Silurian period

Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land. 

The dependence of the ozone column density upon the oxygen level is a matter of considerable interest for it will fix the degree of UV-B penetration through the atmosphere and hence presumably would establish the surface conditions for the emergence of life onto land from the sea during the Silurian period (Berkner and Marshall, 1965). Recent calculations seem to suggest that the column density of O3 exceeded 10% of its current value even at O2 densities as low as 1% PAL (Levine et al., 1981). This behavior (Figure 1) at relatively low O2 densities is attributed to the increased penetration of actinic radiation capable of O2 photolysis in (3) and the enhanced rate of N2O photodissociation... 

In the Cambrian period life began to develop very quickly. For this reason the oxygen concentration increased rather rapidly. Thus, in the late Silurian (420 millions years ago) the oxygen level was as high as 0.1 PAL (Fig. 3) which is termed the second critical level. With the increase of the oxygen concentration the quantity of ozone in the atmosphere increased, together with an increase in the altitude of maximum ozone production. This latter, in the late Silurian period reached 20 km level, which made the spread of life onto dry land possible. At the same time the thermal structure of the atmosphere was drastically changed, which resulted in the appearance of the stratosphere. It was shown previously that our atmosphere has an... 

Hemerythrin. The iron-containing blood pigment of various marine worms Sipunculus, Phascolopsis) as well as the genus Lingula of the brachiopods dating back to the Silurian period (ca. 400 million years ago) and still present in the oceans today. H. is - in contrast to the heme derivatives - a non-heme iron protein (Mr 108000) made up of 8 subunits, each with two Fe(ll) ions directly bound to the protein. Each subunit... 

Palaeozoic The first era of Phanerozoic time. It follows the Precambrian and is subdivided into the Lower Palaeozoic, comprising the Cambrian, Ordovician, and Silurian periods, and the Upper Palaeozoic, comprising the Devonian, Carboniferous, and Permian periods. It extended from about 542 million years ago to about 251 million years ago, when it was succeeded by the Mesozoic era. 

The geology of the area consists of rocks from three time periods (Fig. 1) Silurian (Kingsclear Group), Devonian (Harvey Group) and Carboniferous (Mabou... 

The relative increase in carbohydrates in the Middle Silurian and Middle Devonian is suggested as resulting from a period of increased organic productivity during the deposition of those rocks. 

Devonian Pertaining to or designating that period of the Paleozoic that follows the Silurian and is next below the Mississippian. 

Coal deposits, usually called beds or seams, can range from fractions of an inch to hundreds of feet in thickness. Coals are found in all geologic periods from Silurian through Quaternary, but the earliest commercially important coals are found in rocks of Mississippian age (Carboniferous in Europe). Coals generally... 

The reservoir rocks that yield crude oil range in age from Precambrian to Recent geologic time but rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian, and Ordovician periods are particularly productive. In contrast, rocks of Jurassic, Triassic, Silurian, and Cambrian age are less productive and rocks of Precambrian age yield petroleum only under exceptional circumstances. 

Paleozoic Era The period of time beginning 570 million years ago ending 245 million years ago falls between the Proterozoic and Mesozoic Eras and is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods. 

As stated above, carbonate rocks comprise —30% of the mass of Phanerozoic sediments. Given and Wilkinson (1987) reevaluated aU the existing data on Phanerozoic carbonate rocks, their masses, and their relative calcite and dolomite contents (Figure 38). It can be seen that, as with the total sedimentary mass (Garrels and Mackenzie, 1971a,b), the mass of carbonate rock preserved is pushed toward the front of geologic time. The Tertiary, Carboniferous, and Cambrian periods are times of significant carbonate preservation, whereas the preservation of Silurian and Triassic carbonates is minimal. 

Figure 5 Changes in atmospheric composition over the Phanerozoic period, from 600 million years ago, based on the models of Robert Berner. Oxygen levels (top graph) reached a peak of 35% in the late Carboniferous and early Permian, before falling to 15% in the late Permian. Oxygen levels peaked a second time at 25 to 30% in the late Cretaceous (K) before falling to present atmospheric levels in the Tertiary (T). Carbon dioxide levels (bottom graph) fell from 0.5% in the Silurian (S) to around 0.03% by the end of the Carboniferous. Reproduced with permission from Graham et al.t and Nature.

The Silurian and Early Devonian organic matter is typically sapropelic, mixed, and humic (Fig. 5.3). It was intensely generating hydrocarbons in the south of the region in the Paleozoic, and in the areas of the other basins in the Mesozoic. At present, the organic matter may presumably persist either at the end of the oil window, or in the gas window and low-molecular hydrocarbons. The radioactive shales of the Early Silurian are good source rocks in the central and north-eastern areas of the region. The Late Devonian and Carboniferous periods have provided satisfactory source rocks in the south-west and north-east of the region. 

By the end of the Late Carboniferous period, when the sedimentation cycle was entering its final stage, the highest paleotemperatures were recorded in the south and south-west of the region, reaching 80 °C at the top and 125 °C at the bottom of the Gedinnian sediments. The lowest temperatures were observed in the north of the region 80 C at the Ordovician bottom and 50 °C at the Silurian bottom. By the end of the Cenomanian age, the paleotemperatures rose from 100 to 125 °C at the Silurian bottom and from 80 to no °C at the Devonian. At the present time, the temperature remains at the same level, except for the north-western and south-western areas where it reaches 130-150 °C. 

Fig. 6.8. a Temperature and b maturation histories of the Silurian source shales in the Takhoukht region computed for the main variant with erosion in Fig. 6.2 solid lines) and for the variant without erosion in Fig. 6.7 dashed lines). The temperature of Silurian rocks did not exceed 85 C during the preerosion period and, therefore, the present-day maturation level does not differ in either variant... 

Slow, continuous sedimentation during the period beginning in the Permian resulted in minimal variation in isotherm depths and in the depths of isolines (Fig. 6.17) in the Mereksen area. Our calculations of temperature and organic matter maturity in Ordovician, Silurian, and Devonian rocks at the close of the Carboniferous (approximately 288 Ma) are shown in Table 6.7. These calculations show that at the beginning of the Permian, organic matter in Ordovician and Silurian shales was in the lower part of the oil window (Fig. 6.17). 

It is followed the geological time scale as determined by the International Commission on Stratigraphy (ICS). The ICS has not finished its job and gaps remain, particularly in the Early Paleozoic. Where gaps occur, it is generally followed the Russian system for the Cambrian and the British system for the Silurian. Epochs are subdivided further into ages not listed here. The periods from Cretaceons and older are subdivided into epochs and ages not shown here. 

Silurian A geological period of the Palaeozoic era following the Ordovician period and extending until the beginning of the Devonian period. It began about 444 million years ago and lasted for about 28 million years. 

In general, the greater the geological age of the cmde oil, the higher is its aromatics content (Figure 3.30). Cmde oils from the Tertiary, Silurian and Cambrian periods have especially high levels of aromatics, while Permian, Carbonaceous and Devonian oils are distinguished by very low levels of aromatics. 

According the test result, the Marine shale in Sichuan Basin, with high Maturity, is under thermal pyrolysis period. The Ro of Lower Silurian shale in Sichuan Basin ranges from 1.3%-3.6%. And the Ro of lower Cambrian shale varies between 2.3% and 4.2%. 

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