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Abstract: Can Carbon Isotopes Be Used as a Stratigraphic Tool in Mid-Cretaceous Shallow Water Carbonates?

Jurgen Grotsch, Volker Vahrenkamp

Biostratigraphic resolution in shallow water carbonates is often poor and correlation from shallow platform environments into deep sea sediments is in many instances difficult. In deep marine environments variations in the isotopic composition of ^dgr13C have been demonstrated to provide high-resolution stratigraphic data. Shallow water carbonates, however, which show indication for subaerial exposure(s) are commonly thought to undergo diagenesis causing alterations of the original isotopic signature.

Carbon isotope profiles from the shallow water platform and intrashelf basin facies of the Aptian Shuaiba Formation in Arabia mimic ^dgr13C trends reported from deep marine sections. Even though time gaps of up to 5 Ma exists, evidence for diagenetic alteration of the ^dgr13C variations typical for subaerial exposure is lacking. In order to further investigate this covariation of ^dgr13C isotopic profiles in such different facies, an outcrop section at Mt. Kanala has been measured from an open ocean platform in NW Greece. The profile comprises a time interval from the Barremian to Late Albian. Measured ^dgr13C values fluctuate between -1 ^pmil and +3.5 ^pmil. Even in parts of the section where there is petrographic evidence for severe early eteoric-vadose diagenesis no strong negative ^dgr13C shifts are present. Thus, at Mt. Kanala, as in Arabia, the ^dgr13C profiles covary with those from deep marine sections. Because of the higher subsidence rate of the Gavrovo platform, it provides a more detailed record.

We therefore conclude that ^dgr13C measurements of time-equivalent shallow water limestone successions from the Gavrovo platform in Greece and the Arabian shelf as well as pelagic sections covary during the Barremian to Albian. This suggests that the different depositional environments record secular changes in ^dgr13C composition despite diagenetic overprinting. Hence, chemostratigraphy based on carbon isotope variations is a valuable correlation tool which can greatly improve stratigraphic resolution of Barremian to Albian marine carbonates and, more importantly, allow a platform to basin correlation.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France