OR/15/017 Appendix 2 Sand layers in peat (Dave Long)
|Tappin, D R, Long, D, Carter, G D O. 2015. Shetland Islands Field Trip May 2014 - Summary of Results. British Geological Survey Internal Report, OR/15/017.|
The peat cover on Yell is exceptional, even by Shetland standards with peat extending down to the shoreline where it forms small cliffs. These cliffs along with the abundant peat cuttings provide an opportunity to observe/trace mineragenic layers and using the adjacent organic material to date the event that emplaced them. Various ideas were discussed as to mechanisms to emplace continuous sand layers within the blanket bog at the coast. These mechanisms included: Storm surge, Tsunami deposition, Aeolian (wind blown), Peat slide.
The regional guide (Flinn, 1994) suggested that previously 97% of Yell had been peat covered and that currently 63% has more than half a metre of uncut peat. However he suggests that as this is mostly blanket type peat that based on data from Orkney and elsewhere on Shetland this peat started to accumulate about 3400BP. Radiocarbon dating since by Dundee University have disproved that, demonstrating that the peat has developed over a considerable longer time period. Flinn (1994) notes that in many places in Yell about 10cm above the base of the peat are to be found tree roots and branches up to several centimetres in thickness. In many cases they are of silver birch. He also notes that layers of sand and/or gravel, generally up to 10cm thick, are commonly visible in the peat along the banks of streams high up in the hills. Such occurrences extend down to the shores without change in character, and all appear to be the result of streams overflowing their banks as the result of heavy falls of rain.
Potential depositional mechanisms
Shetland is subjected to strong waves. Deposits attributable to storms have been well reported from cliffs (e.g. Hall et al. 2006). The fact that the source material is potentially on a beach or in shallow water would support an inland thinning of a deposit resulting from winds or waves transporting the grains inland. However it is unlikely to transport cobbles or boulders.
Studies elsewhere suggest that tsunami and storm deposits can be distinguished (Tuttle et al., 2004).
The evidence is strong that Shetland has been struck by the tsunami wave triggered by the Holocene Storegga Slide.
Other than wind-blown sands during strong storms the other source of mineragenic layers within peat deposits are tephra-falls from volcanic eruptions. These events have been noted historically in Shetland (Thorarinsson, 1980) and the deposits found and studied in a variety of deposits (Bennett et al., 1992; Dugmore et al., 1995). The source of the tephra is Iceland. However many often events recorded within the peats are very thin (only a few mm) and often only noted during microscopic examination.
Airfall deposits are usually of even thickness and found at all altitudes, unless they are reworked by downslope waterflow. They therefore do not show the characteristic of the suggested tsunami deposits of thinning inland.
Peat slides (aka bog-bursts) are well known in Shetland. They are a hazard that the local authority has to consider in many aspects of planning; development, infrastructure, civil contingencies. In August 2012 extensive flows of peat occurred following heavy rain at Uradale.
These travelled downslope transporting a vehicle about 400 m (Fig 101) (http://www.shetlandtimes.co.uk/2012/08/22/lands lides-hit-central-mainland-including-uradale- farmhouse). These flows can transport sediment from the peat boulder clay/bedrock interface that may include sands and even gravel. In many cases the peat moves as a single mass or a series of blocks that extend the full thickness of the peat.
Almost all the failure surfaces are at the base of the peat. The morphological features include large blocks, linear compression, thrust features resulting in hummocky terrain and unusual occurrences of mineral debris (Dykes and Warburton, 2008). Studies of peats on Yell have shown that the peat can reach 5 m in thickness and these sliding events have been dated to the mid and late Holocene (Veyret and Coque-Delhuille, 1993).
Images of a major peat slide (19th Sept 2003) at Channerwick in south Mainland show that it can comprise large intact blocks as well as extensive of fluid peat material on which the peat blocks are transported (Winter et al., 2005). The erosion extended down to bedrock where it was able to incorporate other material into the debris flow (Figure 102). This event occurred following very heavy rain falling onto ground that had dried previously. Cracks within the peat provided conduits to the base of the peat where the impermeable bedrock caused the relatively light peat to lift off and flow down the hillside. Once moving the partially saturated peat moves rapidly as a debris slide with peat blocks breaking down into smaller sizes (Nettleton et al., 2005; Winter et al., 2005). These blocks can incorporate mineragenic layers from the bedrock interface and may even deposit them out of sequence through overturning (Fig. 103) (Dykes and Warburton, 2008).
Both the sites examined at Mid-Yell and Vatsetter present coastal peat cliffs with a hummocky terrain landward. It is possible that the sections of sand layers exposed in the cliffs represent a cross-section through a relict peat slide that is now at sea level. The laminated sands and peats at the northern limit of the coastal section examined at Whale Firth are located seaward of very hummocky ground that is suggestive of a major peat slide.
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