SILURIAN DEPOSITION IN EAST SIBERIA AND THE ABSENCE OF LARGE-SCALE EUSTATIC FLUCTUATIONS

Abstract

The concept of time-dependent eustatic fluctuations has been largely recognized, and a number of eustatic events with magnitudes from 20 to 100 m and durations 1 to 3 m. y. (third-order cycles) have been suggested for the Phanerozoic. Eight cycles were distinguished in the Silurian on the basis of sea depth variations in different regions. East Siberia in Silurian time was occupied by a large sea, and its bottom fill has been well documented in many sections. Slow continuous deposition for 10 to 20 m. y. in peritidal environments (≤10 m) recorded in some Silurian sections rules out large-scale eustatic events. The magnitude of the Silurian events could not exceed ∼10−20 m, as follows from analysis of eustatic fluctuations and sedimentation rates in the sections. The absence of large-scale eustatic events in the Cambrian and earliest Ordovician inferred in an earlier study in the East Baltic regions, along with the results from the Silurian sections of Siberia, casts doubt on the existence of rapid large-scale eustatic fluctuations over the greatest part of the Phanerozoic. Considerable sea-depth changes that occurred in Cambrian and Silurian deposition basins at a relatively stable water level are rather of tectonic origin. Rapid crustal uplift and subsidence on the background of slow deepening of the basins is a specific type of tectonic movements on platforms.