A Paleoenvironmental Assessment of the Central Lake Albert Rift Basin — Uganda, East Africa from Exploration Well Geochemical Data

East Africa has experienced a dynamic history of geological, climatological and ecosystem changes since the middle Miocene, which overlap with emergence of the human lineage. Presently, records of these changes are derived from sparse outcrops of lake sediments and volcanic rocks or from deep-sea cores situated at great distances from the continental interior. Detailed and continuous records of past environmental variability are limited, but the tectonic lakes of the East African rift system hold the promise of revealing such long-term changes. In this study, elemental geochemical data from drill cutting samples of a 3392 m deep well drilled on the eastern shores of Lake Albert-Uganda, East Africa are used to investigate a long-term paleoenvironmental history and stratigraphic evolution of the central area of the Lake Albert rift basin-Uganda. The Ngassa-2 well was drilled through loose and coarse sands in the upper 300 m, massive mudstone deposits interbedded with siltstones for middle sedimentary section (300 - 3250 m) and then conglomerate at the base (3250 -3392 m). Results from XRF data and Total Organic Carbon (TOC) are indicative of warm and wet conditions around the late Miocene, later developing into cooler and dryer climatic conditions around the late Pliocene. These dry conditions are possibly related to aridification of East Africa around 3 Ma. Anoxic lacustrine conditions in the early Pliocene are documented by a dramatic rise in TOC and coinciding trends with iron (Fe) for the depth interval 3000 - 3250 m of the Ngassa-2 well. Additionally, integration of geochemical data, lithological observations, seismic data attributes and down-hole gamma ray logs provide evidence of a basin that transitioned from fluvial to generally mixed fluvial-lacustrine and subsequently dominantly lacustrine environment before shifting back to fluvial and shallow lacustrine system in the latest Pleistocene and Holocene. Changes in the variability of XRF and TOC data over the full section may relate to orbital forcing of African climate since the middle Miocene, but confirmation of external forcing on this time scale awaits higher resolution sampling of the Lake Albert sedimentary section.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California