Storegga details

3D image Credit www.ndwp.org

tsunami threat to Scotland

holocene

Bondevik (2003)

Tsunami hazards

 

 

 

 

 

 

 

 

Shell hash in peat at Scatsta

 

 

 

 

 

 

Detail of angular pebbles at Houbans

 

Tsunami deposits

One of the largest Holocene sub-marine slides mapped on Earth is the Storegga slide offshore Norway (Bugge, 1987).

Approximately 3500 km3 material slid out and generated a huge tsunami dated to about 7300 14C yr BP (Bondevik et al., 1997a), or ca 8150 calendar years BP.

Deposits from the tsunami event occur widely on Shetland in both peat outcrops and in lakes . The tsunami eroded peat surfaces and deposited a distinct and wide-spread sand layer that is recognizable in peat outcrops. Also, the tsunami inundated fresh water lakes, leaving a chaotic deposit of sand layers, rip-up clasts, re-deposited lake mud, and marine fossils (Bondevik et al., 2003).

A Sand Layer in Peat

In 1993, David Smith and others identified a distinctive sand layer in peat as much as about 6 m above high tide on the east shore of Sullom Voe, and interpreted it as being a result of a tsunami. Radiocarbon dates of 1-cm slices of bulk peat at contacts above and below the layer indicated an age of ca 5500 14C yr BP, almost 2000 years younger than the Storegga tsunami event.

Bondevik et al. (2003) obtained ages between 7120 60 14C yr BP on seeds just below the tsunami layer on the western shore of Sullom Voe, and 7025 60 14C yr BP from a stick immediately above the sand layer. A twig from within the tsunami deposit to 7320 70. This site at Maggie Kettle's Loch shows the clearest exposure of the deposit, with rip-up peat clasts and stones up to cobble size.

Leaves and seeds extracted from the lake mud just above the tsunami deposit are 7220 70 14C yr BP. The sand layer has the same age as the Storegga tsunami deposit dated in western Norway to ca 7300 14C yr BP (Bondevik et al., 1997a). The previous 14C dates of bulk peat are 1500 to 2000 years too young most likely caused by penetrating roots. The roots transfer current atmospheric CO2-carbon to deeper layers, thus reducing the 14C age of the affected peat (Nilsson et al., 2001).

Tsunami layers have also been identified on Unst at Burragarth and Norwick.

Sea level at ca 7300 14C yr BP was much lower than today on Shetland. Marine deposits or other landforms associated with shore processes do not exist onshore, and the present beach often lies upon peat. Dated submerged peat (Hoppe, 1965) and a modelled sea level curve (Lambeck, 1993) suggest sea level at the time of the Storegga tsunami to be lower than -10 to -15 m. The tsunami deposits have been traced up to 9.2 m above high tide. This demonstrates a vertical run-up of around 2025 m, a doubling of the previously known maximum run-up height for the Storegga tsunami.

Evidence of more recent tsunami may occur at Garth Loch, South Nesting. Sand in peat at around 2 m asl has been dated to between 4925 and 4645 14C BP. Dawson et al. (2006) also report a sand layer from Basta Voe, Yell, at up to 9 m OD. The layer is dated to 1300-1570 years ago and represents possible evidence of the youngest tsunami in Shetland. Together these sand layers represent an important archive of prehistoric tsunami in the North Atlantic.

To date, no coarse clastic deposits have been identified that relate to former tsunami on Shetland. This is a puzzle as movement of massive blocks has been attributed to tsunami in other parts of the world and such blocks should be highly evident in the sheltered inner voes of Shetland.