THE STOREGGA SUBMARINE SLIDES:
EVIDENCE FOR A STOREGGA GENERATED TSUNAMI FROM EASTERN SCOTLAND
By:John Best

Introduction(A brief outline of events)

*The storegga slides (Facts, Figures and Information)

*Storegga 1

*Storegga 2

*Storegga 3

*Slide dimensions

*Holocene Tsunami deposit in Eastern Scotland (Facts, Figures and Information)

*References

                                            Table.1:Bathymetry image of Norwegian basin area.(1)

Introduction

The Storegga submarine slides occurred on the continental slope off the coast of western Norway, extending out into the Norwegian Basin.There where three slide events: the first slide occurred approximately 30,000-35,000 years before present (2).The second and third slides occurred very close together at approximately 7,000 years before present (2).Jansen (1987) stated that earthquakes possibly together with gas released from the decomposition of gas hydrates are considered to be the most likely triggering mechanisms for the slides.This web page is concerned primarily with the second slide as evidence of a tsunami deposit generated by this event can be foundalong the Eastern coast of Scotland.The Storegga slides were a major event and have been described as "thelargest mass movement event to affect theNorthwestEuropean continental margin in the last 50,000 years" (3). The three slide events moved a total of 5580km³ of Cenozoic sediments (2).The first event involved 3880km³ of unconsolidated deposits, the second and third events moved a total of 1700km³ of Quaternary, Neogene and Paleogene sediments (2).The slides originated on the Norwegian continental shelf at an approximate depth of 1000m (3).The slide run out and turbidite flows extend as far as the Aegir ridge.The second slide travelled for 800km and passed through 3500m of water depth, the down draw from this event is believed to have generated the tsunami that struck East Scotland 7,000 years ago.The deposition created by this Tsunami can be seen along the East coast of Scotland as a layer of grey, micaceous, silty fine sand lying within beds of grey silty clay or clayey silt (3).


 
 
 
 

Storegga 1(30,000-35,000 years before present)

The first Storegga slide involved 69% of the total sediments moved by the three slides combined.Jansen (1987) reported that this event probably affected the whole area of the slide scar.The slide surface is smoother than that of the second event, this indicates that the slide was shallow and only affected the upper sediments (2).The slide deposits are made up of poorly consolidated, clayey, probably mainly glacial deposits similar to the undisturbed sediments in the surrounding area (2).The thickness for the slide deposit is estimated to be in the order of 130-200m (4). 


 
 


 
 
 
 

Storegga 2(approximately 7,000 years before present)

The second storegga slide occurred within the first slide scar cutting back 8km into the continental shelf (2).The second slide cut deeper into the seafloor than the first slide, this is reflected by the slide surface morphology, which is rough and blocky (4).This is due to the more consolidated nature of the sediments into which the second slide cut.The second slide had the longest run out of the three slides.Slide deposits have been found as far as the Aegir ridge some 800km from the slide headwall (2).This represents a water depth interval of 3500m from the headwall to the end of the slide run out (2).


 
 


 
 
 
 

Storegga 3(approximately 7,000 years before present)

The third Storegga slide had its headwall at 1,000m water depth (2) inside the second Storegga slide scar.The slide itself was confined to the shallower areas of the first and second slide scars (2).The slide can be traced down to depths of 1500-1600m with possible extensions down to the Gloria valley.The date of the third slide cannot be separated using carbon 14 testing from that of the second slide.


 
 

Slide dimensions (4)


 
 
First Slide
Second Slide
Third Slide
Total
Run out distance/km                350-380      800-850                100-130 850
Area of slide scar/km2
34,000
19,200
6,000
34,000
Total slide-influenced area/km2
52,000
88,000
6,000
112,500
Maximum thickness/m
280
330
430
Average thickness/m
114
88
160
Volume/km3
3,880
1,700
5,580
Volume of deposits left in slide scar today/km3
400
950
1,350
Volume of deposits below 2700m/km3
3,480
750
4,230

 
 
 
 


 
 
 
 
 
 
 
 

Holocene Tsunami deposit in Eastern Scotland

 
The East coast of Scotland has a number of raised shoreline features, these were created during the retreat of the late Devensian ice sheet (3).Subsequent glacioisostatic rebound raised the beaches to there present maximum altitude of 15m above British ordinance datum (3).The raised beaches consist of an accumulation of carse sediments.The carselands are comprised of grey silty clays or clayey silt as was said earlier.Within these deposits is a layer of grey, micaceous, silty fine sand approximately 10cm thick (3).Dawson (1988) reported that the sand layer "seems only to have limited grading" this implies that the sediments were deposited quickly.It has also been noted that the deposit forms an inland facing tapering wedge within the surrounding deposits (3).From this evidence it seems likely that the sand layer is the product of some form of ocean wave event with high levels of run up.The deposit is found between peat sediments, radiocarbon testing has been carried out on biogenic material found in the peat sediments above and below the deposit.The carbon testing indicates that the sand layer was deposited over a brief period of time some 7,000 years before present (3).This date ties in directly with that of the second Storegga slide.The sand layer has also been found to have a high proportion of tychopelagic diatoms, this form of diatom is only found living on ocean floor sediments (3).Dawson (1988) also found evidence of scouring of the underlying deposits, this is a common feature of Tsunami deposits.There has been some argument over the origin of the layer, Smith et al (1985) suggested that the deposit was the result of a storm surge, however storm surges are normally confined to small local areas of deposition whereas the sand deposits cover a much wider area stretching inland (3).From the evidence given it seems that the likeliest explanation for the sand deposit is that it is the result of a major marine incursion caused by the second Storegga submarine slide 7,000 years before present.

 


 
 

References 

  1. The 21st century Atlas. 1999. Trident Press International
  1. Jansen. E. Befring. S. Bugge. T. Eidvin. T. Holtedahl. H. Sejrup. H. P. 1987. Large submarine slides on the Norwegian continental margin: Sediments, transport and timing. Marine Geology. V78. 77-107.
  1. Dawson. A. G. Long. D. Smith. D. E. 1988. The Storegga slides: Evidence from Eastern Scotland for a possible Tsunami. Marine Geology. V82. 271-276.
  1. Bugge. T. Belderson. R. H. Kenyon. N. H. 1988. The Storegga slide. Phil. Trans. R. Lond. A 325, 357-388
  1. Smith. D. E. Cullingford. R. A. 1985. Flandrian sea level changes in the Montrose basin area. Scott. Geogr. Mag. 101: 91-105.

 

Top of page