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.

Debate over the age of the deposits

Previous workers have suggested various ages for the Red Sandstone Group, ranging from Jurassic to Miocene-Pliocene (e.g., Spence, 1954; Pentelkov, 1979; Wescott et al., 1991; Kilembe and Rosendahl, 1992; Damblon et al., 1998). Our geologic investigations of the tectonic and stratigraphic history of the Rukwa Rift Basin indicate that the Red Sandstone Group can be subdivided into two distinct stratigraphic units (Roberts et al., 2004), formally named the Galula and Nsungwe formations (Roberts et al., 2009). A Cretaceous fauna has been recovered from the Galula Formation (O’Connor et al., 2006). The Nsungwe Formation is late Oligocene in age, based on faunal evidence (Stevens et al., 2005, 2006, 2008, 2009a&b) in addition to radiometric dating of intercalated ash beds and detrital zircon geochronology (Roberts et al., 2007, 2009).