Mid Miocene – Early Pliocene depositional environment on the northern part of the Mid-Norwegian Continental Shelf

Abstract

Based on the study of 2D seismic data, this thesis have focused on the depositional environment during the deposition of the Kai formation (Mid-Miocene – Early Pliocene) on the Mid-Norwegian continental margin, in order to increase our knowledge of the evolution of the paleo-environment in the time-period right before the development of the large Northern Hemisphere ice sheets. Based on a seismic stratigraphic analysis, correlated to selected well logs, the deposits comprising the Kai formation were divided into seismic sub-units. The stratigraphy of the formation and the sub-units, as well as the geometry of multiple paleo-sea-floor surfaces have been described and discussed in relation to the development of the ocean circulation pattern in the Norwegian Sea during this time.

The study area were subdivided into an inner- (Trøndelag Platform) and outer (Vøring Basin) part of the continental shelf. The Kai formation is dominated ooze sediments in the deeper basins, and mainly clayey sediments on the inner shelf. Multiple anticlinal highs and structures can be observed within the study area. Based on observations near the flanks of these highs, it is evident that the highs have played a larger role in the distribution and flow pattern of ocean currents under the deposition of the Kai formation. The largest high is the Helland-Hansen Arch, which separates the Kai formation on the inner and outer shelf by an area of non-deposition, located on top of the arch. The relation between the Kai formation and the Molo formation have been included in the study, with alternative interpretations of an undecided sediment package located between the Brygge and Molo formations proposed.

Contouritic deposits, interpreted to be mounded elongated contourite drifts with associating moat structures, were observed within the Kai formation on both the inner and outer part of the shelf, suggesting ocean current activity in the Vøring basin, as well on the Trøndelag Platform. The contourite drifts suggest a north/northeast ocean current pattern, entering the study area from the south, and exiting in the north/northeastern part of the study area. Multiple generations of contourite deposits were found, some separated by unconformities, suggesting changes in current strength and –pattern, as well as possible sea-level oscillation.

Polygonal fault systems have been observed within the Kai formation in the Vøring Basin. The polygonal faults occur in connection with the expulsion of fluids within the ooze sediments in the Vøring Basin, as well as overpressure from the overlying Naust formation.