Orogenic Telescoping and Megabreccia Transport in a Gulf-of-Mexico-Type Allochthonous Salt Complex in the Precambrian Katanga Basin, Congo

Martin P. A. Jackson

ABSTRACT:
The Neoproterozoic Katangan rocks in the Central African Copperbelt represent not only a major source of copper and cobalt but also represent some of the world's most striking and bizarre tectonics. The Katangan rocks in Shaba (Congo) have previously been inferred to represent a conventional fold-and-thrust belt. Instead, we think that they record the phenomenal transporting ability of salt nappes, partly associated with orogenic shortening. No halite has been found in the basal Roan Supergroup. However, the former existence of evaporites is indicated by sabkha facies, crystals and pseudomorphs of gypsum and anhydrite, stratigraphic gaps underlain by collapse breccias, chloride inclusions in ores, hydrothermally propylitized ore hosts, and saline springs. Salt tectonics is the most plausible explanation for spectacular megabreccias underlying at least 25,000 km² and containing exposed megaclasts up to 10 km long. Huge evaporite-megabreccia nappes were extruded in the vanguard of an advancing orogen. This origin explains mechanical aspects that would be paradoxical in a conventional fold-and-thrust belt: (1) the present Roan is too strong and too dense to form the observed structures; (2) the Roan roof was highly extended and fragmented into megaclasts before orogenic shortening; (3) klippen, some of which are undeformed, were transported 25-50 km over a flat plain. Salt tectonics began between 1050 and 950 Ma as coastal sabkhas alternated with marine incursions on an inner carbonate ramp. Small walls and allochthonous sheets containing evaporite-megabreccia began to be emplaced. Between 940 Ma and 850 Ma, larger evaporitic diapirs rose. Between 850 Ma and 650 Ma, nappes of commingled Roan evaporites, Roan carbonate-dominated sediments and ores, and dismembered slabs of younger overburden were expelled northward. Orogeny shortened the klippen and their enclosed diapirs and allochthonous sheets. The resulting picture of a basin containing vast thicknesses of allochthonous salt is not unlike the present-day Gulf of Mexico, except that in the Congo, the basin was subsequently shortened to half its original length in the late Proterozoic.