Late Cenozoic sedimentary outbuilding offshore mid-Norway : sequence stratigraphic analysis

Abstract

Abstract

Cenozoic sedimentary successions along the passive continental margins of the Atlantic Ocean are of central importance in petroleum exploration. The aim of the present study has been to improve the understanding of the dynamics of the Plio-Pleistocene depositional system of the mid-Norwegian continental shelf. This study includes seismic stratigraphic interpretation of over 200 multi-channel 2D regional seismic lines followed by seismic sequence analysis and seismic facies analysis.

Seven seismic sequences together with five seismic facies have been identified and mapped regionally in an area ranging from 610 to 680N, on the basis of reflector terminations and internal reflector configuration within IMain CharismaTM. Three seismic surfaces including Local Downlap Surface (LDS), Regional Downlap Surface (RDS) and Upper Regional Unconformities (URU) are key surfaces mapped along with several internal unconformities in the study area. Chronostratigraphic charts (Wheeler diagrams) have been constructed along two selected seismic sections to obtain better insight into the time relationships of the depositional systems, and their relationships to surfaces of non-deposition, condensation and erosion.

On the basis of seismic sequence analysis the evolution of the Late Cenozoic sedimentary prograding wedge in the northern North Sea and off mid-Norway has been divided into three phases: (1) Initial phase marks the onset of sediment progradation with formation of sequence SS 1; (2) Main phase of large scale sediment outbuilding, SS 2 – SS 6; and (3) Final phase of regional erosion followed by aggradation in SS 7. The ages for the seismic surfaces have been adopted partly from earlier studies and partly assigned using relative age criteria, as there exist no well and biostratigraphic data to give exact ages. Seismic sequence 1 seems similar to the Molo Formation in northern part of the study area and the Utsira Formation in the southern part of the study area both stratigraphically and geometrically, but the exact ages of these three correlated units are not known.

The Plio-Pleistocene prograding wedge succession in offshore mid-Norway is interpreted as the response of a complex interplay between a set of diverse controls: tectonic tilting and climate; sediment supply, primarily glacially derived; sediment transport mechanisms involving downslope and alongslope current activity; and deep and shallow water erosional and depositional processes.