EVOLUTION OF THE OSEBERG FAULT-BLOCK IN CONTEXT OF THE NORTHERN NORTH-SEA STRUCTURAL FRAMEWORK

The central segment (60 degrees 20'-61 degrees N) of the northern Nort h Sea, 170 km in width, demonstrates contrasting basin geometries rela ted to Permo-Triassic and Jurassic rift episodes. In the Oseberg area, representing a part of this segment, major faults of two generations dip in opposite directions. As a consequence the E-tilted Oseberg Faul t-Block of Jurassic origin, approximately 20 km in width, is underlain by a wider (approximately 35 km) W-tilted Permo-Triassic half-graben. The Early Jurassic thickness distribution was governed by a westerly palaeotilt. Mild extensional tectonics influenced Early Jurassic (Rhae tian-Aalenian) deposition, whereas the Middle-Late Jurassic main rift episode is expressed by an accelerated fault-related subsidence from t he Late Bajocian. During the Middle-Late Jurassic the Oseberg area evo lved through a series of high-frequency rift phases, characterized by rotational tilt events separated by periods during which extension was less severe. This allows the subdivision of the Jurassic stretching e pisode into distinct rift phases (4-6 Myr), which involved smaller sca le units corresponding to rotational tilt events (1-2 Myr). The large- scale pattern of Middle-Late Jurassic syn-rift infill is overall one o f upwards fining, reflecting a deepening trend as the rift evolved. Sy n-rotational sediment architecture related to a single rift phase show s systematic variations from a threefold, shallow-marine sandstone-mud stone-sandstone motif(Tarbert-Fm.), through a twofold shallow-marine s andstone-mudstone motif(Heather Fm.) to a twofold deep-marine conglome rate/sandstone-mudstone motif(Draupne Fm.). The vertical stacking of t hese syn-rotational packages reflects an inability of sediment supply to keep pace with the rift-related subsidence? and the temporal evolut ion from a sediment-balanced through a sediment-underfilled to a sedim ent-starved rift-basin. (C) 1998 Elsevier Science Ltd. All rights rese rved.