Abstract
Complex, asymmetric ooliths occur within a thin (5 to 50 cm) carbonate deposit exposed along the northeastern margin of Lake Turkana, an alkaline lake located south of the Kenya-Ethiopia border. The limestone is part of a lower Pleistocene sequence of tuffaceous lake margin sediments (the Koobi Fora tuff) and outcrops over an area of 45 sq km. It is the only laterally extensive, oolitic unit within the 300-m section of Pliocene-Holocene lacustrine and fluvial sediments in the basin. Structurally, the limestone consists of multiple, well-cemented layers, the tops of which in some places exhibit desiccation cracks. Texturally, it ranges from oosparite to biosparite. The unit is composed predominantly of complex ooliths (2 to 80%), biogenic grains (0 to 10%), micritic, low-M calcite cement (0 to 15%), and blocky/sparry low-Mg calcite cements (15 to 90%). The ooliths average 0.8 mm in diameter and consist of nuclei that are unevenly coated by as many as 15 incomplete, overlapping, fan-shaped rims of radial-fibrous, low-Mg calcite. Both texture and mineralogy appear to be primary. Within a single oolith, every fan shows an orientation of fibrous crystals which is unique with respect to adjacent fans. Each fan-like lamina thus represents a different episode of calcite accretion. The ooliths formed on a coastal plain inundated by proto-Lake Turkana during a relatively high stand of lake level. Calcite precipitated only on the up-sides of stationary nuclei, however grains remained at the sediment surface long enough to be moved repeatedly by high energy waves and acquire composite cortices. The occurrence of asymmetric ooids indicates that agitation may play a minor role in oolitic deposition. In quiet water settings, nuclei availability, presence and concentration of organic matter and/or microorganism activity may be the major factors controlling ooid growth and morphology. End_of_Article - Last_Page 989------------
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