The Source-to-Sink Evolution of the Central Lake Malawi (Nyasa) Rift, East Africa and Implications for Deep-Water Lacustrine Reservoirs

Abstract

Deep-water lacustrine reservoirs are important resources in some continental rift systems, but from a global perspective are underexplored. The Central Basin of the Lake Malawi (Nyasa) Rift, East Africa contains a complex assemblage of coarse-grained siliciclastic deposits and sublacustrine fans that may serve as analogues for ancient, buried rifts. An extensive suite of crustal-scale seismic reflection data was acquired in 2015 as part of the SEGMeNT project, which produced outstanding images of the syn-rift section in this most deeply subsided rift segment. The new basin-scale seismic data are augmented by legacy single-channel high resolution reflection data that reveal detailed information on facies geometries and stacking architecture of the deep-water fan systems. Cores collected during the Lake Malawi Scientific Drilling Project provide detailed information on the age and lithologic character of the sedimentary sequences within the rift’s Central Basin. Two major river systems deliver siliciclastic inputs to the basin. The South Rukuru River (drainage area = 11,900 km²) flows east and enters Lake Malawi through an incision in the western border fault of the rift’s Central Basin. The Ruhuhu River (drainage area = 17,230 km²) enters the eastern side of the lake at an accommodation zone margin between the North and Central Basins. These are likely antecedent drainages that may have developed prior to late-Cenozoic rifting. These rivers now deliver sediment to a confined and focused depocenter in the Central Basin. The complex interplay of continental extension, mainly on the border fault systems, and high-frequency and high-amplitude lake levels shifts, produces unique coarse sediment facies stacking architectures, with vertical stacking controlled by hydroclimate, and lateral positioning localized by fault behavior. Focused deposition in the deepest part of the lake (> 700 m water depth) has resulted in an overpressured section within the sediment pile in this area, resulting in a series of mud diapirs. Deep-water channel-levee systems observed in the new seismic data deep in the synrift section indicate that these source-to-sink sink systems been operative in the basin for the past several million years.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018