Geology and Paleoenvironment

The Rukwa Rift Basin preserves one of the only well-exposed, fossiliferous Cretaceous-Neogene continental sedimentary sequences in sub-equatorial Africa. Prior to our work, the significance of the region for paleontological, paleoenvironmental and paleoclimatic reconstructions of African ecosystems during these critical time periods was not recognized.

The Red Sandstone Group is positioned between underlying Karoo Supergroup and the overlying Lake Beds megasequence. Detailed geological investigations of the RSG conducted annually between 2002 and 2009 have produced a wealth of new data on local stratigraphy, lithofacies, alluvial architecture, sedimentary provenance, paleocurrents, clay mineralogy and geochronology. Importantly, this work has resolved a long-standing debate over the age of the deposits, confirming the existence of an extensive middle Cretaceous sequence (Galula Formation) and documenting a previously unrecognized late Paleogene continental sequence (Nsungwe Formation).

The Galula Formation represents a 600-3000 m thick sequence of amalgamated, braided fluvial channel deposits that were deposited across a large braidplain system via multiple parallel channels that had their source in the highlands of Malawi and Zambia. Lithofacies and clay mineralogy indicate that paleoclimate ameliorated during deposition of the RSG, transitioning from tropical semi-arid to tropical humid conditions.

The 400+ m-thick Nsungwe Formation is temporally constrained by radiometrically dated volcanic tuffs. A significant change in depositional environments occurs between the lower alluvial fan-dominated Utengule Member and the upper fluvial and lacustrine-dominated Songwe Member. The Songwe Member preserves a diverse fauna, with abundant ashfall and ashflow volcanic tuffs that were deposited in a semi-arid wetland landscape during the late Oligocene (+/- 24.96 Ma). The Nsungwe Formation provides a new window into the early tectonics and faunal transitions associated with initiation of the “modern” East African Rift System.

For more information on the RRBP geology, see Roberts et al., 2010.

Fossil bearing horizons in the Nsungwe Formation are interpreted as sheet flood deposits associated with a small, flashy discharge river system draining into a local wetland or lake system. Faunal evidence includes a diverse association of such aquatic and semi-aquatic taxa as fish (Gottfried et al., 2008), freshwater crustaceans (Feldmann et al., 2007), frogs (Simons et al., 2006), and molluscs. Facies associations along with the presence of aquatic fauna suggest a variable, semi-arid climate with perennial availability of water (Choh, 2007 and Roberts et al., 2009).