ИСТИНА

Large-scale sea-level changes are supposed to have occurred over most of the Phanerozoic time [Hallam, 1992; Haq et al., 1987; de Graciansky et al., 1998]. Due to important applications for prospecting of oil and gas in stratigraphic traps, third-order cycles with periods T ~ 1-3 Myr and amplitudes of 20-200 m are of special interest. They are often called eustatic events and used as an important marker for time-level correlation. According to paleontological and sedimentological data, in many epicratonic basins, on the carbonate platforms in particular, slow deposition proceeded for a long time at a very shallow depth ≤ 10-20 m. Such conditions existed, e.g., in some regions in East Baltic in the Late Cambrian and Ordovician, in East Siberia in the Ordovician and Silurian, and in the north of the East European Platform from the Middle Carboniferous to the Early Permian. The above deposition depths are characterized by very specific benthic assemblages and depositional features. Sea-level falls of several tens of meters would have resulted in subaerial exposition and erosion of sea bed, while sea-level rises of a similar magnitude would have completely changed the benthic assemblages and sedimentary facies. However, as follows from the analysis of classic stratigraphic successions for the above regions, the deposition at very shallow depth proceeded in them for ≥ 10 Ma without any hiatuses caused by subaerial erosion. A new model has been elaborated which allows us to estimate the changes in the depth of water under the influence of eustatic sea-level fluctuations. A finite rate of tectonic crustal subsidence, the isostatic reaction of the crust on changes in the water load, and the time necessary for the formation and preservation of subaerial erosional features were taken into account. This model allowed us to estimate the maximum magnitudes of eustatic fluctuations for the epochs when no subaerial exposures occurred in the areas under considerations. Some other approaches were also used including the analysis of the accommodation (Fischer) plots. As a result, it has been shown that in the Late Cambrian, over most of the Ordovician, in the Silurian and in the Middle Carboniferous-Early Permian eustatic sea-level changes of a third order did not exceed several tens of meters [Artyushkov et al., 2000, 2007; Artyushkov, Chekhovich, 2001, 2003]. This is considerably smaller than the eustatic fluctuations up to 100-200 m which are commonly supposed for the above epochs. Only at the end of the Ordovician two sea-level falls of ~ 100 m have been identified, which resulted from the formation of a large ice sheet in Gondwana. At the same periods of time, rapid changes of the depth of water, corresponding to third-order cycles by their magnitudes and durations, occurred in many cratonic basins [e.g., Ross, Ross, 1987; Johnson, 1996]. Under relatively stable sea level, they were caused by the crustal uplift and subsidence. Cratonic areas are commonly supposed to be characterized by a quite tectonic regime. The occurrence of rapid vertical crustal movements in them is an unexpected phenomenon. A typical example is the East European Platform in the Bashkirian Age of the Carboniferous [Alexeev et al., 1996]. At the end of the Serpukhovian Age, the platform was covered by a shallow sea with the depth of water ≤ 20 m. In the early and middle Bashkirian its western part emerged above sea level for a few million years which resulted in incision of the Aza River by 100-120 m. By the late Bashkirian, the region was again covered by a shallow sea. This short-term regression is commonly attributed to a sea-level fall of ~ 200 m. However, at the same time a shallow shelf proceeded to exist in the eastern and northern parts of the East European Platform. Hence pronounced regression in its eastern part in the Bashkirian was due to the crustal uplift. Most probably, this was the result of a short-term upwelling convective flow in the mantle. It is necessary to mention that numerous sea-level falls of a fifth order, several tens of thousands of years long and ~ 100 m in magnitude, occurred in the Carboniferous and Permian at the time of formation of ice sheets in Gondwana. However, these short-term regressions resulted only in weak erosion and produced no stratigraphic traps for hydrocarbons. Numerous stratigraphic traps for oil and gas have been formed due to rapid regressions and transgressions of a third order. Their prospecting is commonly based on the assumption on the eustatic mechanism of relative sea-level changes which occurred on a global scale. Tectonic origin of such changes will require a drastic change in the method of prospecting stratigraphic traps. It is necessary to study the development in time in space of rapid vertical crustal movements in each hydrocarbon basin and in the surrounding regions.