DEPOSITIONAL ENVIRONMENT AND LITHOSTRATIGRAPHY OF THE PALEOPROTEROZOIC MOOIDRAAI FORMATION, KALAHARI MANGANESE FIELD, SOUTH AFRICA

摘要

The Paleoproterozoic Mooidraai Formation is an up to 220 in thick succession of marine carbonate rocks that caps the Fe- and Mn-bearing Hotazel Formation in the Kalahari Manganese Field, South Africa. Although it occupies an important stratigraphic position within the upper Transvaal Supergroup, which records major perturbations of the early Paleoproterozoic biosphere, a detailed sedimentological study on the Mooidraai Formation has never been conducted. Here we present a detailed facies analysis that distinguishes eight carbonate and two iron formation lithofacies types. The lower Mooidraai Formation is dominated by carbonate rhythmites and slope breccias deposited on a foreslope, occasionally interbedded with oxide or carbonate fades iron formation. The upper part of the formation reflects various shelf and peritidal environments arranged in shallowing-upward parasequences. Clastic-textured massive dolarenites deposited in shelf and lagoonal environments typically form the base of parasequences and are overlain by subtidal thrombolites, lagoonal to intertidal microbialaminites, and upper intertidal to supratidal smoothly laminated stromatolites. Supratidal intraclast breccias cap shallowing-upward parasequences. Strong base level rise in the lower Mooidraai Formation reflects a transgressive systems tract tied to rapid early subsidence. Together with considerable lateral thickness variation in the following regressive systems tract, this suggests deposition in a basin with significant seafloor relief.