Paleogene tuffaceous intervals, Grane Field (Block 25⧸11), Norwegian North Sea: their depositional, petrographical, geochemical character and regional implications

Abstract

Paleogene volcaniclastic rocks from four wells from the Grane Field (Block 25⧸11), Southern Viking Graben on the western flank of the Utsira High (North Sea), have been studied. The tuffaceous intervals consist of the volcanic phase 1 (58–57 Ma) in the Våle and Lista Formations, and the volcanic phase 2 (55–52 Ma) in the Sele and Balder Formations. Tuff beds of phase 1 are interpreted as having been redeposited by turbidity currents, whereas those of phase 2 reflect normal settling of fall-out pyroclastics in water. Two types of fragments occur, the most abundant being vesicular pyroclasts with irregular shape and dark, microlithic, partly vesicular granules, representing shallow submarine, Surtseyan-type eruptions. The other type of pyroclastic fragments are vitric shards, which originate by quenching granulation during subaqueous eruptions. The tuffaceous intervals have been subdivided into (1) identified pyroclastics based on textural interpretations, and (2) assumed pyroclastics based on geochemical interpretations. The majority of the samples are sub-alkaline basalts and basaltic andesites. The phase 1 tuffs evolved from basalts to rhyolites upwards in the Lista Formation, and phase 2 tuffs consist of sub-alkaline basalts and basaltic andesites of within-plate origin, similar to the contemporaneous Lower Basalts in East Greenland, the Rockall Trough and the Middle Series on Faroes, all linked to the opening of the North Atlantic Ocean.