Tectonics, Taphonomy, and Trilobites: The Cambrian Conasauga Shale Lagerstatte

The Cambrian Conasauga Shale is contemporaneous with the Burgess and Wheeler Shale Lagerstatten of North America and harbors similar trilobite taxa. Unlike those deposits, we found that the Conasauga siliciclastic shales were affected by low-grade metamorphism that altered the preservational quality of the trilobites: none of the preservational grades appeared to be syndepositional in nature as previously reported. Prior reports suggest that the taphonomic condition of the trilobites is excellent resulting from rapid burial in anoxic, shallow marine subtidal environments. While this may be true for parts of the Conasauga Shale complex, the areas we worked in near Rome, Georgia (USA) had low-diversity trilobite assemblages with post-depositional alteration stemming from a younger tectonic history. Taphonomic, paleoecologic and sedimentary analyses indicate that the Shale in our region was deposited in an outer shelf or slope setting characterized by oxic-to-slightly dysoxic marine waters and then compressed by overburden, and finally deformed under low-grade metamorphic conditions resulting from Appalachian orogenic cycles.

Using bulk samples taken along several transects, we found that calcite is rarely encountered in these specimens, and when it did occur, it represented pressure-solution halos or burrow-fillings. Often, trilobite molds were slightly deformed. Petrographic thin section and microprobe analyses indicate that specimens were compressed, laminations were fractured, and fabrics and minerals were consistent with low-grade metamorphism. The iron-oxide rims surrounding trilobites may result from thousands of years of ground water movement, replacing calcite with iron-rich silica.

Rather than syndepositional preservation, we argue that post-depositional taphonomic conditions may rule in the Conasauga. A caveat: The Conasauga is largely unexplored; it represents different water depths and facies, some of which may be untouched by metamorphism, which may explain the suggested Lagerstatte moniker for this Shale.