The Occurrence of a Circular Structure in the Congo Basin
Circular structures can be generated by different geological mechanisms such as volcanic and intrusive processes, diapirism, and sinkholes in areas of karstification. However, the diameter for most of these morphologies in many cases does not reach more than ten kilometers. Ring structures larger than 36 km, the diameter estimated for Omeonga within the Unia River flow, are not common in geological records.
Volcanic calderas or batholiths can display or exceed dimensions similar to those of Omeonga (≥36 km) (e.g., Johnson et al. 2002), as for example, the Meugueur–Meugueur ring structure of the Aïr Massif, which reaches 65 km in diameter (Moreau et al. 1986; Ritz et al. 1996) or the 40 km wide Richat structure (Mauretania) interpreted as updoming induced by a Cretaceous alkaline intrusion (Matton et al. 2005). In the latter case, the structure is marked by regular hogback geometry of the folded beds easily detectable from remote sensing; whereas in Omeonga none of these morphologies is visible. In addition, volcanic activity does not seem to have affected the eastern Kasai Craton, where the Omeonga structure is located. The East African Rift System extends 300 km to the east of the study area (Cahen 1954; Morgan 1983; Choubert and Faure–Muret 1986). On the other hand, volcanism produced by hotspot migration is located further to the northwest (Morgan 1983; Ngako et al. 2005), outside of the Congo cratonic area (Fig. 6). Young or active volcanism in this region is extremely unlikely (Master 2010) and undocumented by seismic or geophysical analyses (Kadima et al. 2011b). Deep-seated magmatic bodies should be detectable in the geophysical maps as suggested by Kadima et al. (2011b); however, in the area, no short wavelength gravity highs were recorded.
Figure 6. Distribution of volcanism on and around the African continent and track of the hotspots; age in brackets; modified after Ngako et al. (2005). Black square: location of the Omeonga structure. (1: West African Craton, 2: Congo-Kasai Craton. 3: Tanzania Craton, 4: Kaapvaal-Kalahari Craton)
Download figure to PowerPoint
Kimberlitic pipes (Cox 1978) have roundish shapes and have been detected to the south of the study area (Milesi et al. 2006). However, they are relatively small (about 4 km in diameter; Jacques 1998) and are characterized by a wide central depression not visible over the Omeonga structure. Therefore, this morphology can be ruled out in the case of Omeonga.
Karst sinkholes can be excluded because carbonate and/or sulfate successions in the area are negligible close to the surface, and sinkholes normally do not display a central ring feature like the one found within the Omeonga ring.
Salt diapirs or domes are frequently circular and are characterized generally by moderate dimensions (up to 15 km in diameter—Aslop et al. 2000), as for example, the salt diapirs of the Great Kavir (Jackson et al. 1990). On the other hand, the stratigraphy of the sedimentary succession of the Eastern Kasai craton shows indeed the occurrence of evaporite beds in the Ituri Group (Daly et al. 1992; Kadima et al. 2011a). According to recent seismostratigraphy, salt diapirism, generating structures of 15–20 km in size, is located at least 2 km below the topographic surface and is related to the Pan-African tectonism (Early Palaeozoic) and, possibly, to Triassic-Early Jurassic tectonic events (Kadima et al. 2011a). Indeed, the sedimentary succession above the Triassic-Early Jurassic U3 unconformity has not been influenced by diapir tectonics (Kadima et al. 2011a). In addition, each evaporite pillow creates negative gravity and magnetic anomalies (Kadima et al. 2011b), which have not been recorded at the Omeonga structure. Hence, although the Omeonga ring is located close to the Loconia High, where the evaporites of the Ituri Group could be shallower by some hundred meters, the geophysical data (Kadima et al. 2011b) allow the exclusion of a near-surface salt diapir.
In the Congo-Kasai Craton, the undulations of the basement floor related to regional deformations, and recorded by the geophysical prospecting (Kadima et al. 2011b), have wavelengths (at least 100 km) too long to fit the Omeonga structure. In addition, they display a clear NW-SE trend (Daly et al. 1992; Kadima et al. 2011b) and a circular shape as the one of the Omeonga structure is unlikely even in the case of dome and basin fold interference pattern (type 1 of Ramsey 1967), which, however, would imply a recurrence of similar structures at a regional scale.
All these geological arguments suggest that igneous intrusion, volcanic activity, salt diapirism, or karst dissolution are not likely mechanisms for the Omeonga structure, highlighting the possibility of an impact origin (French and Koeberl 2010). Indeed, the concentric path of the river, together with the tributary river networks of Omeonga, is similar to the drainage pattern found in many confirmed large impact structures (Mihályi et al. 2008). Examples of a concentric path of a main river are Popigai (Siberia) (e.g., Deutsch et al. 2000) and Carswell (Canada) (e.g., Pagel et al. 1985), whereas an example of drainage around a pronounced peak-ring structure is Gosses Bluff (Australia) (e.g., Milton et al. 1996). Finally, examples of tributary river networks are Manicouagan (Canada) (e.g., Floran et al. 1978) and West Clearwater (Canada) (e.g., Grieve 1978). Complex drainage patterns with both centripetal and concentric small rivers draining from a central uplift and external rim have been found also in the Brazilian tropical region (e.g., Engelhardt et al. 1992; Romano and Crósta 2004; Reimold et al. 2006).
Omeonga as a Peak-Ring Impact Structure
Based on the rejection of seemingly unsupported alternative modes of origin for Omeonga, a possible origin by impact needs to be evaluated.
According to an impact perspective, the continuous depressed arch formed by the 85 km long Unia River and its northern tributaries can be interpreted as a rim basin encircled by an arched rim (Fig. 3b). The rim is discontinuous, but still detectable throughout the basin, indicating a total diameter of about 45 km (Figs. 3b and 5). The centripetal tributaries flowing from the outer rim, better developed in the southwestern and northeastern parts of the structure, may reflect radial structure originating from an impact and then partly obliterated by subsequent erosion. The inner drainage network has probably been conditioned by the main structure within the impact basin rim, a peak-ring whose remnant is the arch-shaped ridge with a diameter of approximately 13–20 km within the Omeonga main structure (Figs. 4b and 5).
Hence, Omeonga could be considered a complex crater structure of “peak-ring basin” type, as expected from its diameter of about 45 km (e.g., Melosh 1989; French 1998). The principal mechanisms that lead to the development of such a structure relate to the collapse of the initial bowl-shaped transient crater, achieved mainly by uplift of the rocks underlying the crater center and flow of material from the wall of the transient crater inward (Melosh 1989). The result is a shallower structure, with a well-developed wall terrace and an inner peak-ring diameter roughly half the rim-to-rim diameter (Melosh 1989).
However, the geographical and climatic position must be taken into consideration. In fact, the Congo Basin has been a tropical region since the Cretaceous, and, therefore, the sedimentary cover has been affected by intense weathering as well as the strong erosion by rainforest rivers. For this reason, the assumed impact structure must have been heavily affected by weathering and erosion, which determined the rather flat morphology of the area (the differences in elevation do not reach 100 m). However, the dendrite morphology, quite common in a mature fluvial environment, here shows an anomalous circular shape compared with the rest of the Congo River catchment; this anomaly indicates that the differential erosion and the drainage affected a circular depression. This, as argued above, may be ascribed to the depression between the peak-ring and rim-to-rim ridges, whose diameters are approximately 20 km and 45 km, respectively.
The Omeonga structure is formed within an Upper Jurassic-Lower Cretaceous sedimentary succession overlaid by Quaternary loose deposits (Fig. 1) included in the seismostratigraphic unit C of Kadima et al. (2011a). Hence, the postulated impact may have occurred as early as the Late Cretaceous. However, the instability of an impact structure in a region of such climatic conditions points to a rapid flattening of the structure and may suggest a younger age, as topography seems to still preserve a morphology akin to a peak-ring basin. However, this may be solved only by future field analysis.