THE DEVELOPING GLOBAL CHRONOSTRATIGRAPHIC SCALE FOR THE CAMBRIAN SYSTEM AND ITS IMPACT ON NORTH AMERICAN STRATIGRAPHY

The definition of GSSPs bounding and within the Cambrian System have profoundly altered concepts of the system as applied in North America. C. D. Walcott’s (1890) definition of the Cambrian base at the Olenellus Zone has been superseded by ratification of the Cambrian base at a horizon coinciding with the appearance of the trace fossil Treptichnus pedum. This decision essentially doubled the length of the Cambrian Period and automatically redefined as Cambrian a long succession of pre-trilobitic stratigraphy previously considered Proterozoic. Ratification of the base of the Ordovician System at a horizon above the base of the Ibexian Series of North America added stratigraphic thickness to the Cambrian that was formerly considered to be Ordovician.

Historically, two sets of nomenclature have been used for regional series and stages of the Cambrian on the Laurentian paleocontinent. Stage and series definitions were based on polymerid trilobite or agnostoid biozones or, alternatively, the absence of trilobites. One set of definitions was developed in restricted inner-shelf facies, and the other set was developed in more open, outer-shelf facies. Common separation of these facies belts by carbonate platform deposits led to the evolution of separate trilobite biofacies. Few taxa in common between the inner and outer shelf regions rendered it difficult to precisely correlate across the continent using trilobite biostratigraphy alone. Imprecise or differing interpretations of boundary positions led to further ambiguity of stratigraphic positions within the North American Cambrian.

The International Subcommission on Cambrian Stratigraphy has recommended a global chronostratigraphic subdivision of the Cambrian that comprises four series and 10 stages, of which half have so far been defined by GSSPs. Internal subdivisions of the Cambrian System adopted globally bear little resemblance to the historical sets of regional nomenclature used in North America. The global chronostratigraphic subdivisions, however, have the advantage of being unambiguous and traceable outside the Laurentian paleocontinent. Using a combination of chronostratigraphic methods it is now possible to overcome the historic impediments to correlation across separate facies belts of Laurentia.