ENVIRONMENTAL CHANGE DRIVEN BY TECTONIC PROCESSES AND CLIMATE SHIFTS AS RECORDED IN THE SEDIMENTARY RECORD OF PALEOLAKE MABABE, NORTHERN BOTSWANA

Lacustrine sediments preserve a record of environmental changes, which are commonly interpreted in terms of climatic shifts. However, in the Mababe Depression, northwest Botswana, tectonic processes along the incipient Okavango Rift Zone (ORZ) also have the potential to affect the formation and stability of ancient lakes. To determine the relative importance of these two types of processes, we used an integrated set of sedimentological and geochemical proxies to develop a model of the paleoenvironmental evolution of the Middle Kalahari desert region.

A major perturbation in lake hydrology and sedimentation occurred between 41 ka and 27 ka, that was caused by movement along faults in the ORZ. This movement diverted river flow from the Mababe Depression and impounded the Okavango River to form the modern Delta. The metal data, especially the alkali and alkali earths show major concentration shifts at times when tectonic activities altered the sediment source to the Mababe Depression. Sediments deposited prior to tectonic events were derived from both local sediments in the Kalahari watersheds and distal sources in the tropical Angolan highlands. After the tectonic event, sediment delivered to the Depression was sourced mainly from distal sources and the new hydrologic regime supported the preservation of higher organic carbon. Diatom abundances in sediments that pre-date tectonic alteration of regional hydrology suggest the persistence of deeper and more stable lake levels. However, post-tectonic sedimentation is characterized diatom abundances and organic carbon concentrations that suggest generally lower lake levels and the development of marsh-like conditions.

In addition to tectonic processes, the sediments also reveal the effects of local and regional climatic shifts over a >64 ka time period. Locally wet climate was characterized in the sediment record by increased sand content derived locally, while regional wet climate brought a high sediment load dominated by clay and silt from the tropics. The record of climatic evens is more evident in the post-tectonic sediments, as low water levels sensitized the lacustrine system to even minor climate changes. Thus, the role of tectonic processes cannot be overlooked when attempting to reconstruct climate records from lacustrine sediments.