EXCEPTIONAL PRESERVATIONS IN NONEXCEPTIONAL CONDITIONS: CONASAUGA FORMATION, MIDDLE CAMBRIAN, WESTERN GEORGIA

Middle Cambrian shales of the Conasauga Fm. in northwestern Georgia contain a conventional shelly trilobite, brachiopod, and hyolithid fauna, admixed with rare soft-bodied and lightly-sclerotized fossils. The latter include red and green algae, demosponges, hexactinellids, calcisponges, and a non-skeletonized trilobite (Naraoia sp.). The exceptional biota also contains very unusual preservations, including a priapulid cololite and coprolite, and a sclerotized trilobite with preserved filamentous (gill) appendages.

Cambrian Konservat-Lagerstätten are not rare, but the Conasauga example is distinctive in several aspects. The exceptional preservations are partially in shallow shelf facies, whereas most other Cambrian Lagerstätten are in outer shelf and slope settings. Most significantly, the Conasauga strata do not appear to indicate catastrophic or unusual events associated with the soft-bodied fossils. This departs from general interpretations of the causes of exceptional preservations, and suggests that heuristic reexamination of conventional Cambrian fossil deposits may reveal additional non-sclerotized biota. Abundant siliceous concretions in the Conasauga, of probable biogenic sources, along with evident limited infaunal bioturbation, may show the physio-chemical cause of exceptional preservations.