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Sediment Exchanges Along the Coastal Margin of the Moray Firth, Eastern Scotland

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Sediment Exchanges Along the Coastal Margin of the Moray Firth, Eastern Scotland Journal ofthe Geological Society, London, Vol. 144, 1987, pp. 179-185, 8 figs., 2 tables. Printed in Northern Ireland Sediment exchanges along the coastal margin of the Moray Firth, Eastern Scotland G. REID' & J. McMANUS Department of Geology, The University, Dundee, Scotland 1 Present address: Department of Oceanography, University College of North Wales, Menai Bridge, Gwynedd LL.59 5EY, UK Abstrart: The Moray Firthis the largest of only three major coastal embayments on the East Coastof Scotland. The relationships between the offshore, coastal margin and fluvial sediments are examined in terms of processes and deposits. These suggest that the large post-glacial accretions of sediment infilling the inner firths are in part attributable to transport of material from the offshore zone. Since the fluvial input to the area appears tobe relatively small and much is retained on estuarine tidal flats it is likely that the offshore contribution is the most important constituent of the deposits of the coastal margin. The other major source is glacial and fluvioglacial material now being reworked by marine activity. Basis of Moray Firth study post-glacial sediments in such areas mayexceed 80 m in Incontrast tothe strongly dissected WestCoast, major thickness. Inthe outer MorayFirth seismicevidence coastal indentations along the East Coast of Scotland are suggests thinner post-and late-glacial accumulations locally limited to the three major embayments of the Forth, the withpatchy distributions resting upona moraine strewn Tay and the largerMoray Firth. Since the rocky stretches of surface (Chesher & Lawson 1983). coastline are sedimentologically relatively inactive,an understanding of the sedimentaryregimes of themajor inlets provides a key to understanding sediment exchanges Hydrographic regime across most of the coast of Eastern Scotland. Investigation The current regime of most of the Moray Firth is typically of the Firths of Forth and Tayhave been reported elsewhere that of a low energy diurnal tide environment. Maps of the (AI-Jabbari et al. 1980; Ferentinos & McManus 1981; J. maximum tidal current velocity for mean spring and mean McManus in preparation). This paper provides an interim neap tides (Lee & Ramster 1981) show that most of the area report on studya of the largest of theEast Coast is subject to a peak current of less than 0.5 m S-'. On the embayments, the Moray Firth with its associated inner firths outer edges of the area andin the entrancesto the innerfirths of Dornoch, Cromarty, Inverness and Beady (Fig. 1). The speeds rise to greater than 1 m S-'. In the outer firth S and basis of the work is an analysis of British Geological Survey SE flood tides reverse to yield N flowing ebb tidal currents. (BGS)offshore sediment samples and geophysical records In the inner firths the tidal components are more notably coupled with calculation of fluvial sediment discharges using E-trending. ratingand flow durationdata. Other information used One of the most notablefeatures of the tidal current includes current meter, waverider and meteorological data pattern is the existence of a flow with virtually no N-S or derived from several sources. westerly component whichoccurs off the Buchan coast (Dooley 1971). This feature is a product of shelter from the ebbcomponent of the tidal flowafforded by the Regional setting configuration of thecoast. Some of theolder models of The coastlines of the MorayFirth area can be loosely circulation in the area feature major eddies(Payne 1963; classified into two types: outer 'hard' coastlines and inner Bohnecke 1922) andthe current anomaly may bethe 'soft' coastlines.These coincidewith the division of the result of such eddyformation (J. Hardistypers. comm.). offshore zone into the Inner Moray Firth and Outer Moray However, the validity of the methods used for these studies Firth (Probert & Mitchell 1980). is uncertainand no definitive circulation patternhas yet The outer coastlines are generally cliff-girt androcky, been published. those of Caithness consisting of OldRed Sandstoneand In an area such as the Moray Firth, characterized by low those of Banffshireconsisting principally of Dalradian tidal currentenergy, winds and waves are critical energy metamorphicrocks, in Banffshire inparticular there is input sources. Where tidal currents are insufficient to erode strongstructural control over the morphology of the sedimentthey may beable to transportmaterial if wave coastline, characteristically producing small pocket beaches. energyhas initiated movement. The importance of such The inner coastline is characterized by large post-glacial processes in areasadjacent to the study area has been accumulations of sediment of a wide particle size range but highlighted by other workers (Owens 1981; Allen 1983). principally of sand and mud. Thefine sand material is found For this area of theNorth Seathere is a statistical typically in large wind blown deposits such as Culbin Sands 50-yearhighest single wave height of 25-28m (Draper and Momch More. Muddy sediments commonly accumulate 1977). Although this value is a useful guide to the extremes in the middleand upper reaches of theinner firths. which can be expected on a wider time scale, it is not very Boreholeand seismicsurveys havedemonstrated that representative of the wave conditions likely to effect 179 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/179/4888790/gsjgs.144.1.0179.pdf by guest on 26 September 2021 180 G. RELD & J. McMANUS 5 4 3 2w Pentland Firth Duncansbv Twi::CAITHNESS Smith's MORAY FIRTH 58N Fig- 1. Map of the Moray Firth showing sites of localities referred to in the text. BF, Beady Firth. sediment transport on a time scale comparable with other water. Local winds are not generally of suacient strength to shelf processes. Analternative approach is based on the be of importance in the offshore zone. Onshore migration of significant wave heightand period values reported from coastaldune systems, as at Culbin, is sedimentologically wave-rider data,and alsoderived from the Hydraulics significant. ResearchStation (HRS) NORSWAM (North Sea Wave Model) wave climate model for the North Sea. These data sources (HRS, BRITOIL, 10s) demonstrate Sediments of the Moray Firth the complexities of wave height and direction distribution of both locally generated waves and swellwaves generated outside the Moray Firth. Values of significant wave height, Previous work periodand the dominant source direction change from When the Institute of Geological Sciences (now the British location to location across the Firth. Numerical models have Geological Survey) began its survey of the offshore zone of beenconstructed to indicate patterns of shallowing and the UK in the early 197Os, the Moray Firth was one of the refraction.These produce wave height losses and wave first areas to be surveyed and sampled. Theaim of the work length reductions, whilst the configuration of the coastlines was prepareto geological maps of thearea. The produces losses of waves from certain source directions. For investigations involved shallow seismic profiling, sidescan example,the southern coast is generally subject to swell sonarsurveys, Shipek grab sampling, gravity coring, waves from between the N and E, the NW coast is subject vibrocoringand limited underwater photography. The to swell from the NE to E, whilst the Inner Firth is subject surface sediment distribution map of the area based on the to swell from only a very limited direction around northeast. Folk classification (Chesher & Lawson 1983) shows several Local wind conditions vary considerably throughout the area discernable trends. The first of these is a close correlation and reflect changes in the regional topography (Plant 1968). betweenincreasing depth and decreasing grain size, Much of thearea is protectedfrom the prevailing reflecting therelative importance of wave energyto south-westerlies by themountains inland. However, this sedimenttransport processes. This is illustrated by the protection is broken in the Inner Firth where the winds can occurrence of muds in the Deeps off the southern coast (the pass along the Great Glen unhindered. Even as far east as deepest ofwhich reachesa maximum depth of around theFindhorn (RAF Kinloss) the SW winds dominate. 230 m). In contrast the sedimentsof the Smith bank (230'W Towards theextreme east and north of thearea wind and 58"lO'N) and similar areas of high ground consist of exposure increases. Along the Caithness coast onshore S or relativelycoarse sands. However, contrary to this general SE winds are dominant in occurrence and magnitude. trend, there are considerable areas of fine sediment in the On site observation has confirmed the r61eof regional inner Moray Firth where the waters are shallow. winds in controlling swash directions,breaker paths and Second,there is amajor input of detritalcarbonate breaker types. All of these are sedimentologically important materialinto the area from the north. The sands of the fortogether theydetermine orientation of thelongshore northern Moray Firthcontain upto 99% carbonate driftand the initiation of sedimentmovement in shallow material. The influence of the detrital carbonate population Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/179/4888790/gsjgs.144.1.0179.pdf by guest on 26 September 2021 SEDIMENTEXCHANGESMORAY ALONGTHE FIRTH 181 isreflected in the distinction between themajor sandy gravel and gravel populations of the area. The gravels of the northwestern area are largely of
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