Evolution of the Oseberg fault-block in context of the northern north sea structural framework

Abstract

The central segment (60°20′–61°N) of the northern North Sea, 170 km in width, demonstrates contrasting basin geometries related 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 Fault-Block of Jurassic origin, ca 20 km in width, is underlain by a wider (ca 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 (Rhaetian–Aalenian) deposition, whereas the Middle–Late Jurassic main rift episode is expressed by an accelerated fault-related subsidence from the Late Bajocian. During the Middle–Late Jurassic the Oseberg area evolved 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 episode into distinct rift phases (4–6 Myrs), which involved smaller scale units corresponding to rotational tilt events (1–2 Myrs). The large-scale pattern of Middle–Late Jurassic syn-rift infill is overall one of upwards fining, reflecting a deepening trend as the rift evolved. Syn-rotational sediment architecture related to a single rift phase shows systematic variations from a threefold, shallow-marine sandstone–mudstone–sandstone motif (Tarbert-Fm.), through a twofold shallow-marine sandstone–mudstone motif (Heather Fm.) to a twofold deep-marine conglomerate/sandstone–mudstone motif (Draupne Fm.). The vertical stacking of these syn-rotational packages reflects an inability of sediment supply to keep pace with the rift-related subsidence, and the temporal evolution from a sediment-balanced through a sediment-underfilled to a sediment starved rift-basin.