PALEOBIOLOGY AND GEOCHEMISTRY OF SILICEOUS CONCRETIONS WITH TRILOBITES AND BROOKSELLA IN THE CONASAUGA FORMATION, MIDDLE CAMBRIAN, NORTHWESTERN GEORGIA

Shaly facies in Middle Cambrian beds of the Conasauga Formation in Floyd County, Georgia, contain abundant, cobble-size, siliceous concretions, often showing well-preserved, high-relief fossils on their external surfaces. Common fossils on the concretions include complete trilobites and isolated trilobites sclerites, hyolithids, and chlorophyte algae. Less common fossils include sponges, rhodophyte algae, Scenella, non-trilobite arthropod fragments, and a unique priapulid worm coprolite. An additional, different group of very common Conasauga fossils are the famous “Brooksella” or “star cobbles,” variously considered to represent cnidarians, sponges, and feeding traces. In some cases (e.g. trilobites, hyoliths), the fossils appear to be adhering to the cobble, whereas “Brooksella” and some sponges appear to comprise the entire concretiion.

Thin-sections show concretions composed of very finely crystalline, slightly silty, argillaceous chert. Concretions are either massive or thinly bedded showing slight size grading. Bioclasts are common internally, with debris including identifiable trilobite sclerites and thin, scattered, fibrous material, possibly hexactinellid or demosponge monaxons. Graded beds in concretions have sharp bottom surfaces overlain by fine quartz silt with minor fine bioclastic debris. One concretion shows distinct concentric structure with drusy calcite infilling, surrounded by darker, poorly-defined rings. The very fine-grained sediment in all specimens indicates deposition in a very low-energy environment, while the presence of slight grading in some suggests occasional turbid flows. The abundant, exclusively benthic fauna, indicates oxygenated conditions, at least at the sediment-water interface. Lack of deformation of bedding at the nodule’s ends indicates that silicification occurred after compaction. The source of silica, as previously reported, may be from siliceous sponge spicules.