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Journal of Earth Science, Vol. 24, No. 6, p. 903–917, December 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0383-5

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 structur-

al 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 transten-

sion 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 lo-

cal ephemeral low-intensity tectonic inversion. Post Oligocene salt diapirism was weak and salt tecton-

ics had a weak influence on sedimentation. Balanced cross-sections of two saliferous horizons crossing

This study was supported by the National Natural Science

Fundation of China (Nos. 40839902 and 40739901).

*Corresponding author: [email protected]

© China University of Geosciences and Springer-Verlag Berlin

Heidelberg 2013

Manuscript received February 11, 2013.

Manuscript accepted May 21, 2013.

different tectonic units from east to west reveal

that the basin tectonic evolution and sediment

filling processes can be divided into three stages

containing seven episodes of rifting, transition

and drifting.

KEY WORDS: salt tectonics, basin evolution,

Gabon Coastal Basin, passive margin, Africa,

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 lati-tude 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 An-guille 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 sali-ferous basin similar to passive continental margin ba-sins 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 com-plex salt tectonic structures. Salt diapirism and its re-lated 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; Nal-pas and Brun, 1993; Jackson and Talbot, 1991), which restricts accurate interpretations of sedimentary se-quences and structural features, and thus full under-standing 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; Dick-son 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 exten-sion zones and compression zones but this is an over-simplification of complex salt tectonics and evolu-tion 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, numer-ical simulations and so on have established a variety of evolutionary models of salt tectonics for studying plas-tic rheological behavior of salt and salt tectonics. From among them evolutionary models involving stretching and shortening tectonics explain the salt tectonic evolu-tion of the Gabon Coastal Basin well. This article ana-lyzes the basin’s structural characteristics, salt structur-al styles and sedimentary facies by using seismic ref-lection profiles and drill logging data to establish a salt tectonic evolutionary model, distinguish tectonic evolu-tionary 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 Gondwa-naland 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 frag-mentation 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 asso-ciated with the Fla plume and Tristan plume (Xiong et al., 2010; Burke et al., 2003; Dalziel et al., 2000; Uchu-pi, 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 Ameri-can Plate and transformed into typical passive conti-

Salt Tectonics and Basin Evolution in the Gabon Coastal Basin, West Africa 905

Deccan 65 Ma

Rajmaha 118 Ma

Kerguelen? 110 Ma?

Thetys

North AmericaPlate

Euriasian Plate

Fla Camp 201 Ma

Afar 31 Ma

Yuca

tan

Tristan 131 Ma

(T3)

(K1)

0 2 000 km

Marion 90 Ma

N

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 deposi-tional systems in Early Aptian times. The basin en-tered 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 subsi-dence to form narrow sea lanes connected with the original South Atlantic Ocean. In Gabon, Gamba sandstone and Ezanga evaporate filled the basin un-conformably 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).

GEOLOGICAL STRUCTURES The Gabon Coastal Basin has double-layered

basement architecture floored by a Precambrian crys-talline basement and a Pre-Cretaceous folded base-ment. 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 Lamba-rene 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 down-ward 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 ex-tended farther westward controlled by the fault which laid the foundation of characteristic east to west zona-tion. (3) Hinge belt III (also known as the Atlantic Hinge zone) developed from Late Cretaceous to Pa-leocene, 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


Table 1 Stratigraphic divisions and basin evolution of the Gabon Coastal Basin

Stages Formation names Thicknesses (m) Lithology Evolutionary

phases Quaternary Akossa

1 500

Sandstone & mudstone

Late drift phase

Neogene

Pliocene N’Tchengue

Mudstone, interbedded sand-stone

Miocene

Messinian

Tortonian

M’Begu

3 000

Serravallian Langhian Burdigalian

Mandarove Aquitanian

Paleogene

Oligocene Chattian Rupelian

Animba 800 Mudstone, interbedded sand-

stone & carbonate rock

Early drift phase

Eocene

Prabonian Bartonian Lutelian Ypresian Ozouri 50 Mudstone & sandstone

Paleocene Thanetian

Ikando 800 Mudstone & sandstone Selandian Danian

Cretaceous

Upper Cretaceous

Maastrichtian Ewongue/Batanga 2 000 Sandstone & mudstone

Campanian Pointe Clairette 2 000 Mudstone, interbedded sandstone

Santonian Anguille 500 sandstone, interbedded mudstone Coniacian

Turonian Azile 1 100 Mudstone, interbedded sandstone & carbonate rock

Cenomanian Cap lopez 2 300 Carbonate rock, sandstone, mudstone

Lower Cretaceous

Albian Madiela

Aptian

Ezanga 1 500 Evaporate Transition phase Gamba

1 600 Sandstone

Dentale Sandstone, interbedded mudstone

Rift phase

Barremian Cardima 700 Sandstone & mudstone

Hauterivian Melania

1 500

Sandstone, interbedded mudstone

Valanginian Lucina Mudstone, interbedded

sandstone Kissenda

Berriasian Sandstone N’DOMBO

Pre-Jurassic Basement Granite, gneiss Pre-rift phase

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

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 up-lifted 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


0 120 km

2o

8o

4 So

Ascen

sion Fz

N. fan

g Fz

S. fan

g Fz

Kango

linea

ment

Bronte

Fz

Dia

nag

otr

ough

Top

oG

rabe

n

Nye

mbe

Ilon

do

hors

t

Dam

inze sy

nclin

e

Dentale

trough

Wes

tern

lim

it

Flex

ure

Atlan

tic

flexure

N’KOM

I

linea

men

t

Gabon

Fz

May

umba Fz

of

Ati

n

Sal

ta

Sette Cama

Mayumba

Liberville

Equatorial Guinea

Cabon

Port-Gentil

A

A’

B

B’

North Gabon

sub-basinInterior sub-basin

Congo Craton

Set

teC

ama

hig

h

South Gabonsub-basin

Gabon

Coastal

Basin

Lam

barenehigh

Gam

ba highV

eragraben

Strik-slip

Cityboundary

Volcanic

Normal fault

Anticline Syncline

Western limit ofAptin salt

Administrative

Basin or sub-basin boundaryrock

fault

Lambarene

10oE

0o

Figure 2. Fault systems and tectonic units division of the Gabon Coastal Basin. Fz. Fault.


Figure 3. Typical salt tectonics in the Gabon Coastal Basin. Azile. Azile Formation; Ang. Anguille Forma-tion; 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 back-ground 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 charac-terized 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: extension- related salt tectonics and extrusion-related salt tec-tonics. 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 dis-placement, salt tectonics can be divided into autoch-thonous and allochthonous types. The main salt tec-

tonic type in the study area is autochthonous salt di-apirism 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 di-apiric 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 depo-sition 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


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 rela-tively weak and coated-continuous sand bodies formed on the top of and around salt anti-clines,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, es-tablishing 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 be-cause 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 se-diment surface, it will result in passive diapirs emerg-ing as salt glaciers.

Extensional tectonic settings almost always occur in rift basins and passive margin basins and exten-sional 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 thick-ness 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


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, am-plification of pre-existing structures commonly occur-ring 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 diapir-ism. The structures of passive diapirs that form under

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 re-tards the formation of new diapirs unless anticlines in the fold belt become deeply eroded (Hudec and Jack-son, 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

Extension with noprecursor structures

Thick saltThin salt

Buried graben fromearly eactive phase

Reactive diapir Salt roller

Passive diapir

Faults become youngertoward center of graben

Growth fault

Breakaway faultsystem

Figure 5. Schematic progressive model of salt tectonics during regional extension, constructed using Geo-sec-2D (Hudec and Jackson, 2007).

Shortening with

percursor salt diapirs

Salt roller

Inverted rollerTeardrop diapirSalt sheet

Diapir stemreactivatedas thrust

Vertical weld atpinched-off stem

Steepened flank Oad3608x

Figure 6. Schematic progressive models of salt tectonics during regional shortening, constructed using Geosec-2D (Hudec and Jackson, 2007).


of kilometers of shortening during the inversion phase that reactivated preexisting diapirs and some shorten-ing 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 evolu-tion 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, comprehen-sive analysis of structural features, regional dynamic field, and the relationship between salt diapirism and the deposition. We recognized five stages of salt tec-tonic 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

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 approx-imately equal to the deposition rate and only occasio-nally less than the deposition rate (Fig. 4).

(d) The basin experienced a tectonic inversion and formed a regional unconformity in the Late Eo-cene to Early Oligocene. The tectonic stress field be-came 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 gravita-tional 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 Ga-bon 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 evolu-tion is relatively low in the South Gabon sub-basin

(b) The early stage of salt burial(weak salt rheomorphic deformation)

(c) Diapir piercement during regional extension(salt diapirism with deposition)

(a) The salt deposit stage (evaporation) (d) Diapir amplification during regional shorteningno( deposition)

(e) The stage in stable tectonic backgroundrate<<(salt diapirism deposition rate)

1 2 3 5 764

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.


0 km

2

4

6

0 km

2

4

6

0 km

2

4

0 km

2

4

0 km

2

4

0 km

2

4

6

0 km

2

4

0 km

2

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6

8

10

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8

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MBEGAPG-IK

ANDO

UPC

AngAz CAP LOPEZ

Az

OZOURI

NTOUM

COMIOUET BEGUIEFOUROU MOUNDONGA

WELLEN’DMBC

MYIM-1 EYHYM-1EYVNM-1

IE-2 IE-1WZ-2 ALNG-1 ABRE-1 WE-1POWEM-1 TCTM-1 KS-2bls

OZITOU-1OZICAL-1 OFAM-1

MANDOROVE

SENONIEN MELANIA

KISSENDACARDITA

CAPLOPEZ

AZILE

CARDITA

Dentale troughSette Cama

highNorth Gabon sub-basinLambarene

high

I n t e r i o rsub-basin

MANDOROVE

Atlantic

DENTALE

GAMBA

EZANGA

EZANGAGAMBA

MADIELA

MADIELA

DENTALEDENTALE

CAPLOPEZAZILE

MELANIAKISSENDA

CARDITA

DENTALE

DENTALE

DENTALE

SENONIEN

MELANIAKISSENDA

CARDITA

CAPLOPEZ

AZILE

CARDITA

DENTALE

GAMBA

EZANGAMADIELA

DENTALEDENTALE

CAPLOPEZAZILE

MELANIAKISSENDA

MELANIAKISSENDA

CARDITACARDITA

CARDITA

CARDITAGAMBA

PG-IKANDO

UPC

AngAz

CAP LOPEZ

Az

OZOURI

NTOUM

COMIOUET

BEGUIEFOUROU MOUNDONGA

WELLEN’D

MBC

EZANGA

GAMBA

MADIELA

NTOUM WELLE

N’DMBC

NEE

NTOUM

COMIOUET BEGUIEFOUROU MOUNDONGA

WELLEN’DMBC


WELLEN’DMBC

PG-IKANDO

UPC

AngAz

CAP LOPEZAz

OZOURI

NTOUM

COMIOUET


WELLEN’D

MBC

EZANGAGAMBA

MADIELA

MELANIAKISSENDA

CARDITA

DENTALE

DENTALEDENTALECARDITA


WELLEN’DMBC

()

the Hinge

belt I

SENONIEN

MELANIA

KISSENDA

CARDITA

CAPLOPEZ

AZILE

CARDITA

DENTALE

GAMBA

EZANGAMADIELA

DENTALEDENTALE

CAPLOPEZ

AZILE

B’BA’A

()I

the Hinge

belt I( )I Ithe Hinge belt I

( )I Ithe Hinge belt I

()I

the Hinge

belt I ( )the Hinge belt I

Dianago trough

CARDITA

NEE

LPCUPC

Ang

LPC

LPCUPC

AngLPC

LPCUPC

AngLPC

DENTALE

North Gabon sub-basin South Gabon sub-basin

Ia. The initial taphrogeny episode in Neocomian(hinge belt I activity period)

Ia. The initial taphrogeny episode in Neocomianhinge belt I activity period( )

Ib(steady bathygenesis, expansion). The taphrogeny-depression episode in the Early Aptian

lake Ib(steady bathygenesis, expansion). The taphrogeny-depression episode in the Early Aptian

lake

IIahinge belt II activity period, parallel unconformity)

. The peneplanation episode in the Middle Aptian( IIa

(hinge belt II activity period, parallel unconformity). The peneplanation episode in the Middle Aptian

IIb( ). The depression episode in the Late Aptian

salt deposited IIb)

. The depression episode in the Late Aptian(salt deposited

IIIa(hinge belt III activity period and salt diapirism). The early drifting episode in Albian-Lutetian

IIIb( ). The tectonic inversion episode in Late Eocene-Early Oligoceneangle disconformity formation IIIb

). The tectonic inversion episode in Late Eocene-Early Oligocene (angle disconformity formation

IIIc( ). The late drifting episode in Late Oligocene-nowadaysextremely westward thermal subsidence IIIc

( ). The late drifting episode in late Oligocene-nowadaysextremely westward thermal subsidence

IIIa(hinge belt III activity period and salt diapirism). The early drifting episode in Albian-Lutetian

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 real-tively 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 tec-tonic 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 tecton-ism (① rift stage, ② transition stage, ③ passive con-tinental 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).


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 environ-ments (Fig. 9a). Basins formed by strong rifting at this early period of the basin formation (Hinge belt I activ-ity period). Graben subsided rapidly and the rate of accommodation increase was greater than the rate of increase of provenance supply. 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

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).

Libreville

Cette Cama

Lambarene

9o 10 Eo

Libreville

Cette Cama

Libreville

Cette Cama

Libreville

Cette Cama

Lambarene

Libreville

Cette Cama

Lambarene

Libreville

Cette Cama

Lambarene

Libreville

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01

o2

o3

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Schematicchannels

Presentor coastline

rivers

Rivers-delta

Semideep-deep lake

City

Land

Coast-shallow lake

Salt lake

Coast-shallowmarine

Actic region

Turbidite fan

Bathyal milieu

Marine delta

Shallow-bathyal milieu

(a) (b) (c) (d)

(e) (f) (g)

Provenancedirection

Faciesboundary

Salt lakecentre

0 60 km 0 60 km 0 60 km 0 60 km

0 60 km 0 60 km 0 60 km

Lambarene Lambarene

9o 10 Eo9o 10 Eo 9o 10 Eo

9o 10 Eo 9o 10 Eo 9o 10 Eo

01

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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.


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). Re-stricted 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 alterna-tive 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 de-veloped 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 depo-sition (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


tectonics reached a mature evolutionary stage with only weak salt tectonic activity (Fig. 8-IIIc).

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 tecton-ics 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 an-gles, 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. ACKNOWLEDGMENTS

This study was financially supported by The National Natural Science Fundation of China (Nos. 40839902 and 40739901) and the Special Fund for Geo-scientific Research in the Public Interest(No. 201211013). The authors thank the two reviewers for

their valuable suggestions, which have dramatically improved this manuscript.

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