Sequence stratigraphy and metre-scale cyclicity of Neoarchean dolomite strata at Bushy Park, Griqualand West Basin, Transvaal, South Africa

Abstract

Deposition of Neoarchaean carbonate platform sediments of the Campbellrand Subgroup (Transvaal Supergroup) are cyclic and exhibits a full range of facies, which extend from subtidal to supratidal deposition. The strata are host to Mississippi Valley-type lead-zinc deposits including the Pering mine and Bushy Park prospect, the latter of which is the focus of this study. Core spanning the top of the Klipfontein Heuwel Formation to the top of the Kogelbeen Formation where described in terms of the sedimentary environments that likely prevailed at the time. Key facies were identified that were used to identify parasequences and depositional sequences, which were based on stacking patterns of shoaling upward sequences and flooding surfaces. Progradational facies indicate sea level rise and retrogradational facies mark sea level fall of different magnitudes. Multiple breccia types were observed including early diagenetic, karst related breccias, slump breccias, as well as structurally controlled collapse breccias. Although an autocyclic origin for the 4th and 5th order parasequences cannot be ruled out, sedimentary parasequences in the Neoarchaean strata at Bushy Park are very similar to those recognised in Phanerozoic carbonate sections where deposition and accommodation space is believed to be controlled by orbitally forced eustatic sea level rise and fall (Milankovitch cycles). This work identifies a need to test the possible effects of Milankovitch cycles not only on other tectonic basins of similar Archean age but also other rocks through time to account for the possibility that Archean cycles are longer in duration than those of the Phanerozoic Eon.