SILICIFICATION OF TRILOBITES IN THE CAMBRIAN WEEKS FORMATION (HOUSE RANGE, UTAH)

Silicification is a mode of preservation in which a fossil is replaced by SiO₂, often in the form of quartz. To contribute to a better understanding of this process, we have studied the patterns of silicification in trilobite exoskeletons (originally low-Mg calcite) from the Weeks Formation (Cambrian Series 3; House Range, Utah). We expect that documenting patterns of silicification will ultimately elucidate the mechanism(s) involved.

The Weeks Formation is dominantly composed of thin-bedded lime mudstones intercalated with shales. Though famous for its diverse assemblages of well-preserved trilobites, including specimens with fossilized guts, it has also yielded a remarkable fauna of soft-bodied invertebrates. Abundant silicified sclerites of trilobites occur in several intervals of the succession. These silicified assemblages differ from the ones comprised of articulated specimens in being dominated by immature individuals (juveniles, more rarely larvae or young adults). Thin-sections of limestone from the Weeks Formation reveal that the trilobite fragments are often incompletely silicified with relict calcite crystals encased in the interior. Silica replacement seems to have produced blocky spherulitic chalcedony crystals (obliterating the original structure of the exoskeleton). This matches observations made on silicified trilobite sclerites recovered after acid (HCl) dissolution of rock samples, which are often thin and fragmentary. Some of these trilobite fragments show internal voids caused by dissolution of the relict calcite on the interior—leaving the outer and inner layers of silica-replaced cuticle. The incomplete silicification of the calcitic exoskeleton of the trilobites from the Weeks Formation suggests a chemical and/or physical evolution of these deposits during diagenesis—leading to conditions no longer conducive to quartz replacement. The overrepresentation of immature individuals in assemblages recovered after acid dissolution is probably artifactual; the partial silicification of larger sclerites, produced fragments too fragile to ‘survive’ the washing of acid residues.