THE DEVELOPING CAMBRIAN CHRONOSTRATIGRAPHIC SCALE: IDEAS ROOTED IN THE WORK OF STIG M. BERGSTRÖM

In recent years, there has been concerted international effort to refine the global geologic timescale/chronostratigraphic scale with boundaries precisely defined at GSSPs. The Cambrian has undergone thorough redefinition, spurred in part by a series of discoveries that have significance for global correlation, and by recognition of regional and intercontinental inconsistencies in historic chronostratigraphic terminology and definition. After much study in the International Subcommission on Cambrian Stratigraphy (ISCS), the traditional, ambiguous, three-fold Cambrian series subdivisions were replaced by four new subdivisions and 10 stage subdivisions, all delineated by widely recognizable criteria.

Much of the influence for how to reconstitute a Cambrian timescale is rooted in ideas developed earlier for the Ordovician by Stig M. Bergström and colleagues. Prior to formal definition with GSSPs, newly proposed stage and series were initially identified by numbers; these were to be replaced later with ratified names. The importance of fossils from deeper water (or cooler water) facies was recognized, as these increase the geographic reach of biostratigraphic guides. The ISCS has relied on agnostoid trilobites, many of them described or well characterized from Sweden, and on conodonts, for biostratigraphic guidance in the upper two Cambrian series.