Pockmarks are a ubiquitous feature on the seafloor of the Norwegian Channel, including the Troll area where they exist despite the absence of evidence of free gas or gas migration. Gas concentrations in the pore water do, however, increase in sediments older than those deposited from the last glaciation. Detailed mapping by remotely operated vehicle (ROV) based multibeam echo sounding and sub-bottom profiler of some selected pockmarks has provided the basis for a sampling and analysis programme. ROV video inspections revealed a lag deposit of gravel and the presence of methane-derived authigenic carbonates within some of them. The radiometric analyses indicate formation about 11 Cal kyrs ago, whereas geotechnical data indicate formation by erosion of the overburden.
From reconstructions of the pressure and temperature history based on both sedimentation history and sea level, this paper proposes that gas hydrates accumulated in the varied lithology of the morainic sediments from the penultimate glaciation during the last glaciation. At the end of the last ice age, about 11 Cal kyr ago, the hydrates decomposed releasing gas that fractured the overlying sediments and escaped to form pockmarks and the authigenic carbonates found within them. Pockmarks asymmetry indicates that they may have been kept open by current activity after the gas was depleted.
The Norwegian Channel (Figure 1) was one of the first areas in which pockmarks were discovered1, 2, 3, 4, and their formation was interpreted as the result of the expulsion of fluids through the seafloor5, 6. Although gas escape has been documented in some North Sea pockmarks7, no such activity has been discovered in the Norwegian Channel? an area extensively surveyed in connection with the hydrocarbon industry? despite their ubiquitous occurrence there. Interest in these pockmarks dwindled due to their lack of activity, but was renewed in connection with the studies of the pressure development at the Troll A platform8 and its possible relationship to natural fluid flow in the area. Several questions concerning pockmarks were posed, the most important being the question of timing: When were they formed, and are they still being formed? Could they be associated with bad weather when no surveys are performed? Another key question is how were they generated. Are they a result of pore water or gas expulsion, or perhaps even due to phenomena on the seafloor? This paper presents background information, the investigations performed and interpretations pertaining to these questions.
The Troll area lies on the western zone of the area influenced by the Tertiary uplift9 of the Norwegian mainland. The sub-horizontal Pleistocene sediments are separated from the un-derlying western-dipping strata, tilted during the uplift, by an unconformity. While most of the uplifted sedimentary rocks have been removed by erosion on the mainland to expose the crystalline basement, their dip decreases towards the west from the Troll area, and the unconformity gives way to a more conformable sedimentary package under the central North Sea10, 11. During the Pleistocene, glaciers repeatedly expanded across Northern Europe, Scandinavia and North America in response to natural variations