New biostratigraphic and microfacies study of the Kotraža successions: Evidence for Early Albian volcanism and the development of an atoll-like reef system in Central Serbia

This study presents a new paleontological data and microfacies analyses of the Aptian–Cenomanian successions of Kotraža (Central Serbia). The studied successions (Section 1 and Section 2) comprises four sedimentary cycles dated by ammonoids and foraminiferal biostratigraphy. Three fining-upward cycles in Section 1 record deepening-trends, whereas the fine-grained Section 2 more distal basin-ward sedimentation.

The Kotraža succession (Section 1) provides the first evidence of Early Albian volcanism (~112 Mio. yr) and the development of an atoll-like reef system around a volcanic island in Central Serbia, reflecting most likely the response of the Inner Dinarides to the mid-Cretaceous tectonic plate reorganization in the Western Tethys Realm. Based on new data, Cycle 1 of the Section 1 records the establishment of deep-marine sedimentation following the drowning of an orbitolinid-bearing carbonate ramp during the latest Aptian. The deposition of deep-water sediments intercalated with mass-transport deposits containing beside volcanic pebbles also different shallow-water organisms, volcanic ashes and sandstones during the Aptian-Albian transition interval, indicate that the volcanic activity triggered progressive deepening and reduction of shallow-water carbonate production, i.e. the drowning of the orbitolinid foraminifera-bearing shallow-water carbonate ramp, restricting shallow-water carbonate production to reefs surrounding the newly formed volcanic island. The Cycle 2 of the Section 1 is characterized by volcanoclastic sandstone turbidites, slumps and mass-transport deposits containing mixed fossil fauna. Fe-rich and glauconitic sandstones with mixed shallow- and deep-water fossil fauna document continued volcanically influence sedimentation and redeposition of shallow-water material into adjacent basinal environments during the emergence of volcanic island the Early Albian. However, the Middle Albian is not proven in the sedimentary succession, and the reason for it remains unclear. The Cycle 3 of Section 1 records a new phase of deep-marine sedimentation during the late Albian and is represented by a fining-upward succession of marlstones and claystones. The decreasing of volcanic ash layers indicates declining volcanic activity, and the appearance of the organic-rich fine-grained sediments reflect low-oxygen conditions associated to late Albian black-shale events. The Section 2 records only one cycle of late Albian transgressive phase characterized by fine-grained succession and decreasing volcanic activity, whereas aboundant planktonic foraminifera indicate open-marine basinal conditions. The uppermost part of the section is characterized by organic-rich sediments which indicate progressively restricted low-oxygen conditions, comparable to late Albian-early Cenomanian intervals recognized in the Western Tethys realm.