RELATIVE ACCEPTANCE OF INTERNATIONAL CHRONOSTRATIGRAPHIC DEFINITIONS IN NORTH AMERICA—A LATE PALEOZOIC PERSPECTIVE

The International Chronostratigraphic Chart and its boundary definitions based on GSSPs has had both broad conceptual impact on North American chronostratigraphy, and specific operational impacts of varying degree depending on the chronostratigraphic unit concerned. For the Late Paleozoic, the associated research has emphasized the paleogeographic location of what is currently North America on what was then the western extremity of Pangea. Numerous paleobiogeographic relationships from this setting have impacted the GSSP selection process and, from a counter perspective, the GSSP process has improved understanding of these relationships.

The degree of variance between North American regional and international chronostratigraphic definitions often reflects the ecological station of the taxa used to define regional concepts, which in turn reflects the degree of their endemism. Open marine, pelagic or nektonic organisms provide more reliable correlation potential any time when there is no circumequitorial ocean to provide rapid dispersal for shallower settings.

Examples of these impacts are demonstrated by contrasting mid-Upper Carboniferous (mid-Pennsylvanian) and Middle Permian GSSP deliberations in relation to the standard North American regional concepts. North American chronostratigraphic units in the Pennsylvanian are based on Andean Realm fusulinid zones and scattered midcontinent reference sections of variable quality. Although excellent for regional correlation in shallower water carbonates, fusulinids have always lacked precision in stage boundary intervals, and the zonal concepts have consistently trumped rock succession in the corresponding reference sections. Cyclic stratigraphy has proven more useful for international correlation. North American regional chronostratigraphic units in the Permian are based on ammonoid zones that were developed with consideration of international correlation, and are tied directly into the stratigraphy of a single basin in west Texas. Ironically, the switch to international standards based on GSSPs has been more readily accepted by Pennsylvanian workers—despite considerable change, than by some Permian workers—in which change has been much less dramatic, with only a shift from ammonoid to conodont zonal boundary definitions.