SIGNIFICANCE OF HIGH FREQUENCY T-R CYCLES IN PART OF PERMIAN BARAKAR FORMATION, WEST BOKARO BASIN, INDIA

Abstract

The West Bokaro basin is famous for its coal seams. During Permian period these coal beds were deposited and there deposition is affected either directly or indirectly by sea level fluctuations. These sea level fluctuations are experienced as transgressions (shoreline shift towards land) and regressions (shoreline shift towards sea). These transgressions and regressions lead to the change of depositional environment near coastal area. During regression, the depositional environment can become fluvial leading to deposition of thick sandstone beds and during transgression the depositional environment can become low in energy leading to deposition of coal, shale carb shale etc. The present work includes the making of lithologs to signify the repetitive nature of certain depositional trend caused by relative sea level fluctuations. It also includes facies identification using Core photographs and identification of 4th order and 5th order parasequences which again help in correlation of lithologs, providing an overall picture of the underground facies arrangement. The facies interpretation from the core photos showed that in almost all logs the strata between coal seams S4 and S1 is either very coarse grained massive sandstone or conglomerate or pebbly sandstone. From the logs it is noticed that the thickness of coal seams reduce from bottom to top of a litholog in all lithologs. It can also be noticed that total 13 coal seams are present in the area (according to the data) from bottom to top of the lithologs but only 8 of them are common (S2-S9), rest are uncommon. The presence of cyclothems in the logs is the most peculiar thing to notice as some particular beds repeat themselves especially between coal seams S1-S5. The change occur as Coal – Carb shale – shale - massive sandstone – Cross laminated Sandstone – Plane laminated Sandstone – Shale – Coal. This cycle is present in almost all lithologs for Seam S1-S5. This pattern has occurred due to relative sea level change which includes both eustatic sea level change and basin tectonism. In this area, tectonism is present due to the glacial rebound after glaciers melted away from talchir conglomerates. Sequence stratigraphy has been used to divide the lithologs into corresponding parasequences of approximately same time intervals and which ultimately 6 helped in correlation of the lithologs to get a better picture of sedimentary rocks underground. Parasequences are relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces. 4th order parasequences are of approximately 8-10 Ma and of approx 20-30m in length and 5th order parasequences are of approximately 5m and of age less than 1Ma. So, Based on the sequence stratigraphy 14 correlations have been done on strike and dip sections in entire study area and thus an underground model of sedimentary deposition has been sketched.