Geochemistry of Miocene lacustrine sediments from the Sokolov Coal Basin (Czech Republic)

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• 作者：Kr&iacute ; bek ; Bohdan ; Strnad ; Mojm&iacute ; r ; Bohá ; cek ; Zbynek ; Sý ; korová ; Ivana ; et. al.
• 关键词：geochemistry ; Sokolov Formation ; Miocene lacustrine sediments
• 刊名：International Journal of Coal Geology
• 出版年：1998
• 期刊代码：24_01665162
• 类别：pw
• 出版时间：October, 1998
• 卷：37
• 期：3-4
• 页码：207-233
• 文件大小：900 K

摘要

Sediments of the upper part of the Sokolov Formation of the Sokolov Brown Coal Basin consist mostly of brown lacustrine clays and claystones enriched in organic matter (2–18%TOC). The lower portion of the stratigraphic profile is formed mostly by kaolinite–illite clays and claystones, while the montmorillonite content increases in the upper portion. The change in the lithology of sediments is caused by the gradual erosion of the weathering crust in the source areas of the clastic material. Inversion of the weathering profile during erosion is manifested geochemically in a decrease in the Al₂O₃/Na₂O ratio and an increase in the SiO₂/Al₂O₃, Na₂O/K₂O and K₂O/TiO₂ ratios from the base to the top of the studied part of the Sokolov Formation. The amounts of trace elements (La, Ce, Nb, Zr, Cr, V, Sn) also decrease in the same direction. On the other hand, the amounts of Rb and Sr increase. The organic matter of the upper part of the Sokolov Formation consists primarily of Types I and II kerogen. Only near the base of the studied part of the formation, i.e., in the coal stringer, kerogen of Type III prevails. The extremely low degree of thermal maturity permits determination of the source of the organic matter and characterization of its accumulation environment. The organic material in the sediments is predominantly of algae origin. Relatively high amount of pentacyclic triterpanes of the hopane series indicates either the occurrence of cyanobacteria as primary producers or bacterial reworking during sedimentation and diagenesis. Organic matter accumulated under the conditions of a fresh-water lake or lake with slightly elevated salinity, in a dysoxic aqueous environment. The alternation of organic carbon-rich layers with layers low in organic matter is explained in terms of changes in the degree of dysoxia of the aqueous environment and a variation in the intensity of oxidation and mineralization of the organic material.