EXCEPTIONAL FOSSIL PRESERVATION IN THE CONASAUGA FORMATION, CAMBRIAN, NORTHWESTERN GEORGIA, USA

Abstract

Mudstones and siliceous concretions in the middle Cambrian Conasauga Formation, northwestern Georgia, contain body and trace fossils showing nonmineralized preservation and represent two temporally and spatially different marine environments. Identifiable, nonbiomineralized taxa include components of a Burgess Shale–type biota with red and green algae, primitive sponges, and the arachnomorph arthropod Naraoia compacta . Also exceptionally preserved are the filamentous appendages of a large ptychopariid trilobite and assemblages of oriented hyolithid tests we interpret as priapulid coprolites and cololites. Exceptional preservation in the Conasauga Formation has multiple causes. The Conasauga contains superabundant siliceous concretions, many with skeletal, trace, and some nonbiomineralized fossils. Shale specimens, especially sponges with preserved details, and whole-body trilobite preservations, often have iron (Fe) oxide halos that resulted from a biochemical cascade including bioimmuration, decomposition gas anoxia, Fe-sulfide crystallization, and Fe oxidation. Preservation of soft tissue is also partly attributable to the well-sorted clay matrix of inner shelf Conasauga shales, which allowed mechanical imprinting of body fossils. Several nonbiomineralized fossils show algal overgrowths, suggesting an additional form of bioimmuration. Exceptional preservation in the Conasauga Formation is relatively poor compared with such better-known Cambrian Lagerstatten as the Burgess and Wheeler Shales; nevertheless, it is significant for three reasons. The siliceous concretions are a rare vehicle for exceptional preservation and feature three-dimensional fossils rather than the more common compressed specimens. The older Conasauga biota occupied a shallow-shelf environment, a setting in which exceptional preservation is poorly understood. The Conasauga Formation extends the geographic range of a Burgess Shale–type biota to the extreme southeastern USA.