Sea level in the Ordovician: Sharp fluctuations in subsidence rates of the Siberian Craton crust

Sea-level change has been commonly interpreted to be of eustatic origin, and many eustatic events were hypothesized for the Phanerozoic, including several 1–3 Myr long cycles in the Ordovician with magnitudes up to 100 or 200 m. However, sea-level change modeling using stratigraphic data from Northern Estonia, which was an area of slow shallow-marine (<10 m) deposition through most of the Ordovician, indicates fluctuations of no more than 20 m. In the Late Ordovician the sea level fell only twice for ∼100 m within 1 Myr during the Gondwanian glaciation. Although the sea level remained relatively stable, there were frequent 100–200 m changes of sea depths we inferred with reference to the time spans of stratigraphic units and intervals between tectonic events estimated reliably against stable durations of East Siberian chronozones (biochrons) of the Ordovician. In the absence of eustatic events, the sea-depth changes most likely resulted from rapid crustal uplift and subsidence. According to correlated well-documented Ordovician sections from East Siberia, the rate of crustal subsidence changed rapidly in different periods and in different places of the area, thus being of a regional scale. The controversy between the sea-level stability and the regional-scale variations in sea depths controlled by rates of crustal uplift and subsidence can be resolved assuming a model of variable eclogitization rates in the lower crust caused by lithospheric stress change. Our inferences undermine the traditional petroleum prediction approach implying formation of depositional traps due to rapid eustatic sea-level change.

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Ordovician sea-level change and rapid change in crustal subsidence rates in East Siberia and Baltoscandia

Artyushkov

Tesakov

Chekhovich

2008

Russian Geology and Geophysics

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Sea level changes and rapid crustal movements in cratonic areas in the Late Paleozoic

Artyushkov

Chekhovich

2011

Russian Geology and Geophysics

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In the Phanerozoic, the sea depth in epeiric sedimentary basins showed considerable variations, often accompanied by regression. In periods of regression and erosion, the subaerially exposed shelf and the adjacent parts of the marine basins gave rise to numerous nonstructural (stratigraphic) hydrocarbon traps. Sea depth variations with a magnitude of up to 100–200 m and 1–3 myr long (third-order cycles) are usually attributed to the eustatic fluctuations of the sea level. To estimate their possible range, a model is proposed which describes the water depth variations as a function of eustatic fluctuations in tectonically subsiding carbonate platforms. We take into account the crustal isostatic response to the changing water load and the finite time necessary for soil and karst formation in the exposed shelf or its upper part. This model allowed analyzing data on the reference sections of the North Timan shallow-water sediments. According to the analysis, the third-order sea level changes in the Middle Carboniferous, Late Carboniferous, and Early Permian did not exceed several tens of meters. During the same period, shorter fluctuations (∼100 kyr) occurred owing to the waxing and waning of large Gondwanan ice sheets. In the first half of the Bashkirian Age (Early Pennsylvanian), regression took place in the East European and North American cratons and then shallow-water sedimentation resumed. This regression is usually attributed to a considerable sea level fall. In some other areas, slow shallow-water sedimentation continued throughout the Bashkirian. This suggests that the Bashkirian regression was due to the crustal uplifts. Short-term uplifts can be explained by ascending convective currents beneath the asthenosphere. In southern North America, they brought an active fluid into the lithosphere. This caused rapid eclogitization-related crustal subsidence in the Arkoma and Anadarko basins as well as intense lithospheric weakening and shortening in the Ouachita Fold Belt.

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Biofacies zonation of Ordovician deposits of the southern Siberian Platform

Stepanova

Byalyi

Каныгин

2015

Russian Geology and Geophysics

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Based on the inventory, revision, and analysis of stratigraphic, paleontological, and lithofacies data on the area of Ordovician deposits in the southern Siberian Platform (Irkut amphitheater), the refined and detailed schemes of biofacies zonation of this paleobasin are substantiated. Schemes of zonation have been compiled for the Nyaian, Ugorian, Kimaian, Mukteian, Volginian, Kirenskian–Kudrinian, Chertovskian, and Baksanian Horizons of the regional Ordovician stratigraphic chart of the Siberian Platform. The schemes present biofacies different in lithologic composition, spread of dominant groups of fauna, and hydrochemical regime (normal-marine salinity, freshwater, or high salinity). It is shown that contrasting changes in paleogeographic environments and the spread of faunal communities under changing environmental conditions were influenced by the transgression–regression cyclicity of the evolution of the Siberian Platform paleobasin and by the proximity of the land. Specific groups of fauna localized in particular facies are described. These groups are regarded as communities that were the first to occupy the littoral zones of epicontinental sea basins in the Ordovician.

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Sea level in the Ordovician: Sharp fluctuations in subsidence rates of the Siberian Craton crust

Cited by 3 publications

References 2 publications

Ordovician sea-level change and rapid change in crustal subsidence rates in East Siberia and Baltoscandia

Ordovician sea-level change and rapid change in crustal subsidence rates in East Siberia and Baltoscandia

Sea level changes and rapid crustal movements in cratonic areas in the Late Paleozoic

Biofacies zonation of Ordovician deposits of the southern Siberian Platform

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