Journal of Earth Science, Vol. 24, No. 6, p. 903–917, December 2013
Printed in China
DOI: 10.1007/s12583-013-0383-5
ISSN 1674-487X
Salt Tectonics and Basin Evolution in the Gabon
Coastal Basin, West Africa
Anqing Chen (陈安清)
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
Chong Jin* (金宠)
Zhejiang Institute of Geology and Mineral Rescources, Hangzhou 310007, China
Zhanghua Lou (楼章华)
Ocean College, Zhejiang University, Hangzhou 310058, China
Hongde Chen (陈洪德), Shenglin Xu (徐胜林), Keke Huang (黄可可), Sihan Hu (胡思涵)
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
ABSTRACT: The Gabon Coastal Basin is a typical saliferous basin located in the middle portion of the
West African passive continental margin. Complex salt tectonics make sedimentary sequences and
structural frameworks difficult to interpret and can lead to difficulties in construction of balanced
cross-sections and reconstruction of basin evolutionary processes. Sedimentary facies and salt structural patterns displaying zonation are based on seismic reflection profiles and drilling data. Two
near-vertical fault systems, NW-SE and NE-SW, caused basin to be subdivided E-W zoning and N-S
partitioning. Scarp slopes and extension faults formed in the Hinge belt III zone where salt diapir
piercement occurred and numbers of salt pillars, salt stocks and salt rollers developed under transtension of coupled near-orthogonal fault systems. The zone east of Hinge belt III is characterized by
small-scale salt domes and salt pillows. To the west are large-scale salt walls and salt bulge anticlines
caused by diapirism promoted by tension and torsion that also resulted in formation of numerous salt
pillars, salt stocks and salt rollers. Our modeling of salt tectonic structures indicates that they were
produced by plastic rheological deformation of salt under regional stress fields that varied during three
distinct phases of extension, compression and re-activation. Hinge belt III was active from Coniacian to
Early Eocene, which was a critical period of formation of salt structures when many extension-related
salt structures formed and salt diapirism controlled the distribution of turbidite fans. Rootless
extrusion-related salt stocks developed throughout the Late Eocene to Early Oligocene as a result of local ephemeral low-intensity tectonic inversion. Post Oligocene salt diapirism was weak and salt tectonics had a weak influence on sedimentation. Balanced cross-sections of two saliferous horizons crossing
different tectonic units from east to west reveal
This study was supported by the National Natural Science
that the basin tectonic evolution and sediment
Fundation of China (Nos. 40839902 and 40739901).
filling processes can be divided into three stages
*Corresponding author: [email protected]
containing seven episodes of rifting, transition
© China University of Geosciences and Springer-Verlag Berlin
and drifting.
Heidelberg 2013
KEY WORDS: salt tectonics, basin evolution,
Gabon Coastal Basin, passive margin, Africa,
Manuscript received February 11, 2013.
Manuscript accepted May 21, 2013.
South Atlantic Ocean.
904
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
INTRODUCTION
The Gabon Coastal Basin lies within the passive
continental margin of West Africa between north latitude 1° and south latitude 4°, bounded to the east by
basement and to the west by the 200 m isobath in the
Atlantic Ocean. The total basin area is about 128 376
km2 (Lin et al., 2010; Liu J P et al., 2008; Guan and Li,
2007; Xiong et al., 2005; Tong and Guan, 2002).
Since the first discovery of hydrocarbon in 1951, a
number of other significant oil fields such as the Anguille marine oil field, the Rabi-Kounga onshore oil &
gas field, and the Olowi marine gas field have been
discovered. The Gabon Coastal Basin is a typical saliferous basin similar to passive continental margin basins in the Persian Gulf, gulf of Mexico, North Sea,
the coast of the Caspian Sea and elsewhere around the
South Atlantic where salt tectonics are important for
hydrocarbon accumulation. The salt formations and
their overlying strata have become deformed by the
buoyancy of salt, differences in load, gravity, thermal
convection, compression and extension to form complex salt tectonic structures. Salt diapirism and its related fractures provide channels and a driving force
for hydrocarbon migration, and abundant traps and
caps for hydrocarbon accumulation (Liu Y L et al.,
2008; Brink, 1974). The irregular shape and density
inhomogeneity of Aptian evaporites combined with
the plastic rheology of halite lead to poor quality
seismic imaging of subsalt formations due to shielding
and interference effects (Hudec and Jackson, 2007;
Tang et al., 2005; Fort et al., 2004; Jia et al., 2003;
Jackson et al., 2000; Jackson and Roberts, 1993; Nalpas and Brun, 1993; Jackson and Talbot, 1991), which
restricts accurate interpretations of sedimentary sequences and structural features, and thus full understanding of basin evolution.
There has been a boom in oil and gas exploration
in passive continental margin basins on both sides of
the South Atlantic in recent years, leadin to extensive
seismic exploration and drilling, and research results
have begun to be published (He et al., 2011; Liu et al.,
2011; Ma Z Z et al., 2011; Sun et al., 2010; Li and
Guo, 2008; Ma J et al., 2009; Dupr et al., 2007; Dickson et al., 2003; Moungueneui et al., 2002; Robert and
Yapaudjian, 1990; Teisserenc and Villemin, 1989).
These studies describe salt tectonics in basins includ ing the Gabon Coastal Basin, dividing them into extension zones and compression zones but this is an
over-simplification of complex salt tectonics and evolution over different periods and in different tectonic units
(Liu and Li, 2011; Ding et al., 2009; Liro and Coen,
1995). Published field observations, drilling,
three-dimensional seismic interpretations, balanced
cross-section constructions, physical modeling, numerical simulations and so on have established a variety of
evolutionary models of salt tectonics for studying plastic rheological behavior of salt and salt tectonics. From
among them evolutionary models involving stretching
and shortening tectonics explain the salt tectonic evolution of the Gabon Coastal Basin well. This article analyzes the basin’s structural characteristics, salt structural styles and sedimentary facies by using seismic reflection profiles and drill logging data to establish a salt
tectonic evolutionary model, distinguish tectonic evolutionary phases, establish the main deformation time and
deformation range, and reveal the sedimentary filling
processes and basin tectonic evolution.
GEOLOGICAL SETTING
The African Plate became the core of Gondwanaland after Late Precambrian Pan-African earth
movements and remained a stable tectonic setting for a
long time until the Gondwana Continent was broken up
by the impact of Mesozoic plumes, that caused fragmentation into the African Plate, the Americas, India,
Australia and Antarctica. The formation and evolution
of coastal basins in West Africa was caused by rifting
between the African and American continents associated with the Fla plume and Tristan plume (Xiong et
al., 2010; Burke et al., 2003; Dalziel et al., 2000; Uchupi, 1989) (Fig. 1). The older northern Fla plume caused
the opening of the North Atlantic from north to south in
Late Triassic–Early Jurassic times, while the younger
southern Tristan plume caused north to south rifting of
the South Atlantic in early Early Cretaceous times. The
equatorial segment of the Atlantic Ocean began to rift
apart in the Albian Stage of Early Cretaceous times and
ultimately led to complete separation of Africa and
South America.
A series of rift basins were generated at the onset
of rifting between the African Plate and South American Plate and transformed into typical passive conti-
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
North America Euriasian Plate
Plate
N
Thetys
Afar 31 Ma
at
an
(T3)
Fla Camp 201 Ma
Yu
c
(K1)
Tristan 131 Ma
Deccan 65 Ma
0
2 000 km
Rajmaha 118 Ma
Marion 90 Ma
Kerguelen? 110 Ma?
Figure 1. Gondwana breakup and plumes position
in Mesozoic.
nental marginal basins when the Atlantic Ocean began
to open during the Late Cretaceous. The Gabon
Coastal Basin began to develop on the east coast of
the South Atlantic in the Berriasian Stage of the Early
Cretaceous and experienced three evolutionary stages
(Liu J P et al., 2008; Xiong et al., 2005; Teisserenc
and Villemin, 1989): a rift phase, a transition phase
and a passive continental margin stage. The rift stage
at the beginning of the Early Cretaceous (140–125 Ma)
initiated splitting from the South American Continent
and resulted in the formation of the Gabon Rift Basin
which was filled by river, delta, and lacustrine depositional systems in Early Aptian times. The basin entered a peneplanation stage marking the end of the rift
stage. In the transition stage in the Late Aptian
(125–116 Ma), separation of the African Plate and the
South American Plate caused obvious crustal subsidence to form narrow sea lanes connected with the
original South Atlantic Ocean. In Gabon, Gamba
sandstone and Ezanga evaporate filled the basin unconformably above the rift sequence. At the passive
continental margin stage or drift phase (116 Ma to the
present), the ocean basin between the African Plate
and the South American Plate widened, and led to
rapid subsidence of the west side of the Gabon Basin,
which filled by the Albian Madiela Formation (Fm),
the Cap Lopez Formation, the Azile Formation, the
Anguille Formation, the Point Clairette Formation,
and the Upper Cretaceous and Cenozoic Batanga
Formation (Table 1).
905
GEOLOGICAL STRUCTURES
The Gabon Coastal Basin has double-layered
basement architecture floored by a Precambrian crystalline basement and a Pre-Cretaceous folded basement. Post-Cretaceous sedimentary cover is mainly
composed of sedimentary strata reaching a maximum
deposition thickness of about 15 000 m of which the
Cretaceous sedimentary thickness is 6 000–10 000 m.
There are two sets of near-vertical fault systems of
with NNW-SSE strike and NE-SW strike which give
the basin an E-W directed zoned tectonic framework
containing N-S directed blocks. The basin may be
divided into four sub-units: Interior sub-basin, South
Gabon sub-basin, North Gabon sub-basin and Lambarene high (Fig. 2).
The NNW-SSE trending faults constitute the
major fault system in the basin, associated with plate
rifting and structural trends in the basin are basically
consistent with strikes of these faults. The tectonic
zonation in the basin is controlled by three tectonic
hinge zones formed by these fault systems from east
to west. (1) Hinge belt I formed at the end of the Late
Jurassic resulting in the Septem-Kama sag, the Vembo
Graben and the Interior sub-basin (Fig. 2) by downward faulting at the eastern boundary and overlapping
at the western boundary, which controlled deposition
in the Neocomian. (2) Hinge belt II developed in the
Aptian resulting in the northern Lambarene high and
the southern Gamba horst (Fig. 2), and the basin extended farther westward controlled by the fault which
laid the foundation of characteristic east to west zonation. (3) Hinge belt III (also known as the Atlantic
Hinge zone) developed from Late Cretaceous to Paleocene, the passive continental margin basin stage
(Fig. 2), and was related to a discordogenic fault as a
structural high caused by differential settlement of
basement of the Gabon Coastal Basin. The east side of
the belt is characterized by shallow-water platform
deposits, the west side by deep-water shelf deposits.
The NE-SW trending fault system belongs is a
set of transform faults (Fig. 2). Some of the larger
fault zones such as the northern Fang fracture zone
which forms the northern boundary fault of the Gabon
Coastal Basin, the middle Enkomi fracture zone that
forms the boundary of the North Gabon sub-basin and
906
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
Table 1
Stratigraphic divisions and basin evolution of the Gabon Coastal Basin
Stages
Formation names
Quaternary
Thicknesses
(m)
Akossa
Pliocene
Messinian
N’Tchengue
Lithology
Sandstone & mudstone
1 500
Tortonian
Neogene
Miocene
Oligocene
Paleogene
Eocene
Paleocene
Upper
Cretaceous
Cretaceous
Ewongue/Batanga
2 000
Campanian
Pointe Clairette
2 000
Santonian
Coniacian
Anguille
Turonian
Azile
Cenomanian
Albian
Cap lopez
Madiela
Ezanga
Gamba
Mandarove
Animba
800
Mudstone, interbedded sandstone & carbonate rock
Ozouri
50
Mudstone & sandstone
Ikando
800
Mudstone & sandstone
Barremian
Cardima
Hauterivian
Melania
Valanginian
Pre-Jurassic
1 100
2 300
1 500
1 600
700
1 500
N’DOMBO
Basement
the South Gabon sub-basin, and the Mayumba fracture
zone that forms southern boundary of the Gabon
Coastal Basin, extend into the oceanic crust to join
transform faults that offset the South Atlantic ridge.
These large transform faults divide the basin into
blocks from north to south.
SALT TECTONICS
Characteristics of Salt Tectonics
Salt rocks have plastic rheology and are likely to
undergo plastic flow during burial because of their
500
Lucina
Kissenda
Berriasian
Late drift
phase
3 000
Dentale
Lower
Cretaceous
Mudstone, interbedded sandstone
M’Begu
Serravallian
Langhian
Burdigalian
Aquitanian
Chattian
Rupelian
Prabonian
Bartonian
Lutelian
Ypresian
Thanetian
Selandian
Danian
Maastrichtian
Aptian
Evolutionary
phases
Sandstone & mudstone
Mudstone, interbedded
sandstone
sandstone, interbedded
mudstone
Mudstone, interbedded
sandstone & carbonate rock
Carbonate rock, sandstone,
mudstone
Evaporate
Sandstone
Sandstone, interbedded
mudstone
Sandstone & mudstone
Sandstone, interbedded
mudstone
Mudstone, interbedded
sandstone
Early drift
phase
Transition
phase
Rift phase
Sandstone
Granite, gneiss
Pre-rift phase
low density, weak compressive strength and small
modulus of elasticity. Seismic and drilling well data
reveal that flow deformation of the Ezanga Fm formed
a large number of salt tectonic structures, including
slightly-uplifted salt domes and several-kilometer uplifted salt diapirs such as salt domes, salt stocks, salt
rolls, salt pillars, salt anticlines, salt pillows and salt
walls and these have developed lots of salt-related
structural traps (Fig. 3). The types of salt tectonics
depend on the structural belt in the basin and are also
characteristics by zonation from east to west. Around
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
907
8o
10oE
120 km
0
Equatorial Guinea
or
eri
Int
sin
-ba
sub
Liberville
ene
bar
Lam
z
North Gabon
sub-basin
Fz
fa
S.
Daminze syncline
Port-Gentil
ure
Flex
Lambarene
Cabon
Fz
b
Ga
on
B
Atlantic
te
B’
Sette
high Cama
2o
Gabon
Coastal
on
Br
I
M t
KO men
’
N ea
lin
Dianago
trough
Western limit
Top
o
A
n
Ny
ho emb
rst
e
Ilo
of
nd
o
o
ng nt
Ka ame
e
lin
abe
ng
gF
A’
h
hig
N
n
. fa
Atin Salta
0o
ns
ce
As
Congo Craton
Gr
ion
Fz
Fz
Basin
Sette Cama
hig
h
Strik-slip
fault
Normal fault
Anticline
Syncline
4oS
Western limit of
Aptin salt
n
be
gra
a
gh
Volcanic
rock
Administrative
boundary
Basin or subbasin boundary
City
ra
Ve
mb
Ga
u
tro
le
nta
De
re
xu
fle
South Gabon
sub-basin
Mayumba
z
aF
b
um
ay
M
Figure 2. Fault systems and tectonic units division of the Gabon Coastal Basin. Fz. Fault.
908
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
Figure 3. Typical salt tectonics in the Gabon Coastal Basin. Azile. Azile Formation; Ang. Anguille Formation; L.P.C.. Lower Pointe Clairette Formation; U.P.C.. Upper Pointe Clairette Formation; P.G.. Port-Gentil
Formation
Hinge belt III transtension of the two sets of fault
systems and gravity slumping in a steep slope background gave rise to a tensional environment causing
salt piercement with salt stocks, salt rollers and salt
pillars. The east flank of Hinge belt III developed
small-scale salt tectonics such as salt domes and salt
pillows and the west flank of Hinge belt III is characterized by large-scale non-pierced low-amplitude salt
anticlines.
Salt tectonic styles can be divided into two types
according to differences in the stress field: extensionrelated salt tectonics and extrusion-related salt tectonics. Seismic interpretation shows that extensional
salt tectonics is the main style of the Gabon Coastal
Basin (Figs. 3a, 3b, 3c). Later compression has had a
weak influence on salt tectonics and salt anticlines
and rootless salt stocks can be found locally (Fig. 3d).
Whether or not there has been significant displacement, salt tectonics can be divided into autochthonous and allochthonous types. The main salt tec tonic type in the study area is autochthonous salt diapirism characterized by continuous distribution,
contrasted with large-scale salt decollement thrusting
and salt window structure formed by allochthonous
salt rafting within the Lower Congo-Congo Fan Basin
(Liu et al., 2011; Jackson et al., 2008; Li and Guo,
2008; Scotese et al., 1999; Teisserenc and Villemin,
1989).
The relationship between deposition and the diapiric uplift rates in the Gabon Coastal Basin shows
that deposition and salt diapirism occurred at the same
time. The mainly formation period of the salt tectonics
was Upper Cretaceous to Oligocene when the rate of
diapric uplift was equal or slightly larger than the rate
of deposition (Fig. 4). The rise of salt pillars and salt
walls affected sedimentation and led to sand deposition around salt structures. Thus salt occlusion traps
and small-scale turtleback anticline salt tectonic
structures were the main traps that formed small-scale
reservoirs (Figs. 3a, 3d). In other areas rates of salt
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
909
Figure 4. Salt tectonic structures with different rates of diapiric uplift compared with deposition rates (Left
figure from Hudec and Jackson, 2007).
diapir tectonics were slower than trates of deposition,
so influence of salt tectonics on deposition was relatively weak and coated-continuous sand bodies
formed on the top of and around salt anticlines,causing low-amplitude salt arches and salt roller
anticlines as the main traps which formed large-scale
reservoirs (Figs. 3b, 3c). Short-term tectonic inversion
of the Gabon Coastal Basin in the Late Eocene caused
a small strengthening effect on former salt tectonics
when salt tectonic activity became weak.
Two Theoretical Models of Salt Tectonic Evolution
in Different Tectonic Settings
Diapir piercement under regional extension
Extension thins and fractures the overburden, establishing a lateral load gradient and weakening the
overburden (Hudec and Jackson, 2007; Vendeville
and Jackson, 1992). Salt undergoes reactive diapirism,
rising up the axis of dismembering graben to fill the
space created by thinning of sediment and separation
of fault blocks. Then the thin and weak salt roof can
be uplifted and shouldered aside by salt buoyancy because salt density is less than that of the overburden.
This phase is termed “active diapirism”. If an active
diapir breaks through the roof and rises up to the sediment surface, it will result in passive diapirs emerging as salt glaciers.
Extensional tectonic settings almost always occur
in rift basins and passive margin basins and extensional salt tectonics is most common in the
above-mentioned two types of basin. Salt tectonics
developed in the passive margin phase of the Gabon
Coastal Basin, and extension caused detachment along
the basement. The extensional salt tectonics model
shown in Fig. 5 can be used to interpret the formation
mechanism of Pre-Oligocene salt tectonics. The main
control on extensional structural styles is salt thickness because of absence of precursor diapirs. Thin salt
caused local detachment in the eastern basin but could
not form diapirs, but thicker salt diapirs and adjacent
910
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
withdrawal basins grew larger in the midwest basin.
Some salt diapirs progressed completely through the
reactive and active stages to become passive diapirs.
Diapir amplification under regional shortening
Shortening leads to buckling of the overburden of
the salt. Ductile salt flows into the lower-pressure core
of a rising anticline and creates a salt-cored anticline,
which can give rise to secondary salt structures, amplification of pre-existing structures commonly occurring above preexisting salt structures (Vendeville and
Nilsen, 1995). The majority of the driving force
comes from tectonic pressure on the salt with
buoyancy playing a minor role in this type of diapirism. The structures of passive diapirs that form under
Extension with no
precursor structures
Passive diapir
shortening contrast with those those formed under
extension. One common shortening salt structure is a
teardrop diapir in which the upper part becomes
largely detached from its source layer, and continued
shortening of a teardrop diapir may reactivate the
weld as a thrust fault (Hudec and Jackson, 2007).
Lateral shortening frequently happened in the
Oligocene inversion tectonic phase of the Gabon
Coastal Basin. Shortening thickens and therefore
strengthens the overburden above the salt, which retards the formation of new diapirs unless anticlines in
the fold belt become deeply eroded (Hudec and Jackson, 2007). The shortening salt tectonic model shown
in Fig. 6 can be used to interpret the mechanism of
Oligocene salt tectonics because there have been tens
Thin salt
Thick salt
Faults become younger
toward center of graben
Buried graben from
early eactive phase
Reactive diapir
Growth fault
Breakaway fault
system
Salt roller
Figure 5. Schematic progressive model of salt tectonics during regional extension, constructed using Geosec-2D (Hudec and Jackson, 2007).
Shortening with
percursor salt diapirs
Salt roller
Teardrop diapir
Salt sheet
Diapir stem
reactivated
as thrust
Vertical weld at
pinched-off stem
Inverted roller
Steepened flank
Oad3608x
Figure 6. Schematic progressive models of salt tectonics during regional shortening, constructed using
Geosec-2D (Hudec and Jackson, 2007).
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
of kilometers of shortening during the inversion phase
that reactivated preexisting diapirs and some shortening salt structures, especially teardrop diapirs.
A Salt Tectonic Evolutionary Model of the Gabon
Coastal Basin
Salt tectonics must be taken into account as a key
factor when constructing balanced cross-sections. It is
necessary to employ the correct salt tectonics evolution modl to accurately reveal the basin evolution
process when balancing cross sections.
We have derived an ideal evolutionary model of
salt tectonics in the Gabon Coastal Basin based on the
ideal evolution models (1) and (2) above, comprehensive analysis of structural features, regional dynamic
field, and the relationship between salt diapirism and
the deposition. We recognized five stages of salt tectonic evolution shown in Fig. 7.
(a) Salt deposition occurred during the Aptian
Stage in a basin transition phase. At this time the
tectonic stress field slowly changed from previous
tensile rifting to tectonic subsidence. The evaporation
rate was greater than the rate of sea water recharge
because of limited connectivity between basin and
ocean and a widespread thick deposit of evaporites,
the Ezanga Formation, was deposited.
(b) Salt burial began in Albian to Turonian times
when the basin was under a weak extensional tectonic
stress field. The east was uplifted and west subsided
slowly accompanied by weakly rheomorphism in the
salt sequence.
(c) The uplift of the eastern basin corresponding
(a) The salt deposit stage (evaporation)
to the active phase of Hinge belt III increased during
enhanced regional extension in Coniacian to Early
Eocene times. Transtensional faults formed by the two
sets of fault systems triggered a lot of salt diapir
piercement. Typical extrusion-related salt tectonics
developed in the extensional tectonic stress field, with
rates of diapirism usually greater than or approximately equal to the deposition rate and only occasionally less than the deposition rate (Fig. 4).
(d) The basin experienced a tectonic inversion
and formed a regional unconformity in the Late Eocene to Early Oligocene. The tectonic stress field became compressive, modifying previous salt tectonic
structures and forming extruded rootless salt stocks.
(e) The tectonic background has been stable from
Miocene to Present. The salt diapir piercement rate is
much smaller than the deposition rate and there has
been no conspicuous effect of salt tectonics on the
deposition of overburden sediment. There has been a
degree of compression and extension led by gravitational collapse in the western Gabon Coastal Basin.
TECTONIC EVOLUTION AND SEDIMENTARY
FILLING PROCESSES IN THE GABON
COASTAL BASIN
The above analysis of salt tectonics showed that
there have been diverse salt tectonic styles in the Gabon Coastal Basin at different times. Characteristic
salt tectonic deformation styles differ significantly
between the South Gabon sub-basin and North Gabon
sub-basin. The degree of salt tectonic evolution evolution is relatively low in the South Gabon sub-basin
(d) Diapir amplification during regional shortening
( no deposition)
(b) The early stage of salt burial
(weak salt rheomorphic deformation)
(e) The stage in stable tectonic background
(salt diapirism rate<< deposition rate)
(c) Diapir piercement during regional extension
(salt diapirism with deposition)
7
5
4
6
1
2
3
1. Salt; 2. overburden; 3. substratum; 4. direction of flow;
5. fault; 6. vertical stress; 7. horizontal stress
Figure 7. The modle of salt tectonics evolution of Gabon Coastal Basin.
911
912
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
North Gabon sub-basin
Ia. The initial taphrogeny episode in Neocomian
(hinge belt I activity period)
NTOUM
South Gabon sub-basin
Ia. The initial taphrogeny episode in Neocomian
( hinge belt I activity period )
NEE
0 km
MELANIA
KISSENDA
CARDITA
WEL LE
M
N’D
BC
NEE
0 km
2
4
CARDITA
2
6
4
Ib . The taphrogeny-depression episode in the Early Aptian
(steady bathygenesis, lake expansion)
Ib . The taphrogeny-depression episode in the Early Aptian
(steady bathygenesis, lake expansion)
0 km
BEGUIE
FOUROU MOUNDONGA
COM IOUE T
NTOUM
8
2
WELLE
CARDITA
MELANIA
KISSENDA
N’DMBC
DE
A
NT
0 km
2
LE
4
6
CARDITA
4
8
IIa . The peneplanation episode in the Middle Aptian
( hinge belt II activity period, parallel unconformity)
IIa . The peneplanation episode in the Middle Aptian
(hinge belt II activity period, parallel unconformity)
0 km
BEGUIE
FOUROU MOUNDONGA
WELLE
N’D
C
MB
2
CARDITA
D E N TA
MELANIA
KISSENDA
D E N TA
LE
4
LE D
0 km
2
E
E N TA L
4
6
CARDI TA
8
IIb . The depression episode in the Late Aptian
( salt deposited )
IIb . The depression episode in the Late Aptian
(salt deposited )
0 km
BEGUIE
FOUROU MOUNDONGA
C
WELLE
MB
N’D
GAMBA
CARDITA
OZO
-IK AN DO
URI
EZ AN GA
UP C
LP C
An g
Az
Z
CAP LOPE
A
MADIEL
GAMB
PG UPC
LP C
An g
Az
T
CO M IO UE
EZ AN GA
U
LP C
An g
Az
PEZ
CAP LO A
MADIEL
BEGUIE
FOUROU MOUNDONGA
C
MB
WELLE
N’D
N TO U M
2
SEN ONI EN
4
GAMB
MIO
UE
T
N TO U M
CAR DITA
AZILE
CAPLOPEZ
MADIELA
2
SENONIE N
GAM BACAR DITA
ALE
DENT
MELANIA
KISSENDA
MANDOROVE
N
EZA
UPC
LPC
Ang
Az
GA
RI DO
OU KAN
-I
PG UPC
L P Cg
An
Az
Z
LOPE
CAP IELA
MAD
M
CO
NTOUM
GAMBA
( the Hinge belt I I I )
A
Atlantic
North Gabon sub-basin
IO
2
ALE
DENT
4
6
CARDITA
8
IIIc . The late drifting episode in late Oligocene-nowadays
( extremely westward thermal subsidence )
WE-1
POWEM-1
0 km
T
UE
0 km
CAPLOPEZ
EZANGA
ALE
DENT
10
IIIc . The late drifting episode in Late Oligocene-nowadays
( extremely westward thermal subsidence )
OZ
4
6
CARDITA
AZILE
6
MBEGA
ALE
DENT
ALE
DENT
IIIb . The tectonic inversion episode in Late EoceneEarly Oligocene (angle disconformity formation )
4
ABRE-1
2
EZANGA
ALE
DENT
MELANIA
KISSENDA
0 km
CAPLOPEZ
8
A
EYVNM-1
MYIM-1 EYHYM-1 IE-2 IE-1WZ-2 ALNG-1
4
6
10
0 km
BEGUIE
FOUROU MOUNDONGA
C
WELLE
MB
N’D
AZILE
AZILE
CAPLOPEZ
MADIELA
GAMBA
CO
E
8
A
NDO
PG -IPCK A
UP C
LP C
Ang
Az
DE NTAL
E
E
IIIa . The early drifting episode in Albian-Lutetian
(hinge belt III activity period and salt diapirism)
0 km
IIIb . The tectonic inversion episode in Late Eocene-Early Oligocene
( angle disconformity formation )
RI
DE NTAL
CARDITA
6
U
OZO
DE NTAL
MELANIA
KISSENDA
4
IIIa . The early drifting episode in Albian-Lutetian
(hinge belt III activity period and salt diapirism)
0 km
2
2
BEGUIE
FOUROU MOUNDONGA
C
WELLE
MB
N’D
VE
DORO
MAN
N
E
I
ON
SEN
( the Hinge ( the Hinge
belt I )
belt I I )
6
KS-2bls
AZILE
CAPLOPEZ
MADIELA
2
4
Lambarene I n t e r i o r
sub-basin
high
TCTM-1
CAR
GAMBA
A
D I TMEL
ANI A
KISS END A
A’
B
ALE
DENT
OFAM-1
0 km
CAPLOPEZ
EZANGA
ALE
DENT
2
ALE
DENT
4
6
CARDITA
8
( the Hinge
( the Hinge belt I I I ) belt I I )
Dentale trough
OZITOU-1
OZICAL-1
AZILE
Sette Cama
high
( the Hinge belt I )
10
Dianago trough
B’
Figure 8. Progressive tectonic evolution of the Gabon Coastal Basin (sections locations in Fig. 1).
and the deformation intensity is relatively weak. In
contrast the degree of salt tectonic evolution is realtively strong in the Nouth Gabon sub-basin and its
deformation intensity is also relatively strong (Fig. 8).
We have constructed two E-W tectonic evolutionary
sections across the Nouth Gabon sub-basin and South
Gabon sub-basin respectively, applying the salt tectonic evolutionary model explained in the preceding
section to the saliferous strata (Fig. 8). We distinguish
seven evolutionary stages in these palinspastic sec tions by studying sedimentary filling characteristics
and referring to the three stages of background tectonism (① rift stage, ② transition stage, ③ passive continental margin stage).
Initial Neocomian Taphrogeny
The South American Plate and the African Plate
began to separate during the Neocomian Stage of the
Early Cretaceous, and consequent rifting and formed a
series of grabens along the present coast (Fig. 8-Ia).
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
Deposition in the Gabon Basin was limited to the
present Interior sub-basin and South Gabon sub-basin,
where the Kissenda Formation and Melania Formation
mainly formed in fairly deep to deep lake environments (Fig. 9a). Basins formed by strong rifting at this
early period of the basin formation (Hinge belt I activity period). Graben subsided rapidly and the rate of
accommodation increase was greater than the rate of
increase of provenance supply.
mainly from the African Continent. Lakes were
shallow and thus the delta deposition migrated
progressively into the central area (Fig. 9b) . Rifting
weakened and the previous taphrogeny transformed to
taphrogeny-depression in this expansionary evolution
stage of lake basins.
Middle Aptian Peneplanation
Rifting terminated in the Middle Aptian when the
tensional stress field changed to compression and
uplift, which led to tectonic inversion of the basin,
causing erosion and peneplanation during a short-term
exposure period, resulting in a parallel unconformity
between the Dentale Formation and the Gamba
Formation (Fig. 8-IIa).
Early Aptian Taphrogeny and Depression
The Early Aptian was a tectonically active stage
in Hinge belt II when episode II rifting occurred and
the basin expanded westward to its present basin
boundary (Fig. 8-Ib). The basin was widely filled by
continental sandstone of the Dentale Formation that
formed in river or delta environments, provenance
10 oE
9o
60 km
(b)
1o
(c)
0
Lambarene
60 km
1o
60 km
(f)
3 oS
0
0
(d)
Land
Semideepdeep lake
Coastshallow lake
Salt lake
centre
Rivers-delta
Coast-shallow
marine
Actic region
Bathyal milieu
Shallowbathyal milieu
Marine delta
Turbidite fan
Facies
boundary
Schematic
channels
Uncertainty
boundary
2o
1o
2o
Cette Cama
Cette Cama
3 oS
0
10 oE
Lambarene
Cette Cama
(e)
Cette Cama
Libreville
0
0
1o
2o
60 km
9o
Libreville
Lambarene
60 km
0
10 oE
9o
Libreville
0
2o
1o
0
3 oS
(a)
10 oE
9o
Cette Cama
Cette Cama
3 oS
60 km
Lambarene
Lambarene
2o
1o
2o
1o
2o
3 oS
Cette Cama
0
Libreville
0
0
0
Lambarene
Lambarene
3 oS
Libreville
0
Libreville
Libreville
10 oE
9o
10 oE
9o
3 oS
10 oE
9o
913
60 km
(g)
Present rivers
or coastline
Salt lake
Provenance
direction
City
Figure 9. Paleogeographic lithofacies maps of the Gabon Coastal Basin. (a) Cretaceous Neocomian Stage;
(b) Early Aptian (Dentale Formation); (c) Later Aptian (Ezanga Formation); (d) Cenomanian–Turonian, (e)
Coniacian–Santonian; (f) Late Cretaceous Campanian Stage–Paleocene; (g) Miocene.
914
Anqing Chen, Chong Jin, Zhanghua Lou, Hongde Chen, Shenglin Xu, Keke Huang and Sihan Hu
Late Aptian Depression
The Aptian was a transition stage between
continental basin and marine basin deposition. The
basement subsided strongly and the Northern
Equatorial Atlantic had yet to open and a
southward-opening narrow sea between the African
Plate and the South American Plate. Transgressive
facies of the Gamba sandstone and the Vembo shale
were deposited in the South Gabon sub-basin,
unconformably overlying rift strata (Fig. 8-IIb). Restricted basins between Cameroon and Angola
connected with the original southern Atlantic Ocean
but were obstructed by the Whale ridge near the
southern boundary of the Namibia Basin. An alternative interpretation is that deposition occurred in an
arid tropical climate (Ma et al., 2011; Teisserenc and
Villemin, 1989). A dual mechanism of intensive
evaporation and intermittent injection of salt
seawater turned the basin turned into a lagoonal
environment and deposited the Ezanga evaporite to a
maximum thickness of 800 m (Fig. 9c).
Albian–Lutetian Drifting
The African Plate and South American Plate
separated completely towards the end of the Early
Cretaceous when the initial oceanic crust of the
South Atlantic formed and spread continuously into a
major ocean basin. During the early drifting process
of the two plates, the western Gabon passive
continental margin subsided rapidly. NE-SW
trending faults and NNW-SSE trending basement
faults formed in the right transtensional stress field
and NNW-SSE trending normal faults and S-N
trending induced transtensional faults developed
above the salt sequence and combined to form a fault
scarp, Hinge belt III.
The unstable tectonic setting and rapid
fluctuation of the Cretaceous global sea level caused
frequent changes to the sedimentary environment of
the Gabon Coastal Basin. Nonstationary sedimentary
construction reflected rapid subsidence of the passive
continental margin in a tensional setting, giving rise
to the Madiela Fm, Cap Lopez Fm, Azile Fm,
Anguille Fm, Point Clairette Fm, Port Gentile Fm,
Batanga Fm, Ikando Fm, Ozouri Fm and Animba Fm.
The deposition centers migrated northward from the
South Gabon sub-basin into the North Gabon
sub-basin and large-scale fault scarps were the
sources of turbidites. ① In Cenomanian to Turonian
times global sea level was rising fast and there was a
deep basin west of the Azile fault scarp and east of
the scarp was a shallow water platform (Fig. 9d); ②
In Coniacian–Santonian times global sea-level was
falling and the large-scale Anguille turbidite fan developed near Port-Gentil west of the Anguille fault
scarp while east was shallow water platform
deposition (Fig. 9e); ③ In Campanian– Paleocene
times, the Pointe Clairette turbidite fan developed
west of the Ewongue fault scarp and the smaller
Batanga turbidite fan west of the Ikando fault scarp
(Fig. 9f). During burial of the salt sequence deposited
in the transition period, transtension of Hinge belt III
provided space for salt diapirism, salt tectonics
developed along with some contemporaneous deposition (Fig. 8-IIIa) so that salt tectonics affected
distributary channel distribution supplying turbidite
fans. Salt tectonic activity in the Nouth Gabon
sub-basin continued to the Miocene, but mainly
occurred in the Albian in the South Gabon sub-basin.
A little salt tectonic activity continued into the
Cenomanian.
Late Eocene–Early Oligocene Tectonic Inversion
At this period the stress field became
compressional instead of the extensional and caused
tectonic inversion. Global sea level dropped and the
Gabon Coastal Basin no longer received sedimentary
deposition and suffered uplift and erosion. Clastic
material was brought into the South Atlantic Ocean
Basin by a deep canyon which cut the continental
shelf and the continental slope. Extrusion-related salt
tectonic structures such as rootless salt stocks formed
during this evolutionary episode (Fig. 8-IIIb).
Late Oligocene–Recent Drifting
After the brief tectonic inversion, during the
Late Oligocene drift episode the tectonic stress field
reverted to extensional. The western part of the basin
steadily subsided steady and the sedimentary center
was still in the North Gabon sub-basin. River, delta
and shore deposit progressed seawards after the
Miocene global sransgression rvent (Fig. 9g). Salt
Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa
915
tectonics reached a mature evolutionary stage with
only weak salt tectonic activity (Fig. 8-IIIc).
their valuable suggestions, which have dramatically
improved this manuscript.
CONCLUSIONS
The salt structural types of the Ezanga
Formation have a zonal pattern: around Hinge belt III
there are salt stocks, salt rolls and salt pillars caused
by numerous extensional faults formed by
transtension of two sets of fault systems and gravity
slumping down steep slopes; the east of the Hinge
belt III are salt domes and salt pillows formed by
small-scale salt tectonics; west side of Hinge belt III
are large-scale non-pierced low amplitude salt
anticlines.
Salt tectonics passed through three stages: 1.
Coniacian–Early Eocene, when rates of salt diapirism
under regional extension were greater than or
approximately equal to deposition rates, salt tectonics controlled sedimentary facies distribution and
gave rise to trap structures. 2. Late Eocene–Early
Oligocene, when rootless extrusion-related salt
stocks developed a results of tectonic inversion in
some areas. 3. Post Oligocene, when salt tectonics
stabilized and rates of salt diapirism became low, so
there was only a weak salt tectonic influences on
sedimentation.
The Gabon Coastal Basin experienced three
phases of tectonic evolution (rift phase, transition
phase and passive continental margin phase) and
seven episodes of tectonic evolution and sedimentary
filling. Two near-vertical fault systems at right angles, a NW-SE striking system and a NE-SW striking
system, give the basin an east-west directed zonal
structure sub-divided north-south directed blocks
forming four secondary tectonic units: the Interior
sub-basin, the South Gabon sub-basin, the North
Gabon sub-basin and the Lambarene high each with
its own characteristic sedimentary filling at different
stages.
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