# New geological insights on the Palaeoproterozoic Sembé-Ouesso Basin in northern Congo-Brazzaville – implications for the Kibaran orogeny, Neoproterozoic rifting and Pan-African overprint

Thomas Fullgraf, Yannick Callec, Guillaume Badinier, Florent Boudzoumou, Benoit Issautier, Fabien Paquet, Jean-Michel Schroetter, Guillaume Vic, Klaus Wemmer

► To cite this version:

Thomas Fullgraf, Yannick Callec, Guillaume Badinier, Florent Boudzoumou, Benoit Issautier, et al. New geological insights on the Palaeoproterozoic Sembé-Ouesso Basin in northern Congo-Brazzaville – implications for the Kibaran orogeny, Neoproterozoic rifting and Pan-African overprint. Journal of African Earth Sciences, 2024, pp.105379. 10.1016/j.jafrearsci.2024.105379. hal-04694616v2

HAL Id: hal-04694616

https://brgm.hal.science/hal-04694616v2

Submitted on 12 Nov 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# New geological insights on the Palaeoproterozoic Sembé-Ouesso Basin in northern Congo-Brazzaville – Implications for the Kibaran orogeny, Neoproterozoic rifting and Pan-African overprint

Thomas Fullgraf<sup>a,*</sup>, Yannick Callec<sup>a</sup>, Guillaume Badinier<sup>a</sup>, Florent Boudzoumou<sup>b</sup>, Benoit Issautier<sup>a</sup>, Fabien Paquet<sup>a</sup>, Jean-Michel Schroetter<sup>a</sup>, Guillaume Vic<sup>a</sup>, Klaus Wemmer<sup>c</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Handling Editor: M Mapeo

**Keywords:**

Sembé-ouesso Basin
Kibaran
Post-Kibaran
Tectonics
Metamorphism
Geochronology
Central Africa

## ABSTRACT

The Palaeoproterozoic Sembé-Ouesso Basin (SOB) is the northeasternmost of six foreland subbasins of the Eburnean orogen; it covers an area of about 64,000 km² in the northern Republic of Congo, southeast Cameroun and the Central African Republic. The structural architecture and tectono-metamorphic history of the SOB have been studied in the Republic of Congo by airborne geophysical interpretation, geological mapping, and petrography, analysis of illite crystallinity and K-Ar geochronology.

The oldest tectono-metamorphic event (D1/M1), which caused the main structuring of the SOB, coincides in age with the last (1.0–1.07 Ga) stage of Kibaran orogeny. Metamorphic conditions decrease from greenschist facies in the east to upper diagenesis in the west. D1 deformation started with west-vertong folding followed by N-S sinistral transpression in discrete shear zones. The tectonic style characterises the SOB as foreland in the external zone of an eastern concealed orogen that either represents the continuation of the Kibaran Belt or forms a separate, as yet unidentified unit.

Subsequent Neoproterozoic extension is marked by two conjugate N-S/NNE-SSW and E-W/NW-SE fault systems and a local pull-apart structure (D2). Coeval mafic magmas were emplaced in the faults of both systems. At regional scale, the extension can be linked to Tonian rifting in the central part of the Congo Basin and within the West Congo Belt.

The influence of the Neoproterozoic (600 Ma) Central African orogeny is mainly restricted to the north-western part of the SOB where the Dja nappe was thrust from the north over the Palaeoproterozoic rocks (D3/M2). Elsewhere, the Pan-African event had no thermal and only minor structural effects on the SOB.

## 1. Introduction

Geological mapping was carried out in 2014-15 by geologists from the French Geological Survey (BRGM) in the north-west of Congo-Brazzaville and supplemented by petrographic, isotopic and geochemical studies (Fig. 1). This resulted in a set of six 1:200,000 scale geological maps accompanied by comprehensive sheet explanations describing the stratigraphy, petrography, structures, geochemistry, age, mineral potential and geodynamic setting of the rocks and lithologic

units (Blein et al., 2017; Callec et al., 2017a-c; Fullgraf et al., 2017; Issautier et al., 2017). Here, we present the results of structural analysis in the Palaeoproterozoic Sembé-Ouesso Basin (SOB), which, in combination with the study of illite crystallinity and K-Ar geochronology provides new insight into the polyphase tectono-metamorphic overprint within the context of the Kibaran and Central-African orogenies and Neoproterozoic rifting.

<footnote><sup>a</sup> French Geological Survey (BRGM), 3 Avenue C. Guillemin, PO 36009, 45060, Orléans, Cedex 2, France</footnote>

<footnote><sup>b</sup> Université de Mariam Ngouabi, Brazzaville, the Republic of Congo</footnote>

<footnote><sup>c</sup> Geoscience Centre University of Göttingen, Goldschmidtstr. 3, 37077, Göttingen, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: t-fullgraf@brgm.fr (T. Fullgraf), y.callec@brgm.fr (Y. Callec), g.badinier@brgm.fr (G. Badinier), florentboudzoumou@yahoo.fr (F. Boudzoumou), b.issautier@brgm.fr (B. Issautier), f.paquet@brgm.fr (F. Paquet), jm.schroetter@brgm.fr (J.-M. Schroetter), g.vic@brgm.fr (G. Vic), kwemmer@gwdg.de (K. Wemmer).</footnote>
## 2. Regional geology

The study area is located in the southeastern foreland of the Paleo-proterozoic Eburnean orogen and later formed the southern foreland of the Central African Fold Belt (CAFB) during the Pan-African event (Bessoles and Trompette, 1980; Castaing et al., 1994, Fig. 1). The rocks in this region consist of 3.1–2.6 Ga Archaean basement overlain by Palaeoproterozoic sedimentary sequences (2.1-1.95 Ga), which were subsequently subjected to low-grade metamorphism and deformation (Fig. 2). Neoproterozoic nappes of the CAFB were thrust over these rocks in the northwestern corner of the area. To the east, the Precambrian rocks are overlain by Cretaceous sandstone and unconsolidated Cenozoic sediments.

The Archaean rocks constitute the Ivindo block and consist mainly of migmatitic gneiss, amphibolite and TTG suites (Thiéblemont et al., 2009). Intercalated greenstone belt sequences comprise chlorite schist and banded iron formations often forming steep ridges in otherwise swampy area. The western lowlands within the Ivindo block are separated by a pronounced N-S escarpment from the eastern highlands with subdued relief underlain by the Palaeoproterozoic metasediments of the SOB. The extensive basin is the northeasternmost of several sub-basins in the foreland of the 2.2-1.95 Ga Eburnean orogen and extends over ca. 64,000 km² from Congo-Brazzaville (where about one third of the area is exposed) via southeast Cameroun to the Central African Republic. The other sub-basins (Booué, Bambékani-Lastourville, Iméno-Mbila, Okondja, Franceville, Ogoué) are located in Gabon and have been studied in some detail (Weber, 1971; Bouton et al., 2009a, b; Ebang Obiang et al., 2009a, b; Prian et al., 1990, 2009a,b). Most of them are

only weakly (if at all) affected by Eburnean deformation and metamorphism up to greenschist facies. An exception are the Abeilles Basin, which shows intense east-vergent folding (Prian et al., 1990) and the Ogoué Metamorphic Complex in the internal zone of the Eburnean Belt. The Francevillean basins are interesting from an economic point of view as they host world-class uranium (Pons, 1971; Bourrel and Pfiffelmann, 1972) and manganese deposits (Martini and Makanga, 2001).

Geological mapping and sedimentary studies in Gabon made it possible to establish a general stratigraphy of these foreland basins (Weber, 1971), which has since been used widely (Thiéblemont et al., 2009, 2014; Bouton et al. 2009 a-d; Préat et al., 2011), even if it does not comply with international standards (IUGS, 2023). The following main units were distinguished from bottom to top (corresponding to the subgroup status in the IUGS classification).

(i) The Francevillian A (FA) is essentially composed of coarse-grained sandstones with locally intercalated conglomerate

(ii) The Francevillian B (FB) consists mainly of finer-grained sediments. Pelites and black shales are locally interlayered with sandstone/conglomerate and/or dolomite. Contemporaneous volcanism is recorded in few places by fine pyroclastic rocks.

(iii) The Francevillian C (FC) is dominated by dolomitic carbonate facies alternating with chert beds, and evaporites.

(iv) The Francevillian D (FD) resembles the FB in composition, however, the proportion of epi- and pyroclastic rocks as well as of lavas is much higher

(v) The Francevillian E (FE) is dominated by sandstones with rare intercalations of shales. Volcanism is absent in this formation.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** The Precambrian to Palaeozoic domains of Central Africa (hatched area = study zone). Extracted from the BRGM 2M-SIG Afrique database (unpublished).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Regional geology of northern Congo-Brazzaville and adjacent areas showing the distribution of the principal Precambrian to Phanerozoic rocks and sediments (BRGM 2M-SIG Afrique, unpubl.).</figcaption></figure>

Further subdivision of these subgroups is given in the literature as FA1, FA1so (=sandstone of FA1) etc. corresponding to the formation and member status in the IUGS.

The schematic cross-section (Fig. 3, Thiéblemont et al., 2009, 2014) illustrates the partly syn-tectonic deposition of the lower FA and FB subgroups in local, fault-bounded troughs, which are interpreted as pull-apart basins (Weber et al., 2016), resulting in high variation in the thickness and composition of sedimentary and pyroclastic rocks. The deposition environment of the FA Subgroup is generally considered to be fluvial in the lower part and deltaic to marine in the upper part. Sedimentary structures (slumps, syn-sedimentary faulting) in predominantly fine-grained turbiditic rocks of the FB Subgroup testify to intense subsidence during progressive rifting, which was locally accompanied by alkaline magmatism. The overlying FC and FD subgroups of regional extent were deposited in a tectonically stable environment and consist mainly of fine-grained clastic and pyroclastic rocks dated at 2083 ± 6 Ma (Horie et al., 2005). The uppermost FE Subgroup is preserved in the Okondja Subbasin only and is considered to be the infill of a local depression that developed at the end of Francevillian sedimentation.

The pre-collisional age of the FC Subgroup and older rocks is constrained by the Massima and Okanda series of the Ogooué Metamorphic Complex in the internal part of the Eburnean Belt.

Initial studies of the SOB were carried out by the BRGM in the 1960s as part of mineral exploration, in which geological mapping was a by-product (Chochine, 1950; Henry and Legras, 1965; Henry and Grès, 1966; Bache et al., 1966, 1967; Grès and Legras, 1967, 1970, Table 1). However, the resulting set of six geological maps at scale 1:200,000 scale remained unpublished. Later, these maps were integrated into the 1:1M geological map of Congo (Dadet, 1969; Desthieux, 1992). According to these authors, the SOB consists of sedimentary rocks that form two main lithostratigraphics units. The basal SO1 series comprises quartzite with local intercalations of conglomerate while the upper SO2 series consists mainly of pelite and sandstone. The results of later studies on magmatism, deformation and metamorphism by Doucet (1982), Vicat and Vellutini (1987) and Vicat et al. (1989, 1995, 1997, 2001) are summarised in Table 1. The regional D1 event is characterised by W-vergent folding and thrusting under low-grade metamorphic conditions. The intensity of deformation decreases from east where isoclinal
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Stratigraphic correlation across the Francevillian subbasins in Gabon (Thiéblemont et al., 2009, 2014). The westernmost Massima and Okanda metamorphic series are part of the Ogooué Metamorphic Complex implying the pre-collisional age of the FC and older rocks.</figcaption></figure>

folds were observed at outcrop-scale to west where gentle folding pre-
vails. A regional N-S sinistral strike-slip fault separates a western from an
eastern block and is associated with conjugate E-W dextral shear and
NE-SW extension. The Eburnean age of the event was suggested by the
c.a. 2070 Ma Mbi granodiorite, which was considered as post-tectonic
intrusion (Vicat et al., 2001). However, mafic igneous rocks of Meso-
proterozoic to Neoproterozoic age are also affected by this event (Vicat
and Vellutini, 1987), implying a Kibaran or Pan-African age. Subsequent
regional sinistral N-S strike slip is recorded in the Douma Shear Zone and
the associated local pull-apart structures such as the Boutazab Graben
(Vicat et al., 1997). Coeval emplacement of the 1167 ± 61Ma per-
aluminous and calcalkaline Sanga Suite along the faults indicates their
development during the Kibaran orogeny (Vicat et al., 2001). Volumi-
nous mafic magmatism has been subdivided in an early late Kibaran to
early Neoproterozoic phase comprising quartz dolerite, metagabbro and
amphibole schist of alkaline (Vicat and Vellutini, 1987) and tholeiitic
composition (Vicat et al., 1997), with the adjacent host rocks undergo-
ing local contact metamorphism. The syn-to post-Ediacaran age of the
second dolerite swarm with tholeiitic composition is constrained by the
hosting Dja diamictite of Sturtian (c. 720 Ma) age, which also constrains
the Pan-African age of D2 greenschist-facies metamorphism and nappe
tectonics in the northeastern part of the study area (Callec et al., 2017a;
Issautier et al., 2017).

**3. Methods**

**3.1. Geophysics data interpretation**

A nation-wide high-resolution airborne geophysical survey was
flown from 2012 to 14 by Asperbras Congo Company at an altitude of
80-120 m, with the N-S flight lines at 500 m and the E-W tie lines at
10,000 m. A five kilometer wide buffer parallel to the national border
was not flown due to security concerns. The processed radiometric data
included K, Th, U single layers, element ratios and ternary imagery
whereas magnetic data comprised reduced to pole total magnetic field
(TMF), analytical signal (AS) and 1st vertical (1VD)/horizontal (1HD)

derivative layers. The interpretation of high-resolution airborne
geophysical data was carried out for a fortnight in a data room by BRGM
geoscientists under the supervision of Asperbras staff to maintain
confidentiality. Thematic shape files were created using ArcGIS software
(Fig. 4) and sent to Asperbras for approval. Despite this complicated and
unusual approach, the method made it possible to identify the main
geophysical domains, the regional structural architecture and the
network of faults and mafic dykes occurring in the Archaean and
Palaeoproterozoic. Furthermore, the comparison of the classified
geophysical data with unpublished geological maps of the BRGM
allowed a first interpretation of the geophysical data with respect to
their geological context.

**3.2. Geological mapping**

Geological mapping and sampling of the SOB was carried out in 2015
by five teams in two campaigns, each lasting around six weeks. The area
is notable for its remoteness, the lack of infrastructure and the fact that it
is predominantly covered by primary forest. Due to intense tropical
weathering, outcrops are mostly limited to rivers, the sources of streams
and few areas of rugged topography. The area was accessed by 4x4
vehicle, boat, helicopter and on foot with the help of flying camps. The
planning of field surveys was based on analysis of archive data, remote
sensing, geophysics interpretation and two reconnaissance campaigns.
Fig. 5 shows the main infrastructure used for access the field and the
position of some 650 localities where rocks were described and partly
sampled.

**3.3. Illite crystallinity and K-Ar age geochronology**

**3.3.1. Sample preparation**

19 samples of siltstone, shale, slate and sericite schist weighing 2–3
kg were sampled for illite crystallinity analysis and K-Ar geochronology.
The rocks were crushed in a shatter mill (vibration mill) for about 20 s
and split into size fractions >63 µm and <63 µm. The fraction <63 µm
was used to extract the <2 µm clay fractions by settling in Atterberg
<table><caption>**Table 1**

Review of the geological history of the SOB according to previous research (qz = quartz, ser = sericite, epi = epidote, chl = chlorite, ab = albite, trem = tremolite, serp = serpentinite, cc = calcite).</caption><thead><tr><th>Orogeny</th><th>Plate tectonic setting</th><th>Age [Ma]</th><th>Sedimenation</th><th>Magmatism</th><th>Tectonics</th><th>Metamorphism</th><th>Reference</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="4">Pan-African</td><td>Collision</td><td>600</td><td></td><td></td><td rowspan="2">D2: Nappe tectonics</td><td rowspan="4">M2: greenschist</td><td rowspan="2">Fullgraf et al., 2017a</td><td rowspan="4">Nappe transport of Dja tillite</td></tr><tr><td>Rift/Drift</td><td>700</td><td></td><td>Mafic magmatism II: olivine dolerite of tholeiitic affinity (intraplate magmatism)</td><td>Gres et Legras, 1967; Jaunatre, 1969; Dostal et al., 1985; Vicat and Vellutini, 1987</td></tr><tr><td></td><td>720</td><td>Dja tillite: mass flow deposited during Sturtian glaciation</td><td></td><td></td><td>Dykes intrude Djab tillite. Magmatic paragenesis: andesine, labradorite, augite/diopside, olivine (often serpentinised)</td></tr><tr><td>Rift related continental intraplate magmatism</td><td>1167–850</td><td></td><td>Mafic magmatism I: quartz dolerite, metagabbro, amphibole schist of alkaline affinity (Vicat and Vellutini, 1987) or tholeiitic (Vicat et al., 1995, 1997)</td><td>contact</td><td>Sonet, 1958; Gres et Legras, 1967; Jaunatre, 1969; Vicat and Vellutini, 1987, Vicat et al., 1995, 1997</td><td>Dykes and sills pre-date Dja tillite but intrude Sangha granite. Metamorphic paragenesis: qz, ser, epi, chl, ab, trem-act, serp, cc, uralite (greenschist facies). Metadolerites record same tectonic overprint as Paleoproterozoic country rock.</td></tr><tr><td>post-Kibaran to pre-Pan-African</td><td></td><td>1167 ± 61</td><td></td><td>Sangha Granite Suite</td><td>Emplacement along major N-S transcurrent faults, magmas of crustal origin</td><td></td><td>Vicat et al., 2001</td><td>Peraluminous and calcalkaline granites with volcanic arc to post-collision affinity. However, anorogenic origin is suggested.</td></tr><tr><td>Kibaran</td><td></td><td></td><td></td><td></td><td>Regional sinistral N-S strike slip, development of Douma shear zone, pull-apart basins (Boutzab Graben)</td><td></td><td>Vicat et al., 1997</td><td></td></tr><tr><td>Eburnean</td><td>Orogenic collaps ? Collision</td><td>2070</td><td></td><td>Mbi granodiorite (Central African Republic)</td><td>Post-tectonic</td><td>M1: greenschist</td><td>Doucet, 1982; Vicat and Vellutini, 1987</td><td>Mbi granodiorite constrains minimum age of tectono-metamorphic event M1. Age of M1 considered to be older than 2070 Ma (Vicat et al., 2001). This is in contradiction to Vicat and Vellutini, 1987: "les dolerites (950 Ma) sont tectonisees conformement a l'encaissant" (i.e. the Francevillean metasediments) implying a Kibaran or Pan-African age of metamorphism</td></tr><tr><td></td><td>Foreland basin</td><td>2200–2100</td><td>Lower sandstone series grading towards the top into marls and carbonate rocks (1500m). Upper sandstone-metapelitic series (700m)</td><td></td><td>D1: W-directed folding and reverse thrusting under low-grade metamorphism, the intensity of deformation decreases from east (isoclinal folds) to west (monoclinal folds). Regional N-S sinistral strike-slip fault close to Douma separating a western from an eastern block. Associated with conjugate E-W dextral shear and NE-SW extension.</td><td></td><td>Cochine, 1950; Sonet, 1958; Gres et Legras, 1967; Jaunatre, 1969; Thieblemont et al., 2007</td><td></td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Example of classified airborne magnetics data (here the analytical signal in combination with the total magnetic field) used to define first order tectonic structures and structural domains in the SOB (Fig. 7).</figcaption></figure>

cylinders. The <0.2 µm fraction was separated from the <2 µm fraction using an ultra-centrifuge. An additional separation of the 2-6µ and 6-10µ fractions was made in sample YE0021 to assess the presence of an older detrital component.

### 3.3.2. XRD analysis

Both the 0.2µ and 2µ fractions were analysed using the XRD technique to determine their mineralogical composition and illite crystallinity (IC). The IC is defined as the peak width at half the height of the 10 Å peak and was calculated using a computer program developed at the University of Göttingen. The digital measurement of the IC was performed by step scan (301 points, 7–10° (2θ), scan step 0.010° (2θ), integration time 4 s, receiving slit 0.1 mm, automatic divergence slit). IC values are given in (Δ°2θ).

The limits between diagenesis/anchizone and anchizone/epizone are 0.600°–0.400° or 0.240° (Δ°2θ). They were verified with the interlaboratory standardization program of Warr and Rice (1994). The given IC values are nearly identical with the crystallinity index standard (CIS) and were determined using the machine parameters recommended by Kisch (1991).

### 3.3.3. Ar isotope analysis

The isotopic composition of argon was measured in a Pyrex glass extraction and purification line coupled to a VG 1200 C noble gas mass spectrometer operating in static mode. The amount of radiogenic 40Ar was determined by the isotope dilution method using a highly enriched 38Ar spike from Schumacher, Bern (Schumacher, 1975). The spike is calibrated against the biotite standard HD-B1 (Fuhrmann et al., 1987).
<figure><img src="image_8.png" /><figcaption>Fig. 5. The study area was accessed by car, boat, helicopter, and on foot. Geological information was recorded in c. 650 localities.</figcaption></figure>

The age calculations are based on the constants recommended by the IUGS, cited in Steiger and Jäger (1977).

# 4. Results

## 4.1. Lithostratigraphy

The new lithostratigraphy of the study area is shown in Fig. 6 and Table 2. The units are only briefly described as they are not the subject of this publication.

Rocks of the Meso-to Neoarchaean Ivindo Inlier were classified into the 3100-2800 Ma Ntem Supergroup, which essentially consists of undifferentiated migmatitic paragneisses intruded by intermediate to felsic granitoids that were metamorphosed into orthogneisses during

Neoarchaean orogeny. Within these gneisses, greenstone and banded iron rocks were combined into the Belinga Group; individual slivers reach of up to 60 km length and 7 km width.

The Francevillian Group was deposited on the Archaean rocks between 2100 and 1995 Ma, after a major hiatus of 700 Ma with no rock record. It is subdivided into the Francevillian A, B and C subgroups, following the nomenclature introduced in Gabon while the subordinate units (i.e. formations) have been given local names (Table 2). The Francevillian A and B subgroups consist mainly of silico-clastic rocks with a predominance of sandstone. Volcanic rocks, which are typical for the Francevillian B in Gabon, have not been recognised. The Francevillian C Subgroup comprises limestone, chert and jasper and is mainly restricted to the area north of the road between Sembé and Douma.

The sedimentary rocks are intruded by a major dolerite swarm
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Simplified geological map of the Sembé-Ouesso area and the cross-section parallel to the national road.</figcaption></figure>

forming dykes and sills (Bissie dolerite = dolerite I); their emplacement predates the first phase of regional deformation and metamorphism (D1).

The Neoproterozoic Dja Group (corresponding to the Dja-Nola Formation of Saggerson and Turner, 1986) of the Oubangi Supergroup overlies the Francevillianan rocks by a thrust contact; it consists of basal pebbly sandstone overlain by glaciogenic diamictite. The unit is an important marker for distinguishing Pan-African tectonics from older structures. The sedimentary rocks were intruded by a second swarm of mafic dolerite dykes (= dolerite II).

The Carnot Group of the Congo Supergroup consists of fine-to coarse-grained quartzo-feldspathic sandstones with conglomerate intercalations that were deposited in the Upper Cretaceous.

## 4.2. Structural-architecture of the SOB

High-resolution airborne magnetic data provide information on the regional structural framework of the SOB whereas field observation of small-scale structures allows establishing the deformation gradient and the detection of local strain variations. The classified geophysical map shows two distinct structural domains in the SOB, separated by a N-S trending first order structure, the Douma Shear Zone (Fig. 6). A third, triangular area with a distinct structural fabric is located to the south of the study area. It is separated from the other areas by two splays in the continuation of the Douma Shear Zone.

### 4.2.1. Eastern domain

The area is characterized by regular open folds of 5–10 km in size with slightly curved NNE-SSW axes. Isoclinal folding and west-vertong thrusts have been noted in the field (Doucet, 1982) but are probably restricted to discrete zones of high strain. The open folds are overprinted by gentle folding around E-W axes, resulting in regional fold interference structures (Figs. 6 and 7). Accordingly, the stereograms show shallow SSW plunging fold axes in the north (AR06), whereas further south they plunge shallow to moderately NE and NNE (AR07, AR08; Fig. 8).

Open, symmetric fold style in area AR06 is indicated by the distribution of S₀ (=bedding) poles; the maximum at 300/72 corresponds to shallow SE-dipping layers, while the remaining beds dip moderately to ESE and WNW. This is in contrast to area AR07, where the maxima of S₀ poles define shallow ESE dipping long limbs (118/33) and moderate WNN-dipping short limbs (285/62), corresponding to asymmetric WNW verging folds. The S₀ poles in area AR08 define a steep southward dip of the AC-plane (187/67) and a shallow north-plunging fold axis (007/23). Two maxima of S₀ poles indicate shallow E-dipping long limbs (088/30) and moderate W-dipping short limbs (301/56) that define asymmetric W-verging folds. The lack of S₁ planes in the eastern domain is partly explained by the predominance of sandstones, in which these structures are only weakly developed due to the low degree of deformation.
<table><caption>**Table 2**
Lithostratigraphic subdivision of the study area. The minimum age of D1/M1 is determined by K-Ar geochronology at 1020–1070 Ma (this study).</caption><thead><tr><th>Age</th><th>Group</th><th>Subgroup</th><th>Formation</th><th>Description</th></tr></thead><tbody><tr><td>Upper Cretaceous (101-66 Ma)</td><td>Congolian</td><td>Carnot</td><td></td><td>fine- to coarse-grained quartz-feldspar sandstone; intercalations of conglomerate</td></tr><tr><td colspan="5"><strong>regional nconformity</strong></td></tr><tr><td>pre-D3/M2 (Pan-African)<br/>Cryogenian (720-635 Ma)</td><td>Oubangian</td><td>Dja</td><td></td><td>dolerite swarm II<br/>pyritiferous shale with polymict lonestones (diamictite), calcareous metapelite, ampellite<br/>basal pebbly sandstone</td></tr><tr><td colspan="5"><strong>regional unconformity</strong></td></tr><tr><td>pre-D1/M1<br/>Paleoproterozoic (2100-1950<br/>Ma)</td><td>Francevillian</td><td>Francevillian C<br/>Francevillian B</td><td>Boutazab<br/>Liouesso-<br/>Seka<br/>Douma</td><td>dolerite swarm I<br/>chert, jasper, micritic limestone with rare stromatolite; locally silicified<br/>fine- to coarse-grained quartz sandstone with minor feldspar; minor<br/>greenish shale with detrital mica<br/>fine-grained to poorly-sorted quartz sandstone forming sequences of<br/>metric to deccametric thickness</td></tr><tr><td></td><td></td><td>Francevillian A</td><td>Nassouk</td><td>in upper portion, intercalations of turbiditic fine- to coarse-grained dark<br/>sandstone<br/>brigit to dark (ampelite) micaceous and pyritiferous shale; locally<br/>calcareous</td></tr><tr><td></td><td></td><td></td><td>Zoulabout</td><td>fine-grained sandstone alternating with pyritiferous white shale<br/>fine-grained, well-sorted quartz sandstone</td></tr><tr><td></td><td></td><td></td><td>Bellevue</td><td>coarse-grained quartz sandstone with intercalations of polymict<br/>conglomerate, white sandstone</td></tr><tr><td></td><td></td><td></td><td>undiff</td><td>sandstone alternating with shale<br/>layered and crossbedded sandstone<br/>coarse-grained quartz sandstone with intercalations of polymict<br/>conglomerate</td></tr><tr><td colspan="5"><strong>regional unconformity</strong></td></tr><tr><td>Meso- to Neoarchean (3100-<br/>2800 Ma)</td><td>Ntem</td><td>Younger orthogneiss suite (2870-<br/>2800 Ma)<br/>Older orthogneiss suite (2950-<br/>2800 Ma)<br/>Belinga (2950-2800 Ma)<br/>Mesoarchean gneissic complex</td><td>undiff.</td><td>banded iron formation, greenstone<br/>(migmatitic) paragneiss, amphibolite</td></tr></tbody></table>

### 4.2.2. Western domain

The structural style in the Western Domain differs significantly from the Eastern Domain although they share some common features. The central part is dominated by the N-S trending Bissie dolerite swarm or megasill of more than 100 km length and 40 km width. The magnetic signal of the mafic rocks considerably disturbs the regional low TMF generated by the sedimentary rocks. However, the relatively sharp eastern contact of the zone and curvilinear high-magnetic anomalies of individual sills/dykes illustrates again folding on a 5 km scale, con- straining the pre-tectonic emplacement of the magmas. The interpreted fold axes and magnetic anomalies change systematically from a northern direction in the southwest to a northeasterly direction in the central part and near the Douma Shear Zone in parallelism with the contact Fig. 7, black solid lines).

This pattern is also reflected in the stereograms by the change in the direction of fold axes. In area AR04, most S₀ poles rotate around a shallow WSW plunging fold axis (255/05) that is parallel to the mag- netic linear anomalies in this sector and coincides with few measured fold axes. The distribution of most S₀-poles defines gentle to open symmetric folding with shallow to moderate NNW and SSE-dipping limbs. The stereogram further illustrates that a number of S₀ poles sys- tematically deviate from the constructed AC plane in the NW and SE quadrants by counterclockwise rotation, which can be explained by subsequent sinistral shear, as indicated by the arrows in the diagram. The rarity of S₁ planes again demonstrates the generally weak defor- mation and/or lack of pelitic rocks where these structures can be recorded.

In area AR05, 10 km further south, the stereogram illustrates the large dispersion of S₀ poles. The pole of the constructed AC-plane defines

the average NE-plunging fold axis, parallel to the trace of the magnetic anomalies and regional fold axial planes (Fig. 8). The offset of many S₀ poles from the constructed AC plane is again attributed to counter- clockwise rotation of the poles by sinistral shear and may also explain the widely varying directions of some measured subhorizontal fold axes. The few S₁ planes dipping SWS indicate NE-SW shortening and therefore clearly are unrelated to the main folding.

The stereogram of area AR10 illustrates mainly shallow dipping bedding planes with strongly varying dip directions, which corresponds to gentle fold style. They poorly define the AC plane, whose pole in- dicates a shallow NE-plunging fold axis related to NW-SE shortening. The considerable dispersion of mainly shallow NE and SW plunging fold axes is interpreted to record local variations in a generally weak stress field resulting in buckling of the rocks. S₁ schistosity planes were not recorded.

In area AR11 at the southern limit of the study area and near the Douma Shear Zone, the S₀ poles define a NW-verging fold with a shallow NE-plunging fold axis. S₁ schistosity planes were again not observed.

### 4.2.3. Douma Shear Zone

This structure in the central part of the SOB changes from a NE di- rection in the north to a southward direction in the south before splitting into several anastomosing branches (Fig. 7). The change in direction of the curvilinear magnetic anomalies and fold style across the contact indicates oblique movements with left-lateral and westward compo- nents, consistent with field observation (Doucet, 1982). Few locations near the structure expose steeply east-dipping sandstones that are locally overprinted by pervasive foliation and mylonitisation. Stereo- grams AR04 and AR05 show a counterclockwise rotation of several S₀
<figure><img src="image_10.png" /><figcaption>Fig. 7. First-order tectonic structures and structural domains in the SOB interpreted from airborne magnetic and radiometric data. The implications for the kinematic movements are presented in the discussion.</figcaption></figure>

poles out of the AC-plane which can be attributed to sinistral shearing.

### 4.2.4. Regional faults, pull-apart basins

The geophysics map shows two fault sets with conjugate strike slip components and a third set of normal faults (Fig. 7). Their chronological order remains unclear at this stage, and the assignment into D2a and D2b is therefore arbitrary.

The first fault set (D2a) consists of conjugate N and NE trending faults that indicate a stress field with NW-SE extension. The largest structure reaches a length of more than 100 km and forms the western limit of the Biessi dolerite swarm. The system also defines the borders of the about 20 km wide and 25 km long Boutazab Graben 5 km north of Biessie, which is interpreted as pull-apart structure opened by a dextral N-S shear (Fig. 7). The basin partly underlies the Dja nappe, constraining
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Presentation of the structural data in stereograms illustrate systematic change in fold style and orientation from east to west. The classification as s1, s2 Fa1, Fa2 structures has been done in individual outcrops.</figcaption></figure>

the pre-Pan-African age and documenting only limited effects of the overthrust rocks on the older, underlying structures.

Trench filling by Francevillean C SG sediments, which are largely absent elsewhere, indicates a phase of post-trench uplift and erosion prior to the arrival of Pan-African nappes. Dolerite dykes in both fault directions were described by Vicat et al. (1997), but do not appear in the magnetic data. This distinguishes them from the strongly magnetic dyke

swarm southwest of Sembé that was emplaced in the second (D2b) conjugate fault system; which developed in a NE-SW extension stress field (Fig. 7).

Individual E-W dykes of this swarm reach again a length of more than 100 km and are slightly affected by Pan-African buckling/gentle folding in the east, which constrains their post-D1, pre-Pan-African emplacement age. The different composition of mafic magmas emplaced into the D2
structures argues for two discrete tectonic events and involvement of different mantle sources or different crystallization conditions. The third group of NW-trending normal faults was identified in the SW corner of the study area. The faults run perpendicular to the direction of maximum D3 extension and could also be part of this event.

### 4.2.5. Dja Nappe

The Pan-African Dja Nappe, overlies the northwestern part of the SOB; it was transported from the north over the Palaeoproterozoic

sediments and the Biessi graben (Fig. 7). The structural inventory and the orientation of the small-scale structures within the nappe and the adjacent underlying basement (stereograms AR01-03) differ significantly from the western and eastern domains. The greenish massive unbedded shales with occasional polymict clasts (i.e. possibly dropstones) of the Dja nappe display a NNW-dipping pervasive schistosity s2. The s0 data of AR02 were determined in sedimentary rocks of the underlying Francevillean B SG and have a similar dip indicating their transposition into the plane perpendicular to the direction of maximum

<figure><img src="image_12.png" /><figcaption>Fig. 9. Illite crystallinity and K-Ar ages of the study area; background: interpreted structural map and classified geophysical data.</figcaption></figure>
shortening during nappe transport. Few measured horizontal fold axes vary widely in direction documenting non-cylindrical fold style.

AR03 combines data from the front of the Dja Nappe and the adjacent southern foreland. The maxima of s0 and s1 poles correspond to subhorizontal bedding planes and a shallow SW-dipping s1 plane. The s0 planes again are widely dispersed, indicating non-cylindrical or interference folding. The poles of the s1 cleavage define an AC plane that indicates subsequent D3 folding around W-plunging axes. Fold axes with horizontal to moderate plunge scatter widely in the NW hemisphere and again evidence non-cylindrical fold style or interference folding.

Few structural data in AR01 southwest of the Dja Nappe document subhorizontal Francevillian rocks with steep NE and SW dipping s1 schistosities.

## 4.3. Metamorphism

The age, degree and gradient of metamorphism across the SOB were determined by 19 samples of pelitic and semipelitic rocks. Fig. 9 illustrates the variation of illite crystallinity (IC) and ages of the 0.2 and 2µ fractions within the SOB. It shows that IC values generally increase from east to west except in the northwestern area affected by Pan-African nappe tectonics. IC values between 0.12 and 0.24 in the east correspond to epizonal (–greenschist) conditions, while IC values of 0.4–0.56 indicate diagenesis in the west. The IC values for the 0.2µ and 2µ fractions hardly differ in 12 samples indicating the absence of a detrital component or a complete thermal reset (Fig. 9, Fig. 10). Higher IC values were determined in the 0.2µ fraction of seven samples, most of which are located in the western SOB, which is marked by diagenetic and anchikal metamorphism. This reflects either prolonged illite growth under waning very-low grade conditions or small proportion of detrital component in the 2µ fraction.

The northwestern area overlain by the Dja Nappe (Figs. 7 and 8, hatched domain), displays epizonal metamorphism corresponding to greenschist facies conditions. Here, the IC values of two samples (SE13171, SE0099) vary only slightly from 1.9 to 2.1. The IC values in three samples few km south of the nappe, vary between 0.38 and 0.54, which corresponds to the lower anchizone and diagenesis, characteristic of the western SOB. This demonstrates that nappe emplacement had only little thermal impact on the southern foreland of the Central African Orogen. However, all three samples have a Pan-African K-Ar age

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** The 0.2µ and 2µ IC values are well correlated with the 0.2µ fraction being slightly increased.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** The 0.2µ vs 2µ K-Ar age diagram illustrates the strong correlation between the ages of both fractions but often slightly higher 2µ ages.</figcaption></figure>

between 532 and 656 Ma, which is consistent with the age determined in the Dja Nappe (574–611 Ma).

The K-Ar ages of the 0.2 and 2µ fractions in the remaining part of the SOB show only minor differences within the individual samples, which in most cases yield slightly higher 2µ ages (Fig. 11, Fig. 9). In contrast, considerable age variations occur at the regional scale in the eastern epizone [797–1015 Ma], the central middle to upper anchizone [934–1010 Ma] and the western diagenesis to lower anchizone [974–1074 Ma] (see Fig. 9).

Two coarser size fractions of detrital origin (2–6 µ and 6–10 µ) were analysed in sample YE0021 from the eastern epizone (Table 3). The ages of 1040 Ma and 1049 Ma are slightly elevated compared to the 0.2µ (1013 Ma) and 2µ fractions (1015 Ma). This is probably due to either incomplete reset of the argon clock during regional metamorphism or subsequent partial argon loss in the smaller size fractions.

The IC-age diagram (Fig. 12) is a second test to detect detrital components. Mixing detrital mica (with low IC and higher ages) and younger low-grade metamorphic mica would result in mixing lines with negative slopes. However, the data yield regression lines with a low correlation coefficient and a positive slope for both size fractions. The age data therefore define a regional tectono-metamorphic event at the turn from the Mesoproterzoic to the Neoproterozoic era. Excluding the outliers (LI19270 and MB3396), the best estimates for the metamorphic ages in the 0.2 and 2µ fractions are 966 ± 30 and 975 ± 27 Ma, respectively. However, assuming partial argon loss, the oldest ages between 1048 and 1074 Ma would be closest to the peak of metamorphism.

## 5. Discussion

Geophysics interpretation in conjunction with geological mapping, illite crystallinity analysis and K-Ar geochronology allows the structural architecture of the SOB to be determined and important parts of its geological history to be reconstructed. The oldest tectono-metamorphic event coincides in age with the last phase of the Kibaran orogeny and is responsible for the main structuring of the SOB. This was followed by two phases of discrete deformation in which conjugate N-S/NNE-SSW and E-W/NW-SE strike slip systems were active and accompanied by coeval mafic magmatism. Nappe transport from the north occurred during the Neoproterozoic Central African orogeny, but had only limited
<table><caption>**Table 3**
Results of the K-Ar isotope and illite crystallinity study.</caption><thead><tr><th rowspan="3">sample</th><th colspan="6">K-Ar isotope analysis</th><th colspan="4">Illite crystallinity</th></tr><tr><th>frac</th><th>K2O</th><th>40 Ar *</th><th>40 Ar *</th><th>age</th><th>2 sigma error</th><th>Dry air</th><th>Glycol</th><th>Dry air</th><th>Glycol</th></tr><tr><th>[µm]</th><th>[wt. %]</th><th>[nL/g STP]</th><th>[%]</th><th>[Ma]</th><th>[Ma]</th><th>φ Δ²Θ</th><th>Metam. degree</th><th>φ Δ²Θ</th><th>Metam. degree</th></tr></thead><tbody><tr><td>LI 13006-1</td><td>&lt;2</td><td>2.79</td><td>100.47</td><td>95.59</td><td><strong>870</strong></td><td>10.9</td><td>1.3</td><td><strong>0.122</strong></td><td>EPI</td><td><strong>0.122</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>1.94</td><td>62.78</td><td>96.24</td><td><strong>797</strong></td><td>8.3</td><td>1.0</td><td><strong>0.160</strong></td><td>EPI</td><td><strong>0.165</strong></td><td>EPI</td></tr><tr><td>LI 14082</td><td>&lt;2</td><td>5.31</td><td>211.40</td><td>99.42</td><td><strong>940</strong></td><td>13.2</td><td>1.4</td><td><strong>0.290</strong></td><td>M-UA</td><td><strong>0.284</strong></td><td>M-UA</td></tr><tr><td></td><td>&lt;0,2</td><td>5.27</td><td>207.99</td><td>99.23</td><td><strong>934</strong></td><td>9.7</td><td>1.0</td><td><strong>0.328</strong></td><td>MA</td><td><strong>0.326</strong></td><td>MA</td></tr><tr><td>LI 14315</td><td>&lt;2</td><td>9.36</td><td>395.20</td><td>99.42</td><td><strong>984</strong></td><td>8.1</td><td>0.8</td><td><strong>0.170</strong></td><td>EPI</td><td><strong>0.175</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>8.91</td><td>367.77</td><td>99.44</td><td><strong>968</strong></td><td>13.5</td><td>1.4</td><td><strong>0.188</strong></td><td>EPI</td><td><strong>0.191</strong></td><td>EPI</td></tr><tr><td>LI 14320</td><td>&lt;2</td><td>7.85</td><td>319.66</td><td>99.29</td><td><strong>957</strong></td><td>11.2</td><td>1.2</td><td><strong>0.179</strong></td><td>EPI</td><td><strong>0.177</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>7.57</td><td>nd</td><td>nd</td><td><strong>nd</strong></td><td>nd</td><td>nd</td><td><strong>0.243</strong></td><td>UA/EPI</td><td><strong>0.249</strong></td><td>UA/EPI</td></tr><tr><td>LI 19142B</td><td>&lt;2</td><td>4.81</td><td>115.38</td><td>98.59</td><td><strong>623</strong></td><td>7.5</td><td>1.2</td><td><strong>0.456</strong></td><td>DIA/LA</td><td><strong>0.421</strong></td><td>DIA/LA</td></tr><tr><td></td><td>&lt;0,2</td><td>4.12</td><td>98.69</td><td>95.50</td><td><strong>622</strong></td><td>6.9</td><td>1.1</td><td><strong>0.507</strong></td><td>DIA/LA</td><td><strong>0.541</strong></td><td>DIA/LA</td></tr><tr><td>LI 19166</td><td>&lt;2</td><td>3.15</td><td>134.99</td><td>98.00</td><td><strong>995</strong></td><td>9.7</td><td>1.0</td><td><strong>0.259</strong></td><td>UA</td><td><strong>0.255</strong></td><td>UA</td></tr><tr><td></td><td>&lt;0,2</td><td>3.97</td><td>163.27</td><td>98.20</td><td><strong>965</strong></td><td>7.5</td><td>0.8</td><td><strong>0.252</strong></td><td>UA</td><td><strong>0.260</strong></td><td>UA</td></tr><tr><td>LI 19270</td><td>&lt;2</td><td>7.99</td><td>378.20</td><td>99.06</td><td><strong>1074</strong></td><td>11.4</td><td>1.1</td><td><strong>0.412</strong></td><td>DIA/LA</td><td><strong>0.448</strong></td><td>DIA/LA</td></tr><tr><td></td><td>&lt;0,2</td><td>8.19</td><td>375.39</td><td>99.70</td><td><strong>1048</strong></td><td>9.7</td><td>0.9</td><td><strong>0.483</strong></td><td>DIA/LA</td><td><strong>0.520</strong></td><td>DIA/LA</td></tr><tr><td>LI 19309</td><td>&lt;2</td><td>5.20</td><td>194.95</td><td>98.02</td><td><strong>897</strong></td><td>9.2</td><td>1.0</td><td><strong>0.194</strong></td><td>EPI</td><td><strong>0.199</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>5.11</td><td>nd</td><td>nd</td><td><strong>nd</strong></td><td>nd</td><td>nd</td><td><strong>0.233</strong></td><td>UA/EPI</td><td><strong>0.234</strong></td><td>UA/EPI</td></tr><tr><td>LI 19344</td><td>&lt;2</td><td>8.69</td><td>365.07</td><td>99.54</td><td><strong>980</strong></td><td>8.7</td><td>0.9</td><td><strong>0.318</strong></td><td>MA</td><td><strong>0.324</strong></td><td>MA</td></tr><tr><td></td><td>&lt;0,2</td><td>8.86</td><td>386.52</td><td>99.52</td><td><strong>1010</strong></td><td>11.3</td><td>1.1</td><td><strong>0.393</strong></td><td>LA</td><td><strong>0.439</strong></td><td>DIA/LA</td></tr><tr><td>LI 19465</td><td>&lt;2</td><td>8.91</td><td>348.98</td><td>99.23</td><td><strong>928</strong></td><td>10.4</td><td>1.1</td><td><strong>0.193</strong></td><td>EPI</td><td><strong>0.209</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>8.65</td><td>327.11</td><td>98.66</td><td><strong>903</strong></td><td>9.8</td><td>1.1</td><td><strong>0.242</strong></td><td>UA/EPI</td><td><strong>0.244</strong></td><td>UA/EPI</td></tr><tr><td>LI 19494</td><td>&lt;2</td><td>9.89</td><td>405.78</td><td>99.52</td><td><strong>963</strong></td><td>9.7</td><td>1.0</td><td><strong>0.256</strong></td><td>UA</td><td><strong>0.275</strong></td><td>UA</td></tr><tr><td></td><td>&lt;0,2</td><td>9.74</td><td>397.19</td><td>99.45</td><td><strong>958</strong></td><td>13.3</td><td>1.4</td><td><strong>0.268</strong></td><td>UA</td><td><strong>0.294</strong></td><td>M-UA</td></tr><tr><td>MB 3396</td><td>&lt;2</td><td>7.63</td><td>7.63</td><td>354.24</td><td><strong>1058</strong></td><td>10.5</td><td>1</td><td><strong>0.326</strong></td><td>MA</td><td><strong>0.320</strong></td><td>MA</td></tr><tr><td></td><td>&lt;0,2</td><td>7.90</td><td>7.90</td><td>nd</td><td><strong>nd</strong></td><td>nd</td><td>nd</td><td><strong>0.332</strong></td><td>MA</td><td><strong>0.329</strong></td><td>MA</td></tr><tr><td>MB 22071</td><td>&lt;2</td><td>6.19</td><td>263.41</td><td>98.35</td><td><strong>991</strong></td><td>19.3</td><td>1.9</td><td><strong>0.408</strong></td><td>DIA/LA</td><td><strong>0.399</strong></td><td>DIA/LA</td></tr><tr><td></td><td>&lt;0,2</td><td>6.75</td><td>281.24</td><td>90.85</td><td><strong>974</strong></td><td>10.8</td><td>1.1</td><td><strong>0.555</strong></td><td>DIA/LA</td><td><strong>0.559</strong></td><td>DIA/LA</td></tr><tr><td>SE 0013</td><td>&lt;2</td><td>4.510</td><td>106.52</td><td>97.30</td><td><strong>615</strong></td><td>7</td><td>1.1</td><td><strong>0.392</strong></td><td>DIA/LA</td><td><strong>0.386</strong></td><td>LA</td></tr><tr><td></td><td>&lt;0,2</td><td>4.570</td><td>106.14</td><td>99.66</td><td><strong>606</strong></td><td>5.7</td><td>0.9</td><td><strong>0.386</strong></td><td>LA</td><td><strong>0.387</strong></td><td>LA</td></tr><tr><td>SE 0099</td><td>&lt;2</td><td>4.650</td><td>109.00</td><td>97.95</td><td><strong>611</strong></td><td>4.8</td><td>0.8</td><td><strong>0.189</strong></td><td>EPI</td><td><strong>0.192</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>5.580</td><td>126.36</td><td>98.41</td><td><strong>593</strong></td><td>7.6</td><td>1.3</td><td><strong>0.213</strong></td><td>EPI</td><td><strong>0.209</strong></td><td>EPI</td></tr><tr><td>SE 3348</td><td>&lt;2</td><td>5.250</td><td>226.85</td><td>98.56</td><td><strong>1002</strong></td><td>8.7</td><td>0.9</td><td><strong>0.330</strong></td><td>L-MA</td><td><strong>0.339</strong></td><td>L-MA</td></tr><tr><td></td><td>&lt;0,2</td><td>5.950</td><td>251.67</td><td>99.98</td><td><strong>986</strong></td><td>12.7</td><td>1.3</td><td><strong>0.366</strong></td><td>LA</td><td><strong>0.360</strong></td><td>LA</td></tr><tr><td>SE 13171</td><td>&lt;2</td><td>4.960</td><td>109.77</td><td>97.15</td><td><strong>582</strong></td><td>4.8</td><td>0.8</td><td><strong>0.214</strong></td><td>EPI</td><td><strong>0.211</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>5.350</td><td>116.54</td><td>98.57</td><td><strong>574</strong></td><td>6.2</td><td>1.1</td><td><strong>0.208</strong></td><td>EPI</td><td><strong>0.212</strong></td><td>EPI</td></tr><tr><td>SE 19474</td><td>&lt;2</td><td>4.050</td><td>103.52</td><td>98.11</td><td><strong>656</strong></td><td>6.7</td><td>1</td><td><strong>0.408</strong></td><td>DIA/LA</td><td><strong>0.361</strong></td><td>LA</td></tr><tr><td></td><td>&lt;0,2</td><td>4.600</td><td>91.75</td><td>98.25</td><td><strong>532</strong></td><td>10.3</td><td>1.9</td><td><strong>0.422</strong></td><td>DIA/LA</td><td><strong>0.384</strong></td><td>LA</td></tr><tr><td>YE 0021</td><td>6-10</td><td>8.390</td><td>385.15</td><td>99.67</td><td><strong>1049</strong></td><td>16.7</td><td>1.6</td><td><strong>0.117</strong></td><td>EPI</td><td><strong>0.118</strong></td><td>EPI</td></tr><tr><td></td><td>2-6</td><td>8.890</td><td>403.10</td><td>99.67</td><td><strong>1040</strong></td><td>10.5</td><td>1</td><td><strong>0.133</strong></td><td>EPI</td><td><strong>0.137</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;2</td><td>8.19</td><td>359.95</td><td>99.91</td><td><strong>1015</strong></td><td>10.3</td><td>1</td><td><strong>0.165</strong></td><td>EPI</td><td><strong>0.163</strong></td><td>EPI</td></tr><tr><td></td><td>&lt;0,2</td><td>7.71</td><td>333.48</td><td>99.55</td><td><strong>1003</strong></td><td>14.5</td><td>1.4</td><td><strong>0.163</strong></td><td>EPI</td><td><strong>0.178</strong></td><td>EPI</td></tr></tbody></table>

## Illite crystallinity - metamorphic boundaries

diagenesis/anchizone: 0,600 - 0,400 Δ²Θ 0,600 diagenesis/0,400 lower anchizone/

anchizone/epizone: 0,240 ± 0,010 Δ²Θ 0,340 middle anchizone/0,290 upper anchizone/0,240 epizone

nd = not determined, EPI = Epizone, UA = upper anchizone, MA = middle anchizone

LA = lower anchizone, DIA = diagenesis

## Analytical parameters

<table><thead><tr><th>Ar-isotopic abundance</th><th>Spike-isotopic composition</th><th>Decay Constants [1/a]</th><th>Potassium</th></tr></thead><tbody><tr><td>40 Ar: 99,6000%</td><td>40 Ar: 0,0099980 %</td><td>l e: 5.81E-11</td><td>40K: 0.011670%</td></tr><tr><td>38 Ar: 0,0630%</td><td>38 Ar: 99,9890000 %</td><td>l b: 4.962E-10</td><td>K2O/K: 0.8302</td></tr><tr><td>36 Ar: 0,3370%</td><td>36 Ar: 0,0009998 %</td><td>l tot: 5.543E-10</td><td>Atomic wt [g/mol]</td></tr><tr><td>Standard Temperature Pressure (STP)</td><td></td><td>Molar Volume</td><td>tot Ar: 39,9477</td></tr><tr><td>0 °C; 760 mm Hg</td><td></td><td>[ml]: 22413,8</td><td>40Ar: 39,9624</td></tr><tr><td></td><td></td><td></td><td>tot K: 39,1027</td></tr><tr><td>Normal Atmosphere (DIN 1343)</td><td></td><td></td><td></td></tr><tr><td>273,15K; 1013,25 mbar</td><td></td><td></td><td></td></tr></tbody></table>

thermal and structural effects on the SOB.

## 5.1. Late mesoproterozoic (Stenian) to Early Neoproterozoic (Tonian) orogeny

### 5.1.1. Pre-tectonic mafic magmatism

The Francevillian rocks of the SOB are intruded by several generations of dolerite sills and dykes. Airborne magnetic data show that the first phase of voluminous mafic magmatism preceded regional metamorphism and D1 deformation, placing the age of emplacement prior to 1070 Ma. Petrography and geochemical analysis classify the rocks as

quartz-bearing metagabbro and dolerite of alkaline affinity associated with a continental intraplate setting (Vicat and Vellutini, 1987). It is possible that these are the far-reaching effects of the Karagwe-Angkole Rift in the eastern Kibaran Orogen, which evolved between ca 1400 and 1180 Ma (Klerkx et al., 1984, 1987; Tack et al., 2010; Fernandez-Alonso et al., 2012). Anomalous asthenospheric heating is proposed as cause for the emplacement of the ca. 1370 ± 40 Ma giant Lake Victoria Dyke Swarm (Mäkitie et al., 2014). According to the authors, the dykes may have been the feeders of continental flood basalts that were subsequently eroded.
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** The IC value vs K-Ar age diagram shows only weak, positive correlation between the two parameters, indicating little influence of detrital components.</figcaption></figure>

### 5.1.2. Tectonics

The K-Ar geochronology shows that the SOB was affected by regional deformation and metamorphism at the turn from the Mesoproterozoic to the Neoproterozoic. Geological mapping and geophysics interpretation revealed the structural subdivision of the SOB into an eastern and western domain, divided by the Douma Shear Zone. The geometry of slightly curvilinear regional folds in the eastern part together with progressive metamorphism from west to east indicates their development during an early stage D1a by WNW to NW-directed movements. Significant deviations in the direction of the measured fold axes in the western domain are attributed to a decreasing stress field that was locally influenced by the composition and geometry of individual rock layers.

The N-S trending Douma Shear Zone in the center of the study area represents a regional structure into which the strain was partitioned during subsequent deformation D1b. To the south, the shear zone splits into several splays whose drag into the main structure suggests left-lateral shearing component. D1a fold axes in vicinity of the Douma Shear Zone are progressively transposed into the direction of the structure thus constraining their older age. Asymmetric folds with moderate to steep E-dipping overturned limbs parallel to the locally developed S₁ schistosity document west-directed transports and in combination with the left-lateral movements the transpressive nature of the shear zone. This is also indicated by a structural unconformity in the southeasternmost branch, which is interpreted as a thrust transporting the Nassouk over the Douma Formation (Fig. 6). Deformation stage D1b thus documents the change from continuous rock deformation to strain partitioning into discrete shear zones during progressive deformation.

### 5.1.3. Metamorphism

Illite crystallinity data document decreasing metamorphism in the Eastern Domain from greenschist facies or higher in the east (epizone) to middle anchizone conditions at the western boundary (Fig. 9). The Western Domain is characterised by diagenetic to lower anchizone conditions, but without a clear jump across the Douma Shear Zone. This indicates that the regional metamorphic gradient remained almost unaffected by the shear and, together with moderate to steep thrust planes and S₁ foliations, argues against a major westward displacement of the Eastern over the Western Domain (i.e. nappe transport).

### 5.1.4. Geochronology

K-Ar isotope analysis of metamorphic illite/sericite shows that the ages of the 0.2 and 2µ fractions vary only slightly in individual samples but have a wide range of 900–1070 Ma across the central-eastern areas not affected by the Pan-African event. The variation is probably due to

partial argon loss, which leads to younger ages. The oldest determined ages of 1058 ± 11 Ma and 1074 ± 11 Ma therefore probably correspond most closely to the age of tectono-metamorphism coincident with a peak of the Kibaran orogeny in RDC dated at 1074 ± 14 Ma (Kokonynangi et al., 2006).

The tectonic style, low-grade metamorphism and K-Ar geochronology classify the SOB as foreland area of an unknown late Mesoproterozoic orogen further east, which later formed the substratum of the Congo basin. Typically, the internal zone of this orogen would be located at a distance of 100–300 km from the foreland. This implies either the presence of an obscured NW-SE trending orogenic belt (comparable to the Irumide belt in Zambia) or an arcuate geometry of the Kibaran orogen, parallel to the suspected subsequent Neoproterozoic rift hypothesised by Daly et al. (1992) and Kadima et al. (2011) (Fig. 13). In the second case, this means that the entire north-eastern part of the Congo Craton was overprinted by the Kibaran event.

## 5.2. Meso-to Neoproterozoic tectonism and magmatism

Geophysical data indicate two conjugate strike-slip fault systems corresponding to two stages of NE-SW and NW-SE extension, respectively; their crustal scale is indicated by mafic dykes emplaced into the faults. The evolution of these systems is placed between D1 (1070-1040 Ma or c. 950 Ma) and the Pan-African D3 nappe tectonics (c. 600 Ma). Vicat et al. (1997) further restricted the timing of magmatism before the deposition of the Sturtian Dia diamictite (720 Ma). The location of the strike slip systems in different parts of the SOB impedes the determination of their chronology, and the current division into D2a and D2b is random. However, the difference in the magnetic properties of the non-magnetic (D2a) and magnetic (D2b) dolerites suggests a different origin and/or petrogenesis of the magmas.

The olivine-orthopyroxene dolerites with tholeiitic affinity are again interpreted as intraplate magmatism (Vicat and Vellutini, 1987; Vicat et al., 1997). At regional scale, the extension can be related either to the development of a NW-SE trending Neoproterozoic rift, which is suggested to underlie the central part of the Congo Basin (De Wit and Linol, 2015, Fig. 13.) or the early rift stage of the Western Congo Belt (Alvarez and Maurin, 1991). Here, extensive bimodal magmatism is recorded in the Sounda Group, whose Nemba Complex (915 ± 8 Ma) consists of similar intraplate tholeiitic basalts and gabbros derived from the primitive mantle with a crustal component (Fullgraf et al., 2015). A fourth generation of intraplate dolerites intruding into the Dia tillite constrains their syn-to post-Sturtian age (Vicat and Vellutini, 1987; Vicat et al., 1997) and documents the longevity of Neoproterozoic extension. The authors did not distinguish between the different generations after D1 which makes it impossible to link them with specific D2-D3 stages of extension.

## 5.3. Pan-African orogeny

The Pan-African orogeny of the Central African Belt had only limited tectonic and thermal impact on the SOB. This is illustrated by the post-D1 Meso-to Neoproterozoic dykes and faults, which remain straight in the west of the study area and are only slightly curved in the east. Airborne magnetic data show some interference pattern in the Eastern Domain indicating buckling or gentle refolding of the Mesoproterozoic folds during the Pan-African event corresponding to NNE-SSW shortening. Mesoproterozoic fold axes to the west were already oriented at high angle prior to the SSE transport of the Dja nappe (suggested by shallow NNW dipping schistosity and bedding planes), preventing the development of fold interference structures. The limited thermal influence is evidenced by the absence of Pan-African K-Ar ages in samples more than eight km from the front of the nappe. The IC values of samples from the Dja nappe indicate epizonal (greenschist) conditions whereas the IC values of the nearest samples in the SOB record lower anchizone metamorphism.
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Geological overview map of Mesoproterozoic rocks of Central and East Africa (extracted from the BRGM 2M-SIG-Afrique, unpublished). Outline of the hypothetical Neoproterozoic rift modified from De Wit and Linol (2015).</figcaption></figure>

5.4. *Regional correlation*

The present study demonstrates the late Mesoproterozoic to early
Neoproterozoic age of the main D1 tectono-metamorphic event in the
SOB. A Palaeoproterozoic age of the D1 event was suggested by Vicat
et al. (2001) in the Bangui area 400 km north-east of Ouesso, where
low-grade schists, considered to be of Archaean age, are intruded by the
2070 Ma Mbi granodiorite (Poidevin and Pin, 1986). However, the
regional D1 greenschist metamorphism and emplacement of the Mbi
granodiorite predate the deposition of the Bangui Series, which has been
correlated with the Francevillian B-E. Both units (Mbi granodiorite,
Bangui Series) are overprinted by a second event D2 under low-grade
metamorphic conditions (Poidevin and Pin, 1986), which can be prob-
ably correlated with the D1 event in the SOB.

The connection with the Kibaran orogen is probable, even though the
nearest outcrops of this orogen are located about 1200 km southeast to
the SOB. An arcuate geometry is required to bring the internal zone of
the belt into proximity with the SOB (Fig. 13). The orogen has a long and
complex history, and debates in recent decades have centred on the
number and timing of tectonic events, the age of the basement and
geodynamic interpretation (Villeneuve et al., 2019 and references
therein). The authors conclude that the northeastern Karagwe-Angkole
Belt (Burundi, Rwanda) evolved in a rift environment, while the cen-
tral belt in eastern Congo is the result of subduction and continental
collision from around 1.38-1.0 Ga. Three tectono-magmatic cycles are
distinguished at about 1380-1375 Ma (D1), 1200-1180 Ma (D2) and
1120-1079 Ma (D3), followed by early Neoproterozoic N-S compression
at about 960 Ma (D4). The D3 cycle represents the culmination of these
events and is structurally characterised by NW-verging mesoscopic and
macroscopic isoclinal folds and reverse faults parallel to earlier D1
fabrics (Kokonynangi et al., 2006). The age of the M3 amphibolite facies
metamorphism has been dated at 1079 ± 14 Ma. Similar NW- to

W-directed transports and K-Ar cooling ages of 1074-1000 Ma in the
SOB suggest that the area was affected mainly by the Kibaran D3 event.

Alternatively, the closure of a concealed NE-SW trending Meso-
proterozoic basin 200 km southeast of the eastern margin of the SOB
could also explain the data (Fig. 13), but further geophysical data are
required to test this model. Contemporaneous voluminous magmatism
during basin evolution from 1200 Ma onwards is suggested by abundant
detrital zircon ages between 1200 and 900 Ma in samples of the over-
lying Phanerozoic Mpioka and Inkisi groups of the Niari Basin (Affaton
et al., 2016). While zircons with ages younger than 930 Ma may origi-
nate from igneous rocks of the Mayombe rift (Fullgraf et al., 2015), no
proximal western source of Stenian (1200-1100 Ma) igneous rocks is
known. The origin of the zircons from the Kibaran belt seems unlikely as
there are very few ages in the 1200–1400 Ma interval, which is a period
of major magmatism in this orogen (Affaton et al., 2016; Villeneuve
et al., 2019).

Voluminous post-tectonic tin-granites were emplaced from 975 to 925 Ma in the eastern Congo following the Kibaran orogeny and her- alded widespread Neoproterozoic rifting, termed the Rodina break-up on a global scale (Zhao et al., 2004). A concealed Neoproterozoic rift underlying the central part of the Congo basin has been identified and outlined using various geophysical methods (Giresse, 1982, Daly et al., 1992; Sandwell and Smith, 1997; Ritsema and van Heijst, 2000; Priestley et al., 2008; Downey and Gurnis, 2009; Crosby et al., 2010; not). The geometry parallels the hypothetical contact between the in- ternal and external zones of the Kibaran belt suggesting that the ancient crustal-scale anisotropies resulting from the Kibaran tectonics have been reactivated. In the SOB, Neoproterozic rifting is documented by two conjugate fault sets intruded by mafic magmas and led to the local development of pull-apart grabens.

The Pan-African nappe tectonics and greenschist facies meta-
morphism in the northwestern part of the SOB are consistent with the
position in the external zone of the Central African Fold Belt (Toteu et al., 2004, 2022). The Dja nappe represents the southernmost part of the Yaounde-Yangana Basin, which evolved during the Tonian to early Ediacaran, underwent metamorphism up to high-pressure granulite facies at c. 600 Ma and was subsequently thrust southward onto the Congo Craton (Toteu et al., 2022). The south-facing nappes locally imprinted pervasive schistosities into the underlying basement, gently folded the foreland and possibly reactivating Mesoproterozoic structures such as the Douma Shear Zone.

## 6. Conclusion

The new structural, metamorphic and geochronological data of the present study lead to a strongly revised model for the geological history of the SOB (Table 4).

* Palaeoproterozoic sedimentary deposition of the Francevillian FA-FC subgroups occurred from 2200 to 2100 in extensive eastern foreland basins of the Eburnean orogen. The minimum depositional age of 2070 Ma is constrained by the intrusion of the Mbi granodiorite in the Bangui region of the Central African Republic.

* Middle to late Mesoproterozoic extension of the SOB produced voluminous pre-tectonic mafic magmatism with alkaline affinity (i.e.

Biessi-Sill swarm). It is related either to the early extension of the Kibaran Belt (1370-1300 Ma, Villeneuve et al., 2019) or to a concealed Mesoproterozoic basin about 100-200 km southeast of the SOB (Fig. 13).

* The regional tectono-metamorphism D1 is bracketed between 1070 and 1000 Ma and varies over 200 km from diagenesis in the west to greenschist conditions in the east. An initial phase of W to NW-directed transports (D1a) was followed by a sinistral N-S transpression (D1b). All this classifies the SOB as foreland of a Mesoproterozoic orogeny, the internal zone of which is inferred about 100-300 km east of the SOB. It could represent the extension of the Kibaran Belt or result from the closure of a concealed Mesoproterozoic basin west of the Kibaran Belt.

* Early Neoproterozoic (?-850 Ma) rifting is recorded by two conjugate strike slip systems associated to NW-SE and NE-SW extension that resulted in local pull-apart structures (D2). Along the regional faults, mafic intraplate magmas were contemporaneously emplaced.

* The deposition of the Djab diamictite in a Neoproterozoic basin north of the SOB is related to the Sturtian event (715-685 Ma). Mafic intraplate magmatism outlasted deposition of the diamictite.

* Pan-African nappe tectonics (D3/M2) occurred around 600 Ma after an uplift phase that eroded the Francevillian C Subgroup and higher

<table><caption>**Table 4**
Revised geological history of the SOB.</caption><thead><tr><th>Orogeny</th><th>Plate tectonic setting</th><th>age [Ma]</th><th>Sedimenation</th><th>Magmatism</th><th>Tectonics</th><th>Metamorphism</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="3">Pan-African</td><td>Collision</td><td>600</td><td></td><td></td><td rowspan="3">D3: Nappe tectonics</td><td rowspan="3">M2: Greenschist</td><td rowspan="3">Nappe transport of Dja tillite<br>Mafic dykes intrude Djab tillite</td></tr><tr><td>Rift/Drift</td><td></td><td></td><td>Mafic magmatism<br>IV: olivine dolerite of tholeiitic affinity (intraplate magmatism)</td></tr><tr><td>Rift related continental intraplate magmatism</td><td>715-685<br>?-850</td><td>Dja diamictite: mass flow deposited during Sturtian glaciation</td><td>Mafic magmatism<br>II + III<br>Sangha Granite Suite</td><td>D2: Transcurrent tectonics (conjugate shear systems), pull apart basins: emplacement of bimodal magmas along major N-S faults</td><td>Contact</td></tr><tr><td>Kibaran</td><td>Collision</td><td>1100-1070</td><td></td><td></td><td>D1b: Regional sinistral N-S strike slip, development of transpressional Douma Shear Zone<br>D1a: W-directed folding and reverse thrusting under low-grade conditions, the intensity of deformation decreases from east (isoclinal folds) to west (monoclinal folds).</td><td>M1: Greenschist</td><td>M1 uplift age is dated by K-Ar illite geochronology. Metamorphism varies from greenschist in Eastern Domain of SOB to Diagenesis in Western Domain.</td></tr><tr><td rowspan="2">Eburnean</td><td>Extension</td><td>1370 - 1300 ?</td><td></td><td>Mafic magmatism<br>I: quartz dolerites, metagabbro, amphibole schist of alkaline affinity<br>Mbi granodiorite (CAR)</td><td rowspan="2">Late/post-tectonic</td><td rowspan="2"></td><td rowspan="2">Biessi megasill pre-dates M1/D1a</td></tr><tr><td>Orogenic collaps ?<br>Foreland basin</td><td>2070<br>2200-2100</td><td>FA: coarse-grained sandstones with locally intercalated conglomerate<br>FB: finer-grained sediments. Pelites and black shales are locally interlayered FC: only local preservation in Boutazab graben</td><td></td><td></td></tr></tbody></table>
stratigraphic units. The FC Subgroup is preserved in the Outazab Basin below the Dja Nappe.

### CRediT authorship contribution statement

**Thomas Fullgraf:** Writing – original draft, Supervision, Investigation. **Yannick Callec:** Supervision, Project administration, Investigation. **Guillaume Badinier:** Investigation. **Florent Boudzoumou:** Investigation. **Benoit Issautier:** Investigation. **Fabien Paquet:** Investigation. **Jean-Michel Schroetter:** Validation, Investigation. **Guillaume Vic:** Investigation. **Klaus Wemmer:** Investigation.

### Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the authors used DeepL software in order to improve readability and language of the manuscript. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

The study was part of the national geological mapping and mineral resources assessment program of the Republic of Congo. The French Geological Survey (BRGM) was commissioned by Asperbras-Congo, the company in charge of the implementation of this program, to carry out the geological mapping part. The authors acknowledge Asperbras-Congo for their logistical support, the provision of geophysical data and the authorisation to publish the results of the SOB. Michel Ville- neuve and an unknown reviewer are thanked for their helpful comments and suggestions.

### References

Affaton, P., Kalsbeek, F., Boudzoumou, F., Trompette, R., Thrane, K., 2016. The pan-african west Congo belt in the republic of Congo (Congo Brazzaville): stratigraphy of the Mayombe and west Congo supergroups studied by detrital zircon geochronology. Precambrian Res. 272, 185–202. https://doi.org/10.1016/j.precamres.2015.10.020.

Alvarez, P., Maurin, J.C., 1991. Evolution sédimentaire et tectonique du bassin protérozoïque supérieur de Comba (Congo): stratigraphie séquentielle du Supergroupe Quest-Congolien et modèle d'amortissement sur décrochements dans le contexte de la tectogènese panafricaine. Precambrian Res. 50, 137–171. https://doi.org/10.1016/0301-9268(91)90051-b.

Bache, J.J., Gres, M., Legras, M., 1966. Mission Ouesso, 3ième campagne. Juillet- decembre 1965. In: BRGM, Internal Report. Bra. 66. A7. 96pp.

Bache, J.J., Gres, M., Legras, M., 1967. Mission Ouesso, 4ième Campagne. Janvier-Mars 1966, 67. A4. BRGM, internal report. BRA., p. 58

Bessoles, B., Trompette, R., 1980. La chaîne panafricaine. In: Zone mobile d'Afrique Centrale (partie sud) et zone soudanaise, 92. Mémoire du BRGM, N'.

Blein, O., Callec, Y., Fullgraf, T., Chevillard, M., Cagnard, F., Bauer, H., Prognon, F., Badinier, Vic G., Boudzoumou, F., Melleton, J., 2017. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Mbomo. Editions BRGM.

Bourrel, J., Piffelmann, J.P., 1972. La province uranifère du bassin de Franceville (République Gabonaise). Miner. Deposita 7, 323–336.

Bouton, P., Thiéblemont, D., Simo Ndounze, S., Goujou, J.C., Kassadou, A.B., Walemba, A., Boulingui, B., Ekogha, H., Moussavou, M., Lambert, A., Roberts, D., Deschamps, Y., Prét, A., 2009a. Carte géologique de la République du Gabon à 1/ 200,000, feuille Franceville - Boumango. Editions DGMG - Ministères des Mines, du Pétrole, des Hydrocarbures, Libreville.

Bouton, P., Thiéblemont, D., Gouin, J., Cocherie, A., Guerrot, C., Tegyey, M., Prét, A., Simo Ndounze, S., Kassadou, A.B., Boulingui, B., Ekogha, H., Moussavou, M., 2009b. Notice explicative de la Carte géologique de la République du Gabon à 1/200,000, feuille Franceville - Boumango. Editions DGMG - Ministères des Mines, du Pétrole, des Hydrocarbures, Libreville.

Bouton, P., Thiéblemont, D., Simo Ndounze, S., Agenbacht, A., Walemba, A., Moussavou, M., Lambert, A., Deschamps, Y., Prét, A., 2009c. Carte géologique de la République du Gabon à 1/200,000, feuille Okondja. Editions DGMG -Ministères des Mines, du Pétrole, des Hydrocarbures, Libreville.

Bouton, P., Thiéblemont, D., Gouin, J., Cocherie, A., Guerrot, C., Tegyey, M., Prét, A., Simo Ndounze, S., Moussavou, M., 2009d. Notice explicative de la Carte géologique de la République du Gabon à 1/200,000, feuille Okondja. Editions DGMG - Ministères des Mines, du Pétrole, des Hydrocarbures, Libreville.

Callec, Y., Schroetter, J.-M., Paquet, F., Issautier, B., Prognon, F., Bruyère, D., Le Bayon, B., Badinier, G., Vic, G., Fullgraf, T., Delpomdor, F., Prét, A., 2017a. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Liouesso-Moloundou. Editions BRGM.

Callec, Y., Bauer, H., Paquet, F., Prognon, F., Issautier, B., Badinier, G., Vic, G., Fullgraf, T., 2017b. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Yengo. Editions BRGM.

Callec, Y., Blein, O., Fullgraf, T., Schroetter, J.-M., Chevillard, M., Le Bayon, B., Cagnard, F., Glaoguen, E., Prognon, F., Badinier, G., Rouzeau, O., Vic, G.C., Boudzoumou, F., Delpomdor, F., Prét, A., Melleton, J., 2017c. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Ewo. Editions BRGM.

Chochine, N., 1950. Carte géologique de reconnaissance de l'A.E.F. & l'échelle du 1/500 000 (feuille Makouk Est) avec notice explicative. Gouvernement Générale Afrique Equatorial Française. Direction Mines et Géologie., Brazzaville, p. 16.

Castaing, C., Feybesse, J.L., Thiéblemont, D., Triboulet, C., Chevremont, P., 1994. Palaeogeographical reconstructions of the Pan-African/Brasiliano orogen: closure of an oceanic domain or intracontinental convergence between major blocks? Precambrian Res. 67, 327–344. https://doi.org/10.1016/0301-9268(94)90095-7.

Crosby, A.G., Fishwick, S., White, N., 2010. Structure and evolution of the intracratonic Congo basin. G-cubed 11 (6), Q06010. https://doi.org/10.1029/2009gc003014.

Dadet, P., 1969. Notice explicative de la carte géologique de la république du Congo- Brazzaville au 1/500 000, 70. Mémoires du BRGM, Orléans, France, p. 103.

Daly, M.C., Lawrence, S.R., Diemu-Tshiband, K., Matouana, B., 1992. Tectonic evolution of the cuvette centrale, zaire. J. Geol. Soc. 149 (4), 539–546. https://doi.org/ 10.1144/gsjgs.149.4.0539. London.

Desthieux, F., 1992. Carte géologique de la République du Congo au 1:1 000 000. Ministère des Mines et de l'Energie du Congo, 2 coupures + notice explicative 27.

De Wit, M., Linol, B., 2015. Precambrian Basement of Congo and its flanking terranes. 19-35. In: De Wit, M., Guillocheau, F. (Eds.), Geology and Resource Potential of the Congo Basin. Regional Geology Reviews, Springer Verlag, Heidelberg, Germany, p. 417. https://doi.org/10.1007/978-3-642-29482-2_2.

Dostal, J., Dupuy, C., Poidevin, J.L., 1985. Geochemistry of precambrian basaltic rocks from the Central African Republic (equatorial Africa). Can. J. Earth Sci. 22, 653–662. https://doi.org/10.1139/e85-072.

Doucet, P., 1982. Mission géologique nord-Congo. In: Permis de Sembé. Rapport de la campagne Novembre 82-Mars 83. S.N.E.A. (P). Direction Mines, D.M.-DUF/PO, p. 105 R.

Downey, N.J., Gurnis, M., 2009. Instantaneous dynamics of the cratonic Congo basin. J. Geophys. Res. Solid Earth 114, B06401. https://doi.org/10.1029/2008JB006066.

Ebang Obiang, M., Nagel, J.L., Abouma Simba, S., 2009a. Carte géologique de la République du Gabon à 1/200 000, feuille Lastoursville. Editions DGMG - Ministère des Mines, du Pétrole et des Hydrocarbures, Libreville.

Ebang Obiang, M., Nagel, J.L., Abouma Simba, S., Tegyey, M., Guerrot, C., Cocherie, A., 2009b. Notice explicative de la Carte géologique de la République du Gabon à 1/200 000, feuille Lastoursville. Editions DGMG - Ministère des Mines, du Pétrole et des Hydrocarbures. Libreville.

Fernandez-Alonso, M., Cutten, H., De Waele, B., Tack, L., Tahon, A., Baudet, D., Baritt, S. D., 2012. The Mesoproterozoic Karagwe-Ankole belt (formerly the NE Kibara belt): the result of prolonged extensional intracratonic basin development punctuated by two short-lived far-field compressional events. Precambrian Res. 216–219, 63–86. https://doi.org/10.1016/j.precamres.2012.06.007.

Fuhrmann, U., Lippolt, H.J., Hess, J.C., 1987. Examination of some proposed K-Ar standards: 40Ar/39Ar analyses and conventional K-Ar data. Chem. Geol. 66, 41–51. https://doi.org/10.1016/0168-9622(87)90027-3.

Fullgraf, T., Callec, Y., Thiéblemont, D., Glaoguen, E., Charles, N., Le Métour, J., Prian, J.-P., Boudzoumou, F., Delhaye-Prat, V., Moreau, F., Kebi-Tsoumou, S., Ndiele, B., 2015. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Dolisie. Editions BRGM.

Fullgraf, T., Blein, O., Le Bayon, B., Cagnard, F., Boudzoumou, F., Joannes, C., Chevillard, M., Melleton, J., Callec, Y., 2017. Notice explicative de la carte géologique de la République du Congo à 1/200 000, Feuille Mékambo-Mintom. Editions BRGM.

Giresse, P., 1982. La succession des sédiments dans les bassins marins et continentaux du Congo depuis le début du Mésozoïque. Science Géologique, Bulletin 35 (4), 183–206. https://doi.org/10.3406/sgeol.1982.1620.

Grès, M., Legras, M., 1967. Mission Ouesso, 5ième campagne. Juin 1966-février 1967. BRGM, Internal report. BRA.67.A5 130.

Grès, M., Legras, M., 1970. Mission Ouesso. Rapport de synthèse (1964-1967). BRGM, Internal report. 70 RME 039 87.

Henry, B., Legras, M., 1965. Mission Ouesso. Rapport de fin de campagne 1964, 7. BRGM, Internal report, p. 110. Bra.65.A.

Henry, B., Grès, M., 1966. Mission Ouesso, 2ième campagne. Decembre 1964 - avril 1965. BRGM, Internal report 66 (A 3), 143. Bra.
Horie, K., Hidaka, H., Gauthier-Lafaye, F., 2005. U-Pb geochronolog and geochemistry of zircon from the Franceville series at Boudoudouma, Gabon. Geochimica et Cosmochimica Acta Conference: 15th Annual V M Goldschmidt Conference 69.

Issautier, B., Chevillard, M., Fullgraf, T., Callec, Y., Le Bayon, B., Schroetter, J.M., Gloaguen, E., Blein, O., Prognon, F., Badinier, G., Vic, G., Bruyère, D., Boudzoumou, F., Delpom dor, F., 2017. Carte géologique de la République du Congo à 1/200 000, Feuille Sembé-Souanké. Editions BRGM.

IUGS 2023. https://stratigraphy.org/guide/index.html.

Jaunatre, A., 1969. Rapport Synthèse Géologique du Ouesso. Rapport C.E.A.

Kadina, É., Ntabwoba, S.S.M., Lucazeau, F., 2011. A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin. Earth Planet Sci. Lett. 304 https://doi.org/10.1016/j.epsl.2011.01.037, 240-25.

Kisch, H.J., 1991. Illite crystallinity: recommendations on sample preparation, X-ray diffraction settings, and interlaboratory samples. J. Metamorph. Geol. 9/6, 665-670. https://doi.org/10.1111/j.1525-1314.1991.tb00556.x.

Klerkx, J., Lavreau, J., Liégeois, J.P., Theunissen, K., 1984. Granitoides Kibariens précoces et tectonique tangentielle au Burundi: magmatisme bimodal lié à la distension crustale. In: Klerkx, J., Michot, J. (Eds.), African Geology. Musée royal de l'Afrique centrale, Tervuren, Belgium, pp. 29-46.

Klerkx, J., Liégeois, J.P., Lavreau, J., Claessen, W., 1987. Crustal evolution of the northern kibaran belt, eastern and central Africa. In: Kröner, A. (Ed.), Proterozoic Lithospheric Evolution. Geodynamics Series. American Geophysical Union, pp. 217-233.

Kokonynangi, J.W., Kampunzu, A.B., Armstrong, R., Yoshida, M., Okudaira, T., Arima, M., Ngulube, D.A., 2006. The mesoproterozoic kibaride belt (katanga, SE D. R. Congo). J. Afr. Earth Sci. 46, 1-35. https://doi.org/10.1016/j.jafrearsci.2006.01.017.

Mäkitie, H., Data, D., Isabirye, E., Mänttäri, I., Huhma, H., Klausen, M.B., Pakkanen, L., Virransal, P., 2014. Petrology, geochronology and emplacement model of the giant 1.37 Ga arcuate Lake Victoria Dyke Swarm on the margin of a large igneous province in eastern Africa. J. Afr. Earth Sci. 97, 273-296.

Martini, J., Makanga, J.F., 2001. Carte métallogénie de la République Gabonaise au 1: 1, 000 000 et note explicative. In: Ministère des Mines, de l'Énergie, du Pétrole et des Ressources hydrauliques du Gabon.

Poidevin, J.L., Pin, C., 1986. 2 Ga U-Pb zircon dating at Mbi granodiorite (Central African Republic) and its bearing on the chronology of the Proterozoic of Central Africa. J. Afr. Earth Sci. 5-6, 581-587. https://doi.org/10.1016/0899-5362(86) 90024-2.

Pons, F., 1971. Indices minéraux et ressources minérales du Gabon : Uranium. In: Plan Minéral de la République Gabonaise, Collectif, 1971, deuxième partie, chapitre X. Direction des Mines du Gabon, Ministère des Mines, pp. 143-148.

Préat, A., Bouton, P., Thiéblemont, T., Prian, J.P., Delpom dor, F., 2011. Palaeoproterozoic high d13C dolomites from the Lastoursville and Franceville basins (SE Gabon): stratigraphic and syndesimetric subsidence implications. Precambrian Res. 189 (1-2), 212-228. https://doi.org/10.1016/j.precamres.2011.05.013.

Prian, J.P., Simeon, Y., Johan, V., Ledru, P., Piantone, P., Coste, B., Eko, N'Dong, 1990. Synthèse géologique et géochimique, potentialités minières du dégrée carre Bououé (Gabon Central), avec carte géologique à 1/200 000, 1. Edition BRGM pour DGMG Gabon, p. 103, 2, 43.

Prian, J.P., Couëffé, R., Thiéblemont, D., Goujou, J.C., Ekogha, H., Boulingui, B., Simo Ndounze, S., Kassadou, A.B., Deschamps, Y., Theunissen, K., 2009a. Carte géologique de la République du Gabon à 1/200 000, feuille Lambaréné. Editions DGMG - Ministère des Mines, du Pétrole, des Hydrocarbures. Libreville.

Prian, J.P., Couëffé, R., Thiéblemont, D., Cocherie, A., Tegyey, M., Castaing, C., Ekogha, H., Boulingui, B., Simo Ndounze, S., Kassadou, A.B., 2009b. Notice explicative de la Carte géologique de la République du Gabon à 1/200 000, feuille Lambaréné. Editions DGMG - Ministère des Mines, du Pétrole, des Hydrocarbures. Libreville.

Priestley, K., McKenzie, D., Debayle, E., Pilidou, S., 2008. The African upper mantle and its relationship to tectonics and surface geology. Geophys. J. Int. 175 (3), 1108-1126. https://doi.org/10.1111/j.1365-246x.2008.03951.x.

Ritsema, J., van Heijst, H., 2000. New seismic model of the upper mantle beneath Africa. Geology 28 (1), 63-68. https://doi.org/10.1130/0091-7613(2000)028%3C0063: nsmotu/3E2.3.co;2.

Correlation of uranium geology between south America and Africa. In: Saggerson, E.P., Turner, L.M. (Eds.), 1986. Technical Reports Series, 270. International Atomic Energy Agency, Vienna, p. 475. ISBN 92 0 145286 1.

Sandwell, D.T., Smith, W.H.F., 1997. Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. J. Geophys. Res. 102 https://doi.org/10.1029/96jb03223, 10-039.

Schumacher, E., 1975. Herstellung von >99,9997% 38Ar für die 40K/40Ar Geochronologie. Geochronologica Chimica 29, 441-442.

Sonet, J., 1958. Carte géologique de reconnaissance de l'A.E.F. à l'échelle du 1/500 000 (feuille Ouesso) avec notice explicative, Gouvernement Générale Afrique Equatorial Française. Direction Mines et Géologie, Brazzaville, pp. 24.

Steiger, R.H., Jäger, E., 1977. Subcommission on Geochronology: convention to the use of decay constants in geo- and cosmochronology. Earth Planet Sci. Lett. 36, 358-362. https://doi.org/10.1016/0012-821x(77)90060-7.

Tack, L., Wingate, M.T.D., De Waele, B., Meert, J., Belousava, E., Griffin, B., Tahon, A., Fernandez-Alonso, M., 2010. The 1375 Ma "Kibaran event" in Central Africa: prominent emplacement of bimodal magmatism under extensional regime. Precambrian Res. 180, 63-84. https://doi.org/10.1016/j.precamres.2010.02.022.

Thiéblemont, D., Castaing, C., Billa, M., Bouton, A., Préat, A., 2009. Notice explicative de la carte géologique et des ressources minérales de la République gabonaise à 1/1000000. Programme Sysmin 8 ACP GA 017, Ministère des Mines, du Pétrole, des Hydrocarbures. Direction Générale des Mines et de la Géologie, p. 384.

Thiéblemont, D., Pascal Bouton, P., Préat, A., Goujou, J.C., Tegyey, M., Weber, F., Ebang Obiang, M., Joron, J.L., Treuil, M., 2014. Transition from alkaline to calc-alkaline volcanism during evolution of the Palaeoproterozoic Francevillian basin of eastern Gabon (Western Central Africa). J. Afr. Earth Sci. 99, 215-227. https://doi.org/10.1016/j.jafrearsci.2013.12.007.

Toteu, S.F., Penaye, J., Djomani, Y.P., 2004. Geodynamic evolution of the Pan-African belt in central Africa with special reference to Cameroon. Canadian Journal of Earth Sciences 41 (1), 73-85. https://doi.org/10.1139/e03-079.

Toteu, S.F., Wit, M.J., Penaye, J., Drost, K., Tait, J., Bouyo, M., Van Schmus, W., Jelsma, H., Moloto-A-Kenguemba, G., Filho, A.F., Lerouge, C., Doucoure, M., 2022. Geochronology and correlations in the Central African Fold Belt along the northern edge of the Congo Craton: new insights from U-Pb dating of zircons from Cameroon, Central African Republic, and south-western Chad. Gondwana Res. 107, 296-324. https://doi.org/10.1016/j.gr.2022.03.010.

Vicat, J.P., Vellutini, P.J., 1987. About the nature and importance of the Precambrian dolerites de Sembé-Ouesso basin. Precambrian Res. 37, 57-69. https://doi.org/10.1016/0301-9268(87)90039-8.

Vicat, J.P., Gioan, P., Albouy, Y., Cornacchia, M., Giorgi, L., Blondin, P., 1989. Mise en évidence sur la bordure ouest du Craton du Congo, de fossés d'effondrement d'âge protérozoïque supérieur masqués par les formations phanérozoïques de la cuvette du Zaire, 309. Compte Rend Académie Sciences, Paris, pp. 1207-1213.

Vicat, J.P., Pouclet, A., 1995. Nature du magmatisme lié à une extension pré-panafricaine: les dolérites des bassins de Combo et de Sembé-Ouesso (Congo). Bulletin Société géologique France 166 (4), 355-364. https://doi.org/10.2113/gsgfbull.166.4.355.

Vicat, J.-P., Pouclet, A., Nkoumbou, C., Seme-Mouangue, A., 1997. Le volcanisme fissural néoproterozoïque des séries du Dja inférieur, de Yokadouma (Cameroun) et de Nola (RCA) - Signification géotectonique. Compte Rend Académie Sciences 325, 471-677. https://doi.org/10.1016/s1251-8050(97)89109-4. Paris.

Vicat, J.-P., Moloto-A-Kenguemba, G., Pouclet, A., 2001. Les granitoïdes de la couverture protérozoïque de la bordure nord du craton du Congo (Sud-Est du Cameroun et Sud-Ouest de la République centrafricaine), témoins d'une activité magmatique post-kibarienne à pré-panafricaine. Compte Rend Académie Sciences 332, 235-242. https://doi.org/10.1016/s1251-8050(01)01521-x. Paris.

Villeneuve, M., Gärtner, A., Kalikone, C., Wazi, N., Hofmann, M., Linnemann, U., 2019. U-Pb ages and provenance of detrital zircon from metasedimentary rocks of the Nyangezie and Bugarama groups (D.R. Congo): a key for the evolution of the Mesoproterozoic Kibaran-Burundian Orogen in Central Africa. Precambrian Res. 328, 81-98. https://doi.org/10.1016/j.precamres.2019.04.003.

Warr, L.N., Rice, A.H.N., 1994. Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data. J. Metamorph. Geol. 12 (2), 141152 https://doi.org/10.1111/j.1525-1314.1994.tb00010.x.

Weber, F., 1971. Indices minéraux et ressources minérales du Gabon: Manganèse. In: Plan Minéral de la République gabonaise, Collectif, 1971, deuxième partie, chapitre IX. Ministère des Mines, Direction des Mines du Gabon, pp. 129-142.

Weber, F., Gauthier-Lafaye, F., Whitechurch, H., Ulrich, M., El Albani, A., 2016. The 2-Ga Eburnean Orogeny in Gabon and the opening of the Francevillian intracratonic basins: a review. Compt. Rendus Geosci. 348, 572-586. https://doi.org/10.1016/j.crite.2016.07.003.

Zhao, G., Sun, M., Wilde, S.A., Li, S., 2004. A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup. Earth Sci. Rev. 67, 91-123. https://doi.org/10.1016/j.earscirev.2004.02.003.