the geology of south raasay dissertation

Jonathan Edwards The Geology of South Raasay

i

The Geology of South RaasayJonathan Edwards

Department of Earth Sciences, Durham University2016

This Dissertation is submitted in Partial Fulfilment of the Requirements for the Degree ‘F600, BSc Geology’


ii

Abstract

A 14km2 area of South Raasay was mapped over an eight-week period in the summer of

2015. Nine sedimentary and three igneous formations were recognised and mapped at

1:10,000 scale. The sedimentary basement is composed of Torridonian aged, braided river

sandstones and conglomerates that were deposited in an alluvial fan system. The alluvial

fan is interpreted to have prograded during the deposition of these rocks. The Torridonian

formations are unconformably overlain by Jurassic age, shallow to deep marine

mudstones and sandstones. Two marine transgressions and regressions are proposed to

have taken place, which correlate with other regional base level changes. Cyclicity was

identified within the Suisnish Mine Formation and it is hypothesised to have been

influenced by orbital mechanisms. Significant hematite alteration of the Beinn na’ Leac

cement was also observed, which is inferred to be a result of hydrothermal fluids exolved

from the adjacent granophyre. The sedimentary succession has been intruded by an

expansive granophyre sill, a micro-gabbroic sill, along with abundant basaltic NE-SW

trending dykes. These igneous rocks are suggested to be part of the North Atlantic

Igneous Province due to their similarity to other Inner Hebridean igneous bodies. During

the Tertiary, regional extension associated with the opening of the North Atlantic resulted

in the development of a series of prominent extensional dip-slip, and conservative strike

slip faults. Many of these faults show strain partitioning relationships such as the Beinn

na’ Leac Fault. Recent, quaternary glaciation is identified by the presence of erratics and

glacial till. Raasay shows little economic promise due to low economic mineral

abundance and high export costs.


iii

Table of Contents

Abstract iiTable of Contents iiFigures iiiAcknowledgements iiiChapter 1: Introduction 1Chapter 2: Stratigraphy 2

2.1 Sequence Stratigraphy of Jurassic Sediments 2Chapter 3: Sedimentology 4

3.1 Eyre Point Formation 53.2 Boulder Valley Formation 63.3 Suisnish Mine Formation 93.4 Suisnish Beach Formation 123.5 Na Fèarns Formation 153.6 Ribbon Formation 163.7 River Footpath Formation 183.8 Borodale Forest Formation 193.9 Beinn na’ Leac Formation 20

Chapter 4: Igneous Geology 244.1 Càrn nan Eun Formation 244.2 Osgaig Point Formation 264.3 Raasay Dyke Complex 11

Chapter 5: Structure 305.1 Introduction 305.2 Brittle Deformation 30

5.2.1 Beinn na’ Leac Fault 305.2.2 Osgaig Fault and Suisnish Fault 325.2.3 Eyre Fault 325.2.4 Other Faults 33

5.3 Dyke Emplacement 33Chapter 6: Geological History and Economic Potential 34

6.1 Geological History 346.2 Economic Potential 35

Chapter 7: Conclusions 37References 39Appendix 1: Arran Fieldwork 40Appendix 2: Raasay Clean Copy Map 46


List of Figures

Grid References and Names

iv

Figure 1: Geometry of Raasay Island. 1Figure 2: Sequence Stratigraphy Interpretation of Jurassic Raasay Sediments 2Figure 3: Geological Column of the Raasay Sedimentary Succession 4Figure 4: Sketch log of Eyre Point Formation 5Figure 5: This section under cross-polarised light showing the Eyre Point

Formation Mineralogy 5Figure 6: Sketch log of the Boulder Valley Formation 7Figure 7: Image showing the Boulder Valley Formation at outcrop scale 7Figure 8: Rose diagram showing clast orientations. Note the E-W modal

abundance along with the spread of data 7Figure 9: Boulder Valley Formation Depositional Environment Block Diagram.

Raasay succession indicated by red pole 8Figure 10: Image showing the paraconformity between the Eyre Point

Formation (below) and the Boulder Valley Formation (above). 8Figure 11: Sketch log of Suisnish Mine Formation 9Figure 12: Stereonet showing bivalve pole orientations. Note the high density

of points close to the great circle. 10 Figure 13: Image Showing Calcified Bivalves within the Wackestone Units 11Figure 14: Field Sketch of Suisnish Mine Formation 11Figure 15: REDFIT spectral analysis curve of the Suisnish Mine Formation with

a 95% chi2 significance line plotted in green. The two statistically significant peaks are highlighted in blue 12

Figure 16: Wavelet spectral analysis plot for the Suisnish Mine Formation with a cone of influence plotted 12

Figure 17: Sketch log of Suisnish Beach Formation 14Figure 18: Sketch log of the Raasay Ribbon Group, showing the three

constituent formations. 18Figure 19: View of the hillside at locality 156 where gradient changes have

allowed the reconstruction of concealed stratigraphic contacts. 18Figure 20: Thin section of the Beinn na’ Leac Formation showing hematite

mineralisation (accented by red shading). 22Figure 21: Stereonet showing joint plane as poles of the Càrn nan Eun Formation 26Figure 22: Image showing contact between the Suisnish Beach Formation and

Càrn nan Eun Granite. Note the feeder dykes present beneath the sill 26Figure 23: Rose diagram showing joint plane strikes of the Càrn nan Eun

Formation 26Figure 24: Thin section under cross polarised light of the Osgaig Point Formation 28Figure 25: Field sketch of Columnular Jointing in the Osgaig Point Formation 28Figure 26: Image Showing Concentric Pillow Lava within the Osgaig Point

Formation. 28Figure 27: Field sketch of the Raasay Dyke and Sill Complex 29

Figure 28: Schematic map showing the major fault names and their displacements in South 31

Figure 29: Schematic map showing the structure present at Beinn na’ Leac. Strain partitioning between dip-slip and strike-slip displacement is evident. Note the distribution and consistent strikes of the fissures. 32

Figure 30: Block diagram showing relative motions of the Càrn nan Eun, Osgaig Point, and Inverarish Forest Fault Blocks. 33

List of Plates

Plate 1: Suisnish Beach Formation1) Field sketch of the Suisnish Beach Formation’s concretions 2) Image showing concretions in the Suisnish Beach Formation 3) Image showing external cast of ammonite in the Suisnish Beach

Formation 15Plate 2: Beinn na’ Leac Formation

1) Image of iron mineralisation, accented in red, showing its curvilinear nature along a slump structure

2) Image of hand specimen showing dendritic iron mineralisation 3) Image of bivalve external cast. Note its size and ribbed morphology 23


Four to six figure grid references are provided in brackets after locations using the notation

(GR: 070 295). The Ordinance Survey code for Raasay (NG) has been omitted from these

grid references, as it remains the same for all locations. All bearing measurements were made

with a magnetic declination of -3°. Location and formation names are italicised (e.g. Beinn

na’ Leac Formation).

Acknowledgements

The author would like to thank Professor Mark Allen for his guidance and support during this

study. Gratitude is also expressed towards the Department of Earth Sciences at the University

of Durham, for providing financial support in the undertaking of this research and supplying

necessary health and safety training. The hospitality of Raasay House is also acknowledged,

without which this project may have not been possible.

v


Chapter 1: Introduction

The Isle of Raasay is a small landmass, located in the Inner Scottish Hebrides. Situated ~3km

off the East coast of the Isle of Skye, the island is 21km long and occupies a total area of

~63km2 (figure 1). It has a total population of ~160 people with the

majority of the populous concentrated in the village in Inverarish

(GR: 5554, 3571), which is positioned on the SW coast of the island.

Raasay’s terrain is rugged, but never mountainous, with large hills

such as Càrn nan Eun (GR: 5581, 3753) and Beinn na’ Leac (GR:

5923, 3671) providing good rocky exposure. The low-lying Borodale

Wood (GR: 5569, 3660), to the South, offers less exposure due to the

vegetation cover, and private land ownership in Inverarish makes

exploring this region difficult. The South of the Island is characterised

by several NW/SE trending normal faults, which have caused deep

gashes in the landscape. These faults have been exploited by glacial

activity and therefore the topographic features are often infilled with

marshy, quaternary material and can be difficult to traverse. Due to

the island’s location on the west coast of Scotland, precipitation and strong winds occur

regularly make fieldwork challenging. A southerly region of the island covering an area of

~14km2 was mapped at 1:10,000 scale over an 8-week period, from the 13 th of August, to the

7th of October, in the year 2015.

This report aims to present a comprehensive overview of the geology of Raasay, using

observations made during the surveying of the area. These observations will then be linked to

the primary scientific literature in order to gain a deeper understanding of how the island’s

geology relates to its regional context.

1

Figure 1: Geometry of Raasay Island

Northern Limit

South Raasay(Mapping Area)

North Raasay

N

1km


Chapter 2: Stratigraphy

2.0 Introduction

The stratigraphy of Raasay consists of Torridonian fluvial sandstones and conglomerates,

capped by a paraconformity, with Jurassic marine sandstones and mudstones resting on top

and a roof of conformable granophyre sill. The main boundary of note is the paraconformity,

which represents and estimated time gap of ~700Ma.

2.1 Sequence Stratigraphy of the Jurassic Sediments

Four sequence boundaries have been interpreted for Raasay’s Jurassic sedimentary succession

(figure 2). These are defined by abrupt changes in the sedimentary facies, usually associated

with base level transgression and a depositional hiatus. The basal boundary of the first

sequence is the paraconformity, which separates the Torridonian basement from the Jurassic

sediments. This sequence is dominated by shelf slope facies that fines upwards over the

course of a Highstand Systems Tract (HST), bound by a maximum flooding surface at its top.

This HST encompasses the Suisnish Mine Formation. The next sequence represents a marine

regression from shelf slope to shallow marine facies in the form of a Regressive Systems

Tract (RST). This includes the Suisnish Beach, Na Fèarns, and Ribbon Formations. Following

on from this, there is a short-lived Transgressive Systems Tract (TST), returning the

depositional environment to a shelf slope facies. This allowed for deposition of the River

2

Figure 2: Sequence Stratigraphy Interpretation of Jurassic Raasay Sediments


Footpath and Borodale Forest Formations. Finally, the uppermost sequence represents a

Lowstand Systems Tract (LST) which resulted in the deposition of the Beinn na’ Leac

Formation (figure 2). These sequence stratigraphic interpretations correlate with Morton

(1989), who also identified four systems tracts over this timeframe; A, B, C, and D (second

column, figure 3).

3


Chapter 3: Sedimentology

3.0 Introduction

Raasay presents a diverse sedimentological record, which indicates several changes in the

geological history of the island. Both carbonate and siliciclastic sediments are represented

across nine formations ranging from fluvial conglomerates, to laminar marine mudstones. The

oldest sequences are the Torridonian age, ‘Eyre Point’ and ‘Boulder Valley’ formations.

These successions were deposited under unidirectional, fluvial

conditions of moderate to high energy, in braided river and

alluvial fan environments respectively. They therefore show

features such as channelized sediments and imbrication. A

significant paraconfomity segregates these sediments from much

younger, Jurassic age sediments on top. These Jurassic

sediments were deposited under much lower energy conditions

in a shallow to deeper marine setting. They therefore

occasionally contain abundant marine fauna including

ammonoids and bivalves. The oldest Jurassic formation is the

‘Suisnish Mine’ formation, of Sinemurian age, which is

deposited on top of the dominant paraconfomity. This is

followed by the slightly younger ‘Suisnish Beach’ formation,

which is of particular interest as it shows a cyclic fluctuation

between a shallow marine, wackestone producing depositional

environment, with a deeper marine, laminar mudstone producing depositional environment.

‘Na Fèarns’ formation, ‘Ribbon formation’, ‘Borodale formation’, and finally ‘Beinn na’

Leac’ formation at the top of the sedimentary succession of Aalenian to Bajocian age. Over

the course of the deposition of the observed Jurassic sediments, there are two main cycles of

base level transgression and regression, which accounts for the variation in grain size of the 4

Figure 3: Geological Column of the Raasay

Sedimentary Succession


sediments, and changes in the apparent biofacies. Recent Quaternary deposits are glacial in

nature consisting of erratics and till material that have infilled areas of topographically low

relief.

3.1 Eyre Point Formation

3.1.1 Spatial Distribution and Outcrop Style

The ‘Eyre Point Formation’ outcrops along the southernmost coast of Raasay from Rubha na

Cloiche (GR: 5629 3372) to the road at North Fèarns (GR: 5891 3547) occupying and

approximate area of 0.7km2. The formation tends to form small ledges (2m-6m high) along

the coast due to the steep gradients with particularly good outcrops

along the EW road that runs along the south coast. Exposure is

limited at South Fèarns due to a steep NE/SW trending slope,

covered by a dense deciduous forest. The Eyre Point Formation was

observed at 9 different localities across the mapped area.

3.1.2 Lithology and Structural Observations

The Eyre Point Formation (figure 4) consists of a clast supported,

medium-sand grade arkose with sub-rounded clasts. The majority of

the formation is well sorted, however there are some anomalous

pebble-grade clasts present that are much larger than the

surrounding matrix. There are also uncommon, poorly sorted,

lenticular units present that are ~4cm thick and up to ~20cm

wide. The mineralogy of the formation is dominated by

quartz (51%) and feldspar (orthoclase (38%) and microcline

(5%)), which is accompanied by muscovite (2%) and biotite

(4%) along with opaque minerals such as hematite (<1%) and

ilmenite (<1%) (figure 5). Cross bedding is observed at many 5

Figure 4: Sketch log of Eyre Point

Formation

Figure 5: This section under cross-polarised light showing the Eyre Point Formation Mineralogy


horizons in the formation and shows an overall eastward palaeocurrent. The high proportion

of orthoclase gives the lithology and overall pink colour. The Eyre Point Formation is very

competent and well joined (~17cm spacing) with some calcified veins.

3.1.3 Regional Context and Interpretations

The Eyre Point Formation shows classic sedimentary structures associated with a braided

river system on an alluvial fan such as cross bedding and channelized sediments. The well

sorted nature would indicate that these sediments were deposited in a fairly distal part of the

alluvial fan, however, the prominent feldspar component to the clasts show that deposition

occurred close to the sediment source. Williams (2001) suggests that these sediments were

derived from the footwall of the extensional Minch fault scarp. This fault also would provide

a gradient change to allow the deposition of the alluvial sediments. Selley (1965) classified

the Torridonian rocks of Raasay into three main facies; red facies, grey facies, and basal

Facies. The observations presented above as well as the stratigraphic location of the Eyre

Point Formation suggests that it belongs to the ‘red facies’, which is also interpreted to be a

braided river depositional environment (Selley, 1965).

3.2 Boulder Valley Formation


The ‘Boulder Valley Formation’ is exposed along the southern flanks of Suisnish Hill to the

North of the road, from Braemore to South Fèarns. It occupies an area of approximately

0.8km2 over two blocks that are offset dextrally by the Eyre Fault. The terrain is steep with

outcrops exposed in cliff sections ranging from 1m to 11m high. Much like the Eyre

Formation, exposure is limited on the East coast of the island, due to a steep incline covered

by deciduous forest that runs from Eyre to South Fèarns. The Formation was observed at 11

localities across the mapped area.

6



The Boulder Valley Formation (figures 6 & 7) is composed of poorly sorted polymict

paraconglomerates (~70% matrix) with a channelized base and four clast compositions:

arenite (39%), arkose (45%), a schistose metamorphic

lithology (15%), and a white metamorphic lithology (1%)

(See figure %). Clasts are well rounded and very poorly

sorted, ranging in size from 2mm to 86mm. The matrix is

sand grade and is similar in composition to the arkosic

Eyre Point Formation. Some lenticular shaped units are

present which are composed exclusively of matrix

material. These range in thickness from ~1m thick (e.g.

locality 63) to ~15cm thick (e.g. locality 125) and tend to

show parallel laminations at their base. The clasts in the conglomerate show an overall E-W

alignment, however there is notable spread in the data (figure 8). The formation is absent of

any fossil.

3.2.3 Interpretations and Regional Context

The Boulder Valley Formation was likely deposited under high energy, fluvial conditions

within river channels. This provides an explanation for the poor sorting of the conglomerates,

7

Figure 6: Sketch log of the Boulder Valley Formation

Figure 8: Rose diagram showing clast orientations. Note the E-W modal abundance along with the

spread of dataFigure 7: Image showing the Boulder

Valley Formation at outcrop scale


rounding and apparent E-W alignment of the clasts, and the presence of lenticular shaped

units. The high degree of variation of clast alignments (figure 8) is likely due to deposition

within a braided system on top of an alluvial fan (figure 9), where flow direction is observed

to vary as a fan matures. The coarse nature of the clasts indicates that deposition occurred

proximal to the source of the sediment of the alluvial fan, a great distance from the fan

margin. This also explains the high orthoclase component within the matrix. The parent rock

for the metamorphic and arenitic clasts is not recognised across the mapped area, however it

is likely that some of the metamorphic material was derived from the Lewisian Complex that

outcrops to the North of the Screapadal Fault and underlies the Eyre Point Formation (e.g.

Storetvedt and Steel, 1977). The Boulder Valley Formation is formally known as the

Stornoway Formation and there is significant debate in the literature (Morrison, 1887;

Woodward, 1914; and Storetvedt, 1977) concerning the formation’s age due to a lack of fossil

material and the paraconformity present at its base. The main age proposals are Torridonian,

Devonian and Permo-Triassic. Observations from this study suggests that the Torridonian age

interpretation is most likely due to several reasons:

- The mineralogy and interpreted depositional environment is similar to that of the Eyre

formation. This infers that both formations were derived from a similar sediment

source, and were deposited in a similar setting - such as a prograding alluvial fan.

- The Boulder Valley Formation is characteristically dissimilar to the Jurassic, marine

sedimentary rocks found in South Raasay. Storetvedt (1978) argues that this is because

the formation is an onshore representation of the Mesozoic marine succession,

however, the compositional difference makes this unlikely.

- An undulating paraconformity was observed between the Boulder Valley Formation

and the Suisnish Mine Formation, indicating that a significant time gap had occurred

before the Jurassic.

8


3.3 Suisnish Mine Formation


The Suisnish Mine Formation outcrops across a locally confined area (~0.3km2) on Suisnish

Hill close to a disused iron hopper that was used for the iron mine works (GR: 5555, 3432). It

is also exposed in South Fèarns (GR: 5815, 3498), however this area is

inaccessible due to a steep, densely forested slope. The best outcrops

occur as cliff sections ~8m high that run parallel with, and on the east

side of the disused railway (GR: 5566, 3464). The formation was

observed at five localities across the mapped area.


The Suisnish Mine Formation (figures 11 and 14) consists of an

interbedded succession of fine grained, dark grey, laminar siltstones

with poorly sorted, sparse biomicrites/wakestones. Bedding is planar

with units varying in thickness from ~10cm to ~140cm. The siltstones

have a micaceous mineralogy (~10%) which is very similar to the composition of the

wackestone matrix. However, the wackestone units sometimes show cross bedding and

contain large (~4cm) calcified bivalves with prominent concentric growth lines (figure 13).

Ammonoids are also present, although rare. The bivalves all have similar orientations (figure

12) and are found in the greatest abundance at the base of the wackestone units (figure 13).

Sub-vertical joints (112/78N) are present which cross cuts the bedding planes however;

jointing is more abundant in the more competent laminar siltstones.


The Suisnish Mine Formation was likely deposited within a marine basin due to the presence

of ammonoids and bivalves. The low energy features such as parallel laminations indicate that

the height of the water column was substantial, though the 9

Figure 11: Sketch log of Suisnish

Mine Formation


presence of bivalves infers that deposition still occurred within the photic zone. The

abundance muscovite and siliciclastic matrix shows that these rocks were deposited fairly

close to a continental sediment input, such as on a continental shelf. The Suisnish Mine

Formation is known formally in the literature as the Ardnish Formation and Lee (1920)

compared the rocks observed in South Raasay with sections at Hallaig Shore and classified

these rocks as Sinemurian age (lower Jurassic) using biostratigraphy.

3.3.4 Cyclic Analysis

The Suisnish Mine Formation represents a rapid changing depositional setting from

wackestone producing, shallow marine environment, to laminar siltstone producing, deeper

marine environment. 10 cycles are observed in South Raasay are three possible interpretations

for these changes: fluctuating; (1) energy, (2) sediment input, and (3) oxygen levels, or

perhaps a combination of the three. Climatic and/or base level allocyclic controls are

potentially therefore a dominant force on the sedimentation.

(1) Fluctuations relating to the energy of the water column associated with base level changes

could lead to the deposition of this formation. During periods of acquiescence, the bivalve

community could flourish and be deposited with a mudstone matrix. During waters that are

more turbulent however, the organisms are less adapted, and so do not survive, while slightly

coarser grained, thicker siltstone units are deposited in their place.

(2) Variations in sediment influx is an alternative hypothesis. Due to the filter feeding nature

of bivalves, an increase in sediment flux and deposition would result in the suffocation of

these organisms and a deposition of thicker units, absent of bivalves. Contrastingly, when

sediment influx was low, bivalve communities could develop and form the bivalve dominant

units.

(3) Cyclicity of the oxygen state of the sediment associated with base level fluctuation could

also allow the deposition of this formation. Transgression would lead to anoxia of the

10


sediments and the deposition of bivalve free units, while regression would allow bivalve

respiration and the deposition of wackestone.

When deposition rate for the formation is assumed broadly constant at outcrop scale, time can

be used as a proxy for height up the section. Therefore, the cyclic nature of the sediments can

be analysed using spectral frequency analysis. Redfit analysis has identified two dominant

frequencies that are statistically significant that may have resulted in the deposition of these

sediments (Figure 15). Wavelet analysis shows a similar bimodal distribution with a dominant

low frequency signal, along with a higher frequency component (figure 16). Wavelet analysis

also shows a slight temporal change in the higher frequency cycles, though this may just be an

artefact of the cone of influence or due to the small dataset size. Unless the bedding planes of

the formation are dated by absolute means, it is not possible to calculate a value for the

frequency of the two cycle cycles, though they can be correlated with other research that has

been done on the subject.

11

Left - Figure 15: Redfit spectral analysis curve of the Suisnish Mine Formation with a 95% chi2

significance line plotted in green. The two statistically significant peaks are highlighted in blueRight - Figure 16: Wavelet spectral analysis plot for the Suisnish Mine Formation with a cone of

influence plotted


Nhnhn

3.4 Suisnish Beach Formation


The Suisnish Beach Formation occurs in two locations across the South of the island. A small

area (~0.6km2) outcrops along the South West coast from Suisnish to Suisnish Point (GR:

5518, 3532), and a much larger area (~1.2km2) is present to the West of the Beinn na’ Leac

fault, along the road (GR: 5813, 3627). Outcrops tend to be floor building, because of the

formation’s soft nature and its shallowly dipping bedding planes. With that said, the

formation has been deeply incised by the Allt Fearns River providing good vertical sections.

The Suisnish Beach Formation was observed at 22 localities across the mapped area.


The formation (figure 17) consists of a dark grey, finely laminated siltstone containing red

coloured nodular structures (plate 1.1 and 1.2) and ammonoids (plate

1.3). The mineralogy of the rocks is difficult to identify, though an

abundance of muscovite mica (~10%) is recognised. The siltstone is

very soft and crumbles easily. Planar bedding varies in thickness from a

few millimetres to ~10cm thick, however most units are at the lower

end of this scale. Bedding planes dip consistently by ~20°, though the

strike is ~30° different between outcrops to the NE and outcrops to the

SW of the island. The red coloured concretions make up ~5% of the

formation and are fine-grained. These features are also much harder and

denser than the surrounding siltstone and show concentric layering.

Multiple concretions often occur along the same bedding plane and

surrounding bedding planes appear to deflect around the structures. The ammonoids observed 12

Figure 17: Sketch log of Suisnish

Beach Formation


in the formation are only ~5.2cm in diameter and show little ornamentation except for some

minor ribbing. In terms of structures, the formation contains one consistent joint plane

throughout (097/82S) which are spaced ~14cm apart. However, the joints do not intersect

more than a few tens of beds and are minor features.


The Suisnish Beach formation was likely deposited within a similar marine setting to the

Suisnish Mine formation; however, the abundant bivalves and wackestones are no longer

present. A hypothesis that explains this change is a marine transgression, which has resulted

in the depositional environment becoming too deep and stagnant for the bivalves to survive.

This would also explain the greater abundance of ammonoids as the biofacies has shifted from

an ‘inner shelf’ environment to a more ‘outer shelf’ environment. The Suisnish Beach

formation is known formally in the literature as the Pabay Shale Formation, which has been

13


biostratigraphically dated using dinoflagellate cysts (Brittain et al., 2010) and ammonites

(Oates, 1978) as upper Sinemurian age. The transgression that is interpreted to have led to the

deposition of this formation is therefore likely a local expression of the regional deepening

that took place during the early Sinemurian (Hesselbo and Coe, n.d.). At this time, many

basement highs across the country were permanently submerged beneath the Jurassic sea such

as the Ordovician-age Durness Limestone (e.g. Farris et al., 1999). In terms of the

concretions, the features are likely syn-depositional, which explains the deflected bedding.

The red colour of the concretions is likely a result of iron mineralisation, which is also

reflected by density and hardness. Therefore, these features would most likely have formed

within the sulphate reduction zone.

3.5 Na Fèarns Formation


The Na Fèarns Formation is a spatially expansive formation that outcrops over an

approximate area of 1.4km2 across the South of Raasay. It is situated along the East coast of

the island at North Fèarns (GR: 5952, 3609), as well as further West, throughout the central

part of the island until Inverarish Hotel (GR: 5645, 3644). Prominent exposure is situated

along Inverarish Burn (GR: 5653, 3699) where the river has stripped away superficial

deposits, however steep valley sides makes this area inaccessible. Exposure is very poor on

the hill to the East of Inverarish Burn (GR: 5771, 3655) because of thick glacial deposits,

though its presence is inferred. Outcrops are in the form of 5m-8m cliff sections and the

formation was observed at 12 localities across the mapped area.


The Na Fèarns Formation is dominated by medium-sand grade, quartz arenite sandstones,

with some siltstone interbeds. The sandstones are thickly bedded (>1m), while the siltstone

14


units are much thinner (~30cm). The formation is generally well sorted and the clasts are sub-

rounded in shape, though there is an overall coarsening upwards trend throughout the units.

Concretions, similar to those observed in the Suisnish Beach Formation are present, though in

a much lower abundance and are dark grey in colour as opposed to rusty red. The mineralogy

of the Na Fearns Formation is mostly quartz (>95%), though the rock’s weathered surfaces

are darker brown in colour which is not what was observed from the other quartz arenite

formation on the island, the Beinn na’ Leac Formation. Hummocky cross stratification is

dominant in places such as at locality 145, where the sediments appear to be subdivided into

discreet ‘packets’. Calcified solitary corals are present within the sandstone units, which have

a diameter of ~0.5cm. These specimens show fibrous radial structures that could be

interpreted as septa. Regarding structure, few joints were present within the formation (~8m

spacing) as it is less competent than other sedimentary lithologies in the area. At locality 67

some slicken-lines were present in the Na Fearns Formation, and this is hypothesised to be

due to the nearby strike-slip fault (see Chapter 5).


The Na Fèarns Formation is hypothesised to have been deposited within a marine

environment; however, this environment is much shallower to the one that created the

underlying Suisnish Beach Formation. This is because the grain size of the rocks is coarser

than the Suisnish Beach siltstones and an overall coarsening upwards is recorded in the

succession. The presence of corals indicates that the formation was deposited proximal to a

reef system, and therefore within the photic zone and above the storm-wave base. This

provides an explanation for the hummocky cross-stratification that has developed due to

bilateral wave action. The Na Fèarns formation is known formally in the literature as the

Scalpay Sandstone Formation; however, relatively less research has been conducted on the

sequence. This is likely due to their inaccessible distribution on the island.

15


3.6 Ribbon Formation


The Ribbon Formation is the oldest member of the Raasay Ribbon Group (figure 18), which

is a thin group of three sedimentary formations that spans a few tens of metres. In map view,

the feature curves with the topography much like a piece of ribbon. Prominent faulting of this

feature has resulting the creation of five main blocks, which are situated from Borodale Wood

(GR: 5575, 3618) to the east coast of the Island. The Raasay Ribbon

Group is particularly visible in the hillside at locality 156 (figure 19),

where gradient changes show the contact relationships. Outcrops of

this Ribbon Formation are often small ledges (~2m2) and are poorly

preserved because of weathering. The Ribbon Formation outcrops

over an area of approximately 0.2km2 and was observed at just 3



The Ribbon Formation consists of well-sorted, matrix supported,

rusty red coloured oomicrite, which is interbedded with darker grey

mudstones. The ooids are pale cream in colour and both ovular and

concentric in nature. The ooids consist of ~30% of the mineralogy.

Muscovite clasts are also present (~25%), while the remaining ~45% is composed of a fine-

grained calcareous mud. Graded bedding is present in the oolite on centimetre scales. Bedding

is planar (dipping 171/16W) and units vary in thickness from ~1.5cm to ~10cm.

16Figure 19: View of the hillside at locality 156 where gradient changes have allowed the

reconstruction of concealed stratigraphic contacts.

Figure 18: Sketch log of the Raasay Ribbon Group, showing the

three constituent formations


3.6.3 Interpretation and Regional Context

The Ribbon Formation’s depositional environment is not easily interpretable. The presence of

öoids would indicate that the formation was deposited within a low latitude shallow marine

environment, where the water would be sufficiently agitated and warm enough to allow the

precipitation of calcite layers about a nucleus. However, this energy level is not reflected in

the muddy matrix which was likely deposited under much lower energy conditions. As a

result, it is hypothesised that the öoids are allochthonous, and were transported from a shallow

marine environment into deeper conditions. The mechanism for this transportation is likely

weather anomalies, such as storm events, which makes cyclic analysis redundant. This would

explain why some units are absent of öoids and the observed centimetre scale graded bedding.

Kearsley (1989) classified the öoids as secondary B4 subclass, because they contain fragile

kaolinite structures that would not withstand the abrasion involved with agitated formation.

As a result, weathering or diagenetic alteration of the original öoids has taken place to

produce these structures. It is probable that the Ribbon Formation marks the start of the

Toarcian sedimentary succession on Raasay as the organic rich and fine grained nature of the

formation characterises the early Toarcian anoxic oceanic event. The Ribbon Formation is

referred to formally as the ‘Raasay Ironstone Formation’, and it is well known for its high

iron content and extraction for ore leading up to and during the First World War (e.g. Draper

and Draper, 1990).

3.7 River Footpath Formation


The River Footpath formation (figure 18) is the intermediate member of the Raasay Ribbon

group. It therefore bears a similar spatial distribution to the Ribbon formation. It occurs

throughout the centre of the mapped area from Borodale Wood (GR: 5619, 3658) to the hill

West of Beinn na’ Leac (GR: 5749, 3690), and on the East coast of the island. The outcrop

17


style is of laterally continuous wall sections that tend to form the Eastern limit of Borodale

Forest due the topographic change associated with abrupt lithological change. The formation

is not very thick and therefore occupies a surface area of just 0.2km2. As a result, this

lithology was only observed at 5 localities across the South of the island.


The River Footpath formation presents a lithology consisting of very fine-grained, laminated

micaceous mudstones. Bedding is often only a few millimetres thick, and the well-developed

laminated texture lends the rock a slight cleavage. The lithology is also very soft, preserving

fingerprints when handled in hand specimen. Muscovite clasts are present in the lithology

(~5%) surrounded by a mud matrix (~95%), and the rock has an overall dark grey to black

appearance.


The very fine grained size of the River Footpath Formation would indicate that this formation

was deposited in very stagnant, low energy conditions. Although no marine fossils were

observed, it’s stratigraphic position would suggest that the palaeoenvironment was still a

marine basin.

3.8 Borodale Forest Formation


The Borodale Forest Formation (figure 18) is the youngest member of the Raasay Ribbon

Group and has a very similar distribution to the River Footpath and Ribbon Formations.

Outcrops tend to be in the form of laterally extensive cliffs between 3m and 6m high. These

cliffs can extend to up to 20m wide and mark the Eastern limit of the Borodale Forest. The

formation outcrops over a surface area of just 0.2km2 and was only observed at 5 localities

across the mapping area.

18



The lithology of the Borodale Forest Formation consists of quartz arenite, similar in

appearance and composition to the Na Fèarns Formation. Parallel bedding is well developed

at the base of the section, while cross bedding is present at the top of the formation. An

overall coarsening upwards trend is also observed from a medium-sand grade to coarse-sand

grade. The mineralogy is mostly quartz (>95%) with some biotite clasts present in low

abundance. The formation’s overall appearance is pale grey.


Due to the similarities with the Na Fèarns Formation, the interpretation for the Borodale

Forest Formation is very similar. A shallow marine environment is inferred to have produced

the texturally mature, sand-grade sediments. An overall increase in the energy environment is

observed which is illustrated by the shift from parallel bedding to cross bedding and

coarsening up section. The Borodale Forest Formation is known formally in the literature as

the Beinn na’ Leac Sandstone Member as part of the Bearreraig Formation.

3.9 Beinn na’ Leac Formation


The Beinn na’ Leac Formation is the most expansive sedimentary formation that occurs on

South Raasay, outcropping over an approximate area of 3.4km2. It forms the bedrock of the

prominent Beinn na’ Leac hill on the East coast of the island (GR: 5915, 3662) and fringes the

southern margins of the Càrn nan Eun granophyre (e.g. GR: 5593, 3706) and Osgaig Point

micro-gabbroic sill (e.g. GR: 5504, 3680). The formation is therefore offset by many of the

strike-slip faults that penetrate the Càrn nan Eun granophyre including the Eyre Fault. The

outcrop style of this formation is greatly dependent on the location. To the West of the island,

the outcrops are floor building making it difficult to identify temporal changes in the rock’s

19


deposition. To the East on Beinn na’ Leac, on the other hand, very high cliffs up to 20m are

present along with treacherous fissures that make exploring this area quite hazardous.

Contacts between the Beinn na’ Leac Sandstone Formation and the Càrn nan Eun Granite are

very easy to identify, as the vegetation that grows on the sandstone is very grass dominated as

opposed to bracken dominated vegetation that grows on the granite. The Beinn na’ Leac

Formation was observed at 38 localities across the mapped area.


The Beinn na’ Leac formation consists of thick units of cross-bedded, medium grained,

arenitic sandstones, with a mature, well-sorted mineralogy dominated by quartz (>95%) and

some detrital orthoclase. Overall, the rock has a rusty

red appearance with a pale grey weathered surface.

At many localities, dark red veins are present (plate

2.2) which are dendritic and curvilinear in nature. In

cross section, these veins show alteration of the

calcite cement to hematite (figure 20) and there is no

difference in grain size across the boundaries.

Furthermore, the formation shows features commonly

associated with dissolution on its weathered surfaces,

such as honeycomb weathering and channels. This is likely because of a calcite component to

the rock’s cement. Fossils are also present in low abundance such as at locality 14, where a

large (13.5cm length from umbo to anterior margin) bivalve external cast is present (plate

2.3). The specimen shows significant ribbing and has a jagged anterior margin. In terms of

structure, the formation is competent and contains many sub-vertical joints, which have

allowed large blocks to fall from outcrops under gravity. The formation dips ~20° degrees

20

Figure 20: Thin section of the Beinn na’ Leac Formation

showing hematite mineralisation (accented by red shading)


NEE, though this varies slightly across the island as the formation has been offset and rotated

by a series of oblique slip faults (see Chapter 6).


The presence of marine fossils shows that the depositional environment is still marine;

however, the energy conditions are much more significant than previously observed in the

Raasay Ribbon Group. This indicates the reverting to a depositional environment similar to

what produced the Na Fèarns Formation. This interpretation can be presented due several

factors. These include the ornamentation and size of the fossils, medium-sand grade grain size

of the clasts, and presence cross-stratification. The dark red veins that occur throughout the

formation are probably post-depositional as there is no variation in grain size and the

21

Beinn na’ Leac Formation Plate

1) Image of iron mineralisation, accented in red, showing its curvilinear nature along a slump structure

2) Image of hand specimen showing dendritic iron mineralisation

3) Image of bivalve external cast. Note its size and ribbed morphology

3

21


compositional difference takes place in the cement rather than the matrix or clasts. Therefore,

it is hypothesised that the veins resulted from fluid migration through the rock that dissolved

the calcite cement and precipitate hematite in its place. Perhaps these fluids exolved from the

stratigraphically near granophyre sill (Chapter 5.1) and thus occurred at a similar time. If this

were the case, however, the mineralisation would be expected to be present other formations

on South Raasay. As a result, more observations need to be made in order to test this

hypothesis. The Beinn na’ Leac Formation is referred to formally in the literature as the

Druim an Fhurain Member (though this spelling is technically incorrect), which is the

youngest member in the Bearreraig Sandstone Formation.

22


Chapter 4: Igneous Geology

The Island of Raasay is host to three main Igneous Formations which outcrop across ~50% of

the mapped area. The most expansive of these is the Càrn nan Eun felsic granophyre which

forms the bedrock of Suisnish Hill and Càrn nan Eun. This granophyre is ‘sheet-like’ in shape

and unconformably rests on top of the sedimentary succession discussed in chapter 4. The

feature was likely responsible for many of the iron enrichment observed across the island due

to exolved hydrothermal fluids. The Osgaig Point micro-gabbroic sill is another significant

igneous formation that is exposed on the West coast of the island. It’s prominent columnar

jointing often resembles that of the famous Giant’s Causeway in Northern Ireland. The

feature was mistakenly identified as a lava plateau in early studies (e.g. Bradshaw and Fenton,

1982), however is more likely to be an intrusive sill due to it’s grain size. Finally, many

basaltic dykes have intruded into the sedimentary rocks of South Raasay, on the South East

coast, and along Allt Fèarns. Much of this igneous activity is interpreted to be a result of

development of the North Atlantic Igneous Province (NAIP) which took place in the mid-

Palaeocene to early-Eocene (Jolley and Bell, 2002). This could potentially explain the

compositional and observational similarities between the Giant’s Causeway and the Osgaig

Point Formation.

4.1 Càrn nan Eun Formation


The ‘Càrn nan Eun Formation’ outcrops across two regions within the mapped area; Càrn

nan Eun to the North West, East of Osgaig (GR: 5591, 3782) and Suisnish Hill to the South,

adjacent to Eyre Point (GR: 5615, 3523). The two regions occupy ~2.8km2 and ~4.1km2

respectively. Both Càrn nan Eun and Suisnish hill are of high relief and show abundant

bedrock exposure. The two areas have also been faulted which has been exploited by erosion,

leaving gullies (~70m deep), many of which have now been drowned resulting in 9 lakes such 23


as Loch Storab. The vegetation cover on these rocks is dominated by thorned plants and long

grasses making the formation’s presence distinctive, even in areas of low exposure. The Càrn

nan Eun formation was observed at 71 localities across the mapped area.


The lithology of these rocks is composed of a coarse-grained granite with a crystalline,

porphyritic texture. The coarse groundmass (2mm –

3mm) makes up ~60% of the rock, while the phenocrysts

(4mm-8mm) make up ~40% of the rock. The lithology

contains four dominant minerals giving it a felsic

composition, and overall pale cream appearance; quartz

(~35%), plagioclase (~26%), orthoclase (~25%), and

biotite (~14%). Quartz and orthoclase tend to form the

phenocrysts whilst plagioclase and biotite make up the

groundmass. Under the microscope, it is observed that intergrowth has occurred between

plagioclase and quartz in this lithology (e.g. Woodward, 1914). Grain size is not consistent

across the intrusion. Where the granite comes into contact with other formations, the

groundmass is significantly finer and darker grey in colour. The phenocrysts are also reduced

in size to ~0.9mm in diameter. This is characteristic of a chilled margin which was observed

in detail at locality 17 (p16). Xenoliths are also present within the granite which range in size

24

Figure 22: Image showing contact between the Suisnish Beach Formation and Càrn nan Eun Granite. Note the

feeder dykes present beneath the sill

Figure 23: Rose diagram showing joint plane strikes of the Càrn nan

Eun Formation

Figure 21: Stereonet showing joint plane as poles of the Càrn

nan Eun Formation


from 1.1m to 2.4m. These xenoliths show planar bedding (~3mm thick) but have been baked

making lithological characteristics difficult to observe. Directly beneath the intrusion, feeder

dykes were observed which have a similar porphyritic composition to the overlying granite

(figure 22). Continued exposure to atmospheric conditions has resulted in the partial

decomposition of ferromagnesian minerals in the granophyre to be replaced with rusty

alteration products. In terms of structure, the Càrn nan Eun Formation is well-jointed due to

its competent nature (figures 21 and 23). Many of these joints are orientated to form vertical

columns (figure 22) and the substantial jointing has led to large blocks of the lithology to have

been dislodged and fall from outcrops.


The Càrn nan Eun granophyre was likely intruded at plutonic depths due to it’s coarse grain-

size. The porphyritic texture shows a two stage crystallisation process, whereby the

phenocrysts had a long time to crystallise before the conditions of the magma chamber

changed and the groundmass cooled at a slightly faster rate. The granophyre sill has been

interpreted to be ‘sheet-like’ in shape (e.g. Woodward, 1914) and it is discordant with the

underlying sedimentary units. This infers that a large time gap has occurred between the

deposition of the Jurassic sediments and the emplacement of the intrusion. The joint planes

are orientated in three main sets. These have likely formed as a result of a negative dilation of

the magma during cooling.

5.2 Osgaig Point Formation


The ‘Osgaig Point Formation’ is situated along the western coast of South Raasay from

Osgaig Point (GR: 5449, 3819) to the ferry terminal (GR: 5447, 3634), covering most of

Osgaig. The lithology occupies ~1.1km2. The area is low lying with poor exposure inland due

25


to drift cover. The best exposure is present along the coastal cliffs with Oskaig Point being

exemplary (locality 22, p20). A significant change in topography trending NNW-SSE marks

the eastern extent of the formation. This topographic anomaly is a result of a significant

strike-slip fault (see Chapter 6). The Osgaig Point Formation was obsered at 16 different



The Osgaig Point Formation consists of a crystalline, medium to coarse-grained lithology

with a porphyritic texture. The medium-grained

groundmass (~1mm) comprises ~80% of the rock and

is composed of pyroxene (55%) and plagioclase (25%)

(figure 24). The groundmass’ overall colour is a dark

grey, which is indicative of a mafic igneous

composition. The remaining 20% of coarse

phenocrysts are all of an olivine composition ranging

in size from 3mm to 5mm. Weathering of these

phenocrysts has let to many being removed from the

groundmass and deposited in the sands of Oskaig beach. The Osgaig Point Formation shows

major columnar jointing (see figure 25, locality 22, p23). Much like with the Càrn nan Eun

26

Figure 26: Image Showing Concentric Pillow Lava within the Osgaig Point

Formation.

Figure 25: Field sketch of Columnular Jointing in the

Osgaig Point Formation

Figure 24: Thin section under cross polarised light of the Osgaig

Point Formation


Formation, xenoliths are present in the Osgaig Point Formation such as at locality 26 (p22).

These show planar bedding and are up to 10m by 6m in size. Alteration from the surrounding

igneous material is once again significant but a sedimentary protolith can be inferred. Finally,

pillow lava


The Osgaig Point Formation’s overall grain size indicates that it was a feature that was

intruded at hypabyssal depths. This must mean that the rock formed intrusively as opposed to

extrusively. The columnular jointing illustrates that the magma experienced a negative

dilation during cooling. The jointing also identifies that the formation is probably a sill, as

these cracks usually form perpendicular to the intrusion’s wall or base. Olivine abundance and

groundmass grain size varies along the coastline, with greater abundances (up to 30%)

towards the south. It is hypothesized that this is because the intrusion has developed a slight

cumulate texture near its base due to the gravity settling of the olivine phenocrysts. Much like

with the granophyre, this intrusion was likely emplaced as a result of the North Atlantic

Igneous Province during the Tertiary.

4.3 Raasay Dyke Complex


The ‘Raasay Dyke and Sill Complex’ occurs across the whole mapping area. They feature in

high abundance on the north side of the road between Na Fèarns and Glen Lodge, along the

southern coast at Suisnish Point, as well as beside the river, west of Beinn na’ Leac. The

formation tends to stand several metres proud of surrounding lithologies, which is inferred to

be due to a greater resistance to weathering. There is very little correlation between the strike

trends of this formation.


27


The Raasay Dyke and Sill Complex (figure 27) takes the form of many discordant and

concordant, fine grained crystalline features, which span

an order of magnitude in thickness (3cm – 3.5m). All

display an equigranular, fine-grained texture and are of a

dark grey colour. These characteristics are diagnostic of

and have led to the interpretation that the lithology is a

mafic basalt. Due to the fine-grained nature of the

lithology it is difficult to quantify the mineral abundances,

however the rock contains both pyroxene and plagioclase.


The igneous dykes observed on Raasay were likely emplaced during the development of the

North Atlantic Igneous Province (see chapter 5)

28

Figure 27: Field sketch of the Raasay Dyke and Sill Complex


Chapter 5: Structure5.1 Introduction

Across South Raasay, several prominent faults have offset the geology causing a repeating of

the stratigraphy (figure 28). This is particularly noticeable in the Ribbon Group sediments

which occurs 5 times throughout the centre of the area. Faulting has also brought the Oskaig

Point Formation adjacent to the Càrn nan

Eun Formation, without which the

formation would not be observed in South

Raasay. Many of these faults have been

exploited by weathering and glacial erosion

resulting in deep scarps in the topography.

An overall tilting of the island is also

hypothesized due to a regular offset of the

bedding measurements in all formations by ~20° to the West. The emplacement of the Raasay

Dyke Complex appears to have been along similar azimuths. This would indicate that their

emplacement was to accommodate an E-W, regional extension event that may be correlated

with similar igneous formations across Northern Britain.

5.2 Brittle Deformation

Six prominent faults were observed on the South of Raasay. These consist of the Oskaig Fault

(from GR: 5442, 3633 to GR: 5475, 3830), Eyre Fault (from GR: 5513, 3735 to GR: 5755,

3409), Beinn na’ Leac Fault (from GR: 5905, 3555 to GR: 5887, 3707), Suisnish Fault (from

GR: 5685, 3499 to GR: 5581, 3396), and two faults which offset the Ribbon Group in the

centre of the mapped area.

5.2.1 Beinn na’ Leac Fault

29

Figure 28: Schematic map showing the major fault names and their displacements in South

Raasay


The Beinn na Leac’ Fault (figure 29) is situated on the South East coast of the island. It trends

NNE-SSW to the west of Beinn na’ Leac (GR: 5857, 3644), before sharply curing to a NW-

SE direction, south of the hill and intersecting the coast. The fault is referred to in the

literature as the ‘Hallaig Fault’ (e.g. Smith et al., 2009), however over last two years it has

been called the Beinn na’ Leac Fault (Morton, 2014). This is a more appropriate name as the

fault defines the Western limit for the Beinn na’ Leac topographic high. Displacement along

the fault is normal in nature which has resulted in the down-throw of the Beinn na’ Leac

Formation adjacent to the older Suisnish Beach Formation. The mean dip of the Beinn na’

Leac fault block is 23°. This is slightly greater than the mean dip of formations on the other

side of the fault (19°). It is therefore apparent that a component of fault block rotation has

occurred which would provide a mechanism for the difference in dips across the fault. If this

is in fact the case, then the fault plane is likely listric at depth. Surrounding formation

thickness is not influenced by the fault, indicating that displacement occurred long after the

deposition of the sediments. Furthermore, the fault has offset many dykes that make up the

Raasay Dyke Complex. As discussed previously, these igneous features were likely emplaced

during the Palaeogene. Therefore, due to cross cutting relationships it can be inferred that the

Beinn na’ Leac Fault is a relatively recent feature that occurred within the last 66Ma. Smith et

30

Figure 29: Schematic map showing the

structure present at Beinn na’ Leac. Strain

partitioning between dip-slip and strike-slip

displacement is evident. Note the distribution and consistent strikes of the

fissures


al. (2009) concluded that the displacement occurred at a similar age, and further interpreted

that the fault was caused by glacial loading of the fault block. It seems unlikely that glacial

loading is the only factor that initiated the faulting. Another factor, for example, could have

been a pre-existing plane of weakness within the Suisnish Beach Formation.

5.2.2 Osgaig Fault and Suisnish Fault

The Osgaig Fault is a curvilinear accessory fault which branches off from the Eyre Fault at

GR: 5514 3737 and curves to NEE-SWW before

intersecting the coastline (figure 30). It has a

normal slip sense, dipping towards the south,

though the displacement is hard to quantify due to

a lack of marker units. The fault’s footwall is

composed of the Osgaig Point Formation, while

the hanging wall is composed of the Beinn na’

Leac Formation.

The Suisnish Fault is similar in many respects to

the Osgaig Fault. It is an accessory fault to the

Eyre Fault showing a normal slip sense and dipping towards the south. It also has a similar

trend and curvilinear form, branching off the Eyre Fault at GR 1568 8349: and intersecting

the coast at GR: 1558 8339. These two faults are particularly interesting due to their

relationship with the Eyre Fault (see 5.2.3).

5.2.3 Eyre Fault

The Eyre Fault is the most laterally extensive fault within the mapped area. It extends from

Oskaig Point to Eyre Point, trending NNW-SSE. Displacement along the fault is oblique,

strike-slip in nature. This is identified from striations observed at locality 144 (GR: 5592

3629). The magnitude of this displacement is estimated at ~640m. What is peculiar about the

31

Figure 30: Block diagram showing relative motions of the Càrn nan Eun, Osgaig Point, and Inverarish Forest

Fault Blocks.


Eyre Fault is that it broadly trends perpendicular to the other major faults in the region, and

has a different slip sense. These characteristics can be explained by a strain partitioning

hypothesis. This hypothesis is applicable as the nearby normal fault, the Osgaig Fault could

be applying stress to the surrounding rocks allowing the strain to partition from a normal slip

sense, to a strike-slip slip sense (figure 30).

5.2.3 Other Faults

Two other dominant faults are present in the mapped area. These are situated around

Inverarish Burn (GR: 5664, 3667 and GR: 5724, 3727) and are largely inferred due to a mask

of superficial deposits. Their inferred trend is broadly N-S, which is roughly in-between the

trend of the prominent dip-slip (e.g. Osgaig Fault) and strike-slip (Eyre Fault) faults in the

area. This likely means that these faults show both dip-slip and strike-slip components if they

formed during the same strain episode.

5.3 Dyke Emplacement

Fine grained, basaltic dykes have intruded into the sedimentary succession across the mapping

area (chapter 4.3). These dykes range in thickness from a few centimetres to several metres.

These features tend to be orientated NE-SW, and therefore likely accommodated some of the

extension that resulted in the development of the faults mentioned above. Indeed, along an

NE-SW transect of Suisnish Beach (GR: 5549, 3420), it was calculated that the crust was

extended by ~4%.

32


Chapter 6: Geological History and Economic Potential

6.1 Geological History

The geological history of South Raasay is summarised in the table below.

AGE EVENT

PRECAMBRIAN1) Torridonian: Deposition of Torridonian sandstones (Eyre Point Formation) within a braided river system on the margin of an alluvial fan.

2) Torridonian: Short time gap followed by the progradation of the fan system resulting in the deposition of coarser fanglomerates (Boulder Valley Formation) on top of a paraconformity.

PALAEOZOIC3) Significant time gap, during which the upper surface of the Boulder Valley Formation is weathered leaving an uneven surface.

JURASSIC

4) Sinemurian: Base level transgression allowing for the deposition of marine shelf mudstones over a HST (Suisnish Mine Formation and Suisnish Beach Formation) on top of a paraconformity.

5) Pleinsbachian: Base level regression (RST) resulting in a coarsening of the sediments to medium-grade sandstones (Na Fèarns, River Footpath, Borodale Forest Formations.)

6) Toarcian: Short-lived transgression (TST) allowing the deposition of a thin band of soft mudstones (Ribbon Formation)

7) Bajorician: Regression resulting in the deposition of the Beinn na’ Leac Formation over a Lowstand Systems Tract.

PALAEOGENE

8) Igneous activity associated with the development of the NAIP, resulting in the emplacement of the Raasay Dyke Complex, Osgaig Point Sill and Càrn nan Eun granophyre sill. Iron vein enrichment in the Beinn na’ Leac formation also takes place.

9) East-West extension brought about by the opening of the North Atlantic resulting in the development of prominent normal dip-slip and strike-slip faults.

10) Regional tilting towards the West.

QUATERNARY

11) Glaciation exploits weaknesses in the bedrock, such as faults, forming valleys and depositing glacial till and erratics.

12) Development of peat bogs.

33


6.2 Economic Potential

6.2.1 Iron

The island of Raasay has a history of exporting iron ore from mid-1916 to the end of the

Second World War (e.g. Draper and Draper, 1990). Signs indicating iron deposits were found

across the mapped area such as rust coloured rivers to iron rich concretions found in the

Suisnish Beach formation. Most of the iron was mined from the ironstone in the ‘river

footpath formation’, and transported to the south coast by a railway. Evidence for the

ironworks and railway are still prominent on the island such as a large hopper at Suisnish

Point (GR: 5549, 3420). The additional cost of transporting the ore off the island by ferry

likely made it uneconomical to mine after the war. In addition, its low iron content (Anderson

and Dunham, 1966) makes it unlikely to be economical in modern markets.

6.2.2 Peat

Raasay has an abundance of peat bogs across the south of the island. Most are fairly small,

spanning only a few tens of metres in diameter. Surrounding the lochs on Càrn nan Eun,

however, the peat deposits are much more expansive and difficult to traverse. Much like with

iron, the peat it is unlikely to be economical in UK markets due to the additional ferry

transportation costs and sporadic nature of the deposits. However, for island residents, the

peat could provide a useful fuel source to generate heat and electricity.

6.2.3 Water

The freshwater lochs of Càrn nan Eun are excellent resources of fresh water. Lochs such as

Loch Storab (GR: 5650, 3863) and Loch na Mna (GR: 5797, 3853) provide fresh water to the

residents of Inverarish.

6.2.4 Granite

34


The expansive granophyre intrusions are a suitable road aggregate resource due to its

resistance to erosion and impermeability. The use of these rocks may be more economical in

road construction on the island, as road stone from elsewhere would not have to be

transported onto the island. However, the demand for more roads of the island is currently

low, and lack of mining infrastructure would act against this resource being extracted.

35


Chapter 7: Conclusions

The most significant conclusions presented in this work are listed below:

1) Two paraconformities are present in the stratigraphic column, between the Eyre Point

Formation and Boulder Valley Formation (time gap of a few million years), and

between the Boulder Valley Formation and Suisnish Mine Formation (time gap of

approximately 700Ma).

2) The Eyre Point and Boulder Valley Formations are concluded to have been derived

from a similar sediment source and deposited under similar conditions to the

Torridonian Sandstones observed in Assynt.

3) The Boulder Valley Formation is interpreted to be of Torridonian age, rather than

Sinemurian age due to its resemblance to the Eyre Point Formation in terms of

composition and interpreted depositional environment.

4) The Beinn na’ Leac Formation has been enriched in hematite in the form of veins.

This hematite was likely sourced from hydrothermal fluids that exolved from the Càrn

nan Eun granophyre.

5) Marine transgression and regression occurred twice during the deposition of the

Jurassic age sediments, which is correlated with regional events such as the early

Sinemurian transgression that flooded the Durness basement high.

6) Two cyclic frequencies have been identified to be forcing the sedimentation on the

Suisnish Mine Formation. These are likely influenced by orbital mechanisms, though

the succession must be dated by absolute means to test this hypothesis.

7) Igneous activity is concluded to have taken place during the Tertiary as part of the

North Atlantic Igneous Province. This explains the similarities between the Càrn nan

Eun Granophyre and other felsic intrusions within the Inner Hebrides such as on

Arran, as well as similarities between the Osgaig Point Formation and Giant’s

Causeway in Northern Ireland.36


8) The Osgaig and Suisnish Faults show normal dip-slip displacements because of strain

partitioning from the neighbouring strike-slip Eyre Fault. This mechanism is also

evident in the Beinn na’ Leac Fault.

9) The fissures present on Beinn na’ Leac are likely minor strike-slip faults that have

been exploited by weathering.

10) The region underwent NW-SE extension during the Tertiary that was accommodated

by the NE-SW trending basaltic dykes and normal faults, and NW-SE trending strike-

slip faults. This strain was likely a result of the opening of the North Atlantic.

11) The area has been subjected to glaciation in recent geological history resulting in the

deposition of erratics and glacial till in topographic lows.

37


References

Anderson, F. and Dunham, K. (1966). The Geology of Northern Skye: FW Anderson and KC Dunham. HM Stationary Office.

Bradshaw, M. and Fenton, J. (1982). The Bajocian 'Cornbrash' of Raasay, Inner Hebrides: palynology, facies analysis and a revised geological map. Scottish Journal of Geology, 18(2-3), pp.131-145.

Brittain, J., Higgs, K. and Riding, J. (2010). The palynology of the Pabay Shale Formation (Lower Jurassic) of SW Raasay, northern Scotland. Scottish Journal of Geology, 46(1), pp.67-75.

Draper, L. and Draper, P. (1990). The Raasay Iron Mine: Where Enemies Became Friends. Heritage Publications.

Farris, M., Oates, M. and Torrens, H. (1999). New evidence on the origin and Jurassic age of palaeokarst and limestone breccias, Loch Slapin, Isle of Skye. Scottish Journal of Geology, 35(1), pp.25-29.

Jolley, D. and Bell, B. (2002). The evolution of the North Atlantic Igneous Province and the opening of the NE Atlantic rift. Geological Society, London, Special Publications, 197(1), pp.1-13.

Kearsley, A. (1989). Iron-rich ooids, their mineralogy and microfabric: clues to their origin and evolution. Geological Society, London, Special Publications, 46(1), pp.141-164.

Lee, G. and Buckman, S. (1920). The mesozoic rocks of Applecross, Raasay, and northeast Skye. Edinburgh: H. M. Stationery off. [printed by Morrisson and Gibb, limited, Tanfield].

Morrison, W. (1887). Precambrian conglomerate of Lewis. Transactions of the Edinburgh Geological Society, 5(2), pp.235-242.

Morton, N. (1989). Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland. Marine and Petroleum Geology, 6(3), pp.243-260.

Morton, N. (2014). Large-scale Quaternary movements of the Beinn na Leac Fault Block, SE Raasay, Inner Hebrides. Scottish Journal of Geology, 50(1), pp.71-78.

Oates, M. (1978). A Revised Stratigraphy for the Western Scottish Lower Lias.Proceedings of the Yorkshire Geological Society, 42(1), pp.143-156.

Smith, D., Stewart, I., Harrison, S. and Firth, C. (2009). Late Quaternary neotectonics and mass movement in South East Raasay, Inner Hebrides, Scotland. Proceedings of the Geologists' Association, 120(2-3), pp.145-154.

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Storetvedt, K. and Steel, R. (1977). Palaeomagnetic evidence for the age of the Stornoway Formation. Scottish Journal of Geology, 13(3), pp.263-268.

Storetvedt, K. (1978). Structure of remanent magnetization in some skye lavas, NW Scotland. Physics of the Earth and Planetary Interiors, 16(1), pp.45-58.

39


Appendix I: Arran Fieldwork

Dyke Extension Exercise

40


Dyke Extension Exercise

41


Logging Exercise

42


Logging Exercise

43


Logging Exercise

44


Logging Exercise

45


Appendix II: Raasay Clean Copy Map

46