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A GEOLOGICAL REPORT OF STRATH, ISLE OF SKYE
Christian J Garvey
Department of Earth Sciences, Durham University
2016
This dissertation is submitted in partial fulfilment of the requirements for the
degree
BSc in Geology
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ABSTRACT
The Isle of Skye is the largest single landmass in the Inner Hebrides. At its heart is the Cuillin
centre, which represents one of a number of Palaeogene igneous central complexes in NW
Scotland. The study area, Strath, lies on its eastern periphery. This report investigates the
geology of the Strath area, based upon fieldwork conducted between 5 th July and 14th August
2015.
Post-Palaeogene erosion has exposed deep crust, which has allowed structural relationships
between igneous and sedimentary units to be more easily investigated. Palaeogene rocks mostly
include intrusive mafic dykes and sills which intrude Pre-Cambrian to Mesozoic sediments.
Most are of Jurassic age, which rest unconformably on Ordovician dolostones, Cambrian
quartzites and Torridonian sandstones. An open arcuate fold is believed to have occurred during
a significant period of deformation in the early stages of the Palaeogene. It is proposed that the
fold is strongly related to the stresses and pressures present in an adjacent caldera, such as the
Eastern Red Hills. The core of the antiform is then intruded by the youngest major Palaeogene
intrusion, the Beinn an Dubhaich Granite.
ACKNOWLEDGEMENTS
I am indebted to many people who have helped me undertake and complete this project. I would
firstly like to thank my supervisor Prof. Colin MacPherson for making such a superb mapping
area available, and also for all the time he has invested during the writing of this report. Further,
the Durham University Earth Science Dept for their significant financial support. Lastly, I
would also like to thank Devon Platt, Jonathon Taylor and Yannick Withoos all of whom made
the atrocious Hebridean summer somewhat bearable.
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CONTENTS
Chapter Page
1. Introduction 4
2. Sedimentary Geology 5
2.1 Stratigraphy 5
2.2 Arkosic Arenite Formation 5
2.3 Quartzite Formation 6
2.4 Camas Malag Dolostone Formation 7
2.4.1 Dolostone T1 7
2.4.2 Dolostone T2 8
2.4.3 Dolostone T3 9
2.5 Lime Breccia Formation 10
2.6 Allt nan Leac Limestone Formation 12
2.6.1 White, Blocky Limestone 12
2.6.2 Micritic Limestone 13
2.7 Boreraig Formation 14
2.8 Suisnish Shale Formation 16
2.8.1 Micaceous Shale 17
2.8.2 Sublithic Calcarenite 18
2.9 Discussion 19
3. Metamorphic Geology 21
3.1 Camas Malag Dolostone Formation 21
4. Igneous Geology 22
4.1 Mafic Dykes 22
4.1.1 Basalt & Dolerite Dykes 22
4.1.2 Gabbro Dykes 24
4.1.3 Discussion 26
4.2 Carn Dearg Igneous Complex (CDIC) 27
4.2.1 Porphyritic Basalt Sill 28
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4.2.2 Microgranite Sill 28
4.2.3 Discussion 29
4.3 Beinn an Dubhaich Igneous Complex (BADIC) 31
4.3.1 Beinn an Dubhaich Granite 31
4.3.2 Discussion 34
5. Structural Geology 35
5.1 Beinn an Dubhaich Igneous Complex Zone 35
5.1.1 Broadford Anticline 35
5.1.2 Beinn an Dubhaich Granite Intrusion 36
5.1.3 Ductile Deformation 39
5.1.4 Discussion 40
5.2 Major Faults 41
5.2.1 “Siliciclastic Wedge” 42
5.2.2 Glen Boreraig 42
5.2.3 Carn Dearg Igneous Complex Zone 42
5.2.4 Discussion 43
6. Economic Geology 44
7. Discussion 44
8. Synthesis & Geological History 46
9. Conclusion 48
References
Appendices
NB: “Figure A/B/C/D/E” correspond to the figures as labelled on the Geological Map of
Strath.
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CHAPTER 1 INTRODUCTION
The Isle of Skye, the largest island in the Inner Hebrides, peels away from the NW coast of
Scotland, separated only by a sea loch, Loch Alsh. The island’s skyline is dominated by the
formidable Cuillin Ridge, which provides Skye’s most dramatic and significant topography.
Central Skye has been the focus of many previous studies, owing to its diverse and varied
geology. The area was first described by MacCulloch in 1816 and has been visited frequently
ever since. This includes the pioneering work of Alfred Harker (1904) in the early twentieth
century.
The Skye Central Complex is one of seven in western Scotland. The central complexes are the
deeply eroded roots of major volcanoes (Emeleus & Bell, 2005). Associated Palaeogene
igneous suites intrude into Proterozoic to Mesozoic age sediments in the form of extensive sills
and well-defined dyke swarms. Geophysical surveys, e.g. Bott & Tuson (1973), conducted over
the Skye Central Complex have indicated the presence of a substantial dense, basic core which
has been proposed to extend to a depth of 14km. White (1988), among others, attributes this
period of intense volcanism to a proto-Iceland mantle plume.
Strath, the studied area, lies to the east of the rugged Cuillin centre, on the opposite side of Loch
Slapin. It covers approximately 10km2 spanning the area between Torrin in the north and the
hamlet of Suisnish on the southern coast. From the elevated vantage point of Beinn an Dubhaich
(235m), there are 360ᵒ vistas; to the north, there are the domed Eastern Red Hills (Beinn na
Caillich 732m); to the south can be seen the smaller Hebridean islands of Eigg and Mull; and
to the west, the great Cuillin outlier Bla Bheinn (928m). The area is sparsely populated, lying
~7miles west of the island’s second largest town, Broadford. Small crofts and peat farms
dominate the landscape. The study area is serviced by a single track road which runs parallel
with Loch Slapin from the B8083 at Kilbride, south to Suisnish.
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The local geology is an excellent representation of Scottish geology. To the north Cambro-
Ordovician carbonates fringe upon the Beinn an Dubhaich Granite. Mesozoic sediments and
mafic intrusions dominate the south.
CHAPTER 2 SEDIMENTARY GEOLOGY
Seven different sedimentary formations have been identified in the mapping area, based on
characteristic composition and form. Where appropriate a formation is sub-divided into
different member lithologies. Interpretations on the likely depositional environment are made
at the end of each sub-section. All seven of these formations are observed in the N-S trending
cross-section (Figure C,D).
2.1 Stratigraphy
To the north carbonates fringe an extensive outcrop of granite. In the east a rare and isolated,
wedge-like outcrop of siliciclastics is observed. It contacts abruptly with the carbonates.
Unconformably deposited on top is a facies series of conformable limestones and shales, which
prevail down to Loch Eishort (refer to Figure D). Type sections and lithological contacts are
exposed well along the eastern coast of Loch Slapin, along which two sedimentary logs were
conducted (refer to Appendix A, B).
2.2 Arkosic Arenite Formation
Arkosic Arenite (AA) outcrops abruptly at the head of the Allt nan Leac valley (O|C 21.1;
GR:[160746,818259]). The rock is significantly exposed along a large north facing break in
slope 15-20m high and extending ~100m laterally (see figure 46, Section 5.2.1).
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Appearance On a fresh surface
the rock appears deep orange to dark buff
colour, and often found weathered to a dark
red/brown (figure 1). The arenite is well
sorted, comprising of grains no larger than
0.7mm, with an overall grain size of
approximately 300μm. The four clast types
identified are: quartz (50%), plagioc lase
(10%), K-feldspar (10%), and lithic fragments (5%). Due to the moderate proportion of
feldspars and the sub-angular texture of the clasts, it can be said that the arenite is
compositionally immature but texturally sub-mature. The unit appears to be fairly massive,
although structures may be disguised by vegetation.
Depositional Environment It is interpreted that these red sands were deposited in a
terrigenous fluvial system of moderate energy. The sand is well cemented and reasonably hard,
most likely to be representative of post-depositional burial and lithification processes. The
presence of lithic fragments, including a small percentage (<1%) of muscovite mica, is
indicative of a metamorphic basement. The significant proportion of intact feldspar grains is a
strong indicator of a proximal, subaerial source.
2.3 Quartzite Formation
Traversing west along the aforementioned break-in-slope, quartzite is exposed after a brief
depression in the topography (GR:[160570,818260]). It is sharply bound to the sandstone in the
east.
Figure 1: Arkosic Arenite. Although the rock is mostly composed of sub-angular quartz, the red colour is believed to
come from the K-feldpar content and the oxidation of iron
within minerals. Lithic fragments appear as black specks.
Thumb nail for scale.
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Appearance The Quartzite represents a
very distinct change in lithology in the area,
outcropping as a homogeneous, white to very
pale grey crystalline rock. Weathered surfaces
often appear polished, which indicate the rocks
hardness. Equigranular quartz clasts are no larger
than 2mm in diameter (figure 2). Towards the
base of the unit, slightly larger spherical quartz
grains (3-4mm) are observed close to a 5mm
wide fracture (GR:[160366,818163]). It is
inferred that quartz pebbles, belonging to a quartz conglomerate, have been dislodged and
loosened by the inherent fracturing.
2.4 Camas Malag Dolostone Formation (CMDL)
The Camas Malag Dolostone Formation is an extensive group of carbonates which dominates
the northern section of the map. It shares contacts with six lithologies, all of which are believed
to be unconformable. The formation has been subject to numerous igneous intrusions and is
subsequently metamorphosed in places (see Chapter 3). The CMDL comprises of three subtly
different members; Dolostone T1, Dolostone T2, and Dolostone T3, (youngest to oldest) (see
Figure C, D). Contacts between them are considered to be gradational. As T1 was the first to
be identified in the field, T2 and T3 are recognised comparatively, by their distinguishing
features.
2.4.1 Dolostone T1
Dolostone T1 is the most extensive member, outcropping well from east to west. The best
exposure of T1 is on the Loch Slapin shore, which is subsequently the rock’s type locality.
Here, it is found unconformably in contact with the overlying Boreraig Formation (Section 2.7),
Figure 2: Fractured, white quartzite. Inset: Where dark
green lichen is present, individual quarz grains become
apparent. Hammer head and thumb nail (inset) for scale.
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separated by a 25cm thick basaltic sill.
Appearance The weathered surface
represents a distinct change in lithology from the
sediments to the south; it exhibits a very rough,
uneven honeycomb texture and lacks distinct
sedimentary structures. On a fresh surface the rock is
dark grey and fine-grained although recrystallizat ion
is evident in places. In stark contrast to the Boreraig
Formation above, it is not fossiliferous but reacts
moderately with acid and is particularly hard. An
impure limestone i.e. a calcic dolomite or dolostone is duly inferred. A defining characterist ic
of the rock is the abundance of small (<40mm) and irregular, dark grey/ black chert nodules
(figure 3). These nodules are randomly spaced within the member and, unsurprisingly, a
positive relationship between the roughness of the dolostone and the concentration of chert
nodules was observed. Bedding planes are prominent.
2.4.2 Dolostone T2
Dolostone T2 generally tends to outcrop closer to the Beinn an Dubhaich Granite than T1, and
is most extensive in the east.
Appearance The most characteristic property of the rock is the randomly spread and
abundant white biogenic nodules (up to 10cm). Although they do not appear to be associated
with the bedding planes, they appear to have once been attached to the dolostone itself. Some
have an inner globe-like feature, which contains many calcite “strands”, enveloped by a further
calcite “wall”. Others, such as figure 4, are elongate and have calcite forming concentrica l ly
around a chert nucleus.
Figure 3: Dolostone T1. Dark grey chert nodules
protrude from the rough weathered surface of the
outcrop. Deep vertical incisions are inferred to be
weathering features (RHS of photo).
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The extent of T2 is limited to the south by a younger brecciated sedimentary unit. A palaeo-
karstic surface is observed close to this contact at O|C 19.4 (GR:[159890,818100]). Here,
individual karsts are infilled with large (2-3cm), clasts of dolostone (figure 5).
2.4.3 Dolostone T3
Believed to be the youngest member, T3 outcrops closest to the Beinn an Dubhaich Granite. It
is not a laterally extensive unit but rather outcrops in patches particularly within the granite
(refer to ‘Dolostone Islands’ in Section 5.1.2).
Appearance Due to its relative proximity to the igneous intrusion, T3 is often partially
to completely recrystallised and has subsequently lost considerable internal structure. On a fresh
surface the unit varies from mid-grey to orange/pink as the granite contact is approached. Thin,
black chert bands, ~4mm thick, are common within the rock.
CMDL Depositional Environment Holistically the CMDL represents an environment
dominated by warm, shallow carbonate seas. Bedded chert common within the dolostone of T3
is attributed to siliceous oozes of deeper water successions. From here it shallows upwards into
a warmer sea of fairly low energy, i.e. a setting favourable to marine benthic organisms such as
corals and sponges found in T2. Likewise, chert nodules present in T1 can form through the
silicification of a nucleus e.g. an ooid or a bivalve. Considering this, it is inferred that this
member was also deposited in a fairly shallow, warm carbonate sea. The formation
Figure 4: Dolostone T2. Large, white biogenic nodules are a
diagnostic feature of this lithology. Inset: Close up of nodules.
White calcite forms around a dark grey chert nucleus.
Figure 5: Infilled palaeokarst at the contact of T2 and Lime Breccia Formation. The infill is primarily composed of
brecciated dolostone.
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predominantly exhibits a vertical change in environment, however lateral variations are evident
as members do not always outcrop in neat parallel bands. The occurrence of palaeo-karsts at
the top of the T2 member is likely to be caused by meteoric fluid interaction. It is analogous to
the surface the CMDL exhibits today, thus represents an erosional unconformity.
Dolomitisation of the whole CMDL is attributed to diagenetic processes.
2.5 Lime Breccia Formation
The Lime Breccia (LB) is a laterally extensive unit, outcropping immediately to the south of
the CMDL and the siliciclastic Quartzite and Arkosic Arenite Formations. The lower boundary
of the LB with the CMDL is marked by a palaeo-karstic surface, which represents an erosional
unconformity (figure 5). The unit pinches out to the west and is not observed further than the
road-river intersection of the Allt nan Leac (GR:[158669,818397]). It is deemed to vary in
thickness from E to W (Figure D).
Appearance Although the rock still maintains a significant lime content, a clear
lithological change is observed due it’s make up of medium to large (some >90mm) (figure 9)
angular clasts of dolostone with small brecciated pieces of chert. The very fine-grained,
blue/grey, calcareous matrix generally constitutes up to 60% of the rock. In terms of
Figure 7: Hjulstrom Curve illustrating the high velocities required (highlighted) in order to erode and transport gravel
and boulder sized sediment. Figure 6: Clast supported Lime Breccia. Lenses of sandy matrix are
highlighted.
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sedimentary structure, the breccia is lacking and bedding planes are often difficult to
distinguish.
Depositional Environment It is evident from the immaturity and angularity of the
clasts that the unit was deposited proximal to source in a turbulent fluvial environment with a
large erosional power (figure 7) ripping up and brecciating the dolostone country rock.
Occasional occurrences of a red/brown, sandy matrix where the breccia outcrops adjacent to
the sandstone (O|C 21.3; GR:[160816,818029]) (figure 6) are inferred to be terrigenous in
nature. This matrix fades into the calcareous matrix, and clasts are undifferentiated between the
two. This is believed to represent post-depositional gravity settling. From the above it is
proposed that a marine regression from carbonate seas to a continental setting has occurred. A
subaerial, continental setting is further supported by the karstic dolostone.
Clast Analysis Clast analyses were conducted at two separate localities, Clast
Count #1 at O|C 21.4 [GR: 160823, 818032] and Clast Count #2 at O|C 19.4 (GR:[159888,
818103]). Data was gathered in order to determine the nature of the breccia at site specific
locations, thus enabling the interpretation of a
depositional environment. Clast Count #1 yielded the
most conclusive palaeo-flow data; strongly indicat ing
a NE-SW trend. This is replicated in 1 out of 2 grids in
Clast Count #2. Significant correlations such as this
Figure 8: Combined palaeoflow data retrieved from Clast Count #1. The data strongly indicates
a NE-SW trend; mean direction 064ᵒ-244ᵒ.
N=200; Internval= 5.
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suggest the occurrence of a distinct NE-SW orientated fluvial channel.
2.6 Allt nan Leac Limestone Formation (ANLL)
The formation’s base onlaps onto the dolostone in the west and the breccia in the east. The
upper extremity is taken at the conformable boundary with the Boreraig Formation, where the
rock becomes less calcareous and are more distinctly bedded. The ANLL has no coastal outcrop
and is poorly exposed in land, hence interpretation is difficult. The boundary with the BRFM
is inferred from limestone associated vegetation, such as ferns, and large, irregular hollows in
the ground. An angular unconformity of approximately 23ᵒ is observed between the CMDL and
the ANLL in the Allt nan Leac. Considering the conformable relationship between the ANLL
and Boreraig Fm, it is likely that an undulating lower bound i.e. an undulating depositiona l
platform of the Lime Breccia or dolostone beneath that, is the reason for the absence of the
ANLL in the west. The formation comprises two lithologies; these are discussed below.
2.6.1 White Blocky Limestone
This member is best (and only) observed in the small river gorge of the Allt nan Leac, 50m
upstream from the road bridge (GR:[158669,818397]).
0
5
10
15
20
25
30
35
0-<10 10-<20 20-<30 30-<40 40-<50 50-<60 60-<70 70-<80 80-<90 >=90
Fre
qu
en
cy
Clast Size (mm)
Histogram: Clast Size Frequency from Clast Count #1
Figure 9: A histogram showing the combined frequency of clast sizes for Grid 1 and Grid 2 of Clast Count #1. A large
proportion of clasts are <10mm, however there is a consistent number of significantly larger clasts present within the rock. It
can therefore be suggested that the fluvial system was highly energetic, in order for the clasts to be transported and
subsequently deposited. When this is considered against the Hjulstrom curve (figure 9), it be approximated that the
depositional setting for this lithology had a significant flow velocity of above 100cm/s.
Page | 13
Appearance Here
the clean, white limestone
juxtaposes with the grey,
rough dolostone. The member
is approximately 10m thick
and is well bedded – regular
15cm intervals – and lightly
fractured. The texture is
grainy but fine and reacts
moderately to acid. A
particular characteristic of the rock are the abundant calcareous growths on the underside of
overhanging beds, growing up to 4mm long (figure 10: inset). There are occasional interbeds
of dark mudstone 2-3mm thick, representing quiescence in limestone deposition.
2.6.2 Micritic Limestone
The dominant member of the ANLL is the micritic limestone.
Appearance The rock is dark to mid grey with occasional chert banding (<10mm
thick), and clusters of small, cubic chert nodules (<5mm), randomly dispersed throughout the
rock. Overall it is fairly homogenous and is not fossiliferous. Although on first appearance the
rock looks similar to the CMDL T1, on closer inspection the rock is absent of prominent
weathering and bedding surfaces.
ANLL Depositional Environment Erosional energy in the system is reduced from
that which deposited the breccia. The white, blocky limestone represents shallow, littora l,
calcareous waters, with brief interludes of clastic deposition halting carbonate production.
Waters become slightly deeper with deposition of the micritc limestone and siliceous chert.
Figure 10: Looking east up the Allt nan Leac valley, the White, Blocky Limestone
forms an unconformity with the CMDL beneath it. The unit is less than 10m thick.
Inset: Calcareous growths are abundant on the underside of the overhanging
blocky, white limestone. Rucksack and pencil tip (inset) for scale. O|C 18.3,
GR:[158669,818397].
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2.7 Boreraig Formation (BRFM)
The BRFM extends laterally across the mapped area, with decent exposures becoming ever
more confined to stream crags as one proceeds from west to east. As the ANLL and Lime
Breccia are absent from the stratigraphy along the shores of Loch Slapin, an angular
unconformity, of approximately 24ᵒ, is observed where the CMDL is truncated by the onlapping
BRFM. This is unconformity similar to that discussed in Section 2.6, thus it can still be reasoned
that the BRFM retains a conformable lower boundary BRFM with the ANLL. A full vertical
exposure of the formation was observed along the west coast and a detailed sedimentary log
was subsequently completed. This was conducted between GR:[158680, 817898] and
GR:[158806, 817590]. (Refer to Appendix B).
Appearance Towards the base of the formation, micaceous silt and mudstone grades
into intensely weathered and finely laminated, mid-grey calcarenite. Calcareous nodules up to
30cm wide and bioclasts approximately 5cm wide are found within the calcarenite. Sedimentary
structures become more prevalent towards the top of the formation. Unidirectional cross-
bedding and wedge-like
bedding is common as the
formation coarsens up into mid-
grey to buff, sandy packestone
and bioclastic grainstones. Very
thin (~3mm thick) drapes are
observed on foresets of cross-
laminations. They appear dark
grey and glassy and accentuate
the sediment structure, and have been interpreted as chert. The packestone itself often contains
well rounded quartz granules 2-3mm diameter, indicative of a terrigenous source. These
blocky, thick beds are often bound by non-calcareous micaceous siltstone. Rare, thin beds
Figure 11: Rudstone found in the upper parts of the Boreraig Fm. Disarticulated
bivalves and fragmented crinoids (5 point symmetry) are Indicative of an
allochthonous death assemblage.
Page | 15
(<5cm) of rudstone (figure 11) are often observed within the cross-laminations, interbedded
between poorly sorted packestone.
Fossils Fossils are abundant within the BRFM. Hollowed out features which
strongly resemble bivalves are observed at O|C 15.1 (GR:[158680,817898]) among other
localities.
Bivalve - Gryphaea arcuata
Gryphaea occur most often as moulds, generally
confined to the calcarenite. They consist of two
articulated valves: a large, gnarly, curved shell and
a smaller, flattened shell. The large valve with its
overhanging beak is a key characteristic of the
genus, and lends itself affectionately to the term
“Devil’s toenail”. The arcuata species was a benthic organism, living on and feeding from the
seafloor. They enjoyed cooler marine conditions, which were optimal for large shell growth.
Subsequently gryphaea abundances can be used as indicators of more temperate rather tha n
tropical waters. Occurrences of the fossil have been constrained to the Upper Triassic to Lower
Jurassic, with Gryphaea arcuata most common around 195Ma (Johnson, 1994).
Palaeocurrent Data Collated and tilt-
corrected palaeocurrent indicators from
unidirectional cross bedding and cross lamina tion
structures reveal a dominant WSW-SW direction of
flow. This orientation coincides with that for the
Lime Breccia, which may suggest a well-established
drainage system in the region.
Figure 13: Tilt adjusted palaeocurrent data for the
Boreraig Formation. The diagram indicates a
strong WSW/SW flow direction. N=20; Interval=2.
(Also refer to p49 Bk2)
Figure 12: Side profile of the gryphaea arcuata. They
are abundant throughout the Boreraig Fm (more so
towards the base) often preserved as a mould.
Page | 16
Depositional Environment In terms of depositional environment, high energy
systems are manifested in the occasional trough cross bedding and fragmented, undifferentia ted
fossils in the upper sequence. Thin deposits of allochthonous death assemblages coincide with
interbedding of terrigenous, low energy silts. This suggests an overall unstable and dynamic
environment, with variable terrigenous output and flow conditions. On the contrary, the lower
sequence is fossil-rich (O|C 18.1 GR:[158552,818331]) and tells a tale of quiescent waters such
as those of a carbonate platform. A setting such as this has enabled the deposition of
rhythmically interbedded bioclast-rich, bioclastic-poor horizons. The proposed calcareous chert
drapes may be indicative of a deep submarine current whereby resettling and grading during
transportation has deposited a chert crust on top of the cross-lamination. However a deep marine
environment contradicts with the more bona fide shallow water sedimentary structures and is
consequently disregarded.
2.8 Suisnish Shale Formation (SSFM)
This formation outcrops extensively throughout the southern half the mapping area, being the
dominant rock type along the south-western and southern coastal sections. The conformab le
contact with the Boreraig Formation to the north runs east, following the path of the Allt Poll
a’ Bhainne from its coastal outlet at GR:[158800,817475].
Two gradational members comprise the formation. Micaceous shale outcrops at the lower and
upper sequence, whereas sub-lithic calcarenite is present in between. Both are mapped as a
single formation owing to limited inland exposure. A sedimentary log of the formation was
conducted between GR:[158748,817372] and GR:[158585,816290] and is located in Appendix
A.
Page | 17
2.8.1 Micaceous Shale
The type section of the micaceous shale is observed at O|C 1.1 (GR:[158748,817372]). Within
the dark grey groundmass, small, shiny grains of mica are observed as specks no larger than
0.5mm, and comprise <7% of the rock. Fissile laminations make up larger beds of 1.5m or
more. The host rock is not calcareous, however large, round calcareous nodules up to 30cm are
frequently observed within the beds (figure 14). Nodules are round and smooth conducive of
concentric growth, while
laminations warp around
them which may indicate
sediment compaction post-
nodule formation. An
average percentage
compaction for the shale
was estimated to be at 92%
(figure 14: inset).
Fossils
Fossils are abundant within the shale member, and are a key characteristic of the rock.
Ammonites, pecten and disarticulated calcite shells are observed throughout.
Cephalopod – Ammonite Arnioceras
The arnioceras is common within the shale and is first observed at O|C 2.2
(GR:[158760,817047]). It has a prominent and dense rib structure, with tight coiling. It is most
often found as a mould, up to 5cm wide. The arnioceras mode of life is relatively standard for
ammonites found in this era. They were pelagic organisms, living in open water of ancient seas.
Many ammonites are thought to have been good swimmers, with flattened, streamlined shells,
the smallest chambers of which filled with air for buoyancy. They subsequently represent
Figure 14: Calcareous nodules (tan-coloured) occurring within the fissile Micaceous
Shale. Inset: Schematic diagram illustrating the formation of the pre-compactional
nodules. i) Calcite fossil deposited in calcareous sand, ii) Focussing of pore fluids and
precipitation of calcite cement in the area around the fossil, iii) Compaction then occurs
after formation. (Adapted from Tucker (2011)).
Page | 18
relatively deep, offshore successions of oxygen poor seafloor (hence the relatively high
preservation rate). Arnioceras has been
constrained to the Sinemurian age within the
Early Jurassic. It occurred c.200-190 Ma (Pálfy,
1999).
Bivalve- Pecten
Pecten moulds are observed towards the top of
the formation. They appear scallop-like with
broad ridges radiating from a central point. At
O|C 8.5 (GR:[160850,815904]) a pecten is
observed to be up to 30cm wide. Synonymous with scallops, they were mobile epifauna l
suspension feeders, thus indicate fairly stable shallow marine successions. Occurrences have
been constrained to be within c.70 to 0.012 Ma i.e. from the Late Cretaceous (Coan, Scott, &
Bernard, 2000).
2.8.2 Sublithic Calcarenite
Micaceous shale grades and coarsens up into a relatively thin (~30m) micaceous calcarenite
member, of fine to medium grade.
Appearance Flakes of mica tend
to be coarse, approximately between 2-
3mm. The member is blocky and
commonly contains large, up to 3m wide,
calcareous nodules. They tend to be
massive and in places show extensive
honeycomb weathering and often occur
Figure 15: Ammonite Arnioceras found within the baked
margin of the Suisnish Shale Fm. They are commonly preserved as moulds, such as this. Pencil nib for scale. O|C
2.2, GR:[158760,817047].
Figure 16: Large doggers of sublithic calcarenite commonly
display significant honeycomb weathering patterns. It is indicative
of calcareous sediment.
Page | 19
adjacent to other massive sand lenses. These have been interpreted as calcite cemented doggers,
forming late during diagenesis (figure 16).
Fossils Crinoid – Small-scale tube-like structures are common within the
massive sandstone beds, such as at
O|C2.4 (GR:[158705,816598]), the type
locality for this lithology. They have
been interpreted as fragmented crinoida l
structures and subsequently point to a
reef environment, perhaps indicating an
overall coarsening upwards
transgression to an offshore sand bar.
Depositional Environment A
fairly abrupt transgression from tidal/nearshore environment is inferred from the top of the
BRFM to the deep marine Suisnish shales. Within the SSFM however, an initial marine
regression is represented; shales grade into massive calcareous sandstones, doggers and
crinoidal remains. Towards the top of the SSFM the calcarenite grades back into shales. This is
suggestive of a marine transgression back towards deeper water.
2.9 Discussion
The occurrence of fossils has allowed a rough biochronology to be applied to some of the upper
beds, of approximately Hettangian to Sinemurian age (c.201 – 191Ma). Thus it is assumed that
the main suite from Lime Breccia to Suisnish Shale were deposited between the Late Triassic
to Early Jurassic. As discussed in the previous sub-sections these Jurassic sediments represent
several marine regression/transgression cycles (Figure E).
The metamorphosed dolostone, which is analogous to the Ordovician Durness Group (Emeleus
& Bell, 2005), provides the dominant bedrock for the mass fluvial deposits of the continenta l
Figure 17: (Left) Fragmented stems of crinoids - or "sea lillies" -
within the sublithic calcarenite. (Right) Diagram of a crinoid with
current going from left to right (BGS, 2016).
Page | 20
Lime Breccia. A marine transgression is observed up the stratigraphic sequence from the
breccia to the Allt nan Leac Limestone. The white, blocky limestone which is interbedded with
thin mudstone points to a littoral, shallow carbonate system. Gryphaea-rich beds at the base of
the Boreraig Formation provide a biostratigraphic age of c.195Ma, and are also significant
indicators of further deepening waters to offshore marine.
Lateral variations of the stratigraphic thicknesses of the Lime Breccia and ANLL is illustra ted
on the west coast where the Boreraig Formation overlaps older Mesozoic strata and
subsequently onlaps onto the dolostone. A WSW to SW palaeoflow within the BRFM and
breccia suggests a well-established sediment routing system to an offshore depocentre.
Following progressive shallowing throughout the Boreraig Fm, there is an abrupt transition to
deeper water across the contact to the Suisnish Shale Fm. Although not observed in the field, it
is widely accepted in the literature (Oates 1978; Hesselbo & Coe 2000) that there is
biostratigraphical evidence of a depositional hiatus and possible angular unconformity between
the formations. Grading up into an offshore sandbar is the likely cause for the calcarenite
member within the SSFM, before a transgression sees a further marine shale sequence
deposited.
The literature regards the Arkosic Arenite and Quartzite as being of Cambro-Ordovician age or
older, analogous to the Torridonian and Eriboll Groups respectively (Emeleus & Bell, 2005).
Observed basal quartz conglomerate within the Quartzite is characteristic of the False-bedded
Member, and considering the close stratigraphic relationship with the Pipe Rock, a nearshore
setting can be inferred. The Torridonian on the other hand is representative of a Precambrian
terrigenous fluvial system. These lithologies are observed elsewhere in NW Scotland and are
considered stratigraphically lower than the dolostone.
Page | 21
CHAPTER 3 METAMORPHIC GEOLOGY
3.1 Camas Malag Dolostone Formation
This formation has not undergone extensive metamorphism, thus in the most part is still
perceived as a sedimentary rock. The Camas Malag Dolostone Formation (CMDL) is divided
into three members, T1, T2 and T3 (see Section 2.4), across which the grade of metamorphism
increases respectively. The formation outcrops well around the periphery of the Beinn an
Dubhaich granite with T3 consistently outcropping closest to the granite contact where all three
members are present.
Appearance (A full lithological description of the
CMDL is made in Chapter 2)
T3 most commonly displays significant marbleisation and
recrystallisation. This is most evident at O|C 22.4
(GR:[160329,818567]). At this locality the rock turns
orange/pink to green as the granite is approached. Nearby at
O|C 22.3 (GR:[160448,818502]) the rock also begins to exhibit
very soft off-white to cream patches. It has a hardness of <2.5.
Interpretations By considering the
proximity of the dolostone to the granite, it is proposed that the soft white mineral is talc and
the colouration of the dolostone to a green colour is inferred to be due to an occurrence of
olivine or diopside. These minerals are commonly associated with low grade contact
metamorphism (figure 18). The literature recognises metamorphism within the aureole, and
these primary observations are concordant with it. Harker (1904) first described the presence
of calc-silicate minerals in the carbonate aureole such as diopside, which develops along with
Figure 18: Marbleised pink/orange dolostone with significant veining.
Veining intensifies as the granite
contact is approached. O|C 23.1,
GR:[158273,818878] (adjacent to
granite contact).
Page | 22
tremolite and forsterite. Subsequent studies of metamorphosed cherty nodules (e.g. Hoersch
(1981)) have yielded a mineralogically distinct talc-bearing boundary between 350-425ᵒC.
CHAPTER 4 IGNEOUS GEOLOGY
Seven different igneous lithologies were observed across the region. Dykes are best exposed on
the shores of Loch Slapin to the west, and Loch Eishort to the south (Figure A). From here some
can be traced in land. There are two igneous complexes which will be discussed; the Beinn an
Dubhaich Igneous Complex (BADIC) and the Carn Dearg Igneous Complex (CDIC), which
dominate the northern and southern regions respectively (refer to Reference Map in Appendix).
4.1 Mafic Dykes
4.1.1 Basalt & Dolerite Dykes
There is a series of NW-SE trending dyke intrusions across the mapping area, most composed
of basalt. The Suisnish Shale Fm (SSFM) harbours one of the most intense number of intact
igneous intrusions. Large expanses of excellent coastal sections and waterfall sections provide
good exposure, such as the outlet of the Allt Poll a’ Bhainne into Loch Slapin
(GR:[158850,817400]). On occasions larger dykes can be followed inland where they outcrop
as abrupt craggy knolls on the landscape, Dun Kearstach (GR:[159650,817450]) being one
example.
The significant majority of the dykes in the area appear very similar. Contacts are sharp with
baked margins present where the basalt is contacts with the country rock. They are orientated
in approximately the same direction, with an average trend of 126 (figure 24) however a minor
NE-SW suite with identical mineralogy is also observed.
Page | 23
Basalt
These dykes tend to be up to 200cm wide. They are largely
composed of very fine-grained dark grey groundmass, micro-
crystalline to aphanitic in smaller outcrops. Small specks of
a white mineral - < 0.5mm - constitutes up to 10% of the rock.
A coarsening from the centre out towards the margins is
observed, especially of the pale minerals (figure 19).
Rare calcite veins and enclaves of coarse calcite crystals are observed in some basaltic dykes.
Interesting examples were observed at O|C 3.2 (GR:[158594,816264]) and O|C 25.1
(GR:[159025,819820]). Equant crystals are up to 5mm and tend to be concentrated towards the
centre of the dyke (figure 20).
Interpretation Pale
minerals are believed to be
clusters of plagioclase feldspar,
while the dark groundmass and
rust-like weathered surface is
suggestive of a significant Fe/Mg
content. Minerals which
constitute this are likely to be
pyroxene and olivine. Calcite is
not commonly associated with igneous petrology. It is however commonly precipitated from
geothermal fluids.
Dolerite
The largest “minor dykes” (defined as being<10m wide) are usually composed of dolerite. The
main dolerite outcrop is manifested as the large, meandering dyke which trends NW from the
Figure 20: Looking south at O|C 25.1; a calcite enclave within a magnetite-
rich basaltic dyke (highlighted). The dyke intrudes into dolostone country
rock, emerging from the granite contact in the background. Inset: Equant calcite crystals up to 5mm. O|C 25.1, GR:[159025,819820]. Notebook and
pencil nib (inset) for scale.
Figure 19: Coarsening of pale minerals
from the edge of the dyke towards the
centre. Pencil for scale.
Page | 24
periphery of the CDIC through Glen Boreraig (O|C 14.1 GR:[158816,817975]). The
mineralogy is largely identical to the basaltic dykes. Weathered surfaces however are rough and
knobbly, indicative of a slightly coarser grain up to 2-3mm.
Interpretation Besides the main mineral constituents of pyroxene (35%), olivine (20%)
and plagioclase (5%), small, bronze coloured nuggets are identified. Interpreted to be
chalcopyrite, they have a metallic lustre and occur in clusters of 1mm.
4.1.2 Gabbro Dykes
Gabbro is only present in large dykes (>10m). It outcrops twice, firstly at Stac Suinish (O|C 5.3
GR:[158562,816269]), and secondly as a very wide dyke (~60m) along the south coast. From
here it can be followed in land to where it forms a large sill-like feature overlooking Loch
Eishort (O|C 7.4 GR:[159957,815659]) (figure 31, 32). The Stac Suisnish Gabbro is particula r ly
interesting and is discussed independently.
General Mineralogy The mineralogy of the gabbro is largely similar to that of the
basalt and dolerite dykes, with olivine, pyroxene and plagioclase being, in that order, the
dominant component minerals. Grains tend to be equigranular in hand specimen, approximate ly
2mm in diameter.
Figure 21: Field sketch of the gabbroic Stac Suisnish jutting out into Loch Slapin. O|C 5.3, GR:[158562,816269].
Page | 25
Stac Suisnish Gabbro The Stac Suisnish Gabbro outcrops as a small headland 10m high
and 13m wide, protruding west into Loch Slapin. It can be traced inland ESE-wards, via a series
of craggy knolls, towards the CDIC. Unlike other mafic rocks, this gabbro displays rhythmic
horizontal layering of coarser, more feldspar-rich layers between thicker, finer and less
feldspar-rich layers (figure 22). It is significantly enriched in plagioclase, constituting up to
15% in the finer layers, and up to 35% in the coarser layers. Layers are randomly spaced and
vary in thickness from 20 to 100cm thick. The contacts are quasi-gradational. Sub-spherical
felsic “blobs” or enclaves occur towards the base of the dyke (figure 23). The mineralogy is
dominated by tabular plagioclase - <7mm- and subhedral and fractured olivine – <4mm.
Interpretation of Stac Suisnish
The Stac Suisnish dyke has been interpreted as an anorthosite layered gabbro. The pale,
feldspar-rich layers are interpreted as secondary injections of felsic-rich melt into the dyke.
Insulated by the hot dyke material from the cool country rock, slow crystallisation has created
a coarse-grained cumulate. Enclaves and “blobs” near the base are inferred to represent upwards
migration of less dense bodies (these being hotter and also compositionally less dense). We
Figure 22 (left): On the southern side of the Suisnish Gabbro dyke, looking north. Rhythmic layering of coarse, plagioclase-rich and
fine, plagioclase-poor rock are clearly visible from a distance. Inset:
Close-up of the gabbro mineralogy. Hammer and pencil (inset) for
scale. O|C 5.3, GR:[158562,816269].
Figure 23 (above): Feldspar and olivine-rich pods at the base of
the dyke exposure. It is coarse-grained and appears that both
minerals have grown together at the same time. Hammer and
pencil nib (inset) for scale. Photo taken at O|C 5.3.
Page | 26
know from the CDIC (refer to Section 4.2) that both felsic and mafic magmas were present in
the vicinity.
4.1.3 Discussion
Based on the mineral characteristics observed, the dykes are believed to have formed from a
tholeiitic melt. Drawing comparisons with observations of similar features adjacent to major
igneous complexes (most notably Arran), it is believed that these dykes were injected as part of
the same magmatic episode as the mafic Cuillin centre to the WNW. The strong NW-SE trend
supports this hypothesis (figure 24). Considering the presence of spherules within the basaltic
dykes (O|C 9.4 GR:[159729,816611]) it is inferred that they were intruded into cool, fluid-r ich,
marine sediments. This is concordant with the conclusions drawn from the depositiona l
environment of the Jurassic sedimentary geology (Section 2.9). The significant dolerite dyke,
along with both gabbro dykes are believed to be large feeder dykes to the sill structure at CDIC
(Figure C).
The Suisnish Gabbro has been repeatedly studied since its first recorded observation in 1902.
Parslow (1976) concludes that the layered dyke was formed in-situ. Considering the slow
cooling rate required, the surrounding country rock must have been maintained at a high
temperature during, and after, the intrusion (this is likely to have been caused by an increased
regional geothermal gradient). It subsequently allowed a combination of gravity settling and
magma flow currents to produce a cumulate texture. The felsic “blobs” identified at the base of
the outcrop are attributed to autobrecciation.
Structurally, it can be inferred that the minimum principal stress axis (σ3) was orientated NE-
SW at the time of intrusion (figure 24). Cross-cutting relationships between NE-SW and the
dominant NW-SE trending dykes were observed at O|C 3.1 and O|C 8.1 (GR:[158698,816615],
GR:[158750,815700] respectively). At both localities it was concluded that the dominant NW-
SE dykes are the younger suite. Rather than inferring a full change in the orientation of the
Page | 27
stress regime in a small amount of time, the older and less frequent NE-SW dykes may either
be attributed to a much older suite of mafic dykes or a product of the Rum Igneous Complex,
which lies just 25km to the SW.
Table 1: Lithospheric extension analysis along the SSFM coastal section (GR:[158774,817489] to GR:[158540,816270]). A
total of 51 dykes were measured.
4.2 Carn Dearg Igneous Complex (CDIC)
Forming a plateau to the north of Carn Dearg, elevation 195m (GR:[159900,816000]), are the
sills which make up the Carn Dearg Igneous Complex (CDIC) (Refer to Reference Map in
Appendix). The CDIC covers approximately 1.5km2. It is composed of two igneous lithologies,
which outcrop entirely within the SSFM. Stratigraphically, the CDIC is emplaced above the
gabbro dyke at Loch Eishort. There is onlapping evidence between the large dolerite dyke,
discussed in Section 4.1.1, and the CDIC which suggests that this dyke is intimately related to
the sill (refer to Figure C).
Locality
Width of
swarm (km)
Number of dykes
per km
Percentage crustal
extension due to dyke
intrusion
Loch Slapin shoreline 1.724 29.58 8.5
Figure 24: Rose diagram showing the orientation of
dykes. Interval=10. N=206. Figure 25: Stereonet of dykes plotted as points to
plane. The mean plane is included and has an
orientation of 145/75E. N=206.
Page | 28
4.2.1 Porphyritic Basalt Sill
In plan view the porphyritic basalt forms a peripheral ring around the igneous complex. It
consistently outcrops as a 2-3m thick layer generally in sharp contact with the SSFM beneath
and microgranite (refer to Section 4.2.2) above.
Mineralogy The rock appears pinky/orange where it has been exposed to subaerial
weathering. A fresh surface reveals a dark-grey, mafic groundmass, peppered with subhedral
plagioclase feldspar phenocrysts up to 4mm long. Mafic minerals are dominant as indicated by
the rusting, whereas minor vitreous plagioclase phenocrysts tend to constitute about 1-5%. The
rock also contains up to 15% of sub-1mm feldspar grains.
4.2.2 Microgranite Sill
This felsic unit is the uppermost and thickest component of the CDIC. It is at least 25-30m thick
and dominates the central part of the complex. A direct contact with the thin mafic sill is
observed for most of the boundary.
Mineralogy The rock is approximately equigranular, with grains ~1mm. The colour
index is between 15-20%, compared to 60-65% for the mafic sill. Pale minerals constitute the
large majority of the rock (~75%); vitreous and pale grey quartz is intergrown with pink to off-
white twinned K-feldspar, and tabular plagioclase present. Smaller, greenish and prismatic
minerals are inferred to be amphiboles. On instances the unit can have a chalky/ashy texture,
this is most likely due to the degradation of feldspar from subaerial weathering.
A more rhyolitic texture was observed at O|C 12.1 (GR:[159665,816104]), on the very western
edge of the sill. Blocky microgranite lies directly on top of an ashy-matrix dominated rhyolite
complete with 1-2mm feldspar phenocrysts. This occurrence is believed to be due to a bottom
contact with the shale country rock (refer to Figure A).
Page | 29
4.2.3 Discussion
Intimate relationships between the felsic and mafic
lithologies are observed on more than one instance.
Firstly at O|C 10.2 (GR:[160200,816932]), felsite veins
are seen intruding into an outcrop of porphyritic basalt.
Secondly, on the screes to the NW of the CDIC,
rounded xenoliths of basic material (up to 40mm) are
found within a felsic host (figure 26). Thus it can be
said that the microgranite post-dates the basalt. Considering the smoothness of the mafic pods
it can be further inferred that both units were intruded within a short period of time of one
another. These relationships may be suggestive of mixing processes, indicating that the two
magmas were available (i.e. molten) at the same time (figure 27, 28).
Figure 26: Porphyritic basalt xenoliths within a
microgranite host rock. Out of situ boulder near
GR:[159500,816660].
Block-like micro-structure; may be indicative of incorporation
mechanism of the olivine i.e. sinking (see red arrow), crys tals in
the groundmass display flow patterns Ol ivine xenocryst displaying high interference colours, and
cons iderable fracturing.
Fine-grained plagioclase
groundmass.
“Flow” s tructure of
plagioclase-rich groundmass around
the phenocryst
Irregular crystal faces
Figure 27: Thin section of the felsic sill (ref. CGM15-15). Sitting
on the cross hairs is a large olivine xenocryst within a
plagioclase-rich groundmass. Field of view: 5mm.
Figure 28: A close-up of thin section CGM15-15. Plagioclase crystals in the groundmass show
orientation and "flow" around the olivine xenocryst, indicating that the olivine was incorporated into the felsic magma while it was still liquid. Irregular faces of the olivne may be suggestive of some
corrosion. Field of view: 3mm.
Page | 30
The sill is believed to have formed as a
result of lateral migration of magma from
the large dolerite and gabbro feeder dykes
(discussed in Section 4.1) in response to a
change in the orientation of the principa l
stress axes (figure 29). Shale (SSFM)
crops out above the basalt sill in a few
locations, such as O|C 11.3
(GR:[159468,816623]). The occurrence is not consistent and a porphyritic basalt-microgranite
contact is much more common. This phenomena has been attributed to an irregular shape of
intrusion; consistent bedding measurements have disregarded a xenolith hypothesis (figure 30).
Structurally, joint data is approximately vertical (Figure B) which could indicates that the
composite sill was
emplaced into the
SSFM after the
beds were tilted
(refer to Figure C).
By considering the
generally fine-
grained and porphyritic nature of the lithologies present, this sill is likely to have undergone
two stage cooling. The relative thinness of the sill is likely to have contributed to the rapid
second stage cooling. Although porphyritic basalt is the most extensive as part of the CDIC,
it is also observed in a few minor basaltic dyke and sill structures. Small lateral expanses of
porphyritic basalt are described at O|C 5.1 and O|C 7.3 (GR:[159103,816733] and
GR:[159293,816352] respectively), and may be inferred to represent multiple thin sheet
Figure 29: Schematic illustration of the orientation of the principle
stress axes at the time of dyke and sheet emplacement. The magma chamber may be inferred to be the Skye Central Complex centre.
Adapted from Emeleus & Bell (2005).
Figure 30: Schematic cross-section through the composite sill. It is proposed that the occurrence
of "Top Shale" is due to the irregular shape of the sill intrusion. The red dashed line indicates a
hypothetical topography.
Page | 31
intrusions, or perhaps the original extent of the CDIC (refer to Figure B for small, isolated
porphyritic sills and dykes).
4.3 Beinn an Dubhaich Igneous Complex (BADIC)
4.3.1 Beinn an Dubhaich Granite (BADG)
The Beinn an Dubhaic Igneous Complex comprises of just one
lithology; the Beinn an Dubhaich Granite (BADG). The granite
which forms the hills to the north of the mapping area, hence
the name, intrudes through the Camas Malag Dolostone Fm
and outcrops extensively to the west and east in a broad arc.
Sharp, vertical to sub-vertical contacts between the two units
are widely observed (figure 33).
Figure 31 (above): Field sketch looking
NW from Loch Eishort towards the CDIC.
The large gabbro dyke and composite sill are seen against the skyline, at the top of
the shale cliffs. O|C 8.3,
GR:[160467;815766].
Figure 32 (right): Comparative photograph of
the adjacent field sketch. Depth of photo ~70m.
Figure 33: Sub-vertical granite-dolostone contact in plan view. The contact zone is
well vegetated. Scale: the gap between
the two lithologies is ~60cm. GR:[159000,818935].
Page | 32
Mineralogy The mineralogy tends to remain fairly constant across the outcrop, with
the designated type lithology found in and around Camas Malag (GR:[158100,819350]). As
seen in the sketch of the thin section (figure 34: SM5), the rock is equigranular. It is reasonably
coarse-grained (~2mm) with a colour index of around 10%. This is indicative of the significant
felsic composition: sub-hedral biotite mica (~7%), pale grey, anhedral quartz (25%), pearly
white, lath shaped plagioclase (15%) which occasionally exceeds 4mm, and pale pink, prismatic
K-feldspar (15-20%). Feldspar grains often display strong twinning on a fresh or tide-polished
surface. Studying the mineralogical content of the rock has led to its identification as an alkali-
rich granite.
A detailed analysis of the BADG was carried out towards the end of the project. Crystal size
and mineral abundances were recorded at almost 100 localities (to see the data transects, refer
to Reference Map in Appendix). Although mineralogy remained fairly constant, texture and
grain size is often quite variable, this is discussed in the proceeding chapter (Section 5.1).
Table 2: Principle mineral properties of the Beinn an Dubhaich Granite. Data was taken from 98 localities spanning the
BADIC.
Thin Section Analysis Thin section analysis has identified further minerals such
as chlorite and zircon (figure 34: SM5) and most notably phlogopite (figure 35: SM1)) which
is experiencing degradation/alteration to chlorite.
The presence of zircon and phlogopite alteration in the same granite sample can be intimate ly
related. Phlogopite, a potassium-magnesium rich biotite, is most commonly found in basic
Quartz Plagioclase K-feldspar
Mean Crystal Size (mm) 1.28 1.91 2.04
Mean Mineral Abundance (%) 27.42 19.29 14.26
Page | 33
igneous rocks. Firstly this is indicative of a mafic xenocryst, implying availability of basic
magmas during emplacement, or incorporation of a small mafic body. Protruding quartz grains
suggest that this inclusion predates the quartz, it may also have been partially melted. Secondly,
if a moderate temperature range of 100-200ᵒC for chlorite geothermometry is inferred (Caritat
et. al., 1993), then it can be postulated that this alteration is as a result of post-emplacement
hydrothermal circulation. Common grains of zircon indicate an enrichment in radioactive
elements such as potassium and thorium. A high radioactive content in turn would allow a large
granite body to generate and retain internally produced heat. This heat could drive and sustain
hydrothermal currents through the crust.
Figure 34: Detailed sketch of the type granite in thin section (ref. SM5). The sample displays a
phaneritic, equigranular texture, dominated by quartz and feldspars. Field of view: 5mm. Sample
location GR:[158090,819500].
PPL XPL Large, s imple-
twinned and
zoned feldspar
Simple-twinning of K-feldspar
Pleochroic biotite mica.
Anhedral and platey
Zi rcon
inclusions
common within feldspars
Chlori te with
anomalous
interference colours, and pale pleochroism
TYPE GRANITE (SM5)
Page | 34
4.3.2 Discussion
The Beinn an Dubhaich granite intrudes through the CMDL Fm and is also found to cross-cut
basaltic dykes. With this in mind it is concluded that this granite unit is the youngest igneous
body in the studied area. Mafic dykes on the other hand are likely to have occurred at a similar
time to the formation of the composite sill.
Mafic inclusions such as that found in SM1 can be attributed to partial melting of assimila ted
basaltic dykes in the dolostone, since mafic rocks have a higher melt temperature than felsic
rocks. Further, hydrothermal influences are widely identified (see Section 4.1.3; calcite within
basalt dykes, and 4.3.1; chlorite in granite) within intrusive igneous bodies. It is therefore fairly
conclusive that these igneous suites were intruded into fluid-saturated upper crust. Emeleus &
Bell (2005) supports this hypothesis, and likewise proposes that magmatic intrusions generated
PPL XPL
Large phlogopite
inclusion. It is has
a birds- eye
mottled texture
The mineral
shows significant
masking of
colour, therefore appears similar
in XPL
Quartz minerals
intrude into the
mineral, suggesting
that the phologopite was
formed earlier and
subsequently
intruded
PHLOGOPITE BEARING
GRANITE (SM1)
GRANOPHYRE (SM4)
Quartz and K-feldspar have crystallised
together to form a
granophyric texture
Crysta l growth is
chaotic and i rregular,
hence the unusual
extinction patterns in XPL
Cons idering the constituent
mineral components, a
granophyre form is likely to be due to cooling of residual
melt. Its formation is also
attributed to the presence of
water.
Figure 35: Two thin sections of the Beinn an Dubhaich Granite: i) (Top) Phologopite bearing granite from GR:[159822,819176] (ref. SM1) and, ii)
(Bottom) Granophyre from GR:[159127,818412] (ref. SM4). Field of view: 5mm.
Page | 35
hydrothermal convection systems, through both the intrusions themselves and the adjacent
country rock.
CHAPTER 5 STRUCTURAL GEOLOGY
This chapter explores the relationship between faulting, folding and the Tertiary igneous
intrusions (discussed in the previous chapter). There are two main structural elements: the Beinn
an Dubhaich Granite intrusion (BADG), which is believed to have been intruded through the
Camas Malag Dolostone Fm (CMDL); and the major unconformity in the east between the
“siliciclastic wedge” and the dolostones.
5.1 Beinn an Dubhaich Igneous Complex (BADIC) Zone
5.1.1 Broadford Anticline
Observations in the field, shown clearly on the
supporting map and cross-section (Figure B and
Figure C respectively), show that bedding becomes
more inclined towards the Beinn an Dubhaich. In
places there is repetition of the stratigraphy in age
order, away from its flanks. This is particularly the
case with the dolostone units (CMDL T1, T2, T3).
The BADG is believed to occupy the core of the
structure. With these observations it is possible to
propose the presence of a large anticlinal antiform.
By interpreting field data using stereonet
projections (figure 37) it is believed to be an open,
Figure 36: Stereonet diagram showing the dip of
bedding across the region. The inclination of the beds
decreases away from the Beinn an Dubhaich Igneous
Complex zone, indicating the presence of an anitform (i.e. CMDL has the steepest dip, whereas SSFM has
the shallowest). To the south, beds dip towards the SW,
whereas to the north they dip to the N.
Bedding planes are colour coded according to
lithology. The CMDL is plotted as one lithology. Arkosic Arenite and Quartzite bedding measurements
are excluded. N=260.
Page | 36
asymmetric fold, with a sense of vergence towards the south (figure 38). The fold has an
irregular amplitude, seeming to drift into a very gentle syncline at Loch Eishort in the south.
A large-scale fold such as this is inferred to have formed at depth. At the time of formation, σ1
appears to have been orientated in a general N-S direction. However the fold axis is not linear,
it is arcuate, seemingly around a point at the centre of the Eastern Red Hills to the north (figure
43).
5.1.2 Beinn an Dubhaich Granite Intrusion
As discussed in Chapter 4, the Beinn an Dubhaich Granite is believed to be the youngest
igneous intrusion in the region. It has steep contacts with the CMDL and is assumed to be boss-
shaped (see Figure C: Geological Cross-Section).
Towards the end of the study, an extensive mini-project was carried out across the granite,
whereby four traverses were made (refer to Reference Map in Appendix). Mineralogy, crystal
size and joint measurements were recorded at 98 localities across the outcrop.
Granite Jointing Joint data are plotted on a stereonet in Figure 39. The point
density contours are concentric, which reveals that jointing within the granite is vertical to sub-
vertical. In turn it is inferred that the granite was intruded almost vertically. There is an
argument however for a slight shift of the data toward the south; a greater number of points lie
Figure 37 (left): Stereonet plot of the Broadford Anticline. The fold axis
(beta) indicates that the fold is plunging shallowly to the west (6-->272).
The axial plane dips moderately to the north thus indicating a fold
vergence towards the south. It has an interlimb angle of ~90ᵒ.
Figure 38 (above): Schematic sketch of the Broadford anticline.
Interpreted from the adjacent stereonet.
Page | 37
on the southern half of the outer great circle as opposed to the northern half. If this is carried, it
would mean that the body was
intruded sub-vertically, in a
southerly direction.
Dolostone Islands
Small, irregular islands of
dolostone are observed occurring
entirely within the granite. They
appear to lie directly on top of the granite, such as at O|C 22.4 ([GR:160329,818567]), a narrow
stream cutting. At this locality the dolostone becomes increasingly metamorphosed over a short
depth to the stream bed (~2m), at which the granite is vaguely visible. These islands, often made
up of CMDL T3, the oldest dolostone unit, are proposed to be relicts of the unroofing of the
CMDL from the granite intrusion beneath. There is also little variation in the strike from the
country rock. Further hypotheses such as dolostone xenolith ‘rafts’ have been largely
disregarded; laboratory tests have concluded that the dolostone has a greater density than that
of the granite, so any ‘rafts’ would therefore have sunk (Harker, 1904).
A crystal size analysis along with several thin section analyses have been interpreted to be in
support of unroofing.
Figure 39: Joints in the Beinn
an Dubhaich Granite plotted
as poles to planes on a stereonet. Point density
contours show an increasing
concentration of points away
from the centre. This
information concludes that the joints tend to be vertical or
sub-vertical. Data was
collected along four transects
of the outcrop. N=217.
Page | 38
SM6
CGM15-13
Figure 401: A contoured grain size map of the Beinn an Dubhaich Granite. Quartz grains are used as a proxy for the modal grain size of each sample. Grain size increases from blue to red (i.e. 0-2mm). Plotted circles indicate localities at which samples
were taken and analysed. White circles are localities at which porphyritic granite was observed. A surface has been interpolated
from point data using Inverse Distance Weighting.
Beinn an Dubhaich Granite: A grain size analysis
Figure 41: Porphyritic vs phaneritic granite of the Beinn an Dubhaich. Clockwise from top left: Porphyritic granites (ref. SM6
GR:[159636;819160], and ref. CGM15-13 GR:[159588,818927]),
phaneritic granite- type granite (ref. SM5 GR:[158090,819500]). Field of
view: 5mm.
SM6: Porphyri tic K-feldspar set
in a fine-grained quartz and
feldspar groundmass.
CGM15-13:
Similar to SM6;
porphyri tic K-
feldspar set in a fine-grained
quartz and
feldspar groundmass.
SM5: Equigranular, phaneritic granite
Thin section analysis
Page | 39
Interpretation Figure 41 is a set of three thin sections from the Beinn an Dubhaich. The
phaneritc sample (SM5; also featured in Section 4.3.1) contrasts starkly against the porphyrit ic
samples observed close to the Beinn an Dubhaich summit (SM6 and CGM 15-13). Figure 40
shows how the grain size has been mapped and contoured, the data has subsequently been used
to interpolate a surface. A null hypothesis that grain size increases concentrically towards the
centre of the BADIC is rejected. Pockets of fine grained porphyritic granite is found towards
the centre of the granite unit, at some of the greatest elevations. Considering this relationship it
can be inferred that these pockets of finer material represent areas close to the dolostone roof
i.e. cool margins. The null hypothesis does not account for the shape/projection of the anticline;
it incorrectly considers the surface of the BADIC as a horizontal slice through a batholith- type
body.
It may also be argued that variations between phaneritic and porphyritic granites are as a result
of a series of multiple intrusions, which form a larger, amalgamated granitic unit. Raybould
(1973) similarlary distinguished different granite types within the area proximal to the summit
of Beinn an Dubhaich, including a porphyrit ic
microgranite, granophyre (see Section 4.3),
and a medium-grained granite.
5.1.3 Ductile Deformation
Ductile deformation tends to be confined to
the periphery of the BADIC Zone, preserved
within mafic dykes of the CMDL. Boudins
and boudinaged mafic dykes are commonly
found along the coast to the south of Camas
Malag, with good examples at O|C 24.4
(figure 42) and O|C 24.6
σ3
σ3
σ1
Ducti le
deformation
s tructures in the
dolostone
σ1
Figure 42: Boudinaged basaltic dyke (highlighted) within
dolostone (CMDL) country rock. The dolostone shows ductile
deformation structures such as gentle folding around the boudins. Boudins indicate extension in an upwards
orientation. O|C 24.4, GR:[158243,818866]. Notebook for
scale.
Page | 40
(GR:[158243,818866] and GR:[158344,818667] respectively). Intense deformation structures
are inferred to be as a result of nearby granite emplacement. Boudinge is a feature associated
with extensional environments. To explain its occurrence, an element of upward thrust with the
intrusion of the granite is inferred, this in turn is proposed to have caused an upward drag of the
country rock. Heating within the aureole will have softened the less competent dolostone
country rock, while causing the more competent basalt dykes to break up into boudins.
5.1.4 Discussion
The Broadford anticline indicates a N-S compressive stress regime. This is not replicated in any
of the igneous suites in the region, which generally represent NE-SW extensional regimes. The
granite is intruded into and cross-cuts the CMDL (see Figure C). The anticline therefore formed
prior to the region’s magmatic episode. Further the slight southerly skew of the granite intrus ion
may be attributed to its emplacement into the core of the already skewed Broadford anticline.
Dolostone islands, which are believed to be relicts of unroofing, further support this. The
arcuate nature of the fold appears to be associated with the centre of the Eastern Red Hills
(figure 43). The emplacement
of such a voluminous amount
of granite such as this, may
have warped a once linear
antiform.
It may be argued that the fold
formed in response to the
forcible intrusion of the Beinn
an Dubhaich granite. However
by looking at proximal dykes,
it is concluded that there is no sign of granite-induced deformation (Ellis, 2007).
Figure 43: The arcuate nature of the Broadford anticline is proposed to be related
to the centre of the Eastern Red Hills. (Google Earth)
Page | 41
A stereonet analysis was conducted comparing the dip of the Suisnish Shale Fm (SSFM) with
the dip of the mafic dykes which are intruded into it (figure 44). The analysis yielded a
hypothesis that the dykes were emplaced into horizontal strata, which were subsequently tilted
following the formation of the antiform (figure 45). However, in the field, dykes showed no
relicts of a compressional stress regime and also a full switch of the regional stress regime (see
Section 4.1.3: σ3 orientated NE-SW; σ1 orientated NW-SE) in the relatively small time between
the emplacement of the mafic dykes and BADG is highly unlikely.
The dykes are inferred to dip in toward the Eastern Red Hills to the N-NNE.
5.2 Major Faults
A series of faults cut through the landscape, offsetting and juxtaposing lithologies. The ones
discussed here have displacements on metre scale or greater.
Figure 44: Stereonet plot of dykes (pink) plotted with SSFM
(grey), as poles to planes. The mean plane of both sets is also
plotted: Dykes 145/75E; SSFM 145/13W. N=285.
Figure 45: Schematic sketch illustrating the relationship highlighted in the previous
figure. Hypothesis: i) Dykes intruded
vertically into horizontal strata ii) Post-
emplacement tilting of ~15ᵒ (due to folding).
This is one way of explaining the observed field relationship between the dykes and
SSFM.
Page | 42
5.2.1 Siliciclastic ‘Wedge’
As discussed in
Chapter 2, a
wedge-like
outcrop of
quartzite and
arenite appears
at the head of
the Allt nan Leac valley. The juxtaposition of siliciclastic sediments with carbonates on all
sides, along with a significant break in slope of approximately 10-15m, is believed to indicate
a thrust fault (figure 46).
5.2.2 Glen Boreraig
Two strike slip faults running just off parallel, orientated NNW-SSE, are inferred in Glen
Boreraig. A central block is pushed northwards by approximately 100m, it is bound to the west
by a sinistral fault and to the east by a dextral fault. In the field, offsets between the Allt nan
Leac Limestone (ANLL) and Boreraig Fm (BRFM) are inferred from the occurrence of
limestone specific vegetation (e.g. fern) and indicative large holes eroded out by groundwater.
5.2.3 Carn Dearg Igneous Complex Zone
On the southern periphery of the CDIC there are a number of steep stream gullies draining into
Loch Eishort. Here, prominent outcrops of porphyritic basalt sill are used as offset markers
against the pale, upper felsic unit and the grey shales below. Six strike-slip faults are duly
identified; most are orientated NNW-SSE with a dextral sense of slip. At O|C 12.3
(GR:[160222,816003]) a NNE-SSW trending dyke is inferred to lie directly on the fault line.
As the faults displace component units of the CDIC in a brittle sense, faulting is believed to
occur post-emplacement. On occasion fault planes have utilised pre-existing planes of
Figure 46: Field sketch of the break in slope which reveals the Arkosic Arenite. View looking south
from the north side of the Allt nan Leac valley. O|C 21.1, GR:[160779,818352].
Page | 43
weakness within the upper crust by following the sense of dyke intrusions (e.g. at O|C 12.3, and
the long porphyritic dyke which extends from GR:[160300,816150] to GR:[160340, 816500]).
5.2.4 Discussion
Faults are mainly inferred based on topographical features and distinct scarps in the landscape.
The siliciclastic thrust is deemed to be a thin-skinned fault, continuing to moderate depth, in
order for the Torridonian sandstone to thrust over the thick Cambro-Ordovician dolostone.
Although no direct evidence was found in the field, a normal fault along the contact between
the Quartzite and Arkosic Arenite may be stipulated in order to bring the units adjacent to one
another. From clast counts conducted within the adjacent Lime Breccia, it is known that the
sandstone was exposed subaerially, as the isolated block it still is today, during the late Triassic
(figure 48; also refer to Reference Map in Appendix). As the break in slope faces to the north,
the fault plane is inferred to dip towards the south.
Although the strike-slip faults in Glen Boreraig and around Carn Dearg are generally orientated
in the same direction, faults from Glen Boreraig do not propagate through the Suisnish Shale
Fm (SSFM). This suggests that this set of faulting was activated during the Early Jurassic, prior
to shale deposition. If so, it indicates that the region has a history of N-S compressiona l
tectonics; the Broadford anticline being the most prominent example.
Those in the CDIC Zone, are fairly minor and insignificant and are inferred to represent post-
emplacement settling and cooling of the complex or recent, minor tectonic movement in the
upper crust.
Figure 47:
Clast Count 2.
GR:[159888,81
8103]. At this
locality, the Lime Breccia is
adjacent to the
dolostone. No.
of clasts: 78.
Figure 48: Clast
Count 1. O|C 21.4
(GR:[160823,818
032]). At this locality, the Lime
Breccia is adjacent
to the "Siliciclastic
Wedge" of
sandstone and quartzite.
No. of clasts: 100.
Page | 44
CHAPTER 6 ECONOMIC GEOLOGY
The close relationship between the Beinn an Dubhaich granite and the dolostone country rock
poses the most significant economic potential of the Strath area. The area around Kilchrist and
Strath Suardal is littered with old and long-abandoned marble quarries. The dolostone has been
hornfelsed to form the well-renowned, decorative “Skye Marble”. The formation of diopside,
serpentine and brucite gives the rock a yellow and green appearance. The Skye Marble
Company still quarries this rock to the south Torrin, and produces up to 20,000 tonnes of rock
mass a year. Most of the product however is in the form of crushed chips, which is used mainly
for agricultural lime and aggregate (Emeleus & Bell, 2005).
Further economic potential is also associated with the observed Mesozoic sedimentary
sequences. These successions are considered to be analogous to those in the offshore Hebridean
Basin, a site for major hydrocarbon potential (Morton, 1989).
CHAPTER 7 DISCUSSION
This chapter will briefly discuss and draw comparisons between the ideas and interpretat ions
from the literature and the fieldwork conducted in this project.
The Beinn an Dubhaich Granite is one of the most intensely studied small intrusions in Britain.
In line with field observations it is widely considered that it post-dates the Broadford Anticline,
intruded at c.54Ma (Holness, 1992), anad is subsequently the youngest unit of the Eastern Red
Hills igneous centre. This suppports the theory that the southerly skew of the granite was
inherited from the skew of the anticline. There is much debate however over the form of the
granite. Although it is believed to be a steepsided boss which extends to depth, work by King
(1960) argues that the granite has a sheet-like form. Dolostone islands are believed to be as a
Page | 45
result of differential erosion of the granite to reveal underlying sediments. However, magnetic
surveys conducted by Hoersch (1979), reveal that the Beinn an Dubhaich granite is likely to
extend to ~500m, at which point it is underlain by a thick (~14km), dense, gabbro root which
spreads beneath the whole central complex.
Arcuate folds, proximal and concentric to a caldera, such as the Broadford Anticline, are also
found on Mull (Cheeney, 1962) and may be attributed to magmatic pressures from the central
reservoir (Sigurdsson & et al., 2015). This phenomena would also explain the irregular
amplitude of the fold.
Although the Broadford Anticline and the thrust responsible for the “Siliciclastic Wedge”
(widely referred to as the Kishorn Thrust) share similar stress orientations (i.e. N-S compressive
stress), they most likley did not form at the same time. This is for several reasons: i) Folding is
indicative of a ductile environment in the deep crust as opposed to brittle thrusting. ii) The
thrust doesn’t propagate through the Jurassic sediments, which are believed to have all folded
together. It implies that the thrust predates the fold. iii) Clast counts in the Lime Breccia indicate
that the sandstone was subaerial by the Late Triassic.
The Kishorn Thrust is widely regarded to be associated with the Moine Thrust sheet. As is
observed in the field, the sole thrust cuts through underlying imbricated Cambrian and
Torridonian strata (McClay & Coward, 1981).
Palaeoflow data within the Jurassic sediments (Chapter 2) points to an offshore depocentre to
the WSW-SW. This hypothesis is concordant with Morton (1989) and Hesselbo, Oates, &
Jenkyns (1998) analysis of the Jurassic stratigraphy in NW Scotland. Syn-sedimentary
deposition occurred with large-scale rifting and the formation of half-graben structures in the
Hebrides Trough. The succession relevant in this report are constrained within the Hettangian
(c.201-199Ma) and Sinemurian (c.199-191Ma) stages by Morton (1989).
The igneous activity in Skye and NW Scotland is heavily asscoiated with the opening of the
Page | 46
North Atlantic and the emplacement of a proto-Iceland mantle plume. With the use of
radiometric dating, it is largely confined to 62Ma and 55Ma i.e. the Palaeogene Epoch (Emeleus
& Bell, 2005). Regional crustal inflation from a proposed plume, during and potentially post-
emplacement, may be the cause of i) the north easterly dip of dykes (145/75NE), and ii) the
sourtherly vergence of the Broadford Anticline by a similar amount (90/64N).
CHAPTER 8 SYNTHESIS & GEOLOGICAL HISTORY
Skye, like much of NW Scotland, has a very rich geological history. Throughout the study
various hypotheses have been introduced and discussed. The following chapter forms a
scientific model based on all of these interpretations and the initial observations made in the
field.
1) Ordovician Camas Malag
dolostones (purple) are deposited in an offshore
sedimentary basin, likely to be located in a tropical,
warm carbonate sea. This represents a shift from the
largely terrigenous depositional setting of the
Quartzite and Arkosic Arenite Fms (orange), upon
which they are unconformably deposited. A metamorphic basement is inferred from analys is
of lithic fragments within the sandstone. It is likely to
be Lewisian Gneiss (grey folds), which is the
prominent basement rock in NW Scotland.
2) Post-Ordovician After the dolostone
formation is deposited, the Kishorn Thrust is initia ted.
It propagates through to the surface, thrusting
Figure 49: 1) CMDL deposited unconformably on top of Arkosic Arenite and Quartzite Fms.
Figure 50: 2) Thin-skinned thrust, up throwing dolostone and sandstone.
Page | 47
Quartzite and Arkosic Arenite on top of the dolostone. The “Siliciclastic Wedge” is duly
formed.
3) Late Triassic-Early Jurassic Following a
period of erosion, which brought the “Siliciclast ic
Wedge” and dolostone to the surface, a large
succession of sediments is unconformably deposited
(grey). From the analysis of the Lime Breccia it is
known that the Arkosic Arenite, Quartzite and
dolostone were exposed subaerially during the Late Triassic. The succession represents an
overall marine transgression, implying the continued evolution of an offshore depocentre
during deposition. This is explained by the occurrence of a syn-sedimentary basin to the SW.
4) Early Palaeogene Mesozoic sediments
are folded to form the Broadford Anticline. It is an
arcuate fold, concentric to the Eastern Red Hills,
likely to be related to the formation of a caldera.
5) Palaeogene Continued and
increased igneous and volcanic activity in the region results in the emplacement of a series of
igneous suites into the country rock: i) Emplacement of NW-SE trending mafic dykes (pink),
ii) Some act as feeder dykes to the composite sill of
the CDIC (smaller red), iii) Intrusion of the Beinn an
Dubhiach Granite (large red) as the youngest igneous
suite. It occupies the core of the Broadford Anticline.
All suites are intruded into fluid-saturated crust. The
system remains in a shallow marine setting.
Figure 51: 3) Erosion followed by deposition of Mesozoic sediments.
Figure 52: 4) Regional folding.
Figure 53: 5) Suites of igneous rock intrude the country rock (grey).
Page | 48
6) Post-Palaeogene-Igneous-Activity Regional uplift due to crustal inflat ion
concentrated in a region to the N, causes tilting of the
whole sequence of ~15-20ᵒ. Subsequently, the
anticline is given an apparent southerly vergence, the
Beinn an Dubhaich Granite is skewed to the south,
and the suite of mafic dykes inherit an inclinat ion
towards the NNE/NE.
7) Post-Palaeogene to Quaternary Continued uplift and erosion emplaces the deep
crust at the surface. Sedimentary successions above
the Suisnish Shale are removed the Beinn an
Dubhaich is unroofed to reveal the granite core. Most
recently, glaciation has since sculpted the landscape,
carving out the dramatic landscape seen on Skye
today. Peat and glacial moraine is deposited. (Dashed
red line indicates current topography).
CHAPTER 9 CONCLUSION
To conclude the study, this chapter will present the main results, a brief discussion of the
methods used and areas which could potentially be carried forward into future research.
The geological model aims to synthesise and collate all the information discussed in the report.
Strath is a great example of the geological diversity the NW of Scotland has to offer. In
summary, Pre-Cambrian to Mesozoic age sediments are intruded by a significant episode of
igneous activity, which occurred during the Palaeogene. Strath lies on the periphery of the Skye
Figure 54: 6) Regional uplift and tilting towards the south.
Figure 55: 7) Erosion and glaciation form the landscape observed today.
Page | 49
Central Complex, therefore although it is affected by suites of intrusions, the geology is not
dominated by it, and its rich sedimentary history is preserved. Hypotheses have been drawn
directly from field observations and are aided by analytical techniques conducted in the lab.
Thin sections have been used to gather a further understanding of the rocks and geologica l
relationships in question, in turn supporting or disproving the null hypothesis.
To take the project forward, further, more detailed petrographical analysis, along with the use
of radioactive dating of igneous suites, such as the CDIC and the Beinn an Dubhaich Granite,
would enable the study to be more conclusive about their emplacement histories. A further
aspect to focus attention would be the Broadford Anticline. This is the most ambiguous of the
significant features in the area and its history is the most difficult to decipher. Research should
be carried out to determine whether the anticline actually is a product of the early formation of
a caldera. If this is correct it would be worth conducting further research into its full relationship
with the Eastern Red Hills, using analogous examples from other central complexes, such as
Mull. A proto-Iceland mantle plume in the North Atlantic has been attributed to the regional
tilting. However, considering the regional principal stresses, a question to ask is why does the
dominant NW-SE dyke swarm interact almost perpendicular to this phenomena (i.e. proto-Mid-
Atlantic Ridge)?
REFERENCES
Bott, M. H., & Tuson, J. (1973). Deep structure beneath the Tertiary volcanic regions of Skye, Mull and Ardnamurchan, north-west Scotland. Nature (Physical Sciences), Vol.242
114-116. British Geological Survey. (2016, January 5). Crinoids. Retrieved from BGS: Discovering
Geology: http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/crinoid.html Caritat, P., Hutcheon, I., & Walshe, J. L. (1993). Chlorite Geobarometry: A review. Clays and
Clay Minerals Vol.41 No.2, 219-239.
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Cheeney, R. F. (1962). Early Tertiary fold movements in Mull. Geological Magazine Vol.99, 227-232.
Ellis, N. V. (2007). Chapter 2: The isle of Skye. In Extracts from An Introduction to the
Geological Conservation Review. Volume 4: British Tertiary Volcanic Province (pp. 1-5). JS Publications.
Emeleus, H. C., & Bell, B. R. (2005). British regional geology: the Palaeogene volcanic districts of Scotland (Fourth edition). Nottingham: British Geological Survey.
Harker, A. (1904). The Tertiary igneous rocks of Skye. Glasgow: Published by order of the lords
commissioners of His Majesty's Treasury. Hesselbo, S. P., & Coe, A. L. (2000). Jurassic sequences of the Hebrides Basin, Isle of Skye,
Scotland. In J. R. Graham, & A. Ryan, Field Trip Guidebook. (pp. 41-58). Dublin: IAS. Hesselbo, S. P., Oates, M. J., & Jenkyns, H. C. (1998). The lower Lias Group of the Hebrides
Basin. Scottish Journal of Geology 34.1 , 23-60.
Hoersch, A. L. (1979). General structure of the Skye Tertiary igneous complex and detailed structure of the Beinn an Dubhaich Granite from magnetic evidence. Scottish Journal
of Geology Vol.15, 231-245. Holness, M. B. (1992). Metamorphism and fluid infiltration of the calc-silicate aureole of the
Beinn an Dubhaich Granite, Skye. Journal of Petrology, Vol.33, 1261-1293.
Johnson, A. L. (1994). Evolution of European Lower Jurassic Gryphaea (Gryphaea) and contemporaneous bivalves. Historical Biology. Vol. 7, no. 2, 167-186 .
King, B. C. (1960). The form of the Beinn an Dubhaich granite, Skye. Geological Magazine Vo.97, 326-333.
McClay, K. R., & Coward, M. P. (1981). The Moine thrust zone: an overview. Geological
Society, London, Special Publications Vol.9.1, 241-260. Morton, N. (1989 6.3). Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland.
Marine and petroleum geology, 243-260. Oates, M. J. (1978 Vol. 42 No.1). A revised stratigraphy for the western Scottish Lower Lias.
In Proceedings of the Yorkshire Geological and Polytechnic Society (pp. 143-156).
Geological Society of London. Pálfy, J. e. (1999). Integrated ammonite biochronology and U-Pb geochronometry from a basal
Jurassic section in Alaska. Geological Society of America Bulletin, pp1537-1549. 111.10.
Parslow, G. R. (1976 Vol.40). The Suisnish layered dyke. The Mineralogical Magazine, 683-
693. Raybould, J. G. (1973). The form of the Beinn an Dubhaich Granite, Skye, Scotland. Geological
Magazine, 341-350. Sigurdsson, H., & et al. (2015). The encyclopedia of volcanoes. Elsevier. White, R. S. (1988). A hot-spot model for early Tertiary volcanism in the N Atlantic. In A. C.
Morton, & L. M. Parson, Early Tertiary volcanism and the opening of the NE Atlantic. (pp. 393-414). Geological Society of London Publications No.39.
Page | 51
APPENDICES
A) Sedimentary Log #1 (Suisnish Shale Formation)
Facies: 1) Terrigenous 2) Lagoonal 3) Intertidal 4) Reef
5) Foreshore 6) Open shelf 7) Basinal
Page | 52
B) Sedimentary Log #2 (Boreraig Formation)
Page | 53
C) Beinn An Dubhaich Grain Size Map Full
Page | 54
D) Reference Map
Page | 55
Arran Fieldwork
19th- 24th June 2015
Page | 56
E) Corrie Foreshore Geological Map
Page | 57
F) Sedimentary Log of the Corrie Foreshore
Page | 58
G) Detailed Sedimentary Log of the Upper Corrie Foreshore
Page | 59
H) Clast Analysis of the Corrie Foreshore
Page | 60
I) Geological Map of Coastal Dyke Swarm
Page | 61
J) Dyke Count
Page | 62
K) Dyke Count cont.
Page | 63
L) Dyke Analysis
Page | 64
M) Geological Map of Drumadoon Point
Page | 65
N) Cairn Map along the Drumadoon Foreshore
