Integrated Stratigraphy of the ~1.2 Ma Lake Malawi (East Africa) Scientific Drill Core 1B: Exploring New Evidence for Transitions in Lake-Basin Type

Abstract

The rift lakes of East Africa have transitioned from serving as modern analogs for prolific oil-producing basins in Brazil, West Africa, and China to occupying prominent positions as new exploration targets. Despite the excitement surrounding discoveries in this frontier (e.g., Lake Albert/Uganda), much remains unknown about petroleum potential in young lacustrine rifts characterized by elevated heat flow. Lake Malawi, a deep tropical lake comprised of several linked half-grabens, represents an end-member in the spectrum of continental rifts that explorers might encounter in the geologic record. In 2005, the Lake Malawi Drilling Project recovered a ~380 m drill core from the lake’s central basin that provides a well-preserved, ~1.2 Myr vertical section of the uppermost portion of the basin’s notional source kitchen. To clarify depositional history, we generated a detailed observation-based sequence stratigraphic framework for the core. The analysis relied on assessment of physical, biological, and chemical attributes to define lithofacies, as well as examination of facies stacking patterns to determine the arrangement of parasequences. Key surfaces (e.g., sequence boundaries, major flooding) are challenging to pinpoint in strata from distal (profundal) environments, but clues exist in grain size trends, bedding thicknesses, ostracode assemblages, and the diversity and abundance of algae. The results indicate the presence of decameter-scale parasequences that vary depending on stratigraphic level in the core. An important change in depositional character occurs at ~700 ka, when parasequences transition from indistinct to distinct, and ostracodes that reflected a fluvial influence on the depositional environment are replaced by ostracodes and authigenic minerals consistent with a much deeper lake. Although the boundary is not marked by an obvious disconformity, this lithostratigraphic and biofacies transition is attributable to a shift from an overfilled to a balanced-filled lake-basin type. Changes in sediment and water supply to the basin, available accommodation, or the hinterland landscape are often responsible for prompting changes in lake-basin type in continental rifts. At Lake Malawi, new provenance information for the drill core provided by detrital zircon age assemblages suggests that the cause for this transition may relate to tectonic influences on the configuration of the lake’s watershed and a concomitant rise in sill elevation.

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