PALEOECOLOGY AND SEDIMENTARY PETROLOGY OF THE CAMBRIAN CONASAUGA FORMATION (COOSA VALLEY, NW GEORGIA)

The chert concretions and shales of the Middle Cambrian Conasauga Formation, located in NW Georgia within the Coosa Valley, record a rich and occasionally exceptionally preserved fossil biota. Despite sporadic study during the past century, the identification of biofacies and the origin of the chert remain poorly known. The Fall 2007 paleontology course at the University of West Georgia investigated both of these topics during a semester-wide class project.

The formation appears to contain several biofacies, each associated with distinctive lithofacies. The dominant lithofacies, fossiliferous shales, includes an agnostid and ptychopariid trilobite-dominated facies. A monospecific hyolithid biofacies occurs in the most fissile shales. The rarest facies, a calcareous shale, bears thin, platy and irregularly bedded limestones bearing occasional in situ chert concretions; upon dissolution of the limestone, they contained a diverse assemblage of inarticulated brachiopods, hyolithids, and trilobites. Although in situ concretions did not bear fossils, weathered concretions included a distinctive fauna not otherwise found in the shales, dominated by brooksellids, possible rhodophyte algae, trilobites (especially Asaphiscus gregarious), hyolithids, and a variety of trace fossil burrows. Many of these species co-occur in the dissolved limestone.

Thin section analysis revealed that fossils were preserved both on the surface and throughout concretions; relict bedding was also observed. Scanning electron microscopy (SEM) and elemental analysis (EDS) of cherts and residue from limestone dissolution and shale maceration revealed no unambiguous sponge spicules preserved in any lithology. The faunal association between the chert concretions and limestones and the petrological analysis both point to the origin of the chert via post-depositional diagenetic alteration of limestone interbeds within the shale. The source of the silica remains unknown and could include reactivation of completely dissolved hexactinellid (brooksellid) sponge spicules or silica-rich meteoric groundwater.