The Geology of the Minches, Inner Sound and Sound of Raasay

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. The north ment trend has influenced structures in the sedi­ western margin of the basin south of Kebock Head mentary cover. The granulite-grade Lewisian is marked by a steep bathymetric scarp following gneisses of the Loch Laxford Scourian separate the south-east coastline of the Outer Hebrides two regions of dominantly quartzo-feldspathic along the line of the Minch Fault which here gneisses and schists to the north and south. The throws down the less resistant Mesozoic rocks granulites are characteristically denser, more against Lewisian basement. highly magnetic (Powell, 1970) and of significant­ It should be noted that the new bathymetric ly higher P-wave velocity (Hall and AI-Haddad, map presented in this report provides no support 1976) than the gneisses and schists which were for Dearnley's (1962) conjecture that a supposed strongly reworked during Laxfordian deformation line of 200 m-deep troughs might be the bathy­ and metamorphism. The Lewisian geology of the metric expression of a NW-SE-trending sinistral Outer Hebrides shown in Figure 3 is simplified transcurrent fault running up the centre of the from recently published IGS maps. The main Little Minch. structural grain is again NW-SE, although in north The major influence on the bathymetry, aside Lewis an older Scourian (2600-2800 Ma) NNE­ from the primary control of the solid geology SSW trend predominates. The eastern coastline (Figure 3), has been the pre-glacial drainage pattern of the Outer Hebrides closely parallels the Outer as mapped by Ting (1937) and Godard (1965) Isles Thrust zone, which in Pre-Cambrian and again (Figure 4). The major watershed which then ran in late Caledonian times thrust dense granulites from Trotternish to Harris, some 25 km south­ westwards over less dense amphibolite gneisses and west of the Shiant East Bank basement ridge is still granites (McQuillin and Watson, 1973). The thrust expressed as a bathymetric high, although it was zone is made up of mylonites and other cataclastic glacially breached in the Sound of Shiant. rocks, and normally dips eastwards at about 25°. During the Quaternary glaciation, Ice preferentially followed the fluvial valleys and Torridonian overdeepened them, especially at the more These essentially unmetamorphosed Precambrian constricted points, for example at the north of the sediments rest on the Lewisian basement of the Inner Sound, and to the east and west of the mainland and are divided into an older Stoer Shiant East Bank. Although the overdeepened Group, and a younger Torridon Group (Stewart, valley of the Inner Sound has been partially infilled 1975). The ages of the two groups, determined by by glacial sediments, it is nevertheless, at 320 m, Moorbath (1969) using the Rb-Sr method on the deepest water on the UK Continental Shelf. Ice shales, are about 995 Ma and 810 Ma respectively also scoured Mesozoic sediments from the down­ (Brook and others, 1977, p. 494). Maximum thrown sides of the major faults, for example in thicknesses are 2.3 km for the Stoer Group and the Inner Sound, and along the south-eastern coast 7 km for the Torridon Group, with an intervening of the Outer Hebrides, south of Kebock Head. angular unconformity of about 25°. Stewart (1969, fig. 2) suggested that thicknesses would have been even greater to the west, and it is likely, therefore, that several kilometres of Torridonian SOLID GEOLOGY (sensu la to ) are still preserved below the Minches. ONSHORE GEOLOGY The low-grade metamorphosed Sleat Group The sedimentary rocks of north-west Scotland, in south-east Skye occurring within the Kishorn resting on the metamorphic Lewisian gneisses, span nappe may be in part the lateral equivalent of the the time interval from the Late Precambrian, about Torridon Group. 950 Ma ago, to the present. An important phase of Palaeogene volcanic activity is represented by the Moinian igneous rocks of Skye and its neighbouring islands. The Moine Succession of dominantly arenaceous Some aspects of the onshore geology are and argillaceous meta-sediments reaches a summarised below. considerable thickness which IS difficult to estimate due to structural complexity, and occurs Lewisian above the gently eastward-dipping Moine Thrust on These high-grade gneisses, derived dominantly the mainland (Figure 3). These metamorphic rocks, from pre-2600 Ma igneous intrusions with minor which are not found farther west than the outcrop metasediments and meta-volcanics, were last of the thrust, may possibly be the stratigraphical reworked during the Laxfordian orogeny, about equivalents of the Torridonian (Soper and Barber, 1800 Ma ago. They crop out in two major areas 1979). trending NNE-SSW, one on the mainland of north-west Scotland and the other forming most of Cambro-Ordovician the islands of the Outer Hebrides. On the main­ Up to 1600 m of Cambro-Ordovician rocks lie on land, NW-SE structures predominate; these the low angle unconformity above the Torridonian

5 ______~~~------~OOTO~W~------_,~~N

Figure 6 Shallow seismic profiles

6 -g 0.O~"Nc;;sw____------__u_n?

.! ~:~ ea Qu 7Pl 7 Q~~jQu Pt' .'" 0.3 j f 0.4 ! 0.5-'------

-g W Line 6 E 0 0.0 ~ 0.1 .S 0.2 pt T .~ 0.3 0.4 ....f 0.5

Une 6 (cont.) NE

::::;;'pt .Si~:~lsea~p"~47vv/.Y 0.2 J 771 .~ 0.3 f 0.4 ....~ 0.5~------J

NW Lina7 SE l~:~~seao;:Ter," .1< 02 • _._ ~ " .~ . j j\ Qu ~ ~ 0.3 .' > J " " -; 0.4 ! 0.5-L--______~

SW Une' NE loo~e.ti'l 0.1 " Intrusion .E 0.2 ".",,'" ~ pt Qu Quaternary ,~,. 0.3 j Jurassic ~ 0.4 l O.5-L------______~ pt Perm"" Triasslc pt·j Permo-Triassic·Jurassic

T Totridonian base_nt

L Lawislan basement W 10 line E ~ W Une11 E -g 0.0 F Fault 0 Sea " T g ~ 0.1 L I! .£ .r: .. 0.2 .. g 0.3 .g >- ,. Horizontal scale 0.4 ~ 0 km 10.5 ,!

7 (Swett, 1969), but the post-Caledonian tilt to the The overlying Bearreraig Sandstone of the ESE of 50 _200 makes it unlikely that they are Middle Jurassic (Bajodan) (Morton and Hudson, now preserved beneath the Minches. Much of the 1964) comprises massive crossbedded marine underlying Torridonian was probably also eroded sandstones with clays and shales. It shows great from the Minches during the same period. variation in thickness and lithology in the south of Skye and Raasay, near to a contemporary shore­ Permo-Triassic line (Morton, 1965), whereas in northern Skye, The red beds at Stornoway, first described as of farther from the source of sediments, more 'Primary' age by Macculloch (1819, vo!. 1, uniform conditions of deposition prevailed. p. 195; vo!. 3, 'General map') were subsequently The Great Estuarine Series, also of Middle assigned to the Torridon Sandstone or Old Red Jurassic (Bathonian) age, overlies the Bearreraig Sandstone (Morris on , 1888; Steavenson, 1928; Sandstone, and is a mainly non-marine sequence of Jehu and Craig, 1934; Kursten, 1957). Modern sandstones, shale and limestones, attaining a maxi­ sedimentological techniques (Steel, 1971; 1974a; mum thickness of about 200 m in the north Skye 1974b; Steel and Wilson, 1975) were used recently (Hudson, 1962). Faunal and petrological studies to support the suggestion originally put forward (Hudson, 1963; 1964) indicate an environment of by Stevens (1914; see also Peach and Horne, 1930, deltas built into brackish-water lagoons, in what pp. 84-85) that the beds are more probably of New was probably an early Minch basin, since there ~ed Sandstone age. ;Palaeomagnetic pole vectors appears to have been a small landmass to the west, (Storetvedt and Steel, 1977) also suggest a Triassic in the area of the Outer Hebrides, as well as a larger age, and do not support the older dates previously one to the east. As Hudson (1964. p. 524) stated, put forward. Furthermore, the discovery of mixed the Middle Jurassic palaeogeography 'looks polarity appears to rule out deposition during the remarkably like present-day Scotland without the Kiaman interval of reversed geomagnetic polarity, Tertiary volcanics'. which lasted from the Upper Carboniferous Upper Jurassic rocks, mainly in the form of (Stephanian) until about the end of the Permian blue-grey clays and shales of Oxfordian to (about 300-230Ma), with the exception of a short Kimmeridgian age, crop out on land mainly in period of normal polarity at about the Carbon­ northern Skye, and are up to 100 m thick. These iferous-Permian boundary (McElhinny, 1973). rocks present a return to fully marine conditions. The Stornoway Formation consists mainly of Other occurrences of J urassic rocks are present at conglomerates deposited in alluvial fans, and has an Applecross and Gruinard Bay, where Lower Lias apparent stratigraphic thickness of about 4 km. sediments unconformably overlie the Triassic, and Modelling of the Bouguer anomaly suggests, on the Shiant Islands where Upper Lias strata crop however, that only about 1 km is preserved in a out. westward-tilted wedge down thrown against the Lewisian (R. McQuillin, personal communication). Cretaceous The discrepancy between these two thickness \ The widespread absence of Upper J urassic and estimates can be accounted for by the strong I complete absence of Lower Cretaceous throughout diachronism of the sequence (see also Steel and , the Hebrides is due to their non-deposition or Wilson, 1975, fig. 5), as the locus of the fault­ erosion prior to the Cenomaman transgression generated sedimentation shifted westwards with (Upper Cretaceous), which laid down greensands time. The Stornoway Formation displays the best now found as small outliers throughout the evidence in the Minches of syn-depositional fault­ Hebrides, including Raasay, Scalpay and central ing in the Permo-Triassic, a process which is and southern Skye. Additionally in Skye, a 5 m­ believed to control the infill of major basins both thick limestone is considered to be altered chalk in the Minches and in other areas off western (Richey, 1961, p. 39). Scotland (Evans and others, 1981). Other Triassic outcrops comprise minor outliers Tertz"ary on the mainland (Richey, 1961), and minor Hebridean Palaeogene igneous actIVIty is well pockets and patches in Skye and Raasay (Steel and represented in Skye, beginning with basal tuffs and others, 1975), where they underlie Jurassic rocks, followed by effusion of flood basalts over a and are preserved as infills of hollows in the mature, base-levelled landscape. On north Skye, Triassic topography. five separate groups of lavas are recognised, preserved by later faulting. Anderson and Dunham Jurassic (1966) argued that the 1300 m thus preserved may J urassic rocks are extensively exposed in Skye and approach the maximum thickness of the pile, and Raasay, where the cumulative thickness is over it is also clear that the two lowest groups formerly 900 m. The Lower Jurassic (Lias) is up to 400 m extended much farther than the present area of thick and consists of a dominantly marine fades of north Skye. Towards the end of the period of shales and calcarenites deposited in a shallow sea basalt eruption the basic Cuillin centre was bordering a low-lying landmass in roughly the intruded. The age of these events is believed to be present position of the Scottish mainland (Hallam, mid-Palaeocene, about 59 Ma ago (MacIntyre and 1965; MacCallum, 1971). others, 1975; Brown and Mussett, 1976). The 8 basalts and Cuillin mass are reversely magnetised, (Upper Hettangian to Lower Sinemurian) age, on in good agreement with the reversed geomagnetic line 2 (Figure 5), about 0.7 s TWT above the base polarity interval preceding anomaly 26, and which of the Jurassic. This implies that around 1 km of lasted from 61-58Ma (Hailwood and others, Lower Lias is preserved here, a figure comparable 1979). with the 900 m drilled in the Mochras borehole The emplacement of the Cuillin centre and (Woodland,1971). cessation of basalt eruption were followed by Between 58° 20'N and 58° 30'N there is a intrusion of a basic sill complex into the Jurassic gentle ridge trending approximately E-W, sediments in northern Skye. The aggregate vertical mappable on deep seismic profiles (Figure 5, line thickness of the leaves of the complex is about 1), sparker traverses (Figure 6, line 1) and 250 m, and the individual sills frequently transgress discernible from the Bouguer anomaly map (Figure the sediment bedding-planes, so as to maintain a 7). On this ridge Borehole 72/33 cored black mud­ roughly constant depth below the base of the lavas. stones dated as Pleinsbachian-Toarcian (Upper A period of injection of basic dykes, trending NW­ Lias), about 0.5 s TWT above the base Jurassic SE followed, and last came the intrusion of the reflector. The total thickness of Lias here is thus Western and Eastern Red Hills granitic centres, of the order of 700 m. perhaps as late as the early Eocene, about 52Ma To the north of the ridge, the character of the ago (Brown and Mussett, 1976). Lower Jurassic changes from acoustically No Tertiary sediments, other than thin inter­ transparent to a series of good reflectors, suggest­ basaltic horizons, are recognised onshore. ing a lithological change from shales and mud­ stones to a more arenaceous or calcareous sequence. The top of this Jurassic 'well-bedded' acoustic OFFSHORE GEOLOGY sequence can be mapped as an approximate base The Minches are underlain by two separate post­ mid-Jurassic horizon, by correlation with its out­ Carboniferous sedimentary basins about 2-2Y2 km crop near Borehole 72/33 (Figure 3). deep occupying the North Minch and Little Minch The strongly reflective sequence below the respectively (Figure 3). These are separated by the Jurassic is interpreted as Permo-Triassic. Its base, Shiant East Bank structural high. The basins form generally about 0.5 s TWT below the base Jurassic half grabens bounded to the west by the Minch horizon (Figure 5), is poorly defined on the deep Fault. seismic reflection sections and has been extrapolat­ ed downwards from outcrops determined by North Minch sparker sections (Figure 6) calibrated by boreholes. The line-drawing interpretations of representative It is distinguished from the Torridonian by the seismic reflection lines in the North Minch (Figure generally rugged topography of outcrops formed 5, lines 1-3; Figure 6, lines 1-3) show that the by the latter and by the absence of reflectors on basin is asymmetric. It is faulted along its western sparker sections (Figure 6). In the centre of the margin only, and plunges gently NNE towards basin, infrequent, steeply dipping poor reflectors the basin outlined by gravity low 'C' of Bott and on deep seismic records can be seen below the flat­ Watts (1970) west of the Orkneys. lying reflector picked as the base of the Permo­ South of about 58° 20'N the base Jurassic Triassic; these suggest an angular unconformity reflector marks the bottom of an acoustically with the underlying Torridonian. The isolated transparent sequence up to 0.7 seconds of two­ coastal outliers of Permo-Triassic on the mainland way travel time (TWT) thick. Below is a sequence show that the unconformity is highly irregular, and of strong reflectors with no well-defined base, but that the overlying beds comprise locally-derived possibly more than 0.5 s TWT thick. These seismic material. Even in the more distal environment of characters are well illustrated by the right-hand the basin centre such an unconformity is unlikely section reproduced in McQuillin and others (1979, to make a good reflector, so the reflector picked fig. 7/22). The two acoustic sequences are typical as the base may well be somewhat above the actual of shales overlying sandstones, and this interpreta­ base of the Permo-Triassic, perhaps by 0.1-0.2 s tion is corroborated by the lithologies of the bore­ TWf. hole sample sites in the area. The dates assigned to the samples suggest that the base of the Jurassic Shiant East Bank Ridge coincides with the base of the acoustically trans­ The existence of an apparently continuous well­ parent sequence, on the assumptions that i), that defined basement ridge, trending NW-SE, there is little or no marine Rhaetian; and ii), that cropping out on the seabed and connecting Rubha the red sandstones are Triassic or older (perhaps Reidh on the mainland to Kebock Head on Lewis, including rocks of Rhaetian age). as shown on earlier published maps (Eden and If the identification of the base Jurassic others, 1971; IGS, 1979), does not agree with the horizon is approximately correct, there must be a current re-interpretation. The ridge is not continu­ great thickness of Lower Lias in the southern part ous at surface to Kebock Head, as shown by the of the North Minch Basin. Borehole 76/55 well bedded, westerly-dipping acoustic sequence at encountered shales and mudstones of Lower Lias the western ends of sparker lines 4 and 5 (Figure 6)

9 which are now interpreted as J urassic and correlat­ of the Hebrides Trough (McQuillin and Binns, ed with the smooth bathymetry east of Kebock 1973; 1975; Binns and others, 1974). At outcrop Head. This does not preclude the existence of a on and around northern Skye the J urassic rocks basement ridge at depth. form a saucer shaped depression centred on Loch The Shiant East Bank Ridge consists of an Snizort, . and largely covered by early Tertiary inlier of Precambrian rocks that form irregular out­ basalts. The sediments are heavily intruded by sills, crops on the seabed making the gentle shoal of probably the leaves of one major sill complex, Shiant East Bank. The shape and trend of this which can readily be mapped offshore using structural high are poorly defined, but it appears sparker and shipborne magnetometer traverses in to be roughly dome-shaped, with younger sedi­ combination with the bathymetry, since they ments encircling the inlier and dipping radially out­ form prominent features on the seabed, often wards away from it. The inlier is mainly with a pronounced dip and scarp slope. The Torridonian and the two boreholes (71/13 and intrusions are probably continuous at shallow 78/3) which provide the main evidence for this can depth over the whole area, resulting in very poor be tentatively ascribed to the Stoer and Torridon quality multichannel seismic reflection sections Groups on their lithology (G. Blackbourn, personal (such as, Figure 5, line 4). South of about 57° communication). However, the discovery of 30'N the sills are less frequent, and an attempt supposedly tOn situ acid gneiss at the southern can be made to map the base Jurassic and base margin of the bank during Vickers Pisces sub­ Permo-Triassic, employing similar criteria to those mersible dive P71/7B (Eden and others, 1973) used for mapping the North Minch. A tentative remains a problem (Figure 3). It is possible that the two-way time map for the lower horizon has been Loch Maree Fault runs through the basement high published (McQuillin and Binns, 1975, figure 5), on a NW-SE trend, bounding an area of relatively and parts of deep seismic line 5 (Figure 5) have shallow Lewisian to the south-west, as suggested by been reproduced previously (Smythe and others, the NW-SE trending gravity high of around 1972, figure 2; McQuillin and Bacon, 1974, 30 mGal (Figure 7). figure 2) to show reflection quality. The Permo-Triassic infill below northern Skye Inner Sound and Sound of Raasay is up to 1.5 km thick, and is probably bounded by The Lewisian of north Raasay and Rona is fault­ a fault or faults, trending NW-SE, below bounded to the east by the offshore continuation Trotternish (Smythe and others, 1972; Hall and of the Raasay Fault northwards up the Inner Smythe, 1973. figure 1). Thus at Staffin Bay, Sound. Lewisian crops out on the seabed as an Jurassic rests directly on Torridonian. West of inlier over an area 5 to 10 km north of Rona north-west Skye, in the area of 57° 30/N, 7°W, a (Figure 6, line 10) and, on the evidence of the structural high, trending NNE-SSW, may bring 35 mGal gravity high north-east of Trotternish, pre-Jurassic rocks to outcrop at the seabed (Figure probably forms a buried ridge trending north-west 6, lines 7 and 8). North-west of this ill-defmed from there. The westerly-dipping post-Torridonian anticline the sediments dip consistently WNW sediments below the Inner Sound cannot be towards the Minch Fault. Recent drilling, (Bore­ divided into discrete Permo-Triassic and Jurassic hole 80/14, 57° 42.9/N, 6° 54.6/W) undertaken sequences. The horizontal orange and yellow band­ since the completion of the geology map (Figure ing shown on the solid geology map (Figure 3) 3), has shown that these sediments consist of indicates schematically that the lowest part of the clays and lignites of probable Oligocene age, and succession here is expected to be Permo-Triassic. may equate to those of the Canna Basin (Smythe The outcrop of Permo-Triassic to Jurassic between and Kenolty, 1975). Broadford Bay and Loch Kishorn has been Pisces dive P71/6A in the same area of WNW­ delineated using bat1}ymetry, so its extent north­ dipping sediments recovered a sample of mudstone eastwards into Loch Kishorn is speculative. with miospores assigned to the Upper Carboniferous Between Trotternish and Raasay, the apparent­ (Eden and others, 1973, pp. 19-21) (figure 3). ly unfaulted westerly-dipping sequence seen in the This fauna is probably derived, as are the Carbon­ Sound of Raasay can be interpreted as lower to iferous miospores found in sample SH 207 (Binns middle Jurassic by extrapolation from the eastern and others, 1974, pp. 19, 36) of Zechstein age, coastline of north Skye. Jurassic rocks here have obtained at a similar location near to the Minch overstepped the fault-bounded Permo-Triassic of Fault east of Benbecula. These Carboniferous northern Skye, to rest on Torridonian (Smythe miospores, together with the erratics of Carbon­ and others, 1972), and north of Rona the Jurassic iferous limestone found on the Outer Hebrides rests directly on Lewisian. West of the 6°W Uehu and Craig, 1925, p. 639), imply that meridian the Jurassic sequence is heavily intruded Carboniferous rocks may still be preserved below by Tertiary sills. the Little Minch; however, no in situ Carbonifer­ ous sediments have yet been proved along the Little Minch western seaboard of Scotland between the Rona The sedimentary basin below the Little Minch and Ridge, west of the Shetlands (Ridd, 1980), and northern Skye is the northerly extension of the Sea Inninmore Bay, Morvern (Richey, 1961).

10 The Mz'nch Fault Tertiary Intrusive Igneous Rocks This major normal fault system forms the western The high density of dolerite sills in the Little margin of the Permian to Tertiary sedimentary Minch has been mentioned above. The basement basins of the Sea of the Hebrides and Minches. A rocks of the Shiant East Bank may have formed a most remarkable feature of the fault complex is its northward limit to the intrusion of the sills from parallelism and close proximity to the Outer Isles the Skye centre. Thrust, which dips eastwards at around 30°. The The aeromagnetic map (Figure 8) shows a thrust is believed to have been active during both number of linear negative anomalies, trending the Proterozoic and the early Palaeozoic (Mendum, NNW-SSE, of which the most prominent is that 1979). It has clearly acted as a locus for the late running from Loch Ewe across the North Minch. Palaeozoic and younger normal movements on the This anomaly is attributed to the Loch Ewe dyke Minch Fault, which presumably intersects the which modelling suggests is about 500 m wide with thrust a few kilometres below sea-level. its top at a depth of around 1 km. Two seismic The Minch Fault is a single fracture, clearly sections (reproduced in McQuillin and others, defined bathymetrically and magnetically from 1979, fig. 7/22) show diffraction patterns and Barra Head (at the southern tip of the Outer minor faulting due to this major dyke. It appears Hebrides) to Kebock Head (Figure 2) as a result of to terminate in the south at the Loch Maree the physical contrast between Lewisian gneiss and Fault, and can be traced northwards to the top of J urassic mudstones. The fault is difficult to the continental slope at 59° 25'N, 6° 30'W on identify only where Tertiary sills intrude the Sheet 1 of the aeromagnetic map of Great Britain. J urassic adjacent to the fault or, locally, the A swarm of large, reversely magnetised dykes Lewisian to the west (Mackinnon, 1974). From can also be inferred from the aeromagnetic map. Kebock Head, the fault runs northwards to These run through central Lewis at depth, and into Stornoway, where the downthrow of the Stornoway the Little Minch in the area of the Shiant Islands. Formation is taken up along two faults trending This swarm passes through the Outer Roag Tertiary NE-SW. South of the Eye Peninsula, Borehole igneous complex postulated by Bullerwell (1972) 78/4 cored Jurassic black shales with ammonite from inspection of the same map. fragments. The structural relations between this J urassic outcrop and the Triassic Stornoway Formation are not entirely clear, but as is shown schematic ally on Figure 3, the two main segments of the Minch Fault described above may be linked QUATERNARY GEOLOGY at depth (or at outcrop) across the mouth of Stornoway Bay. The Jurassic proved by Borehole GLACIAL AND POST GLACIAL SEDIMENTS 7S/4 just north of the inferred fault dips south to south-east. Introduction The main branch of the Minch Fault continues Quaternary sediments occur mainly in the North en echelon NNE from the eastern coastline of Minch, the Inner Sound and the Sound of Raasay, the Eye peninsula, but its character in this area and the Little Minch. These areas are separated by is very different. Instead of the consistent dip regions with considerable rock exposure. In places, of the downthrown sediments westwards towards the Quaternary is as thick as 150 m (Figure 9). the fault, the beds on the eastern, downthrown This thickness has been calculated from shallow side dip basinwards (Figure 5, line 2; Figure 6, seismic profiles using an assumed propagation lines 1, 2). It is noteworthy that this section of the velocity of sound through drift of 1.S km/so On fault has no close link with the Outer Isles thrust, most seismic records, the bedrock surface can be unlike the section farther south. The fault in this distinguished as a major reflector at a level of region has been mapped from the change in stratigraphic discordance at the base of the structural patterns on sparker sections such as Quaternary. The depth to bedrock is greatest in those of Figure 6, lines 1 and 2, and by fault tlae glacially over-deepened troughs, such as the diffraction patterns on multichannel seismic data Inner Sound. Figure 10 shows contours on the base (not shown here). These observations are of the Quaternary in metres below OD. corroborated by the ages of the sediments on Study of sparker profiles gives some indication either side of the fault. To the north of the area of the character of the Quaternary sediments. shown in Figure 3 (Borehole 77/S, 5So 50.3'N, Different sediment types tend to display character­ 6° 6.3'W), to the west of the fault, cored Lower istic seismic textures on the sparker records, a Jurassic, which when related to seismic sections, property which Binns and others (1974) used in an constrains the outcrop west of the Minch Fault to attempt to describe sediment types in the Sea of be Lower Jurassic or older. By constrast, the out­ the Hebrides. Boreholes which cored Quaternary crop east of the Fault in the northern North Minch sediments have been used to correlate lithology is no older than Middle Jurassic, and may even with geophysical characteristics. Unfortunately include Cretaceous sediments. only four boreholes in the Minches provide

11 7' s"W N

Scale

L-____~ ______~ ______~ ____~ ____~~km

• I.G.S. Borehole t

58" 30' la X2/33• 2b

2a la 72/32 1b ..... 3b \ S Tolsts Head

5a V 4b 4aA 5b 76/56 ~6b ~ Rhu COlgach

8a "'9b 5S'N \ 8b

~Q,. SHIANT ISLES Rubha Reidh

118

~ +Gairloch 1~ L WESTER ROSS

Trotternish

57' 30' ~ - ~ 14b 1 Inner Cl') Sound RAASAY~ V D ~LP~Y

Figure 11 Location of quaternary seismic profiles

12 I.G.S. BOREHOLE line l-North Minch 72/33 1a 1 km tv NE lb Upper laver of strong and Sea floor Horizontal reflectors merge continuous horizontal reflectors graduallv into jumbled and hummocky area 100ms I 1illr"ms \ ... - ~\ -----~ ~~ .~ 200 ms 'r~-? ----~~> -.....7 200..,. Strong and continuous horizontal reflectors, slightly dipping and ~ rising, and wfth minor hummocks Rockbead7 Geological~ structures beiow rockhead Jumbfed and hummocky reflectors

Line 3-North Minch 3a 3b Line 2-North Minch I.G.S. BOREHOLE Sea floor 2b 72/32 21i1 ..100 rns - Jumbled and hummocky reflectors / 100 ms

Upper laver' of :~~~t~~:izontal r::~O~f,~~~erC:~;e~t ------2"! --:: Rookbead Middle layer of jumbled Jumbled and hummocky and hummocky reflectors reflectors throughout 200 rns 200ms

..... \.>:)

Line 4-North Minch line 5-North Minch

~ .. 4b 5a 5b lG.5. B~1%~OLE

Sea~loor I Sub· horizontal hummocky reflectors ntal 100 ms lOOms /100 ms S8.~\or J SUb'horr hummocky 'efl.cto", 100ms \ I -r --.~ ---~ 1 -::::::-~ .. ~ 200ms-- - ~ -=----.:;_:::::::- = 200--;;;.1 £OOms - -. 200 ms

'\ Rockhead Rockhead

Line 6-North Minch Sea floor

Upper layer is ....acoustically transparent Rockhead apart from a few weak, horizontal but continuous reflectors g~;:~iO~k~ Figure 12 Line drawings of Quaternary seismic profiles Line 7-North Minch 7a~ 7b

line 8-North Minch

8a 120 rn. 120ms 8b Strong reflector lOOms 160ms

Jumbled and hummocky reflectors lOOms --:;::

240

Jumbled and hu"mmocky reflectors 280ms Layers above and below major reflector are largely structureless, almost acoustically .- !fi=~~u~U~:r;i;~n~r~!lt:t~' ..po.

Line 9-North Minch 9a 9b

'Sea floor Jumbled and hummocky reflectors hoo ms \ I lOOms

--... " ~ -~ ...... -~- ~. ~-:::-- - ~ r--.. -'"'~ :::---.. ~~ ~- ..r--. _~---.. 200ms 1" 7"/2"00"" R..lh&ad ! Dipping strata below rockhead

Upper 1ayer of strong Une 10-North Minch, Rubha Reidh to Loch Ewe horizontal reflec10rs 10a

ms '""'\ ;Oms Rockh&ad Strong honzonim reflectors Lower layer of jumbl&d (Rockh&&d ~ in multiple) and hummocky reflectors

, ...... --. line 11-lnner Sound and Sound of Raasay, Rubha Reidh to Gairloch 11a160ms 160 Very strong reflector Sea floor ms

'40ms 40 m~ :::::.:::::;::: 280ms 280

Roclihaad Layers above and below maior reflector ate almost structureless. apart from WMk and discontinuous horizon~ reflectors 160mo \60ms11b Sea floor 1200ms \ \ ~ 200ms \ --- '''''- - -....:..-.~~~ ~ 240 ... ~~ -r 280 ms -- Rockhead 280 m

line 12-Gairloch to Rona \60ms l~a 160 ms~ilirUc,w,;~;,;p.;TaY~----~ Upper and lower layers have Fairly continuous horizonta.l reflectors .,,----.,-.. - ... - horizontal reflectors 200ms 200ms strong reflector Sea floor 240ms ~.~ / l'l L 240"" I~'280~~- ~~ /-- 280ms u. 0;;;;;;;;.._____ ~ ~ -====----_____ 20

Strong internal refl~tQt's in tower layer Rockh$8d Jumbied and hummocky reflectors

lOOms t lower ~fayer'structlireless-:apaii-ff-Om discontinuous, horizontal line 13-Sound of Raasay Sea floor :tec.tors Very st,o~,ellector 13a'60ms / 160ms 240

200"", .. --: 280 ms

320 ~ L ~~~ ~~~:~Ies~I~~~~::k 240 ms; and discontinuous horizontal reflectors Rockh.... d

Largely structurelsss, but with weak and discontinuous horizontal reflectors Sea floor ~3b 160ms / "" :=::;;;:;;;;;::;;;;;~=~~"" ~~~ :, -- - /' 240 msl 1----->-...... ------=--.....-...... --...... -.,240 ms ~"'\c:::::?Rockhead Line 14-lnner Sound Partially exposed rock sill of Inner Sound 14a 160 m. lOOms

Sea floor 200ms 200ms

:40ms 24() ms

2~~~ 280 ms!

320ms

,Oms

Thick succession of largely structureless sediments with numerous discontinuous horizootal reflectorS Sea floor ms (Approximately 200 m of sediment here) 200ms

~ms

ms Line 15-Little Minch i280 Oms15b ---- 120m Rockhead Jumbled and hummocky reflectors 360ms

/' 24() m.~~

-0'1 line 16-little Minch,Loch Snizort Exposed rock sill of Loch Snizort 6840 ms

ms

_2' __ ~ ------"....-- "'''' 160ms 0'" :::=:--: Jumbled and hummocky reflectors, merging below with the rock.head. 200 which is often indistinct 100 ms

indistinct Rockhead more clearly discernable

line 17-Little Minch. Loch Dunvegan ~~~~rn~SV::Zi~~r reflectors 178160ms 17b ----- Thin ~irnoAnt rnV4'!r

Sea floor Continuous horizontal reflectors 24() 1'M

280ms

Jumbled and hummocky 320 ms reflectors in lower layer 320ms Quaternary sequences sufficiently thick for calibra­ Lithology Depth Thick- Micro- tion of seismic profiles so that interpretation of below ness palaeontology seismic records over a wide area is necessarily seabed (m) tentative. Selected seismic profiles across the (m) Minches are shown in Figures 11 and 12. Fine grey sand 0 25 Present day boreal In the Sea of the Hebrides, Binns and others with shell frag- fauna. ments, becoming (1974) recognised four formations within the more muddy with Quaternary, each possessing its own seismic increasing depth 'texture'. At the base, Formation 1 was observed Stiff grey-brown 25 5 Fewer cold water ele- to rest directly upon bedrock. It was identified in clay with shell ments than below. boreholes as till, with a characteristic texture of fragments and Climatic amelioration - jumbled and hummocky reflectors. Above it, small pebbles more temperate forms Formation 2 consisted of firm, poorly sorted sandy Sandy clay with 30 7 Upper part: Increased muds with scattered pebbles, recognisable by a cobbles near base number of cold water generally structureless seismic texture with a few forms widely spaced horizontal reflectors. This formation Lower part: Late glacial fauna of mixed is best developed along the courses of major ice cold and warm water flows, and although micropalaeontological forms assemblages show it to be of marine origin, it Rockhead 37 appears to have been deposited in close association with ice. Formation 3 was recognised on the With allowance made for the distance between seismic records by closely spaced horizontal the borehole and the seismic line, there is quite a reflectors which fade laterally and cannot be reasonable correlation between the two. The late correlated with any lithological feature in the glacial shelly clays of the lower part of the boreholes. Samples show it to consist of poorly sequence which are tentatively interpreted as sorted sandy muds, somewhat similar to Formation boulder clays agree with the Formation 1 identifi­ 2, but with fewer pebbles. Micropalaeontological cation from the geophysical data, while the upper evidence and a radio-carbon dating of 8680 ± muddy sands appear to be Formation 3 deposits, 250 years BP for Formation 3 deposits suggest it passing upwards into Recent sands at the sea floor. to be of late to post glacial age and of periglacial Line 2 also shows lateral changes in this area with origin, perhaps laid down in part a short distance the Formation 1 sequence developing reflectors from a retreating ice front. Formation 4 consists of Formation 3 type at the base of the sequence of a thin layer of modem sediment, a few metres towards the south-west. or less in thickness. Borehole 72/32, near Tolsta Head (Figure 11), penetrated more than 27 m of grey-brown clay, North Minch probably till, beneath a thin veneer of Recent This is by far the largest area of Quaternary sedi­ sand, which correlates well with reflectors of ments, extending northwards from the margins of Formation 1 type of seismic line 3. Borehole the Shiant East Bank. The maximum thickness of 76/55 was located at the junction of seismic lines sediment is present in two overdeepened troughs 4 and 5, both of which display a pattern of fine­ which run to the east and west of the Shiant East scale hummocky but sub-horizontal reflectors Bank, possibly along the paths of pre-glacial drain­ throughout the whole Quaternary sequence. At age courses. North of Greenstone Point (Figure 2), this site, the borehole penetrated over 65 m of the bathymetric map shows a ridge, trending N-S, dark grey, tenacious sandy and pebbly clay above which has no expression in the rockhead contours. bedrock. Here, the lithology is of Formation 2 It appears to be a medial moraine deposited type, but the seismic profIle seems to be between between coalescing ice flows emerging from the Formations 1 and 2 in character. To the south of Inner Sound and the Loch Broom-Gruinard Bay Borehole 76/55, line 6 shows only a thin layer of area. deposits of Formation 2-type above thick tills of Seismic line 1 (Figure 12) in the North Minch Formation I-type. passes within 1 km of Borehole 72/33 which In the southern part of the North Minch, to penetrated 37 m of Quaternary sediment before the west of the Shiant East Bank (lines 7 and 8), reaching rockhead. The nearest point on the line to thick Formation 2 deposits are present, with only the borehole indicates 32 m of sediment. The thin underlying patches of Formation 1 deposits. upper 11 m of the seismic section shows strong and Line 9 crosses the morainic ridge north of continuous horizontal reflectors of Formation 3 Greenstone Point and shows well developed type, while below, a mass of jumbled and Formation 1 sediments. Line 10, from the minor hummocky reflectors can be tentatively identified sediment body around Rubha Reidh, reveals as Formation 1. The borehole log and micro­ Formation 1 tills and Formation 3 deposits, the palaeontology report can be summarised as tills being best developed opposite the mouth of follows: Loch Ewe.

17 Inner Sound and Sound of Raasay at least, the four formations cannot be regarded The second largest area of Quaternary sediment in as chronostratigraphic units. the Minches occupies the Inner Sound and the A possible explanation for fades variation Sound of Raasay. Although glacially eroded to a within the Quaternary sediments of the North depth of 410 m below sea level and partly infilled Minch comes from a consideration of sea by 90 m of sediment, the Inner Sound has a rock levels in late-glacial times. Figure 13 shows a sill at its mouth with a sediment cover of less than proposed late-glacial/early post-glacial shore­ 20 m. The neighbouring Sound of Raasay possesses line for north-west Scotland, dated at 12 000 a rock sill with 30 to 40 m of sediment cover, years BP, the date of withdrawal of the main behind which up to 100 m of sediment have ice sheet. The shoreline is based upon a accumulated in the trough. 120-m submergence of the Uists (see Ritchie, Although not proved lithologically by borehole 1966; Valentin, 1953), and isostatic rebound data, the deep glacial troughs in the Inner Sound occurs within a dome centred on the southern­ and Sound of Raasay are considered to be filled by western Highlands, with a curved hinge-line thick Formation 2 deposits (lines 11, 12, 13, 14), running through the Little and North Minches, to with underlying patches of Formation 1 overlying cross the mainland south of the Durness area. bedrock. A strong, generally horizontal reflector The difficulties of estimating the inter-relation­ is developed within Formation 2 sediments in this ships of isostatic and eustatic changes in an area area, but the significance of this horizon is at like north-west Scotland were discussed by present uncertain. Bishop (1977), but even this sketched interpreta­ tion demonstrates that a 120-m submergence of Little Minch the Uists, when interpolated around the margin Quaternary sediments are, in general, not as thick of the dome of rebound into the North Minch, in the Little Minch as they are in the North Minch would put its floor (with an average depth today the Inner Sound, or the Sound of Raasay. Never­ of about 120 m) very close to sea level in late­ theless an overdeepened channel with thicker glacial-early post-glacial times. With such even sediments exists north of Vaternish Point, topography, the location of the coastline in the representing a major ice path from Loch Snizort. North Minch is difficult to define; farther south, The rock sill of Loch Snizort is only thinly buried greater relief makes the palaeogeography easier to and is partly exposed. It separates the thicker determine. The deeply eroded glacial troughs, such Quaternary sediments of the loch from those of as those in the Little Minch and the Inner Sound, the Little Minch where the thinner Quaternary formed a large system of salt-water lochs opening sequence is due to the absence of Formation 2 southwards into the Sea of the Hebrides. In the deposits, even within the deep troughs, as seen on deeper parts of the North Minch, seawater may line 15. have penetrated around or below a waning ice Borehole 71/7, at the mouth of Loch Snizort, sheet in late-glacial times. On lines 1 and 2, located at the northern end of seismic line 16 morainic sediments of Formation 1 are found on (Figure 11), proved 34 m of dark olive grey higher ground, and pass with deepening water boulder clay over rockhead. Line 16 shows into Formation 3 deposits, laid down contemp­ jumbled and hummocky reflectors of Formation 1, oraneously with the tills in shallow water close to with only thin overlying horizontal reflectors of the ice front. The possible existence of till. above Formation 3. Some development of Formation 3 Formation 3 deposits on line 2 may be the result occurs round the coast of Skye at Loch Dunvegan of slight fluctuations in the complex balance of (line 17), but thick Formation 2 only occurs land and sea levels in late-glacial times causing within the rock sill in Loch Snizort. relatively large regressions or advances of the coastline over the flat North Minch. Correlation To the south, interpretations of the seismic The seismic profiles from the northern North records show greater similarities to the Sea of the Minch are generally more difficult to interpret Hebrides. Although not proved lithologically by than are those from farther south, and give rise borehole data in the Minches, the deep glacial to problems apparently not encountered by Binns troughs have thick Formation 2 deposits (lines 7, and others in the Sea of the Hebrides. There, it 8, 11-14) with underlying patches of Formation 1 was possible to arrange the four formations in over rockhead. As deep seawater may have pene­ chronological order. Line 1 from the North Minch, trated under the ice in these troughs during late­ however, apparently reveals Formation 1 tills glacial times, Binns' conclusion that in the Sea of passing laterally to the north-west into Formation the Hebrides Formation 2 represents waterlain 3 sediments. Line 2 also shows lateral changes in glacial deposits, is also compatible with the this area, this time towards the south-west, with a evidence from the Minches. On seismic profiles, Formation 1 sequence developing seismic character­ Formation 2 deposits can be seen as far north as istics of Formation 3 reflectors at the base. the Eye Peninsula in line 6, where they form a thin Although the lithological reality of these 'seismic layer above a much thicker lower till. However, facies' cannot be confirmed without further bore­ geophysical records with characteristics similar to hole evidence, it appears that, in the North Minch Formation 1 correlate with Formation 2 lithology 18 7'W S'W

~-i&>...... •.... Land

5B"N

=

in Borehole 76/55 a short distance farther north. SURFACE SEDIMENTS One feature of Formation 2 sediments in the Minches, not mentioned by Binns, is the develop­ Introduction ment of a strong reflector within this formation. Various relationships between the upper and lower The surface sediments are those deposits present at parts of the formation are shown on lines 7, 8, the seabed and formed under the Recent sediment­ 12 and 13. A major reflector associated with a ary environment. The surface sediment throughout lithological change is also present on line 2 in the most of the Minches consists of a thin layer of North Minch. This can be traced from the North material derived from deposits of glacial age, re­ Minch into the mouth of the Inner Sound, where sorted to varying degrees under post-glacial and it disappears (line 14). Without lithological or Recent marine conditions, during which only the palaeontological evidence, the significance of finer fractions have been subjected to a significant this feature is not known. amount of transport. Direct detrital contributions The relatively thin Quaternary sediments of the do not appear to be important except very near to Little Minch are explained by the absence of the coasts, but quite large areas are covered by Formation 2 deposits even in the deep troughs local biogenic sediment. Some influx of sand-grade (line 15). Constriction of the ice flow in the material from outside the Minches is indicated in narrow Little Minch channel may not have the coastal areas of the extreme north of the North favoured deposition, for sediment thicknesses Minch. The fundamental textural characteristics only start to increase to the south, in the broader of the surface sediments are summarised ip Figure Sea of the Hebrides (Binns and others, 1974). 14, where the five sediment categories are simple

19 N

Scale

10 30 Cape Wrath 'm t

Bull 01 lewis 58" 30'

NORTH MINCH

TRAVERSE A

M 282 M296 M 311 M 315 NS 2()2 Lochinver STORNOWAY BASIN

TRAVERSE B M 251

M 146 5S"N

'it;, SHIANT ISLES

TRAVERSE C M 105

+ Gairloch

WESTER ROSS

RONA

i 08 J~ S1" 30'

Inner i~I:) Sound ('/) RAASAY V \) ~LP~

r

Figure 15 Location of sediment sample traverses

20 STORNOWAY BASIN (SOUTH) SHIANT EAST BANK

DEPTH (m) M247 M248 M33 M147 NS180 M250 M251 M146 o ,.---r-.---.----~------__r-.__---__,,_-----_,_------_._-_r Sea-level 40 80 120 140~~~~~~~~~~~~~~~~~~~~~ __~~,.~~~~~~~~~~~~~~~~

M 247 M248 M33 M147 NS180 M250 M251 M145

GRAB

o

Rockhead

50

100

150

Gravel

200 i~\V{/l Sand

Mud

250

300

Centimetres

Figure 16 Variation of surface sediments along traverses

21 TO LSTA TIUMPAN HEAD HEAD BROAD BAY STORNOWAY BASIN NORTH MINCH ENARD BAY DEPTH (m) M273 NS189 NS188 M222 M224 NS202 M317 M282 M296 M315 M278 Sea-level

M273';:':;:;;::" NS189 NS188 M222 M224 NS202 M317 M282 M 296 M315 M278 GRAB -:~:.:.::}: ~~~); :.:::: ...':'.: 1~g;} ",-0-' .0' BI RI ~ O.a.·~ o . ",oe .. :--_··~O... • tv tv

...... : u : l1li Gravel [iJ0

Sand

200 Mud Centimetres LITTLE MINCH SKYE!SHIANT PLATFORM SHIANT EAST BANK RUBHA LOCH EWE REIDH

DEPTH (m) M92 NS182 M245 M96 M94 M202 O+---.------.------r---__~--_,------,--- M89 I M~2 :71 Sea-level 40 80

180120~===Jb~~G[21~±1[ili~iiiGllfSill[[dilit~22E2~2K2221i212IDillt~JJiE[2[t=£i2~::::::IIdtlI2ki3Z2GITE:~~~~~§S2J2211JjSWdSdlli£iilif1S~ :I:

M75 MS1 M104 M105 M237 M245 M96 M94 M 202 M92 M91 M89 NS182 M292 ...... "";':.. ; o:··~ ' .. GRAB - :~.. ::~: ~':'i-' ., [hl... ~-~- [ill] ~- . ~=- m. --- .":.. ' .. ·.·0 t-..:l 01 g 0:;=0 ij I.>:> ~ [I~-'-!- .~ 50 I ~"".~ i I

100

Gravel

150 [~t}(/l Sand

Mud

200

250

Centimetres amalgamations of fifteen types defined by Folk parent material, they are not closely inter-related (1965). Comparison of sediment distribution with to each other. Till-like sediments survive in the bathymetry shows that gravels are found in Minches to intermediate depths, where they are shallower water, muds in the deepest hollows and neither altered by the strong bottom currents of sheltered lochs, while sands occur at intermediate shallower water nor masked by the mud deposition depths. of the deep hollows. Nor are they to be found on Figure 15 shows the location of sediment the more exposed floors of the northern North sample traverses across the Minches, and Figure 16 Minch or southern Little Minch. summarises the variability of the surface sediment layer. Many core samples penetrate the surface Suspension deposited muds layer to reach the sediment below. Where variation On the C-M diagram, samples from the mud­ occurs, coarser surface sediments generally pass floored hollows and lochs fall into the suspension downwards into finer material. The contrast is deposit field. Visual observations from a manned most dramatic on the shallow banks north of Skye submersible in a deep hollow in the Little Minch and off the coastal headlands. In deeper water the (Eden and others, 1973) suggest that net sedi­ muddy sands, which cover much of the central mentation is in progress, a dusting of fine sediment North Minch and Little Minch, generally prove to in filling and covering shell debris on the sea floor. be no more than 0.8-1.0 m thick, and pass Indication that the muds are derived from exposed gradually downwards into more muddy sediment. glacial clay or till comes from the study of their The soft muds of the deep hollows differ from mineralogy by X-ray diffraction (Bishop, 1977). most other samples from the Minches. No textural Both the glacial clays on the banks and also clay contrasts with depth were encountered, and only fractions from deeper mud deposits consist of a occasional slight colour changes were present. similarly proportioned mixture of chlorite, illite Palaeomagnetic investigation of muddy cores and calcite. The older sediment displays a slightly from the Inner Sound (Bishop, 1975) reveal a greater chlorite content, as is consistent with clays maximum sedimentation rate within this material of direct glacial origin, while mud from semi­ of about 0.60 m per 1000 years, with the rate enclosed bodies like the Inner Sound of Loch decreasing slightly towards the present. Such a Broom, has a little more illite, possibly due to rate of accumulation suggests that the muds may some modern input. Bathymetry alone is not be only 7-10 m or less in thickness. Probably the sufficient to explain fully the distribution of Recent sediments in the Minches are rarely more suspension-deposited mud, because some mud­ than 10 m thick. floored areas, especially Loch Snizort and the Sound of Raasay, are no deeper than the sand Sands and muds related to tills covered floor of the north of the Little Minches. Figure 17 is a classification of surface sediment In these cases, mud deposition is partly explained types according to their mode or origin. A very by the shelter of the surrounding land, and partly large area is covered by sand and mud with by the net circulation pattern of these broad and textural properties similar to those of tills. In deep mouthed lochs (Craig, 1959; Milne, 1972), Figure 18 the graphic mean grain sizes of grab with an inshore-flowing bottom current of denser samples have been plotted against their coarsest saline water in which mud may be suspended and grain sizes (the 1% value on the cumulative curve). carried in from offshore sources. Bishop (1975) The resultant C-M diagram has genetic fields has shown how much a current could be which have been taken, with some modifications, responsible for falling sedimentation rates south­ from Schlee (1973). The degree of sorting tends wards along the Inner Sound. The muds of the to increase towards the C-M line. In Zone A, hollows and lochs contain minor quantities of farthest from the line, Schlee has plotted known fine sand-sized quartz and mica, and foraminifera tills, the sediment type with most disparity are abundant. between mean and coarsest sizes. Numerous Minches surface sediment samples fall within this Lithic sand same area. Zones D, E and B are transitional zones, In the more exposed southern and northern not used by Schlee, between the till-field and the extremities of the Minches, till-like muddy sands suspension- and traction-fields. All three zones are pass gradually into cleaner fine-grained sand, filled by dense spreads of data points which lead possessing textural characteristics that plot into from the till-field into the suspension mud, Zone F of the C-M diagram, which has been traction sand and gravel fields, zones G, F and C. associated with sediments laid down after transport All till-like sediments, from zones A, D, E and B, by bottom traction. However, there is no evidence have been mapped together on Figure 17. The that the sands have undergone any significant spread of data points on Figure 18 displays greater transport. They may represent either a less muddy, continuity between the till-field and the sands, more current-agitated derivative of till-like sedi­ muds and gravels than exists among these three ments, or are possibly derived from periglaciaI sediment end members themselves, suggesting deposits. The textural and mineralogical properties that although they are all related to a till-like of these dominantly lithic sands do not suggest

24 6·W

N Sand and gravel size shell debris [IIIll]]] of local origin

Dominantly lithic gravel, derived from till or exposed bedrock.

~ Sands and muds, closely •L5 related to tills

Re-sorted lithic sand with relatively 58· 30' ~ little gravel or shell. Uttle transport EEJ Well sorted shell sand influx

Suspension deposited § muCls

I .,/X ...... /N.r;H..:::: , .... :...... , .

ill!!!!!!!'!...... Lochinver

WESTER ROSS

57"

Scale

r

Figure 17 Genetic classification of surface sediment

25 -... B ... -9 !

B _5+-______~~A

-4 .. o • .. -.. .\ -3 . .

-2 , I:.e •• e ...... ;.:. -1 .---;'"'"' .• 0 . ----. . 0;------+---•• ...... 2 '.. . .. 3 G . e-,. .:•. 4 '. '"

5

6

7

8

9

9 8 7 6 5 4 3 2 o -1 -2 -3 -4 -5 -6 -7 -8 -9 Graphic mean grain size

Figure 18 Surface sediment C-M diagram that sediment movement is taking place, and side­ transported for some distance. The most likely scan sonar records display no evidence of sand source of the shell sand is from the shelf west of transport. the Outer Hebrides, which, however, has not yet Shell sand been mapped in detail. Shell debris generated in Towards the coast of Lewis, the lithic sands of the this area could be transported along the Lewis North Minch gradually give way to a distinctive coast towards the Butt of Lewis and be drawn into body of shell sand, consisting of well sorted and the Minches by strong tidal currents around the very worn shell fragments of medium sand grade. headland caused by the unusual tidal gradient. This sand is similar to the shell sands that can be Arrival of high water on the Minches side is 15- seen on the Atlantic coast of Lewis, and which has 20 minutes later than that on the Atlantic coast. accumulated in wide beaches near Tolsta Head and A combination of tidal and coastal currents near in Broad Bay. In contrast to the biogenic sand the shore (Ministry of Defence, 1958; 1973) found elsewhere in the Minches, there is no carries the sand as far south as Broad Bay, where it evidence that this deposit is locally derived; on the is impounded. Tidal currents are less strong contrary, it bears every indication of having been immediately south of the Eye Peninsula.

26 Lithic gravels debris in the Minches corresponds with the general Lithic gravels form the surface sediment on much pattern of carbonate distribution on Inner Shelf of the Shiant East Bank and around coastal head­ areas (Milliman, 1974), elaborate biogenic assem­ lands; no gravel deposits occur at greater depths. blages being common near rock exposures swept Lithic (as opposed to biogenic) gravel is more by strong currents and away from major river abundant around the coasts, and the lithology of mouths. the pebbles matches that of the adjacent coasts. Lewisian metamorphic rocks dominate along the Hebridean coasts, Torridonian sandstones and Lewisian along the mainland coast, and Mesozoic sediments and basic igneous rocks occur north of Skye. The Shiant East Bank similarly displays a close relationship between geological outcrop and pebble lithology; Mesozoic, Tertiary, igneous and Torridonian pebbles are present. To a certain extent, details of the distribution of rock types among the pebble gravels of the banks reflects probable glacial transport, although this can be more clearly seen in the pebble component of the till-like sediments from greater depths, where lithologies indicate the source areas of the ice sheets which once covered the Minches. On the banks, a thin veneer of pebble gravel often passes downwards into pebbly clay, and not directly onto bedrock, and the surface deposit probably represents the winnowed residue of a once thicker sediment layer. The bottom currents of these shallow areas are strong enough to be responsible for the removal of the finer fractions, much of which, it is thought, has come to rest in the neighbouring mud-floored hollows. The slowing down of the rate of sedimentation of the muds over the last few thousand years, as revealed by the palaeomagnetic properties of the Inner Sound cores, may be partly related to the increasing protection afforded to finer sediment in shallow­ er areas by a growing cap of gravel and sand.

Locally derived shell sands and gravels In shallow areas away from the coasts, pebble gravels are replaced by a sediment consisting of sand and gravel-sized biogenic debris, in which the contribution of lithic material often falls below 5% (by volume). This sediment, like the pebble gravel, forms a thin veneer over rockhead or glacial clay and although it is best developed on Mesozoic and Tertiary igneous ground in the Little Minch and southern East Bank, it also occurs on the sea­ ward side of the Cape Wrath Bank. A variety of biogenic debris is present, from unbroken shells to comminuted and unrecognisable fragments, with juvenile forms common. The sediment represents the debris of a diverse living community of modem benthonic species on banks which are productive fishing grounds. Bivalve shells make the greatest contribution to the sediment together with gastro­ pods, echinoderms, worm tubes and crustaceans. Solitary corals were found at two stations in the Little Minch. For the most part, this sediment is poorly sorted, but immediately south of the Shiant Isles reworking has produced a small area of well­ sorted shell sand. The distribution of biogenic

27 REFERENCES Geikie,J. 1878. On the glacial phenomena of the Long Island, or Outer Hebrides. Quart. J. Geol. Soc. Allerton, H. A. 1968. An interpretation of the gravity of London, Vol. 29,532-570. the North Minch, Scotland. MSc Thesis, University of -". George, T. N. 1966. Geomorphic evolution in Hebridean Scotland. Scott.]. Geol., Vol. 2, 1-34. Durham [Unpublished] . Anderson, F. W. and Dunham, K. C. 1966. The geology of Godard, A. 1965. Recherches de geomorphologie en Ecosse du Nord-Ouest. (Paris: Societe d'Editions Northern Skye. Mem. Geol. Surv., 216 pp. Baden-PoweU, D. F. W. 1938. On the glacial and inter­ les Belles Lettres.) 701 pp. glacial marine beds of northern Lewis. Geol. Mag., Hailwood, E. A., Borck, W., Costa, L .• Dupeuble, P. A., Muller, C. and Schnitker, D. 1979. Chronology and Vol. 75,395-409. biostratigraphy of northeast Atlantic sediments, Binns, P. E., McQuillin, R. and Kenolty, N. 1974. The DSDP leg 48. 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28 MacKinnon, A. 1974. The Madadh Rocks: a Tertiary Schlee,J. 1973 . .Atlantic continental shelf and slope of olivine dolerite sill in the Outer Hebrides. Scott. I. the United States: sediment textures of the north­ Geol., VoI. 10,67-70. eastern part. Pro! Pap. U.S. Geol. Surv., No. 529L. McQuillin, R. and Ardus, D. A. 1977. Exploring the 64 pp. geology of shelf seas. (London: Graham and Smythe, D. K. and Kenolty, N. 1975. Tertiary sediments Trotman.) 234 pp. in the Sea of the Hebrides. I. Geol. Soc. London, and Bacon, M. 1974. Preliminary report on seismic Vol. 131,227-233. reflection surveys in sea areas around Scotland, 1969-, Sowerbutts, W. T. C., Bacon, M. and McQuillin, R. 1973. Rep. Inst. Geol. Sci., No. 74/12, 7 pp. 1972. Deep sedimentary basin below northern Skye - andBarclay,W. 1979. An introduction to seismic and the Little Minch. Nature, Phys. Sci., Vol. 236, interpretation. (London: Graham and Trotman.) 87-89. 199 pp. Soper, N.J. and Barber, A.J. 1979. 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