Sequence and carbon isotopic stratigraphy of the Neoproterozoic Roan Group strata of the Zambian copperbelt

Abstract

The Neoproterozoic Roan Group in northern Zambia is host to numerous world class stratiform sediment-hosted Cu ores that occur around the transition from basal continental clastics to an overlying shallow marine succession. The latter can be considered in terms of six sedimentary facies deposited within a mixed clastic and carbonate barred basin margin environment. Sequence stratigraphic analysis, based on palaeo-bathymetric cycles defined by vertical facies changes in sections constructed from detailed logging of diamond drill cores, allows the definition of seven sedimentary sequences. Accommodation for the lowermost two sequences, which host the bulk of the copper ores, was generated by half-graben development during active extension. The subsequent two sequences record denudation of tectonically generated relief and evolution to a laterally extensive, low-relief, basin margin carbonate platform during a period of tectonic quiescence. The uppermost sequences that coincide with the Mwashia Subgroup, record the resumption of tectonically generated accommodation that may reflect the onset of the breakup of Rodinia. Carbon isotopic profiles through the Roan succession in Zambia show two secular δ 13C excursions of more than 10‰ to values of <−5‰ that can be correlated to the global curve for the Cryogenian. The upper excursion occurs below the Sturtian Grand Conglomérat and is therefore interpreted to record the Islay anomaly. The lower excursion is interpreted to record the Bitter Springs stage, which occurs prior to the Sturtian glacials in a number of other Cryogenian sections where, as is the case in the Roan Group section in Zambia, it is not associated with glacial strata.