Home – Home – Jornals – Russian Geology and Geophysics 2002 number 10

The concept of time-dependent eustatic fluctuations has been largely recognized, and a number of such fluctuations, with amplitudes of 20 to 100 m and durations of 1 to 3 Ma (third-order cycles), have been inferred for the Phanerozoic. Eight cycles of this type were distinguished for the Silurian on the basis of water-level curves for different regions. East Siberia in Silurian time was occupied by a large sea basin whose fill has been well documented in many sections. Some Silurian sections show slow deposition for 10 to 20 Ma at shallow water depths (~10 m), which rules out significant third-order eustatic cycles. The amplitude of these cycles in Silurian time could not exceed ~10-20 m, as follows from analysis of fluctuation curves and sedimentation rates. Earlier data on East Baltic regions did not show large eustatic cycles in the Cambrian and early Ordovician, which, along with the results from the Silurian sections of Siberia, casts doubt on the existence of rapid and strong eustatic fluctuations over the greatest part of the Phanerozoic. Considerable rapid sea depth changes occurred in a number of Cambrian and Silurian deposition basins at a relatively stable water level. Therefore, the slow subsidence of these basins was accompanied by rapid tectonic uplift or subsidence of the crust, which can be interpreted as a specific type of tectonic movements on platforms.