THE NATURE OF SEA DEPTH CHANGES IN EPICONTINENTAL SEDIMENTARY BASINS OF EAST SIBERIA IN THE SILURIAN

Abstract

It is commonly recognized that sea level experienced strong global-scale systematic changes in the geologic past. Its 20-100 m fluctuations in 1-10 m.y. (third-order) cycles are most often detected using Fischer plots based on thicknesses of meter-scale cycles in sections of shallow-marine carbonate facies. However, the classical Fischer plots constructed for different regions of East Siberia show a considerable mismatch, though eustatic changes are supposed to be globally synchronous. This mismatch is caused by time-dependent variations in durations of elementary cycles asynchronous in different sections. Therefore, a great number of earlier inferred global eustatic events apparently never existed in reality. We obtained more reliable accommodation plots with the thicknesses of 0.5 m.y. synchronous chronozones as elementary units in the Silurian section of East Siberia. According to these plots, eustatic fluctuations never exceeded ±5-7 m throughout the Silurian, which is far below the accepted magnitude of 30-130 m. The Silurian stratigraphic subunits and intervals between tectonic events were constrained owing to stable durations of the chronozones. Our plots reveal strong lateral variations of crustal subsidence rates over East Siberia. These variations were controlled neither by the flexural response of lithosphere to changes in horizontal stresses nor by the mantle topography dynamics, but rather by metamorphic phase change and related consolidation of the mafic lower crust. Subsidence rates occasionally became times faster or slower within ∼0.5 myr intervals, the contrasts surprisingly great for platforms. Phase change rates may have been controlled by deviatoric stresses in addition to temperature and fluid migration. This mechanism can be responsible for subsidence rate variations reported for many intracratonic basins.