Triassic depositional systems of the norther North Sea – an alternative interpretation from cores

Since the early exploration of the northern North Sea, Triassic sediments have been assigned to terrestrial depositional environments. Evaluation of core data from approximately one hundred wells in the Norwegian and UK sectors of the northern North Sea (58 o–62o N), has challenged this interpretation. The sediments of the Smith Bank and Teist formations filled the deepest, syn-rift parts of the basin which in Early Triassic time were the areas closest to the Norwegian mainland, after which the centres of major subsidence shifted farther west. The mudstones of the Smith Bank and Alke formations formed as subaqueous, distal, fine-grained sediments in front of the proximal sand systems of the Lunde and Skagerrak formations. The latter accumulated as two major clastic wedges of deltaic sands derived from major valley systems emerging from the Sognefjord and Hardangerfjord/Boknfjord areas. Standing water was present during progradation of the depositional cones from the east. The depositional pattern was: alluvial to deltaic plain, delta front, subaqueous. Smaller volumes of sediment were shed off the western margin of the basin. The validity of the direct and indirect indicators of standing bodies of water, such as the rare marine and brackish-water fossils, laminae of micritic carbonate, clasts of finely laminated micritic limestone, subaqueous mass flows, wave ripples, intervals of mudstone with high GR readings correlated over semi-regional and regional areas, pyrite, possible submerged lacustrine sand flats and sabkha deposits, as well as the marine incursions into the central part of the basin during deposition of the Lunde and Skagerrak formations, are discussed.