Stable Carbon Isotope Chemostratigraphy of Shallow-Marine Carbonates, Lower Cretaceous Adriatic Platform, Croatia: What Is the Curve Telling Us?

Abstract

Application of δ¹³C chemostratigraphy in shallow-marine carbonates has commonly been regarded as problematic due to susceptibility of platform carbonates to diagenetic alterations. We analyzed the δ¹³C signal from the 700-m-thick Hauterivian to Albian Adriatic Platform from Mljet Island, Croatia which underwent little post-Mesozoic burial. A total of 715 bulk-rock samples were collected at 1 meter intervals from micrite matrix of mudstone and wackestone. Packstone, grainstone and dolomite were excluded due to their susceptibility to diagenetic alteration. δ¹³C values generally vary from -1.5 to -3.5 ‰ VPDB, and δ¹⁸O from -1 to -3.5 ‰ VPDB. Compared to the range of values from coeval Tethyan pelagic carbonates, Adriatic Platform stable isotopes are negatively shifted ~2 to 3 ‰ VPDB, suggesting early meteoric fluids or late stage Paleogene upland-sourced meteoric fluids as possible culprits. However, comparison with δ¹³C curves from pelagic sections shows that the major excursions are reproduced in the platform succession, including the oceanic anoxic events OAE1a, 1b, and 1c. The Adriatic Platform δ¹³C curve also shows no clear correlation between the negative excursions and disconformities. Furthermore, some of the major negative excursions, including the OAE1b and 1c lie above the nearest unconformities, ruling out early diagenesis as a cause of these and most other δ¹³C excursions in intervals that are lacking the emergence features. The study suggests that the Lower Cretaceous platform-top sediments may be better recorders of global carbon cycling than their aragonite rich Neogene counterparts, and that similar Cretaceous platforms that underwent little post-Mesozoic burial could provide detailed δ¹³C records that may be shifted wholesale to lighter values, but still preserve the remnant of the original oceanic signal.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018