Abstract
Modeling of the evolution of freshwater diatoms and the mechanism of their distribution in the Neogene lake system in Southwestern Bulgaria is accomplished for the first time. The main purpose is the determination of the major stages in the evolution of the Mio-Pliocene diatom floras developed in the graben structures in this realm. Various data were used for the realization of this purpose: palaeontological, biostratigraphical, mineralogical, petrological and sedimentological. Outcrops and core drills from Stanitsi, Beli Breg, Sofia, Chukurovo, Palakaria, Kostenets, Razlog, Simitli, Sandanski, Strumeshnitsa, Gotse Delchev, Satovcha Basins and Valkosel-Tuhovitsa depression were characterized. In total, 660 diatom species, varieties and forms have been described from these lake systems. This surprisingly high diversity is even comparable with marine biodiversity for the Eastern Paratethyan Bay on the territory of Northeastern Bulgaria (546 species, varieties and forms) during the Miocene. The oldest known lacustrine diatoms from the sediments in Southwestern Bulgaria are Middle Miocene in age (Simitli Basin — Simitli Formation), the youngest — Early Pliocene. Three major stages in the evolution and distribution of centric diatom genera can be distinguished during Middle Miocene-Pliocene. Diatom analyses, complemented by structural and geodynamic investigations could demonstrate the impact of different palaeoenvironmental factors on the development of the diatom flora. We found that the process of nutrient levels, the salt content, temperature and the changes in the depth of the lake correlated significantly with patterns in the diatom assemblages, based on complex of lithostratigraphic and biostratigraphic investigation, as well as different statistical techniques. The inference model of Sofia Basin could be useful in describing the colonization of diatoms in the lacustrine basins on the territory of Southwestern Bulgaria.
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