METHODOLOGY TO CALIBRATE NUMERICAL AGES FOR CARBON ISOTOPE EVENTS: BARREMIAN-ALBIAN

Chemostratigraphic events in the rock record are spatial geochemical excursions defined at inflection points or gradient changes in stratigraphic profiles. Observable variations in chemical rock properties such as isotopes and trace elements are traceable between stratigraphic sections. Data shifts reflect paleoenvironmental changes recorded in the rock record. Cretaceous carbon isotope signals define oceanic anoxic events (OAEs) and are subdivided into successive increments or ‘events’ (Cs). These increments are chronostratigraphic markers because they represent oceanographic and depositional conditions, and hence, incur time significance. Such profiles have chronostratigraphic utility and potentially can be numerically dated. Because direct correlation of many events with interbedded radiometrically dated beds is not possible, other methods must be applied to interpolate numerical ages.

Analysis of Barremian-Albian isotope events illustrates a method to objectively interpolate numerical ages. Graphic plots of three sections successfully integrate carbon isotope events with established numerically dated chronostratigraphic markers. Carbon isotope event segments were defined in Tethyan Cismon and Rotter Sattel Barremian-Albian sections. The Aptian events were subsequently identified in the Mexican Santa Rosa Canyon where Albian events were added. Chemostratigraphic events are integrated with fossil biozones and magnetochrons in each section. These bioevents and chronozones enable carbon events to be integrated with a comprehensive Cretaceous time scale. The interpolated numerical ages of isotope events enables chronostratigraphic interpretation of sections where fossil zones are not recorded.