Cherts are nodular masses and beds of microcrystalline quartz that replace carbonate sediment or opaline biologic oozes of diatoms or radiolarians during burial. The replacement can occur during early to deep burial, so chert is younger than the time of host sediment deposition by variable amounts not easily determined. The quartz itself contains no radionuclides that can be used for age-dating the time of crystallization. Detrital impurity grains may contain datable material, but the age is that of the source area and not of the sediment. The value of cherts in age-dating relates to the unsurpassed preservation that silicification affords fossils.
Silicification involves a slow micron-by-micron replacement process. Silicified fossils better survive the alteration effects of subsequent burial, uplift, and weathering. Acid dissolution of carbonate frees isolated examples, and these probably represent the majority of invertebrate fossils used to establish the paleontological record and...
KeywordsFluid Inclusion Geological Timescale Acid Dissolution Hydrothermal Quartz Paleontological Record
- De Wever, P., Dumitrica, P., Caulet, J. P., Nigrini, C., and Caridroit, M., 2001. Radiolarians in the Sedimentary Record. Amsterdam: Gordon and Breach Scientific Publishers, p. 533.Google Scholar