FOSSIL HOT-SPRING TRAVERTINE IN THE TURKANA BASIN, NORTHERN KENYA: STRUCTURE, FACIES, AND GENESIS

The Ngakoringora Ridge is a large 300 m long linear mound oflimestone dolomitized limestone and chertthat rises abruptly from the desert floor on the southwestern edge of the Lothidok Hills west of Lake Turkana Kenya The origin of the ridge has been controversial It has previously been considered either a hot spring deposit or an uplifted inlier of pre Cenozoic marine carbonates Interpretation has been hampered by the extensive diagenetic alteration of the rocksand the lack of identifiable fossils A preliminary examination of the ridge morphology and the facies and fabrics of the rocks confirms a hydrothermal origin The carbonates contain radial calcite fans micritemicrosparite laminae stromatolites coated grains peloids intraclasts Mn shrubs and other fabrics that characterize hot spring travertines Many of the carbonates have been partly silicified and dolomitized The ridge is interpreted to be a fissure ridge travertine precipitated from thermal watersthat discharged along a medial fissure Fluids flowed laterally from five mainmounds that were probably active at different times Crystal fabricsalong the ridge crestare compatible with abiotic precipitation from alkaline spring waters undergoing rapid degassing of CO2 Microbially influenced fabrics including stromatolites become more common distally Silicified plants filamentous microbial mats and thin chert bedsare locally present indistal slope settings Plant silicification and chert formation may have taken place inshallow terrace pools from spring fluids undergoing cooling and evaporation The Ngakoringora Ridge formed after the faulting and tilting that formed the Lothidok Hills but its age is difficult to constrain Silicification and dolomitization of the carbonates resulted from contact with hydrothermal fluids and possibly from circulating ground water orlake water after deposition Hydrothermal activity in rifts migrates with the evolving structural configuration In the Kenya Rift this is evident as a migration of hydrothermal activity toward the rift axis Fossil spring deposits can provide much useful paleoenvironmental information eventhough they are of small lateral extent