Fossil Hot-Spring Travertine in the Turkana Basin, Northern Kenya: Structure, Facies, and Genesis
Robin W. Renaut, Chris K. Morley, Brian Jones, 2002. "Fossil Hot-Spring Travertine in the Turkana Basin, Northern Kenya: Structure, Facies, and Genesis", Sedimentation in Continental Rifts, Robin W. Renaut, Gail M. Ashley
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The Ngakoringora Ridge is a large (300 m long), linear mound of limestone, dolomitized limestone, and chert that 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 holier of pre-Cenozoic marine carbonates. Interpretation has been hampered by the extensive diagenetic alteration of the rocks and 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, micrite-microsparite 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 waters that discharged along a medial fissure. Fluids flowed laterally from five main mounds that were probably active at different times. Crystal fabrics along the ridge crest are 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 beds are locally present in distal slope settings. Plant silicification and chert formation may have taken place in shallow 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 or lake 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 even though they are of small lateral extent.
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Sedimentation in Continental Rifts
Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.