ORIGINS OF MAGADI-TYPE CHERT: NEW CLUES FROM THE HSPDP LAKE MAGADI DRILL CORES

Abiogenic cherts, so-called “Magadi-type cherts”, formed from magadiite or sodium silicate gels in alkaline environments, have been studied from outcrop samples in the Lake Magadi basin for decades. More recent studies indicate a possible biogenic formation of chert precursors as well. With the 2014 drilling of sediment cores to bedrock in the Magadi Basin by the Hominin Sites and Paleolakes Drilling Project (HSPDP), hundreds of samples of chert, up to ~1 million years in age, have been collected and analyzed by scanning electron microscopy and elemental analysis, petrographic thin section observation, and X-ray diffraction. We are documenting a variety of chert origins from the HSPDP Lake Magadi cores, including: (1) relict crystal pseudomorph structures showing replacement of sodium silicates (magadiite) and other chemical precipitates, (2) replacement of evaporite crystals such as trona and nahcolite, (3) newly discovered “labyrinth patterns” indicating lithification of synsedimentary siliceous gels, (4) replacement of magnesium calcite in peloidal mudstones along well-defined fronts, (5) patchy replacement and cementation of diatomaceous deposits (6) replacement and cementation of carbonate fossils (gastropods and ostracodes), (7) silicification of plant fragments with preservation of plant structures, (8) silicification of quartzose-zeolitic mudstones, and (9) chalcedony filled voids and fractures. Virtually all chert-forming processes occurred early in the diagenetic history, except (8) silicified mudstone, where compaction has rotated and aligned plant fragments, and (9) where chalcedony formed after lithification. Understanding the timing and origins of the Magadi cherts is crucial to their use for radiometric dating and the improvement of outcrop to core correlations. Chert also provides important clues about paleoenvironmental conditions in the Lake Magadi basin.