COMPLEX SEM FABRICS IN SILICIFIED TRILOBITES FROM THE CAMBRIAN WEEKS FORMATION: CLUES TO THE PROCESS OF SILICIFICATION

The quality of preservation of silicified fossils is known to greatly vary among fossil sites, but the reasons for that remain unclear. We report on low fidelity silicification of trilobite sclerites from the Cambrian Weeks Formation, Utah, that helps illuminate the often-ignored process of silicification. Although relatively rare on crack-out surfaces, the genus Hardyoides is common in the silicified material, possibly owing to its smaller size. Although its sclerites (1-3 mm) are usable for taxonomic classification, their fine details are missing and specimens are often incomplete. Closer inspection reveals that the silica often continues beyond the sclerite for 10s of microns into the surrounding carbonate-shale matrix, acting as a cement. The limit of the sclerite proper is thus obscured, with a variety of clay-to-silt sized detrital grains embedded in the silica. Larger silt-sized embedded grains include mica and euhedral to skeletal Na-silicate (albite?).

Coating sclerites, and sometimes the matrix, we found two different forms of biofilm. Crystalline-looking biofilm mats, threads, and filaments are composed of Si-O-Al, minor C, and therefore interpreted as silicified biofilm associated with the original silicification. Rare cocci-like spheres are associated with these silicified biofilms. Ropy to smooth biofilm mats composed of C-O, with a trace of N are more likely modern, and certainly post-silicification. Additional features include silicified (or dissolved) pyrite framboids (~10 µm), and rhombohedral to tabular molds (dolomite and/or plucked Na-silicate). All of these features are found by close examination in the SEM at >1000X, not the more common <20X.

The process of silicification is poorly understood. We hypothesize an organic-rich micro-environment to set the conditions for silicification. Usually, the source of organic material is the fossil and silicification stops at its boundary. The presence of silicified biofilms suggests an additional organic source in these samples, that allowed continued silica precipitation into the surrounding matrix, thus decreasing the quality of replication of the sclerites. We invite comment and discussion on ways to test this hypothesis, and wonder how common such features really are in silicified fossils.