Local mollusc standard zones of Quaternary sediments from the Bulgarian Black Sea shelf

Abstract

The only boundary which can be used to differentiate the Bulgarian Black Sea shelf into stratigraphic zones is the spatial distribution of mollusc fauna. This boundary is bathymetrically determined and divides the central morphological shelf zone into two uneven parts given inner and outer shelf. Another boundary is drawn which follows roughly the parallel of Cape Kaliakra and divides it into two uneven parts. The sediments of Low Pleistocene series were not deposited in the inner shelf zone and here a stratigraphic gap is established there which coincides with Chaudian Age. A similar gap at the time of Early Chaudinian Subage is established in the outer zone of the shelf where the Dreissena rostriformis tschaudae local mollusc Range-zone is indicated. In both shelf zones the sedimentation continues by formation of the Middle Pleistocene Old Euxinian and Uzunlarian Regional Stage in whose sediments the corresponding are Didacna crassa pontocaspia – Dreissena euxinica and Didacna crassa pontocaspia – Abra ovatа local mollusc Parallel Range-zones. The Upper Pleistocene (Karangatian) mollusc zones are established only at the inner parts of the Bourgas Bay and there three: Corbula gibba – Rissoa parva, Paphia senescens – Cardium tuberculatum and Cardium edule – Bittium reticulatum local mollusc Parallel Range-zone are found out which correspond to the three Substages of the Karangatian Regional Stage. The succession of the mollusc zones from the inner shelf is interrupted by a barren zone which corresponds to the continental deposits sedimented at the time of the Early New Euxinian Substage. During the greater part of the Late Pleistocene Subepoch in the zone of the outer shelf a stratigraphic gap is established. In the composition of the zone community of the next in superposition Dreissena polymorpha – Dreissena rostriformis distincta local mollusc Parallel Range-zone two associations are established – one in the inner and another in the outer shelf. The climatic changes which took place after the Würm glaciation restored the connection of the Black Sea with the World Ocean. In the Black Sea basin this Holocene transgression causes uplift of the sea level and increasing of the salinity to its recent values. The latter determines the successional changes of the mollusc communities which from the beginning of the Holocene became more halophilic. The community of Hydrobia ventrosa – Monodacna caspia caspia local mollusc Parallel Range-zone and Mytilus galloprovincialis local mollusc Range-zone comprise the sediments of Old Chernomorian Substage. The two New Chernomorian Spisula subtruncata triangula and Modiolus phaseolinus local mollusc Range-zone are fingering spatially in the zone of the outer shelf together with increase of the bottom at practically one and same composition of the zonal community.