Variability, characteristics and origins of hydrothermal-related silica deposits in the Kenya Rift Valley

This research will examine Quaternary (the last 2.6 million years) deposits composed of nearly pure silica (cherts, opaline silica) precipitated from silica-rich waters derived at least in part from springs in the Kenya Rift Valley. Samples will be collected mainly from the central and southern parts of the rift where siliceous deposits are common, with supplementary samples available from previous studies in the Suguta Valley in northern Kenya and from core materials in the southern Kenya Rift. These areas include a variety of silica-rich deposits formed in sublacustrine to playa (saline shallow ephemeral lakes) and wetland settings, as well as in hot springs and adjacent environments. Opaline silica is also widely present in several Pleistocene and Holocene formations, where it appears to be related to permineralisation of spring-fed terrestrial ecosystems. Silica is also present as gels formed in the shore zone of one spring-fed saline lake.

The research adopts four broad approaches: 1) an attempt to recognize and describe the major types of silica-rich deposits (excluding biological sediments such as diatomites) that are present in the Kenya Rift, including their physical characteristics in outcrops and hand specimens as well as their microscopic features; 2) a study of their major and trace element geochemistry, including spatial variations between spring, stream, wetland or lake settings; 3) and 4) developing models that should help to explain the formation of the silica-rich deposits that will, in turn, assist with the interpretation of similar sediments elsewhere.

The project will achieve these broad aims through systematic field and laboratory studies based on known locations where silica has been deposited and through an exploration of new locations where structural factors (faults, basement trends) favour silica deposition. The deposits available include layered and massive cherts (nearly puresilica rocks), laminated stromatolitic (finely layered with possible biological influences) cherts, probably associated with shallow saline lakes. Other examples include structureless spring cherts formed below former lakes. Cherts have also formed in spring vents and in nearby spring-fed marshes and soils, with silica also accumulating in plant stems following evapotranspiration.

The study will include basic descriptions of outcrop setting in relation to other deposits, field geometries of the siliceous sediments as well as their hand specimen characteristics, microscopic features and geochemistry. The resulting data will be used to develop models of deposition and the controlling factors that influence silica formation. Uranium series dating will also assist in determining the timing of deposition