R. Boe et al., ELONGATE DEPRESSIONS ON THE SOUTHERN SLOPE OF THE NORWEGIAN TRENCH (SKAGERRAK) - MORPHOLOGY AND EVOLUTION, Marine geology, 146(1-4), 1998, pp. 191-203
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.