member in the study area to the sequence chart of Hardenbol et al. (1998) is tentative. The gamma ray log response of the Asse Member (in the North in wells L02-04, K06-01 and K07-02, Fig. 3.7a, in the South S02-01, S02-02 and S05-01, Fig. 3.7b) shows a coars-ening upward megatrend. Exception is well K07-02, which shows overall fining upward grain sizes (Fig. 3.7a). The sediments of the Asse Member can tentatively be divided into two, possibly three sequences (Fig. 3.7b). In the South (wells S02-02 and S05-01), the lower sequence of the
Lu1
+ _
South North
BFB
tectonicuplift?
possible winnowingof sediments
b)
cu a fucu
BFB = Area overlying the Mesozoic Broad Fourteens basin
fucu
Schematic grain size
Schematic log response
_?
BFB
Yp10
Yp10
a)
Lu1
BFBYp10
Lu2?
Fig. 3.9
a) Interpretation of the
sequence architecture of
Lutetian sequence Lu1
in the Brussels Sand and
Marl Member (Dongen
Fm.).
b) Interpretation of the
sequence architecture of
Lutetian sequence Lu2
in the Brussels Marl
Member (Dongen Fm.).
Sequence stratigraphic interpretation of log correlations
Chapter 3
61
Asse member starts with a fining upward interval. This transgressive sequence is very thin close to the southern margin of the Broad Fourteens Basin. The transgressive sequence is followed by a (coarsening upward) highstand systems tract. The second sequence shows a fining upward trend (Fig. 3.7b). Biostratigraphic control of the member is poor, but the interval is thought to cover nannoplankton zones NP16 and NP17 (Van Adrichem Boogaert and Kouwe, 1997). This would suggest that the member covers the eustatic sequences Lu3, Lu4 and Bart1. The long-term eustatic sea level movements during deposition of sequences Lu3, Lu4 and Bart1 are regressive (Fig. 2.1). This long-term trend seems to be reflected by the succession. 3.3 Late Eocene tectonic uplift
The onset of tectonic activity associated with the Pyrenean phase was dated Late Eocene (Letsch and Sissingh, 1983; Van Wijhe, 1987a; 1987b). This dating was probably based on the age of the youngest sediments (Asse Member) below the regional erosional hiatus resulting from the uplift. It shows a maximum age of 37.0 Ma (coinciding with the base of the Priabonian), based on an esti-mated age of 43.4-37.0 Ma of the Asse Member (biozone NP16 and possibly NP17, Van Adrichem Boogaert and Kouwe, 1997). No other pulses of Middle to Late Eocene tectonic activity have been reported from the study area.
4. Discussion
The sequence stratigraphic interpretation of the well log correlations suggests that tectonic activity could have started at the beginning of the Lutetian. The northernmost wells (K06-01 and L02-04) were not affected by erosion and uplift during deposition of the Lutetian Brussels Marl Member. The internal grain size variations of the Brussels Marl Member in these wells closely reflect the eustatic sea level curve (Fig. 3.7a). In contrast, in all other analysed wells, the Brussels members display grain size trends, which deviate from those of the two northernmost wells. The occurrence of these local variations in grain size is interpreted to be caused by local tectonic activity during deposition of the Brussels and Asse members (Fig. 3.7). Alternatively, the grain size variations might indicate that these sequences are diachronous. There is, however, no indication of erosion or non-deposition within the succession.Moving from the North towards the Broad Fourteens Basin area, the grain size trend of the second sequence (Lu1) of the Brussels Member changes progressively from a fining upward, to constant, to a coarsening upward trend (Figs. 3.7a, 3.9a). Additionally, the wells in the South of the study area (S02-01, S02-02 and S05-01) display a distinct regressive, coarsening upward grain size trend in the Brussels Sand Member. This suggests a local relative sea level drop, and a reduction in ac-commodation space, during deposition of the second sequence (Lu1) of the Brussels Marl Member in the wells closest to the margin of the Mesozoic Broad Fourteens Basin (K12-01, L13-01 and the wells in the South). Because the eustatic sea level curve (Hardenbol et al., 1998) and the north-ernmost wells do not indicate a eustasy-controlled regression during this period, but a significant sea level rise, the regressive signal in the other wells is probably the result of tectonic forcing. The tectonic activity possibly occurred in the form of uplift of the centre of the study area, possibly re-stricted to a small area lying over the Mesozoic Broad Fourteens Basin only (Fig. 3.9a). The uplift could have been fault-bounded or accommodated by flexure. The uplift resulted in a reduction of the local accommodation space and therefore in a regressive signal in the wells close to the centre
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of the study area. Associated winnowing or erosion of the unconsolidated Middle to Upper Eocene sediments in the centre of the uplifted area would also result in a regressive signal (Fig. 3.9a).The third sequence (Lu2) of the Brussels Member is regressive in wells K06-01 and L02-04, which is conform the eustatic sea level trend (Fig. 3.7a). In contrast, sequence Lu2 is transgressive in the wells closer to the northern margin of the Mesozoic Broad Fourteens Basin area, which points at a local increase in relative sea level (Figs. 3.7a, 3.9b). This might be the result of formation of a lo-cal depression after the previous phase of compression, due to the relaxation effect discussed in the previous chapter (Fig. 3.9b). The variability in sequence architecture of the remaining sediments of the Bartonian Asse Member suggests that the tectonic activity continued during the deposition of the Asse Member.The tectonic activity during the Lutetian and Bartonian demonstrates that the onset of the Pyrenean tectonic pulse in the southern Dutch North Sea was preceded by other episodes of small-scale tectonic activity. The tectonic pulses in the Dutch offshore are time-equivalent to Lutetian and Bartonian phases of uplift of the Brabant and Artois Blocks in Belgium, which have been reported by Vandenberghe et al. (2004). This would suggest that the inferred tectonic activity is of regional importance.The Lutetian-Bartonian tectonic pulses in the southern Dutch North Sea are only perceptible as de-viations of the local relative sea level from the eustatic sea level curve. The tectonic activity caused local changes in subsidence rates and possibly even uplift. The local sea level deviations are re-flected in the Middle to Upper Eocene sedimentary succession, such as observed for the Lutetian Brussels members. Local tectonic overprinting of eustatic sequences started at the beginning of the Lutetian (Lu1, circa 48 Ma), which is much earlier than the previously assumed first period of Late Eocene tectonic activity, which started during the Early Priabonian (circa 37 Ma) with the onset of the Pyrenean phase. Even before the Lutetian, during the Late Ypresian, tectonic activity occurred in the area. This period of tectonic activity is, however, of a different character. It will be discussed in Chapter 4. 5. Conclusions
Sequence stratigraphic interpretation based on well logs provides a high-resolution correlation of the siliciclastic sediments of the Palaeogene in the southern Dutch North Sea. Although such inter-pretations have to be applied with proper care, they appear to be a fairly reliable tool to discrimi-nate between eustatic sea level variations and local tectonic movements and to date the latter.The Late Eocene tectonic activity started much earlier than previously assumed. Previously, it was thought that the area was tectonically quiet until the onset of the Pyrenean phase, which started during the Early Priabonian (circa 37 Ma). However, based on the present study, it can be con-cluded that tectonic activity in the study area already started during the Early Middle Eocene (be-ginning of the Lutetian, circa 48 Ma). Moreover, there are indications that tectonic activity started even earlier, during the late Ypresian. The episode of tectonic activity can be correlated to tectonic uplift in the Brabant and Artois Blocks in Belgium, which suggests that it might be of regional significance.The results shown in this Chapter are an illustration of the occurrence of small-scale tectonic activ-ity during periods, which were previously thought to be tectonically calm. In Chapter 4, the Ypre-sian episode of tectonism preceding the Lutetian, which is also such an example of small-scale tectonism, is elaborated on.
Sequence stratigraphic interpretation of log correlations
