Sequence paleogeography and coal accumulation in epicontinental basin

Based on an investigation of the Benxi-Shanxi formations in the southwestern Shandong province,we studied the forming mechanisms of key stratigraphic boundaries in an epicontinental basin and the coal-accumulating characteristics in the paleogeographic background of isochronous stratigraphic frameworks.In total,we sequenced three types of stratigraphic boundaries(i.e.,regional unconformities,the shifting surface of transgressions caused by basin basement subsidence,and fluvial erosional surfaces including exposed surfaces as well as correlative conformities,which are caused by forced regression)and proposed a model for coal-accumulation.The results show that in the high accommodation space of the barrier coast system in the lower Benxi-Taiyuan formations,the base level was higher than sedimentary boundaries,parasequences were bounded by a marine flooding surface and formed during the semi-cycle of base level falling that was influenced by high-frequency fluctuations of base level,and coal seams were thin with multistoried patterns.In this region,the coal accumulating center was located in the transitional zone of the barrier island and tidal-flat.In the low accommodation space of the fluvial delta in the upper Shanxi formation,the base level coincided with sedimentary boundaries,parasequences formed during the semi-cycle of base level rising were bounded by erosional surfaces and a marine flooding surface,and the coal accumulating center was located in the lower deltaic plain and interdistributary lacustrine environment.In this region,the surplus accommodation space was relatively high.

HIGH-RESOLUTION SEQUENCE DIVISION AND TRANSGRESSIVE EVENTS COAL FORMATIONIN SOUTHEASTERN NORTH CHINA EPICONTINENTAL BASIN

The sea level changes in the Paleozoic epicontinental basin were of high frequency and the transgressive events were episodic.This kind of sea level changes controlled both the basin filling and the coal accumulation within the basin.Coal accumulation in the transgressive process is one of the typical features of the North China epicontinental basin.High resolution sequence analysis to the Permo carboniferous coal series based on the core observation indicates that the epicontintental coal bearing succession can be divided into 1 tectonic sequence (the original type of basin filling), 3 supersequences, 12 third order sequences and about 48 parasequences.The third order sequences are of dual configuration sequences that are composed of transgressive systems tract in the lower part and highstand systems tract in the upper part.The transgressive systems tracts are relatively thin in thickness and have fewer parasequences (commonly 1 to 2), while the highstand systems tracts are thick, generally consist of several parasequences.

Deposition of coal-bearing measures and coal accumulation under marine transgression 'events' in North China epicontinental sea basin

In recent years, studies show that coal formed during marine transigression process is also important in coal accumulation basin. Still, the characteristics of coal formation is more clearly demonstrated by the coal formation in TST in the sequence stratigraphic framework. Based on the study of the characteristics of coal formation in epicontinental coal accumulation basin, the viewpoint that coal was formed under marine transgression was put forward by the authors and the mechenism thereof was also analyized. Marine transgression resulted in the forming of swamp and then the peat swamp. And the transgression later on put the peat formed in advance in deep water environment to form coal in the end. The important stratigraphic horrizons, such as coal seams, marine facies layers, formed with marine transgression are of the property of isochronism.

A Comparative Study of the Coal-forming Characteristics of Marginal Sea Basins and Epicontinental Sea Basins

The coal-forming characteristics,as well as the similarities and differences between epicontinental sea basins and continental marginal sea basins developed during different time periods,were analyzed in this study by adopting comparative analysis thoughts and methods.The results obtained in this study revealed that epicontinental basins and marginal sea basins are both characterized by the main development of thin coal seams or extremely thin coal seams.In addition,changes in sea levels were determined to be the main controlling factors for coal formation,and there were similarities in the continent-sea interactions and coal-forming sedimentary systems of the different basins.However,there were also significant differences observed in the sea level change events,basin basement structural characteristics,coal seam stability levels,accumulation and aggregation characteristics,and the migration patterns of coal-forming materials.For example,the marginal sea basins in the South China Sea were found to be characterized by strong tectonic activities,diversity and complexity.The basin structures showed complex patterns of depressions,uplifts and concave or sag uplifts,which tended to lead to greater complexity in the paleogeographic patterns of the coal formations.This had subsequently resulted in complex coal-forming processes and paleogeographic characteristics,in which the coal-forming zones displayed bead-like distributions,and the enrichment areas and centers were scattered.The practical significance of studying the similarities and differences of the coal-forming characteristics between epicontinental basins and marginal sea basins is that the results can potentially be used to guide the predictions of coal-measure coal seam distributions in South China Sea,as well as provide valuable guidance for future explorations of natural gas reservoirs related to coal measures in the South China Sea area.

EXTENSION OF SEQUENCE STRATIGRAPHIC MODELAND SEQUENCE STRATIGRAPHIC ANALYSIS IN LIMNIC DEPOSITIONAL BASIN

The architectural patterns of sedimentary succession are diverse in different depositionalbasins. The sedimentary architecture and geological condition of such basins asepicontinental sea, intraplate limnic basins, etc., differ cIearly from those of continentalmargin basin. Extension, complement and perfection of sequence stratigraphic models are needed in the studies of various depositional basins based on the classical sequence model. This paper, for this reason,expounds the thought, principles of sequence division, methodology and technology of the study of sequence stratigraphy in epicontinental and limnic basins.