Abstract

Early Miocene (c. 18 Ma) hominoid sites (Proconsul) from Rusinga Island, Lake Victoria occur in palaeosols and volcaniclastic strata deposited in a semi-arid, seasonal climate on the flanks of the active, low-relief edifice of the Kisingiri volcano. Palaeosols interbedded with sequences of primary pyroclastic deposits have stable carbon isotope values indicative of C3 vegetation from semi-arid environments. Isotopic values of palaeosol organic matter and pedogenic carbonate from the Rusinga Group have average δ13C of – 23.8 ± 0.8 and – 7.7 ± l.l‰, respectively, considerably heavier than average C3 vegetation. These isotopic values were most likely caused by reduced photosynthetic fractionation under water-stressed conditions. Prolonged dry seasons and semi-arid precipitation levels for Rusinga Group fossil soils are also supported by the profile depth of calcareous horizons and vertic clay structures.

The principal fossil-bearing units of the Rusinga Group (Kiahera and Hiwegi formations) are dominated by sandy strata which are interbedded with palaeosols and have features indicative of hydromagmatic pyroclastic deposition. Pyroclastic surge features include very low-angle cross-bedding, low amplitude and long-wavelength dune bedforms, moderate to poorly sorted layers of tuffaceous and pebbly sandstones, and common, isolated cobbles and boulder clasts of Precambrian basement rocks with associated impact sags. These features are interpreted as the deposits of pyroclastic surge dune bedforms and ballistically implaced volcanic ejecta, both produced by powerful hydromagmatic explosions from the Kisingiri volcano. According to this interpretation, the initial stages of this carbonatite-nephelinite volcano had repeated episodes of hydromagmatic eruptions which built a large (15–20 km radius), low-relief tuff ring or maar volcano. The palaeosol sequences interbedded with the primary pyroclastic deposits represent periods of volcanic quiescence lasting hundreds to thousands of years. Thus, Proconsul and associated fauna and flora inhabited a frequently disturbed landscape that experienced repeated catastrophic volcanic deposition in an overall semi-arid seasonal climate.

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