On Climate, Weathering, and Siliciclastic Sedimentation in Tropical Lacustrine Rift Basins

McGlue, Michael; Ivory, Sarah; Ellis, Geoffrey; Boehlke, Adam; Blome, Margaret; Cohen, Andrew; Lyons, Robert; Scholz, Christopher

Although conceptual source-to-sink models have existed for many years, a dearth of sufficiently long-duration and well-dated sediment archives has prevented more rigorous examinations of the sediment routing system that links tropical rift basins with their hinterlands. In particular, feedbacks between climate change and weathering are still poorly understood. Using data from the Lake Malawi Drilling Project (East Africa), we examined the relationships between climate, vegetation, fire, and sedimentation over what is believed to be the Penultimate Glaciation (PG) and Marine Oxygen Isotope Stage 5e (MIS 5e). To this end, we developed complementary datasets on a stratigraphically well-constrained ~40 m section of drill core, including: terrigenous particle size, clay mineralogy, fossil pollen, and charcoal fragments. The results show that climatic controls on hinterland forest composition and grassland extent may be critically important to patterns of weathering, associated sedimentary processes, and the accumulation of siliciclastics downdip. In general, peaks in insolation correlate with high precipitation and elevated ratios of tree-to-herb pollen. Silty detritus dominates at an insolation peak during the PG, when drought-intolerant evergreen trees were abundant on the hinterland landscape. By contrast, a peak in sandy detritus at MIS 5e correlates with palynological evidence that shows the hinterland was dominated by open canopy, drought-resistant deciduous trees. Pronounced wildfires at this time may have further conditioned the hinterland for sediment transport, prior to heavy seasonal rainfall. Clay minerals, particularly the ratio of kaolinite to smectite, track the change in forest composition, with their highest values coinciding with the evergreen phase, consistent with heavy leaching driven by extended wet seasons during that time, whereas leaching is lower and more variable during MIS 5e. During intervals of low insolation, aridity prevailed upon the Lake Malawi catchment, leading to grassland development and clay minerals dominated by smectite. Our results have implications for source rock quality in rifts, particularly for predicting patterns of siliciclastic dilution of organic-rich muds and nutrient recharge of surface waters during lake level highstands. The results are also relevant for understanding lacustrine reservoirs, especially with respect to the prediction of conventional deepwater sands and unconventional shale plays.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013