The Division of High- resolution Sequences and the Transgressive CoalFormation in the Epicontinental Basin of the Western Shandong Province

Key words: 
• high-resolution sectuence stratigraphy /
• coal formation in the transgressive Prrocess /
• base一level cycle /
• Western Shandong Province

Abstract: The high一frequency sea level changes in the North China epicontinental basin from the lzte Carboniferous to Early Permian are the main factozs controlling the basin一filling and coal accumulation. The mal一bearing succes- lion, therefore, has distinct cycles. The sea level changes in the North China epicontinental basin were composite sea level changes. The long一term。level changes are superposed访shorter一term cycles, the mid一term and shop，一term sea level changes. There were short一term reti,ogressive events in the process of long term transgres- live period, and multiple short periodic transgreSSive events superposed an the long一term transgressive period. Study shows that the base一level cycles in the North Ghina epicontinental basin may be substitute the relative sea level cycles, because the high一frequency sea level changes in the basin are the integrative results of the sea lev- e1 changes, the tectonin subsidence, the climate changes and the sediment supply. The later three factors, the tec- tonics, the climate and the sediment supply~relatively stable. The sea level changes, thus, become the domi- nant factor. Using the thought and methodology of high一resolution sequence stratigraphy, the authors investigate the characteristics of the high一resolution sequence stratigraphy of the coal measures in the Western Shandong, identi- fy and divide the mid一long term and mid一short term Ease一level cycles. Three long一term base一level changes lead to the formation of three third order sequences. The mid一term base一level cycles are approximately oorre- sponded to the systems tracts. Large一scale marine flooding events were caused by the mid一term base一level rise. Because of the hysteresis of the deposition, the thick一bedded limestanes were depceited in the period of maximum base一level rises, which occurred three times in the history of the epicontinental basin, representing the depositions of three times of maximum base一level rises. The outstanding feature of the paralic succession is the alternative oc- currences of the marine and terrestrial depositions, forming multiple cycles. That the marine layers directly super- posed on the malheds is another feature of the deposition in the epicontinental basin. Eighteen short一term base一 level cycles may be recognized in the epiwntinental succession of the Western Shandong. Most of cycles are asym- metric, seperated by episodic transgressive event surfaces, peatification event surfaces and large swpe of scouring surfaces. The short一term base一level cycles are of high frequency. The identification and come王ation of short一terrrt base一level cycles help to the division of high一resolution se- quence. The study of the epicontinental paralic base一level cycles in the Western 5handong indicates that the eigh- teen short一term Ease一level cycles arc corresponded to eighteen fifth一order sequences corresponding to the parasequences.The episodic transgressive events have great effect on the coal accumulation in the epicontinental basin; coal formation in the transgressive process is the outstanding feature of the coal formation in the Western shandong coalfields. The transgressive coalbeds in the long一term base一level cycles develop in the transgressive systems tracts, while the transgressive ooalbeds in the short一term base一level cycles occur in the high stand sys- terns tracts.