Strontium-isotope stratigraphy of Upper Cretaceous platform carbonates of the island of Brač (Adriatic Sea, Croatia): implications for global correlation of platform evolution and biostratigraphy

Abstract

The chronostratigraphy of Coniacian–Maastrichtian platform carbonates exposed on the island of Brač and the adjacent mainland has been revised, based on numerical ages derived from strontium-isotope stratigraphy (SIS) of low-Mg calcite of rudist shells. The Dol intra-platform basin formed during the mid-Coniacian–early Santonian. The base of the prograding Pučišća Formation is of mid-Santonian age (84.9 Ma) in the southeast, and late Middle Campanian (77.3 Ma) in the northwest of the island, indicating a progradation rate of the platform margin of ca. 2.5 km/myr. Subaerial exposure of the platform occurred during the latest Middle Campanian and is coeval with a major drop in sea level reported from the Boreal Realm, North America, and the southern Tethyan margin. The base of the Sumartin Formation is revised here to the earliest Late Campanian (ca. 75 Ma). At its top, the formation contains rudist-bearing limestones of latest Maastrichtian age (65.4–65.0 Ma). The exact position of the K/T boundary cannot be drawn due to the lack of material suitable for SIS, and to the absence of diagnostic fossils in restricted innermost-platform deposits of the Liburnian Formation, which follows conformably over the rudist-bearing Sumartin Formation. Based on the revised chronostratigraphy of platform evolution, and particularly on the numerical ages that constrain the progradation of the Pučišća Formation, the stratigraphic ranges of characteristic Tethyan rudist bivalves and benthic foraminifers are re-evaluated.