Upper Jurassic – Lower Cretaceous platform-to-basin integrated stratigraphy across the Bulgarian/Serbian border

This report presents the results of integrated microfossil biostratigraphy, facies and microfacies analyses with the purpose of age determination, correlation and tracing out of the carbonate platform-to-basin transition in the Callovian to Valanginian carbonate sequences across the Bulgarian – Serbian border.
   The Upper Jurassic and Lower Cretaceous are of wide occurrence in the western Bulgaria and eastern Serbia. The sediments deposited in a bathymetrically differentiated basin, associated with the gradual emergence of the southern landmass and the formation of the Central Moesian Basin. The shallow-water sections are located in the southwestern prolongation of the Western Moesian Carbonate Platform and belong to the West Srednogorie Unit (Bulgaria) and Vidlić/Tepoš Zone (Serbia). The main part of the carbonate platform is represented by the limestones of the Slivnitsa Formation (Bulgaria) and the Crni Vrh Limestones (Serbia). Both formations are built up by thick-bedded to massive light grey to whitish organogenic and less common micritic limestones containing a large number of benthic foraminifers and algae, colonial corals, rudists, brachiopods, crinoids, gastropods and other benthic forms. The age interval is Callovian to Valanginian based on foraminifera and calcareous dinocyst. Six successive foraminiferal zones are recorded. Seven facies (facial zones) with specific microfacies types are superposed within the platform carbonates: homoclinal ramp (peloidal); reef and perireef (bioclastic); subtidal lagoon (nonfossiliferous and oncoidal), intertidal flat (fenestral and foraminiferal); subtidal lagoon (foraminiferal); reef (Bacinella and Lithocodium) and slope (bioclastic). The carbonate platform deposits are covered by the clayey limestones and marls of the Salash Formation of Valanginian to Early Hauterivian age.
   The Callovian to Valanginian peri-platform pelagic carbonates were deposited on the northern Tethyan continental margin. In the Western Balkan Unit (Bulgaria) the pelagic record consists of the sediments of the Yavorets, Gintsi and Glozhene formations. Their correlatives in the Stara Planina-Poreć Zone (Serbia) are Kamenica, Pokrovenik and Rosomać formations. These are micritic and clayey nodular pelagic limestones formed in relatively deep basin conditions under quite low rates of sedimentations. Starting from the Late Berriasian, the basinal carbonate accumulation was quickly replaced by hemipelagic alternation of clayey limestones and marls which continued up to the Hauterivian (Salash Formation in western Bulgaria and Ržana Formation in eastern Serbia). Diverse ammonites and planktonic microfossils such as calcareous dinocysts, calcareous nannofossils and calpionellids were applied for detail zonations, stage and substage subdivisions. For the Oxfordian–Valanginian interval twelve calcareous dinocyst zones, five calcareous nannofossil zones and seven calpionellid zones are recorded. In the basin facies six microfacies within the pelagic carbonates are superposed: filamentous, Globuligerina-Radiolarian, Saccocoma, Globochaete and calpionellid and spicule microfacies. Stable sedimentary environment persisted during the whole Late Jurassic. Since the Late Berriasian a clear bathymetrical tendency occurred in the pelagic carbonates from west to east – platform slope, basin and a periphery of flysch trough.
   The carbonate platform sedimentation started with the formation of a homoclinal ramp in the Callovian and passed through a rim platform during the early Kimmeridgian. The platform evolution includes three main stages – stepwise progradation, aggradation and retrogradation during the late Kimmeridgian to Valanginian. The phase of platform drowning started in distal portions of the platform. The drowning phases are documented by erosional surfaces, hiatuses and condensed glauconitic beds. The drowning of the platform shows westward youngering from the earliest to Late Valanginian.