Taphonomic bias of hydrothermal silicification in biodiversity patterns of Cambrian shelly pavements from the Iberian Chains, NE Spain

Abstract

Silicified fossils are ideal for taxonomic studies because they can be extracted without damage using acids. However, permineralization commonly introduces taphonomic biases: as silica neomorphism occurs, silicified fossils are also susceptible to loss of microstructural fidelity and diagnostic characters useful for taxonomic determination. These processes are studied in the traditional ‘lower‒middle Cambrian’ transition of the Iberian Chains, where the macrofossil content includes variable abundances of trilobites and linguliformean brachiopods. In contrast, acid etching of partly silicified limestone interbeds offers a quite different biodiversity pattern, dominated by trilobite larvae, chancelloriids and sponge spicules, accompanied by locally abundant calcite- and phosphate-walled brachiopods, echinoderm holdfasts, and psammosphaerids and serpulids encrusting disarticulated sclerites. Petrographic, geochemical (chondrite-normalized REE, gull-wing patterns), cathodoluminescence (prominent yellow emission bands between 560 and 580 nm, and weaker blue bands between 440 and 500 nm) and Raman spectral data (main peak at 465.2 cm−1 and secondary ones at 205.5 and 128.1 cm−1) yielded by silicified microfossils reveal that quartz precipitation was induced by a distinct episode of acidic hydrothermal activity, close to 100 °C. The event is linked to a broadly penecontemporaneous tectonic breakdown event, where fissures served as conduits for silica fluids.