Macro-, Meso-, and Microstructures of Microbialites from Lake Turkana, East African Rift: Paleoenvironment and Diagenesis
Lake Turkana Rift sediments preserve a variety of macro-, meso-, and microstructures in the Plio-Pleistocene microbialites outcropping near the modern lake margins. Published works, focused on the northern (Nachakui) and central (Koobi Fora) basins have classified Lake Turkana microbialites as stromatolites. However, some of these “stromatolites” actually contain laminae that are thrombolitic, dendritic, and stromatolitic all within the same individual. These mesostructures can then be subdivided by their microstructures into tussocks, clots, fine laminations, and branching fabrics; any of which may or may not contain microbial filaments. Macrostructures of these microbial forms can be described as bioherms, biostroms, and oncolites. Individual oncoids are the most common deposit and nucleate on basaltic pebbles, cobbles, or on shells. In some cases, these oncoids will coalesce up-section forming biostroms and, in a few areas, oncoid sizes will also increase up-section. In the South Basin of Lake Turkana, three discrete, mounded stromatolites were mapped.
Oncoids and larger microbialite mounds show little diagenetic alteration after initial calcite cementation, unlike the other carbonate facies that are abundant along the margin of the South Basin of the lake. Lack of diagenetic alteration and advanced dissolution in these microbial mound samples, which are presently subaerially exposed, suggests oncoid/microbialite lithologies have low initial porosity and permeability. From three porosity and permeability measurements, porosity ranges from 14-39%, with permeability ranging from 0.47-22.8 md. Where alteration has occurred, such as in the case of the precipitation of rhombic dolomite crystals, the cements are in vugs that were synsedimentary and formed as part of the primary ‘reefal’ or mound system.
Turkana Basin oncoids and stromatolites likely formed
where alluvial stream channels provided a cobble substrate for nucleation.
Smaller oncoids that nucleated on shell material formed near shore, where shell
material was available. The large mounded stromatolite outcrops are interpreted
as having formed along spring seeps adjacent to faults and/or fractures. All
three mounds are oriented N/NE-S/SW, consistent with the orientation of faults
in the area. Modern hydrothermal deposits are found on North and Central
Islands, and a fossilized hot spring was described by Renaut et al.
(2002)
along the western side of lake, supporting this interpretation.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California