| Description |
Many studies present climate change as the principal cause of the most prominent shifts in depositional environments in the Omo-Turkana Basin (eastern Africa) during the Pliocene and Pleistocene Epochs. For example, some studies relate the development of major lake sequences to presumed high precipitation during Milankovich's eccentricity maxima. The hypothesis of this dissertation is that major shifts in depositional facies in the basin took place only when tectonic activities allowed, including the development of at least four of the basin's most prominent lake sequences at about 4.0 Ma, 3.5 Ma, 3.2 Ma, and 2.0 Ma. Corroborating field evidence includes pronounced changes in the basin's depositional facies in association with tectonic-related features such as prominent syndepositional faulting and local to basin-wide basaltic volcanism about 4.0 Ma, 3.2-3.3 Ma, and 2.02-2.18 Ma. A distinction between climatic and tectonic influences on ancient habitats in the Omo-Turkana Basin is critical in paleontological and paleoanthropological studies because the sedimentary record of this basin and associated fossils and archeological artifacts are widely cited as among the best examples of the role of climate in the evolution of modern African mammals and humans at large. The hypothesis presented in this dissertation illustrates geographic location and tectonic-related physiography as significant factors in the Omo-Turkana Basin in addition to regional and global climate. This dissertation submits that substantial deposition of Pliocene and Pleistocene strata in the Omo-Turkana Basin was initiated and sustained primarily by rift tectonics. The Omo River, with headwaters in the Ethiopian volcanic highlands, is presented as the main supplier of both sediments and water in the basin since the Early Pliocene. The geologic record shows that the river has remained perennial, regularly draining outside the basin, even reaching the Indian Ocean, and at times flooding the basin to form expansive tectonic lakes. Precession- to eccentricity-scale astronomical forcing of climate seems to have played a subordinate role in modulating the sediment supply and the formation of extensive lakes except for second- to third-order fluctuations in lake levels-a rise by a few tens of meters possibly for a few thousand years. |
