Syn-rift and synorogenic olistostromes and their role in interpretation of tectonic evolution of the Neoproterozoic Lufilian orogen as background for comparative studies

The Lufilian belt is a part of the Neoproterozoic-Lower Palaeozoic Pan-African orogenic system of southern and central Africa. Its sedimentary sequence of the Katanga Supergroup (880-500 Ma) is composed of five groups: Roan and Nguba that represent rifting stages and Kundelungu, Fungurume, and Biano, which reflect evolution of the synorogenic foreland region. The occurrences of olistostromes, traditionally called the Katangan “megabreccias”, are a prominent feature of the Lufilian belt architecture. Some olistostrome bodies reach thickness of 2000 m and contain huge blocks of Katangan rocks, some of which host the famous copper-cobalt orebodies of the Dem. Rep. of Congo sector of the Central African Copperbelt. They were previously considered as tectonic mélanges (“friction breccias”) marking regional decollement zones related to thrusting during the Pan-African orogenesis. However, these fragmental rocks were recently shown to be of sedimentary origin and to form two extensive olistostrome bodies, which shed new light on the stratigraphy and tectonic evolution of the Lufilian belt. Borehole cores available in this copper mining region reveal fine details of their stratigraphic, sedimentological and structural relations.
   The main lines of evidence for the olistostrome genesis are following: (1) lack of pervasive shearing that would point to tectonic fragmentation; (2) textures and structures diagnostic for subaqueous sediment gravity flows ranging from debris flows to turbidites; (3) roundness and provenance of clasts, and lateral facies gradients implying erosion, abrasion and unroofing of the Katangan source rocks elevated in the source areas; (4) lower boundaries of fragmental bodies are not tectonic but stratigraphic; (5) injections of unconsolidated conglomeratic matrix filling open joints in megablocks and fragmented slide sheets, which represent olistoliths and olistoplaques.
   The older olistostrome unit is a disorganised to locally organised syn rift complex with olistoliths reaching five metres across. The clasts were derived from the uplifted rift margin and redeposition resulted from mass-wasting (rockfalls producing sedimentary breccias), sliding of solitary blocks, and pebbly to cobbly debris flows, some of which evolve to turbidites. The underlying strata are deformed by slump folds, and the succeeding lowest part of the olistostrome contains slump-generated debris flows with fragments of the dismembered slump beds. This olistostrome succession consists of three complexes typified by matrixsupported debris flow conglomerates with Roan clasts. Some of the conglomerate beds pass upwards to normally graded turbidite layers and are accompanied by solitary slump beds. The three conglomeratic assemblages are separated by two intervals of sedimentary breccia composed of allochthonous angular Roan blocks interpreted as mass-wasting debris redeposited into the basin by high-volume sediment-gravity flows. The breccia bodies document unroofing of the source area in that the older one contains dolomite clasts derived from the upper Roan strata, and the younger breccia consists of quartzite fragments sourced from the lower Roan.
   Synorogenic olistostromes in the Fungurume Group deposited in the foreland basin of the Lufilian Belt and derived from the Katangan nappes thrust northwards are composed of nappe-derived olistoliths and olistoplaques up to several kilometres in size embedded in debris-flow conglomerates. They represent all units older than the Fungurume Group. Olistostrome at Kambove overlies a turbidite sequence, distally grades to a conglomerate complex and includes olistoliths of red-bed strata, which in the Katanga Supergroup occur in the foreland region. This is interpreted as recycling of the foreland sediments involved in a successive orogenic phase due to advancement of the orogenic front from the south and migration of the foreland depocentre during a punctuated orogenesis.
   Acknowledgements: This paper has been partly supported by the Polish Ministry of Science and Higher Education, grant no N N307 249733.