Types of Parasequence and Law of Coal Accumulation of Coal-bearing Sequence in the Paleozoic Epicontinental Sea Basin of Western Shandong Coalfield
摘要: 小层序（Ｐａｒａｓｅｑｕｅｎｃｅ）为Ⅲ级含煤层序的内部构成单元—体系域单元。研究表明，海陆交替型含煤层序可识别出稳定的能作为区域对比的周期性小层序，其顶底界面多为事件型海侵面和泥炭化事件界面。根据小层的成因特点和含煤情况，将小层序划分为：波浪作用为主形成的总体向上变细小层序、潮汐流作用为主形成的含煤小层序（有向上变细、向上变粗又变细两类）、河流作用为主受潮汐流影响形成的具有向上变粗又变细的二元结构的含煤小层序。小层序类型和小层序组的形成与内陆表海海平面变化特点具有十分密切的关系，海水的退却致使沉积体系进入废弃阶段而泥炭沼泽化，标志着小层序发育的终结。因此，研究小层序的形成与研究煤聚积规律具有密切关系。Abstract: Parasequence is a internal composing unit of the third order coal-bearing sequence, namely a system tract unit. The present study shows that stable periodic parasequences that can be used as regional correlation can be recognized in the paralic coal series. The superfaces and subfaces of these parasequences are mostly surfaces of event transgression or peatification. Based on the genetic characteristics and coal bearing conditions the parasequences may be classified into four types: wave controlled upward fining parasequences, tide controlled upward fining coal bearing ones, tide controlled upward coaring and thus fining coal bearing ones, and fluvial controlled tide influenced upward coarsing and thus fining coal bearing ones. The types of parasequences and the formation of parasequence sets are closely related to the sea level change of epicontinental sea. The retreat of sea water and peatification of depositional system mark the end of the growth of parasequences. The research on the formation of parasequences, therefore, is closely related to the study of coal accumulation law. The present research shows that, with parasequences being the basic drawing units within the sequence stratigraphic frameworks of the coal bearing basin, the paleogeography in the coal forming period can be more precisely reconstructed, the nature of peat swamps can be more correctly identified. This method can provide more reasonable scientific basis for the evaluation and prediction of coal resources and their associated mineral deposits. Hence, the classification of parasequence types suitable for the characteristics of coal bearing basin in China is of a new route for the analysis of coal accumulation. The sequence stratigraphy advances the theory and methodology system of the coal geology and provides useful tools for the prediction of coalfields.
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