FORMAL BARREMIAN–ALBIAN CHEMOSTRATIGRAPHIC UNITS DEFINED BY ROCK PROPERTIES AND STANDARD SECTIONS

Chemostratigraphy uses geochemical data to subdivide and correlate sedimentary stratigraphic successions. Chemostratigraphic Units (CUs) are a new category comparable to lithostratigraphic and biostratigraphic units but not yet formally defined in stratigraphic guides. Geochemical signals of organic matter, major, minor, and trace elements, and isotopes have been recognized as widespread, even synchronous intervals in stratigraphic sections since the 1970’s at least. For example, Cretaceous organic carbon-rich black shales define oceanic anoxic events (OAEs) and are correlated in deep sea and outcrop sections.

Six chemostratigraphic attributes and procedures define CUs objectively: 1) geochemical elements that produce reliable stratigraphic signals such as TOC, δ¹³C, δ¹⁸O, δ¹³C/ δ¹⁸O; 2) chemical signals with distinct identifiable quantifiable excursions; 3) closely spaced samples (e.g, 50 cm or less); 4) events defined at bases of excursions, at inflections, at peaks, or as interval between excursions; 5) events integrated with independent criteria, e.g., bioevents, magnetozones, or well log properties; and 6) events correlate from reference sections to other sections.

Barremian–Aptian carbon isotope zones were defined in Italy and Switzerland at Cismon and Rotter Sattel. Subsequently Aptian–Albian carbon isotope zones were identified in Mexico at Cañon Santa Rosa. Each section has well-documented microfossil biozones and magnetozones that enable the chemostratigraphic units to be integrated with a comprehensive Cretaceous time scale. These carbon-isotope zones are synchronous correlation units because they represent oceanographic and depositional events, and hence, incur time significance.