MIOFLORAL ASSEMBLAGE OF THE SUBSURFACE LOWER GONDWANA ROCKS OF PARMIAN IN PARTS OF MAHANADI BASIN, INDIA
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Abstract:
The Gondwana basins of the Mahanadi River basin, located almost at the heart of India, have a regional NW-SE trend. The principal coalfields falling in the Mahanadi master basin are Korba, Mand-Raigarh and Ib-river. The Gondwana sediments of the Korba Coalfield extend south-easterly through the Mand-Raigarh Coalfield and continue to the Ib-river Coalfield. Of these, the Korba and the Ib-river Coalfields have gathered importance because of large reserves of coal and have witnessed the pulse of mining activity since a long time. Compared to this, the Mand-Raigarh Coalfield was the last to attract attention and remained practically neglected - exploration and exploitation starting much later. On the platform of the above-mentioned background, the palynological history of the subsurface coal and associated sediments of Mand-Raigarh Coalfield is yet to be known as per available information. The present work, including samples from five boreholes, not only records miofloral population of the basin for the first time, but also provides with a definite palynostratigraphic frame to the subsurface Gondwana sediments. The palynostratigraphic columns thus erected may serve as reference columns for the recognition and correlation of the disturbed and sliced coal bearing formations in this sub-basin. Seven miofloral assemblage zones have been delineated for the drill core sediments based on parameters such as, first appearance, relative abundance / scarcity of the palynoforms, dating from the earliest Permian (Talchir Formation) up to the close of Permian (Raniganj Formation). i) Parasaccites - Plicatipollenites assemblage zone (corresponding to Talchir Formation), ii) Sulcatisporites - Brevitriletes assemblage zone (corresponding to lower Barakar Formation ), iii) Sulcatisporites - Rhizomaspora assemblage zone (corresponding to upper Barakar Formation due to presence of minor amount of Densipollenites), iv) Densipollenites - Striatopodocarpites assemblage zone (corresponding to Barren Measures Formation), v) Striatopodocarpites - Densipollenites assemblage zone (corresponding to lower Raniganj Formation), vi) Striatopodocarpites - Densipollenites - Sulcatisporites assemblage zone (corresponding to upper Raniganj Formation) and vii) Sriatopodocarpites - Alisporites assemblage zone (corresponding to uppermost Raniganj Formation to lowermost Kamthi Formation). On the basis of the qualitative miofloral composition, earlier workers have compared the coal bearing formations of the Korba and the Ib-river Coalfields. The present work is a necessary step for filling in the gap between these two coalfields on the west and east, respectively, in understanding the regional palynostratigraphy of the Gondwana sediments in the master basin. Surface morphological features, such as microshrinkage cracks, vertical root and root impressions have been observed in sediments of Raniganj Formation, which are suspected to represent a palaeoso).
