Treptichnus pedum in the Tres Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America

摘要

The index ichnofossil Treptichnus pedum Seilacher 1955 and four types of Microbially Induced Sedimentary Structures (MISS) variants of wrinkle and elephant skin structures - are the first fossils described from the Tres Marias Formation, topmost unit of the Bambui Group in south-central Brazil. Despite the stratigraphic importance and widespread occurrence of this unit in the Sao Francisco Craton and adjacent Brasilia Fold Belt, its age is still poorly constrained between the Neoproterozoic and Early Paleozoic. This discovery is significant because the first appearance datum (FAD) of T. pedum has been used to define the Ediacaran-Cambrian boundary. Hence, the presence of T. pedum indicates an early Paleozoic age for the uppermost Bambui Group and means that the Ediacaran-Cambrian limit must lie deeper in the Bambui Group, somewhere between this level and that of the terminal Ediacaran index-fossil Cloudina sp. in the Sete Lagoas Formation, near the base of the group. Available age determinations on detrital zircon grains and chemostratigraphic correlations are consistent with an Ediacaran to Cambrian age for this group. Detailed paleontological prospection throughout the group may now proceed within a better constrained time frame and hopefully reveal additional fossil evidence of Cambrian as well as Ediacaran life in the Bambui Group.