Acid processing of pre-Tertiary radiolarian cherts and its impact on faunal content and biozonal correlation

The numbers of radiolarians visible in thin sections of chert-rich rocks are commonly an order of magnitude greater than the numbers observed on the surfaces of fragments etched by hydrofluoric acid (HF) and typically orders of magnitude greater than the numbers of individuals found in HF-processed residues. Destruction of radiolarians during both diagenesis and HF processing severely reduces faunal abundance and diversity and affects the taxonomic and biostratigraphic utility of chert residues. The robust forms that survive the processing represent only a small fraction of the death assemblage, and delicate skeletal structures used for species differentiation, commonly preserved in limestone radiolarian faunas, are either poorly preserved or dissolved in many coeval chert residues. First and last occurrences of taxa in chert sequences are likely to be coarse approximations of their true stratigraphic ranges. Precise correlation is difficult between biozonations based solely on index species from cherts and those constructed from limestone faunas. Careful selection of samples in sequence, use of weaker HF solutions, and study of both chert and limestone faunas should yield better biostratigraphic information.