TRACE FOSSIL DISCOVERIES IN THE CHILHOWEE GROUP: PROGRESS REPORT AND IMPLICATIONS FOR THE CAMBRIAN AGRONOMIC REVOLUTION AND EDIACARAN-CAMBRIAN BOUNDARY IN THE SOUTHERN APPALACHIANS

Recent discoveries of trace fossils in the Chilhowee Group, a thick succession of sparsely fossiliferous clastic rocks recording rift-to-drift tectonics associated with origin of the Iapetus Ocean, include ichnotaxa not previous recorded from these rocks and suggest an alternative chronostratigraphic interpretation. Previous studies employing field identifications of rare, poorly preserved trace fossils and questionable identification of two key specimens (one trace, one body), concluded that the middle-upper Unicoi Fm. was early Cambrian, unnamed Stage 3. These reports placed the Ediacaran-Cambrian boundary in the lower Unicoi Fm.

Based a remarkable unit consisting of a heavily bioturbated sandstone facies having abundant vertical burrows, we suggest that the Cambrian Agronomic Revolution is present in the middle Hampton Fm, unnamed Stage 2. Trace fossils from the lower Hampton Fm and upper Unicoi Fm consist exclusively of morphologically simple, shallow horizontal burrows, consistent with Fortunian Age. Those of the mid-Unicoi, along with a body fossil are consistent with upper Ediacaran. Unequivocal evidence of burrowing trilobites does not appear in the succession until the level of the Murray Mbr of the Erwin Fm (low to mid unnamed Stage 3). Based on the Bonnia-Olenellus zone fauna of the Helenmode Mbr, the top of the Erwin Fm straddles the unnamed Stage 3-4 boundary.

Specifically our discoveries include, in ascending stratigraphic order: 1) middle Unicoi (?Treptichnus isp. and a small tubular Ediacaran-type body fossil); 2) upper Unicoi (?Helminthoidichnites isp. and ?Psammichnites isp.); 3) lower Hampton (?Trichophycus isp., ?Helminthoidichnites isp. and ?Psammichnites isp.); 4) the middle Hampton bioturbation acme (CAR) (heavily burrowed interval; with Teichichnus cf. T. palmatus, Syringomorpha isp. and Diplocraterion paralleum, occurring in varied combinations at different locations); 5) Nebo Mbr of the Erwin (typical basal Cambrian “piperock”); and the Murray Mbr (Rusophycus isp. and Palaeophycus tubularis, among other forms). Although trace fossil occurrences are strongly controlled by facies, and sample size and preservation are significant factors, overall stratigraphic distribution provides insights into chronostratigraphy and paleoecology of this important interval.