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Article

Composition of Pre-Salt Siliciclastic Units of the Lower Congo Basin and Paleogeographic Implications for the Early Stages of Opening of the South Atlantic

by
João Constantino
1,
Pedro A. Dinis
2,*,
Ricardo Sousa Gomes
3 and
Mário Miguel Mendes
2,4
1
Research and Development Centre Sonangol SA, Luanda P.O. Box 1316, Angola
2
University of Coimbra, MARE—Marine and Environmental Sciences Centre/ARNET—Aquatic Research Network, Earth Sciences Department, Rua Silvio Lima, Pólo II, 3030-790 Coimbra, Portugal
3
University of Coimbra, Earth Sciences Department, Rua Sílvio Lima, Pólo II, 3030-790 Coimbra, Portugal
4
Faculty of Sciences and Technology, Fernando Pessoa University, Praça 9 de Abril, 4249-004 Porto, Portugal
*
Author to whom correspondence should be addressed.
Geosciences 2025, 15(5), 189; https://doi.org/10.3390/geosciences15050189
Submission received: 15 March 2025 / Revised: 9 May 2025 / Accepted: 19 May 2025 / Published: 21 May 2025
(This article belongs to the Section Sedimentology, Stratigraphy and Palaeontology)
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Abstract

:
The Lower Congo Basin (LCB) is a rift-type basin with petroleum systems that developed at the western African margin in association with the opening of the South Atlantic. Two pre-salt siliciclastic units of the LCB, Lucula (uppermost Jurassic to Lower Cretaceous) and Chela (Aptian) formations, were sampled in deep wells and outcrops. Heavy mineral assemblages, XRD mineralogy and geochemistry indicate prevailing source in high rank metamorphic rocks from western regions of the Lower Congo Belt. However, sediment composition reveals some provenance heterogeneity. For the Chela Formation, occasionally abundant amphibole in the heavy mineral fraction, coupled with relatively high Fe and Ti proportions, suggest that it formed when deeper crustal units were exhumed. The Lucula Formation collected in outcrops have composition substantially different from Lucula and Chela samples collected in deep wells, indicating distinct provenance and the incorporation of recycled material. A significant diagenetic overprint compromises the interpretation of compositional features in terms of paleoclimate. The presence of a chemical component with dolomite, halite and diverse sulphates and the stratigraphic position of the Chela Formation at the transition to a thick evaporitic succession are compelling evidence of deposition under warm and dry conditions, which are probably more extreme than those associated with the original stages of rifting recorded by the Lucula Formation.

1. Introduction

During the Early Cretaceous, the breakup of Gondwana and the opening of the South Atlantic were responsible for the separation of the African and South American continents. The Atlantic margins of these continents show several Mesocenozoic sedimentary basins that host enormous amounts of hydrocarbons [1], with the Lower Congo Basin (LCB) being one of them [2,3,4]. In the LCB, as in the Espirito Santo Basin in the west Atlantic conjugate margin, the rifting processes were responsible for the development of a series of tilted blocks followed by a sag basin, where a thick siliciclastic succession accumulated before Aptian salt deposition [5,6]. In the South American margin, south of the Espirito Santo Basin, important reserves of hydrocarbons are known in the sedimentary succession of Santos Basin covered by thick salt-units (pre-salt), [7]. The pre-salt siliciclastic units of the LCB are also considered good potential reservoirs in sealed structures. However, due to the lack of well-constrained exposures, these units have not been investigated in detail.
In this research, we present integrated compositional data, comprising geochemistry, bulk mineralogy and heavy mineral suites of the most voluminous sand–gravel pre-salt siliciclastic units of the LCB. Studied samples were retrieved from deep wells (up to ~4500 m) drilled in offshore and onshore portions of the basin, and from strata that has been ascribed to early stages of deposition in the LCB and are presently exposed in coastal Angola (Figure 1). Such integrated data should help establish source areas during different phases of basin evolution, compositional transformation through exogenous processes, and how diagenesis may have affected coarse porous units.

2. Geological Setting

2.1. The LCB and the Opening of the South Atlantic

The LCB is located in the Central Segment of the South Atlantic, between the Rio Grande and the Ascension fault zones [9,10]. The opening of the South Atlantic started south, around the Jurassic–Cretaceous transition, and progressed northward, culminating in the full separation of Africa and South America by late Aptian or Albian times [11,12].
Although the timing of the earliest stages of oceanic crust formation is not clear, it must have formed south of the Walvis ridge short before the volcanic eruptions associated with the Paraná-Etendeka flood basalts [13,14]. By these times, the Central Segment of the South Atlantic was probably analogue to the present-day East African Rift Systems, with elongated lakes and important lateral sediment supply [12]. The beginning of seafloor spreading in the Central Segment of the South Atlantic probably took place during the Aptian–Albian transition, hence, it coincided approximately with the connection of the salt basins of the South Atlantic with the North Atlantic [11].
The initial stages of the opening of the South Atlantic was accompanied by the formation of alluvial fans and small streams feeding rift lakes [10,15]. Deposition extended north and south-ward during the Early Cretaceous as volcanic activity, mostly centred in the Walvis–Rio Grande fault Zone, characterizes the Valanginian to Hauterivian in southern sectors of the Central Segment [10]. The early Aptian was essentially a period of uplifting, responsible for a hiatus in most of the African margin [9]. After resuming clastic/carbonate deposition and the formation of an evaporitic unit, marine environments, with incursions from north and south, persisted on both South Atlantic margins [4,10].

2.2. Basement of the LCB

Onshore, the LCB stands on the West Congo Belt, one part of Araçuai–West Congo Orogen that extends westward in Brazil, and its basement units (Figure 1). The Araçuai–West Congo Orogen is genetically associated with 800 to 500 Ma (~Pan-African) tectonic events and the formation of the São Francisco–Congo paleocontinent [16,17,18]. The West Congo Belts stretches for over 1300 km from southwestern Gabon to northwestern Angola with 150–300 km wide. It reveals increasing metamorphic grade from the Neoproterozoic foreland in the east to medium/high-grade towards the west. The older units, usually affected by higher grade metamorphism, are thrusted eastward over more recent formations [8,19].
In this research, we adopted the regional litostratigraphic frame presented by Tack and colleagues [8]. The Mesoproterozoic and Neoproterozoic West Congo Supergroup covered by a major angular unconformity, the Paleoproterozoic Kimezian basement (~2.1 Ga), which is correlated across the South Atlantic with the Mantiqueira and Juiz de Fora Complexes in Brazil [17,20]. The Kimezian Supergroup consists mostly of gneisses, migmatites and amphibolites [21]. In a flexural zone of West Congo Belt, close to the Angola–Democratic Republic of the Congo border, the Kimezian basement is intruded by the Noqui-type peralkaline granites (~1000 Ma), which are ascribed to volcanic–plutonic magmatism, associated with incipient rifting [8].
The West Congo Supergroup begins with the thick volcano-sedimentary succession of the Zadinian Group, dominated by diverse siliciclastic rocks [19], which pass upward to (meta) basalts (~0.93 Ga). The Zadinian Group is covered by the Mayumbian Group (~0.92–0.91 Ga), mostly composed of felsic volcanic–plutonic rocks with subordinate volcano-sedimentary and sedimentary intercalations. The bimodal magmatic suite of these units has been associated with continental flood volcanism [8]. The overlying West Congolian Group can be sub-divided into two major units: (1) a passive-margin platform succession comprising siliciclastic and carbonate rocks, including marine diamictites and intercalated basalts; (2) a syn-orogenic to post-orogenic (relative to Pan-African) molasse-type succession dominated by red-beds. Overall, these units are sourced from the Archean Congo Craton and the Paleoproterozoic Kimezian basement [22].

2.3. Stratigraphy of the LCB

The sedimentary succession of the LCB is dated to between the Late Jurassic and the Neogene. It has a thickness of 2500–4000 m on land and ~5000 m offshore, which can be organized into pre-rift, syn-rift (I and II) and post-rift phases (Figure 2). The syn-rift can also be divided into the “fault” and “sag” phases of [23].
The pre-rift phase lasted from the Late Jurassic until the early Cretaceous, being recorded mainly by coarse grained terrigenous continental deposits. The syn-rift phase I, usually considered to be from the “Neocomian” to Barremian, is linked to lake systems where the structural lows were partially filled by turbidites that passed laterally and upward to organic-rich shales (Bucomazi Formation). These thick units (locally more than 1500 m) are considered source rocks for oil accumulation [24,25]. The subsequent syn-rift II phase, in the Barremian–Aptian age, is associated with marine incursions that originally progressed from the south [26]. The beginning of the post-rift phase is recorded by an evolution towards an evaporitic succession designated in the LCB as the Loeme Formation or saline Loeme [26,27]. This unit acts as a regional seal between the syn-rift lacustrine petroleum system and the overlying marine petroleum system. Contrasting the Loeme Formation, the Cretaceous is recorded mainly by marine carbonate and fine-grained siliciclastic deposits (Pinda, Vermelha and Iabe formations; [15]).
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Figure 2. Simplified stratigraphy of the LCB based on da Costa and colleagues [28] (A) and images of the Lucula Formation in exposures (B,C).
Figure 2. Simplified stratigraphy of the LCB based on da Costa and colleagues [28] (A) and images of the Lucula Formation in exposures (B,C).
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Lucula and Chela formations are the main porous siliciclastic intervals deposited before the Loeme Formation [29,30,31,32]. The Lucula Formation (up to 450 m thick) is composed of conglomerates, sandstones and lutites, which become more abundant towards the top of the unit. It records deposition by alluvial fans that evolved upward to the river systems and has been attributed to the Tithonian and “Neocomian” ages [26,33]. The Lucula Formation has been associated with pre-rift and early syn-rift tectonic phases in the LCB [27,34].
The Chela Formation (Aptian) consists mainly of white to grayish very fine-grained to coarse-grained sandstones with intercalations of conglomerate and lutite, with common fern spores. The unit has great lateral continuity, with a maximum thickness of ~90 m offshore. The facies associations of the Chela Formation point to dominant deposition in fluvial and lacustrine environments [26,33]. According to Braccini and colleagues [26], fossil assemblages also suggest intervals under marine influence. The upper strata of the Chela Formation (5 to 10 m onshore) are composed of mudstones with some anhydrite intercalations that appear to evolve vertically towards the Loeme salt unit (upper Aptian). It is generally considered that the Chela Formation covers the break-up unconformity in the LCB [15,27] and marks the transition to the post-rift phase [15].

3. Materials and Methods

3.1. Studied Samples

Most samples were obtained from cuttings and cores of deep wells drilled in onshore and offshore portions of the LCB that are stored at the National Agency of Oil, Gas and Biofuels (Luanda, Angola). It was also possible to access the exposure of the Lucula Formation. A selection of 15 samples from drilling wells, 10 of Chela Formation, 5 of Lucula Formation, and 8 samples of the Lucula Formation collected in 4 outcrops was used in this research (Figure 1 and Table 1). The studied region is organized in 3 drilled sectors (from north to south, Cabinda, Soyo and Mucula Low) and the onshore outcrops.

3.2. Geochemistry

A chemical composition of a fraction finer than 2 mm was determined using a portable X-ray fluorescence instrument (XRF) Thermo Scientific (Waltham, USA) Niton XL3t 900 GOLDD+. The instrument has a silicon drift detector with a built-in silver anode measuring at a maximum of 50 kV and 200 μA. The hybrid Compton–fundamental calibration (HCF—TestAll; [36,37]) was adopted in sample measurements. Samples were ground into powder and hand-compressed in sample cups with a polypropylene film at one end. Chemical composition was determined with an acquisition time of 120 s per run. All samples were measured three times, and average results were adopted. Except for the major elements in geological material (Si, Al, Fe, Mg, Ca, K, Ti and P), elements with concentrations below the detection limit in at least one third of the samples were discarded.
In addition, split aliquots of the 0.063–0.5 mm fractions, which was used for the separation of heavy minerals, of eleven selected samples were also analyzed at the accredited commercial laboratory of Actlabs using the analyses package 4E-Expl. Major elements (expressed as oxides) and a few selected minor elements (Sr, V, Y, Zr) were determined by Inductively Coupled Plasma Mass Spectrometry (ICP). Samples were mixed with a flux of lithium metaborate and lithium tetraborate, fused in an induction furnace and poured into a solution of 5% nitric acid containing an internal standard until completely dissolved. A set of elements (REE, Cr, Rb, Th, Sc) were analyzed by Instrumental Neutron Activation Analysis (INAA). Because a part of Ba may result from barite used in drilling muds, their concentrations were not considered. Analyses using ICP after total digestion in a mixture of HClO4, HNO3, HCl and HF were adopted for Cu, Ni and Zn.

3.3. XRD Mineralogy

Mineralogy using X-ray diffraction (XRD) was obtained using an Aeris (Malvern PanAnalytical) device with Cu K-alpha radiation at 15 kV, 40 mA and 0.15406 nM wavelength. Non-oriented aggregates of the studied samples, previously ground into powder, were analyzed in the range 2 to 60° (2 θ) with 0.02173° step-size at a velocity of 3.3°/min. The identification of the mineral species in each sample was based on the recognition of sets of characteristic reflections. Quantitative estimates of the proportions of different minerals were performed with Profex 5.4.1 software (https://www.profex-xrd.org/ (accessed on 14 January 2025)) through Rietveld refinement.

3.4. Heavy Minerals

Heavy minerals were obtained from split aliquots measuring 0.063–0.5 mm fractions were separated by wet and dry sieving from sandstone samples after manual disintegration. Separation of the heavy mineral fraction was performed with Na-polytungstate heavy liquid at a density of 2.90 g/cm3. The weight percentages of the 0.063–0.5 mm fraction in the sample and of the heavy mineral concentrates in this fraction were recorded to estimate the total heavy mineral abundances.
Heavy mineral concentrates were then mounted on glass slides with Canada Balsam. The mineralogy of 100 to 180 transparent grains was determined in each slide under the petrographic microscope, according to their optical and morphological properties. In one mica-rich sample, it was possible to identify only 40 grains. Biotite and non-identifiable weathered grains were not considered, and opaque minerals were counted independently. Heavy mineral proportions are classified as dominant (>50%), secondary (5–50%), minor (1–5%) and rare (<1%).

3.5. Data Treatment

Statistical treatment was performed using the software JMP Pro 17. Principal Component Analysis (PCA) was applied to compositional data in order to understand the association between compositional variables and highlight the similarities between samples [38]. Different sets of chemical and mineralogical variables or combinations of the two were adopted, continually ensuring that the number of samples was at least close to two times the number of variables.

4. Results

4.1. Mineralogy

The XRD mineralogy of the studied pre-salt units is heterogenous, with quartz, feldspars, phyllosilicates, carbonates and sulphates among the predominant components (Table 2). Quartz is always the most abundant mineral in the Lucula Formation (37–99%), constituting more than 50% of samples and more than 81% in deposits collected from outcrops. Potassium-feldspar and plagioclase occur in secondary to minor proportions, being absent in a few samples and showing the highest proportions in the Mucula Low sector (reaching 15.3 and 18.3%, respectively). In well samples, carbonates (mostly calcite, up to 11%) are more abundant in the Mucula Low sector. In the Cabinda sector, barite (<9%), also occurs, likely derived from drilling muds. One outcrop sample reveals minor apatite (4%).
A more diversified XRD mineralogy, which is more dependent on the sampled sectors, is observed in the Chela Formation. In samples collected from the southern well of Soyo sector, the contents of quartz (6–19%) are substantially lower or comparable to feldspar (9–58%). These deposits are the most enriched in phyllosilicates, in general, there is a predominance of mica (up to 26%), and they contain variable proportions of diverse sulphates, such as anhydrite (<31%), natrojarosite (<8%) and gypsum (<6%). The two deposits sampled in the northern well of the Soyo sector are distinguished by their enrichment in quartz (41–43%), more plagioclase than K-feldspar, the presence of some halite and garnet and the absence of carbonates. Conversely, deposits from Mucula Low are distinguished by their high dolomite content (36–54%), comparable quartz, K-feldspar and plagioclase (11–28%) and low proportion of phyllosilicates (<6%).
The heavy mineral fractions represent 0.16–6.01% of the studied samples (0.45–12.85% of 0.063–0.5 mm fraction; Supplementary File). The average values in these formations are not substantially different. The proportions of opaque minerals are highly variable (0.01 < opaque/transparent < 3.58). The lowest and highest heavy mineral concentrations were determined in samples from the Chela Formation collected in the Soyo sector. The two samples with highest concentrations came from the same site (LC4, samples 4.1C and 4.2C) and revealed very low content of opaque minerals.
The transparent suites of the Lucula Formation samples retrieved from drilling wells are largely dominated by garnet (86–98%), usually with minor amounts of apatite, kyanite, zircon, rutile, staurolite and sillimanite (Figure 3). Samples of the Lucula Formation collected in outcrops yield heavy mineral assemblages either dominated by kyanite (30–72%) or apatite (<51%) or with mixed kyanite–zircon or kyanite–garnet–epidote assemblages. Rutile, tourmaline and sillimanite can also be found in secondary amounts.
Garnet is either largely dominant or found in secondary amounts in the heavy mineral fractions of the Chela Formation (39–96%). In the Soyo sector, green amphibole can be the most abundant mineral or occur only secondary to trace amounts (<44%). Samples collected in the Mucula Low sector and the southern drilling well of the Soyo sector also yield secondary, and sometimes relatively high, contents of apatite. Zircon and rutile are always found in minor to lower secondary proportions. Kyanite, staurolite, sillimanite, epidote and tourmaline were found only in a few samples and always in secondary to trace proportions.

4.2. Geochemistry

The full geochemical results for the Chela and Lucula formations are presented in Supplementary File. The XRF geochemistry of samples obtained in drilling wells is comparable, but part of the Chela Formation sample is relatively enriched in Ca and Mg, and depleted in Si and Al, reflecting the presence of an authigenic carbonate-sulphate component (Figure 4). Deposits from the Cabinda sector tend to be slightly enriched in Mn and depleted in Ca relative to those from the Mucula Low sector. Lucula samples collected from outcrops are distinguished by high contents of Si, coupled with P, Zr and occasionally also Fe, V and Cr, and low contents of elements that tend to be concentrated in phyllosilicates (e.g., Al, K, and Rb) and carbonates/sulphates (Ca, Mg and Sr).
The chemical composition of the 0.063–0.5 mm fractions of the drilling well samples is relatively homogenous, but, in general, there is higher variability in the Chela formations than the Lucula formations (Figure 4). No clear differentiation between sampled sectors was detected. The Lucula deposits collected in outcrops are again distinguished by their high Si, P and Zr contents (except for one sample with 97.4% SiO2), while Al, Mg, Ca, Na and K tend to be less abundant than in samples from drilling wells.

5. Discussion

5.1. Source Units During South Atlantic Opening

The geochemistry of siliciclastic sediments, and in particular, multiple ratios of elements that tend to be non-mobile during weathering, provides important information about the nature of source units and have been frequently used in provenance studies [41,42,43,44,45,46]. With adequate calibration and data analysis tools, the composition from XRF methods can be considered rigorous for most major elements found in siliciclastic materials [47]. But, the limited number of non-mobile elements that were measured by XRF in the studied samples compromise the application of classic discrimination diagrams based on element ratios. ICP/INAA data are more informative. Using the composition of fine sand fractions, tectonic setting cannot be assessed with the diagrams proposed by Bhatia and Crook [48], which place most samples in or close to the Continental Island Arc field (Figure 5), not compatible with a rift basin at a continental margin. Geochemical data, however, reveal that the studied deposits can be either enriched in mafic or felsic material relative to the UCC. This is confirmed by variable plagioclase/K-feldspar and heavy mineral assemblages.
The clay mineral assemblages are also compatible with variable amounts of detritus derived from felsic and mafic units, as demonstrated by the ratios of smectite (commonly associated with Fe-Mg-rich rocks) to illite + chlorite (typically associated with the early stages of weathering of felsic rocks) in deposits with relatively low kaolinite [52].
Although some samples of the Chela Formation from the Soyo sector are relatively enriched in amphiboles, pyroxenes absence and only secondary amphibole can be associated with a rift shoulder not strongly dissected (Figure 6). In general, samples collected from drilling wells yield mainly garnet or reveal comparable garnet and amphibole concentrations, both indicative of a high-grade metamorphic source. A different provenance must be considered for the Lucula samples collected in outcrops. Here, the high kyanite proportions, occasionally coupled with epidote and frequently with secondary amounts of sillimanite, staurolite, zircon and rutile, indicate other metamorphic sources and some enrichment in felsic material. Using heavy mineral data, three different provenances came out clearly in the PCA biplot for the two main components (Figure 7). In this diagram, the studied deposits appear clustered according to their relative enrichments in (1) garnet, (2) amphibole, and (3) kyanite + zircon + stauroline + sillimanite.
The predominance of garnet and amphibole, occasionally coupled with apatite, are compatible with high grade metasedimentary sources from the Paleoproterozoic Kimezian basement [18,21,54,55,56,57]. The Paleoproterozoic basement in the western locations of the Mayombe chain, in Congo-Brazzaville, was affected by multiple garnet-formation events, with the earlier Eburnean generation and Pan-African overgrown, thus contrasting with eastern locations where garnet appears to be restricted to Pan-African or Eburnean events with lower reached temperatures [58]. Hence, although associated with different genetic conditions, garnet can also be derived from the overlying units of the West Congo Supergroup. Abundant garnet in present-day beach and river sediments at the latitude of LCB was ascribed to recycling of Mesocenozoic deposits originally fed by units of the Lower Congo Belt [59]. An uplifted tectonic block coeval from the deposition of the Chela Formation may have been involved in sediment generation at the Soyo sector, accounting for the high amphibole content.
Different provenance should be considered for the deposits attributed to the Lucula Formation that were sampled in outcrops. Kyanite is recognized in several units of the Araçuai–West Congo Orogen, being associated with the exhumation of high-pressure subduction-related rocks [58,60]. Abundant kyanite is reported in a graphite schist from the Araçuai orogen of Brazil [61], but plausible kyanite-rich units deprived of garnet and amphibole were not identified in the studied region. Sediment sources may have also involved eastern parts of the West Congo Belt and older units of the Angola Block of the Congo Craton. Epidote is a common mineral in present-day sediments, which is primarily derived from granitoids of the Eburnean basement in the Angola Block [61,62]. It can also be linked to the epidote–amphibolite facies that is considered most compatible with the Pan-African collision in the region [58].

5.2. Weathering, Recycling and Diagenesis

By promoting the decomposition of the least durable minerals at surface environments and the leaching of mobile elements, weathering influences both the chemistry and mineralogy of siliciclastic deposits. Weathering intensity is best assessed with fine grained fractions [63,64]. The clay mineral assemblages that were obtained for pre-salt formations are heterogenous, and their interpretation in terms of weathering processes is not straightforward [52]. The clay fractions of the Lucula Formation (not including exposure deposits that should be influenced by post-depositional decomposition) are characterized by low kaolinite proportions. Part of the samples from the Chela Formation are also depleted in kaolinite, which is compatible with the limited weathering expected at precursory conditions and the dry environment associated with the formation of evaporitic units. Garnet, the most abundant heavy mineral in most samples, is vulnerable to weathering and can be completely decomposed in wet and hot equatorial climates [65]. Thus, heavy mineral’s assemblages of samples collected in wells provide additional evidence of limited weathering.
The higher contents of kaolinite in the clay fraction of some coarser strata should reflect diagenetic processes related to the decomposition of feldspar in porous medium [52,66]. These processes have been reported for near surface environments during early burial or after structural inversion [67]. It can also be associated with the presence of fluids with high CO2 contents, possibly resulting from the maturation of organic matter, and the transformation of detrital clay minerals that were already present in the sedimentary units [68,69]. Such post-depositional transformations explain the occurrence of kaolinite in dolomitic sandstone strata. If CO2 is abundant in the interstitial fluids and Mg derived from chlorite is also present, as happens with the studied deposits [52], the diagenetic generation of kaolinite and dolomite may occur [69].
There is more evidence that diagenetic transformations play an important role in sediment composition. Although the sedimentary units selected for the research are siliciclastic, some yield a non-detrital component that is reflected in the proportion of carbonates (dolomite and calcite), sulphates (gypsum, anhydrite, natrojarosite) and halite, and in the contents of Ca and Mg. The PCA performed with chemical data indicates that this component, probably associated with diagenetic precipitation in the voids of sandy deposits, accounts for a significant proportion of compositional variability (Figure 7).
Deposits of the Lucula Formation presently exposed onshore are also influenced by recycling. These processes probably account for the enrichment in zircon and rutile and depletion in garnet, and they explain the way samples spread in the diagram Zr/Sc versus Th/Sc, contrasting with those collected in drilling wells (Figure 5). A comparison of compositional features often used to evaluate weathering intensity reinforce the hypothesis of recycling influence. With increasing weathering intensity, both the proportions of kaolinite and the ratios of Al2O3/K2O must increase. However, onshore Lucula deposits enriched in kaolinite may reveal low Al2O3/K2O (Figure 8). In these deposits, the scarcity of clay fraction and the relative abundance of K-feldspar, which yield similar concentrations of K2O and Al2O3, are likely effects of recycling, thus explaining the lack of correlation kaolinite-Al2O3/K2O.

5.3. Paleogeographic Implications

The relatively homogenous heavy mineral suites in well samples, made up mostly of minerals common in high-grade metamorphic rocks that can be linked to units of the neighbouring West Congo Belt, is clear evidence of environmental conditions characterized by very small detritus supply systems, such as alluvial fans or small streams. The Paleoproterozoic Kimezian Supergroup of the LCB basement was affected by multiple events of garnet formation and should be considered a major source of sediment for pre-salt formations. Garnet abundances in samples of the Lucula Formation collected in northern offshore locations of the LCB, immediately to the east of an uplifted region (Figure 1), are indicative of an important supply from west during the early rifting stages.
Compositional data suggest that the prevalence of sedimentary sources in the close vicinity of the LCB prevailed until the early post rift, recorded by the Chela Formation. Hence, although the depositional environments where the Lucula and Chela formations were formed are different [15,26,27,31], provenance did not change much from the pre-rift to early sin-rift in most regions of the LCB. However, the amphibole enrichment coupled with a relatively high content of Fe and Ti in some samples of the Chela Formation from one investigated sector (Soyo), indicates a contribution from a different source enriched in mafic material (Figure 9). Such a contribution is best represented in two separated samples of the Chela Formation from the same drilling well, suggesting the interfingering of deposits with distinct source areas.
The deposits attributed to the Lucula Formation that are presently exposed onshore are puzzling. The relatively high K/Al ratios could be attributed to a source in the per-aluminous Noqui type granite, presently exposed near the sampling locations. But this source is not compatible with the high quartz and silica contents. Combined geochemical and mineralogic features indicate a completely different provenance, more heterogenous and with a recycled component, unlike the buried strata of the Lucula Formation (Figure 9). Although the deposits sampled in outcrops have been associated with the base of the LCB infill in north-west Angola, one cannot rule out the possibility that they resulted from subsequent reworking of the Lucula Formation or even younger strata. Contrastingly, the Chela Formation, which is more than 10 Ma younger than the Lucula Formation, does not show evidence of sediment recycling. These compositional features are compatible with a structural context promoting the accumulation and preservation of the sedimentary successions in restricted subsiding sectors.
Because weathering is closely related to climate, an assessment of weathering intensity could be used to interpret climatic conditions [70,71,72,73]. But, it was demonstrated that some features typical of intense weathering result from post-depositional transformations and cannot be associated with wet and warm paleoclimatic conditions. The palynological data available in drilling logs provide same additional information. The presence of fern spores can be regarded as an indicator of humid conditions [74,75]. Nevertheless, the ferns also include plants adapted to dry habitats. According to Abbink [76], the possible parent plants of Matonisporites Couper was very well adapted and preferred dry warm conditions. Thus, the occurrence of fern spores assigned to the form-genera Matonisporites in the studied levels of the Chela Formation suggests a warm climate with seasonal dry conditions.

6. Conclusions

Compositional features of the uppermost Jurassic to Lower Cretaceous coarse siliciclastic units of the Lower Congo Basin (LCB) were analyzed. Data on geochemistry (major and minor elements), bulk mineralogy and heavy mineral suites articulate coherently, allowing for a comprehensive evaluation of the main factors that control the composition of the pre-salt Lucula and Chela formations. Buried deposits of the LCB are mainly fed by high rank metamorphic rocks, with the Lower Congo Belt, which presently crops out in the close vicinity, as the most likely source. Despite some variability, especially in the Chela Formation, the heavy mineral assemblages suggest that provenance did not change much from the sin-rift to early post-rift phases. However, units laying on the LCB basement that are presently exposed onshore and have been attributed to the early sin-rift had different provenance and their formation involved recycling processes. The assignment of these deposits to the Lucula Formation needs to be reconsidered. Bulk mineralogy and geochemistry indicate that post-depositional diagenetic transformations strongly influenced the composition of the pre-salt units of the LCB, compromising an interpretation of paleoclimatic conditions based on sediment composition. More data are required to understand how diagenesis affected the potential of the pre-salt coarse siliciclastic strata for hydrocarbon storage.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/geosciences15050189/s1. Supplementary File: Compositional data obtained from pre-salt siliciclastic deposits of the Lower Congo Basin.

Author Contributions

Conceptualization, P.A.D.; methodology, P.A.D.; validation, J.C., P.A.D. and M.M.M.; formal analysis, J.C., P.A.D. and R.S.G.; investigation, J.C., P.A.D., R.S.G. and M.M.M., resources, J.C. and P.A.D.; data curation, J.C. and P.A.D.; writing—original draft, J.C., P.A.D. and M.M.M.; visualization, P.A.D., J.C. and R.S.G.; Supervision, P.A.D.; project administration, J.C. and P.A.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work had the support of national funds through Fundação para a Ciência e a Tecnologia, I. P (FCT), strategic programmes fundings under the projects UIDB/04292/2020 (https://doi.org/10.54499/UIDB/04292/2020) and UIDP/04292/2020 (https://doi.org/10.54499/UIDP/04292/2020) granted to MARE.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Acknowledgments

Our thanks go to oil and gas geologist Lumen Sebastião, from ANPG, Angola, for his support in sampling from drilling records, and Anselmo de Mendonça Sicato, for his support during field work. We would also like to thank Carlos Maia (DCT, UC) for his excellent collaboration in the XRD analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological setting of the study area in SW Africa. (A) Onshore lithology; basement units after Tack and colleagues [8]. (B) Structural interpretation based on Total Magnetic Intensity Reduced to Pole (TIM−RTP) with indication of sectors considered in text separated by dashed lines. Location of the sampling points is also indicated.
Figure 1. Geological setting of the study area in SW Africa. (A) Onshore lithology; basement units after Tack and colleagues [8]. (B) Structural interpretation based on Total Magnetic Intensity Reduced to Pole (TIM−RTP) with indication of sectors considered in text separated by dashed lines. Location of the sampling points is also indicated.
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Figure 3. Heavy mineral assemblages of Lucula and Chela formations.
Figure 3. Heavy mineral assemblages of Lucula and Chela formations.
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Figure 4. Geochemistry of the Lucula and Chela formations. (A) XRF results for different sectors of the LCB. (B) Range of values obtained for the fine sand fraction (0.063–0.5 mm) of selected samples. Samples concentrations normalized to the Upper Continental Crust according to Rudnick and Gao [39] and Hu and Gao [40].
Figure 4. Geochemistry of the Lucula and Chela formations. (A) XRF results for different sectors of the LCB. (B) Range of values obtained for the fine sand fraction (0.063–0.5 mm) of selected samples. Samples concentrations normalized to the Upper Continental Crust according to Rudnick and Gao [39] and Hu and Gao [40].
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Figure 5. Interpretation of source rock nature for pre-salt siliciclastic units based on geochemical features of selected samples. (A) Triangular plot V-Ni-Th used to access the contribution of different source rocks [49]. (B) Triangular plot La-Th-Sc [48]. (C) Ratios Th/Sc versus Cr/Th as estimators of and felsic/mafic contributions [50]. (D) Ratios Zr/Sc versus Th/Sc as an indicator of mafic and felsic components, and recycling effect [51].
Figure 5. Interpretation of source rock nature for pre-salt siliciclastic units based on geochemical features of selected samples. (A) Triangular plot V-Ni-Th used to access the contribution of different source rocks [49]. (B) Triangular plot La-Th-Sc [48]. (C) Ratios Th/Sc versus Cr/Th as estimators of and felsic/mafic contributions [50]. (D) Ratios Zr/Sc versus Th/Sc as an indicator of mafic and felsic components, and recycling effect [51].
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Figure 6. Ternary plots with heavy mineral assemblages. Fields in the left diagram from the continental block provenance of Garzanti and colleagues [53]. ZTR is the summed proportion of zircon, rutile and tourmaline. The size of the symbol for each sample is proportional to heavy mineral concentration in the sample (small: 0–1%; medium: 1–3%; large >3%).
Figure 6. Ternary plots with heavy mineral assemblages. Fields in the left diagram from the continental block provenance of Garzanti and colleagues [53]. ZTR is the summed proportion of zircon, rutile and tourmaline. The size of the symbol for each sample is proportional to heavy mineral concentration in the sample (small: 0–1%; medium: 1–3%; large >3%).
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Figure 7. Biplot for PCA obtained with major elements (A) and heavy mineral data (B) for pre-salt siliciclastic units.
Figure 7. Biplot for PCA obtained with major elements (A) and heavy mineral data (B) for pre-salt siliciclastic units.
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Figure 8. Relation between kaolinite proportion in the clay fraction of sand deposits [52] and the ratio Al2O3/K2O, two parameters that tend to increase with weathering intensity.
Figure 8. Relation between kaolinite proportion in the clay fraction of sand deposits [52] and the ratio Al2O3/K2O, two parameters that tend to increase with weathering intensity.
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Figure 9. Biplot for PCA obtained with a combination of selected chemical and mineral data. amph: amphiboles; sulf: sulphates; K-f: K−feldspar; carb: carbonates; gr: garnet; plg: plagioclase; Qz: quartz; ky: kyanite; ZTR: zircon + tourmaline + rutile. Arrows indicate compositional trends, relative to most common pre-salt siliciclastic deposits (with ellipse envelope), ascribed to enrichment in felsic sediment material or recycling effects (reflected by high contents of Si, quartz and ultrastable heavy minerals) and the enrichments in mafic material (reflected by high contents of Fe, Ti and amphiboles).
Figure 9. Biplot for PCA obtained with a combination of selected chemical and mineral data. amph: amphiboles; sulf: sulphates; K-f: K−feldspar; carb: carbonates; gr: garnet; plg: plagioclase; Qz: quartz; ky: kyanite; ZTR: zircon + tourmaline + rutile. Arrows indicate compositional trends, relative to most common pre-salt siliciclastic deposits (with ellipse envelope), ascribed to enrichment in felsic sediment material or recycling effects (reflected by high contents of Si, quartz and ultrastable heavy minerals) and the enrichments in mafic material (reflected by high contents of Fe, Ti and amphiboles).
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Table 1. List of samples used in the present research.
Table 1. List of samples used in the present research.
SiteSampleMGSFm.SectorDepth (m)
Well LC11.1CCS (0.3)ChelaMucula Low3063.0
Well LC11.2CCS (0.9)ChelaMucula Low3081.0
Well LC11.3CCS (0.9)ChelaMucula Low3144.0
Well LC11.4LMS (1.2)LuculaMucula Low3441.0
Well LC11.5LMS (1.1)LuculaMucula Low3468.0
Well LC11.6LMS (1.4)LuculaMucula Low3486.0
Well LC33.2CMS (1.5)ChelaSoyo1288.0
Well LC33.3CMS (1.8)ChelaSoyo1292.0
Well LC33.4CMS (1.7)ChelaSoyo1292.0
Well LC33.5CMS (1.6)ChelaSoyo1296.0
Well LC33.7CMS (1.5)ChelaSoyo1313.5
Well LC44.1CCS (0.9)ChelaSoyo1203.0
Well LC44.2CMS (1.1)ChelaSoyo1205.0
Well LC2424.1LFS (2.2)LuculaCabinda3325.4
Well LC2424.2LFS (2.3)LuculaCabinda3325.4
S4CC1MS (1.1)LuculaOutcrop~0
S4CC2MS (1.4)LuculaOutcrop~0
S2CC3MS (1.6)LuculaOutcrop~0
S3CC4CS (0.5)LuculaOutcrop~0
S1CC5MS (1.8)LuculaOutcrop~0
S4CC6MS (1.5)LuculaOutcrop~0
S2CC7MS (1.2)LuculaOutcrop~0
S3CC8VCS (−0.7)LuculaOutcrop~0
MGS: mean grain-size (ϕ scale between backets). Measurements obtained with the graphical method of Folk and Ward [35]. FS: fine sand; MS: medium sand; CS: coarse sand; VCS: very coarse sand.
Table 2. Mineralogy of Lucula and Chela formations.
Table 2. Mineralogy of Lucula and Chela formations.
XRD MineralogyHeavy Minerals
SampleFm.QzK-FPlgPhylCarbSulfHalOth%HMZTRGrEpAmpKySilApOth
1.1CChela1513116532001.612.076.00.00.00.00.022.00.0
1.2CChela2819134350011.403.271.60.00.02.11.121.11.1
1.3CChela 1.042.080.01.02.02.00.09.04.0
1.4LLucula4914111771011.571.095.00.00.01.00.03.00.0
1.5LLucula5214121642001.661.197.80.00.00.00.00.01.1
1.6LLucula37151818120003.371.096.00.00.00.00.02.01.0
3.2CChela65502990100.556.468.20.00.91.80.920.01.8
3.3CChela135802900003.391.940.90.043.50.00.013.60.0
3.4CChela19281817133110.276.658.20.019.70.00.015.60.0
3.5CChela11200203513010.616.050.70.01.30.00.740.01.3
3.7CChela990311239001.322.238.90.043.30.00.013.91.7
4.1CChela4110231400754.351.096.00.01.00.01.01.00.0
4.2CChela4410129011956.014.093.01.00.00.00.02.00.0
24.1LLucula61501928140.825.086.00.00.01.01.04.03.0
24.2LLucula75001618000.600.090.00.00.00.00.06.04.0
CC1Lucula81131000051.1234.30.60.60.058.03.01.22.4
CC2Lucula9622000050.5747.90.00.90.043.63.40.04.3
CC3Lucula9720100050.7729.50.01.60.050.811.50.85.7
CC4Lucula9910000000.4839.00.013.00.041.03.01.03.0
CC5Lucula8555100042.3915.35.58.32.824.31.840.53.7
CC6Lucula9440200002.1924.40.00.80.066.90.80.07.3
CC7Lucula9900000013.5613.023.022.02.030.07.02.01.0
CC8Lucula9720100000.1618.00.04.00.072.00.02.04.0
Qz: quartz, F-K; K-feldspar; Plg: plagioclase; Phyl: phyllosilicates; Carb: carbonates; Sulf: sulphates; Hal: halite; Gr: garnet; Fe-Ti Ox: Fe-Ti oxides; Oth: others; %HM: heavy mineral percentage in sample; ZTR: zircon + tourmaline + rutile; Gr: garnet; Ep: epidote; Amp: amphibole; Ky: kyanite; Sil: sillimanite; Ap: apatite.
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Constantino, J.; Dinis, P.A.; Gomes, R.S.; Mendes, M.M. Composition of Pre-Salt Siliciclastic Units of the Lower Congo Basin and Paleogeographic Implications for the Early Stages of Opening of the South Atlantic. Geosciences 2025, 15, 189. https://doi.org/10.3390/geosciences15050189

AMA Style

Constantino J, Dinis PA, Gomes RS, Mendes MM. Composition of Pre-Salt Siliciclastic Units of the Lower Congo Basin and Paleogeographic Implications for the Early Stages of Opening of the South Atlantic. Geosciences. 2025; 15(5):189. https://doi.org/10.3390/geosciences15050189

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Constantino, João, Pedro A. Dinis, Ricardo Sousa Gomes, and Mário Miguel Mendes. 2025. "Composition of Pre-Salt Siliciclastic Units of the Lower Congo Basin and Paleogeographic Implications for the Early Stages of Opening of the South Atlantic" Geosciences 15, no. 5: 189. https://doi.org/10.3390/geosciences15050189

APA Style

Constantino, J., Dinis, P. A., Gomes, R. S., & Mendes, M. M. (2025). Composition of Pre-Salt Siliciclastic Units of the Lower Congo Basin and Paleogeographic Implications for the Early Stages of Opening of the South Atlantic. Geosciences, 15(5), 189. https://doi.org/10.3390/geosciences15050189

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