Base-Olenekian Boundary GSSP: Is There an Optimal Correlation Marker?

Ammonoid and conodont biostratigraphy and carbon isotopic excursions (CIE) provide excellent correlation potential for Lower Triassic successions around the world. So why don’t we have a ratified GSSP for the base-Olenekian Stage? Possible diagenetic control on the isotopic record, differing biostratigraphic methods, taxonomic issues, and personalities of ammonoid and conodont workers have all restricted progress. Another issue relates to the apparent imbalance in duration of the Induan (~1.1 Myrs) and Olenekian stages (~3.8 Myrs). Finally, the two stages and four informal substages are incongruous with three Lower Triassic depositional sequences. This interval has been sampled, studied and scrutinized more than most because it includes Earth’s greatest extinction event and subsequent biotic recovery. There is a consensus that the base-Olenekian boundary could be marked by the first occurrence (FO) of the conodont Novispathodus waagenisensu lato or near the base of the Rohillites rohila ammonoid zone at the Mud section, Spiti Valley, northwest India or the west Pingdingshan section, South China. Historical considerations suggest precedence for the FO of the ceratite ammonoid Flemingites as a marker, but F. barghavai first occurs low in the Ceratite Marls at the Nammal section in the Salt Range at a level that would make the Induan even shorter. Flemingites has not been identified in the Canadian Arctic where Tim Tozer defined the Griesbachian, Dienerian, Smithian and Spathian substages, but F. reticulatus occurs in one locality in northeastern British Columbia (NEBC) within the Sverdrupites Zone and associated with the Dienerian conodonts Neospathodus dieneri and Ns. cristagalli. This level can be shown to be well below a CIE P2 peak at Nammal and NEBC. In the Salt Range and at the Mud section, but not in most other locations, Nv. waageni sensu lato morphotypes also occur well below the P2 peak. The base-Olenekian may be better marked by the CIE P2 peak. Recent studies indicate that carbonate and organic CIEs are paired and that the CIE P2 marker can be identified in most sections from China to Canada, including in the subsurface of northeastern British Columbia. At the latter location the P2 peak occurs at a level dated by astronomical tuning (and geochronology elsewhere) as ~250.9 Ma and associated with the conformable maximum regressive surface of the lower Montney Formation, just below the first occurrences of Nv. waageni sensu lato and Ns. posterolongatus. The P2 peak at the Mud section occurs in association with the FO of Eurygnathodus hamadei, Ns. posterolongatus and Rohillites rohila. The P2 peak at west Pingdingshan also occurs very close to the FO of Ns. posterolongatus and Eu. hamadei. The guidelines of the International Commission on Stratigraphy indicate that a GSSP can be defined at a point marked by any event that exhibits optimal correlation potential. Perhaps this optimal marker for the base-Olenekian is the CIE P2 peak with ammonoids and conodonts providing secondary markers for correlation. Finally, a third stage (base-Spathian?) could be established at a level associated with a P3 CIE and corresponding with major changes in both ammonoids and conodonts.