Changing Fluvial Environments and Vertebrate Taphonomy in Response to Climatic Drying in a Mid-Triassic Rift Valley Fill: The Omingonde Formation (Karoo Supergroup) of Central Namibia
The Omingonde Formation of Central Namibia is a redbed succession infilling a half-graben that developed on the upland plateau of southern Gondwana in the early mid-Triassic. Field studies of the rocks and vertebrate fossils of these strata are used to reconstruct the changes that occurred in the rift valley landscapes over approximately 10 My, and show how these changes affected the life habits and preservation of terrestrial reptiles that inhabited the valley at that time. The early rift basin contained extensive lakes that were filled rapidly with conglomeratic sands deposited on alluvial fans prograding from the active boundary fault. These fans subsequently drained into an axial braided river system. With cessation of downfaulting, the fault scarp retreated and gradients in the basin became gentle enough to convert the braid plain to one or more meandering rivers confined by extensive floodplains. The climate changed from sub-humid at the beginning of rifting, through semi-arid for most of the Omingonde times, to arid at the onset of the overlying Etjo Formation sedimentation. The influx of abundant loessic silt, possibly a peripheral effect of the Triassic "megamonsoon," significantly increased floodplain accretion and the burial potential of surface bones on the Upper Omingonde floodplains. The major control of fluvial style and floodplain accretion rates in the Omingonde basin was subsidence caused by episodic movements of the boundary fault. However, the association of desiccated and mummified carcasses with thick beds of loessic silt suggests that climatic aridity was the overriding factor controlling the preservation and taphonomic style of vertebrate remains.