Origin of reddened and melted zones in Pleistocene sediments of the Olorgesailie Basin, southern Kenya Rift

Hardened red zones and rare melt rocks are puzzling and rare features in the Middle Pleistocene sediments of the Olorgesailie Formation of the southern Kenya Rift. We propose that combustion metamorphism and melting produced these zones. This is evidenced by field observations, chemical composition, X-ray diffraction and melting experiments. Plant-rich beds and disbursed plant matter served as fuel. Probably lightning generated surface fires that ignited carbonaceous beds exposed in gullies. The burning then proceeded underground. Melt rocks and red thermally metamorphosed zones within beds distinguish combustion metamorphism from reddening of paleosols by surface vegetation fires. Diagenetic oxidation has removed much of the carbonaceous material from the paleosols and marsh deposits, except where they are included in dense metasediments. Extraordinarily high temperatures of about 1200(o)C, which are essential to producing the melt rocks, required plant matter buildup, burial of the plant fuel under at most a few metres of sediment, and then rapid burning under dry conditions. This sequence of events points to one or more extreme droughts between 601,000 and 493,000 years ago, the period represented by upper Member 12 and Member 13 of the Olorgesailie Formation. Combustion metamorphism was not recognized in early studies at Olorgesailie, partly because it is not a widely taught nor published subject. Other Pleistocene East African basins and similar sites may contain as yet unrecognized products of combustion metamorphism that may prove of paleoclimatological importance.