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Plymouth University
Drake Circus
PLYMOUTH
PL4 8AA
School of Geography, Earth and Environmental Sciences
Ordovician Volcanism and Palaeoenvironment
Of the Tryfan Area, Snowdonia, Wales
January 2020
Word count (full report): 10,957
Word count (bare text): 9,060
Grade = 62%
ii
1
Abstract, Introduction, Methodology
Abstract
A twenty-four-day mapping project was completed in an area centred around Tryfan in
Snowdonia, Wales. Four Formations represent a period of varying, voluminous, volcanic
activity in the Upper Ordovician in a coastal environment.
The Cwm Tryfan Formation consists of tuff, shale and volcaniclastic members, lain in a
rapidly evolving shallow marine environment with periodic local volcanic activity.
Breccia, rhyolite, shale and tuff compose the Tryfan Formation, representing a period of
extensive volcanic eruptions S.E of the study area around Snowdonia, deposited in a coastal
area.
The Llyn Bochlwyd Formation comprises of ignimbrite, shale, volcaniclastic and tuff which
were formed in a shallow marine setting with nearby explosive volcanic eruptions. Fossils
found within this formation give the age of the geology as 457.5 – 455.8 Ma, in the
Soudleyan stage of the Upper Ordovician.
A basaltic submarine eruption produced a brecciated pillow lava member within the Clogwyn
Formation, representing a change in the chemical composition of the magma source.
The Geology of the area was deformed by the Caledonian orogeny producing the Tryfan
anticline, and cleavage observed predominantly in shale.
Introduction
For this project twenty-four days of mapping was conducted around Tryfan in Snowdonia,
Wales. A 1:10,000 geological map was produced, and notes were taken in the field. Primary
research was used with Secondary sources to produce a report detailing the
palaeoenvironment and geological history of the area.
During the early Ordovician there was two eruptive cycles (Howells et al. 1991) that were
caused by the subduction of the Iapetus Ocean underneath the Avalonian microcontinent
(Howells et al. 1991). Volcanism lasted 2-3 million years between 460-455 Ma (Howells et al.
1991), producing mostly rhyolitic rocks in the Cwm Tryfan, Tryfan and Llyn Bochlwyd
Formations and some basaltic rocks observed in the Clogwyn Formation.
2
Aims of this report is to explain the primary findings from the field study and to describe
formations and members. The report aims to describe the structures within the study area
and to explain the palaeoenvironment in the Upper Ordovician.
Methodology
Twenty-four days of mapping in the summer was completed around the area North if the A5
to the East and West of Tryfan, and to the area around the peak of Glyder Fach in the South.
A combination of surface and outcrop mapping was used. A map was produced to 1:10,000
scale in the field, and data was taken with a Compass Clinometer and recorded in a
notebook. The base map was based on the 1976 Ordnance Survey map. Thin sections were
used to study some of the members, and rock samples were collected from the field.
Secondary research was used with primary data to produce an interpretation of the geology
and geological history. A risk assessment was completed prior to starting the field work. A
lack of thin sections for all of the members is a limitation. Only a small number of dip
measurements was collected on the Eastern side of the Tryfan anticline, this could have
reduced the accuracy of the stereonets, and interpretation made of the fold including the
axial plane. Some areas of the field were inaccessible due to topography.
3
Formation and Member Descriptions
Cwm Tryfan Formation
The Cwm Tryfan Formation is found in many exposures around the valley to the East of
Tryfan consisting of conformable shale volcaniclastic and tuff and is 500m thick. An anticline
is found within the centre of the formation, increasing its apparent thickness. This is the
oldest of the mapped formations and it has a conformable upper boundary with the Tryfan
Formation. In literature this formation is part of the Gwern Gof Tuff Formation (Howells et al.
1991).
Figure 1; A map showing the location of the Cwm Tryfan Formation
N
4
Cwm Tryfan Tuff Member
Overview
The Cwm Tryfan Tuff Member is found from pont Wern-gof on the North side of Cwm Tryfan
GR: 674/605, to the South side GR: 667/506. It is found in two beds with conformable
boundaries with the Cwm Tryfan Volcaniclastic, and Cwm Tryfan Shale. It is faulted in the
South side of Cwm Tryfan. The thickness of the member is 150-300m. There is a
conformable boundary between tuff and the shale members in the centre of Cwm Tryfan at
GR: 670/595, this is one of the type localities for this member. The other type locality has
clear cross bedding at GR: 674/596.
Description
In large exposures the Cwm Tryfan Tuff Member is dark grey and are massive exposures,
with visible bedding which is 1-2cm thick. The exposures are generally large on the scale of
2-10m.
It generally has a fine ash matrix and is well sorted (Fig.2). However, there are some
exposures that have angular medium-coarse grains that are poorly sorted. Some exposures
have clasts between 5-50mm (fig.3) and make up to 50% of the rock mass. These are quartz
feldspar in composition. The grains in this exposure are medium grain and are angular.
There is 60% visible quartz and 30% weathered feldspar grains. Other accessory minerals
make the rest of the rock mass. In fresh exposures it has a pale green colour with white
weathered angular plagioclase feldspar grains. Most beds have clear bedding (Fig.2), and in
one exposure at GR: 674/596, there is visible cross bedding (Fig.4 + Fig.5). This shows a
coarsening upwards sequence and a palaeoflow towards 160° S.
Interpretation
Cross bedding and sedimentary structures in an exposure at GR: 674/596, show that some
of the Cwm Tryfan Tuff Member has been reworked (Howells et al. 1991). The cross
bedding indicates that is was deposited in a shallow marine environment with enough wave
energy to produce cross bedding, however there was interludes with low enough wave
energy to produce shale (Plint, 1995) elsewhere in the Cwm Tryfan Formation. The cross
bedding indicates that the member is younging to the N.W, and there was a N.E-S.W
trending basin (See pg.52). Howells et al. 1991, agrees with this hypothesis, stating that the
formation was deposited in shallow-marine conditions on a Westerly facing palaeoslope.
The grains in this area are angular, indicating that they are immature have not been
transported far from its original source. The grey colour of the exposure and the presence of
5
quartz feldspar indicate that the Tuff was from the same rhyolitic primary source as the
volcanic materials observed in the Tryfan and Llyn Bochlwyd formations. The fine ash matrix
in most of the exposures show that this member was likely formed by an ash-fall (Howells et
al. 1979) (Howells et al. 1991) (Fig.43). Consecutive thin ash layers produce finely laminated
exposures (Fig.2). The member has no examples of being welded. This member was formed
as a result of a nearby series of eruptions which deposited beds of ash in a clastic and shale
dominated shallow marine environment. Due to a high rate of deposition the youngest part of
the member on the East side, where cross bedding was observed, was deposited sub
aqueously where the upper layers were deposited sub aerially.
A log (Fig.5) of the exposure showing trough cross bedding (Fig.4) at GR:674/596, shows a
coarsening upwards sequence. This indicates an increase in depositional energy (Hunt and
Tucker, 1992), which could be the result of regression.
In literature this member is referred to as the Gwern Gof Tuff Formation (Howells et al.
1991). The member, and the rest of the formation, is younging to the N.W, so it is both
younger and older than the Cwm Tryfan Shale and Cwm Tryfan Volcaniclastic Members.
Figure 3; A tuff exposure that has visible clasts and has a
different texture to other tuff exposures within the
Formation. GR: 668/590.
E W
Figure 2; An exposure of Tuff showing clear bedding
and has a well sorted ash matrix. GR: 671/597.
EW
6
N
Figure 4; A picture showing clear cross bedding in a tuff exposure. Palaeoflow to A log of this area is in
Fig.5. GR: 674/596
S
7
Figure 5; A stratigraphic log showing an exposure of Cwm Tryfan Tuff, with clear planer bedding
and trough cross bedding
Length(cm)
8
Cwm Tryfan Volcaniclastic Member
Overview
The Cwm Tryfan Volcaniclastic Member is found in many outcrops around Cwm Tryfan.
From Tryfan Fach in the North GR: 671/601, to the ridge to the South of the valley at GR:
666/584. It has conformable boundaries with the Cwm Tryfan Shale and Tuff Members and
the thickness of this member is 200m. The type locality for this member is at the base of
Tryfan Fach GR: 671/601, as there are many clear easily accessible outcrops around the
area.
Description
Large, 3-10m, weathered exposures of this member are dark grey and have clear bedding
structure. The bedding is usually on the scale of 0.5-1m, however some exposures show
individual beds as fine as 5-10mm. Some exposures show clear cross bedding (Fig.6) and
trough cross bedding (Fig.7).
It is medium grained in most exposures and is well sorted. Yet other exposures have small
layers that a very coarse and poorly sorted (Fig.6). Less than 1% of exposures have some
clasts that between 2-10mm (Fig.6), these have no preferred orientation. The grains are
generally well rounded to sub angular. Quartz is visible and makes 20-30% of the rock mass,
and white weathered feldspars make 10% of the rock mass. There are 10% green chlorite
grains in some exposures.
Figure 6; A clear exposure of cross bedding below planar bedding. Palaeoflow direction of 116° S.E. GR:
671/590. Photo courtesy of Loizos Geordgiades.
20cm
N.W S.E
9
At GR: 672/597, within a bed of the Cwm Tryfan Shale Member there is a 3-5m thick
volcaniclastic layer. It is observable around 50m in length and has a clear conformable
boundary with the shale member (Fig.8). The clastic within 30cm of the boundary has clear
cleavage similar to the shale which gets weaker further away from the boundary.
Figure 7; An exposure showing clear examples of trough cross bedding in a large
exposure of volcaniclastic. GR: 666/585.
E W
50cm
10
Interpretation
Cross bedding, trough cross bedding and mature, sub-rounded grains indicate that the
original material has been reworked. Medium-coarse grains show that it was deposited in
high energy environment. Cross bedding was used to produce a palaeoflow direction of
N.W-S.E, which helps prove the presence of a N.E-S.W trending welsh basin (See pg.52). It
also indicates that the younging direction is to the N.W which is confirmed by structures seen
in the Cwm Tryfan Tuff Member and the Llyn Bochlwyd Formation.
In literature this member is referred to as being part of the Gwern Gof Tuff Formation
(Howells et al. 1991). The original material likely originated from the edge of the Snowdon
caldera (Howells et al. 1991). The depositional environment for this member is a shallow
marine shelf environment, 0-30m below sea level and inner-infralittoral (Howells et al. 1991).
A log of a volcaniclastic exposure (Fig.9) shows a coarsening upwards and fining upwards
sequence indicating a change in the energy of the environment (Hunt and Tucker, 1992). It
shows how the palaeoenvironment of the area varied over time.
Figure 8; A conformable boundary between the Cwm Tryfan Volcaniclastic Member and the Cwm
Tryfan Shale Member. The volcaniclastic shows cleavage close the boundary. GR: 672/597
N.WS.E
50cm
11
Figure 9; A stratigraphic log showing a volcaniclastic exposure showing variation between different
environments.
Length (cm)
12
Cwm Tryfan Shale Member
Overview
The Cwm Tryfan Shale Member is found in two faulted beds in Cwm Tryfan. It bounds both
clastic and tuff and has a visible conformable boundary with the Cwm Tryfan Tuff Member at
GR: 670/595. Most of its exposures are highly eroded relative to other members in the area.
This makes many areas of shale appear as gullies, and have vegetation covering the
outcrops. The thickness of the Westerly bed is 80m, and the larger bed has a thickness of
200m. The type locality for this member is an exposure near the path to the North of Tryfan
Fach GR: 672/600.
Description
Exposures are generally small, on the scale of 1-5m, and appear dark grey to black and
have clear slaty cleavage and some show faint bedding. The cleavage is 5-20mm thick.
Exposures have some rusty coloured rocks. It has a silty grain size and is very well sorted.
There are visible quartz grains that make 20-30% of the rock mass, and other grains are too
fine to be identified. In fresh exposures it is dark blue/grey in colour. There are no clasts and
no fossils found in this member.
At GR: 672/597 there is a thin, 10 metre, volcanic clastic exposure within a larger shale bed.
There is a clear exposure showing a clear conformable boundary between the shale and the
Cwm Tryfan Clastic Member (Fig.8). The clastic within 30cm of the boundary shows faint
cleavage.
Towards the south side of Cwm Tryfan GR:669/589 there is a shale exposure that is heavily
weathered and white in appearance (Fig.10). This indicates a high percentage of feldspar in
this exposure. The exposure also has two different visible cleavages.
Interpretation
Shale is silty grained which indicates that it was deposited in a low energy environment. This
is different to the other members of clastic and tuff that are found within this formation. Black
shale is often deposited in anoxic waters (Middleton, 2003). There are no fossils found within
this member which provides further evidence that it was formed in an anoxic environment.
This information indicates that the palaeoenvironment was offshore or lagoonal. Other
members within the Cwm Tryfan Formation were deposited in a near shore environment
indicating that this member was to, leading to the conclusion that this member was
deposited in a lagoonal environment rather than an offshore zone.
13
The area around GR:669/589 shows two distinct cleavages. Some other exposures in the
surrounding area also show this. This could indicate there has been pressure applied to this
area in different directions in different periods (See pg.48).
W
E
20cm
Figure 10; An exposure of shale that is weathered white with a high proportion of
feldspar. There are also two visible cleavages.
14
Tryfan Formation
The Tryfan formation consists of four conformable members, three of which are volcanic in
origin. A period of high levels of volcanic activity is represented by this formation, with the
presence of rhyolite flows, rhyolitic breccias and tuffs. The Tryfan formations boundaries are
at the base of the East and the West of Tryfan, just South of the peak of Glyder Fach, and
the Northern side of Tryfan. In literature, this formation is named the Capel Curig Volcanic
Formation.
Figure 11; A map showing the location of the Tryfan Formation.
15
Tryfan Rhyolite Member
Overview
The Tryfan Rhyolite member is found continuously from the North base of Glyder Fach, to
the North side of Tryfan. It is also in the south east of Tryfan and is found in a large, almost
circular area. There is a visible boundary between rhyolite and a tuff exposure, and a visible
sudden change between the two members. There is also a boundary that is visible on the
peak of the Far South Peak of Tryfan GR: 663/592, in the direction of S.W-N.E, between
rhyolite and tuff. The bed which crosses N-S across Tryfan, has an apparent thickness of
100-250m. The type locality of this members boundary with the Tryfan Tuff Member is on the
North ridge of Tryfan at GR: 665/601.
Description
Rhyolite appears light grey in colour in weathered exposures with no cleavage. The
exposures are generally massive, not faulted and on the scale of 2-20m. Most of the
exposures, have clear flow banding (Fig.12) which distinctly differentiates rhyolite from other
rock types in the area. The observed flow banding was on the scale of 2-5m and the majority
was tight to isoclinal and upright (Fig.13).
Figure 13; Different rhyolite flow banding, with the axial
plane marked with a red line. This shows that they’re
isoclinal and upright. This could indicate that the flow was
roughly N-S or vice versa.
W
1m
E
2m
50cm
E
Figure 12; showing a clear example of flow
banding in a Tryfan Rhyolite Member
exposure on the south side of the far south
peak. GR: 661/589
1m
NS
16
In fresh exposures rhyolite is a dark grey/blue colour and the crystals are poorly sorted and
medium/fine in size, with an aphanitic ground mass. A combination of field observations and
microscope analysis show that there is around 70% quartz, 10-50μm in size (Fig.15), which
makes up the groundmass of the rock. Plagioclase feldspar crystals are weathered white,
make up 15% of the rock, and are between 500-1000μm (Fig.15). Orthoclase feldspar
crystals are between 200-1000μm, sub-rounded, and represent 10% of the rock mass.
Under the microscope the feldspar crystals are euhedral (Fig.15). There are around 5%
minerals that are difficult to identify but have a second order interference and are colourless
in plain light, which could indicate that they are muscovite mica. There are no visible
structures under the microscope.
Some exposures have >10% angular rhyolite clasts that are around 1cm in size. There are
some bombs visible closer to the rhyolitic breccia, around the peak of Glyder Fach, which
are circular and around 5cm in diameter, and get more numerous closer to the boundary
At GR:656/586, on the N.W slope of Glyder Fach, there is an exposure, 25m in length and
has accessible exposures on the base of the cliff face, that has a different lithology to the
surrounding rhyolite. The exposure is not large enough by itself to be mappable. In
weathered exposures it has a pale pink colour and a darker pink in clear exposures, this is
caused by the presence of orthoclase feldspar. It has roughly 5% black minerals, that are
Figure 15; A view under the microscope of a plagioclase mineral surrounded by an
aphanitic quartz groundmass. GR: 666/587
17
biotite. Moving east from these exposures the visible exposures start to show characteristics
common of rhyolite described above. This is a conformable change without a hard boundary,
the visible orthoclase gradually becomes sparser, until the exposures show the same
characteristics described above. This is a small exposure of micro-granite.
In some exposures of the rhyolite, within 20m of the upper boundary to tuff on the western
side of the Tryfan Rhyolite Member, there is a quartz nodular horizon (Fig.14). The nodules
are densely packed and range between 5-40mm in diameter. These exposures have a very
week visible layering. The nodular horizons are around 20-40m in size, and the type locality
is on the north ridge of Tryfan, around 665/600. Columnar jointing is found above heathers
terrace on the East side of Tryfan.
Interpretation
On the south side of the Tryfan Rhyolite Member, there is a rhyolitic breccia member which
has exposures on the peak of Glyder Fach GR:659/584. This was formed during the same
period of volcanic activity as the Tryfan Rhyolite Member, and from the same source
material (Howells et al. 1979). The boundary is conformable, however due to the terrain
none of the boundary is easily accessible. The relationship between the breccia and the
Tryfan Rhyolite Member, shows that the source volcano was to the south of this area (see
pg.58).
The whole area is younging to the N.W.W, so the Tryfan Rhyolite Member is younger than
the observed members, and formations to its west.
18
Figure 14; Pictures of an area with a nodular horizon on the boundary of tuff and rhyolite. GR: 665/600.
20cm
N.W
N
S
S.ES N
1m
50cm
19
Glyder Rhyolitic Breccia Member
Overview
The Glyder Rhyolitic Breccia Member is found in a series of outcrops, around the peak of
Glyder Fach. The member is 700m from E-W and has a boundary around 50m to the east of
Castell y Gwynt. There is a visible unconformable boundary between the Cwm Tryfan Shale
member and the Glyder Rhyolitic Breccia Member, on the East side of Glyder Fach at
GR:660/596. The breccia has less clasts, the closer to the boundary. East of the peak of
Glyder Fach there is an exposure which shows all the characteristics of the Glyder Rhyolitic
Breccia Member, making it the type locality GR: 659/584.
Description
The visible outcrops are light grey, have little structure and are relatively large, up to 30m.
There are large angular fragments, that range between 1-50cm (Fig.16), and are easily
visible, in most outcrops the fragments protrude from the matrix (Fig.17). In the majority of
outcrops these fragments make around 50% of the rock, but closer to the boundary with
shale, on the S.E side of the exposure, it is around 20%.
The matrix of this member is light grey, and fine grained. There are some visible quartz
crystals, that make roughly 60% of the matrix. The clasts have the same lithology as the
Figure 17; A large outcrop, that has many
angular fragments and is representative of
the rest of the exposures. GR: 657/583
W
1m
Figure 16; An example of a large angular clast (30cm), which protrudes
from the matrix. GR: 658/584. Photo courtesy of David Rowlands.
E
20
Tryfan Rhyolite Member. Some exposures show a faintly visible flow structure (Fig.18),
indicating that it is an epiclastic volcanic breccia (Fisher, 1960). The fragments are 1-50cm
and are angular.
Interpretation
The Glyder Rhyolitic Breccia Member was formed during the same period of volcanic activity
as the Tryfan Rhyolite Member. This is shown by the breccia having the same mineral
composition of the rhyolite and is hypothesised by (Howells et al. 1979). A reason that this
area is a breccia and there are no clasts further along the flow, into the rhyolite could be that
a topographic feature obstructed the flow (Howells et al. 1979). In (Fitch, 1967), it is
hypothesised that there was a volcanic vent situated somewhere on what is now Glyder
Fach (See pg.55). This could mean that the centre of the eruption was where this member
is, rather than towards Snowdon as originally hypothesised.
Volcanic breccia’s around Snowdonia are discussed in Howells et al. 1979 and Howells et al.
1991, refers to the outcrop as GFB, Glyder Fach Breccia.
Figure 18; A picture of a volcanic breccia outcrop that shows faintly visible flow structure. The red
lines indicating the direction and shape of the flow. GR: 659/584. Photo courtesy of David Rowlands.
21
Tryfan Rhyolitic Tuff Member
Overview
The Tryfan Rhyolitic Tuff Member is found flanking both sides of the Tryfan Rhyolite
Member, separated in some small areas by the Tryfan Shale Member. It is found on the
North of Tryfan near Llyn Ogwen GR:664/602 to Castell Y Gwynt, near the peak of Glyder
Fach GR:654/582.
It has an observable boundary with the Tryfan Rhyolite Member (Fig.19) GR: 665/600. This
boundary is also visible on the peak of the Far South Peak GR:663/591. The boundaries
with the Tryfan Rhyolite Member and the Glyder Rhyolitic Breccia Member are both not
directly visible in the field. The outcrop thickness of this member is 150-300m, with a true
thickness of 300m for the bed to the East of Tryfan and 150m for the Bed to the West of
Tryfan. The type locality of the Tryfan Rhyolitic Tuff Member is on the North base of Tryfan
GR:668/603.
E W
2m
Figure 19; A view facing upwards onto the North ridge of Tryfan. The top right of
the exposure is the Tryfan Rhyolitic Tuff Member, and the bottom left is the Tryfan
Rhyolite Member. GR: 665/600
22
Description
Tryfan Rhyolitic Tuff appears light grey and massive in exposures, most have clear fracturing
and exposures are up to 50m in size. The exposures have faintly visible bedding, on the
scale of 5-20cm. Weather feldspars often give the tuff a white appearance, and it is light grey
in fresh exposures. It has a poorly sorted fine ash grain size and has between 10-20%
coarse grains. Angular quartz grains make up 20% of the rock mass. Some exposures have
5-10mm lapilli that make up to 20% of the rock mass, these are very fine grained with no
identifiable minerals. Pores around 1-2mm are visible in some fresh outcrops. Rarely, >1%,
there are quartz feldspar clasts up to 5cm.
Interpretation
A light grey colour and the presence of quartz and feldspar indicate that the material
producing this tuff was originally from the same volcanic source that produced the Tryfan
Rhyolite Member. The angular fragments, faint bedding, lack of cross bedding, indicate that
the Tryfan Rhyolitic Tuff member is a lapilli fallout tuff (Fig.43). This is because there is no
evidence that the material has been reworked as is seen in the Cwm Tryfan Tuff. This
member is not a welded tuff as it has no visible fiamme, and none of the fragments are
welded. This was the result of explosive eruption with a high plume.
The Tryfan Rhyolitic Tuff Member was lain before and after the Tryfan Rhyolite Member. The
time between the two beds being deposited was not long, as there is only a small amount of
shale deposited either side of the Tryfan Rhyolite Member. The layers within this formation
are younging to the N.W.W. In literature this member is described under the name the Capel
Curig Volcanic Formation (Howells et al. 1991). It is interpreted to have been erupted from
the Llweyn centre to the North of the study area.
23
Tryfan Shale Member
Overview
The Tryfan Shale Member is found in thin beds, bounding both sides of the Tryfan Rhyolite
Member between the rhyolite and the Tryfan Tuff Member. It is found around the peak of
Tryfan GR:666/596, and it is exposed on the bottom of the North ridge of Tryfan
GR:667/603. This member is generally significantly more eroded than the Tryfan Rhyolite
Member and the Tryfan Tuff member. This means that the Tryfan Shale Member is often
seen as a gully and has a lower topography than the surrounding exposures. The Tryfan
Shale Member has a conformable boundary with the Tryfan Tuff Member, and the Tryfan
Rhyolite Member. The type locality for these boundaries is on the North ridge of Tryfan
GR:665/598. The beds are 20-50m thick.
Figure 20; A view to of Tryfan with a layer of Tryfan Shale Member, seen with a lower topography than then
surrounding Tryfan Rhyolite Member (left) and the Tryfan Tuff Member (right). GR:665/595.
N.W
S.E
24
Description
Most of the Tryfan Shale Member is eroded more than the surrounding members and often
appears as small gullies (Fig.20). In clear exposures it appears dark grey and has clearly
visible cleavage.
The Tryfan Shale Member is very well sorted and has a silty grain size, with 10% visible
quartz grains. There is a clear slaty cleavage with layers between 2-10mm. It is Dark
grey/blue in clear surfaces, and some weathered exposures have a faint red rust colour.
There is no evidence of fossils found in this member. Towards the south of Tryfan, the shale
pinches out.
Interpretation
The Tryfan Shale Member is found between two different volcanic rock types, rhyolite and
tuff. Both members appear to be lain extrusively, indicating that the Tryfan Shale Member
was deposited in relatively shallow water or a flood plain, and not in deep water. The
presence of water lain members within the same group indicates that this member, and
formation, was deposited close to the sea, which is evidence that it was deposited in a
coastal lagoonal environment (Howells et al. 1991). However, the member could have been
lain in a floodplain or river delta (Arthur and Sageman, 1994).
This member is found above and below the Tryfan Rhyolite Member meaning it was
deposited before and after the volcanic eruption that produced the member. (Howells et al.
1991) describes this member as a siltstone intercalation.
25
Llyn Bochlwyd Formation
The Llyn Bochlwyd Formation is found from the Western base of Tryfan, to Y Gribin, West of
Llyn Bochlwyd, and consists of four different members. It represents a gap in volcanism, and
several large eruptions towards the Western side of the formation. The thickness of this
member is between 600-900m.
Figure 21; A map showing the exposures of the Llyn Bochlwyd Member
26
Bochlwyd Shale Member
Overview
The Bochlwyd Shale member is found in many beds around Llyn Bochlwyd, from GR:
658/601 to GR: 653/583. It has conformable boundaries with the Bochlwyd volcaniclastic
member and the Bochlwyd Felsic Tuff Member. Many of the outcrops of this member are
highly weathered and appear with a lower topography than the surrounding members. The
thickness is between 50-150m and the type locality for this member is a small outcrop near a
path around GR: 658/600, where Brachiopod fossils were found.
Description
The shale appears grey to brown in colour with fine, 2-10mm, clearly visible slaty cleavage in
relatively small exposures around 1-5m. It has silty to very fine grains, that are very well
sorted, with no clasts. Thin section analysis shows a silty groundmass (Fig.22) which is
quartz and feldspar which constitutes 50% and 10% of the rock mass respectively. Larger
quartz grains, around 100μm, make around 15% of the rock mass. A highly deformed
mineral that has high second order interference colours, makes up 15% of the rock mass.
This is likely muscovite mica; however, it is difficult to specify. Around 10% of the rock is not
identifiable from a thin section.
Figure 22: A picture of a thin section of shale from the Bochlwyd Shale Member,
showing a silty grain size. GR: 658/598.
27
At GR: 658/600, just south of Llyn Ogwen, there is a series of small outcrops that have
numerous visible brachiopod (Fig.23). These appear to be from one species that have
between 16-18 ribs and are 2-2.5cm in size. A poorly preserved fossil found could be part of
a coral fossil or a bryozoan.
Interpretation
To the West of Llyn Bochlwyd there is a series of beds of this member and the Bochlwyd
Felsic Tuff Member. This indicates that there was a series of repeated changes in the area
that resulted in the shale being deposited rather than the Tuff. This could be a series of
intermittent explosive volcanic events depositing beds of tuff in an offshore marine setting.
The fossils found within this member likely belong to the taxon of Dinorthis berwynensis.
(Howells et al. 1991) shows that this taxon of brachiopod is found in the Lower Rhyolitic Tuff
Formation, which is the name used in some literature for this formation, and the specimens
have an equal number of ribs, 16-18, as the samples found in the shale at GR: 658/600
(Fig.23). The age of this taxon is limited to 457.5 to 455.8 Ma (Howells et al. 1991). This
gives an accurate age of this member and formation. The presence of organic matter
differentiates this from the black shale that is found within the Cwm Tryfan Formation, that
was anoxic with no fossils. A small part of what could be a fossilised Coral or Bryozoa, may
show that the depositional environment for this member was in the photic zone close to the
shoreline. Howells et al. 1991 states that the presence of this taxa of fossil shows that the
depth of the water is less than 10m.
Figure 23: Dinorthis berwynensis found around GR:658/600.
28
Shale is representative of a low energy environment as it is fine grained. It is also known to
be a marine environment, as there are brachiopods found within the member. This could
prove that the member was deposited in a lagoonal environment or extensive tidal mud flats
(Arthur and Sageman, 1994). (Howells et al. 1991) states that some siltstone beds in this
area are formed by low density turbidity flow currents and are deposited at a depth between
60-100m. This is unlikely however due to the presence of fossils and the nearby Bochlwyd
Volcaniclastic Member that indicates a shallow, nearshore environment. However, the beds
of the Tuff to the West of the ignimbrite may have been deposited in a deeper, offshore
environment.
29
Bochlwyd Volcaniclastic Member
Overview
The Bochlwyd Volcaniclastic Member is found across the formation, conformably bounding
all of the members in the Llyn Bochlwyd Formation. It is near to Ogwen Cottage in the North
to close to Bwlch y Ddwy Glyder in the South. The typical outcrop is relatively large from 5-
20m. The type locality for this member is just to the south of Llyn Ogwen GR: 655/602 and
has cross bedding.
Description
The boundaries between the other members within this formation are conformable. The
conformable lower boundary with the Bochlwyd Ignimbrite Member is observed in two
locations once to the East of Llyn Bochlwyd GR: 656/593, and again at GR: 655/600. The
beds have a thickness of 70-100m to the West of Llyn Bochlwyd and 200m to the East of
Llyn Bochlwyd. To the south of Llyn Ogwen, the volcaniclastic appears to be up to 600m
thick, however there is not many exposures in this area. This could also be as a result of the
secondary fold (See pg.48).
In large outcrops the member appears massive with a light grey in colour and has some
rusty red colour. Bedding is obvious on a large scale between 0.5-3m and some exposures
have visible cross bedding which shows a palaeoflow to the N.W and the S.E (See pg.52). It
also certifies that the members and formations in the Tryfan area are younging to the
N.W.W. Fresh exposures appears dark grey in colour (Fig.24) and are medium to coarse
grained and is poorly sorted. Plagioclase crystals are visible in clear exposures as white
grains mostly around 1mm, and up to 5mm. Thin section analysis shows that there is quartz
grains that are 200-500μm in size, and a quartz dominated groundmass, meaning quartz
makes around 70% of the rock mass. Plagioclase is clearly visible with distinguishing
polysynthetic twinning (Fig.25), which consists of around 20% of the rock mass. A mineral
that is generally very small, ~50μm, appears with a second order interference colour and is
highly deformed. This is difficult to specify the mineral however it is likely a muscovite mica,
which comprises 5% of the rock mass. There is around 5% unknown accessory minerals.
The visible grains have no preferred orientation and are mostly sub-rounded with >20%
angular grains.
Interpretation
Fossiliferous Bochlwyd Shale was deposited during the same geological period as the
volcaniclastic, and cross bedding was found in many exposures of the member. This
indicates that the Bochlwyd Volcaniclastic Member was deposited in a shallow marine
30
environment, above the fair-weather base (Howells et al. 1991). A lack of bioturbation and
muds within this member gives evidence that it was deposited in a period of high
sedimentation. This also provides a reasoning for the lack of fossils within this member,
when they are found on the lower boundary of the Bochlwyd Ignimbrite Member, in a death
assemblage (See pg.33), and in a bed of shale which has upper and lower boundaries with
volcaniclastic. The death assemblage had Orthid brachiopods which shows that it is a
shallow marine depositional environment (Howells, 1991).
The mineralogical composition of the member is similar to that of the Tryfan Rhyolite
Member. This implies that the source of the volcaniclastic material is extrusive rhyolite,
which is younger than the Bochlwyd Volcaniclastic Member. (Howells et al. 1991) concludes
that this material is eroded from the Western edge of the caldera and the tuffs formed by the
volcanic activity and that it was deposited in a marginal marine environment. The sub
rounded and angular grains prove that this is a rather immature sediment and shows it was
deposited close to the material source with little transportation, implying that the material
originated from the nearby caldera and tuffs.
5cm
Figure 24; A fresh exposure of volcaniclastic. The visible white grains are plagioclase. GR:
652/585
31
10-50cm thick graded beds are found in an area to the S.W of Llyn Bochlwyd (Fig.26).
These show a fining upwards sequence with a planar top. (Howells et al. 1991) says that
these may indicate turbidity-current deposition, however it is specifically talking about
exposures to the West of this member. A lack of large clasts likely disproves this hypothesis,
nevertheless it does give evidence for changing relative sea level (Howells et al. 1991).
In literature (Howells et al. 1991) this member is part of the Cwm Eigiau Formation and the
Lower Rhyolitic Tuff Formation.
Plagioclase
Large quartz grain
Figure 25; A thin section of a volcaniclastic sediment, with clear quartz and plagioclase grains.
32
Length(m)
Figure 26; A stratigraphic log showing a transgressive event and a series of different nearshore
environments that resulted in the deposition of this member.
33
Bochlwyd Ignimbrite Member
Overview
The Bochlwyd Ignimbrite Member is found from Ogwen Cottage to Bochlwyd Buttress, to the
S.W of Llyn Bochlwyd. It has an observed thickness between 100-250m. The outcrops are
massive and generally on the scale of 5-10m, and up to 60m on Bochlwyd Buttress.
The Bochlwyd Volcaniclastic Member bounds the Bochlwyd Ignimbrite Member on both
sides. The type locality for this boundary is visible on the top of a small hill to the East of Llyn
Bochlwyd GR: 656/592.
Description
The type locality for this member is between Ogwen Cottage and the Bochlwyd Butress GR:
654/590. Bochlwyd Ignimbrite Member outcrops appear grey in large scale, with no
structures, and a rough texture. Weathered feldspars show as white patches in some parts
of the outcrops. Some exposures have clasts from 1-30cm, that make up most of the rock
mass.
The matrix of the rock is fine grained, well sorted, and light grey in colour. The matrix has
quartz and feldspar. There a visible Fiamme, around 1-5cm in some exposures that are
black flat structures (Fig.27). The member has a varying amount of clasts, which get sparser
from the East to the West side of the bed. The clasts are rhyolitic in composition. There is an
exposure on the East side of Llyn Bochlwyd where the Bochlwyd Ignimbrite Member has a
boundary with the Bochlwyd Volcaniclastic Member. Here there are clasts that make 90% of
the rock mass (fig.28). Some exposure have visible quartz nodules, that are between 1-
10cm. Vesicles are found in some exposures around GR: 654/590, which are 1-5mm in size.
Large quartz veins are that cut through the exposures. Some welded fragments show that
this is a welded ignimbrite. At GR: 654/590 there is an exposure of the boundary between
the ignimbrite and the Bochlwyd Volcaniclastic Member, with a death assemblage. It is
around a metre thick and was full Orthid brachiopods. There was no fluidal structures in this
member.
Interpretation
A stratigraphic log (fig.29) shows the top of the ignimbrite member. This shows a bed
containing fossils on the bottom of the layer, showing it is younging to the N.W.
Fiamme, flattened pumice fragments and a lack of fluidal structures show that this Member
was formed by a Pyroclastic density current (PDC) (Fitch, 1967). The death assemblage at
GR: 654/590 was caused when the pyroclastic flow covered a marine environment, causing
34
many Brachiopods to become fossilised at the base of the member. This, along with the
change in density of clasts moving East to West, shows that the member is younging to the
N.W. This was likely a shallow marine environment, similar to that found in the Bochlwyd
Shale Member. Meaning that the bottom of the PDC was deposited in a marine environment,
then built up above the water.
Howells et al. 1991, calls this member the Pitt’s Head Tuff. (Fitch, 1967) predicts that the
vent for the source if this PDC was on the peak of Glyder Fach. However, (Howells et al.
1991) states that the ignimbrite was erupted from the Llwyd Mawr volcanic centre to the S.W
with significant evidence to back this up.
Figure 27; Black Fiamme structures seen in a fresh exposure of the
Bochlwyd Ignimbrite Member
35
Figure 28; An exposure of the ignimbrite at its lower boundary with a high
percentage of clasts compared to other exposures. GR: 656/592.
W E
36
Figure 29; A stratigraphic log showing 62m of the bottom (youngest side) of the Bochlwyd Ignimbrite
Member.
Length(m)
37
Bochlwyd Felsic Tuff Member
Overview
The Bochlwyd Acidic Tuff Member is found in four, relatively thin beds to the West of Llyn
Bochlwyd. The beds have an apparent thickness of 50-150m and are repeatedly bedded
with the Bochlwyd Shale Member. This often means that this members outcrops appear 2-
10m in size as they are less weathered than the surrounding beds. The type locality for this
member is found to the West of Llyn Bochlwyd at GR: 654/594.
Description
Outcrops of the Bochlwyd Felsic Tuff Member are light grey and have bedding that is visible
on a larger scale. The Matrix is made of medium ash, which is very well sorted. There was
no bombs or large clasts found in this member. The boundaries with the Bochlwyd Shale
Member are covered with vegetation, but they are conformable boundaries. The light colour
indicates a high percentage of weathered feldspar; however an exact percentage is difficult
to obtain.
Interpretation
Several repeated beds with the Bochlwyd Shale Member, and the conformable boundaries
between them indicate that there was a series of Parasequences or volcanic events that
deposited layers of ash, forming tuff.
This member was likely lain by a subaqueous ash flow (Howells et al. 1991). Shale is
bounded to this member, which is deposited in a deep marine environment. This may show
that the tuff was deposited in the same low energy environment as shale. A lack of cross
bedding and other sedimentary structures provides evidence for this hypothesis.
In literature this member is part of the Lower Rhyolitic Tuff Formation and is often referred to
as an acidic tuff. However, acidic is an outdated term and it should be referred to as felsic.
38
Clogwyn Formation
Found on the Western side Llyn Bochlwyd, this formation represents a period of basaltic
volcanic activity. The formation overlies the Llyn Bochlwyd Formation and is the youngest of
the studied formations. It shows a difference in material than the formations to the East, as
they were mostly rhyolitic. However, it was still formed during the same period of volcanic
activity in the second eruptive cycle (Howells et al. 1991) (See pg.57)
Clogwyn Basalt Member
Overview
The Clogwyn Y Tarw Basalt Member (CTBM) is observed on to of a ridge to the West of Llyn
Bochlwyd GR: 651/594, this is also the type locality for the CTBM. The outcrops are
generally large, 5-10m, and it has a thickness of around 70m (Howells et al. 1991). It
overlays the Llyn Bochlwyd Formation to the East. The outcrop is independent on the
topography, which limits the expanse of it.
Description
In large outcrops the CTBM has a very chaotic structure (Fig.30) and can vary largely
between the different outcrops. It has an unclear mafic pillow lava, on the scale of 1-2m and
is highly brecciated (Fig.30). It is grey with a faint red colour in more massive blocks.
Between the pillows is a silty grained infill (Fig.31) which is dark grey. The massive blocks
are angular and generally between 1-20cm in size. One exposure shows a more massive
structure with one block being near 3m (Fig.32). There is a structure that is near vertical,
around 80° to the East, which is too variable to be measured. The rock mass has a very fine-
grained aphanitic texture, with <10% phenocrysts that are feldspars and a
Interpretation
Pillow lavas are visible within this member, and it is mostly very fine grained, which indicates
that this was formed during a sub marine volcanic eruption (Howells et al. 1991). This is also
supported by the fact the CTBM has an unconformable lower boundary with shale, which
was deposited in a marine environment. In literature this member is described as being
basaltic, based on field observations in other areas, especially towards Snowdonia (S.W),
and chemical analysis (Howells, 1991). This can explain the minerals of pyroxene and
feldspar that were observed in the field. Howells et al. 1991 describes this member as auto-
brecciated which would explain the chaotic structure.
Despite the difference in composition, this member was formed during the same period of
volcanic activity as the formations to its East, as described by (Howells et al. 1991). In
39
literature the CTBM is part of the Bedded pyroclastic Formation (BPF). Rhyolitic materials
were erupted before after and during the deposition of the BPF, within the local area
(Howells et al. 1991). This indicates that this member was erupted from a different vent than
the rhyolitic materials, hypothesised by (Howells et al. 1991). The CTMB is likely close to the
original source vent (Howells et al. 1991), which was likely eroded away due to post
depositional events.
W E
50c
m
Figure 30; A large outcrop of CTBM showing a heavily brecciated structure, with a
vertical flow structure GR: 651/594.
40
E
20cm
W
Figure 31; A dark grey very fine grained, highly laminated, rock forming the infill between the
pillows GR: 651/594.
41
E W
2m
Figure 32; A large exposure of CTBM, showing a vastly different appearance to the exposure
shown in Fig.1. The blocks are massive, with no structure GR: 651/594.
42
Structures
Figure 33; A map and cross section showing the structures of the study area.
43
Dip Measurements
All the dip measurements are between N.W.W and S.E.E, the largest proportion
show a relatively high dip towards the N.W.W, found to the West of the Tryfan
anticline. Some are dipping shallowly to the S.E.E that are taken from the Eastern
side of the Tryfan anticline.
Figure 34; A stereonet showing all dip measurements taken in the field. Plotted poles to planes.
(Appendix A).
44
Tryfan Anticline
An anticline is found on the East side of Tryfan, in literature it is known as the Tryfan
Anticline. A stereonet of dip measurements taken around both limbs of the fold (Fig.34)
shows a clear trend of dips to the N.W.W, found on the Western limb, and S.E.E, found on
the Eastern Limb. The stereonet also shows that there was a dip direction towards the
S.S.W, which indicates the Tryfan Anticline is plunging in this direction, with a shallow angle
between 4-8 degrees. The dips to the West of the anticline are higher than the dips to the
East of the anticline indicating that the axial plane is inclined to the N.W.W.
Figure 34; A stereonet showing dip measurements from either side of the Tryfan
Anticline, and the data used in the stereonet. The Measurements are all within the Cwm
Tryfan Formation.
45
Cleavage Measurements
Cleavage measured in the mapping area is consistent and shows a single trend,
steeply dipping to the N.W. Cleavage is predominantly taken from shale in the Llyn
Bochlwyd Formation and the Cwm Tryfan Formation. The cleavage is likely a result
of the compression caused during the Caledonian orogeny, as it is also in the same
direction as the axial plane of the fold (See pg.47).
Figure 35; A stereonet showing cleavage plotted poles to planes. The average of the
measurements is shown by the blue line; Strike: 218.7, Dip: 72.9. (Appendix B).
46
Axial Plane
This stereonet shows the axial plane of the Tryfan anticline, calculated from
measurements taken either side of the fold axis. The axial plane dips steeply
towards the N.W. A lack of measurements taken on the Eastern limb of the Tryfan
anticline may limit the accuracy of the axial plane calculation.
Figure 6; A stereonet showing a select group of dips taken close to the center of the fold,
on each limb. A red line shows the axial plane of the fold; Strike: 197.7, Dip: 77.5.
(Appendix C).
47
Fold axial plane and cleavage
The average of the cleavage measurements, taken throughout the mapped area, is
similar to the axial plane of the Tryfan anticline. This indicates that the cleavage
found in the Tryfan area is axial planar cleavage showing that it was formed during
the same period of tectonic activity as the Tryfan anticline.
Figure 37; A stereonet showing the axial plane (Fig.36) and the mean vector average of
the cleavage measurements (Fig.35). Plotted planes from poles.
48
Braid and McCaffrey, 1999, hypothesises that there is a synform with an axial surface trace
of ENE-WSW, that is half way between Llyn Bochlwyd and Llyn Ogwen. This could be the
cause of why units mapped in the area appear to bend to the N-W. It may also give a reason
to why some members in the Llyn Bochlwyd Formation, seem to get wider to the North.
Faults
There are several normal faults around the studied area, cutting through different formations
and varying in size. A type locality for faults within this area would be on the East side of
Tryfan GR: 668/591, where there is a clearly visible offset between two layers of Cwm Tryfan
Tuff (Fig.38). This is a relatively large fault, that is clearly visible from an aerial view (Fig.39),
with a length of 1.5km and an average offset of 60m. The hanging wall of this fault is on the
Southern side, and the foot wall on the Northern side.
On the N.E side of Tryfan (Fig.40) GR: 669/596 there is an exposure that shows a small
normal fault, with a throw of 2cm, and it has a dip of 59°. The direction of throw is almost
perpendicular to the direction of the bedding, which makes this likely to be a normal fault.
This fault was striking E-W, which is the same as some of the larger faults on either side of
Tryfan including the one seen in Figure 1 and 2. This may indicate that the larger faults are
also normal faults caused by the same processes. Two faults to the East of Llyn Bochlwyd
could have been formed as a graben structure which would explain the observed offset.
However, there is nothing directly indicating that our observed faults are strike slip or not, as
no indicating structures were observed such as slickensides. Further observations and
mapping around the local area could provide more evidence to firmly decide what type of
faults these are.
Some small strike-slip faults are found around the west of the study area. These are almost
right angles to the small fold hypothesised by (Braid and McCaffrey, 1999). This could
indicate that they were formed during the same period of compression as the smaller fold.
In literature there are descriptions of faults in this area which are formed syndepositionally,
by the Snowdon Graben. This was a ~40km wide, complexly faulted tectonic depression,
that was caused by an East-West extension between 458-452 Ma (Kokelaar, 1992). This
was later rotated clockwise to where they are today (Kokelaar, 1988) which means the faults
would have been originally orientated N-S. The throw of the faults is now E-W, because the
beds have been tilted during folding.
49
S N
Figure 38; A picture of Tryfan taken from the Eastern side, around GR: 673/5900. The throw of the fault is
clearly visible as the lighter grey tuff layer (highlighted green) is offset by it. The dashed red line indicates
the fault.
50
Figure 39; An aerial view of the West side of Tryfan that shows a fault that is clearly visible in the
landscape. This is backed up by mapped offset. (Map data ©2019 Google)
51
8cm
S N
Figure 40; A small normal fault showing a 2cm throw. Hanging
wall is on the left-hand side. GR: 669/596.
52
Paleocurrent data
Cross bedding is common in exposures East and West of Tryfan. This shows (Nichols, 2013)
a bipolar distribution (Fig.41) of paleocurrent directions, at 180° to each other, flowing N.W
and S.E. This shows unidirectional currents and is likely the result of the alternating currents in
shore-line deposits (Selley, 1968). However, this is not definite as bipolar currents can also
uncommonly be caused by off-shore winds (Selley, 1968), and stream flow producing antidune
bedding (Selley, 1968).
According to Howells et al. 1991, during the time of deposition, in the Ordovician, there was a
N.E-S.W trending basin. Tidal flows in this basin would produce sediments showing
paleocurrents, N.W-S.E, which is what is observed in exposures in the Tryfan area, adding
evidence to the paleocurrents being as a result of tidal flows.
Figure 41; A rose diagram showing the direction of palaeoflow measured around the mapped
area. (Appendix D).
53
Geological history
Cwm Tryfan Formation
The youngest formation in the study area was the Cwm Tryfan Formation, which shows a
period of relatively uncommon explosive volcanic activity, producing ash, that is seen in the
Cwm Tryfan Tuff Member. The tuff is formed by ash falling out of explosive plumes and
landing is a coastal, shallow-marine environment (Fig.43). The Cwm Tryfan Shale and
Volcaniclastic Members indicate that the palaeoenvironment was a shallow marine coastal
area. Due to local erosion and deposition this varied between a low energy anoxic
environment, producing black shale, to a higher energy tidal location, depositing
volcaniclastic.
Figure 42: A palaeomap showing the possible environment of Snowdonia in the Upper Ordovician.
Modified from modern maps of the North island of New Zealand. (Map data ©2019 Google)
N
54
The environment was likely similar to that shown in the N.W of (Fig.42), where constant
deposition could have caused stagnant lagoons to be formed and destroyed over time. This
palaeomap is equivalent of the Snowdonia area in the Ordovician (Gibbons, 1998).
In literature the Cwm Tryfan Formation is referred to as the Gwern Gof Tuff Formation. It
was deposited towards the center of the first volcanic cycle (Howells et al. 1991) and was
formed 458 to 457 Ma (Brenchley, 1992) during the Soudleyan stage (Howells et al. 1991)
(Known as the Sandbian in current literature (Cohen, 2019).
Tryfan Formation
The Tryfan Formation represents a period of voluminous volcanic activity, which produced
the Tryfan Rhyolite, Tuff and Glyder Rhyolitic Breccia Members. The thickness of the
volcanic material in this formation is around 500m. The Tryfan Tuff Member is the result of
ash fallout from a large local volcanic eruption (Fig.43). The Tryfan Rhyolite Member was
likely formed by an extrusive lava flow (See pg.58), representing a large volcanic event.
Figure 43; A diagram showing the formation of fallout ash tuffs, comparable to
the formation of the Cwm Tryfan Tuff. Modified from (Plint, 1995).
55
Glyder Rhyolitic Breccia also represents a large volume of volcanic material that was
produced during the same time period which may have been deposited on the Snowdon
Caldera margin (Fig.43). A lack of sedimentary structures caused by rapid deposition in this
formation mean that the palaeoenvironment is difficult to determine from the study area.
In literature, this formation is a part of the Capel Curig Formation and represents the climax
of the first eruptive cycle (Howells et al. 1991). It was deposited around 457 Ma (Brenchley,
1992), at the end of the Soudleyan stage (Howells et al. 1991). (Howells et al. 1991) shows
that the study area was in a transitional depositional environment. As explained in the unit
description, the Glyder Rhyolitic Breccia Member was formed during the same volcanic
activity as the Tryfan Rhyolite Member, and a topographic feature blocked (Howells et al.
1979) the flow of large materials around the S.S.E of the study area. This indicates that the
source of the material was from the S.S.E and likely from the Snowdon Center. The
topographic feature that prevented the flow of large material to the North of Glyder Fach
could be the edge of a caldera.
Figure 44; A modified map from (Howells et al. 1991), showing the location of the Snowdon
caldera relative to other modern locations and the studied area.
Approximate setting of the studied area
Cwm Idwal Syncline
56
Below Glyder Fach on the N.W face GR: 656/586 there is a microgranite exposure (See
pg.16) which is formed from the same magma source as rhyolite but cooled intrusively
(Haldar, 2013). This may indicate that during the period of volcanic activity there was a
volcanic vent somewhere around Glyder Fach, also hypothesised by (Fitch, 1967).
Llyn Bochlwyd Formation
Sedimentary beds that are lain after the Tryfan Formation and the Llyn Bochlwyd Formation
are the Bochlwyd Volcaniclastic and Shale Members. These beds were formed during a
period of low to none volcanic activity, and sperate two periods of higher volcanic activity.
The Palaeoenvironment of these beds were a shallow marine environment with a water
depth of less than 10m (Howells et al. 1991). In literature, these are part of the Cwm Eigiau
Formation and they represent the period between the first and second eruptive cycles
(Howells et al. 1991).
Further to the West is the Bochlwyd Ignimbrite Member, this is the result of a pyroclastic
density current. It has a thickness of 150m and is likely from a single explosive volcanic
event. A death assemblage on the youngest (East) side of the layer shows that it was
originally a shallow marine environment.
This member in literature is part of the Pitts Head Tuff Formation (Howells et al. 1991) .
Roberts, 1965 suggested that this formation was erupted from a caldera on Llwyd Mawr
GR:505475, which is around 20km S.W of the study area. This member could have been the
result of an eruption of a vent centred around Glyder Fach (Fitch, 1967), which may have
relation to the microgranite observed North of Glyder Fach.
West of the Bochlwyd Ignimbrite Member, there is a series of beds of tuff and shale. This
was likely developed in an offshore low energy environment with sporadic volcanic
eruptions. The tuff beds are relatively thin, 50-100m, and could have been formed by ash
fallout being deposited in the low energy environment. In literature, this is described as the
Lower Rhyolitic Tuff Formation and was formed from volcanism originating in the Snowdon
caldera (Howells, 1991).
57
Clogwyn Formation
This formation consists of the Colgwyn Basalt Member which cuts across the Bochlwyd
Shale Member, making it unconformable. It is an auto-brecciated pillow lava flow, which
shows that it was formed in a marine environment. This member represents a change in the
chemical properties of the magma source.
In literature, this member is referred to as the bedded pyroclastic formation and represents a
basaltic volcanic phase in the second eruptive cycle (Howells et al. 1991). The basaltic
material was from the same magma source as the earlier rhyolitic formation but erupted from
different vents.
Faulting
Normal faults were likely formed syndepositionally in the Ordovician (See pg.48). This was
because of a graben system, developed because of tectonic extension. This is known in
literature as the Snowdon Graben System.
Caledonian Orogeny
Around 450 Ma, a continental collision began between the Avalonian and Baltican tectonic
plates. This caused the closure of the Tornquist Sea between the two plates. In the area of
Snowdonia, this caused folding and some low-grade metamorphism. During this time period
the Tryfan anticline was formed and cleavage observed in the shale members (See pg.47).
A possible second fold was caused after the Caledonian orogeny causing the observed N-S
fold.
58
Discussion
Source of Tryfan Rhyolite Member
The Tryfan Rhyolite Member is shown in literature and published maps to be intrusive
(Howells et al. 1991) (Howells et al. 1985). However, this can be argued against as there is
evidence pointing towards this member being extrusive.
By all accounts, the Tryfan Rhyolite Member is called a rhyolite and not a granite or
microgranite Howells et al. 1991) (Howells et al. 1985). A sample of rhyolite from the field
area, observed under microscopes shows the same mineral composition of rhyolite (Haldar,
2013). Haldar, 2013, says that rhyolite is an extrusive rock, and the intrusive equivalent
would be granite or microgranite. This is in direct disagreement with maps and papers that
state that is intrusive.
The rhyolite is fine to medium grained indicating that it cooled quickly, indicative of an
extrusive lava flow. Flow banding is also observed in this member, but this can also be
explained of the magma being cooled in a lava dome. However, the bedding layer of rhyolite
does not seem to indicate the shape of a dome, rather that of a consistently thick extrusive
flow or Sill. Quartz nodules are found in two locations within the rhyolite on the North and
South side of Tryfan GR: 662/589, 667/. These are generally formed when magma or lava
comes in contact with water, which also indicates that it was lain extrusively.
The Glyder Rhyolitic Breccia Member was formed during an extrusive eruption (Howells et
al. 1979). It was likely from the same source vent as the Tryfan Rhyolite Member, but the
local topography meant the large clasts did not flow to the North of Glyder Fach. The large
cliff face makes this boundary inaccessible, but this hypothesis points towards the rhyolite
being formed extrusively.
As sill could possibly produce the shape of the exposure, yet after the Llyn Bochlwyd
Formation the volcanic activity in the area was basaltic in composition (Howells et al. 1991).
This would have meant the sill formed within a short time span after the surrounding Tryfan
Tuff was deposited. Flow banding is found rarely in sills but the quartz nodules are difficult to
explain. It does not cut across other bedding layers so is not a dyke. No metamorphic
aureole was observed around the rhyolite, potentially providing evidence against it being a
sill.
Microgranite was observed on the North face of Glyder Fach, this was conformable with the
Tryfan Rhyolite Member with a gradual reduction in the amount of pink plagioclase feldspar
59
visible in the exposures moving Eastwards. This could have formed intrusively and may
represent the boundary of a vent between intrusive microgranite and extrusive rhyolite.
Literature Formations
Figure 45; A map showing the formations as they are in literature. (Howells et al. 1991)
60
In literature there are six formations that comprise our mapping area (Fig.45). These are
separated into formations that have distinct volcanic events. For example, the Cwm Eigiau
Formation represents the gap between the first and second eruptive cycles and the Pitts
Head Tuff shows an Ignimbrite eruption. The map as part of this study, consists of different
formations of members. The mapped members help give an extra layer of detail to the
geology and can provide extra information. However, it could be argued that the formations
used in literature provide a better grouping than that used in this study.
61
Conclusion
The geological exposures around the Tryfan area, demonstrate extensive Upper Ordovician
volcanism, deposited in a predominantly shallow marine environment. Rhyolitic eruptions
include explosive tuff and ignimbrite forming events and extrusive rhyolite flows. Basaltic
volcanism produced a member in the West of the area, in the form of a sub-marine eruption.
Structures in the area indicate that normal faulting occurred syndepositionally, and that a
secondary tectonic event caused some strike-slip faults in the West of the study area. The
Caledonian Orogeny caused folding in Snowdonia, producing the Tryfan anticline and the
cleavage that is in the study area.
Acknowledgements
____
References
62
A. Guy Plint. (1995) Sedimentary Facies Analysis: A Tribute to the Research and Teaching
of Harold G. The International Association of Sedimentologists. P.172,175,182.
A. Williams. 1963. The Caradocian brachiopod faunas of the Bala District, Merionethshire.
Bulletin of the British Museum of Natural History (Geology) 8(7)
Arthur, M. A., & Sageman, B. B. (1994). Marine Black Shales: Depositional Mechanisms and
Environments of Ancient Deposits. Annual Review of Earth and Planetary Sciences, 22(1),
p.537
BAIRD, A. W., & McCAFFREY, K. I. W. (1999). Polyphase deformation and metamorphism
in the Llyn Ogwen area of Snowdonia, North Wales. Journal of the Geological Society,
156(1)
Brenchley, P. J. (1992). A geologic time scale 1989, by W. B. Harland, R. L. Armstong. A. V.
Cox. L. E. Craig, A. G. Smith and D. G. Smith. Cambridge University Press, Cambridge.
Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) The ICS International
Chronostratigraphic Chart. Episodes 36: 199-204.
Fisher, R. (1960). CLASSIFICATION OF VOLCANIC BRECCIAS. Geological Society of
America Bulletin, 71(7), p.974.
Fitch, F. J. (1967). Ignimbrite volcanism in North Wales. Bulletin Volcanologique, 30(1), 206,
208, 209, 216
Geological Survey on the six-inch scale by M.F. Howells, B.E Leveridge, R. Addison, C.D.R
Evans, and M.J.C. Nutt in 1970-79. E.G. Smith and I.P. Stevenson, District Geologists. 1:25
000 Geological Sheet published 1985.
Gibbons. (1998). Rhyolitic volcanic corridors in magmatic arcs: comparing North Wales and
North Island, New Zealand. Terra Nova, 10(6)
Haldar, S. (2013). INTRODUCTION TO MINERALOGY AND PETROLOGY. [S.l.]:
ELSEVIER.
Howells, M F, Reedman, A J, and Campbell, S D G. 1991. Ordovician (Cardoc) marginal
basin volcanism in Snowdonia (north-west Wales). (London : HSMO for the British
Geological Survey.)
Howells, M. F., Leveridge, B. E., Addison, R., Evans, C. D. R., & Nutt, M. J. C. (1979). The
Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs
related to emplacement environment. Geological Society, London, Special Publications,
8(1), 614
63
Hunt, D., Tucker, M.E., 1992, Stranded Parasequences and the forced regressive wedge
Systems Tract: deposition during base-level fall. Sedimentary Geology 81
Kokelaar, P. (1988). Tectonic controls of Ordovician arc and marginal basin volcanism in
Wales. Journal of the Geological Society, 145(5) p.770
Kokelaar, P. (1992). Ordovician marine volcanic and sedimentary record of rifting and
volcanotectonism: Snowdon, Wales, United Kingdom. Geological Society of America
Bulletin, 104(11), 1454
Middleton, G V. (2003). ENCYCLOPEDIA of SEDIMENTS and SEDIMENTARY ROCKS.
Kluwer Academic Publishers. P.83
Nichols, G. (2013). Sedimentology and Stratigraphy. New York, NY: John Wiley & Sons.
Appendix
64
Appendix A – All dip measurements Appendix B – All cleavage measurements
65
Appendix C – Dips on each limb of the Tryfan anticline
Appendix D – Palaeoflow directions

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Geology dissertation Tryfan and Glyder Fach Snowdonia

  • 1. i Plymouth University Drake Circus PLYMOUTH PL4 8AA School of Geography, Earth and Environmental Sciences Ordovician Volcanism and Palaeoenvironment Of the Tryfan Area, Snowdonia, Wales January 2020 Word count (full report): 10,957 Word count (bare text): 9,060 Grade = 62%
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  • 3. 1 Abstract, Introduction, Methodology Abstract A twenty-four-day mapping project was completed in an area centred around Tryfan in Snowdonia, Wales. Four Formations represent a period of varying, voluminous, volcanic activity in the Upper Ordovician in a coastal environment. The Cwm Tryfan Formation consists of tuff, shale and volcaniclastic members, lain in a rapidly evolving shallow marine environment with periodic local volcanic activity. Breccia, rhyolite, shale and tuff compose the Tryfan Formation, representing a period of extensive volcanic eruptions S.E of the study area around Snowdonia, deposited in a coastal area. The Llyn Bochlwyd Formation comprises of ignimbrite, shale, volcaniclastic and tuff which were formed in a shallow marine setting with nearby explosive volcanic eruptions. Fossils found within this formation give the age of the geology as 457.5 – 455.8 Ma, in the Soudleyan stage of the Upper Ordovician. A basaltic submarine eruption produced a brecciated pillow lava member within the Clogwyn Formation, representing a change in the chemical composition of the magma source. The Geology of the area was deformed by the Caledonian orogeny producing the Tryfan anticline, and cleavage observed predominantly in shale. Introduction For this project twenty-four days of mapping was conducted around Tryfan in Snowdonia, Wales. A 1:10,000 geological map was produced, and notes were taken in the field. Primary research was used with Secondary sources to produce a report detailing the palaeoenvironment and geological history of the area. During the early Ordovician there was two eruptive cycles (Howells et al. 1991) that were caused by the subduction of the Iapetus Ocean underneath the Avalonian microcontinent (Howells et al. 1991). Volcanism lasted 2-3 million years between 460-455 Ma (Howells et al. 1991), producing mostly rhyolitic rocks in the Cwm Tryfan, Tryfan and Llyn Bochlwyd Formations and some basaltic rocks observed in the Clogwyn Formation.
  • 4. 2 Aims of this report is to explain the primary findings from the field study and to describe formations and members. The report aims to describe the structures within the study area and to explain the palaeoenvironment in the Upper Ordovician. Methodology Twenty-four days of mapping in the summer was completed around the area North if the A5 to the East and West of Tryfan, and to the area around the peak of Glyder Fach in the South. A combination of surface and outcrop mapping was used. A map was produced to 1:10,000 scale in the field, and data was taken with a Compass Clinometer and recorded in a notebook. The base map was based on the 1976 Ordnance Survey map. Thin sections were used to study some of the members, and rock samples were collected from the field. Secondary research was used with primary data to produce an interpretation of the geology and geological history. A risk assessment was completed prior to starting the field work. A lack of thin sections for all of the members is a limitation. Only a small number of dip measurements was collected on the Eastern side of the Tryfan anticline, this could have reduced the accuracy of the stereonets, and interpretation made of the fold including the axial plane. Some areas of the field were inaccessible due to topography.
  • 5. 3 Formation and Member Descriptions Cwm Tryfan Formation The Cwm Tryfan Formation is found in many exposures around the valley to the East of Tryfan consisting of conformable shale volcaniclastic and tuff and is 500m thick. An anticline is found within the centre of the formation, increasing its apparent thickness. This is the oldest of the mapped formations and it has a conformable upper boundary with the Tryfan Formation. In literature this formation is part of the Gwern Gof Tuff Formation (Howells et al. 1991). Figure 1; A map showing the location of the Cwm Tryfan Formation N
  • 6. 4 Cwm Tryfan Tuff Member Overview The Cwm Tryfan Tuff Member is found from pont Wern-gof on the North side of Cwm Tryfan GR: 674/605, to the South side GR: 667/506. It is found in two beds with conformable boundaries with the Cwm Tryfan Volcaniclastic, and Cwm Tryfan Shale. It is faulted in the South side of Cwm Tryfan. The thickness of the member is 150-300m. There is a conformable boundary between tuff and the shale members in the centre of Cwm Tryfan at GR: 670/595, this is one of the type localities for this member. The other type locality has clear cross bedding at GR: 674/596. Description In large exposures the Cwm Tryfan Tuff Member is dark grey and are massive exposures, with visible bedding which is 1-2cm thick. The exposures are generally large on the scale of 2-10m. It generally has a fine ash matrix and is well sorted (Fig.2). However, there are some exposures that have angular medium-coarse grains that are poorly sorted. Some exposures have clasts between 5-50mm (fig.3) and make up to 50% of the rock mass. These are quartz feldspar in composition. The grains in this exposure are medium grain and are angular. There is 60% visible quartz and 30% weathered feldspar grains. Other accessory minerals make the rest of the rock mass. In fresh exposures it has a pale green colour with white weathered angular plagioclase feldspar grains. Most beds have clear bedding (Fig.2), and in one exposure at GR: 674/596, there is visible cross bedding (Fig.4 + Fig.5). This shows a coarsening upwards sequence and a palaeoflow towards 160° S. Interpretation Cross bedding and sedimentary structures in an exposure at GR: 674/596, show that some of the Cwm Tryfan Tuff Member has been reworked (Howells et al. 1991). The cross bedding indicates that is was deposited in a shallow marine environment with enough wave energy to produce cross bedding, however there was interludes with low enough wave energy to produce shale (Plint, 1995) elsewhere in the Cwm Tryfan Formation. The cross bedding indicates that the member is younging to the N.W, and there was a N.E-S.W trending basin (See pg.52). Howells et al. 1991, agrees with this hypothesis, stating that the formation was deposited in shallow-marine conditions on a Westerly facing palaeoslope. The grains in this area are angular, indicating that they are immature have not been transported far from its original source. The grey colour of the exposure and the presence of
  • 7. 5 quartz feldspar indicate that the Tuff was from the same rhyolitic primary source as the volcanic materials observed in the Tryfan and Llyn Bochlwyd formations. The fine ash matrix in most of the exposures show that this member was likely formed by an ash-fall (Howells et al. 1979) (Howells et al. 1991) (Fig.43). Consecutive thin ash layers produce finely laminated exposures (Fig.2). The member has no examples of being welded. This member was formed as a result of a nearby series of eruptions which deposited beds of ash in a clastic and shale dominated shallow marine environment. Due to a high rate of deposition the youngest part of the member on the East side, where cross bedding was observed, was deposited sub aqueously where the upper layers were deposited sub aerially. A log (Fig.5) of the exposure showing trough cross bedding (Fig.4) at GR:674/596, shows a coarsening upwards sequence. This indicates an increase in depositional energy (Hunt and Tucker, 1992), which could be the result of regression. In literature this member is referred to as the Gwern Gof Tuff Formation (Howells et al. 1991). The member, and the rest of the formation, is younging to the N.W, so it is both younger and older than the Cwm Tryfan Shale and Cwm Tryfan Volcaniclastic Members. Figure 3; A tuff exposure that has visible clasts and has a different texture to other tuff exposures within the Formation. GR: 668/590. E W Figure 2; An exposure of Tuff showing clear bedding and has a well sorted ash matrix. GR: 671/597. EW
  • 8. 6 N Figure 4; A picture showing clear cross bedding in a tuff exposure. Palaeoflow to A log of this area is in Fig.5. GR: 674/596 S
  • 9. 7 Figure 5; A stratigraphic log showing an exposure of Cwm Tryfan Tuff, with clear planer bedding and trough cross bedding Length(cm)
  • 10. 8 Cwm Tryfan Volcaniclastic Member Overview The Cwm Tryfan Volcaniclastic Member is found in many outcrops around Cwm Tryfan. From Tryfan Fach in the North GR: 671/601, to the ridge to the South of the valley at GR: 666/584. It has conformable boundaries with the Cwm Tryfan Shale and Tuff Members and the thickness of this member is 200m. The type locality for this member is at the base of Tryfan Fach GR: 671/601, as there are many clear easily accessible outcrops around the area. Description Large, 3-10m, weathered exposures of this member are dark grey and have clear bedding structure. The bedding is usually on the scale of 0.5-1m, however some exposures show individual beds as fine as 5-10mm. Some exposures show clear cross bedding (Fig.6) and trough cross bedding (Fig.7). It is medium grained in most exposures and is well sorted. Yet other exposures have small layers that a very coarse and poorly sorted (Fig.6). Less than 1% of exposures have some clasts that between 2-10mm (Fig.6), these have no preferred orientation. The grains are generally well rounded to sub angular. Quartz is visible and makes 20-30% of the rock mass, and white weathered feldspars make 10% of the rock mass. There are 10% green chlorite grains in some exposures. Figure 6; A clear exposure of cross bedding below planar bedding. Palaeoflow direction of 116° S.E. GR: 671/590. Photo courtesy of Loizos Geordgiades. 20cm N.W S.E
  • 11. 9 At GR: 672/597, within a bed of the Cwm Tryfan Shale Member there is a 3-5m thick volcaniclastic layer. It is observable around 50m in length and has a clear conformable boundary with the shale member (Fig.8). The clastic within 30cm of the boundary has clear cleavage similar to the shale which gets weaker further away from the boundary. Figure 7; An exposure showing clear examples of trough cross bedding in a large exposure of volcaniclastic. GR: 666/585. E W 50cm
  • 12. 10 Interpretation Cross bedding, trough cross bedding and mature, sub-rounded grains indicate that the original material has been reworked. Medium-coarse grains show that it was deposited in high energy environment. Cross bedding was used to produce a palaeoflow direction of N.W-S.E, which helps prove the presence of a N.E-S.W trending welsh basin (See pg.52). It also indicates that the younging direction is to the N.W which is confirmed by structures seen in the Cwm Tryfan Tuff Member and the Llyn Bochlwyd Formation. In literature this member is referred to as being part of the Gwern Gof Tuff Formation (Howells et al. 1991). The original material likely originated from the edge of the Snowdon caldera (Howells et al. 1991). The depositional environment for this member is a shallow marine shelf environment, 0-30m below sea level and inner-infralittoral (Howells et al. 1991). A log of a volcaniclastic exposure (Fig.9) shows a coarsening upwards and fining upwards sequence indicating a change in the energy of the environment (Hunt and Tucker, 1992). It shows how the palaeoenvironment of the area varied over time. Figure 8; A conformable boundary between the Cwm Tryfan Volcaniclastic Member and the Cwm Tryfan Shale Member. The volcaniclastic shows cleavage close the boundary. GR: 672/597 N.WS.E 50cm
  • 13. 11 Figure 9; A stratigraphic log showing a volcaniclastic exposure showing variation between different environments. Length (cm)
  • 14. 12 Cwm Tryfan Shale Member Overview The Cwm Tryfan Shale Member is found in two faulted beds in Cwm Tryfan. It bounds both clastic and tuff and has a visible conformable boundary with the Cwm Tryfan Tuff Member at GR: 670/595. Most of its exposures are highly eroded relative to other members in the area. This makes many areas of shale appear as gullies, and have vegetation covering the outcrops. The thickness of the Westerly bed is 80m, and the larger bed has a thickness of 200m. The type locality for this member is an exposure near the path to the North of Tryfan Fach GR: 672/600. Description Exposures are generally small, on the scale of 1-5m, and appear dark grey to black and have clear slaty cleavage and some show faint bedding. The cleavage is 5-20mm thick. Exposures have some rusty coloured rocks. It has a silty grain size and is very well sorted. There are visible quartz grains that make 20-30% of the rock mass, and other grains are too fine to be identified. In fresh exposures it is dark blue/grey in colour. There are no clasts and no fossils found in this member. At GR: 672/597 there is a thin, 10 metre, volcanic clastic exposure within a larger shale bed. There is a clear exposure showing a clear conformable boundary between the shale and the Cwm Tryfan Clastic Member (Fig.8). The clastic within 30cm of the boundary shows faint cleavage. Towards the south side of Cwm Tryfan GR:669/589 there is a shale exposure that is heavily weathered and white in appearance (Fig.10). This indicates a high percentage of feldspar in this exposure. The exposure also has two different visible cleavages. Interpretation Shale is silty grained which indicates that it was deposited in a low energy environment. This is different to the other members of clastic and tuff that are found within this formation. Black shale is often deposited in anoxic waters (Middleton, 2003). There are no fossils found within this member which provides further evidence that it was formed in an anoxic environment. This information indicates that the palaeoenvironment was offshore or lagoonal. Other members within the Cwm Tryfan Formation were deposited in a near shore environment indicating that this member was to, leading to the conclusion that this member was deposited in a lagoonal environment rather than an offshore zone.
  • 15. 13 The area around GR:669/589 shows two distinct cleavages. Some other exposures in the surrounding area also show this. This could indicate there has been pressure applied to this area in different directions in different periods (See pg.48). W E 20cm Figure 10; An exposure of shale that is weathered white with a high proportion of feldspar. There are also two visible cleavages.
  • 16. 14 Tryfan Formation The Tryfan formation consists of four conformable members, three of which are volcanic in origin. A period of high levels of volcanic activity is represented by this formation, with the presence of rhyolite flows, rhyolitic breccias and tuffs. The Tryfan formations boundaries are at the base of the East and the West of Tryfan, just South of the peak of Glyder Fach, and the Northern side of Tryfan. In literature, this formation is named the Capel Curig Volcanic Formation. Figure 11; A map showing the location of the Tryfan Formation.
  • 17. 15 Tryfan Rhyolite Member Overview The Tryfan Rhyolite member is found continuously from the North base of Glyder Fach, to the North side of Tryfan. It is also in the south east of Tryfan and is found in a large, almost circular area. There is a visible boundary between rhyolite and a tuff exposure, and a visible sudden change between the two members. There is also a boundary that is visible on the peak of the Far South Peak of Tryfan GR: 663/592, in the direction of S.W-N.E, between rhyolite and tuff. The bed which crosses N-S across Tryfan, has an apparent thickness of 100-250m. The type locality of this members boundary with the Tryfan Tuff Member is on the North ridge of Tryfan at GR: 665/601. Description Rhyolite appears light grey in colour in weathered exposures with no cleavage. The exposures are generally massive, not faulted and on the scale of 2-20m. Most of the exposures, have clear flow banding (Fig.12) which distinctly differentiates rhyolite from other rock types in the area. The observed flow banding was on the scale of 2-5m and the majority was tight to isoclinal and upright (Fig.13). Figure 13; Different rhyolite flow banding, with the axial plane marked with a red line. This shows that they’re isoclinal and upright. This could indicate that the flow was roughly N-S or vice versa. W 1m E 2m 50cm E Figure 12; showing a clear example of flow banding in a Tryfan Rhyolite Member exposure on the south side of the far south peak. GR: 661/589 1m NS
  • 18. 16 In fresh exposures rhyolite is a dark grey/blue colour and the crystals are poorly sorted and medium/fine in size, with an aphanitic ground mass. A combination of field observations and microscope analysis show that there is around 70% quartz, 10-50μm in size (Fig.15), which makes up the groundmass of the rock. Plagioclase feldspar crystals are weathered white, make up 15% of the rock, and are between 500-1000μm (Fig.15). Orthoclase feldspar crystals are between 200-1000μm, sub-rounded, and represent 10% of the rock mass. Under the microscope the feldspar crystals are euhedral (Fig.15). There are around 5% minerals that are difficult to identify but have a second order interference and are colourless in plain light, which could indicate that they are muscovite mica. There are no visible structures under the microscope. Some exposures have >10% angular rhyolite clasts that are around 1cm in size. There are some bombs visible closer to the rhyolitic breccia, around the peak of Glyder Fach, which are circular and around 5cm in diameter, and get more numerous closer to the boundary At GR:656/586, on the N.W slope of Glyder Fach, there is an exposure, 25m in length and has accessible exposures on the base of the cliff face, that has a different lithology to the surrounding rhyolite. The exposure is not large enough by itself to be mappable. In weathered exposures it has a pale pink colour and a darker pink in clear exposures, this is caused by the presence of orthoclase feldspar. It has roughly 5% black minerals, that are Figure 15; A view under the microscope of a plagioclase mineral surrounded by an aphanitic quartz groundmass. GR: 666/587
  • 19. 17 biotite. Moving east from these exposures the visible exposures start to show characteristics common of rhyolite described above. This is a conformable change without a hard boundary, the visible orthoclase gradually becomes sparser, until the exposures show the same characteristics described above. This is a small exposure of micro-granite. In some exposures of the rhyolite, within 20m of the upper boundary to tuff on the western side of the Tryfan Rhyolite Member, there is a quartz nodular horizon (Fig.14). The nodules are densely packed and range between 5-40mm in diameter. These exposures have a very week visible layering. The nodular horizons are around 20-40m in size, and the type locality is on the north ridge of Tryfan, around 665/600. Columnar jointing is found above heathers terrace on the East side of Tryfan. Interpretation On the south side of the Tryfan Rhyolite Member, there is a rhyolitic breccia member which has exposures on the peak of Glyder Fach GR:659/584. This was formed during the same period of volcanic activity as the Tryfan Rhyolite Member, and from the same source material (Howells et al. 1979). The boundary is conformable, however due to the terrain none of the boundary is easily accessible. The relationship between the breccia and the Tryfan Rhyolite Member, shows that the source volcano was to the south of this area (see pg.58). The whole area is younging to the N.W.W, so the Tryfan Rhyolite Member is younger than the observed members, and formations to its west.
  • 20. 18 Figure 14; Pictures of an area with a nodular horizon on the boundary of tuff and rhyolite. GR: 665/600. 20cm N.W N S S.ES N 1m 50cm
  • 21. 19 Glyder Rhyolitic Breccia Member Overview The Glyder Rhyolitic Breccia Member is found in a series of outcrops, around the peak of Glyder Fach. The member is 700m from E-W and has a boundary around 50m to the east of Castell y Gwynt. There is a visible unconformable boundary between the Cwm Tryfan Shale member and the Glyder Rhyolitic Breccia Member, on the East side of Glyder Fach at GR:660/596. The breccia has less clasts, the closer to the boundary. East of the peak of Glyder Fach there is an exposure which shows all the characteristics of the Glyder Rhyolitic Breccia Member, making it the type locality GR: 659/584. Description The visible outcrops are light grey, have little structure and are relatively large, up to 30m. There are large angular fragments, that range between 1-50cm (Fig.16), and are easily visible, in most outcrops the fragments protrude from the matrix (Fig.17). In the majority of outcrops these fragments make around 50% of the rock, but closer to the boundary with shale, on the S.E side of the exposure, it is around 20%. The matrix of this member is light grey, and fine grained. There are some visible quartz crystals, that make roughly 60% of the matrix. The clasts have the same lithology as the Figure 17; A large outcrop, that has many angular fragments and is representative of the rest of the exposures. GR: 657/583 W 1m Figure 16; An example of a large angular clast (30cm), which protrudes from the matrix. GR: 658/584. Photo courtesy of David Rowlands. E
  • 22. 20 Tryfan Rhyolite Member. Some exposures show a faintly visible flow structure (Fig.18), indicating that it is an epiclastic volcanic breccia (Fisher, 1960). The fragments are 1-50cm and are angular. Interpretation The Glyder Rhyolitic Breccia Member was formed during the same period of volcanic activity as the Tryfan Rhyolite Member. This is shown by the breccia having the same mineral composition of the rhyolite and is hypothesised by (Howells et al. 1979). A reason that this area is a breccia and there are no clasts further along the flow, into the rhyolite could be that a topographic feature obstructed the flow (Howells et al. 1979). In (Fitch, 1967), it is hypothesised that there was a volcanic vent situated somewhere on what is now Glyder Fach (See pg.55). This could mean that the centre of the eruption was where this member is, rather than towards Snowdon as originally hypothesised. Volcanic breccia’s around Snowdonia are discussed in Howells et al. 1979 and Howells et al. 1991, refers to the outcrop as GFB, Glyder Fach Breccia. Figure 18; A picture of a volcanic breccia outcrop that shows faintly visible flow structure. The red lines indicating the direction and shape of the flow. GR: 659/584. Photo courtesy of David Rowlands.
  • 23. 21 Tryfan Rhyolitic Tuff Member Overview The Tryfan Rhyolitic Tuff Member is found flanking both sides of the Tryfan Rhyolite Member, separated in some small areas by the Tryfan Shale Member. It is found on the North of Tryfan near Llyn Ogwen GR:664/602 to Castell Y Gwynt, near the peak of Glyder Fach GR:654/582. It has an observable boundary with the Tryfan Rhyolite Member (Fig.19) GR: 665/600. This boundary is also visible on the peak of the Far South Peak GR:663/591. The boundaries with the Tryfan Rhyolite Member and the Glyder Rhyolitic Breccia Member are both not directly visible in the field. The outcrop thickness of this member is 150-300m, with a true thickness of 300m for the bed to the East of Tryfan and 150m for the Bed to the West of Tryfan. The type locality of the Tryfan Rhyolitic Tuff Member is on the North base of Tryfan GR:668/603. E W 2m Figure 19; A view facing upwards onto the North ridge of Tryfan. The top right of the exposure is the Tryfan Rhyolitic Tuff Member, and the bottom left is the Tryfan Rhyolite Member. GR: 665/600
  • 24. 22 Description Tryfan Rhyolitic Tuff appears light grey and massive in exposures, most have clear fracturing and exposures are up to 50m in size. The exposures have faintly visible bedding, on the scale of 5-20cm. Weather feldspars often give the tuff a white appearance, and it is light grey in fresh exposures. It has a poorly sorted fine ash grain size and has between 10-20% coarse grains. Angular quartz grains make up 20% of the rock mass. Some exposures have 5-10mm lapilli that make up to 20% of the rock mass, these are very fine grained with no identifiable minerals. Pores around 1-2mm are visible in some fresh outcrops. Rarely, >1%, there are quartz feldspar clasts up to 5cm. Interpretation A light grey colour and the presence of quartz and feldspar indicate that the material producing this tuff was originally from the same volcanic source that produced the Tryfan Rhyolite Member. The angular fragments, faint bedding, lack of cross bedding, indicate that the Tryfan Rhyolitic Tuff member is a lapilli fallout tuff (Fig.43). This is because there is no evidence that the material has been reworked as is seen in the Cwm Tryfan Tuff. This member is not a welded tuff as it has no visible fiamme, and none of the fragments are welded. This was the result of explosive eruption with a high plume. The Tryfan Rhyolitic Tuff Member was lain before and after the Tryfan Rhyolite Member. The time between the two beds being deposited was not long, as there is only a small amount of shale deposited either side of the Tryfan Rhyolite Member. The layers within this formation are younging to the N.W.W. In literature this member is described under the name the Capel Curig Volcanic Formation (Howells et al. 1991). It is interpreted to have been erupted from the Llweyn centre to the North of the study area.
  • 25. 23 Tryfan Shale Member Overview The Tryfan Shale Member is found in thin beds, bounding both sides of the Tryfan Rhyolite Member between the rhyolite and the Tryfan Tuff Member. It is found around the peak of Tryfan GR:666/596, and it is exposed on the bottom of the North ridge of Tryfan GR:667/603. This member is generally significantly more eroded than the Tryfan Rhyolite Member and the Tryfan Tuff member. This means that the Tryfan Shale Member is often seen as a gully and has a lower topography than the surrounding exposures. The Tryfan Shale Member has a conformable boundary with the Tryfan Tuff Member, and the Tryfan Rhyolite Member. The type locality for these boundaries is on the North ridge of Tryfan GR:665/598. The beds are 20-50m thick. Figure 20; A view to of Tryfan with a layer of Tryfan Shale Member, seen with a lower topography than then surrounding Tryfan Rhyolite Member (left) and the Tryfan Tuff Member (right). GR:665/595. N.W S.E
  • 26. 24 Description Most of the Tryfan Shale Member is eroded more than the surrounding members and often appears as small gullies (Fig.20). In clear exposures it appears dark grey and has clearly visible cleavage. The Tryfan Shale Member is very well sorted and has a silty grain size, with 10% visible quartz grains. There is a clear slaty cleavage with layers between 2-10mm. It is Dark grey/blue in clear surfaces, and some weathered exposures have a faint red rust colour. There is no evidence of fossils found in this member. Towards the south of Tryfan, the shale pinches out. Interpretation The Tryfan Shale Member is found between two different volcanic rock types, rhyolite and tuff. Both members appear to be lain extrusively, indicating that the Tryfan Shale Member was deposited in relatively shallow water or a flood plain, and not in deep water. The presence of water lain members within the same group indicates that this member, and formation, was deposited close to the sea, which is evidence that it was deposited in a coastal lagoonal environment (Howells et al. 1991). However, the member could have been lain in a floodplain or river delta (Arthur and Sageman, 1994). This member is found above and below the Tryfan Rhyolite Member meaning it was deposited before and after the volcanic eruption that produced the member. (Howells et al. 1991) describes this member as a siltstone intercalation.
  • 27. 25 Llyn Bochlwyd Formation The Llyn Bochlwyd Formation is found from the Western base of Tryfan, to Y Gribin, West of Llyn Bochlwyd, and consists of four different members. It represents a gap in volcanism, and several large eruptions towards the Western side of the formation. The thickness of this member is between 600-900m. Figure 21; A map showing the exposures of the Llyn Bochlwyd Member
  • 28. 26 Bochlwyd Shale Member Overview The Bochlwyd Shale member is found in many beds around Llyn Bochlwyd, from GR: 658/601 to GR: 653/583. It has conformable boundaries with the Bochlwyd volcaniclastic member and the Bochlwyd Felsic Tuff Member. Many of the outcrops of this member are highly weathered and appear with a lower topography than the surrounding members. The thickness is between 50-150m and the type locality for this member is a small outcrop near a path around GR: 658/600, where Brachiopod fossils were found. Description The shale appears grey to brown in colour with fine, 2-10mm, clearly visible slaty cleavage in relatively small exposures around 1-5m. It has silty to very fine grains, that are very well sorted, with no clasts. Thin section analysis shows a silty groundmass (Fig.22) which is quartz and feldspar which constitutes 50% and 10% of the rock mass respectively. Larger quartz grains, around 100μm, make around 15% of the rock mass. A highly deformed mineral that has high second order interference colours, makes up 15% of the rock mass. This is likely muscovite mica; however, it is difficult to specify. Around 10% of the rock is not identifiable from a thin section. Figure 22: A picture of a thin section of shale from the Bochlwyd Shale Member, showing a silty grain size. GR: 658/598.
  • 29. 27 At GR: 658/600, just south of Llyn Ogwen, there is a series of small outcrops that have numerous visible brachiopod (Fig.23). These appear to be from one species that have between 16-18 ribs and are 2-2.5cm in size. A poorly preserved fossil found could be part of a coral fossil or a bryozoan. Interpretation To the West of Llyn Bochlwyd there is a series of beds of this member and the Bochlwyd Felsic Tuff Member. This indicates that there was a series of repeated changes in the area that resulted in the shale being deposited rather than the Tuff. This could be a series of intermittent explosive volcanic events depositing beds of tuff in an offshore marine setting. The fossils found within this member likely belong to the taxon of Dinorthis berwynensis. (Howells et al. 1991) shows that this taxon of brachiopod is found in the Lower Rhyolitic Tuff Formation, which is the name used in some literature for this formation, and the specimens have an equal number of ribs, 16-18, as the samples found in the shale at GR: 658/600 (Fig.23). The age of this taxon is limited to 457.5 to 455.8 Ma (Howells et al. 1991). This gives an accurate age of this member and formation. The presence of organic matter differentiates this from the black shale that is found within the Cwm Tryfan Formation, that was anoxic with no fossils. A small part of what could be a fossilised Coral or Bryozoa, may show that the depositional environment for this member was in the photic zone close to the shoreline. Howells et al. 1991 states that the presence of this taxa of fossil shows that the depth of the water is less than 10m. Figure 23: Dinorthis berwynensis found around GR:658/600.
  • 30. 28 Shale is representative of a low energy environment as it is fine grained. It is also known to be a marine environment, as there are brachiopods found within the member. This could prove that the member was deposited in a lagoonal environment or extensive tidal mud flats (Arthur and Sageman, 1994). (Howells et al. 1991) states that some siltstone beds in this area are formed by low density turbidity flow currents and are deposited at a depth between 60-100m. This is unlikely however due to the presence of fossils and the nearby Bochlwyd Volcaniclastic Member that indicates a shallow, nearshore environment. However, the beds of the Tuff to the West of the ignimbrite may have been deposited in a deeper, offshore environment.
  • 31. 29 Bochlwyd Volcaniclastic Member Overview The Bochlwyd Volcaniclastic Member is found across the formation, conformably bounding all of the members in the Llyn Bochlwyd Formation. It is near to Ogwen Cottage in the North to close to Bwlch y Ddwy Glyder in the South. The typical outcrop is relatively large from 5- 20m. The type locality for this member is just to the south of Llyn Ogwen GR: 655/602 and has cross bedding. Description The boundaries between the other members within this formation are conformable. The conformable lower boundary with the Bochlwyd Ignimbrite Member is observed in two locations once to the East of Llyn Bochlwyd GR: 656/593, and again at GR: 655/600. The beds have a thickness of 70-100m to the West of Llyn Bochlwyd and 200m to the East of Llyn Bochlwyd. To the south of Llyn Ogwen, the volcaniclastic appears to be up to 600m thick, however there is not many exposures in this area. This could also be as a result of the secondary fold (See pg.48). In large outcrops the member appears massive with a light grey in colour and has some rusty red colour. Bedding is obvious on a large scale between 0.5-3m and some exposures have visible cross bedding which shows a palaeoflow to the N.W and the S.E (See pg.52). It also certifies that the members and formations in the Tryfan area are younging to the N.W.W. Fresh exposures appears dark grey in colour (Fig.24) and are medium to coarse grained and is poorly sorted. Plagioclase crystals are visible in clear exposures as white grains mostly around 1mm, and up to 5mm. Thin section analysis shows that there is quartz grains that are 200-500μm in size, and a quartz dominated groundmass, meaning quartz makes around 70% of the rock mass. Plagioclase is clearly visible with distinguishing polysynthetic twinning (Fig.25), which consists of around 20% of the rock mass. A mineral that is generally very small, ~50μm, appears with a second order interference colour and is highly deformed. This is difficult to specify the mineral however it is likely a muscovite mica, which comprises 5% of the rock mass. There is around 5% unknown accessory minerals. The visible grains have no preferred orientation and are mostly sub-rounded with >20% angular grains. Interpretation Fossiliferous Bochlwyd Shale was deposited during the same geological period as the volcaniclastic, and cross bedding was found in many exposures of the member. This indicates that the Bochlwyd Volcaniclastic Member was deposited in a shallow marine
  • 32. 30 environment, above the fair-weather base (Howells et al. 1991). A lack of bioturbation and muds within this member gives evidence that it was deposited in a period of high sedimentation. This also provides a reasoning for the lack of fossils within this member, when they are found on the lower boundary of the Bochlwyd Ignimbrite Member, in a death assemblage (See pg.33), and in a bed of shale which has upper and lower boundaries with volcaniclastic. The death assemblage had Orthid brachiopods which shows that it is a shallow marine depositional environment (Howells, 1991). The mineralogical composition of the member is similar to that of the Tryfan Rhyolite Member. This implies that the source of the volcaniclastic material is extrusive rhyolite, which is younger than the Bochlwyd Volcaniclastic Member. (Howells et al. 1991) concludes that this material is eroded from the Western edge of the caldera and the tuffs formed by the volcanic activity and that it was deposited in a marginal marine environment. The sub rounded and angular grains prove that this is a rather immature sediment and shows it was deposited close to the material source with little transportation, implying that the material originated from the nearby caldera and tuffs. 5cm Figure 24; A fresh exposure of volcaniclastic. The visible white grains are plagioclase. GR: 652/585
  • 33. 31 10-50cm thick graded beds are found in an area to the S.W of Llyn Bochlwyd (Fig.26). These show a fining upwards sequence with a planar top. (Howells et al. 1991) says that these may indicate turbidity-current deposition, however it is specifically talking about exposures to the West of this member. A lack of large clasts likely disproves this hypothesis, nevertheless it does give evidence for changing relative sea level (Howells et al. 1991). In literature (Howells et al. 1991) this member is part of the Cwm Eigiau Formation and the Lower Rhyolitic Tuff Formation. Plagioclase Large quartz grain Figure 25; A thin section of a volcaniclastic sediment, with clear quartz and plagioclase grains.
  • 34. 32 Length(m) Figure 26; A stratigraphic log showing a transgressive event and a series of different nearshore environments that resulted in the deposition of this member.
  • 35. 33 Bochlwyd Ignimbrite Member Overview The Bochlwyd Ignimbrite Member is found from Ogwen Cottage to Bochlwyd Buttress, to the S.W of Llyn Bochlwyd. It has an observed thickness between 100-250m. The outcrops are massive and generally on the scale of 5-10m, and up to 60m on Bochlwyd Buttress. The Bochlwyd Volcaniclastic Member bounds the Bochlwyd Ignimbrite Member on both sides. The type locality for this boundary is visible on the top of a small hill to the East of Llyn Bochlwyd GR: 656/592. Description The type locality for this member is between Ogwen Cottage and the Bochlwyd Butress GR: 654/590. Bochlwyd Ignimbrite Member outcrops appear grey in large scale, with no structures, and a rough texture. Weathered feldspars show as white patches in some parts of the outcrops. Some exposures have clasts from 1-30cm, that make up most of the rock mass. The matrix of the rock is fine grained, well sorted, and light grey in colour. The matrix has quartz and feldspar. There a visible Fiamme, around 1-5cm in some exposures that are black flat structures (Fig.27). The member has a varying amount of clasts, which get sparser from the East to the West side of the bed. The clasts are rhyolitic in composition. There is an exposure on the East side of Llyn Bochlwyd where the Bochlwyd Ignimbrite Member has a boundary with the Bochlwyd Volcaniclastic Member. Here there are clasts that make 90% of the rock mass (fig.28). Some exposure have visible quartz nodules, that are between 1- 10cm. Vesicles are found in some exposures around GR: 654/590, which are 1-5mm in size. Large quartz veins are that cut through the exposures. Some welded fragments show that this is a welded ignimbrite. At GR: 654/590 there is an exposure of the boundary between the ignimbrite and the Bochlwyd Volcaniclastic Member, with a death assemblage. It is around a metre thick and was full Orthid brachiopods. There was no fluidal structures in this member. Interpretation A stratigraphic log (fig.29) shows the top of the ignimbrite member. This shows a bed containing fossils on the bottom of the layer, showing it is younging to the N.W. Fiamme, flattened pumice fragments and a lack of fluidal structures show that this Member was formed by a Pyroclastic density current (PDC) (Fitch, 1967). The death assemblage at GR: 654/590 was caused when the pyroclastic flow covered a marine environment, causing
  • 36. 34 many Brachiopods to become fossilised at the base of the member. This, along with the change in density of clasts moving East to West, shows that the member is younging to the N.W. This was likely a shallow marine environment, similar to that found in the Bochlwyd Shale Member. Meaning that the bottom of the PDC was deposited in a marine environment, then built up above the water. Howells et al. 1991, calls this member the Pitt’s Head Tuff. (Fitch, 1967) predicts that the vent for the source if this PDC was on the peak of Glyder Fach. However, (Howells et al. 1991) states that the ignimbrite was erupted from the Llwyd Mawr volcanic centre to the S.W with significant evidence to back this up. Figure 27; Black Fiamme structures seen in a fresh exposure of the Bochlwyd Ignimbrite Member
  • 37. 35 Figure 28; An exposure of the ignimbrite at its lower boundary with a high percentage of clasts compared to other exposures. GR: 656/592. W E
  • 38. 36 Figure 29; A stratigraphic log showing 62m of the bottom (youngest side) of the Bochlwyd Ignimbrite Member. Length(m)
  • 39. 37 Bochlwyd Felsic Tuff Member Overview The Bochlwyd Acidic Tuff Member is found in four, relatively thin beds to the West of Llyn Bochlwyd. The beds have an apparent thickness of 50-150m and are repeatedly bedded with the Bochlwyd Shale Member. This often means that this members outcrops appear 2- 10m in size as they are less weathered than the surrounding beds. The type locality for this member is found to the West of Llyn Bochlwyd at GR: 654/594. Description Outcrops of the Bochlwyd Felsic Tuff Member are light grey and have bedding that is visible on a larger scale. The Matrix is made of medium ash, which is very well sorted. There was no bombs or large clasts found in this member. The boundaries with the Bochlwyd Shale Member are covered with vegetation, but they are conformable boundaries. The light colour indicates a high percentage of weathered feldspar; however an exact percentage is difficult to obtain. Interpretation Several repeated beds with the Bochlwyd Shale Member, and the conformable boundaries between them indicate that there was a series of Parasequences or volcanic events that deposited layers of ash, forming tuff. This member was likely lain by a subaqueous ash flow (Howells et al. 1991). Shale is bounded to this member, which is deposited in a deep marine environment. This may show that the tuff was deposited in the same low energy environment as shale. A lack of cross bedding and other sedimentary structures provides evidence for this hypothesis. In literature this member is part of the Lower Rhyolitic Tuff Formation and is often referred to as an acidic tuff. However, acidic is an outdated term and it should be referred to as felsic.
  • 40. 38 Clogwyn Formation Found on the Western side Llyn Bochlwyd, this formation represents a period of basaltic volcanic activity. The formation overlies the Llyn Bochlwyd Formation and is the youngest of the studied formations. It shows a difference in material than the formations to the East, as they were mostly rhyolitic. However, it was still formed during the same period of volcanic activity in the second eruptive cycle (Howells et al. 1991) (See pg.57) Clogwyn Basalt Member Overview The Clogwyn Y Tarw Basalt Member (CTBM) is observed on to of a ridge to the West of Llyn Bochlwyd GR: 651/594, this is also the type locality for the CTBM. The outcrops are generally large, 5-10m, and it has a thickness of around 70m (Howells et al. 1991). It overlays the Llyn Bochlwyd Formation to the East. The outcrop is independent on the topography, which limits the expanse of it. Description In large outcrops the CTBM has a very chaotic structure (Fig.30) and can vary largely between the different outcrops. It has an unclear mafic pillow lava, on the scale of 1-2m and is highly brecciated (Fig.30). It is grey with a faint red colour in more massive blocks. Between the pillows is a silty grained infill (Fig.31) which is dark grey. The massive blocks are angular and generally between 1-20cm in size. One exposure shows a more massive structure with one block being near 3m (Fig.32). There is a structure that is near vertical, around 80° to the East, which is too variable to be measured. The rock mass has a very fine- grained aphanitic texture, with <10% phenocrysts that are feldspars and a Interpretation Pillow lavas are visible within this member, and it is mostly very fine grained, which indicates that this was formed during a sub marine volcanic eruption (Howells et al. 1991). This is also supported by the fact the CTBM has an unconformable lower boundary with shale, which was deposited in a marine environment. In literature this member is described as being basaltic, based on field observations in other areas, especially towards Snowdonia (S.W), and chemical analysis (Howells, 1991). This can explain the minerals of pyroxene and feldspar that were observed in the field. Howells et al. 1991 describes this member as auto- brecciated which would explain the chaotic structure. Despite the difference in composition, this member was formed during the same period of volcanic activity as the formations to its East, as described by (Howells et al. 1991). In
  • 41. 39 literature the CTBM is part of the Bedded pyroclastic Formation (BPF). Rhyolitic materials were erupted before after and during the deposition of the BPF, within the local area (Howells et al. 1991). This indicates that this member was erupted from a different vent than the rhyolitic materials, hypothesised by (Howells et al. 1991). The CTMB is likely close to the original source vent (Howells et al. 1991), which was likely eroded away due to post depositional events. W E 50c m Figure 30; A large outcrop of CTBM showing a heavily brecciated structure, with a vertical flow structure GR: 651/594.
  • 42. 40 E 20cm W Figure 31; A dark grey very fine grained, highly laminated, rock forming the infill between the pillows GR: 651/594.
  • 43. 41 E W 2m Figure 32; A large exposure of CTBM, showing a vastly different appearance to the exposure shown in Fig.1. The blocks are massive, with no structure GR: 651/594.
  • 44. 42 Structures Figure 33; A map and cross section showing the structures of the study area.
  • 45. 43 Dip Measurements All the dip measurements are between N.W.W and S.E.E, the largest proportion show a relatively high dip towards the N.W.W, found to the West of the Tryfan anticline. Some are dipping shallowly to the S.E.E that are taken from the Eastern side of the Tryfan anticline. Figure 34; A stereonet showing all dip measurements taken in the field. Plotted poles to planes. (Appendix A).
  • 46. 44 Tryfan Anticline An anticline is found on the East side of Tryfan, in literature it is known as the Tryfan Anticline. A stereonet of dip measurements taken around both limbs of the fold (Fig.34) shows a clear trend of dips to the N.W.W, found on the Western limb, and S.E.E, found on the Eastern Limb. The stereonet also shows that there was a dip direction towards the S.S.W, which indicates the Tryfan Anticline is plunging in this direction, with a shallow angle between 4-8 degrees. The dips to the West of the anticline are higher than the dips to the East of the anticline indicating that the axial plane is inclined to the N.W.W. Figure 34; A stereonet showing dip measurements from either side of the Tryfan Anticline, and the data used in the stereonet. The Measurements are all within the Cwm Tryfan Formation.
  • 47. 45 Cleavage Measurements Cleavage measured in the mapping area is consistent and shows a single trend, steeply dipping to the N.W. Cleavage is predominantly taken from shale in the Llyn Bochlwyd Formation and the Cwm Tryfan Formation. The cleavage is likely a result of the compression caused during the Caledonian orogeny, as it is also in the same direction as the axial plane of the fold (See pg.47). Figure 35; A stereonet showing cleavage plotted poles to planes. The average of the measurements is shown by the blue line; Strike: 218.7, Dip: 72.9. (Appendix B).
  • 48. 46 Axial Plane This stereonet shows the axial plane of the Tryfan anticline, calculated from measurements taken either side of the fold axis. The axial plane dips steeply towards the N.W. A lack of measurements taken on the Eastern limb of the Tryfan anticline may limit the accuracy of the axial plane calculation. Figure 6; A stereonet showing a select group of dips taken close to the center of the fold, on each limb. A red line shows the axial plane of the fold; Strike: 197.7, Dip: 77.5. (Appendix C).
  • 49. 47 Fold axial plane and cleavage The average of the cleavage measurements, taken throughout the mapped area, is similar to the axial plane of the Tryfan anticline. This indicates that the cleavage found in the Tryfan area is axial planar cleavage showing that it was formed during the same period of tectonic activity as the Tryfan anticline. Figure 37; A stereonet showing the axial plane (Fig.36) and the mean vector average of the cleavage measurements (Fig.35). Plotted planes from poles.
  • 50. 48 Braid and McCaffrey, 1999, hypothesises that there is a synform with an axial surface trace of ENE-WSW, that is half way between Llyn Bochlwyd and Llyn Ogwen. This could be the cause of why units mapped in the area appear to bend to the N-W. It may also give a reason to why some members in the Llyn Bochlwyd Formation, seem to get wider to the North. Faults There are several normal faults around the studied area, cutting through different formations and varying in size. A type locality for faults within this area would be on the East side of Tryfan GR: 668/591, where there is a clearly visible offset between two layers of Cwm Tryfan Tuff (Fig.38). This is a relatively large fault, that is clearly visible from an aerial view (Fig.39), with a length of 1.5km and an average offset of 60m. The hanging wall of this fault is on the Southern side, and the foot wall on the Northern side. On the N.E side of Tryfan (Fig.40) GR: 669/596 there is an exposure that shows a small normal fault, with a throw of 2cm, and it has a dip of 59°. The direction of throw is almost perpendicular to the direction of the bedding, which makes this likely to be a normal fault. This fault was striking E-W, which is the same as some of the larger faults on either side of Tryfan including the one seen in Figure 1 and 2. This may indicate that the larger faults are also normal faults caused by the same processes. Two faults to the East of Llyn Bochlwyd could have been formed as a graben structure which would explain the observed offset. However, there is nothing directly indicating that our observed faults are strike slip or not, as no indicating structures were observed such as slickensides. Further observations and mapping around the local area could provide more evidence to firmly decide what type of faults these are. Some small strike-slip faults are found around the west of the study area. These are almost right angles to the small fold hypothesised by (Braid and McCaffrey, 1999). This could indicate that they were formed during the same period of compression as the smaller fold. In literature there are descriptions of faults in this area which are formed syndepositionally, by the Snowdon Graben. This was a ~40km wide, complexly faulted tectonic depression, that was caused by an East-West extension between 458-452 Ma (Kokelaar, 1992). This was later rotated clockwise to where they are today (Kokelaar, 1988) which means the faults would have been originally orientated N-S. The throw of the faults is now E-W, because the beds have been tilted during folding.
  • 51. 49 S N Figure 38; A picture of Tryfan taken from the Eastern side, around GR: 673/5900. The throw of the fault is clearly visible as the lighter grey tuff layer (highlighted green) is offset by it. The dashed red line indicates the fault.
  • 52. 50 Figure 39; An aerial view of the West side of Tryfan that shows a fault that is clearly visible in the landscape. This is backed up by mapped offset. (Map data ©2019 Google)
  • 53. 51 8cm S N Figure 40; A small normal fault showing a 2cm throw. Hanging wall is on the left-hand side. GR: 669/596.
  • 54. 52 Paleocurrent data Cross bedding is common in exposures East and West of Tryfan. This shows (Nichols, 2013) a bipolar distribution (Fig.41) of paleocurrent directions, at 180° to each other, flowing N.W and S.E. This shows unidirectional currents and is likely the result of the alternating currents in shore-line deposits (Selley, 1968). However, this is not definite as bipolar currents can also uncommonly be caused by off-shore winds (Selley, 1968), and stream flow producing antidune bedding (Selley, 1968). According to Howells et al. 1991, during the time of deposition, in the Ordovician, there was a N.E-S.W trending basin. Tidal flows in this basin would produce sediments showing paleocurrents, N.W-S.E, which is what is observed in exposures in the Tryfan area, adding evidence to the paleocurrents being as a result of tidal flows. Figure 41; A rose diagram showing the direction of palaeoflow measured around the mapped area. (Appendix D).
  • 55. 53 Geological history Cwm Tryfan Formation The youngest formation in the study area was the Cwm Tryfan Formation, which shows a period of relatively uncommon explosive volcanic activity, producing ash, that is seen in the Cwm Tryfan Tuff Member. The tuff is formed by ash falling out of explosive plumes and landing is a coastal, shallow-marine environment (Fig.43). The Cwm Tryfan Shale and Volcaniclastic Members indicate that the palaeoenvironment was a shallow marine coastal area. Due to local erosion and deposition this varied between a low energy anoxic environment, producing black shale, to a higher energy tidal location, depositing volcaniclastic. Figure 42: A palaeomap showing the possible environment of Snowdonia in the Upper Ordovician. Modified from modern maps of the North island of New Zealand. (Map data ©2019 Google) N
  • 56. 54 The environment was likely similar to that shown in the N.W of (Fig.42), where constant deposition could have caused stagnant lagoons to be formed and destroyed over time. This palaeomap is equivalent of the Snowdonia area in the Ordovician (Gibbons, 1998). In literature the Cwm Tryfan Formation is referred to as the Gwern Gof Tuff Formation. It was deposited towards the center of the first volcanic cycle (Howells et al. 1991) and was formed 458 to 457 Ma (Brenchley, 1992) during the Soudleyan stage (Howells et al. 1991) (Known as the Sandbian in current literature (Cohen, 2019). Tryfan Formation The Tryfan Formation represents a period of voluminous volcanic activity, which produced the Tryfan Rhyolite, Tuff and Glyder Rhyolitic Breccia Members. The thickness of the volcanic material in this formation is around 500m. The Tryfan Tuff Member is the result of ash fallout from a large local volcanic eruption (Fig.43). The Tryfan Rhyolite Member was likely formed by an extrusive lava flow (See pg.58), representing a large volcanic event. Figure 43; A diagram showing the formation of fallout ash tuffs, comparable to the formation of the Cwm Tryfan Tuff. Modified from (Plint, 1995).
  • 57. 55 Glyder Rhyolitic Breccia also represents a large volume of volcanic material that was produced during the same time period which may have been deposited on the Snowdon Caldera margin (Fig.43). A lack of sedimentary structures caused by rapid deposition in this formation mean that the palaeoenvironment is difficult to determine from the study area. In literature, this formation is a part of the Capel Curig Formation and represents the climax of the first eruptive cycle (Howells et al. 1991). It was deposited around 457 Ma (Brenchley, 1992), at the end of the Soudleyan stage (Howells et al. 1991). (Howells et al. 1991) shows that the study area was in a transitional depositional environment. As explained in the unit description, the Glyder Rhyolitic Breccia Member was formed during the same volcanic activity as the Tryfan Rhyolite Member, and a topographic feature blocked (Howells et al. 1979) the flow of large materials around the S.S.E of the study area. This indicates that the source of the material was from the S.S.E and likely from the Snowdon Center. The topographic feature that prevented the flow of large material to the North of Glyder Fach could be the edge of a caldera. Figure 44; A modified map from (Howells et al. 1991), showing the location of the Snowdon caldera relative to other modern locations and the studied area. Approximate setting of the studied area Cwm Idwal Syncline
  • 58. 56 Below Glyder Fach on the N.W face GR: 656/586 there is a microgranite exposure (See pg.16) which is formed from the same magma source as rhyolite but cooled intrusively (Haldar, 2013). This may indicate that during the period of volcanic activity there was a volcanic vent somewhere around Glyder Fach, also hypothesised by (Fitch, 1967). Llyn Bochlwyd Formation Sedimentary beds that are lain after the Tryfan Formation and the Llyn Bochlwyd Formation are the Bochlwyd Volcaniclastic and Shale Members. These beds were formed during a period of low to none volcanic activity, and sperate two periods of higher volcanic activity. The Palaeoenvironment of these beds were a shallow marine environment with a water depth of less than 10m (Howells et al. 1991). In literature, these are part of the Cwm Eigiau Formation and they represent the period between the first and second eruptive cycles (Howells et al. 1991). Further to the West is the Bochlwyd Ignimbrite Member, this is the result of a pyroclastic density current. It has a thickness of 150m and is likely from a single explosive volcanic event. A death assemblage on the youngest (East) side of the layer shows that it was originally a shallow marine environment. This member in literature is part of the Pitts Head Tuff Formation (Howells et al. 1991) . Roberts, 1965 suggested that this formation was erupted from a caldera on Llwyd Mawr GR:505475, which is around 20km S.W of the study area. This member could have been the result of an eruption of a vent centred around Glyder Fach (Fitch, 1967), which may have relation to the microgranite observed North of Glyder Fach. West of the Bochlwyd Ignimbrite Member, there is a series of beds of tuff and shale. This was likely developed in an offshore low energy environment with sporadic volcanic eruptions. The tuff beds are relatively thin, 50-100m, and could have been formed by ash fallout being deposited in the low energy environment. In literature, this is described as the Lower Rhyolitic Tuff Formation and was formed from volcanism originating in the Snowdon caldera (Howells, 1991).
  • 59. 57 Clogwyn Formation This formation consists of the Colgwyn Basalt Member which cuts across the Bochlwyd Shale Member, making it unconformable. It is an auto-brecciated pillow lava flow, which shows that it was formed in a marine environment. This member represents a change in the chemical properties of the magma source. In literature, this member is referred to as the bedded pyroclastic formation and represents a basaltic volcanic phase in the second eruptive cycle (Howells et al. 1991). The basaltic material was from the same magma source as the earlier rhyolitic formation but erupted from different vents. Faulting Normal faults were likely formed syndepositionally in the Ordovician (See pg.48). This was because of a graben system, developed because of tectonic extension. This is known in literature as the Snowdon Graben System. Caledonian Orogeny Around 450 Ma, a continental collision began between the Avalonian and Baltican tectonic plates. This caused the closure of the Tornquist Sea between the two plates. In the area of Snowdonia, this caused folding and some low-grade metamorphism. During this time period the Tryfan anticline was formed and cleavage observed in the shale members (See pg.47). A possible second fold was caused after the Caledonian orogeny causing the observed N-S fold.
  • 60. 58 Discussion Source of Tryfan Rhyolite Member The Tryfan Rhyolite Member is shown in literature and published maps to be intrusive (Howells et al. 1991) (Howells et al. 1985). However, this can be argued against as there is evidence pointing towards this member being extrusive. By all accounts, the Tryfan Rhyolite Member is called a rhyolite and not a granite or microgranite Howells et al. 1991) (Howells et al. 1985). A sample of rhyolite from the field area, observed under microscopes shows the same mineral composition of rhyolite (Haldar, 2013). Haldar, 2013, says that rhyolite is an extrusive rock, and the intrusive equivalent would be granite or microgranite. This is in direct disagreement with maps and papers that state that is intrusive. The rhyolite is fine to medium grained indicating that it cooled quickly, indicative of an extrusive lava flow. Flow banding is also observed in this member, but this can also be explained of the magma being cooled in a lava dome. However, the bedding layer of rhyolite does not seem to indicate the shape of a dome, rather that of a consistently thick extrusive flow or Sill. Quartz nodules are found in two locations within the rhyolite on the North and South side of Tryfan GR: 662/589, 667/. These are generally formed when magma or lava comes in contact with water, which also indicates that it was lain extrusively. The Glyder Rhyolitic Breccia Member was formed during an extrusive eruption (Howells et al. 1979). It was likely from the same source vent as the Tryfan Rhyolite Member, but the local topography meant the large clasts did not flow to the North of Glyder Fach. The large cliff face makes this boundary inaccessible, but this hypothesis points towards the rhyolite being formed extrusively. As sill could possibly produce the shape of the exposure, yet after the Llyn Bochlwyd Formation the volcanic activity in the area was basaltic in composition (Howells et al. 1991). This would have meant the sill formed within a short time span after the surrounding Tryfan Tuff was deposited. Flow banding is found rarely in sills but the quartz nodules are difficult to explain. It does not cut across other bedding layers so is not a dyke. No metamorphic aureole was observed around the rhyolite, potentially providing evidence against it being a sill. Microgranite was observed on the North face of Glyder Fach, this was conformable with the Tryfan Rhyolite Member with a gradual reduction in the amount of pink plagioclase feldspar
  • 61. 59 visible in the exposures moving Eastwards. This could have formed intrusively and may represent the boundary of a vent between intrusive microgranite and extrusive rhyolite. Literature Formations Figure 45; A map showing the formations as they are in literature. (Howells et al. 1991)
  • 62. 60 In literature there are six formations that comprise our mapping area (Fig.45). These are separated into formations that have distinct volcanic events. For example, the Cwm Eigiau Formation represents the gap between the first and second eruptive cycles and the Pitts Head Tuff shows an Ignimbrite eruption. The map as part of this study, consists of different formations of members. The mapped members help give an extra layer of detail to the geology and can provide extra information. However, it could be argued that the formations used in literature provide a better grouping than that used in this study.
  • 63. 61 Conclusion The geological exposures around the Tryfan area, demonstrate extensive Upper Ordovician volcanism, deposited in a predominantly shallow marine environment. Rhyolitic eruptions include explosive tuff and ignimbrite forming events and extrusive rhyolite flows. Basaltic volcanism produced a member in the West of the area, in the form of a sub-marine eruption. Structures in the area indicate that normal faulting occurred syndepositionally, and that a secondary tectonic event caused some strike-slip faults in the West of the study area. The Caledonian Orogeny caused folding in Snowdonia, producing the Tryfan anticline and the cleavage that is in the study area. Acknowledgements ____ References
  • 64. 62 A. Guy Plint. (1995) Sedimentary Facies Analysis: A Tribute to the Research and Teaching of Harold G. The International Association of Sedimentologists. P.172,175,182. A. Williams. 1963. The Caradocian brachiopod faunas of the Bala District, Merionethshire. Bulletin of the British Museum of Natural History (Geology) 8(7) Arthur, M. A., & Sageman, B. B. (1994). Marine Black Shales: Depositional Mechanisms and Environments of Ancient Deposits. Annual Review of Earth and Planetary Sciences, 22(1), p.537 BAIRD, A. W., & McCAFFREY, K. I. W. (1999). Polyphase deformation and metamorphism in the Llyn Ogwen area of Snowdonia, North Wales. Journal of the Geological Society, 156(1) Brenchley, P. J. (1992). A geologic time scale 1989, by W. B. Harland, R. L. Armstong. A. V. Cox. L. E. Craig, A. G. Smith and D. G. Smith. Cambridge University Press, Cambridge. Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. Fisher, R. (1960). CLASSIFICATION OF VOLCANIC BRECCIAS. Geological Society of America Bulletin, 71(7), p.974. Fitch, F. J. (1967). Ignimbrite volcanism in North Wales. Bulletin Volcanologique, 30(1), 206, 208, 209, 216 Geological Survey on the six-inch scale by M.F. Howells, B.E Leveridge, R. Addison, C.D.R Evans, and M.J.C. Nutt in 1970-79. E.G. Smith and I.P. Stevenson, District Geologists. 1:25 000 Geological Sheet published 1985. Gibbons. (1998). Rhyolitic volcanic corridors in magmatic arcs: comparing North Wales and North Island, New Zealand. Terra Nova, 10(6) Haldar, S. (2013). INTRODUCTION TO MINERALOGY AND PETROLOGY. [S.l.]: ELSEVIER. Howells, M F, Reedman, A J, and Campbell, S D G. 1991. Ordovician (Cardoc) marginal basin volcanism in Snowdonia (north-west Wales). (London : HSMO for the British Geological Survey.) Howells, M. F., Leveridge, B. E., Addison, R., Evans, C. D. R., & Nutt, M. J. C. (1979). The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment. Geological Society, London, Special Publications, 8(1), 614
  • 65. 63 Hunt, D., Tucker, M.E., 1992, Stranded Parasequences and the forced regressive wedge Systems Tract: deposition during base-level fall. Sedimentary Geology 81 Kokelaar, P. (1988). Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society, 145(5) p.770 Kokelaar, P. (1992). Ordovician marine volcanic and sedimentary record of rifting and volcanotectonism: Snowdon, Wales, United Kingdom. Geological Society of America Bulletin, 104(11), 1454 Middleton, G V. (2003). ENCYCLOPEDIA of SEDIMENTS and SEDIMENTARY ROCKS. Kluwer Academic Publishers. P.83 Nichols, G. (2013). Sedimentology and Stratigraphy. New York, NY: John Wiley & Sons. Appendix
  • 66. 64 Appendix A – All dip measurements Appendix B – All cleavage measurements
  • 67. 65 Appendix C – Dips on each limb of the Tryfan anticline Appendix D – Palaeoflow directions