Trilobite Transport, Taphonomy and Turbidites: the Cambrian Conasauga Shale Lagerstätte Revisited  

In 1916, Charles Walcott suggested that the Conasauga Shale (Coosa River, northwestern Georgia, USA) was Middle to Upper Cambrian in age.  His initial assessment has not been refuted.  The Conasauga Shale is composed of inner shelf (Middle Cambrian) and mid-shelf (Upper Cambrian) strata, composed of abundantly mineralized trilobites along with other Burgess Shale-like organisms of Walcott renown.  Exceptional trilobite preservation in the Conasauga lagerstätte is thought to occur in relatively oxic, shallow-water facies – in direct contrast to the anoxic shelf-to-slope facies represented by the Wheeler and Burgess Shale lagerstätten.  We hypothesise that the Conasauga Shale also may contain outer shelf-to-slope deposits similar to the Wheeler Shale. 

Here, we interpret these deep-water deposits as turbiditic facies composed of thinly laminated siliciclastic shales interbedded with siltstones.  In these putative turbiditic beds, impressions of trilobite elements (e.g., free cheeks, genal spines and thoracic segments) are common, and complete trilobites are rare.  Molted elements are usually size-sorted and occur as mono-element assemblages, suggestive of current sorting in turbiditic flows.  When complete specimens occur, the dominant trilobite is Elrathia antiquata, a species that may live in exaerobic conditions like its southwestern relative, E. kingii.  In thin sections, iron-oxide pseudomorphs after pyrite are present, an indication of rapid trilobite burial in relatively anoxic conditions.  Thus, the Conasauga lagerstätte may also contain trilobite hardparts that were entombed via deep-water turbidites, not unlike other Cambrian lagerstätten.