Tectonic and climatic controls on sedimentation during deposition of the Sinakumbe group and Karoo supergroup, in the mid-Zambezi Valley Basin, southern Zambia

Abstract

Sediments of the Ordovician to Devonian Sinakumbe Group (∼210 m thick) and overlying Upper Carboniferous to Lower Jurassic Karoo Supergroup (∼4.5 km thick) were deposited in the mid-Zambezi Rift Valley Basin, southern Zambia. The Sinakumbe-Karoo succession represents deposition in a extensional fault-controlled basin of half-graben type. The basin-fill succession incorporates two major fining-upward cycles that resulted from major tectonic events, one event beginning with Sinakumbe Group sedimentation, possibly as early as Ordovician times, and the other beginning with Upper Karoo Group sedimentation near the Permo-Triassic boundary. Minor tectonic pulses occurred during deposition of the two major cycles. In the initial fault-controlled half-graben, a basin slope and alluvial fan system (Sikalamba Conglomerate Formation), draining southeastward, was apparently succeeded, without an intervening transitional facies, by a braided river system (Zongwe Sandstone Formation) draining southwestward, parallel to the basin margin. Glaciation followed by deglaciation resulted in glaciofluvial and glacio-lacustrine deposits of the Upper Carboniferous to Lower Permian Siankondobo Sandstone Formation of the Lower Karoo Group, and isostatic rebound eventually produced a broad flood plain on which the coal-bearing Lower Permian Gwembe Coal Formation was deposited. Fault-controlled maximum subsidence is represente by the lacustrine Upper Permian Madumabisa Mudstone Formation. Block-faulting and downwarping, probably due to the Gondwanide Orogeny, culminated with the introduction of large quantities of sediment through braided fluvial systems that overwhelmed and terminated Madumabisa Lake sedimentation, and is now represented by the Triassic Escarpment Grit and Interbedded Sandstone and Mudstone Formations of the Upper Karoo Group. Outpourings of basaltic flows in the Early Jurassic terminated Karoo sedimentation.