Petrographic Criteria for the Recognition of \u27Magadi-Type\u27 Cherts

The Magadi cherts, inorganic lacustrine deposits from the Lake Magadi area, Kenya, are widely used as a modern analog to explain the origin of ancient inorganic cherts. Formed in a highly alkaline lake, as the result of a transformation from the sodium silicates minerals, magadiite and/or kenyaite, to quartz, the Magadi cherts possess a distinctive set of textural characteristics that allow them to be distinguished from cherts of different origin with only a limited number of samples. Textural characteristics that are diagnostic of the Magadi cherts and that can be used as a test for the occurrence of ancient \u27Magadi-type\u27 cherts are: 1) groundmass textures, which lie on a continuum from equigranular mosaics to grid-works, 2) variable concentrations of groundmass inclusions which occur as washes, clots, and fragments, 3) morphology, location, and orientation of cracks, fenestrae, and their filling phases, and 4) crystal molds. The transformation from magadiite to chert is accompanied by a 25% volume loss which is accommodated by the formation of shrinkage cracks, both internal and external, most similar in morphology and infill to subaqueously formed shrinkage cracks in clays and concretions, respectively. Textural and paragenetic criteria appear to be a valid test for the presence of \u27Magaditype\u27 cherts. Because important paleoenvironmental interpretations are made based on the presence of \u27Magadi-type\u27 cherts, it is important to have diagnostic tests to aid in its recognition. Further field work, and comparisons with lacustrine cherts of intermediate age are clearly needed at this point