A TUFA “EVENT” IN AN ARID RIFT VALLEY (KENYA); CLIMATE OR TECTONICS?

Large fossil tufa deposits are found in the equatorial East African Rift Valley and most are associated with faults and rift-related fractures. The deposits appear to be localized in both time and space. The region is semi-arid (P is 500-800 mm/yr and ET is ~2500 mm/yr) with bi-annual monsoon rains and inter-annual increases in precipitation during ENSO years. On the long term, wet-dry cycles in the low latitudes are thought to be related to Milankovitch precession cycles (19-23 ka). It is not clear whether these large tufa deposits are a record of climate change (e.g., increased spring discharge linked to a wetter climate) or if they record a tectonic event (e.g. a phase of faulting that induced a change in CO2 flux).

Recent analysis of Middle Pleistocene rift basin sediments in the Baringo-Bogoria basin revealed a 2.2 m-thick (1 km2) tabular-shaped paludal tufa deposit overlying fluvial gravels and silty lake-margin flats and overlain by smectitic lacustrine clays. The carbonate occurs in the lower portion (~40 m) of the Kapthurin Formation (Member K3) which is well dated by volcanics, spans ~40 kyr and contains at least two lake cycles. The K3 tufa appears to be centered on a minor rift fault and the deposit was later offset by post-depositional normal faulting. This fault was the likely the conduit for calcium carbonate-rich spring water sourcing the tufa. The deposit is tabular and composed of 4 distinct subhorizontal beds (from 10-80 cm thick) and an interbedded paleosol. It is generally of porous texture with clay-filled root traces or burrows and is fossiliferous containing freshwater bivalves, gastopods, ostracod and algal (chara?) molds, and peloids. The tufa mineralogy is calcite.

The bed geometry and the lithofacies context suggest the tufa was laid down on dry fluvial and lake margin flats and later flooded as the lake level rose. Its estimated age is ~525 ka which places it at time of climate change, i.e., global warming between a glacial stade (isotope stage 14) and interstadial (isotope stage 13). We cannot discount the possibility of a tectonic origin for the tufa, but the depositional sequence (including the paleosol) suggests that the spring-fed carbonate was not “instantaneous”, but formed slowly over a substantial period of time (thousands of years) during a time of climate change from dry to wet.