Variations in depositional environments of the Paleogene Firkanten Formation across Adventdalen, from Operafjellet to Breinosa
Sammanfattning: The lowermost Paleogene clastic infill of the Central Spitsbergen Tertiary Basin, the coal bearing FirkantenFormation, has been the subject of study for researchers in the past but little focus has been on thenortheast edge of the basin until now. Due to the economic potential of the coal seams in the formation,the Norwegian mining company, Store Norske Spitsbergen Grubekompani (SNSG), has for several yearsconducted core drilling in key locations in basin, and for decades, they have given an access to their corematerial to researchers.Drill cores and laboratory results from SNSG is used for sedimentological and geochemical investigationfor the purpose of comparing the depositional environment in the Operafjellet and Breinosa mountains,to give a better understanding of the depositional setting at the northeast edge of the basin.Six cores from each mountain were chosen as they characterized the overall lithology of the area, with acomplementary field log and observations from Operafjellet mountain. All cores were logged in detail(1:20), and special attention was put on the coal where lithotyping was done on cores with intact coalseams. Furthermore, laboratory results from SNSG, containing ash and sulphur results from the coalseams, were used to support correlation and to investigate the quality of the coal at the basin edge.In the past, 5 coal seams have been recognised in the Todalen Member, the lowermost member ofFirkanten Formation. Two new coal seams, one from the top of Todalen Member and another from theuppermost Endalen Member of the formation are presented with detailed description and names todistinguish them from other seams.Results show that the Firkanten Formation is deposited in coastal plain to shallow-marine setting andsignificant lithological differences are observed in the two areas despite their relatively short distancefrom each other, this is especially noticeable in the Todalen Member where there are coal deposits ofhigher quality and greater thickness and relatively thicker foreshore to backshore deposits in Operafjelletthan in Breinosa. The overall trend of much thicker deposits in Operafjellet suggest a relatively greateraccumulation rate and accommodation space at the edge of the basin.The Todalen Member in Operafjellet is suggested to be deposited on a backshore tidal flat withinterfingering upper-shoreface deposit, and in same member further south in Breinosa, there is evidenceof deposition on foreshore to proximal lower shoreface. The overlying Endalen Member is suggested tobe deposited on lower to uppers shoreface with a small regression at the top of the member allowing forbackshore tidal flat deposits with peat accumulation in the top of the formation in Operafjellet.Facies association distribution and unit thickness along with sulphur and ash conserved within the coal,strongly supports increased marine influence (within the coal) and deeper marine facies in the south ofthe study area. This indicates a general NW-SE orientation of the coastline and sediment input from thenorth/north west.
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