Explosive Volcanism in the Kenyan Rift: A Tephrostratigraphic Perspective

Traditionally, tephrostratigraphic work in eastern Africa has provided chronological control on archaeological formations, helping to piece together temporal and spatial trends in human evolution. The result is that tephrostratigraphic frameworks in the rift valley are patchy, focus mainly on the earlymid Pleistocene and the sources of volcanic deposits are rarely defined, leaving recent volcanic histories incomplete and poorly dated or correlated. Lake sediment records offer an opportunity to establish more comprehensive records of volcanism. In this study, two ~Holocene sediment core sequences, Lake Chala (Tanzania) and Lake Naivasha (Kenya), are investigated for the presence of cryptotephra deposits to provide better constraints on eruption histories. The complete Holocene tephrostratigraphy for Lake Chala includes 26 cryptotephra deposits and two visible tephra layers deposited between 13,350 and 63 cal yrs BP. Cryptotephra analysis has almost doubled the number of tephras known in the full (~260 kyr) Lake Chala sediment record. In fact, Lake Chala provides the most detailed Holocene eruption record for the Kenyan Rift thus far, documenting local off-rift volcanism and a single distal event from the Central Kenyan Rift. Similarly, Lake Naivasha reveals three previously unknown explosive events (7350-4817 cal yrs BP) from the nearby volcanic system of Longonot. The combined major, minor and trace element datasets presented here are one the first of its kind and will prove valuable to future work in the region aiming to establish correlations. This study assesses the suitability of Lake Chala and Lake Naivasha for low- and high-resolution cryptotephra sampling and presents preliminary work into the effectiveness of XRF and machine learning techniques for the more rapid identification of microscopic deposits. The findings presented in this study demonstrate that low-energy lake environments with small catchments and a neutral pH, can reveal significant amounts of volcanic detail. However, these example lakes are not found frequently throughout the rift, thus all lake records have a part to play in developing a more complete tephrostratigraphic framework for the Kenyan Rift.