A geophysical and geochronological assessment of coastal dune evolution at Aberffraw, North Wales

• Main Author: Bailey, Simon David
• Published: University of London 2003
• Subjects: 577.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.533191

Aberffraw is a lkm wide and 3km long transgressive dune field on the Isle of Anglesey, North Wales, that extends inland along a northeast-southwest trending valley from a southwest facing beach, Traeth Mawr. The prevailing wind is from the southwest and both the parabolic dunes and the vaUey within which they lie are subparallel to the prevailing wind. The dune field at Aberffraw includes two foredunes ridges and three rows of active parabolic dunes. At the landward end is a lake, Llyn Coron, which has been formed by the dunes migrating up the valley and damming the river, Mon Ffraw. Rates of parabolic dune migration, calculated from aerial photographic analysis using a new linear fit method for compound dune forms, range from a minimum of 0 m1yr to 3.6 mJyr, with an average migration rate of 1 m1yr. Ground Penetrating Radar (GPR) was used to identify sedimentary structures in the subsurface of the two foredune ridges, four parabolic dune ridges and the associated interdune areas. Surveys at 100 MHz and 200 MHz allowed the resolution of both the gross dune field stratigraphy (bounding surfaces, erosion surfaces) and the smaller sedimentary structures (e.g. cross-stratification, foresets, cut and fill troughs), together with the water-table and basement. Interpretation of the GPR profiles used a combination of the radar facies technique and a radar sequence boundary stratigraphic approach to construct relative chronologies for sections of the stratigraphy. Detailed intersecting GPR surveys by Llyn Coron revealed a relative chronology of six sand packages younging successively towards the NE, bounded by erosion surfaces representative of relic lake beds. Large landward dipping erosion surfaces are interpreted as incursions by the lake, with a raised water-table resulting from highstand phases initiated by dune damming. Smaller troughs and planar based scours are interpreted as periods of deflation to a lowered water-table associated with lowstand phases of the lake. Prograding stacks of landward dipping foresets are interpreted as continued dune migration throughout these events. Hand augering was used to sample specific packages identified from the GPR profiles for Optically Stimulated Luminescence (OSL) dating to provide absolute ages for sand deposition. Samples were also acquired from the neighbouring site of Newborough Warren for comparison, and for radiocarbon dating. Quartz grains were dated using a single aliquot regenerative dose (SAR) protocol that allowed for sensitivity changes and monitoring of thermal charge transfer. Thirty-four dates ranged from AD 1281 for a 5 m deep sample to AD 1981 for a modern bleached sample, demonstrating good precision in contrast with the radiocarbon technique over this time scale. and confirming the relative chronology constructed from the GPR data. The OSL dates for sand movement at Aberffraw show good correlation with both local and regional temporal trends in sand movement for coastal dune areas in NW Europe, with a positive correlation between periods of sand movement and periods of increased storm frequency and severity associated with the North Atlantic Oscillation (NAO) and climatic deterioration during the Little Ice Age cooling event.