This study is based on biostratigraphic analysis of upper Cainozoic strata in eight exploration and production wells from the Tampen area (Snorre and Visund fields), and one well from the Troll field. Dating of the units is based primarily on planktonic and benthic foraminifera. Eleven fossil assemblages have been defined in sediments from the Lower Oligocene to the Pleistocene. In addition, strontium isotope, lithologic and petrophysical log analyses have been performed, and the studied wells have been correlated along regional 2-D and 3-D seismic lines. In the Troll area the Pleistocene rests unconformably on the Lower Oligocene. The upper part of the Upper Oligocene is absent in all the Tampen wells. In the Visund area (block 34/8) there is a hiatus of more than 2 m.y. between Oligocene and Lower Miocene strata, and in the Snorre area (blocks 34/4 and 34/7) there is a hiatus of more than 18 m.y. between Oligocene and Upper Miocene deposits. The Neogene section has been subdivided into five major lithologic units. In the Visund area, a Lower Miocene unit (1) of predominantly
fine-grained, silty sediments has been identified. A major hiatus separates this unit from the overlying Utsira Formation (2), which in the northern North Sea comprises a thick lower part composed of quartzose sand and a thinner upper part of glauconitic sand. The main sands of the Utsira Formation are not present in any of the studied wells, but preliminary results from well 35/11-l indicate a Late Miocene to possible latest Middle Miocene age for this unit. The glauconitic part of the Utsira Formation (Late Miocene to earliest Early Pliocene in age) over Iies the Oligocene strata in the Snorre area and the Lower Miocene deposits in the Visund area. To the east it may drape over the main Utsira Formation sands or partly interfinger with these. It is overlain by a basal upper Pliocene unit (3) consisting of gravity flow deposits. Co res from this unit exhibit icerafted
pebbles and have a glad o-marine affinity. A thick complex of Upper Pliocene prograding wedges (4) downlap the basal Pliocene unit in the Tampen area and the Utsira Formation in the eastern part of the basin. It is unconformably overlain by a Pleistocene unit at the top (5). An important feature of the Neogene succession is a large incised valley/canyon system which developed in a north-westerly direction from block 35/8 (off Sognefjorden) to about 62?N. This erosive system cuts in to the basal Upper Pliocene unit in block 34/3 and is thus much younger than has been previously suggested.