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Microfossils have been discovered in cavity-fill and replacement silica that occurs between chert-breccia clasts in 1200-million-year-old paleokarst at the top of the Mescal Limestone, central Arizona, and in approximately 800-million-year-old paleokarst at the top of the Beck Spring Dolomite, southeastern California. Microbial communities on Precambrian (>550 million years ago) land may have been extensive enough to affect weathering, erosion, sedimentation, and geochemical processes.
Horodyski et al. (Fri,) studied this question.