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 Firkanten Formation, has been the subject of study for researchers in the past but little focus has been on the northeast 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 years conducted core drilling in key locations in basin, and for decades, they have given an access to their core material to researchers. Drill cores and laboratory results from SNSG is used for sedimentological and geochemical investigation for 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 a complementary 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 coal seams. Furthermore, laboratory results from SNSG, containing ash and sulphur results from the coal seams, 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 of Firkanten Formation. Two new coal seams, one from the top of Todalen Member and another from the uppermost Endalen Member of the formation are presented with detailed description and names to distinguish them from other seams. Results show that the Firkanten Formation is deposited in coastal plain to shallow-marine setting and significant lithological differences are observed in the two areas despite their relatively short distance from each other, this is especially noticeable in the Todalen Member where there are coal deposits of higher quality and greater thickness and relatively thicker foreshore to backshore deposits in Operafjellet than in Breinosa. The overall trend of much thicker deposits in Operafjellet suggest a relatively greater accumulation 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 with interfingering upper-shoreface deposit, and in same member further south in Breinosa, there is evidence of deposition on foreshore to proximal lower shoreface. The overlying Endalen Member is suggested to be deposited on lower to uppers shoreface with a small regression at the top of the member allowing for backshore 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 of the study area. This indicates a general NW-SE orientation of the coastline and sediment input from the north/north west.