Study of the Late Cenozoic depositional environment and seismic anomalies on the Trøndelag Platform using 3D seismic data

Abstract

This thesis has focused on the depositional environment during the development of the Naust Formation in order to improve our understanding of the glacial history of the mid-Norwegian continental shelf, using 3D-seismic data from the Trænabanken and Sklinnadjupet area along with regional 2D-lines. Seismic anomalies have also been investigated to increase our understanding of fluid migration affecting the Naust Formation. A seismic stratigraphy of the area has been established and correlated to earlier works in adjacent areas, resulting in 5 seismostratigraphic units, N (oldest), A, U, S and T (youngest).

Eleven seismic horizons in different stratigraphic levels have been mapped for geomorphological features, identifying glacial features such as cross-shelf troughs, MSGL, flutes, hill-hole pairs and iceberg plough-marks. These features suggests a dynamic glacial history on the mid-Norwegian continental shelf over the past 2.8 Ma. Iceberg plough-marks have been identified early in Naust N time (1.5-2.8 Ma), which may indicate the presence of calving ice along the Norwegian coast. Morainal deposits along with an increase in deposition of debris flows suggest a change in depositional environment taking place during Naust N time, where glacial processes became more dominant. This may be the first indication of the ice sheet reaching the paleo-shelf edge within the study area. Cross-shelf troughs, MSGL and flutes have been located on the base of unit A, S and T as well as on the seafloor. This suggests the presence of several fast-flowing ice streams in the period from ~1.5 Ma to the last glacial maximum. The Vestfjorden paleo-ice stream appears to have reached all the way down to the Skjoldryggen area during the Elsterian glaciation. However, the buildup of Trænabanken probably resulted in a change in flow pattern during the two last glaciations, where the Vestfjorden paleo-ice stream drained throughout Trænadjupet while Sklinnadjupet was dominated by ice flowing from the east.

The massive loading by the thick glacial deposits of the Naust Formation resulted in polygonal faulting of the underlying Brygge Formation in the study area. This lead to dewatering, which may be the main source of fluids in the area along with fluids migrating through deep-seated faults from deeper sources. Vertical discontinuities in the seismic data suggests vertical fluid flow within pipes or through fractures created by overpressure within the Naust Formation, while some bright spots and paleo-pockmarks indicate that as the fluids encounter permeable layers the flow was freely following the stratigraphic boundaries of the prograding wedges.