Volcanically influenced calcareous palaeosols from the Miocene Kiahera Formation, Rusinga Island, Kenya

Abstract

The Miocene Kiahera Formation at its type locality on Rusinga Island in the Kenyan part of Lake Victoria consists of a sequence of alluvial sedimentary and pyroclastic strata and calcareous palaeosols. The 80 m thick formation was formed by braidplain aggradation and pyroclastic airfall in response to volcanism from the nearby Kisingiri volcano. Numerous fining-upward sequences approximately 5 m to 15 m thick make up the bulk of the Kiahera Formation. A complete sequence contains sandy and conglomeratic beds at the base which fine upward to sandy siltstones, calcareous palaeosols and airfall tuffs. The fining-upward sequences may have been produced by aggradation during phases of volcanic eruption. During major eruptive episodes, scoriaceous nephelinite detritus flooded the alluvial system to create the lower, coarse-grained beds. As eruptive activity waned, airfall ash and overbank flood deposits accumulated on the landscape forming the upper, fine-grained beds of the sequence. Calcareous soils formed on these highly alkaline ashes between large ash fall events. The calcareous palaeosols are grouped broadly into two main types: tuffaceous clayey Inceptisols with distinct Ck horizons (Chido palaeosols) and sandy Inceptisols consisting of nephelinitic rock fragments and subordinate amounts of tuff (Okoto palaeosols). Both types show only weak to moderate soil development interpreted to be a result of episodic rejuvenation by fresh airfall ash and flood deposits. Comparison of depth of the calcareous subsurface horizon from these palaeosols with those in calcareous modern soils from the Serengeti Plain that also developed on alkaline volcanic ash allow for rainfall estimates of between 550 mm a −1 to 750 mm a −1 for the Kiahera palaeosols. This together with root traces, soil structure, and major element chemistry of the two palaeosol types are evidence of a landscape with sandy, stream-side woodland soils (Okoto palaeosols) and clayey, floodplain grassy woodland soils (Chido palaeosols).