Recent progress by the International Subcommission on Cambrian Stratigraphy toward development of a globally applicable chronostratigraphy has resulted in a four-series, ten-stage model for the Cambrian time scale. So far, two series (Terreneuvian and Furongian) have been defined and named, and four stages (Fortunian, Drumian, Guzhangian, and Paibian) have been defined and named.
Work toward defining GSSPs of undefined stages and series is well underway. The first appearances of cosmopolitan agnostoid trilobites make excellent datum horizons on which to base finely resolved global correlations in the upper half of the Cambrian System, but precise correlations on a global scale in the lower half of the system are less secure. A combination of correlation methods can aid in making correlations intercontinentally, and carbon isotope chemostratigraphy (ã13C) has proved to be a particularly valuable tool where regional biostratigraphic guides are difficult to match from one paleocontinent to another.
Ten distinct ã13C excursions have been named, but others also can assist in correlation. To resolve key horizons in the lower half of the Cambrian System, it is necessary to use (at a minimum) trilobite, small shelly fossil, and perhaps archaeocyath ranges. These can be calibrated against the global ã13C curve. Radiometric age dates that help constrain internal boundary positions of the Cambrian are limited at present.