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Lower Cretaceous carbonate deposits of Mt. Svilaja, Croatia: biostratigraphy vs. chemostratigraphy

Two chronostratigraphically equivalent successions (Milešina and Jarebinjak) of Cretaceous (Aptian– Albian) Adriatic Carbonate Platform (AdCP) deposits exposed only 7 km apart differ significantly in their facies composition and Aptian emersion features. Both successions reveal similar microfacies characteristic of the Lower Aptian (OAE 1a interval), but differ in their thickness. They are both composed of wackestones and oncoid-bioclastic floatstones with Bacinella irregularis RADOIČIĆ and requieniid rudist fragments. The beginning of the Lower Aptian succession at Jarebinjak has a rudist assemblage (Offneria sp., Praecaprina sp., and Glossomyophorus costatus MASSE et al.) within bioclastic floatstones indicating an Early Aptian age. The thicker Lower Aptian interval at Jarebinjak, as well as the caprinid assemblages indicate an open sea influence and a greater accommodation space, probably as a consequence of synsedimentary tectonics. At Milešina, several metres thick successions of algal wackestones and peloid-miliolid packstone- grainstones to wackestones directly overlie Lower Aptian strata. Thin layers of breccia or abundant charophytes, reflecting an increased fresh water influence, indicate subaerial exposure and regressive trends that are common in this interval elsewhere in the AdCP area. At Jarebinjak, in contrast, this interval with 7 to 8 thin beds of clay and marl associated with subaerial exposure features, exhibits more distinct regressive trends. Limestone deposits within this occasionally emergent horizon are characterized by algal wackestones and rare species of Mesorbitolina foraminifera. Above the emergence horizon, the Upper Aptian– Lower Albian successions at both localities contain micritic limestones with variable amounts of peloids and skeletal grains (miliolids and ostracods), irregularly alternating with thin layers of peloid- intraclastic-skeletal packstone-grainstones and peloid wackestonepackstones with molluscs (commonly gastropods) and benthic foraminifera (e.g., Mesorbitolina texana (ROEMER)). Increased fresh water influence is indicated by abundant charophytes (identified as Munieria grambasti sarda CHERCHI et al.). The layers with charophytes also contain dasyclad alga Salpingoporella and indicate that fossil dasyclads can inhabit brackish environments. Stratigraphic determinations are based on benthic microfossil assemblages, including foraminifera and dasyclad algae, which are very good paleoenvironmental indicators. However, their usefulness in biostratigraphy can suffer from low resolution and poor correlation with standard biochronologic scales based on planktonic foraminifera, calcareous nannoplankton and ammonites. Therefore, this research also involved a chemostratigraphic study including stable isotope analyses that proved useful for stratigraphic correlation between the examined successions and improved their age determination, despite limitations due to potential masking of global marine isotope signatures in restricted depositional environments. The results include refinements in the placement of the Barremian–Aptian boundary, recognition of the OAE 1a (in the Lower Aptian strata) and OAE 1b (straddling the Aptian–Albian boundary) intervals, and correlation with carbon- isotope stages C1 to C8. This provided critical information for correlating these Mt. Svilaja strata to other coeval successions that span the time interval of major global oceanographic changes and carboncycle perturbations associated with Lower Cretaceous oceanic anoxic events.

Lower Cretaceous, carbonate deposits, biostratigraphy, chemostratigraphy, Mt. Svilaja, Croatia

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