Holocene litho- and biostratigraphy of the NW Black Sea (Romanian shelf)

摘要

Detailed lithological and micropaleontological studies were performed on several cores from the Romanian Black Sea shelf, located in front of the Danube Delta, in front of the Razelm-Sinoe lagoonal complex, close to the Eforie beach, as well as on the Romanian inner shelf. All the studied cores are situated at a water depth varying between 12 m and 67 m. The purpose of the investigations was to reveal Upper Holocene lithological changes and the associated calcareous nannoplankton fluctuations in a very shallow marine setting such as the Romanian Black Sea inner shelf. The youngest Holocene unit or "Shallow Unit" is composed of alternating sequences of mud, silt, clay and coquina layers, containing marine mollusc faunas. The upper part of the Shallow Unit contains blooms of the nannoplankton species Emiliania huxleyi, while its lower part yields, besides common E. huxleyi, rare specimens of Braarudosphaera bigelowii. The Shallow Unit overlies a "shell hash layer", mainly made by coquinas, containing mixing marine and brackish mollusc faunas. Towards the upper part of this level, monospecific assemblages with B. bigelowii were observed in a layer rich in carbonate acicular crystals. The Shallow Unit partly corresponds to the Unit 1 (finely laminated coccolith ooze) and Unit 2 (sapropelic sediments) of slope and basinal settings of the Black Sea. The layer rich in carbonate acicular crystals could be a shallower equivalent of the oldest layer of aragonite bands placed at the lower part of the Unit 2 in deep parts of the Black Sea. In the analyzed cores, the oldest lithological unit (Unit 3, the muddy clay sediments deposited in a brackish to freshwater environment) is characterized by the deposition of green-yellowish clay devoid of any Holocene calcareous nannoplankton species and containing brackish mollusc faunas. Based on nannofloral fluctuations, an increasing salinity from the base towards the upper part of the Shallow Unit may be supposed, together with the progressive establishment of a stable marine setting, similar to today.