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Records of Early Cretaceous Late Hauterivian-Early Albian Oceanic Anoxic Episodes Within Shallow-Marine Carbonates, Adriatic Platform, Croatia


To evaluate the record of Early Cretaceous Hauterivian-Early Albian anoxic events and their effects on a shallow platform stratigraphy, we focused on a Bahama-like, Adriatic Platform of Croatia. Seven sections were logged noting the bed thickness, lithology, sedimentary structures, grain size, shape, sorting, fossil content, and any diagenetic features. Two continuous sections were sampled every meter for carbon and oxygen isotope analyses of bulk carbonate matix. The facies identified include breccias and green mudrock (subaerial exposure), microbial and planar laminite (intertidal-supratidal), barren lime mudstone (restricted lagoon), peloid-skeletal packstone and grainstone (high-energy shoal), peloid-skeletal wackestone and packstone (moderately shallow subtidal), skeletal wackestone, mudstone and floatstone (low energy subtidal facies), and platy laminated lime mudstone and fine pellet packstone (deeper lagoon facies). The carbon isotope values of the bulk carbonate matrix range between −3‰ to +4‰ PDB. At the meter-scale sampling interval employed in this study, generally there is no systematic shift from light to heavy d13C values beneath emergence surfaces, which suggests that the C-isotope excursions are not the product of near-surface diagenesis. The C-isotope curve shows a minor positive excursion in the late Valanginian, likely corresponding to the Faraoni episode. It is followed by an overall stepwise increase in C-isotope values that ends up with a negative excursion at the Barremian-Aptian boundary (Taxy episode). The subsequent early Aptian Selli episode is clearly identified and is characterized by a large positive excursion in C-isotope values. It is followed by a pronounced negative excursion, after which the C-isotope values undergo some small fluctuations before undergoing a negative, followed rapidly by a positive excursion marking the onset of OAE1b (Paquier event), after which d13C values undergo a step-wise decrease. The studied Adriatic Platform sections do not directly record any black shale deposits that correlate with the above events. However, OAE1a is evident on the outer part of the Adriatic platform as poorly oxygenated lagoonal deposits. In contrast, the Adriatic platform did not undergo incipient drowning during the other late Hauterivian-early Albian events, and the platform remained shallow, and above the level of incursion of basinal waters.