Salt diapir and gas leakage on Tiddlybanken

MAREANO is well into its 2013 Barents Sea cruise. We have just finished our work on Tiddlybanken, where one of the video lines was planned to cross a previously described gas leakage site. The gas leakage was found as well as other exiting structures on the seabed.

The gas leakage was known from earlier Russian investigations, in which also Norwegian geologists and biologists participated. Tiddlybanken is however so special that also MAREANO decided to visit it.

Figure 1: MAREANO’s 2013 investigation area in the Barents Sea. The salt diapir and seabed gas leakage occurs on Tiddlybanken.

Old salt deposits

A map published by the Geological Survey of Norway 10 years ago shows that the bedrock beneath the gas leakage consists of evaporite (mainly salt but also gypsum and other minerals). Thick layers of evaporite were deposited at the bottom of the Barents Sea approximately 300 million years ago. This was caused by evaporation of sea water, resulting in deposition of minerals at the seabed. Thick layers of other sediments were deposited on top of the salt layers during later periods.

Salt is less heavy than other sediments. When thick layers of salt are buried deeply enough, the salt will start to move upwards through the overlying sediments just like the bubbles in a lava lamp. One major difference is that the process will take tens of millions of years instead of a few minutes as in the lava lamp.

Figure 2: Map showing hills and valleys on Tiddlybanken. The shallowest bank areas are shown in brown (c. 180 m), while the deepest water areas are grey (c. 340 m). Video transects are shown as white lines, each 700 m long. Yellow square shows the video line at the gas leakage site. The location of this area is shown in Figure 1.

Holes in the seabed and gas leakage

Gas leakages from the seabed are often found in areas of salt diapirs, such as on Tiddlybanken. The area has a special topography, with several hills and topographic depressions. These may have been created by glaciers during the last ice age, but can also be related to rising salt below the seabed. When passing the area, leaking gas was recorded by the echo sounder. Video records from the seabed showed large holes, which may have been created by leaking gases or fluids, or which may result from salt dissolution. Shallow seismic data collected along the video line showed a topographic depression in the interpreted bedrock surface, which may have been created by leaking gases or fluids.

Figure 3: Picture of the seabed in the area of the gas leakage (see Figure 2).The bottom sediments consist of a thin layer of mud, sand and gravel. We do not know with certainty what occurs below, but the bedrock map suggests that it may be salt. Burrowing animals have several places made holes in the seabed. The holes seen in this picture are however more than half a metre in diameter and do not resemble animal burrows. They may be created by gas or fluids leaking trough the seabed and dissolving salt in the underground. The distance between the two red laser dots is 14 cm.

Figure 4: Shallow seismic line acquired by bottom penetrating sonar (Topas) onboard G. O. Sars. The seismic line runs along the trace of the video line and shows where gas leaks from the seabed (observed in echosounder data) and where large holes occur in the seabed (close to the topographic depression). Figure 2 shows the position of the line.