CLIMATE CONTROLS ON WATER LEVELS OF LAKE MALAWI (AFRICA) OVER THE LAST 1.3 MYR: IMPLICATIONS FOR LACUSTRINE RIFT PETROLEUM SYSTEMS

We present geophysical and geochemical drill-core data from Lake Malawi, a large East African lacustrine rift basin. Over 600 m of drill-core was collected at 2 sites. The longest core (collected in 590 m modern water depth) recovered ~380 m of continuous lacustrine sediment, recording a highly variable lake level history over the last ~1.3 Ma. Today the lake is fresh, stratified, corrosive to carbonate at depth, and hydrologically open with an outflow to the south. These conditions are characterized in the core by low-density, organic-rich, carbonate-poor, finely laminated siliceous muds. This is in stark contrast to the lowstand deposits observed in the core, which are characterized by high-density, low organic carbon, carbonate-rich muds and sands, indicating a well-mixed, slightly alkaline lake. Lowstand delta seismic facies in as much as 500 m modern water depth corroborate the core results, and indicate rapid, high-magnitude, and high-frequency lake level change throughout the Late Quaternary. We benchmark key lowstand events by integrating the sediment core and seismic data sets, and then use Principal Component Analysis (PCA) of the drill-core data to develop a PC(1)-lake level transform function. PC(1) indicates that over 20 lowstands, more than 400 m below modern levels, occurred over the ~1.3 Ma record. Energy-balance hydrologic modeling suggests that a ~60% drop in precipitation relative to modern is required to sustain hydrologic equilibrium during these lowstands, requiring a significant reorganization of atmospheric circulation to generate this magnitude of lake level change. These results have significant implications for exploration in tropical lacustrine rift basins. Similar lake-level variability likely occurred in the Tertiary Albert and Lokichar Basins of East Africa and the Cretaceous pre-salt South Atlantic Basins. Such marked variability likely played a fundamental role in developing source, reservoir, and seal facies in these and other similar hydrocarbon provinces.