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