DIAGENESIS OF PLIO-PLEISTOCENE LACUSTRINE CARBONATES FROM LAKE TURKANA, KENYA
Four stages of diagenesis (Stage I-IV) are recognized petrographically: I-Isopachous bladed calcite cement precipitated on the lake floor during carbonate accumulation, II- Partial dissolution and neomorphic reprecipitation of sublacustrine calcite in the littoral zone, III- Shallow burial and replacement by sublacustrine dolomite, and IV- Renewed calcite precipitation and dedolomitization in the presence of meteoric fluids. Most of the carbonate lithologies contain evidence for 3 or 4 of the diagenetic stages; however, microbial deposits/oncoids underwent little diagenetic alteration after initial calcite cementation. Allochems include peloids, ostracods, microbes, gastropods, bivalves, ooids, coated grains, diatoms, and detrital grains such as quartz, volcanic glass, and volcanoclastic fragments. All allochems are micritized, and the fossils generally do not retain their original microstructure. Dissolution of fossils and peloids created rare to common moldic porosity and common to abundant microporosity but poor permeability in most samples.
All the calcite cements are considered to be low magnesium, even though calculated MgO concentrations in the south basin are as high as 5.9 weight percent, indicating that another sink for magnesium is required. Dolomitization is a shallow subsurface phenomenon in Lake Turkana (215 cm below the sediment-water interface), where microbial metabolic processes create the proper water chemistry for dolomite production. In general, these rock samples were not subaerially exposed during diagenetic Stages I-IV. Pore fluid chemistry was probably partially controlled by lake water chemistry, subaqueous or near shore spring seeps, and the metabolic processes of microbes.