BASE PLIOCENE VELOCITY INVERSION ON THE EASTERN VORING MARGIN - CAUSES AND IMPLICATIONS

The Plio-Pleistocene wedge off mid Norway is characterized by a pronou nced increase in sonic velocities versus depth. At its base, a velocit y inversion of similar to 0.55 km s(-1) exists with respect to the und erlying unit. We propose that the inversion may be a result of the lat e stage margin evolution, in which both the pre-depositional physiogra phy of the margin and the depositional history of the wedge itself hav e been key elements. The glaciation of the uplifted mainland and the c yclic advancement of glaciers over the shelf led to the formation of t he Plio-Pleistocene prograding megasequence. Underlying sequences may have been overpressured due to the rapid deposition of the wedge and b ecause shales at the base of the wedge, deposited in a distal deltaic environment, represented a seal for water drainage. Overpressure provi des sediment instability, and is an attractive mechanism to explain in tra-sequence deformation and partly lateral mass movements.