CONDITIONS OF FORMATION OF SEDIMENTARY THICKNESS
Definition of concepts facies & quot ;, facies analysis
Accumulation of precipitation, in which hydrocarbons may occur, occurred under certain physical and geographical conditions.
Regular alternation of rock complexes allows us to judge the periodic change in the conditions of sedimentation and the general direction of changes in these conditions at different periods. To express the change in the composition of the deposits of a certain stratigraphic segment on the area of its distribution, depending on the conditions of sedimentation, the term "facies" was introduced into geology.
Currently, there are more than 100 different definitions of the term "facies". Most researchers invest in the concept of "facies" the unity of the genetic type of rocks and the environment of its formation. The most capacious and concise is the definition proposed by N.V. Logvinenko : "Faction is an atmosphere of sedimentation, modern or ancient, embodied in sediment or rock."
By the physico-geographical conditions (environment) is understood all the conditions and nature of the sedimentation environment, for example: subaerial or subaqueous environment; confinement to some geomorphological elements of the land; the nature of the basin (lake, lagoon, sea) and its probable depth; position in a certain part of the basin (coastal, on the open shelf, bathyal, in a stagnant zone, etc.); remoteness from the coastline; dynamics of the environment; conditions of life and burial of organisms, etc.
In general, the conditions of sedimentation are determined by the relief, climate, tectonics and the features of the development of life on Earth in this period. The geology section that considers the physical and geographical conditions of sedimentation is called facies theory, and the ways of reconstructing these environments for past periods in the history of the Earth are called facies analysis.
In the facial analysis, the method of actualism is widely used. This method of scientific knowledge of the geological history of the Earth, the reconstruction of processes and the environment of the past by using the regularities revealed in the study of modern geological processes.
Tectonics and sedimentation
Among the factors that determine the conditions for the formation of sedimentary rocks and the patterns of their formation, the leading position is occupied by
vertical movements of the earth's crust of a variable sign and varying amplitude, or oscillatory motion.
The leading role of tectonics in the lithogenesis process is reflected in numerous works of scientists [17, 35, 12, 36, etc.]. In the summary of B.K. Proshlyakov and VG Kuznetsova , the significance of tectonic movements for the conditions of formation of sedimentary strata amounts to the following.
Oscillatory movements cause transgression and regression of marine waters and, consequently, movement of shorelines. Together with the change in the position of the shore, the composition of the sediment changes. For example, the accumulated strata of sandy sediments with transgression are replaced by argillaceous silt deposits. Vibrational movements can lead to the formation of shallow water reservoirs in which, with intensive evaporation, terrigenous sedimentation can be replaced by salt formation.
Oscillatory tectonic movements within the land lead to a change in the location of the demolition area, the change in the basis of erosion, which in turn affects the composition of the accumulated sediment.
Tectonic movements have a profound effect on precipitation accumulation rate and their power. This influence is effected through the directionality and velocity of vibrational movements, the duration of the stable state of the tectonic regime, the surface relief of the sedimentation basin (land or bottom of the reservoir), direction and speed of water flows.
The rate of accumulation and the thickness of the sediments depend to a large extent on the amount of incoming clastic material. In cases where its number is insignificant, the deflection is not able to provide a high rate of accumulation and power of the sediment. If there is an abundant supply of sedimentary material that exceeds the required amount to compensate for deflection, there will be a shallowing of the basin and a change in the conditions of sedimentation, and ultimately accumulation may be replaced by denudation. Maximum capacity and sedimentation rates in large water basins are typical for compensated deflection areas (depressions, troughs). With the regional long-term immersion of the territory, powerful, huge sedimentary strata are formed.
Tectonic mode largely determines the shape and the size of the sedimentary bodies. Sedimentary rocks are mostly in the form of layers and layers of considerable length, with relatively parallel surfaces. In this case, between the terms layer and layer & quot ;, according to N.V. Logvinenko , there is a certain difference.
Plast - a slab-like body of sedimentary rock, representing the unit of subdivision of the section by the material composition (clay layer,
sandstone, limestone). The formation of beds is associated with the vibrational movements of the earth's crust, with the shifting of the basin of the basin, the change in the basis of erosion.
Layer - part of the formation. It is formed under the influence of local factors: the effects of currents, disturbances, composition and intensity of the introduction of detrital material, the periodicity of the precipitation of matter from the aqueous solution, the vital activity of organisms.
Tectonic oscillatory movements are one of the main causes of the layered structure sedimentary strata, alternating between rocks of different composition. The boundary between the layers can be expressed quite clearly, this means that the change in one sedimentation environment of another was accomplished quite quickly, that is, the vibrational tectonic movements did not occur smoothly, but intermittently, with pauses accompanied by stabilization of sedimentation environments when the layer consists of one layer.
Tectonic movements also influenced the post-sedimentation transformations of sedimentary rocks. The zones of faults with the development of intensive fracturing within their boundaries were channels, along which hot mineralized solutions came from deep horizons, thanks to which a variety of processes of new mineral formation, metasomatism and recrystallization occurred.
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