Sue Dawson

Rapid Holocene relative sea-level changes in Gruinart, Isle of Islay, Scottish Inner Hebrides

Litho- and biostratigraphic analyses undertaken in the Gruinart estuary, central Islay, reveal a detailed sedimentary record of Holocene relative sea-level changes and high-energy flood events during the last 10 000 years. During the Lateglacial-Holocene transition relative sea level had fallen to below c. 0.5 m OD and remained at this depth until c. 8830 14C years BP. Thus, the early-Holocene minimum sea level is here, relatively well constrained altitudinally. A sustained and largely uninterrupted rise in relative sea level took place after c. 8500 14C years BP, reaching a maximum altitude of c. 4.5 m OD, and remained at an elevation of c. 4 m OD until c. 2000 14C years BP. Both the recorded maximum altitude of the Main Holocene transgression and the late age of the relative marine regression are incompatible with published shoreline uplift isobase models for this area.

Historical Climatology and coastal change associated with the ‘Great Storm’ of January 2005, South Uist and Benbecula, Scottish Outer Hebrides

Abstract This paper describes some of the changes that affected the coastlines of South Uist and Benbecula following the ‘Great Storm’ of 11 January 2005. The paper considers historical changes in patterns of winter storminess and addresses the issue of whether or not there has been a recent increase in regional storminess, manifested partly by this particular storm. The paper concludes on the basis of available Greenland ice core chemistry data and historical climate data, together with recent meteorological data, that no such increase has taken place. This surprising conclusion is further discussed in relation to current thinking on the nature of recent climate change.

Barrier islands on bedrock: A new landform type demonstrating the role of antecedent topography on barrier form and evolution

Barrier islands commonly occur in coastal plain and deltaic settings, and a variety of behavioral and stratigraphic models exist. In this paper, a 90 km long barrier island system in the Outer Hebrides of Scotland is identified for the first time. Uniquely among barrier islands, the system rests directly on a planar bedrock (gneiss) surface (a strandflat). This bedrock surface has a profound influence on the morphology of the barrier island chain, and exerts the dominant influence on barrier island form and evolution during transgression. Topographical irregularities on the bedrock surface control both plan and profile morphology of the barrier islands. Under sea-level rise, by barrier migration, sediment is redistributed onshore and alongshore in a strongly three-dimensional manner, determined by topographic variability in the bedrock. The barrier islands represent an endpoint in transgressive barrier island geomorphology that is controlled by the bedrock surface: they are at the upper end of the recognized tidal amplitude for barrier islands; the offshore wave regime is very high-energy; they lack a shoreface; and neither ravinement surface nor transgressive sand sheet are developed. Sediment supply is dominated by offshore-derived skeletal carbonate.

Onshore tsunami sediment transport mechanisms inferred from heavy mineral assemblages

The aim of this study is to discuss and to extend the characterization of (palaeo)tsunami deposits, and their source materials, based upon the detailed study of their heavy mineral assemblages. Results obtained from three distinct locations (Portugal, Scotland and Indonesia), different coastal contexts and chronologies (the tsunami events studied took place at 1500 cal. yr BP, AD 1755 and 2004) are summarized and discussed in order to contribute to the sedimentological study of onshore (palaeo)tsunami deposits. Results indicate that heavy mineral assemblages primarily reflect local specific conditions. For example, in the Portuguese sites, ca. 90% of the heavy mineral population consists of tourmaline+andalusite+staurolite, whereas in the Scottish samples garnet+amphiboles can be dominant in 90% of the assemblage, where at the Indonesian study site amphiboles+andalusite were the most frequent minerals. The application of Principal Component Analysis for each site reveals that the first two components explain at least 55% of the total variance. In the three studied areas, hydraulic sorting by density was observed and a higher presence of the denser heavy minerals of the assemblages was detected. However, it is important to stress that the sediment source plays a key role in the establishment of the heavy mineral assemblages of the (palaeo)tsunami deposits. In this study, relationships between the likely source sediments and the (palaeo)tsunami deposits were described and whenever possible sediment sources were clearly identified. Furthermore, it was also possible to detect the backwash signal using the analysis of the heavy minerals (e.g. higher frequency of denser minerals or variation in the presence of rounded or euhedral zircon). The work presented here, in contributing to the enhancement of sedimentological criteria presently available to recognize and differentiate extreme marine inundation deposits, also highlights new areas for future research.

A Late Holocene Tsunami at Basta Voe, Yell, Shetland Isles

Abstract A distinctive sand layer enclosed within Holocene peat is described from Basta Voe, Isle of Yell, Shetland Isles. The sand layer, that can be traced considerable distances inland and up to a maximum altitude of c. + 9 m OD, is here interpreted as having been deposited by a former tsunami. AMS dating appears to indicate that the tsunami occurred between 1300 – 1570 cal years BP. At present, the tsunami has no known source mechanism although the most likely mechanism is thought to have been an offshore slump or slide. The available information would presently seem to indicate that this tsunami was restricted to the eastern coastline of Shetland. The relatively young age of this inferred tsunami is of considerable importance to planners and engineers concerned with estimating coastal flood frequency and magnitude.

Submarine “Sandstorms” and Tsunami Events in the Indian Ocean

Abstract The December 26, 2004, disastrous tsunami event in the Indian Ocean may have come as a shock and even a surprise to the communities outside geosciences. However, tsunami events are a natural part of the geosystems in regions of submarine earthquakes, and they will continue to hit the region even in the future. Here we present evidence of former tsunami events both on Sri Lanka and on the Maldives. Submarine “sandstorms” driven by tsunami waves brought littoral deposits down into caves 21 to 38 m below sea level. Radiocarbon dates allow the comparison with related tsunami signals in fens and lagoons, and historical documentation. The terrible December 26 event has raised a general awareness of tsunami events. It has also generated the installation of necessary warning systems. We are in urgent need, however, of a long-term tsunami chronology for a realistic tsunami hazards assessment.

A Pliocene age and origin for the strandflat of the Western Isles of Scotland: a speculative hypothesis

A series of very wide (up to 15 km) raised shore platforms in the Scottish Hebrides are identified and described for the first time and are considered part of a high rock platform shoreline in the western isles of Scotland described by W. B. Wright in his classic Geological Magazine paper a century ago as a ‘preglacial’ feature. Subsequent interpretations suggesting that the platforms were produced during the Pleistocene are rejected here in favour of a speculative hypothesis that the features are part of the well-known strandflat that is extensively developed across large areas of the northern hemisphere. It is argued that the Scottish strandflat developed during the Pliocene and was later subjected to extensive Pleistocene glacial erosion such that only a few areas of platform have survived in the Scottish Inner Hebrides (ice-proximal) while they are well-preserved in the Outer Hebrides (ice-distal). Support for a Pliocene hypothesis is provided by the marine oxygen isotope record for this time interval which points to prolonged periods of relative sea level stability as would be required for the production of such wide features. This hypothesis for the formation of a Scottish strandflat not only provides an elegant explanation for the origin and age of the raised rock platform fragments that occur throughout the western isles of Scotland, but it may also have relevance for other coastal areas of the northern hemisphere (e.g. Norway, Greenland, Alaska) where the strandflat is a well-developed feature.

Barrier island geomorphology, hydrodynamic modelling, and historical shoreline changes: an example from South Uist and Benbecula, Scottish Outer Hebrides

Abstract Dawson, A.G.; Gómez, C.; Ritchie, W.; Batstone, C.; Lawless, M.; Rowan, J.S.; Dawson, S.; Mcilveny, J.; Bates, R., and Muir, D., 2012. Barrier island geomorphology, hydrodynamic modelling, and historical shoreline changes: an example from South Uist and Benbecula, Scottish Outer Hebrides. A partly quantitative reconstruction is provided of the evolution of Gualan Island, a barrier island located between South Uist and Benbecula in the Scottish Outer Hebrides, using historical maps, aerial photographs, and Lidar (light detection and ranging) data. Geomorphological changes over the last approximately 200 years are described together with quantitative changes in the dimension of the barrier island, including rates of shoreline retreat. A series of digital terrain models (DTMs) provided the boundary conditions for a two-dimensional (2D) ocean circulation tide-surge model simulating water level and wave conditions associated with a highly destructive storm that took place during January 2005. During this storm event, the central part of the barrier island was overtopped by waves. Validating the hydrodynamic model against eye-witness and field evidence obtained after the 2005 storm allowed simulation of a range of potential future breaching scenarios. Thus with the same storm conditions a large barrier breach 500 m wide would result in wave heights rising by 0.8–0.9 m on hitherto sheltered shorelines.

The application of microtextural and heavy mineral analysis to discriminate between storm and tsunami deposits

Abstract Recent work has applied microtextural and heavy mineral analyses to sandy storm and tsunami deposits from Portugal, Scotland, Indonesia and the USA. We looked at the interpretation of microtextural imagery (scanning electron microscopy) of quartz grains and heavy mineral compositions. We consider inundation events of different chronologies and sources (the AD 1755 Lisbon and 2004 Indian Ocean tsunamis, the Great Storm of 11 January 2005 in Scotland, and Hurricane Sandy in 2012) that affected contrasting coastal and hinterland settings with different regional oceanographic conditions. Storm and tsunami deposits were examined along with potential source sediments (alluvial, beach, dune and nearshore sediments) to determine provenance. Results suggest that tsunami deposits typically exhibit a significant spatial variation in grain sizes, microtextures and heavy minerals. Storm deposits show less variability, especially in vertical profiles. Tsunami and storm quartz grains had more percussion marks and fresh surfaces compared to potential source material. Moreover, in the studied cases, tsunami samples had fewer fresh surfaces than storm deposits. Heavy mineral assemblages are typically site-specific. The concentration of heavy minerals decreases upwards in tsunamigenic units, whereas storm sediments show cyclic concentrations of heavy minerals, reflected in the laminations observed macroscopically in the deposits.

Geological Recognition of Onshore Tsunami Deposits

The study and understanding of coastal hazards is a fundamental aspect for most modern societies. The consequences of extreme events such as tsunamis are being regarded as major threats for coastal regions. The sedimentological record provides a database useful to characterize and evaluate recurrence of tsunamis, which contributes to assessing the vulnerability of any coastal area to this natural hazard. Thus, the enhancement of our ability to recognize (palaeo) tsunami specific signatures in coastal sediments, through the application of diverse sedimentological techniques, is of unquestionable interest.