STRATIGRAPHIC AND RADIOMETRIC CONSTRAINTS ON RIFT-RELATED VOLCANISM, TERMINAL NEOPROTEROZOIC GLACIATION, AND ANIMAL EVOLUTION

The absence of a well-resolved biostratigraphic framework for terminal Neoproterozoic strata has resulted in the general view that animal evolution post-dated Neoproterozoic glaciation by up to tens of millions of years, although radiometric constraints on the last of the ice ages were lacking. New field and carbon isotopic observations in northern Virginia reveal that deposition of post-glacial cap carbonate of the Fauquier Formation and widespread volcanic flows of the Catoctin Formation were synchronous. Geochronologic and paleomagnetic data indicate that the rift-related volcanics in eastern Laurentia were emplaced between 580 and 555 million years ago, thus providing the youngest known age limit for high latitude Neoproterozoic glaciation. This interval falls within uncertainty of radiometric constraints for both glaciogenic and fossiliferous strata in Avalonia, supporting the contrasting view that Ediacaran animal evolution was coincident with widespread glaciation shortly before the Precambrian-Cambrian boundary.