ELONGATE DEPRESSIONS ON THE SOUTHERN SLOPE OF THE NORWEGIAN TRENCH (SKAGERRAK) - MORPHOLOGY AND EVOLUTION

Multibeam echosounder surveys conducted on the southern slope of the N orwegian Trench between Norway and Denmark, show that elongate depress ions interpreted as current-modified pockmarks are widespread in water depths of 100-500 m, in the Holocene-Recent succession. The long axes of the depressions are oriented at an angle of about 15 degrees downs lope, relative to the depth contours. The long axes appear to almost e xactly parallel the effective bottom currents in this part of the Skag errak. The present current directions were probably established about 4000 years ago, following a major shift in the oceanographic circulati on pattern. The most characteristic feature of the depressions is thei r large size, with maximum depths of 45 m, widths of 400 m and lengths of 2 km. Strong bottom currents continuously transport particles thro wn into suspension by the seepage process away from the pockmarks and prevent further deposition. Continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high se dimentation rates are thought to be responsible for the exceptional si ze of many depressions. The shallow gas probably has a mixed biogenic/ thermogenic origin. however, the thermogenic component is considered t o be most important for the formation of pockmarks. Seepage from perme able layers in the Jurassic-Cretaceous bedrock is evidenced by linear rows of depressions above these horizons, while pockmarks above sub-su rface faults indicate gas seeps also from there. In shallow-water area s, the shape and size of pockmarks are locally dependent on their loca tion with respect to giant sand waves, which trend sub-parallel to the depth contours and are interpreted to be of late Weichselian to early Holocene age. In deeper-water areas, the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls. (C) 1998 Elsevier Science B.V. All rights reserved.