Placement of the Stage 5, Series 3 GSSP (Approximately the "Lower-Middle" Cambrian Boundary):  An Update  

Placement of the “Lower-Middle” Cambrian boundary has been, and still is, controversial.  It is difficult to correlate the boundary, as presently recognized, in different regions of the world.  The “Lower-Middle” Cambrian working group of the Cambrian Sub-commission of the ICS is presently tackling the problem.  At present the traditional boundaries, recognized by the last appearance of Olenellus (olenellids) in Laurentia; first occurrence of Paradoxides (paradoxids) in Baltica and the Mediterranean; and the last occurrence of Redlichia in China, are not suitable.

After vigorous discussion about the attributes of the different potential biohorizons, each member of the Working Group was asked to vote for two likely FAD horizons.  Working Group members include representatives from Australia, Canada, China, England, Germany, Korea, Russia, Spain, Sweden, and the United States.  Twenty-three out of 25 members voted, their first choice was given a full point and their second choice a half point.  The members suggested that two biohorizons are the most likely candidates for the definition of Stage 5/Series 3 lower boundary, which is roughly equal to the “Lower-Middle” Cambrian boundary.  The favoured horizon is the FAD of Oryctocephalus indicus, both in total points (12.5) and first place votes (11). The second favoured horizon is the FAD of Oryctocara granulata in total points (9.5) and in second place votes (11).

The location of the potential GSSP is still being considered.  There are two possible GSSP locations using the FAD of O. indicus.  The most likely location is in the Kaili Formation at the Wuliu Section near Balang, Guizhou, China where it occurs just above the LAD of Redlichia, matching the traditional boundary in eastern Gondwana.  The second location is in the Emigrant Formation at the Split Mountain, Nevada, USA, where the boundary is located 15 m above the last occurrence of olenellid trilobites.  However, the Split Mountain section is condensed; correlation to other locations in Nevada indicates a stratigraphic thickness of 30­–50 m between the FAD of O. indicus and the LAD of olenellids.  At present, the Kuonamka Formation at the Molodo River section, Siberian Platform, Yakutia, Russia is the only likely location for a GSSP using the FAD of Oryctocara granulata.  This section is very well exposed, but is condensed and remote; the climate restricts access to two months per year.

There are many problems that hinder the progress in establishing a GSSP.  First and foremost is the correlation between palaeogeographical regions.  To perform this correlation, some other problems need to be addressed, and more data collected.  Problems include:  (1) the taxonomic identification (and agreement) and ranges of O. indicus and O. granulata; (2) the paucity of data on carbon and sulfur isotopes, REE, magnetostratigraphy, and sequence stratigraphy; and  (3) the taxonomic evaluation (and data collection) of brachiopods, hyolithids, acritarchs, molluscs, and small shelly fossils.