Catastrophic fluid escape venting-tunnels and related features associated with large submarine slides on the continental rise off Vesterålen–Troms, North Norway

Video inspection of the seabed on the continental rise (ca. 2200 m water depth) off Vesterålen–Troms, North Norway has revealed several remarkable features, including large tunnels, small tunnels, chimney tunnels, carbonate crusts and bacterial mats. The structures occur in landslide areas with slide blocks up to 2 km long, 1 km wide and 100 m high. We hypothesize that the large seabed tunnels, in and between slide blocks and with opening diameters up to 1 m and apparent inner diameters of 10–20 cm, are formed by dewatering of fine-grained sediments buried by the slide masses and possibly venting of underground gas reservoirs related to sliding and loading. This process probably occurred instantaneously after sliding. Numerous small tunnels with diameters up to a few centimetres, in slide blocks of compacted sediments, may have formed by dewatering following sudden pressure release or by decomposition of gas hydrates following off-loading of sediments due to sliding. Bacterial mats and carbonate crusts indicate active leakage of subsurface fluids and methane through the seabed and the gas hydrate stability zone, either from thermogenic sources or from shallow reservoirs. Cemented chimney tunnels up to 15 cm tall, and with inner diameters up to a few centimetres occur both on slide blocks and on flat and even seabed. The presence of apparently both inactive and active chimney tunnels indicates that gas and fluid release is a process that has been periodically active, possibly from before sliding up to the present.