Palaeoenvironmental setting of rudists in the Upper Cretaceous (Turonian–Maastrichtian) Adriatic Carbonate Platform (Croatia), based on sequence stratigraphy

Rudist bivalves are abundant in Upper Cretaceous Adriatic carbonate platform deposits in biostromes of varying biodiversity and density that alternate with foraminiferal wackestones to packstones. Their distribution is reviewed with respect to the sequence stratigraphic framework. The Turonian–Senonian succession of the platform comprises three third order sequences: (1) Turonian–Middle Santonian; (2) Middle Santonian–Upper Campanian; (3) Upper Campanian–Upper Maastrichtian. After the early Turonian transgression, which is recorded in all areas investigated, the Adriatic carbonate platform had a regionally varied history. Our analysis shows that hippuritid individuals here grew preferentially in relatively open, outer shelf conditions that prevailed while carbonate production was catching up with sea-level rise following platform foundering. By contrast, densely packed and laterally extensive radiolitid clusters formed in more restricted platform settings where carbonate production tended to keep up with relative sea-level rise and yielded successive shallowing-upward cycles. This pattern contrasts with that known from more northern Tethyan regions (such as the Pyrenees and SE France), where hippuritids frequently dominate the restricted platform top deposits. The difference may reflect a greater tendency to hypersalinity on the broad interiors of the pure carbonate platforms in central and southern Tethyan regions, though other possible explanations have been offered.