THE ORIGIN OF WATER-DEPTH CHANGES IN PAST EPEIRIC SEAS

Abstract

As inferred from numerous available seismostratigraphic and lithological data [1, 2, and others], Phanerozoic epeiric seas experienced frequent and significant water-depth changes. Identification of regularities in these changes and their physical mechanisms is one of the basic geodynamic problems. The third-order cycles with amplitudes of ~20‐100 m and a duration of 1‐3 Ma are of particular importance for oil and gas prospecting and geological correlation. Major eustatic sea level fluctuations are usually considered the main responsible factor [1 and others], although some researchers admit the influence of tectonic movements. Rapid eustatic fluctuations ( ≤ 1‐3 Ma) with an amplitude of ~20‐100 m can be largely attributed to the formation of large ice sheets and their degradation. In the Phanerozoic, these processes were rare, but the third-order cycles were almost constant. Therefore, the eustatic origin of the third-order cycles is sometimes doubtful [2]. Numerous eustatic third-order fluctuations were previously proposed for the Cambrian‐initial Ordovician and Silurian. Recently, it was established, however, that sea level fluctuations did not exceed 10‐20 m during the former epoch [3] and significant eustatic sea level changes with a sharp regressive phase and harmonic-type fluctuations did not occur during the Silurian [4]. Rapid water-depth changes recorded in some basins during these time intervals were in fact related to tectonic movements. In this work, we estimate maximal values of Silurian eustatic sea level fluctuations of an arbitrary form and consider the mechanism responsible for changes in the crustal subsidence rate in eastern Siberia. The history of this region was characterized by the existence of extremely shallow (0- to 5-m-deep) sea basins or spacious shoals and semiisolated (0- to 10-mdeep) lagoons located beyond the shoal zone [5].