The East Siberian basin in the Silurian: evidence for no large-scale sea-level changes

According to conventional ideas, numerous eustatic sea-level changes occurred during the Phanerozoic with amplitudes 20–100 m and periods T=1–3 Myr (third-order cycles). At the same time, it is widely recognized that tectonic factors have an important role in such water-depth changes. Rapid, large-scale sea-level changes have been suggested for the Silurian, for which detailed and continuous stratigraphic successions are available in the East Siberian sedimentary basin, ∼2×106 km2 in size. These show slow deposition at depths ≤5–10 m proceeded for 10–20 Myr in many regions and enable the mathematical analysis of maximum eustatic sea-level changes, with periods of T=1–3 Myr (third-order), that could have occurred during the Silurian. The amplitudes of such events could not exceed ∼20–30 m, and, for some epochs, could not be larger than 8–10 m. At the same time, rapid water-depth changes of up to ∼100 m occurred in some other cratonic areas. As the Silurian had a relatively stable sea level, these must be attributed to rapid crustal uplift and subsidence. The absence of third-order eustatic fluctuations with amplitudes ≥10–20 m has also been demonstrated for the Cambrian and earliest Ordovician. It is probable that no significant sea-level changes also occurred at many other epochs, while rapid changes in water depth resulted from large-scale, vertical crustal movements.