Base Pliocene velocity inversion on the eastern Vøring margin— causes and implications

The Plio-Pleistocene wedge off mid Norway is characterized by a pronounced increase in sonic velocities versus depth. At its base, a velocity inversion of 0.55 km s−1 exists with respect to the underlying unit. We propose that the inversion may be a result of the stage margin evolution, in which both the pre-depositional physiography of the margin and the depositional history of the wedge itself have been key elements. The glaciation of the uplifted mainland and the cyclic advancement of glaciers over the shelf led to the formation of the Plio-Pleistocene prograding megasequence. Underlying sequences may have been overpressured due to the rapid deposition of the wedge and because shales at the base of the wedge, deposited in a distal deltaic environment, represented a seal for water drainage. Overpressure provides sediment instability, and is an attractive mechanism to explain intra-sequence deformation and partly lateral mass movements.