Paleohydrological Signatures and Rift Tectonics in the Interior of Gondwana Documented from Upper Permian Lake Deposits, the Mid-Zambezi Rift Basin, Zambia

Abstract Paleoenvironmental signatures deduced from the sedimentary record of the Upper Permian Madumabisa Formation in the Mid-Zambezi Basin indicate moderate climate and positive hydrologic balance in continental interiors of Gondwana (50-60° S) during the Late Permian. The inferred climate conditions are congruent with the prolific Late Permian terrestrial ecosystems that characterized south-central Africa and equivalent latitudes in Gondwana. Our results do not support the severe aridity and extreme seasonal temperature variations simulated by numerical climate models. The Madumabisa Formation is a lacustrine sequence that represents the maximum extensional phase of the Mid-Zambezi Rift basin. The basin preserves > 5 km of continental sedimentary rocks, of which the Upper Permian Madumabisa Formation comprises freshwater lacustrine deposits up to 700 m thick. Deposition occurred in a fault-controlled asymmetric half graben formed by passive rifting driven by plate boundary stresses that originated from the southern edge of Gondwana. The sedimentary sequence is dominated by black and dark greenish gray mudstones and limestones interbedded with minor gray siltstones and sandstones. The mudstones are predominantly internally massive or bioturbated, but thin-bedded and thinly laminated dark shales with moderate quantities of organic matter and carbonate occur in the lower part of the sedimentary sequence. Laminated limestones are interbedded with the mudstones and comprise fossil-rich beds and inorganic micrite beds precipitated from lake waters, most likely during periods of low clastic supply. Minor current-rippled siltstones and sandstones indicate that silt-laden flows reached deep parts of the basin. Judging from the distribution of the remains of the Madumabisa Formation, the ancient “Madumabisa Lake” covered 140,000 km2, and was larger than most modern lakes. The mainly dark color of the rocks and thinly laminated shales and carbonate content suggest that the lake was deep and experienced periodic, perhaps seasonal, thermal stratification. Abundant biogenic remains including conchostracods, bivalves, fish, and algae indicate a highly productive freshwater lake. Lack of features indicative of emergence and desiccation, such as evaporites, soil features, mudcracks, or cavities, suggests that the lake was perennial. Preserved plant debris including leaves and wood logs indicate that uplands in the watershed were vegetated. For most of the history of the lake system, the perennial lake remained hydrologically closed because of a steady subsidence and moderate axial sediment supply. At the end of the Permian, pulses of basinal shortening caused shallowing of the lake. Basin uplift associated with postulated basin inversion led to changes in sediment supply patterns from axial to rift flanks, as shown by paleocurrent directions. The lake sediments were eroded and buried rapidly, as indicated by the lack of paleosol features, and the lake was filled by thick, red conglomeratic sandstones of the Lower Triassic Escarpment Grit Formation. Contrary to predictions of numerical climate models, abundant moisture and cool (to moderately warm) temperatures would have been essential to sustain a large, perennial, freshwater lake such as “Madumabisa Lake” was, deep within the continental interior of south Gondwana.

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