Stratigrahy of the Kundelungu supergroup

Overview

The sediments in the Lufilian belt and its foreland were deposited during the Neoproterozoic, on a Palaeo- to Mesoproterozoic basement. The Katangan sediments started to be deposited in an intra-cratonic rift (Porada and Berhorst, 2000; Unrug, 1988) or in an epicontinental marine embayment (Jackson et al., 2003). The underlying pre- Katangan basement is poorly studied in Katanga and what is known in northern Zambia has been documented by Key et al. (2001) and Rainaud et al. (2002). These sediments were deformed during the Lufilian orogeny (ca 560 – 550 Ma; Cahen et al., 1984; Kampunzu and Cailteux, 1999; Porada and Berhorst, 2000). The Lufilian fold-and-thrust belt and its foreland are bordered to the west by the Mesoproterozoic Kibaran belt and to the east by the Paleoproterozoic Bangweulu block.

The Katanga Supergroup consists of a 5 to 10 km-thick sequence that can be subdivided into three groups based on two regionally extensive diamictites (Figure 5). From the bottom to the top, the Katanga Supergroup is divided into the Roan, the Nguba and the Kundelungu Groups (Cailteux et al., 2005). Sedimentation of the Katanga system started in a continental Roan rift basin after ~880Ma (Armstrong et al., 2005), with a basal conglomerate (Cailteux, 1994). The Roan is divided into four subgroups, i.e. the R1 to R4 Subgroups. The R1 Subgroup, known as the “roches argilo-talqueuses (R.A.T.)”, consists essentially of massive or irregularly stratified detrital formations with hematite present as authigenic plates and red pigment, attesting to the primary oxidising conditions (Cailteux, 1994).

Towards the contact with the R2 (Mines Subgroup), there are indications that sedimentation took place in an evaporitic environment. The sedimentary transition to the Mines subgroup is at certain localities continuous, whereas at other localities a tectonic breccia developed at the contact. The tectonic breccia formed during detachment of the Mines Subgroup, which was aided by fluidisation of evaporitic material, probably present near the top of the R.A.T. subgroup (Cailteux and Kampunzu, 1995). The importance of salt tectonics to explain the large observed breccia bodies in the Lufilian belt is further stressed by Jackson et al. (2003). The Mines Subgroup (R2) is a carbonate unit that contains the richest stratiform copper-cobalt mineralisation, which occur at two different stratigraphic levels. The overlying R3 (Dipeta Subgroup) is subdivided in four formations, each characterised by predominantly argillaceous and siliciclastic beds at the base and by predominantly carbonate beds at the top (Cailteux, 1994). The transition to the overlying Mwashya Subgroup is again marked by a tectonic breccia which developed at the contact. The Mwashya was deposited between 760 and 735Ma (Master et al., 2005). The continuous stratigraphic sequence for the Roan group, as proposed by François (1974) and Cailteux (1994), is contested by some authors. Porada and Berhorst (2000) agree that the R.A.T. was deposited on the pre-Katangan basement, but they believe that the overlying Mines, Dipeta and Mwashya subgroups form a platform facies association, which became a Copperbelt-type tectonostratigraphical succession through the development of foreland propagating thrust faults. Wendorff (2000a+b, 2005) does not interpret the brecciated contacts as of tectonic origin. He proposed they should be called conglomerates that were derived from erosion of advancing thrust sheets during the Lufilian orogeny. The R.A.T. and the Dipeta Groups are olistostromes, deposited at the border of the developing Lufilian orogen (Wendorff, 2002a+b, 2005).

In contrast to the disagreement about the stratigraphy of the Roan, all authors agree that sedimentation of the Nguba group started with the deposition of a diamictite (the ‘Grand Conglomérat’) that likely formed part of the Sturtian glacial deposits (Kampunzu et al., 2005) (Table 1). The ‘Grand Conglomérat’ thickens towards the north (François, 1974) and is at least observed as far north as Pweto, which is a town to the north of Dikulushi (Cahen, 1954). The Likasi Subgroup, with at its base the ‘Grand Conglomérat’, contains a mixture of shale and dolomite in the south and a lateral facies change towards pure shale in the north of the Kundelungu basin (François, 1974; Batumiké et al., 2006). The Kakontwe limestone, in the middle of the Likasi Subgroup, is only observed in the south, towards the Lufilian belt. This limestone formation could be a cap carbonate, confirming the interpretation of the ‘Grand Conglomérat’ as a glacial tillite (Porada and Berhorst, 2000). The Monwezi Subgroup was deposited towards the end of the Nguba and consists of more detrital lithologies, with relatively thin arkose layers in the north and thick, slightly carbonatised pelites and fine sands in the south (François, 1974; table 1). Transition to the overlying Kundelungu Group is again marked by a diamictite (the ‘Petit Conglomérat’) that forms part of the Kalule Subgroup. This conglomerate layer could have been deposited during the Marinoan-Varanger ice age. The Kalule Subgroup is similar to the Likasi Subgroup, since it also contains a thin cap carbonate (the ‘Calcaire Rose’) overlying the ‘Petit Conglomérat’. The Kalule Subgroup becomes more detrital towards its top, with the deposition of dolomitic siltstones, sandy shales and pink oolitic limestone. The ‘calcaire rose’ is, unlike the Kakontwe limestone from the Likasi Subgroup, continuous from the south to the north, with a layer thickness of around 5m (François, 1974). The Kiubo Subgroup overlies the Kalule Subgroup and consists of sandstones and shales (Cailteux et al., 2005). The top of the Kundelungu group is formed by the Plateau Subgroup that consists of shales and arkoses.

Historically, the mineralized sections of the R.A.T. and the Mines Subgroup have been subdivided in different sections based on their appearance (e.g. François, 1974). The R1 Sub-group forms the so-called R.A.T. rouge. The Mines Subgroup (R2) has been subdivided into three formations that are subdivided into different niveaus. Formation R2.1 is subdivided into niveau R2.1.1 or the so-called R.A.T.gris, in niveau R2.1.2 that consists of subniveau R2.1.2.1 or DStrat and subniveau R2.1.2.2 or R.S.F. and niveau R2.1.3 or the R.S.C. The formations R2.2 and R2.3 are also called S.D. and C.M.N, respectively.