Sedimentation and fossil preservation in the Nyanza Rift System, Kenya

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Abstract
Summary: Two major factors led to the accumulation of fossiliferous strata in the Nyanza Rift System—volcanism and tectonism. Several volcanoes have been active in, or near, the Nyanza Rift Valley during the past 20 Ma. Arranged in rings round these volcanoes were 3-D annular lithofacies dependent for their origin on various volcanic, geomorphic and pedogenic factors. Near-vent facies are characterized by predominantly coarse clastic deposits which accumulated on steep slopes (>10°) in upland areas. They usually show few signs of pedogenesis. Further from the vent (4–15 km radially) the apron facies are represented by finer-grained volcaniclastic strata, often with significant amounts of pedogenic overprint but little evidence of deposition from, or in, water. These apron facies accumulated on gentler (3–10°) slopes, both by subaerial fallout and by radial reworking of the near-vent facies. Even further from the volcanic centre are the arena facies which are generally fine-grained and waterlain in virtually flat-lying countryside (0–3° slopes). Primary volcanic ashes are occasionally present in the arena facies. The bulk of these volcanogenic sediments accumulated on dry land. Their deposition was not especially related to existing sedimentary basins. The second major factor which was responsible for the genesis of at least part of the Nyanza sediments was the formation of basins by tectonic activity associated with rifting. Depressions were formed in which fluvial and lacustrine sediments accumulated. The bulk of these sediments represent the products of erosion of the scarp shoulders of the rift, the sloping half-graben blocks, and the upstanding volcanic forms within the rift and on its shoulders. The distribution of fossil vertebrate species in western Kenya is closely related to depositional facies. Terrestrial assemblages are particularly distinct from aquatic faunas and generally occur in strata which accumulated subaerially as part of the apron and arena facies. Aquatic faunas are found in the fluvial and lacustrine sediments. In addition, the geochemical composition of the source rocks is an important factor in fossil preservation. Sediments derived from rocks rich in lime and low in silica contain abundant vertebrate and invertebrate fossils, while those that are silica-rich and lime-poor yield remarkably few faunal remains.