CHEMOSTRATIGRAPHY AND THE NORTH AMERICAN STRATIGRAPHIC CODE: IS THERE A REASON FOR FORMAL UNITS?

Chemostratigraphy (the use of geochemical signals for stratigraphic correlation) has been an active area of research for more than 50 years. It was first widely applied to the late Cenozoic in the form of δ¹⁸O records in time series. These were used as paleoclimate proxies and to facilitate correlation at both local and global scales. Since this beginning, numerous chemical records have been used as proxies for Earth processes that otherwise do not leave a record. Despite its wide application, chemostratigraphy does not have a standardized nomenclature nor do the recognized units have an accepted hierarchy.

The North American Commission on Stratigraphic Nomenclature (NACSN) is currently investigating the feasibility and desirability of formalizing the nomenclature and rank of chemostratigraphic units. Such units and their hierarchy would be described in the North American Stratigraphic Code. This would provide consistency in the use of chemostratigraphic units and would facilitate correlation and further research on geochemical signals.

Most chemostratigraphic units that have been described to date have many of the characteristics of chronostratigraphic units. They have a distinct duration in time. Examples include the late Cenozoic isotope “stages” equated with glacial and interglacial episodes and the Paleocene-Eocene Thermal Maximum, recognized globally by a distinct δ¹³C excursion or “spike”. Each unit also started as a material stratigraphic unit in a core or section at a specific geographic location. As with all material stratigraphic units, these chemostratigraphic units, as initially defined, were independent of time, only being correlated with the geologic time scale later.

Not all chemostratigraphic units will be recognized as chronostratigraphically significant but many geochemical signals in the sedimentary record may be of at least local or regional value in facilitating physical correlation. Establishment of formal units and a formal hierarchy could provide standard procedures and criteria as well as editorial guidance for the recognition and definition of chemostratigraphic units. Discussion from the stratigraphic community is invited.