Geological-Structural Setting of Massif and the Levels of Quartz - Sulphide M...
Dissertation
1. 1
University of Aberdeen
Geological Map Project 2015-2016.
GL4023
Geological Report and History of Northen
Straithaird with Reference to Paleostress
Benjamin Robert Duncan
Masters of Geology (fourth year)
2. 2
Abstract
The findings of this report are from seven weeks of field work mapping the northern part of
the Straithaird peninsula on minimal prior knowledge. The report correlates field work
geological formation boundaries and descriptions to those of the British Geological Survey
(BGS) and finds some of the geological boundaries to be slightly different. It also suggests
more formations and members in some formations and excludes some formations and
members in other locations. Generally the findings of the report are largely conformable with
the established understanding of the geology within the area. The report also focuses on being
able to link the joint orientation data of both igneous and sedimentary units to known events
of extension and compression, seen from the presence of the igneous bodies and folds
identified from bedding dip changes in the sedimentary rocks. These findings aide to back up
the geological history of the area presented within this report.
Introduction
The area focused on in this report and outlined by the accompanying A1 map is of a 22km2
section of the northern part of the Straithaird peninsular in Skye (Fig. 1). The fieldwork for this
report was carried out over a period of seven weeks, with minimal prier research. Post field
work research has been done to tie in this study to previous works. The area includes large
igneous intrusions of both basic and acid nature, basalt lava flows forming hills with thicknesses
up to 1000ft (Almond, 1960) and sedimentary successions of Jurassic age (Morton and Hudson,
1995). This report will give a description and interpretation of each rock unit encountered (and
correlated to existing units), give a description of the geological structure and history of the
area and also look at fracture data collected from a variety of lithologies and locations to see
what conclusions can be gathered on the stresses that different rocks have undergone.
Figure. 1. Location map. Highlighted red square defines area mapped. Inset: Location of
Figure 1.
3. 3
Description of rock units
The young igneous and older sedimentary rocks are outlined in the tables that follow (Table. 1-
5). These correlate field descriptions with the British Geological Surveys’ (BGS) descriptions
and suggest a name accordingly. All the formations and members described in these tables can
be found on the accompanying map.
To correlate more accurately, some of the field work formations have been merged within the
sedimentary succession. Some BGS formations were either merged during fieldwork or were
not seen. Within projected boundary areas a few sections have been marginally redefined due
to updated interpretation of how bedding dip relates to topography. In some cases boundaries
have been redefined from grouping beds previously thought to be in one formation into the
adjacent.
From these changes the BGS sometimes has more formations present. In other cases this study
believes that more formations can be identified or formations can be broken down into more
specific members than the BGS has currently defined.
The Great Estuarine Group in table 3 sometimes merges all its formations. This is for areas
where there is very sparse outcrop. Also in Table 3 the Lealt formation only defines the different
members found from this report within the two northern streams (Allt na Dunaiche and Allt
Aigeinn). Elsewhere outcrop is too sparse to identify and the members have therefore been
merged a few hundred meters to the South of Allt na Dunaiche.
BGS name Field
work
name
Reports
name
Description
Dolerite
and Basalt
Dykes
Alkali
Olivine
Basalt and
Dolerite
Dykes
Basalt
and
Dolerite
Dykes
Small
Mafic
Intrusions
Fine to medium-grained dark volcanic dykes cut all
formations present and themselves (Fig. 2.) to reveal
several later stages of mafic intrusions.
The dykes are made of a dark dull groundmass
containing black shiny minerals, identified as biotite, and
various dull dark green minerals, possibly olivine,
hornblende or pyroxene. Some dykes contain milky
white rectangular phenocrysts identified as plagioclase
feldspar. Most have chilled margins present, rimed
sections of finer, orange grains and perpendicular
fractures. These margins are normally a few cm wide.
There is also direct fault evidence from linear boundaries
revealing displacement.
Granite Granite Granite Coarse to very coarse light igneous rock in the north east.
It is a white crystalline rock composed dominantly of
feldspars and quartz with some small brown minerals,
probably biotite. Within the rock there is veining of the
same felsic composition.
From the compositional make up and crystal size this
rock has been classified as granite. The veining of similar
composition suggests multiple intrusion events in
4. 4
forming the body. The whole body is highly fractured
and a direct study of fracture strikes and dips was taken
to see if any conclusions could be made about varying
paleostress from the coarse edge of a large intrusion,
compared to the very course section a few hundred
metres within it.
There is also a body of coarse granite, with very similar
looking composition to the west of this granite that is
likely to be directly related.
Granite,
Grano-
phyric
Grano-
diorite
Porphyry
Coire
Uaigneich
Grano-
phyre
Hybabyssal light intrusive igneous rock that fills the base
of the Coire Uaigneich valley. Composed of plagioclase
feldspar, quartz, micas and a brown/green dull mineral
likely to be hornblende. Name widely accepted (Almond,
1960. Brown, 1963. Dickin et al, 1981. Wagner et al,
1953).
The rock has ridges running through it with some
sections showing very high density fracturing and others
low. Within some of the less fractured sections clasts are
visible within the intrusion (Fig. 3.).
The clasts have a similar texture to the granophyre but
range in colour from brown to light and dark grey. The
brown clasts appear to be med-grained sandstone with
sub-rounded grains. The grey clasts appear to be siltstone
and the dark grey appears to be basalt. There are also
small pebbles of quartz present. The clasts differ in relief,
some being completely weathered out, up to 3 cm indents
and range in size from a 1cm up to 50cm long. All the
clasts have rounded boundaries with the granophyre,
some are highly spherical. Also present in the rock are
dark bands 15cm long of sub-planar shape, defined
boundary on top and unclear bellow, possibly composed
of biotite (Fig. 4.). Smaller banding of lighter grains also
visible.
The felsic rock to the north west of the map has a very
similar texture to this rock and has been classified as the
same rock due to likely parent source.
Gabbro Gabbro Gabbro This plutonic, dark, igneous rock dominates the north
west of the area. Composed of dull green minerals,
possibly olivine, pyroxene and epidote; white minerals,
feldspars, likely to be plagioclase. Also present are dark
red minerals, likely to be pyroxene. Sections of this
mineral are all joined up in a subhedral crystals with
60/120 cleavage exist and are likely to be a type of
amphibole. Fracture strike orientations were impossible
to take due to very coarse nature of rock.
Volcanic
Breccia
(not
identified
Angular
Conglom
-erate
Volcanic
Breccia
(NG5383
2115)
Matrix supported, conglomerate with angular clasts.
Only in contact with basalt dyke. The single outcrop is a
few metres long and a few cm thick, located within one
of the lower flows of An Stac.
5. 5
in map
area)
The yellow matrix is made of silt and clay grains, is very
well sorted and does not contain carbonates. The loosely
reverse graded clasts are dominantly basalt but there are
sandstone clasts with medium, rounded, spherical grains
and also small black brittle flint-like clasts, possibly
obsidian, along with holes from weathered out clasts
(Fig. 5.).
Alkali
Olivine
Basalt
(lava
flows)
Chilled
Margin
Basalt
and
Dolerite
Chilled
Margin
Dolerite
and
Basalt
Along the edge of the main gabbroic intrusion is a
transition into a dark igneous rock with an average
crystal size below 3mm. Islands of gabbro are present
within this area and islands of this rock are present within
the main gabbro body.
Alkali
Olivine
Basalt
Mugearite
Picro-
basalt
Basalt and
Hawiite
Basalt
lava flow
Basalt
Lava
Flows
The hills Slat Bhienn and An Carnach are made of dark
dull volcanic bodies of rock structured in layers, several
metres thick. Plagioclase phenocrysts are present in some
places. Some sections have dark green bands in a zebra-
like fabric and look like they have undergone light
metamorphism. In other places dark or light green
minerals are visible: olivine, pyroxene or epidote and in
other places light grey and white minerals are visible,
feldspars, all without signs of imbrication. Some sections
have ridges running across the surface both curved and
straight and fractures are present throughout with fairly
consistent density. Some sections have irregular white
blobs that are harder than the basalt.
Table. 1. Igneous lithologies; Small Mafic Intrusions, Granite, Coire Uaigneich Granophyre,
Gabbro, Volcanic Breccias, Chilled Margin Dolerite and Basalt and Basalt Lava Flows.
Figure. 2. Multiple dyke events, Notebook 2 page 39
6. 6
Figure. 3. Clasts in Coire Uaigneich Granophyre, Notebook 2 page 121
Figure. 4. Dark bands in Coire Uaigneich Granophyre, Notebook 2 page 126
7. 7
Figure. 5. Volcanic Breccia, Notebook 2 page 120 (Location redefined to NG5383 2121)
Interpretation of igneous rocks in Table. 1.
Small Mafic Intrusions
The small mafic intrusions reveal several stages of extension within the area. Due to the mafic
dykes cutting the other igneous bodies in different concentrations it reveals ongoing mafic
dyking events throughout all the periods of igneous activity. From carrying out a study of the
coastline dykes over a distance of 2.5km it revealed 1.4% extension in the area revealed in these
small mafic intrusions (data and calculations in appendix).
8. 8
Granite
The granite was formed from felsic magma cooling slowly in plutons while subsurface. The
granite contains the lowest number of mafic intrusions revealing that it is the youngest major
rock present.
Coire Uaigneich Granophyre
The Coire Uaigneich Granophyre is a felsic rock that cooled faster than the large granite plutons
to the north. The clasts found within it are probably xenoliths from the underlying country rock
carried up in the intrusion. This could explain the apparent variation of clasts present and the
smooth yet sharp boundaries seen with the porphyry. This means that the composition of the
intrusion has been effected from the layers it has partially dissolved, similar to the formation of
I or S type granite. The dark bands could be due to alignment of mica while flow movement
was still happening or it could be due to a smearing effect as a basalt clast was partially melted.
The small white banding could be alignment of muscovite or sericite or a section of quartz
being smeared. There are basalt dykes present cutting the whole section but not in as high a
quantity as in the sedimentary sections elsewhere.
Gabbro
The gabbro formed as a mafic pluton-like body and is from the same source as the basalt lava
flows. Some red minerals suggests re-melting from there non-crystalline appearance. As one of
the common minerals is likely to be epidote this also suggests re-melting. This could have been
due to fresh hot intrusions passing close by already formed gabbro creating slight re-melting
and crystallisation. A convecting intrusion could explain this as suggested by Weedon from
Carr (1961) in relation to the Southern Cullin.
Volcanic Breccia
This rock was formed from ash and rock partials falling down a crack in a lava flow. This
reveals an explosive eruption creating fragments of basalt, at a time after, or during, the basalt
flow events.
Chilled Margin Dolerite and Basalt
The gabbro described above transitions into dolerite and basalt. The BGS have identified this
section to be a highly metamorphosed part of the lava flows. However, due to the finding of
gabbroic islands within this area this report suggests that some of the previously interpreted
lava flows are in-fact better defined as chilled margin dolerite and basalt that have been
metamorphosed due to inner convection within the gabbro allowing thermal heating after initial
crystallisation. Islands of sedimentary rock were also seen within this section.
Basalt Lava Flows
From the distinct beds, dipping with the topography, it seems evident that these basalts are from
lava flow events. The different types of basalt can be explained by different flow events having
slightly different compositions. The ridges observed could be due to flow banding from cooling
during movement of the magma. The white blobs could be related to the granophyre as some
9. 9
type of felsic veining. Due to the proximity of the large gabbroic intrusion it can be assumed
that all the lavas have undergone light thermal metamorphism, more obvious in some places
than others (Almond, 1960).
A section to the northwest of Choire a caise was originally interpreted as chilled margin basalt
however to better reflect the presence of some of this basalt being from lava flows an inferred
boundary has been added to better reflect the true interpretation.
BGS name Field work
name
Report name
(Thickness)
Description
Staffin Shale
Formation;
Camasunary
Siltstone
member
Camasunary
Sandstone
member
Scaladal
Sandstone
member
Tobar Ceann
Siltstone
member
Calcareous-
sand
dominant
section
Upper
member
Lower
member
Staffin Shale
Formation
(70m)
Upper
member
Lower
member
This is the first sedimentary stratigraphic section
below the lava flows located around Abhainn
Cille Mhaire. The formation contains calcareous
and non-calcareous sand beds, mud beds and silt
beds.
The upper member is dominantly non-
calcareous white sands with some calcareous
beds. These sands are massive, fine grained and
well sorted. Beds are large, over three metres,
with few mud and silt interbeds. Some fossils
found in non-calcareous white sand beds, ribbed
ammonites (Fig. 6.) and bi-valves (Fig. 7.). All
beds are planar without evidence of any folding.
Wavy lamination is present in the mud inter-
beds. Upper member lower limit defined by
dominance of calcareous beds.
The lower section is dominantly the same white
calcareous or non-calcareous sands with a
higher proportion of calcareous beds. The grain
size on average is smaller (mud-vfs) and there
are more mud and silt beds. The fossils in this
section are bi-valves, gryphea (Fig. 8.) and
belomnites (normally indents). Possible burrows
present along with dark nodules. (Fig. 9). The
lower limit of this formation is the first
appearance of coarse sand beds.
Staffin Bay
Formation
Sandstone 1 Staffin Bay
Formation
(13m)
Sparse outcrop showing med-coarse grains, well
sorted, finely bedded, non-calcareous sandstone.
Correlated to BGS formation to give name.
Bottom boundary of formation defined by very
dark mud beds.
Table. 2. Youngest sedimentary rocks in succession; Staffin shale formation and Staffin bay
formation.
12. 12
Interpretation of formations in Table. 2.
Staffin Shale Formation
The upper member has been formed in a shallow marine environment of medium energy
revealed by presence of ribbed ammonites. Occurrences of small belomnites and due to beds
having smaller grain sizes, the lower member seems to have been formed in a lower energy
environment. This reveals a deeper open marine environment and shows a period of relative
elevated sea levels.
Staffin Bay formation
Medium energy environment below the tidal zone but above the fair weather wave base of a
beach front.
BGS name Field work
name
Report
name
(thickness)
Description
Great
Estuarine
Group;
Skudburgh
Formation
Kilmaluag
Formation
Duntulm
Formation
Valtos
Sandstone
Formation
Lealt
Shale
Formation
Elgol
Sandstone
Formation
Cullaidh
Shale
Formation
Muddy
Limestone
dominant,
broken into
formations
elsewhere;
Very Dark
Mudstone
Sandstone 2
Mudstone
Sandstone 3
Mudstone 2
Sandstone 4
Limestone
Sandstone 5
Siltstone
Mudstone 3
Limestone
2
Dolostone
Mudstone4
Sandstone 5
Mudstone 5
Sandstone 6
Mudstone 6
Great
Estuarine
Group
(Gest)
(122m)
This group is normally broken into formations
but within the central north section of
Straithaird too few outcrops and complex
folding make it impossible to do so. This
report sees no reason to disagree with the BGS
interpretation, as first termed by Judd (1878).
However it does not see fault evidence and
attempts to correlate more formations through
the area of no outcrop that the BGS simply
leaves as being within the general group.
It contains sandstones and mudstones along
with limestones, some of which are
dolomitised. Chert present in some of the
limestones picks out the bedding, to reveal
complex folding.
Area between Camasunary Fault and Slat
Bheinn lava flows contains beds of this
formation, possibly Kilmaluag, Duntulm and
Valtos Sandstone Formation as found from
following average bedding data either side of
Slat Bheinn and assuming the presence of a
syncline (boundaries not defined on map).
Skudburgh
Formation
Very Dark
Mudstone
Skudburgh
Formation
(11m)
Very dark mudstone with planar beds containing
internal wavy lamination. The lower limit of this
formation is the limit of the very dark mud bed.
13. 13
Kilmaluag
Formation
Sandstone 2
Mudstone
Sandstone 3
Kilmaluag
Formation
(12m)
This formation consists of multiple beds with
med-very fine sand with sub-rounded, sub-
spherical grains and some thin mud interbeds.
Lower down the formation mud beds become
dominant with silt beds present and rare thin
sand beds. The sand layers show evidence of
having eroded into their underlying mud layers
and some cracks in the mud have been infilled
with the same sand.
The silt beds appear in two metre- thick sections
and are massive with 1mm mud layers picking
out planar bedding. The mud has planar
lamination and some wavy laminated red
sections (iron oxide?) that seem bleached in
places. 2cm thick very dark mud beds are present
near the base. The lower limit of this formation
is the appearance of calcareous beds.
Duntulm
Formation
Mudstone 2 Duntulm
Formation
(8m)
Calcareous mudstone dominant with some sand
beds present which have erosional contacts with
the mud beds. There are also small felsic
intrusions present alongside dykes.
The mudstone is dark red with lighter bands,
lamination is planar in places but often wavy.
The smaller sand layers present are made of very
fine grains and have thin mud sections (1mm)
picking out planar beds. Some of the beds are
calcareous and often of a silty composition.
From the bedding data observed it suggests that
this formation is part of the sediments exposed
to the west of Slat Bheinn.
Lower limit is dominant lithology change to
sandstone.
Valtos
Sandstone
Formation
Sandstone 4 Valtos
Sandstone
Formation
(9m)
Composed of sandstone with thin mud interbeds,
large portions are calcareous.
Sands have very fine, well sorted, well rounded
grains with thin mud interbeds. Planar bedding
looks disturbed in some places, mud beds are
wavy and discontinuous with wavy lamination.
Some disturbed bedding, normally planar.
Lower limit of formation is the coral limestone
bed below.
- Limestone Coral
Limestone
Formation
(2m)
Pure limestone with texture that looks like coral.
Only two outcrops were seen.
High calcareous component with large calcite
crystals present (8x5mm), crystals are more
common in upper part of beds. Beds are wavy at
top with dark shell fragments present. Thin beds
at base getting thicker at top, sub-planar. Lower
limit at base of limestone beds.
14. 14
Lealt
Shale
Formation
Sandstone 5
Siltstone
Mudstone 3
Limestone
2
Lealt Shale
Formation
(59m)
Very Fine
Sand
member
(14m)
Siltstone
member
(6m)
Mud and
Limestone
member
(8m)
Dominantly made of siltstone and sandstone
beds with mud interbeds and transitioning to
being finer- grained towards the base.
Harris and Hudson (1980) breaks the formation
into two members. However, due to a section of
low topography change and favourable bedding
dip along a stream, a very large quantity of good
outcrop is exposed. It has dominant sections of
very fine sand, siltstone, mudstones/limestone,
dolostone, mudstone and very fine sandstone.
Therefore instead of just two members this
report suggests a possible six members can be
defined.
White sandstone with very fine to medium, sub-
rounded, spherical grains with mud interbeds. In
places very thinly bedded, beds picked out by
yellow impurities. Also contains 20cm
structureless beds made of very well- sorted fine
grains. The mud beds are wavy and
discontinuous with some disturbed bedding
towards base. Member lower limit defined by
silt beds taking the place of the sandstone.
Silt beds with mud beds also present.
Dominantly non-calcareous silt with some
calcareous sections. The mud beds present are
all calcareous and are highly weathered. The silt
beds have both planar and wavy lamination
while the mud beds have only sub-planar. The
base of this member is defined by appearance of
beds including bivalve imprints.
Mudstones, which in places grade to muddy
sandstone, with yellow limestone inter-beds
progressively becoming more dominant down
strata.
The rock is a grey, poorly-sorted, silty mudstone
with fissile planar sections and wavy laminated
sections. It is made of a dark matrix of clay
partials (60%), light quartz grains (35%), small
shiny mineral (2%), muscovite and other light
grains (3%). It is non-calcareous with some non-
calcareous concretions in distinct beds. Some
beds are massive beds and 2mm white veins of
quartz are present. There are very high density
8mm bivalve imprints in some mud beds,
possibly Necmiodon, as also seen by Harris
(1980). The imprints show minimal variation in
size (Fig. 9.).
15. 15
Dolostone
Mudstone 4
Sandstone 5
Mudstone 5
Dolostone
member
(4m)
Mudstone
member
(14m)
Very Fine
Sandstone
member
(13m)
The limestone is yellow, calcareous and soft
with mud concretions and thin mud interbeds
with varying thickness present. The bottom of
this member is defined by absence of yellow
limestone and bi-valve rich beds.
This mottled grey and white textured rock looks
like it may be formed from small skeletons,
however it does not react with acid. It is a
crystalline limestone, probably dolomite. It
contains dark internal layers composed of mud
in 1mm bands. This member ends when the
dolostone is no longer present.
Non-calcareous, clay partials 60%, with grains
up to fine sand, quartz 35% and muscovite is
also present 2%. It has a silky appearance and is
very dark with finely laminated and dominantly
planar bedded with some non-calcareous
nodules.
There is a small tight fold observed but it is very
local without bedding being influenced. The
bottom of this member is defined by the
appearance of large very fine sand beds.
Very fine sandstone with thin mud beds,
luminescent colours visible in dark mud bands
and beds appear structureless. Lamination is
dominantly planar and the dark bands that are
present are probably made of organic matter.
Luminescent colours could indicate cracking of
bitumen having taken place to release light
hydrocarbons that stain with these colours. The
base of this formation is defined by the
appearance of large medium sand beds.
Elgol
Sandstone
Formation
Sandstone 6 Elgol
Sandstone
Formation
(16m)
White and yellow sandstone containing thin
beds of mud.
The sand is made of medium, poorly sorted, sub-
rounded, quartz dominant grains. Bedding is
dominantly planar containing darker and lighter
sections with some of the dark sections
containing pebble lag, particularly towards the
top of the beds. The mud layers contain planar
lamination and look like thin roots are present in
places. The lower limit of the formation is with
the appearance of thick beds of mudstone.
Cullaidh
Shale
Formation
Mudstone 6 Cullaidh
Shale
Formation
(5m)
Very dark fissious mudstone.
The rock is a black and red mudstone with wavy
lamination and it is darker towards its base. The
black layers leave a brown streak and seem
16. 16
likely to be bitumen. The rock is highly fractured
and readily breaks into cubes. The lower limit of
this formation is defined by first appearance of
white sandstone.
Table. 3. Great Estuarine group; Skudburgh Formation, Kilmaluag Formation, Duntulm
Formation, Valtos Sandstone Formation, Coral Limestone Formation, Lealt Shale Formation,
Very fine sand member, Siltstone member, Mud and limestone member, Dolostone member,
Mudstone member, Very fine sandstone member, Elgol Sandstone Formation and the
Cullaidh Shale Formation.
Figure. 9. Bi-valves, Notebook 1 page 114.
17. 17
Interpretation of formations in Table. 3.
Great Estuarine group;
No evidence has been found contrary to the interpretation made by Harris (1989) who explains
the mouth-bar hydrodynamics that deposited this group. There would be changes of water
depth, energy and salinity creating the different formations. From the changing grainsize it
appears that the group represents both transgressive and regressive events.
According to BGS, this group also appears on the west of the map but the formation(s) visible
can’t be distinguished. The beds here have a lithology of 60% very fine sand and 40% mud.
Internal structure varies between fissile lamination being very discontinuous and planar or wavy
bedding. Calcite crystals are present in calcareous and non-calcareous beds next to limestones
(Fig. 10.).
Figure. 10. Sedimentary section of unknown formation within Gest. Notebook 2 page 71.
Skudburgh Formation
This is a low energy environment with a high content of organic matter likely to reveal a coastal
swamp-like area.
18. 18
Kilmaluag Formation
The dominance of mud reveals a low energy environment. The filled-in cracks are likely due
to aerial exposure creating mud cracks that shortly after were covered by sand transported by a
period of higher energy. The darker layers suggest deposition of organic matter in an anoxic
environment. Therefore a normally low energy environment that undergoes areal exposure
followed by higher energy events could be explained by estuarine mud flats.
Duntulm Formation
Low energy environment located close to a low clastic supply to allow clear enough water to
produce carbonates. Likely to be detritus due to mud content making carbonate formation
unlikely in this area.
Valtos Sandstone Formation
The sand and mud repeated pattern suggests prolonged periods of higher energy and shorter
periods of low energy likely to reveal a shoreline or lagoonal environment depending on relative
sea level. The disturbed bedding in sand and discontinuous wavy lamination in muds could be
due to de-watering during compaction.
Coral Limestone Formation
A shallow sea covered the area to allow coral reefs to form and create these sediments.
Lealt Shale Formation
Low energy and very low energy environment with varying proximity to a calcareous source,
possibly revealed by the dolomitised section of what could be a reef subjected to a flow of Mg
rich water. One section containing dominance of one species of bi-valve reveals conditions only
suitable for this species, Necmiodon, which is a fresh water species. This likely reveals the
conditions on the inland side of a reef where a freshwater lagoon could have formed creating a
low energy low salinity environment likely to only be suitable for one species of bi-valve. Due
to the large quantity of small bi-valves it seems that they have an abundant food source and low
risk of predation, so no need to grow bigger. Due to apparent absence of bi-valves in lower
members it suggests low energy environments that have less favourable conditions for life,
possibly they are less well oxygenated, this could be formed by the river source supplying fresh
water changing course or a sea level rise moving the salinity equilibrium further inland. The
fold could simply be due to be a soft sediment deformation process and possibly related to a
movement of the Camasunary Fault.
Elgol Sandstone Formation
This is a medium energy environment with occasional high energy events. Suggesting flood
events as revealed from the pebble lag layers that require raised levels of flow. It is likely that
this environment is close to shore and shallow, allowing plants to be growing rooted into the
ground and still remaining in the photic zone.
19. 19
Cullaidh Shale Formation.
These beds reveal a low energy environment with organic matter deposited in places suggesting
proximity to shore but below the storm weather wave base.
BGS name Field work
name
Report
name
(Thickness)
Description
Bearreraig
Sandstone
Formation
Garantiana
Shale
member
Druim an
Fhuarain
Sandstone
member
Sandstone 7;
Top member
Middle
member
Lower
member
Upper
Bearreraig
Sandstone
Formation;
(37m)
Top member
Middle
member
Lower
member
The BGS have defined this formation as one
whole unit with internal variation. From this
study it appears possible to break the formation
into two formations, each with identifiable
members. The BGS have also interpreted the
formation to thin till it disappears in the north,
however no field evidence for this was seen –
due to the lack of outcrop in the north section
of the map.
The white sand ranges from fine to course
grained and generally is made of rounded,
poorly sorted quartz. There are three possible
members identifiable within this formation.
The top member can be identified by planar
bedding being dominant in the thin beds of fine
to medium sand. It has shell fragments and hard
ridges cutting across bedding and mud nodules
and pebbles can be seen picking out bedding in
places. Burrows present, rounded gently
curving tunnels made of same white sand (Fig.
11.A.). The lower limit of this member is the
dominance of cross bedding.
The middle member can be identified from
trough cross bedding being dominant and some
thin mud inter-beds being present, the mud can
also be seen picking out bed boundaries. This
can be seen as amalgamation within the cross-
beds. The beds are larger than those above and
well-rounded quarts pebbles (up to 1.2cm
diameter) can be seen commonly, no fossils or
burrows were seen (Fig. 11.B.). The lower limit
of this member defined by absence of mud
amalgamation within cross-beds.
The lower member is the plainest, it has larger
beds and less pebbles present made of sorted,
sub-angular, coarse grains. It has some
structureless beds however cross-bedding is
still dominant but with the absence of mud
20. 20
amalgamation. No fossils were seen (Fig.
11.C.). This members lower limit is marked by
the appearance of calcareous sands (Fig. 12.).
Calcareous
Sandstone;
Upper
member
Lower
member
Lower
Bearreraig
Sandstone
Formation;
(28m)
Upper
member
Lower
member
Dominantly white, poorly sorted, very fine to
course, angular, sand with a carbonate cement
and planar bedding. Yellow sections are also
present and these tend to be sorted, well
rounded, med grained massive beds. Pebble
layers are present throughout both sections. It
can be grouped into upper and lower members.
Upper member contains calcareous and non-
calcareous beds that are interbedded, large and
normally structureless. In calcareous sections
shells can be seen, small belemnite guards
(7mm diameter) along with calcite crystals and
quartz pebbles. Beds are often disturbed and
have possible dish structures and convolute
lamination present. Lower limit of member is
with last large structurless beds.
All beds have either planar or cross-bedding.
Virtually all beds are calcareous and belemnite
guards are far more common and far larger, up
to 22mm diameter (Fig 13.). Quartz is still a
key component but less than above member.
Lower limit of member, and formation, defined
by appearance of mud lithology.
Table. 4. Upper Bearreraig Sandstone Formation and Lower Bearreraig Sandstone
Formation.
21. 21
Figure. 11. Upper Bearreraig Sandstone formation. A, left, Top member, Notebook 1 page 116, B, centre, Middle member, Notebook 2 page
4, C, right, Lower member, Notebook 2 page 12 (Note different scales)
22. 22
Figure. 12. Transition zone between upper
and lower Bearreraig Sandstone. Notebook 2
page 140.
Figure. 13. Lower Beareraig Sandstone,
lower member. Notebook 1 page 123.
23. 23
Interpretation of formations in Table. 4.
Upper Bearreraig Sandstone Formation
Medium energy environment formed from migrating near-shore/river bars allowing storm
events to leave pebble lag. In the middle member amalgamation within cross-beds reveals a
high and low energy environment with different unidirectional flows (Figure. 14.). This reveals
a tidal environment. The lowest member is a high energy environment, still near shore but a
less tidally dominant. The Paleoflow variations could have been influenced by the major
Camasunary, Screapadal and Applecross faults in the area as stated previously by Morton
(1983).
Figure. 14. Stereonet showing paleoflow within Bearreraig Sandstone, Upper and Lower.
Lower Bearreraig Sandstone Formation
Similar medium energy shallow sea environment but with more carbonate supply, possibly
located down shore from a reef area. Belemnites reveal a more open marine environment. The
bottom member suggests a higher energy environment shown from belemnite size increasing,
to allow them to cope with harsher conditions. The quartz component also drops revealing less
clastic supply. This could reveal the lack of close proximity to a river supplying clastic sediment
and a transition into a shallower environment.
24. 24
BGS
name
Field work
name
Report name
(Thickness)
Description
Scalpay
Shale
Formation
Mudstone 7
Sandstone 8
Scalpay
Shale
Formation
(19m)
This formation has very few outcrops but
appears to be made of dark grey siltstones and
mudstones.
Beds are massive but one outcrop shows faint
lamination present along with some non-
calcareous, white and black, concretions. In
one location an imprint of a ribbed section of
a bi-valve is exposed. Some small medium
grained sand beds are seen within the mud
sections. Quartz veins 1-3mm across cut the
rocks. This formation ends when the grey silt
and mud beds are no longer present.
Pabbly
Shale
Formation
Mudstone 8 Upper
Pabbly Shale
(18m)
This formation has very few outcrops and is
dominantly fine sandstone.
The rock has fine to coarse, sub rounded, sub
spherical grains with structureless beds.
Some sections are calcareous. Within the
section it looks like a fault has been active
from looking at the map and assuming bed
thickness continuity. The south east section
of these beds are hard, white, don’t react with
acid and contain dark nodules. This
formations lower limit is when the dominant
lithology becomes mudstone.
Siltstone 2 Lower Pabay
Shale
(46m)
This formation also has very few outcrops
and is dominantly mudstone. The rock is
dark, weathered grey, all non-calcareous with
very fine and faint sub-planar fabric-like
lamination with white veins of quartz cutting
irregularly. The lower limit of this formation
is with the appearance of structureless
siltstones.
Broadford
Beds
Formation
Siltstone 3
Siltstone 4
Broadford
Beds
Formation
(138m)
This formation contains mudstones,
sandstones and limestones but is dominantly
composed of siltstones.
These are composed of very well sorted, silt
grains with a large clay component. It is white
and crumbly, non-calcareous, possibly
composed largely of illite. Bedding is hard to
see but picked out by faint light and dark
layering in places. Some mud inter-beds are
present with clear internal sub-planar
lamination. Some calcareous beds, up to 0.4m
thick, are present that tend to be made of
sorted silt grains. Erosional contacts are
present in places normally below sand beds.
25. 25
There is also a single siltstone outcrop in the
north east showing large structureless beds of
grey silt and mud with calcareous nodules.
The BGS accept this area as part of the
Broadford Beds (Searl, 1992) and this report
finds nothing contradictory to that
conclusion.
Table. 5. Scalpay Shale Formation, Upper Pabbly Shale, lower Pabay Shale and the
Broadford Beds Formation.
Interpretation of formations in Table. 5.
Scalpay Shale Formation
The black concretions are likely to be segments of organic matter, as these occur in lumps and
haven’t been broken down into a fine layer it shows rapid deposition and an environment likely
to be fairly close to shore yet below the storm weather wave base and low energy.
Upper Pabbly Shale
Low energy environment, further offshore than the scalpy shale formation as shown from the
lack of organic matter chunks. The south east section contains a baked calcareous section
forming an impure marble, there are old marble quarries within a few km of the location.
Lower Pabay Shale
Grain size increases slightly so likely to be closer to shore but still a low energy environment.
Broadford Beds Formation
A dominantly low energy environment with periods of higher energy allowing erosional
contacts. Likely to reveal a transgressive and regressive sea level with each regression event
recorded by the sand beds with erosional contacts.
26. 26
Geological Structure and History
The area can be broken up into two main sections, old sedimentary rocks and young igneous
rocks. Within the sedimentary rocks there is one succession, discontinuous to the west due to a
large fault. Some folding is present, especially near the igneous bodies. There is also one fault
visible to the far east of the map. The clearest paleoenvironment identified is a tidal delta but
there is a great variety of sedimentary rocks including sands, shales and limestones suggesting
other paleoenvironments. All the sediments have undergone light thermal metamorphism
resulting from the igneous activity which can be broken down into four main events; extrusive
basalt lava flows, large gabbroic intrusion, large granitic intrusions and small dyke swarms that
are dominantly mafic. There is also one small outcrop that looks like a volcanic breccia.
Sedimentary
Looking at the history of the older sedimentary sections providing the base of the area, it can
be seen that they have been laid down in a varying coastal environment of a climate similar to
the Mediterranean today (Holmden et al, 2003). According to Morton and Hudson (1995) these
are Jurassic sediments laid down in a half grabon system with a faulted western margin. The
clearest environment indication is in the Beareraig Sandstone Formation where there is
amalgamation of mud within sand cross-bedding, suggesting an environment of both low and
high energy and therefore likely to be a tidal dominated area. Also the paleocurrent data from
the cross beds shows a highly variable orientation in line with a tidal dominated environment
(Fig. 14.). Other sections suggest lagoonal environments from the limestone and fine sediment
deposition and some areas suggest more open marine and deep water environments from the
appearance of small belemnites and dark shale beds.
All these beds were then buried, lithified, tilted (and faulted), uplifted and eroded as shown in
Fig. 15. steps 1 and 2. There is a major fault present somewhere separating the succession in
the far west of Slat Bheinn from the near west sedimentary succession. During field work this
was interpreted to run under the basalt lava flow (Fig. 16.). This was because the sedimentary
successions in the east didn’t seem correlatable with those to the west of Slat Bheinn. From
research however it appears that the fault forms between the formations west of Stat Bheinn
(Fig. 17.) and is called the Camasunary Fault (Seal from Hudson, 1983). It is a normal fault that
was active during the deposition of the Jurassic sediments but has been inactive throughout the
tertiary period (Butler et al, 1994). The sediments have been generally correlated using field
descriptions of the formations in question along with the BGSs interpretation to create a
comprehensive interpretation of the sedimentary succession in the area as previously covered
in this report.
Due to the positive topography (allowing lava flows south) it can be assumed that the sediments
experienced compressional forces to create an area of high topography rather than a basin.
Therefore the Camasunary fault was originally thought to be a thrust fault associated with
compression. However from research it has become apparent that it is a normal fault related to
the half grabon extensional system prior to compression (Morton and Hudson 1995. Butler et
al, 1994).
27. 27
Figure. 15. Initial interpretation of the Coire Uaignerich History, Notebook 1 page 153.
28. 28
Figure. 16. Original cross-section cutting Slat Bheinn. (Original on back of south west field map)
Figure. 17. Relevant part of updated cross-section cutting Slat Bheinn. Shows new location of fault,
change in angle and presence of syncline. (Full cross-section shown on accompanying map)
29. 29
Igneous
Looking closer at the structure of the igneous section we see the main lava flow is from a high
point around Slat Bheinn. The flows decrease in height and therefore flow south. Due to no
other sediment being visible between the lava flows and the Jurassic sediments and due to the
change in bedding angle, we know that erosion or a long period of non-deposition took place
before the igneous activity began, around 59Ma (Thrasher, 1992).
Initially field work suggested that the lava flows and large gabbro intrusion, both of mafic
composition, were from two similar events within the same period, An Stac gabbro and lavas
and then Blabheinn gabbro and lavas (Fig. 15.). From further research it appears that one main
source is responsible for the flows (Goodenough, 2006). Current understanding is that the lava
flows visible today formed first and were then metamorphosed by the magma chambers that
formed within the older flows and now are seen as gabbro (Fig. 18.). The lava flows only appear
metamorphosed in certain areas, most of them require thin section analysis to detect
metamorphic minerals present and have maintained their original mineralogy and texture
(Almond, 1960.).
It seems that some later eruptions were explosive in order to create a volcanic breccia that has
infilled a crack in the basalt near An Stac. The fact that there is a recorded paleo-crack here
suggests that the rock was weaker in this particular area, possibly also explaining why the valley
formed at this weak point.
31. 31
Fracture Analysis
Fracture data can be very important for industries, e.g. fracture continuity and orientation
between different layer interfaces has a pronounced effect on oil, gas and water migration
(Helgeson et al, 1991). This study intends to develop understanding of fractures within the
northern Strathaird area in order to understand the geological history better and potentially aid
exploration in locations where these formations have been buried to significant depths to allow
production of hydrocarbons.
When a critical level of tensile stresses is applied to brittle rock it fractures and causes jointing.
The frequency and occurrence of joints is dependent upon the physical properties of the rock
bed, the surrounding rock beds, the bed thickness and the degree of tensile stress (Hobbs, 1967).
The cause of these tensile stresses can be due to several changing factors like tectonics,
changing temperatures causing contraction or expansion, or increasing pore fluid pressure. As
a result of these properties and stresses many of the rocks in the area covered by this paper are
heavily fractured by joints. These fractures reveal paleostresses, and can be used along with an
understanding of the relative ages of the rocks, to help unravel the geological history of the area
and form interpretations based on their evidence.
Almost all of the fractures present are joints, defined here as fractures lacking field evidence of
any lateral movement and they tend to appear as interlocking systematic joint systems with
some non-systematic joint sets in certain locations (basalt flow locations). These joints that
exhibit no shear displacement are a type of extension fracture (Bahat et al, 2005).
In this study a broad view has been taken of the fracture orientations in the whole area, with six
data sets from within sedimentary rocks, four from within granitic rocks and three from within
basalts. This was done with the intention to allow comparison of both lithology and age. The
minimum number of fractures counted in a section was sixty, while in some locations around
three hundred strikes were recorded. The data collected has been sorted into 10o
bins and put
into rose diagrams (Tables. 6-7) for analysis as follows, original data can be found in the
appendix. For the interpretation of these joints sigma 1 (most compressive stress) is assumed to
be the vertical stress.
Igneous
In Table 6 (A,B,C) Slat Bheinn (A) and An Stac (C) show pronounced orthogonal jointing with
a dihedral angle of around ninety degrees. They seem to show multiple orthogonal jointing
systems that range from 120 to 160 degrees. An Carnach’ (B) joints are less simply distributed.
Slat Bheinn (A) and An Stac (C) are significantly higher up in the lava flow plato than An
Carnach. This is why they have similar fracture strike orientations. When comparing An Stac
and Stat Bheinn directly we see that although they both have a set of NE-SW joints and a set of
NW-SE joints their primary strike orientation is reversed. This could suggest two separate joint
forming stress events, one where An Stac was more affected and one where Slat Bheinn was
more affected.
32. 32
This could be due to the particular lava flow orientation at each location. An Stac is made of
SW flows and has dominantly NE-SW faults suggesting an extensional force SW. Slat Bheinn
is made of SW flows and has dominantly NW-SE faults suggesting an extensional force SW.
This shows that the direction of flow is the same as the extensional force shown in the fractures.
Bai et al. (2002) reveals that orthogonal cross jointing can be formed by one stress field, without
rotation of 90o
. It seems likely that these lavas have only undergone one principle stress to
create both their joint sets, orientation defined by flow direction which is 90o
different between
the flows.
An Carnach (B) shows deeper lava flows that have a less defined flow direction. The data was
taken in a 600m transect along the edge of the cliffs now called An Carnach. Therefore the
expected dominant flow would have been NE and the subsequent dominant fracture orientation
would be expected to be NW-SE. This is what the data shows. A secondary strike orientation
of NNE-SSW suggests a NEE flow recorded and a tertiary strike orientation NE-SW suggests
a secondary flow direction SE. These paleo-flows are all consistent with outcrop appearance
and general geological understanding. Also as the An Carnach flows are older they are more
likely to have been subjected to more paleostress history, possibly convoluting the fracture
strikes even more than the multiple lava flow orientations.
In Table 6. (D,E,F,G). the granitic rocks appear to show conjugate joint systems with dihedral
angles less than 90 degrees. This general difference in joint system could be due to the
mineralogical and crystal size differences between granite and basalt along with the lack of
obvious flow within the granites.
The Choire a Casise graniphyra (D) shows the most varied joint sets. This is likely to be because
of the irregular form of the intrusion allowing inconsistent cooling. From the line like structure
of the intrusion it would be expected that the dominant stress recorded would be E-W, from 80o
– 100o
, this is the case. The other main set orientations could be from later extension of the
other felsic rocks. Different compositional segments (and therefore strengths) within the rock,
depending on the dissolved xenoliths, could also affect the strike orientations.
As the granites (E,F,G) were the last igneous intrusive event they reveal the last extensional
forces recorded in the area. The two finer granites (E,F) show much cleaner joint sets than the
course grained granite (G). This is because fractures are less hindered from following their
natural orientation in finer grained rocks than in courser grained rocks. Both granites E and F
show extensional forces dominantly away from the centre of their respective intrusive bodies.
This could be because cooling is faster around the edge of the intrusion so that while the edge
cooled and became brittle extension was still occurring and the centre of the intrusion still being
injected with felsic magma.
33. 33
A. Slat Bhinn, NW-SE primary, NE-SW
secondary (n=237)
B. An Carnach, NW-SE primary, NE-SW
secondary, less clear cut (n=348)
C. An Stac, NE-SW primary, NW-SE
secondary (n=60)
D. Choire a Casise Graniphyra,
NNE-SSW and NNW-SSE
primary, E-W secondary (n=90)
E. NW Granite, NNE-SSW
primary, E-W secondary
(n=137)
F. NE Fine Granite, NNW-SSE
primary, NE-SW secondary
(n=138)
G. NE Course Granite, NW-SE
and NNW-SSE primary, NE-SW
and NEE-SSW secondary (n=90)
Table. 6. Fracture strikes in basalt (A,B,C) and felsic (D,E,F,G) rocks.
34. 34
Fig. 19. shows strike and dip data taken from within the NW granite. It shows very spread out
data for both crystal sizes. The two slightly more concentrated areas reveal the NNW-SSE
orientation trend already highlighted by rose diagrams (Table. 6 F,G.). This stereonet also
reveals that the joints have a higher proportion of steep dips in both fine and course granite.
Within the data there are small clusters of more concentrated areas in both the fine and coarser
granites, these are clearer within the finer granites.
The clusters all show steeply dipping plains revealing that the joints forming as very similar
sets with other joints are likely to have steep dipping angles, some of 90o
. This is means that
fluid will be able to migrate vertically through the granite and in locations where this young
tertiary granite is located it seems likely that they will not act as seals.
Figure. 19. This stereonet shows the poles to the fracture joint plains within the NE finer and
coarser granite. Data is very evenly spread with two slightly more concentrated sections
(large circles). The fine grained granite shows three main clusters (circled red). The courser
granite also shows three main clusters (circled blue).
35. 35
Table. 7. Fracture strikes in sedimentary rocks.
A. Limestone of Choire a Caise, N-S primary,
NW-SE secondary (n=85)
B. SE Lower Bearerag Sandstone, NE-SW
primary, NW-SE secondary (n=130)
C. Staffin Shale of Abhamin Cille, NW-SE
primary, NE-SW secondary (n=184)
D. Mudstone of Allt na Dunaiche , NNE-SSW
primary, NE-SW, NW-SE and NEE-SWW
secondary (n=71)
E. Within Lealt, NWW-SEE primary, N-S
secondary (n=101)
F. SW Broadford beds, NE-SW primary,
NW-SE secondary (n=157)
36. 36
Sedimentary
Table. 7. (A-F) shows the fracture strikes from six of the older sedimentary rocks. Out of these,
four of them show a clear primary strike orientation, two of which have an orthogonal system,
one of which suggests a conjugate system, one suggests only one set joints and the final two
reveal multiple sets of joints.
It is already known that the sediments have undergone compaction to form a high topographic
area along with the Camasunary fault. Evidence for the extension that occurred during the
Mesozoic in the area is in the form of the igneous intrusions present, along with local
compaction on the sediments from the intruding magma. The temperature change due to the
igneous activity could also cause stresses in the sediments from thermal expansion or
contraction.
The Gest limestone sediments at Choire a Caise (A) with their single dominant set of joints
reveal the main paleostress’ within this bed to be almost north-south at the time of joint
formation. It is likely that the secondary fracture strikes here are only surface cracks and not
true subsurface cracks because of their small number. There are highly variable bedding
readings over this area and it reveals the dominant stress as being almost perpendicular to
bedding in the Allt na Dunaiche sediments and in line with an extensional force (least
compressive force) being located somewhere in the gabbro body.
In different lithologies faulting can occur at different times as shown by Bahat (2005). This
means faults will form at different stages throughout a tensile stress folding event and means
that strikes will be recorded at different orientations as the beds are moved relative to the
principle stress. Two of the sediments (B + C) show orthogonal jointing systems. The
sediments are located at the bottom of the map, furthest away from the igneous intrusions. There
strikes differ 20o
, this can be explained by jointing events taking place during the folding event
that is revealed by the changing bed dip orientations within the Staffin Shale. Possibly the
compressional forces that created the change of bedding within the Staffin Shale are the same
forces that created one of these joint sets, as folds create areas of extension perpendicular to the
principle stress. While extension was occurring within the Staffin Shale compression was
occurring within the Lower Beareraig Sandstone meaning that joints here would form with a
90o
orientation difference. As folding continued these beds became offset by 20o
.
The mudstone in Allt na Dunaiche (D) has sets of joints in most orientations. The primary joint
set is orientated NNE-SSW which could be from extension revealed by the gabbroic extension
events. Bedding varies to the north and evidence of compressional forces is a likely reason for
the other joint sets orientations.
The northern Lealt fractures (E) show a conjugate system with a clear primary strike orientation
NWW-SEE. This could also be due to the Mesozoic extension as a granite body is to the north.
The bedding orientation here is not the same as in other places suggesting that folding and/or
faulting is present and the forces required for this to take place could also be responsible for the
jointing.
37. 37
The final rose diagram to be analysed is in the oldest sedimentary rocks, the Broadford beds
(F). Several sets of joint orientations are recorded. The primary orientation lines up with the
large gabbroic intrusion to the NW of the beds, it’s likely that these joints formed from the
Mesezoic extension related to the gabbro to form. The secondary set is aligned NW-SE and
could have formed due to earlier compression in this orientation meaning that the least
compressive stress was almost perpendicular and allowed joints to form in this orientation.
Thus the fault orientations examined in this report can be used to add evidence to the
geographical history of the area and the timing of compressional and extensional events
compatible with the fracture strike data. It does however appear that the areas paleostresses
have been dominated by the Mesezoic local extension, so they are not a good analogue for any
locations where these beds have been buried to significant depths for oil exploration.
This report recommends follow up work to be undertaken to investigate the joint strikes of
many more sedimentary beds throughout the area. This would give opportunity to see if
different joint responses to stresses can be identified within sedimentary layers to try to
understand how these rocks might react in a different location with a different stress history.
Fracture extent, dip, spacing and width should also be taken for more reliable and full analysis.
Conclusion
This report finds that it is possible to break up certain formations, as previously defined by the
BGS, into more defined units, specifically within the Lealt and the Berreraig sandstone
formation. The report also finds that joint orientations within the igneous sections correlate with
the individually specific expected extension. The joints within the sedimentary sections can be
somewhat explained by the tertiary extension but some of the sets reveal that they were formed
from older tectonic stresses. Thus this report finds the geological history of the area to be one
of; sedimentary deposition in a dominantly of a near shore environment with varying sea level,
followed by a period of compression creating folds and a high topography area then a period of
non-deposition occurred until extension allowing mafic lava flows and later felsic bodies to
intrude the area.
38. 38
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