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Bio- and chemostratigraphy of the Lower Cretaceous carbonate platform deposits of Mt. Svilaja, Croatia

Two chronostratigraphically equivalent successions (Milešina and Jarebinjak) of the Cretaceous (Aptian–Albian) Adriatic Carbonate Platform (AdCP) deposits exposed only 7 km apart at Mt. Svilaja (Croatia) differ significantly in their facies composition and Aptian emersion features. Both successions reveal similar microfacies characteristic of the Lower Aptian oceanic anoxic event 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. However, the beginning of the Lower Aptian succession at the Jarebinjak locality has a species-rich rudist assemblage embedded within bioclastic floatstones. On weathered outcrop surfaces, caprinid, caprotinid and probably monopleurid rudist transverse sections can be recognized, indicating an Early Aptian age: Offneria sp., Praecaprina sp., and Glossomyophorus costatus MASSE et al. The caprinid assemblages, as well as the thicker Lower Aptian interval at Jarebinjak, indicate an influence of open sea circulation and a greater accommodation space, respectively, probably as a result of synsedimentary tectonism. At the Milešina locality, several metres thick successions of algal wackestones and peloid– miliolid packstone–grainstones to wackestones directly overlie Lower Aptian strata. Evidence for regressive trends and subaerial exposure, which are common in this horizon elsewhere in the AdCP region, are also present here, but as thin layers of breccia or abundant charophytes, reflecting an increased fresh water influence. In contrast, at the Jarebinjak locality, this horizon exhibits more distinct regressive trends, evidenced as 7 to 8 thin beds of clay and marl associated with subaerial exposure features. The limestone beds 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 calcareous mud, peloids and skeletal grains (rare miliolids and ostracods). These deposits irregularly alternate with thin layers of peloid–intraclastic–skeletal packstone– grainstones and peloid wackestone–packstones with fragments of molluscs (mostly gastropods) and benthic foraminifera (e.g., Mesorbitolina texana (ROEMER)). Increased fresh water influence is indicated in the Lower Albian level at Milešina by abundant charophytes, identified as Munieria grambasti sarda CHERCHI et al. The strata with charophytes also contain dasyclad alga Salpingoporella, indicating that fossil dasyclads can inhabit brackish environments. Stratigraphic determinations were based on benthic microfossil assemblages, including foraminifera and dasyclad algae, which are very good paleoenvironmental indicators, but their usefulness in biostratigraphy can suffer from low resolution and poor correlation with standard biochronologic scale based on planktonic foraminifera, calcareous nannoplankton and ammonites. Therefore, this research also included a chemostratigraphic study involving stable isotope analyses of homogenous micritic matrix samples. Despite limitations due to potential masking of global marine isotope signatures in restricted depositional environments and subsequent diagenetic modifications, the documented variations in carbon isotope compositions, especially when considered relative to oxygen isotope values, proved useful for stratigraphic correlation between the examined successions and for improving their age determination. Examples include refinement 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 of MENAGATTI et al. (1998). The results provide critical information for correlating these Mt. Svilaja strata to other coeval successions that span the time interval of major global oceanographic changes and carbon-cycle perturbations associated with Early Cretaceous oceanic anoxic events.

Biostratigraphy ; Chemostratigraphy ; Lower Cretaceous carbonate deposits ; Mt. Svilaja ; Croatia

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