Paleo-tsunamis in the Swedish sedimentary records

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Mِrner, Nils-Axel۱; Dawson, Sue۲
Holding Date 07 October 2008

During the deglacial period, Sweden was characterised by an extremely high seismic activity. This is recorded both in the magnitude of individual events and in the frequency of events. The Swedish Paleosismic Catalogue includes 58 events established by multiple criteria and dated by varves or radiocarbon.
In 1995, we found evidence of the first tsunami event, linked to the autumn 10,430 vBP paleoseismic event. Shortly after, a second tsunami event was documented in association with the 9663 vBP paleoseismic event at Hudiksvall. By now, we have a record of 16 tsunami events from Sweden. Our tsunami events are always established in association with records of paleoseismic events. At the time of deglaciation, the sea level was significantly higher over the coasts of Sweden. Hence, there was water enough to create quite large tsunami waves.
The tsunami breaks in over land, flooding land-surfaces and invading existing lakes. In the lakes, we record special tsunami signals; usually in the form of sandy layers in graded bedding (gravel-sand-silt-clay) as an odd layer in the normal lake deposits. These tsunami beds contain microfossils dominated by pelagic species typical for the water where the tsunami was generated (sea in the west, and sea or lake in the Baltic). Often, it is possible to distinguish both an on-swash bed and a back-swash bed. The back-swash bed is usually the most significant one. In the off-shore region, sandy-gravelly beds are formed.
They range from tsunami beds via hybrids between tsunamis and turbidites to true off-shore turbidites. In varved clay sequences, the turbidides can be dated as to a single varve year. Several unusual and odd varves, previously classified as "drainage varves", can now be understood in terms of paleoseismic turbidites; i.e. "seismites". Sometimes we record separate tsunami shorelines, erosion level and accumulation deposits. The height of the breaking wave is recorded in the difference between the sea level at the time of the tsunami and the height of lakes into with the run-up water or over-washing reached depositing tsunami layers. The distance of the land area between the shore and an invaded lake is also significant.
The spatial range of the tsunami event is another important parameter. The West Coast data include 5 tsunami events; at 12,400, 11,600, 11,250, 1600 and 900 BP. The first three evens were significant, especially the 12,400 BP event. The last two events need further studies. The 900 BP event seems to be connected to the sudden silting-over of two ships of Viking-type dated at ~1100 AD. The Baltic data include 11 tsunami events dated at 10,430 vBP, 9663 vBP, 9428 vBP, 8600 cBP, 8000 cBP, 6100 cBP, 4000 cBP, 4000 cBP, 3250 cBP, 2900 cBP and 2000 cBP. Many of these events will be explored during Excursion No. 11, parts A and B.