A COMMON SENSE CHECK ON STAGE BOUNDARY PLACEMENT

Many of the classic stage boundaries on the geologic time scale were placed at distinct biostratigraphic breaks associated with unconformities. Since these time-significant surfaces, which separate older strata below from younger strata above, commonly occur at distinct lithologic breaks, they were readily mappable in outcrop, as well as in the subsurface on geophysical logs, by a large number of geoscientists working in government, academia, and industry. Recently, however, the International Commission on Stratigraphy has undertaken the Global Boundary Stratotype Section and Point (GSSP) initiative, which is an effort to redefine the lower boundaries of each stage on the geologic time scale. Since one of the prime directives of this initiative is to find conformable facies successions, and thus not use unconformities, this methodology is in direct contrast to the classic placement of these boundaries. The direct results of this initiative are new stage boundaries placed at lithologically non-distinct stratigraphic positions which are identifiable by only a very limited number of biostratigraphic specialists. Furthermore, the original stage boundaries, and associated biostratigraphic breaks while still there, are now lost within the underlying, or in some case overlying, newly defined geologic stages.

The impact of the newly defined Turonian Stage boundary, as well as recently proposed Coniacian Stage boundary, on the Cretaceous succession in the U.S. Western Interior and U.S. Gulf Coast highlights these issues. In both cases, the stage boundaries were moved from distinct lithologic and biostratigraphic beaks, easily defined in outcrop and the subsurface, to lithologically non-distinct stratigraphic positions with biostratigraphic breaks, which now can only be defined by limited number of biostratigraphic specialists. Furthermore the stratigraphic breaks at the base of the Greenhorn/Upper Eagle Ford and Niobrara/Austin are still there, but now just lost within the underling Turonian and Coniacian Stages. So truth be told, are GSSP stage placement changes actually progress to the greater geologic community, or simply change for the sake of change?