General Organic Features in the Phanerozoic

Sea level changes can be reliably described for the last 600 x 106 yr. Using data on the total area of inundated territories of the present-day continents and considering possible transformation of the hypsographic curve of the Earth's surface (Figure 4), it is possible to reconstruct sea-level fluctuations during the Phanerozoic. The results reveal the rising trend of the sea level for almost a billion years with the average rate about 0.5 m 106yr. The general tendency was, however, complicated by significant transgressions and regressions:

Thus, the Vendian period (c. 600 x 106 yr BP) is usually regarded to be a geocratic era, during which the sea level was rather low. In this period, the total area of the oceans could exceed the present-day one by c. 32 x 106km. With the corresponding volume of all oceans taken as

1352 x 106km3 and due account of its gradual increase as a result of degassing, the average depth should be about 3.44 km (Figure 4). It is worth noting that possible changes of water-exchange processes at the Earth's surface could cause sea-level fluctuations of rather high amplitude (about 100 m) and shorter period, as evidenced by the traces of the Early Cambrian glaciation.

The highest sea-level rise took place in the Ordovician (c. 500 x 106yr BP). The most extensive sea transgression in the Earth's history marked by the marine sediments reached its maximum in the middle of the period. More than 72 x 106km2 of present-day continents, or 50% of land, has been flooded. During the Ordovician transgression, sea-level rise was probably more than 250 m and its rate amounted to 8 m 10_ yr. More than 83% of the total surface of our planet was under water. Average depth of the oceans declined to 3.12 km, probably due to reduced size of the oceanic basins.

The end of the Ordovician was marked by sea regression during the Taconian phase ofthe Caledonian orogeny. The sea level rapidly became more than 200 m lower and the area of the oceans declined to about 391 x 10 km2. According to geological data, during the Caledonian orogeny, the transition zones of the oceans could be transformed into young platforms, which adjoined the ancient shields, thus increasing the land area. A tendency toward the consolidation of separate continental shields in Central and Southern Asia was also obvious.

Figure 4 Hypsographic curve showing sea-level fluctuations during the Phanerozoic.

Global paleogeographic and stratigraphic data suggest that after the Caledonian (Taconian) orogeny within Europe and Northern America, the Devonian should be characterized by the general transition from epicontinental marine conditions to continental ones. Old red sandstone was accumulated within intermontane depressions of Scotland, Asia, and Northern America. On a boundary of Siluric and Devon periods, maximum of a significant transgression fell on the turn of the Silurian and the Devonian (c. 395 x 106yr BP).

During the epoch of Hercynian orogeny, further transformation of continental margins took place, resulting probably in the consolidation of Europe and Asia and the expansion of other continents. Increase of the land area and reduction of the area of oceans in combination with the ongoing input of juvenile water from the mantle could cause the deepening of the World Ocean.

In the Early Carboniferous, a significant part of the east European platform was drained to become a wetland with numerous lakes and rivers where coal beds were formed. The accumulation of coal-bearing strata under the particularly humid climate of the Carboniferous was interrupted by the Hercynian orogeny. During the next Perm-Triassic epoch, the new red continental facies became widespread from New Jersey to Tasmania. The traces of continental glaciation of that time are found in Antarctica, India, Australia, and Southern America.

During the Saalian phase of the Hercynian orogeny (the Perm), a new cycle of transgressional sea-level rise occurred with mean rate up to 8.5 m 106yr have been revealed. The transgression had ended about 240 x 106yr BP when the area of oceans totaled about 411 x 106km2.

The end ofthe Perm and the beginning ofthe Triassic was a geocratic period accompanied by regression of the sea from about 32 x 106km2 of land area. During the Triassic, there were only minor fluctuations of sea level, which was generally rather low.

The onset of the Jurassic was marked by another sea transgression, which coincided with the initial phase of the Alpine tectonic and volcanic epoch. The sea level was on the rise for c. 40 x 106 yr with a different intensity (2-6 m 10-6 yr). The second half of the Jurassic was the time of the ocean regression, which continued till the beginning of the Cretaceous. The sea level lowered by 100-120 m, which could be partly explained by possible accumulation of compatible amount of water in the inland drainage water bodies and epicontinental seas. In the Late Devonian (370350 x 106yr BP), the area of such seas, including geosynclinal ones, could amount to 50-60 x 106km2. The larger parts of the Russian Plain, western Siberia, and the Far East were marine basins at those times, and the geosynclinal seas occupied Southern Europe, Kazakhstan, the Urals, and northeastern Asia. Continental regime was characteristic to the extreme northwest of the Russian Plain, several uplands in its central part, as well as middle Siberia.

During the Cretaceous, the second largest transgression of the Phanerozoic took place. The Cretaceous transgression, the probable maximum of which took place at 90-97 x 106yr BP, was accompanied by sea-level rise up to 150 m. About 38 x 106km2 of land area was inundated, that is, 36% of the present-day land. The total area of the oceans was approximately 415 x 106 km2, or 81% of the Earth's surface (Figure 5).

In the last quarter of the Cretaceous period, another sea regression began (c. 100 m), which was interrupted by a short and rather small transgression in the Early Paleogene and then proceeded approximately to the Middle Paleogene. This regression distinctly coincided with several phases of the Alpine orogeny.

Distribution of sedimentary rocks of marine origin on the continents illustrates the fact that the sea level fell more than 150 m since the Cretaceous. The rate was about 1.5 mm 10-3yr. This suggests the progressive deepening of the oceanic depression, which increased its volume by nearly 0.5 km3 yr-1 despite rather high rates of marine sedimentation.

Investigations of the sea-level dynamics in the geological time show a rather close correlation between its pronounced fluctuations and tectonic and igneous processes. As a rule, the regressions of the oceans occurred during the orogenic periods, which led to the significant reorganization of the Earth's surface, increased relief contrasts, and thus augmented the volume of oceanic depressions.

At the same time, the general tendency of land area growth for the last 600 x 106yr developed. Investigations of the probable sea-level fluctuations during the Phanerozoic indicate that according to the generalized geological data the maximum rate of regression could exceed 10 x 10-6 myr-1. Thus the volume of oceanic depressions could increase by up to 4km3yr-1, on average by 1.1 km3 yr-1. At the same time the accumulation of sediments could reduce the intensity of this process.

According to calculations, the mean rate of transgressions typical for inter-orogenic periods of the Phanerozoic could be as high as 14 m 10-6yr, averaging about 4 m, or 4 x 103mmyr-1. High rates of the sea-level rise could be to a large extent the result of sedimentation.

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