ROAD-TESTING GSSPS IN THE MIOCENE COASTAL PLAIN OF MARYLAND AND VIRGINIA, USA

The perfect Global Stratotype Section and Point (GSSP) does not exist. Every GSSP is a compromise between what is desirable and what is possible. A Global Stratotype Section is a specific place that can be visited by stratigraphers and plotted on a map. The “Point” is perhaps a more difficult concept. It is a tangible point and, at that one location, the point in space defines an instant in time. Everywhere else, identifying that instant in time is an ideal. Correlation proceeds by whatever means are available and appropriate.

The ultimate goal behind the establishment of GSSPs is to make unambiguous communication easier among stratigraphers and among all geoscientists. GSSPs are established, for the most part, in open marine deposits. Extending correlations into nearshore and marginal marine deposits, such as the mid-Atlantic Coastal Plain, allows rigorous testing of GSSP utility.

For the Miocene Series, the GSSPs for four stage boundaries have been established, and two boundaries still await formal designation. For now, there is ambiguity in discussing the lower and middle Miocene of the Coastal Plain, and this ambiguity will remain until a GSSP for the Langhian Stage is ratified. When it, or any, GSSP is ratified there will be winners and losers. For example, when the GSSP for the base of the Tortonian Stage (upper Miocene) was selected, some stratigraphers found that sediments that they used to call “uppermost middle Miocene” and become “lowermost upper Miocene.”

Communication relative to the Oligocene-Miocene boundary was made much easier when the GSSP for the base of the Aquitanian Stage was ratified in 1996. Since that time, much scientific progress has been made in orbital tuning, paleomagnetic interpretations, decay constants, biostratigraphy, chemostratigraphy, and boots-on-the-ground geologic mapping. Orbital tuning of the Global Stratotype Section yielded a new numerical age (23.03 Ma) with respect to the original reference (23.80 Ma). Now, Coastal Plain stratigraphers must be wary of indirect numerical ages assigned near this boundary: were they assigned before or after the recalibration?

The road test is how difficult or easy it is for Coastal Plain stratigraphers to communicate that the rocks didn’t change, just their chronostratigraphic adjective; the ranges didn’t change, just their calibration.