Report 83/6 INSTITUTE OF GEOLOGICA L SCIENCES The geology of t he Minches, Inner Sound and Sound of Ra asay • • • • • • • • • • • • /I • • • • • • • • • • • • • • • • • • • • •• INSTITUTE OF GEOLOGICAL SCIENCES Natural Environment Research Council Report 83/6 The geology of the Minches, Inner Sound and Sound of Raasay J. A. Chesher, D. K. Smythe and P. Bishop © Crown copyright 1983 ISBN 0 " 884268 4 London Her Majesty's Stationery Office 1983 CONTENTS Introduction The sea areas Offshore investigations Aims and methods of interpretation Bathymetry 3 Solid geology 5 Onshore geology 5 Offshore geology 9 Quaternary geology 11 Glacial and post-glacial sediments 11 Surface sediments 19 References 28 FIGURES 1 Survey coverage 2 2 Bathymetry In pocket 3 Solid geology In pocket 4 Pte-glacial drainage 3 5 Deep seismic profiles 4 6 Shallow seismic profiles 6-7 7 Bouguer anomaly map In pocket 8 Aeromagnetic anomaly map In pocket 9 Quaternary sediment thickness In pocket 10 Depth to rockhead map In pocket 11 Location of Quaternary seismic profiles 12 12 Line drawings of Quaternary seismic profiles 13-16 13 Possible late glacial shorelines 19 14 Surface sediment map In pocket 15 Location of sediment sample traverses 20 16 Variation of surface sediments along traverses 21-23 17 Genetic classification of surface sediment 25 18 Surface sediment C-M diagram 26 Bibliographical riference CHESHER, J. A., SMYTHE, D. K. and BISHOP, P. 1981. The geology of the Miches, Inner Sound and Sound of Raasay. Rep. Inst. Geol. Sci., No. 83/6. Authors J. A. Chesher, BSc, PhD, D. K. Smythe, BSc, FRAS Institute of Geological Sciences, Murchison House, West Mains Road, Edinburgh EH93LA P. Bishop, BSc, PhD University College, London, WCl 11 The geology of the Minches, Inner Sound and Sound of Raasay j. A. Chesher, D. K. Smythe and P. Bishop INTRODUCTION Observations from the Vickers manned sub­ mersible Pisces were made in 1970 and 1971 THE SEA AREAS (Eden and others, 1971; 1973) and detail geo­ The Minches, for the pUlposes of this report, physical investigations of the Little Minch were comprise the sea area from Skye and the Scottish made by Glasgow University in 1971 on board mainland to the Outer Hebrides and bounded to the RRS John Murray (Smythe and others, 1972), the north and south by latitudes 58° 40'N and and in 1972 aboard the MV Calanus. 57° 30'N. The Little Minch lies between Skye and The results of some other commercial and the Outer Hebrides islands of Harris and North academic surveys mentioned below have not been Uist, whereas the North Minch is the larger sea explicitly incorporated into the present account area east of Lewis. The narrow strips of water on for reasons of confidentiality and/or duplication of either side of the islands of Raasay and Rona are data. During the early 1970's, several commercial the Sound of Raasay to the west, and the Inner deep seismic surveys were shot in the Minches and Sound to the east. Sea of the Hebrides, but as the area has proved to be 'non-prospective' for hydrocarbons, no such work has been undertaken recently. A provisional OFFSHORE INVESTICA TIONS Bouguer anomaly map of the North Minch was Prior to the 1960's, understanding of the offshore compiled from the results of Durham University geology of the region advanced very little, and it cruises in 1967 and 1968 (Allerton, 1968; Bott and was believed that the Minches were simply under­ Watts, 1970), and some airgun reflection records lain by a gentle NNE-SSW-trending shallow basin were obtained west of Skye during a University of Mesozoic sediments, perhaps up to 800 m deep College, London, cruise in 1972 (E. J. W. Jones, (compare, for example, MacCulloch, 1819, vol. 1, personal communication). p. 443; vol. 3, pI. 32, fig. 3; MacKinder, 1907, pp.74-75; Hallam, 1965, pp. 410-411; and AIMS AND METHODS OF INTERPRETA TION Donovan, 1968, p. 9). George (1966, p. 16), The aim of the pre-Quaternary section of the however, alluded to the necessity of offshore geo­ report is to present the solid geology. The deeper physical exploration, which began with the region­ (pre-Permian) structure below the Minches is al aeromagnetic surveys flown for the Institute of mentioned only briefly, as this would require Geological Sciences (IGS) in 1964 and 1965. detailed discussion of the reflection and The marine survey work on which this report refraction surveys on and around Skye by IGS is based began in 1968 with a reconnaissance cruise and Glasgow University (Smythe and others, by the RRS John Murray carrying .out geophysical 1972), and the presently confidential reflection traverses on a NW-SE and NE-SW grid, at a surveys, mainly in the North Minch, made by spacing of about 10 km (Figure 1). The equipment several oil companies. However, the interpretation consisted of a 1000-joule sparker, side-scan sonar, of the solid geology, undertaken jointly by Chesher echo sounder, gravity meter and magnetometer and Smythe, is compatible with our unpublished (McQuillin and others, 1968). A marine deep interpretations of the deeper geology; and the seismic survey of the Hebrides was shot in 1969 by Quaternary section of the report by Bishop is Seismograph Service Ltd for IGS, and five of those based on work done during a NERC studentship lines fall into the area covered by this report based jointly at University College, London and (McQuillin and Bacon, 1974). This was followed in IGS. 1970 by a shallow sampling programme and The techniques used to compile an offshore further geophysical profiling, using the RV Vz'ckers geology map from geophysical and rock-sample Venturer (Kenolty, 1971; Eden and others, 1971). data are described in McQuillin and Ardus (1977). Drilling, together with additional sampling and geo­ Examples of problems of interpretation in the physical work, was undertaken from MV White­ Hebrides-Minches region are described further in thorn between 1970 and 1973 (Chesher and McQuillin and others (1979, pp. 104-109). The others, 1972; Institute of Geological Sciences, particular characteristics of the interpretation of 1974), and a limited programme of further drilling the Minches region presented in this report are: has since been carried out (Institute of Geological i excellent geological control is provided by the Sciences, 1977; Evans and others, 1981). land outcrops; 7° Geophysical traverse line Sample station o I.G.S. borehole 58° 30' 0 10 20km , ! , UTM PROJECTION 58°N 58°N " WESTER ROSS 57° 30' N 5° 30' Figure 1 Survey coverage ii the unusually complex bathymetry is a by re-locating profiles to match the bathymetry, considerable aid, both in geological intrepretation, which was surveyed by the Admiralty using the and in correcting navigational errors (see be la w) ; much more accurate Decca Hifix system (McQuillin and Ardus, 1977, p. 21); iii the Decca Main Chain hyperbolic radio­ iv the density of profile and sample coverage is navigation system, used for most of the surveys, is very high, relative to most other areas on the UK here very poor. Systematic errors of up to 1 km in continental shelf, but the quality of the seismic line positioning have been corrected where possible records is relatively low by present standards. 2 THE SUBMERGED BOW PRE-GLACIAL DRAINAGE OF THE MINCHES Probable former o 10 20 km river Courses L'-----'-,-----', Butt of Lewis Major pre-glacial ........ watershed Sea-bed deeper than 100 m (Slightly modified after Godard 1965) N 58°N 58"N WESTER ROSS 57° lI' Figure 4 Pre-glacial drainage BATHYMETRY led to a gently shelving coastline, which along the eastern margin is marked by large embayments. A bathymetric contour map of the Minches (Figure The Shiant East Bank, which reaches 40 m below 2) has been prepared from Hydrographic Office sea level, separates the North Minch Basin from the data, supplemented by echo sounder traces Little Minch Basin to the south. obtained during the IGS surveys. The bathymetry of the Little Minch is more The North Minch consists of a fairly shallow complex, but is essentially basinal with an average basin averaging 120 m in depth below sea level. depth of 100-140 m below sea level. The bathy­ The absence of faults controlling the margins has metric variability is largely due to the resistant 3 Borehole S Line 1 72/33 N 11J "--"'--~___ J_ur_a= ... :::.c:- __________ .... d !is'' 4O'N urw OOOW !is'' 4O'N ~ I Torridonktn ~ Permo~Triass\c I I I I j .5 ----- 2 ~. .~ 2 :r o 5 10km I I I o 10 20km 13 3 i I I I f- ~ Borehole o W Line 3 E 0 0 W +76/55 Line2 E ?B~~B'S.~ I!:::= ~ /'! r '.=-:: --t'!!'rno-Tri.ssi~ BaseJU~- d l Torridonian j 58°N .5 Torridonian ~2 2 2 2 ~. Minch Faul! .">- :i" o 5 10km o 5 10km ~3 I 3 3 I I I 31-c. ~ Minch Fault .. .j:>. .-L________________________________________ ~I 4 4~----------------------------------~ S Line 4 N o Wse. Line 5 E lase Jurassic Reflect()(s not ' ..... ~ • Basalt outcrop ------ ~=:o~to ~ No reflectors ...... __ visib4e ? Base P",mo-~ ... ~~"-~ .. _beneath 1 ~basalts .5 ----- I ~ ? Torridonian ~ 2 ? Torridonian d' .'">- s· ~ o 5 10 km o 5 10 km 1 I , ! I I ~ 3 ~ f- ~ 57° 10'N t· ..', !.L. -[~, Figure 5 Deep seismic profiles. Diagrammatic representation of seismic lines 1 and 2 courtesy of British Petroleum and line 3 by Shell UK basic sills of early Tertiary age which intrude the include foliation, fold axes, shear zones and dyke Mesozoic sediments to give rise to numerous banks, swarms (Bowes, 1969). It is possible that this base­ knolls and arcuate scarp and dip ridges.