Lower Cretaceous Getic Carbonate Platform: transition from photozoan to heterozoan carbonate factory (South-eastern Serbia)

Key words: microfacies, carbonate factory, Getic Carbonate Platform

The Getic Carbonate Platform (GCP) represents a Mesozoic sedimentary system developed on the southern margin of the European plate. It is coeval with other complex and well-studied carbonate platforms of Western Tethys, such as the Helvetic, Jura and Moesian carbonate platforms. Continuous sedimentation within the GCP during Late Jurassic–Early Cretaceous resulted in several thousand meters thick successions of carbonate and mixed siliciclastic-carbonate successions, cropping out in Romania, Serbia and Bulgaria. The stratigraphic evolution of the depositional system from a shallow-water carbonate platform to a mixed system influenced by terrigenous input, and finally the recovery of the shallow-water carbonate platform was studied in southeastern Serbia, in the proximity of Dimitrovgrad.

Facies analysis integrating the field data with microfacies petrography was conducted on more than 300 thin-sections within 12 stratigraphic logs. Sedimentary analysis additionally incorporated geochemical data from Oxygen and Carbon isotope analysis (δ¹³C, δ¹⁸O), SEM-EDS and Rock-Eval pyrolysis, depending on the characteristics of the sedimentary portion.

The Lower Cretaceous sedimentary succession of Dimitrovgrad was divided into three time slices (Berriasian-Valanginian, Valanginian-Hauterivian, and Barremian-Aptian) characterized by the lithology, either pure carbonates or mixed siliciclastic-carbonates, and type of the carbonate factory (photozoan carbonates dominated by the light-dependent skeletal carbonate producers or heterozoan association with filter-feeder biota).  The variations of distinctive sedimentary features are the results of terrigenous input, tectonic activity and climate changes.

The Berriasian-Valanginian interval is dominated by the photozoan carbonate factory with Bacinella/Lithocodium, rudists, calcareous and siliceous sponges, which formed patch reefs. Within this interval the episodes of subaerial exposure were followed by platform drowning. The subsequent increase in terrigenous supply disturbed the carbonate platform leading to a transition to the heterozan carbonate factory of the Valanginian-Hauterivian interval. The filter-feeders such as bryozoans, crinoids, large bivalves and serpulids were the main carbonate producers. A common characteristic of this mixed siliciclastic-carbonate system was the deposition of ooid-rich facies as a result of the chemical factory taking the lead during high levels of seawater acidification. The recovery into a renewed shallow-water photozan carbonate platform is marked by rudist, calcareous and siliceous sponge rich facies with reefs of Barremian-Aptian interval.

The GCP represents a complex sedimentary system reflecting the Early Cretaceous tectonics of the Vardar–Ceahlau-Severin interaction domain, global climatic warming, ocean acidification and nutrient input during global perturbations of the carbon cycle and regional sedimentary regimes. A similar evolution pattern was recognized in other Western Tethys domains, confirming the influence of the same controlling factors.