Hauterivian Biogenic Buildups of the Bodrak River Right Bank Area, Crimean Mountains

The paper analyzes the factors affecting spatial distribution of the Low Cretaceous biogenic buildups in the Crimean Mountains. For the case of the Hauterivian paleobasin buildups, two most plausible hypotheses have been chosen out of the explanations, published earlier by different authors. These are an anoxic hypothesis of buildups control and a hypothesis of an increasing terrigenous export resulting from transgression, active block tectonics and differences of the surface relief. The hypothesis of the terrigenous export caused by active tectonics can be tested by applying methods of geological survey of biogenic buildups. In such a way, if in transgressive overlapping of certain biogenic buildups there is revealed an increase in an amount of terrigenous material in the reef limestone or inhibition of coral growth, the hypothesis of terrigenous export will be proved. In the present paper the altitude harp method and the facial profiles analysis have been applied to buildups of the Bodrak River right bank area. The study reaffirms the Early Hauterivian transgression for this area; however, its rates are quite moderate (of about 5?10 m during the time span corresponding to two ammonite zones). The parts of buildups which correspond to the sea level rising show no signs of degradation, quite the reverse, they have the most complete development. These results argue against the hypothesis of buildups control by a terrigenous export and are the evidence of classical positive dependence between the area downwarping and reef growth. Thus, only the anoxic scenario remains most credible explanation for buildups distribution in the Hauterivian of the South-Western Crimeans. A trend to an exclusion of corals from the adaptive zone of reef-builders, which existed during all the Lower Cretaceous, was caused by global warming (similarly to modern situation). This trend resulted in destroying coral communities, in the first place at the expense of their heterotrophic component. As a consequence, there occurred anoxic events. In the case of the Early Hauterivian of the Bodrak river right bank area those used to be local, merely confined to inter-buildup depressions, hence later overgrowing into the global ones. It has been shown that biogenic buildups of the Hauterivian Bodrak-Alma interfluve can be classified as buildups of sandbank barrier zone. Low depths and off-shore location, the absence of foreslope bioclastic aprons, rapid lateral migration of buildup facies, and the presence of waveprotected areas are characteristic of these buildups. Besides, the paleogeographic situation was additionally complicated by the presence of many intrusions building positive topographic forms, all providing favourable environment for scleractinian communities.