Distribution of Albian clastic deposits in the Benguela basin (Angola ...

Bull. Soc. géol. Fr., 2009, t. 180, no 2, pp. 117-129

Distribution of Albian clastic deposits in the Benguela basin (Angola): evidence of a Benguela palaeocurrent? DIDIER QUESNE1, ANDRÉ BUTA-NETO2, DOMINIQUE BENARD1,3 and MICHEL GUIRAUD1 Key-words. – Tuenza Formation, Albian, Kwanza basin, Albian palaeocurrents, Benguela current, Angola.

Abstract. – The Albian clastic Tuenza Formation of the Benguela basin (south Angola) is described and its geographic distribution analysed in detail. From these observations, the existence of an Albian “PalaeoBenguela current” is proposed. This would imply that a marine longshore current reworked the sand transported by palaeorivers and settled it northward along the coast of the young Atlantic Ocean (during the Albian). A parallel between Albian clastic deposits and the modern sediments deposited by the Benguela current is drawn, focusing on the striking resemblance between the two currents, i.e. the actual Benguela current and the Albian marine current, and the associated sedimentary bodies.

Distribution des sédiments détritiques albiens dans le bassin de Benguela (Angola) : mise en évidence d’un paléocourant de Benguela ? Mots-clés. – Formation Tuenza, Albien, Bassin de la Kwanza, Courant de Benguela, Angola.

Résumé. – La formation albienne détritique de Tuenza du bassin de Benguela (Sud Angola) est décrite et sa distribution géographique est analysée en détail. À partir de ces observations, l’existence d’un “ paléocourant de Benguela “ est proposée. Ceci impliquerait qu’un courant marin longeant la côte a remanié le sable apporté par des paléofleuves et l’a déposé le long de la côte du jeune océan Atlantique pendant l’Albien. Un parallèle entre ces dépôts détritiques albiens et les sédiments actuels déposés par le courant de Benguela est proposé, en soulignant la forte ressemblance entre ces deux courants et les dispositifs sédimentaires associés.

INTRODUCTION The discovery of giant oil fields in the Atlantic margin basins of Angola has focused attention on the different control factors affecting Cretaceous sedimentation. Although petroleum companies have performed many studies on seismic profiles and exploration drillings, these works essentially concern offshore areas [Brognon and Verrier, 1966; Tillement, 1987; Hudec and Jackson, 2004], so Angolan geology remains poorly understood onshore. The only data available is work by Mascarenhas Neto [1961], and more recently, by Duarte-Morais and Sgrosso [1992, 2000] and Morais and Sgrosso [1993]. All these authors have contributed to define the stratigraphic framework of the Kwanza sedimentary basin (fig. 1 and 2), although most of their research has focused on the northern part of the basin (between latitudes 8o and 11o south). This work focuses on the Benguela Basin (south Angola), which represents the southern part of the Kwanza basin (fig. 1). The aim here is to focus on the clastic dominated Albian Tuenza Formation (fig. 2), which, due to the civil war (1960 – 2002), has not been extensively studied [Amaral

and Seyve, 2000]. The study area is located in the southern part of the Benguela basin, between the town of Catumbela and the village of Cuio (fig. 3). This sedimentary area is located between the Pan African basement to the east, and the Atlantic Ocean to the west. In order to study the Tuenza Formation from south to north, five outcrop areas were selected for the quality of the exposures of the Lower Cretaceous deposits: Cuio, Dombe Grande, Tchimalavera, Santa Clara and Catumbela (fig. 3).

METHOD The outcrops have been studied by reconstructing several sections, in order to define the situation and the importance of the Tuenza Formation in the sedimentary infill of the Benguela basin. Palaeocurrents have been systematically measured, when possible, on crest ripples, trough cross beddings and foresets.

1. Université de Bourgogne, UFR des Sciences de la Terre et de l’Environnement, UMR 5561 Biogéosciences, Dijon. France. [email protected], [email protected] 2. Universidad Agostinho Neto, Luanda, Angola. André[email protected] 3. Bénevise, 26 410 Treschenu-Creyers, France Manscrit déposé le 30 avril 2007 ; accepté après modifications le 19 juin 2008 Bull. Soc. géol. Fr., 2009, no 2

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THE SPATIAL EVOLUTION OF THE TUENZA FORMATION To study the spatial evolution of the Tuenza Formation, the sedimentary facies were observed on a south-to-north transect along the coast, particularly in the areas of CuioDombe Grande, Tchimalavera, Santa Clara, and Catumbela.

Description of the Tuenza Formation

FIG. 1. – Location of the study area. FIG. 1. – Localisation du secteur étudié.

FIG. 2. – Partial chronostratigraphic chart of the Kwanza Basin. FIG. 2. – Charte chronostratigraphique partielle du bassin de la Kwanza. Bull. Soc. géol. Fr., 2009, no 2

The Tuenza Formation rests on the Binga Formation, which is made of lagoonal and probably late Aptian – early Albian azoic mudstones (fig. 2), and passes progressively upward to the Catumbela Formation (Albian in age, fig. 2). The areas chosen to study the Tuenza Formation in the Benguela basin, illustrate this feature. The Catumbela Formation is made of sandy, locally oolitic limestones, containing some ammonites and numerous foraminifera. This Formation represents a shoreface palaeoenvironment.

ALBIAN CLASTIC DEPOSITS IN THE BENGUELA BASIN (ANGOLA)

In all the outcrops where it can be seen, the Tuenza Formation is characterized by its clastic nature. The CuioDombe Grande section can be used as a reference to describe the Tuenza Formation (fig. 4) because of its complete development in this area. The Formation reaches here a thickness of 40 m. The high cliffs of the Cuio-Dombe Grande outcrop display sedimentary structures such as 2D and 3D dunes of variable sizes (fig. 4 and 5). These structures occur from the base to the top of the section, which shows two main lithological facies:

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– Facies 1: medium to coarse grained sandstones (0.2 to about 2 mm in grain size), commonly well sorted, mainly composed of quartz and micas; – Facies 2: erosive and poorly sorted levels of conglomerates containing pebbles and cobbles originating from the basin basement (quartz, granite and metamorphic rocks). The only fossils identified in either of the two facies are marine gastropods (Nerinea) observed within 2D megaripples at the 11 m level in this section, and in other places of the Tuenza Formation.

FIG. 3. – A) location of the studied areas and sections, B) simplified geological map. FIG. 3. – A) Localisation des secteurs étudiés et des coupes, B) Carte géologique simplifiée. Bull. Soc. géol. Fr., 2009, no 2

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FIG. 4. – The Dombe Grande-Cuio section (location, see fig. 3 and 7). C = clay; Fs = fine sandstone; Ms = medium sandstone and Cs = coarse sandstone and conglomerate. FIG. 4. – Coupe de Cuio-Dombe Grande (localisation, voir fig. 3 et 7). C = argile ; Fs = grès fins ; Ms = grès moyens et Cs = grès grossiers et conglomérats.

Systematic measurement of paleocurrent directions on the different bedforms highlighted a remarkable agreement between them (fig. 6). The 2D and 3D dunes suggest that traction is the main mechanism of transport. As 2D dunes facies contain locally marine gastropods, we assume they have been deposited in marine environment. 3D structures, systematically associated with coarse sandstones, reflect a more proximal and energetic environment. We suggest that they may indicate fluvial stream action. In summary, the Tuenza Formation reflects a marine environment influenced sometimes by fluvial conditions. Such configuration is typical of a deltaic environment, when detrital sedimentation (sandstones and conglomerates) is dominant as in the Cuio area. The Tuenza Formation shows the two main facies (facies 1 and facies 2) in all sections observed in the study Bull. Soc. géol. Fr., 2009, no 2

area, from Cuio to Catumbela. The only differences observed are the thickness of the formation, and the respective distribution of the two facies. Moving northward from the Cuio section previously described (fig. 6), the main sections can be defined as follows: – in the Dombe Grande sections (fig. 7 and 8), the Tuenza Formation gradually disappears. The sandstones diffuse into the Catumbela Formation that is characterized by thick limestones (grainstone with foraminifera, such as Hedbergella). The sedimentary structures measured are foresets in the few clastic migrating bedforms seen in this area (fig. 6); – in the Tchimalavera area (fig. 3, 7 and 9), the Tuenza Formation (25 m thick), is represented at the base by a coarse conglomerate of basement pebbles (facies 2), followed


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FIG. 5. – A : partial view of the Tuenza Formation on the Dombe Grande-Cuio track; B : 2D dune in the Tuenza Formation. Progradation toward N055oE. FIG. 5. – A : vue partielle de la formation Tuenza depuis le chemin de Dombe Grande à Cuio ; B : dune 2D dans la formation Tuenza. Progradation vers le N055oE.

FIG. 6. – Rose diagrams of the measured sedimentary structures in the Tuenza Formation. FIG. 6. – Roses diagrammes des structures sédimentaires mesurées dans la formation de Tuenza. Bull. Soc. géol. Fr., 2009, no 2

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FIG. 7. – Simplified geological map of the Dombe Grande-Cuio, Tchimalavera and Santa Clara areas. FIG. 7. – Carte géologique simplifiée des secteurs de Dombe Grande-Cuio, Tchimalavera et Santa Clara.

by fine-grained sandstone (facies 1) with a few sedimentary structures (2D ripples essentially). Paleocurrent directions are shown on figure 6. The Tuenza Formation is overlaid by the Catumbela Formation (12 m thick), mainly composed of dolomitic limestone, sometimes interrupted by fine-grained sandstone layers. As in the Cuio section, the clastic Tuenza Formation is again dominant in the Tchimalavera area, while it is absent from the Dombe Grande area;

oolithic limestones characterise this formation. As in the Dombe Grande area, the Tuenza Formation is not clearly individualized in the Santa Clara sections. It is represented by a fine to medium-grained sandstone (facies 1) that diffuses into the Catumbela Formation through sandy limestones and thin sandstone layers showing locally ripples and small dunes (detail in figure 10). The directions measured in the section are shown in the figure 6;

– in the Santa Clara area (fig. 3 and 7), salt tectonic affects the deposits [Buta-Neto, 2006]. Cross sections (fig. 10) show the predominance of the Catumbela Formation, which rests above a shale level (1 to 5 m thick), crossed by secondary gypsum veins. Dolomitic and sometimes

– finally, at the very north of the study area, near the town of Catumbela (fig. 3 and 11), the predominant formation is the Catumbela Formation, outcropping in huge cliffs, 150 to 300 m thick. Here, the Tuenza Formation is not clearly individualized. Clastic sedimentation, represented

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by sandstone (facies 1) or conglomerate (facies 2) layers, is diluted in the thick carbonate sedimentation, as in the Dombe Grande and Santa Clara areas. No sedimentary structures have been observed. Interpretation This study highlights an important feature of the Benguela basin: the Tuenza Formation only outcrops in some areas of the basin (fig. 7). When considering this formation in detail, its mode of deposition changes from one area to another: it is most common to the south of the Cuio-Dombe Grande area and in the Tchimalavera area, where the formation grades into the Catumbela Formation north Cuio-Dombe Grande area, Santa Clara and Catumbela areas. Due to the lack of biostratigraphic data from the Tuenza Formation, it is impossible to date precisely these detritic facies, and consequently to correlate the different sections. The

Tuenza Formation can nevertheless be relatively dated, thanks to its situation. As observed on the field, it is closely related to the Catumbela Formation (mainly below, and some times between), of which a Lower and Medium Albian age is given by the ammonites observed in the Dombe Grande area [Tavares, 2006]. As shown by the sedimentary structures (fig. 6), two families of paleocurrent directions exist. The first family (on average: N130oE – N310oE) is associated with the sedimentary structures that are assumed to be influenced by fluvial dynamics (3D bedforms observed in the Cuio-Dombe Grande area; fig. 4 and 5). The second family (on average NNE– ENE) is associated with the 2D bedforms of marine origine (fig. 6). This feature can be interpreted by assuming the existence of palaeorivers reaching the Albian Atlantic coast

FIG. 8. – (A) Photographic panorama and (B) field section near Dombe Grande (location, see fig. 3 and 7). FIG. 8. – (A) Panorama photographique et (B) coupe dans le secteur de Dombe Grande (localisation, voir fig. 3 et 7). Bull. Soc. géol. Fr., 2009, no 2

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FIG. 9. – The Tchimalavera section (location, see fig. 3 and 7). FIG. 9. – Coupe de Tchimalavera (localisation, voir fig. 3 et 7).

(fig. 12). These rivers carried sand and pebbles from the basement to the young Atlantic Ocean. The vertical distribution of the two sedimentary facies contained into the Tuenza Formation suggests the action of a current. The sediments were settled in deltas and probably reworked at the coast by marine palaeocurrents. This mode of deposit may have formed coastal sandbodies (facies 1), migrating parallel to the coast under the action of a longshore current as indicated by the second family of paleocurrent directions (NNE-ENE; fig. 6). Incisions (a few kilometres long, hundred meters large and up to fifty meters high) can be observed on the field in the Benguela basin (fig. 13). Some of them incise the basement as well as the Aptian Sal massivo Formation (fig. 2), and they are filled with Late Cretaceous shales. Thus they occur after the Aptian and before the Late Cretaceous. When concerning the basement, they are orientated following the main faults, whereas on the sedimentary infill of the basin, their direction may change. During the Early Cretaceous and afterwards, these incisions could have served as outlets for clastic sediments transported by palaeorivers, from the continent to the sea. The deep-sea fans associated Bull. Soc. géol. Fr., 2009, no 2

to these paleorivers are observed by the petroleum geologists in offshore areas, and may be rather similar to those studied in the gulf of Guinée [Anka and Seranne, 2004]. At the mouth of a palaeoriver, Albian sedimentation was strongly influenced by the clastic part of the deposits. Farther along the coast, as the influence of river-born clastic deposits decreased progressively, carbonate sedimentation could become dominant (fig. 12). In such a case, as suggested by observations, a marine longshore current would have been strong enough to transport the sand northward along the Early Cretaceous shoreline (fig. 6 and 12). This interpretation explains the distribution of the Tuenza Formation. It is predominant in two areas: Cuio-Dombe Grande and Tchimalavera, where the two sedimentary facies can be equally observed. The Tuenza Formation progressively decreases at Dombe Grande north, at Santa Clara, and Catumbela (fig. 12), where the sedimentary facies 1 is predominant. The marine current, deduced from the sedimentary structures observed along the sections, can be set in the context of the opening phases of the young Atlantic Ocean. This current may have been determined by the opening of

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ocean passages in the southern part of the Atlantic Ocean, as a consequence of the break-up of Gondwana and the divergence of South America and Africa, because currents are strongly sensitive to paleogeographic evolution [Poulsen et al., 1998; Martin-Chivelet et al., 2003]. When created, the current, channelled between West Africa and the east of South America, may have travelled northward along the coasts of Africa [Francis and Frakes, 1993] (fig. 14). It is important to note the striking similarity between these observations and the modern distribution of clastic sediments transported by present-day rivers (Cacuaco River, Coporolo River, Catumbela River, fig. 3). Satellite photographs (fig. 3, 7 and 15) show this distribution and highlight the role of the present-day Benguela current, the

eastern boundary current of the South Atlantic subtropical gyre [Peterson and Stramma, 1991; Wedepohl et al., 2000]. The Benguela current is generated by the SET (South-East Trade Wind), the equatorial part of which follows the Namibian coast (Namibia upwelling), and then the Angolan coast [Shannon and Nelson, 1996; Hagen et al., 2001]. The Benguela current causes vigorous coastal erosion [Wefer et al., 1998]. It reworks the sediments carried to the coast by present-day rivers in typical wave-dominated deltas (fig. 15A), and builds sandy barriers, several kilometre long, as in Lobito (fig. 3), Mussolo (fig. 15), and Luanda. These coastal sedimentary bodies are made of sand, gravels and pebbles, similarly to the Tuenza Formation containing sandstones and conglomerates. The

FIG. 10. – (A) photographic panorama, (B) interpretative scheme, and section near Santa Clara area (location, see fig. 3 and 7). FIG. 10. – (A) Panorama photographique, (B) schema interpretatif et coupe dans le secteur de Santa Clara (localisation, voir fig. 3 et 7). Bull. Soc. géol. Fr., 2009, no 2

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present-day Benguela current is thus proposed as a modern analogue of the coastal processes that gave rise during Early Cretaceous times to the deposition and distribution of the Tuenza Formation. CONCLUSION This study shows that the Albian Tuenza Formation, which settled in coastal palaeoenvironments, may have been affected by a longshore marine current. This observation reveals rather similar conditions during the Albian and modern periods.

FIG. 11. – The Catumbela section (location, see fig. 3). FIG. 11. – Coupe de Catumbela (localisation, voir fig. 3). Bull. Soc. géol. Fr., 2009, no 2

This result is interesting because (1) it suggests the existence of a “PalaeoBenguela current”, which could have been active during the Albian in the young Atlantic Ocean and (2), it may offer interesting research perspectives for oil companies, as these results can help in predicting Albian reservoir quality, geometry and distribution.

Acknowledgements. – This work has been financed by Total E & P Luanda. We thank E. Vennin, A. Préat and B. Tessier for reviews that greatly improved the manuscript.


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FIG. 12. – Distribution scheme of the Tuenza Formation during the Albian, and representation of a marine tractive current reworking clastic deltaic deposits. FIG. 12. – Schématisation de la distribution de la formation Tuenza durant l’Albien, et représentation du courant marin tractif remaniant les dépôts détritiques deltaïques.

FIG. 13. – Situation and orientation of three main incisions in the Benguela basin. FIG. 13. – Situation et orientation de trois incisions majeures dans le bassin de Benguela. Bull. Soc. géol. Fr., 2009, no 2

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FIG. 14. – Late Albian paleogeographic reconstruction [modified from Barron, 1987 and Funnel, 1990], with the main palaeocurrents [from Francis and Frakes, 1993]. FIG. 14. – Reconstitution paléogéographique de l’Albien supérieur [modifié à partir de Barron, 1987 et Funnel, 1990], avec les principaux paléocourants [à partir de Francis and Frakes, 1993].

FIG. 15. – (A) Modern distribution of the deltaic deposits of the Coporolo river, due to the Benguela current. (B) Sand barrier developed due to the present-day Benguela current, in the Mussulo lagoon, near Luanda. FIG. 15. – (A) Distribution actuelle des dépôts deltaïques du fleuve Coporolo, due au courant de Benguela, (B) barrière sableuse due à l’actuel courant de Benguela, lagune de Mussulo, à côté de Luanda.



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