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The 1.000 m thick Redbeds sequence of the Congo River Basin (CRB): a generally overlooked testimony in Central Africa of post-Gondwana amalgamation (550 Ma) and pre-Karoo break-up (320 Ma)

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L. Tack
L. Tack
L. Tack
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Damien Delvaux at Royal Museum for Central Africa
Damien Delvaux
K.E. Kadima at University of Lubumbashi
K.E. Kadima
Franck Delpomdor at Illinois State Geological Survey, Champaign, United States
Franck Delpomdor
  • Illinois State Geological Survey, Champaign, United States
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Geological Society of London Fermor Meeting 2009: Rodina: Supercontinents, Superplumes
and Scotland. Edinburgh, 6-13 September 2009.
The 1.000 m thick Redbeds sequence of the Congo River Basin (CRB): a
generally overlooked testimony in Central Africa of post-Gondwana
amalgamation (550 Ma) and pre-Karoo break-up (320 Ma)
L. Tack1, D. Delvaux1, E. Kadima2, F. Delpomdor3, A. Tahon1, P. Dumont (†)1, M.
Hanon1, M. Fernandez-Alonso1, D. Baudet1, S. Dewaele1, E. Cibambula4, V. Kanda
Nkula4 & Ch. Mpiana4
1 Royal Museum for Central Africa, B. 3080 Tervuren, Belgium; e-mail: luc.tack@africamuseum.be
2 UNILU (Université de Lubumbashi), Lubumbashi, Democratic Republic of the Congo
3 Free University of Brussels, Brussels, Belgium
4 UNIKIN (Université de Kinshasa), Kinshasa, Democratic Republic of the Congo
The present-day Congo River Basin (CRB) is a continent-scale feature coinciding with a gravity low. It
consists of a pile of less than 2.000 m loose and/or poorly consolidated sedimentary rocks, spaced in
time between the base of the Karoo Supergroup (ca. 320 Ma) and the Holocene. These sequences
rest on a pre-Karoo basement, which is known in outcrop along the edge of the CRB. The CRB is part
of a large cratonic block encompassing several Archaean nuclei, supposedly welded together between
2.1-1.8 Ga as a result of Eburnean-aged collisional orogeny. Throughout the late-Palaeoproterozoic
and Mesoproterozoic this block remained united. As a result of Gondwana amalgamation at ca. 550
Ma, the block was bordered by Pan African collisional high-grade terranes to the N (“Central African
Orogenic Belt”) and to the E (“East African Orogenic Belt”). The W and the SE rim of the CRB form
foreland domains of respectively the “Araçuai/West Congo” and “Katanga/Zambezi” Pan African
orogenic accretionary belts. In both forelands, Neoproterozoic tabular sedimentary sequences were
largely preserved and define, respectively, the West Congo and Katanga Supergroups. The
uppermost unit of these Supergroups is made up of a ca 1.000 m thick Redbed-facies sequence,
respectively the “Inkisi” and “Plateaux” Subgroups (the latter also known as “Kilungu Lupili” and
recently renamed “Biano”). Other tabular or moderately deformed Neoproterozoic sedimentary
sequences are known, exposed in several intracratonic aulacogenes around and/or in the CRB. Each
of these sequences has been defined as a distinct Supergroup (e.g. the Mbuji-Mayi Supergroup in the
DRC Kasai region). The most prominent is the Lindi/Ubangi Supergroup exposed to the N and NE of
the CRB. Its uppermost unit (the “Banalia Arkoses”) is also a ca. 1.000 m thick Redbed sequence. In
places, these Redbeds overlie unconformably the folded Neoproterozoic sequences of the Pan-
African West Congo and Katanga belts and are thus post ca. 550 Ma (paroxysm of Pan African
orogeny). Therefore, they can no longer be considered as Precambrian but are Phanerozoic in age.
The Redbeds are themselves overlain by the ca. 320 Ma Karoo tillite (base of the Karoo
Supergroup).The Neoproterozoic subtabular sequences exposed in the Lindi/Ubangi – Katanga and
West Congo regions dip under the CRB cover deposits. This is confirmed by various geophysical
exploration works and four drillholes in the CRB (including the ca. 2.000 m deep, cored Samba- and
Dekese drillholes). Estimates of the ca. 1000 m Redbeds thicknesses observed along the edge of the
CRB compare well with the minimum 871 m given for the Samba cores. The Dekese cores confirm
that the Redbeds are overlain by the Karoo Supergroup. Re-interpretation of the abundant seismic
reflection profiles obtained in the 1970-ies and early 1980-ies in the framework of hydrocarbon
exploration programmes in the CRB groups the Redbeds sequences and overlying Karoo Supergroup
together into a single seismo-stratigraphic unit bound by regional unconformities, thus indicating a
similar geodynamic history. The Redbeds are characteristic of lacustrine and fluvio-deltaic (semi)arid
palaeo-environments in contact with atmospheric oxygen as indicated by their colour due to iron
oxides/hydroxides coating around detrital grains. Still, the palaeo-depositional history of the CRB
Redbeds is poorly constrained. Sedimentation is ascribed to the development of a subsiding basin
under extensional regime after Gondwana amalgamation, spanning the whole CRB and (large parts
of) its rim. As such, the Redbeds form a more than 1600 km long intracratonic blanket with remarkable
similarities in depositional facies. Attempts to better constrain the age of the Redbeds (post-Pan
African but pre-Karoo) are subject to controversial interpretations of scarce and scattered data,
including, amongst others, preliminary SHRIMP on detrital zircons or 40Ar/39Ar on detrital muscovites.
Notwithstanding all these arguments, on the synthetic geological map of the DRC (1974) and on many
more recent documents and compilations derived from it, the CRB Redbeds are still indicated as part
of the “upper Precambrian (615-645 Ma)”.
... During the pre-rift phase , the southwestern subduction of Gondwana induced an NW-SE compressional stress related to the Gondwanide orogeny (Johnston, 2000). This was inferred in the Paleozoic Inkisi Group, South West Congo (Nkodia et al., 2020;Tack et al., 2008;Kadima et al., 2011). From the Late Jurassic to the Early Cretaceous, i.e. during the syn-rift phase, the West African margin experienced a NE-SW oriented extension related to the opening of the Atlantic Ocean (Rabinowitz and LaBrecque, 1979;Nürnberg andMüller, 1991 andSalomon et al., 2015), also highlighted by Viola et al. (2012) as an ENE-WSW extension. ...
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... It is not clear how these series connect laterally in the CB as the biostratigraphic control is inexistant. Tack et al. (2008) and Fernandez-Alonso et al. (2015) correlated the Biano, Banalia and Inkisi units as a single distinct lithostratigraphic Group between the Neoproterozoic and Karoo Supergroups. We can now propose to extend this to incorporate all the Red Beds in the CB, defining a new lithostratigraphic supergroup that we propose to name Aruwimi Supergroup, from the name of the river where Verbeek (1970) described for the first time to the Aruwimi sequence. ...
The Congo Basin: Stratigraphy and subsurface structure defined by regional seismic reflection, refraction and well data
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  • GLOBAL PLANET CHANGE
The Congo basin (CB) occupies a large part of the Congo craton, which derived from the amalgamation of different cratonic pieces. As intracratonic basin, it initiated, possibly as a failed rift, in late Mesoproterozoic and evolved during the Neoproterozoic and Phanerozoic in an intraplate setting. For this reason, the CB can be considered a natural laboratory for investigating the processes that govern the long–term evolution of continental interiors. In this study, we reconstructed the stratigraphy and tectonic evolution of the CB using all available and geological seismic data (reflection and refraction seismics, borehole and field data). We interpreted almost 2600 km of seismic reflection profiles and well log data located inside the central area of the CB (the “Cuvette Centrale”). The obtained model will be further constrained by aeromagnetic and satellite gravity data and density measurements from rock samples in a next paper. Results show that the depth to the basement varies quite significantly, defining a series of structural highs and depocenters that developed throughout the history of the basin. The major controlling factors for the development of the CB are, besides the deep geodynamic processes, the inherited heterogeneity of the pre-Neoproterozoic basement, the tectonic evolution of Rodinia, Gondwana and Pangea amalgamation and breakup, and environmental conditions influenced by the drifting through the South Pole towards its present-day equatorial position and global climatic fluctuations between icehouse and greenhouse conditions.
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... C'est une formation rocheuse essentiellement constitué des grès arkosiques [14] d'origine fluviatile présentant, trois (3) termes répétitifs [15] : (i) grès grossiers associés à galets de quartzitiques, de roches magmatiques et métamorphiques de forme elliptique, qui parfois montrent dans sa partie inférieure des conglomérats ; (ii) : grès grossiers massifs à stratification entrecroisées ; (iii) : alternance grès fins à très fins micacés à stratifications planes parallèles et à rides de courant. L'âge du groupe de l'Inkisi est suggéré pré-Karoo (320Ma) [16], car en Angola il est surmonté par les dépôts du Karoo d'âge Permien [17]. Les âges obtenus sur l'analyse des zircons détritiques les plus jeunes se situent entre 558 et 851 Ma [18,19]. ...
Cartographie par traitement d'image satellitaire des linéaments du groupe de l'Inkisi en République du Congo : implications hydrogéologique et minière
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Full-text available
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This study aims to map, by processing the Landsat 8 OLI satellite image, the lineaments of the Inkisi group in the southern of Brazzaville and in the department of Pool. The objective followed is thirstily to make a statistical analysis of the lineaments and compare them with the data obtained in the field, in order to 69 Afrique SCIENCE 16(4) (2020) 68-84 Timothée MIYOUNA et al. understand the fracturing organization in the Inkisi group. The second objective is to identify the zones with high density of fracturing and their relationships with the hydrographic network, and finally to orient the establishment of borehole with high operating rate and the mining exploration. The methodology implemented is basing on the processing of a Landsat 8 OLI satellite image using the techniques of Colored Compositions, Principal Component Analysis, Band Ratios and Sobel Directional Filter followed by an automatic extraction of the lineaments using the LINE algorithm in PCI Geomatica software. The results obtained show that the Inkisi group underwent lineaments, which are organized into two main systems oriented NW-SE and NE-SW. These lineaments correspond to faults which have the same directions as those highlighted by recent work and our study in the field. These lineaments are organizing into networks and corridors, which define large zones or fracturing corridors, which control the Loufoulakari, Djoué and Congo rivers, which drain the study area. These networks and corridors isolate polygons with low or even medium fracturing density. This fracturing density gives to the Inkisi group a good aquifer and reservoir potential. The implementation of borehole with good operating flow as well as the exploration for mineralization should target the fractures and the corridors with high density of fractures oriented NW-SE and NE-SW.
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... The depositional age of this formation is currently interpreted as Palaeozoic but older than Permian (Frimmel et al., 2006;Tack et al., 2008). This ~1 000 m thick redbed sequence was deposited in semiarid lacustrine and fluvio-deltaic palaeoenvironments. ...
Palygorskite-bearing fracture fills in the Kinshasa area, DR Congo – an exceptional mode of palygorskite vein development
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Tectonic fractures in Palaeozoic strata of the Kinshasa area, DR Congo, locally host palygorskite-bearing veins and associated calcite occurrences. The palygorskite deposits are typically massive, with a varying degree of alignment of clay particles, a higher quartz content than the arkose substrate, and a variable amount of smectite (montmorillonite). The associated calcite occurrences are macrocrystalline coatings and infillings, and more fine-grained calcite veins with cataclastic texture. The calcite coatings and infillings formed from solution in earth surface conditions, as recorded by their stable isotope signature. The palygorskite-dominated deposits in the fractures formed at a later stage, in a setting without indications of authigenic mineral formation related to hydrothermal activity or to low-temperature interaction of solutions with the local substrate. The veins most likely formed by vertical infiltration of suspended matter in fractures that extended to a post-Palaeozoic palaeosurface, during or after deposition of palygorskite-bearing Upper Jurassic to Early Cretaceous sediments. This represents an exceptional mode of palygorskite vein development, unrelated to any form of mineral authigenesis that is typically invoked to explain vein-type occurrences of palygorskite and related minerals.
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The Cambro-Ordovician Gondwana alluvial megafan in Central Africa: Insights from the Paleozoic sandstones of the Inkisi group, Congo Republic and Democratic Republic of the Congo.
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The Paleozoic arkoses of the Inkisi Group in both the Republic and the Democratic Republic of the Congo has been investigated in order to examine whether they can represent an alluvial fan originating from the Gondwana supermountains in Northern and East Africa. This study focuses on outcrops’ description, lithofacies, bounding surfaces, architectural and sequential organization of sedimentary bodies and paleocurrents. The aim is to determine the nature of the Inkisi sandstones, the provenance of its sediments and the depositional setting, in order to test whether the East African orogen was also the sediments source for the formation of sedimentary fan in the western part of Central Africa. The study shows that the Inkisi Group consists of various sandstone lithofacies, amalgamated by conglomerates. The sediments are immature composed of quartz and feldspars associate with lithic fragments and micas. The sediments originated from the erosion of the West-Congolian belt and the Congo Craton, and were transported by braided fluvial system under mainly upper flow regime and secondarily lower flow regime. Sediments were deposited in the Inkisi basin, which was extensional and affected by burial. The Inkisi sediments setting up in a proximal to medial distributary zone of an alluvial megafan. The flows were directed southward, as opposed to paleofans originating from the Gondwana supermountain in Northern and East Africa. Thus, the Inkisi sandstone constitutes a new alluvial megafan distinguished from the Gondwana superfan derived from the Gondwana supermountain. It originated from the Pan-African belt of Mayombe and the Congo Craton. We name this megafan “the Cambro-Ordovician alluvial megafan of Central Africa”.
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Neotectonics of Brazzaville and Kinshasa: linking Congo Basin seismicity and in situ stress in the Inkisi Group
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Full-text available
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The Congo Basin has been affected by several earthquakes for which the in-situ stress has not yet been reported. This study aims to determine the in-situ stress related to earthquakes in the Congo Basin, particularly those located in the north portion of the Republic of Congo (RC) and in the northwest portion of the Democratic Republic of Congo (DRC). The combined analysis of seismic history of the Congo Basin and of in-situ paleo-stress in the Inkisi Group allowed us to distinguish onshore earthquakes that are linked with preexisting zones of fractures on the continent and offshore earthquakes that are directly linked with transform faults. The Inkisi Group has been affected by two phases of strike-slip tectonics. The first phase, with a direction of compression N142°, is a result of the Gondwanide orogenesis in the Paleozoic. The second phase, with a compression direction of N078°, is related to the present-day stress of earthquakes in the Congo Basin. This phase is still active and is likely attributable to ridge push from the opening of the Atlantic Ocean. It is therefore appropriate that infrastructure construction in Brazzaville and Kinshasa considers seismic risk in the Inkisi bedrock of this area. As an example, we note that several masonry fences along the Congo river have developed fractures.
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Flower structures in sandstones of the Paleozoic Inkisi Group (Brazzaville, Republic of Congo): evidence for two major strike-slip fault systems and geodynamic implications
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Few studies have reported field descriptions of flower structures associated with strike-slip faults. This study describes and illustrates flower structures near Brazzaville (Republic of Congo) and explains their implication for the tectonic history of the Paleozoic Inkisi Group. Field observations show that the Inkisi Group is affected by two major strike-slip fault systems. The oldest system is dominated by north-northwest–south-southeast striking sinistral strike-slip faults and minor east–west striking dextral strike-slip faults. The youngest system consists of dominant northeast–southwest striking dextral strike-slip faults and minor northwest–southeast striking sinistral strike-slip faults. Flower structures within these major strike slip faults show four types of arrangements that likely depend on fault growth, propagation and damage zones: (i) flower structures associated with wall damage zones; (ii) flower structures associated with linking damage zones; (iii) flower structures associated with tip damage zones; and (iv) “hourglass” flower structures. Paleostress analysis reveals that both major fault systems originated from two differently oriented pure strike-slip regime stress stages. The first stage, which engendered the first major fault system, developed under northwest–southeast compression (i.e, σ1 = 322°). This phase probably coincided with north–south collision in the southern part of Gondwana in the Permo-Triassic and the Late Cretaceous compression times. The second stress stage, creating the second major fault system, developed under east–west (i.e, σ1 = 078°) compression. This phase is correlated with compression from the east–west opening of the Atlantic Ocean in the Miocene times.
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Congo River sand and the equatorial quartz factory
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Strike-slip deformation in the Inkisi Formation, Brazzaville, Republic of Congo
Article
Full-text available
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Evidence of strike-slip deformation in the Inkisi Formation was overlooked for a long time. After controversial characterization, this paper demonstrates that the Inkisi Formation underwent at least two phases of strike-slip deformation accompanied with a compressive component, that created faults that help determine paleostress. Field observations permitted to characterize NW-SE trends sinistral strike-slip faults systems and NE-SW oriented dextral strike-slip faults systems. The strike-slip faults are associated with flower structures in profile view and with damage zones along tips, wall and linking zones in plan view. Both faults systems (sinistral and dextral) initiated from joints, which show similar orientations with faults. Many kinematic indicators of slip sense or extension have enabled to determine the stress stages and the evolution of the structures. The first tectonic phase has a horizontal maximum principal compressive stress σ1 of 319°±21,1/03, which probably have a potential correlation with far-field stress propagation which occurs during the subduction of Gondwana south margin in the Permo-Trias. The second tectonic phase with a slightly inclined maximum principal compressive stress σ1 of 264°±22,3/12 potentially results from the intraplate stress propagation, due to the opening of the Southern Atlantic Ocean.
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