The abundance of diatoms in Neogene lacustrine sediments and their apparent scarcity in Paleogene deposits have long perplexed geoscientists, but siliceous shales from Eocene lake beds of central British Columbia provide new insights. Major element geochemistry, X-ray diffraction patterns, and relict diatom frustules suggest that Eocene beds at Horsefly, McAbee, and Princeton originated as lacustrine diatomite that underwent diagenetic alteration to produce siliceous shale. The combination of high SiO2 and low Al2O3 values and the presence of opal-CT X-ray diffraction peaks provide a distinctive geochemical fingerprint for biogenic silica deposits that have been remineralized. The discovery of diatomaceous geochemical signatures in siliceous shales may prove to be a useful tool for extending the geologic record of diatoms, perhaps helping to reduce the apparent discrepancy between fossil evidence and evolutionary interpretations based extant species.

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