old red sandstone rocks of great britain · 2014. 10. 11. · introduction to the old red sandstone...
TRANSCRIPT
Chapter 1
Introduction to the Old Red Sandstone
of Great Britain
W.J. Barclay
From:Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. and Wilby, P.R. (2005) The Old Red Sandstone of Great Britain, Geological Conservation Review Series, No. 31, Joint Nature Conservation Committee, Peterborough, 393 pages, illustrations, A4 hardback, ISBN 1 86107 543 XFor more information see: http://www.jncc.gov.uk/page-2936
Introduction
3
INTRODUCTION
The Old Red Sandstone is one of the two major‘red-bed’ sequences of sedimentary rock inGreat Britain, the other being the youngerPermo–Triassic rocks that were formerly termedthe ‘New Red Sandstone’ (see the companionGCR volume by Benton et al., 2002) todistinguish them from the Old Red Sandstone,rocks that are about 150 million years (Ma)older. In the early days of geological research inthe 1830s, the Old Red Sandstone was includedin the Carboniferous System, but soon after wasgiven separate status and accorded a Devonianage, in recognition of its equivalence to the
marine Devonian rocks of south Devon andCornwall.
The GCR sites described in this volume arerepresentative of the continental Old Red Sandstonefacies in Great Britain. The rocks are mainly of whatis now formally defined as Devonian age (about418 to 362 million years (Ma) old), but accordingto modern definitions extend back into the SilurianPeriod, perhaps locally into the Wenlock Series(424 Ma). They also extend upwards into what isnow defined as the early Carboniferous at less than362 Ma (see Figure 1.4, ‘Stratigraphical frameworkfor the Old Red Sandstone’, this chapter).
The Old Red Sandstone crops out principallyin five areas in Great Britain (Figure 1.1), which
Figure 1.1 Simplified sketch map showing the principal Devonian outcrops of Great Britain. Marine Devonianstrata are confined to south-west England, the remainder being sedimentary rocks of Old Red Sandstone faciesand volcanic rocks. Caledonian (Ordovician to Late Devonian) intrusive rocks are not shown.
Introduction to the Old Red Sandstone of Great Britain
4
broadly reflect the original sedimentary basins inwhich they were deposited. These are:
• the Orkney and Shetland islands and north-east Scotland (the Orcadian Basin);
• the Midland Valley of Scotland (in anamalgamation of several basins of which thelargest was the Strathmore Basin);
• the Scottish Borders and Northumberland(the Scottish Border Basin);
• the southern Lake District (the Mell FellTrough); and
• south Wales, the Welsh Borderland and Bristol(the Anglo-Welsh Basin).
Figure 1.2 shows the stratigraphical distributionof the main Old Red Sandstone sequences.
Traditionally, the base of the Old RedSandstone in the Anglo-Welsh Basin was placedat the base of the Ludlow Bone Bed, a thin,
lenticular, phosphatized ‘lag deposit’ markingthe top of the Silurian Ludlow Series. However,the international agreement at the MontrealDevonian Symposium in 1972 to define the baseof the Devonian System in the fully marine,graptolite-bearing succession exposed at Klonckin the Czech Republic, at the base of theMonograptus ultimus Biozone (e.g. House,1977) now places the basal parts of the Old RedSandstone in the modern Silurian System. Thestrata from the Ludlow Bone Bed up to the baseof the modern Devonian System, which is as yetpoorly defined in the Old Red Sandstone,belong to the PÍídolí Series, the fourth, upper-most series of the Silurian System (White andLawson, 1989). The age intervals (or stages) ofthe Devonian Period, also defined in the marinerocks of Europe, are applied to the terrestrial OldRed Sandstone succession with some difficultybecause of its absence of marine fossils.
Figure 1.2 Stratigraphical distribution of the main Old Red Sandstone sequences of Great Britain. Tectonicevents and their timing are from Soper and Woodcock (2003). Ages are from Williams et al. (2000). Small solid bars indicate the principal volcanic rocks. Individual chapter introductions provide more detailedstratigraphical distribution charts. (HV – Hoy Volcanic Member; MF – Mell Fell Conglomerate Formation; RC –Ridgeway Conglomerate Formation.)
Scope
5
GCR SITE SELECTION
The selection of Geological Conservation Review(GCR) sites described in this volume was carriedout in the 1980s and 1990s, following thecriteria set out in Ellis et al. (1996). The mainreasons for qualification of a site for a particularGCR site selection category are:
• international importance – for example, thesite may be important because it is a typelocality for a geological time period, rock unitor fossil species, or is of historical importancein the development of geological science;
• possession of unique or exceptional geologicalfeatures;
• national importance because a site is represen-tative of a feature, event, process or rock bodythat is fundamental to the understanding ofthe geological history of Great Britain.
SCOPE
The GCR sites were selected according tothematic GCR ‘Blocks’, the present volumedescribing the ‘Non-marine Devonian’ GCRBlock, which consists of 64 ratified GCR sites,together with a small number of potential GCR sites. The site descriptions are arrangedgeographically, from north to south, in areas thatcorrespond to the original depositional basins.The sites are listed in Table 1.1, together with theprincipal criteria for their selection. Many of thesites have features that satisfy several selectioncriteria. Furthermore, there are numerous OldRed Sandstone sites that have been independentlyselected for other GCR palaeontological ‘Blocks’.These sites are described in the companion GCRvolumes on fossil fishes (Table 1.2; Dineley andMetcalf, 1999) and Palaeozoic palaeobotany(Table 1.3; Cleal and Thomas, 1995).
Introduction to the Old Red Sandstone of Great Britain
6
Sit
e A
ge
Sel
ecti
on
cri
teri
a O
rca
dia
n B
asi
n
Eas
ter
Ro
va H
ead
M
id-D
evo
nia
n
Spec
tacu
lar
sea-
cliff
exp
osu
res
of c
on
glo
mer
ates
. Fo
ota
bro
ugh
to
Wic
k o
f Wat
snes
s M
id-D
evo
nia
n
Co
nti
nu
ou
s se
a-cl
iff e
xpo
sure
s o
f Wal
ls F
orm
atio
n.
Th
e C
lett
s, E
xnab
oe
Mid
-Dev
on
ian
C
on
tin
uo
us
sea-
cliff
exp
osu
res
of
cycl
ic l
acu
stri
ne,
flu
vial
an
d a
eolia
n f
acie
s o
f th
e B
rin
dis
ter
Flag
sto
ne
Form
atio
n.
Also
a fo
ssil
fish
GC
R s
ite.
M
elby
: Mat
ta T
ain
g to
Lan
g R
igg
(P)
Mid
-Dev
on
ian
Fo
ssil
fish
GC
R s
ite.
Sp
ecta
cula
r se
a-cl
iff a
nd
fore
sho
re e
xpo
sure
s o
f lac
ust
rin
e M
elby
Fo
rmat
ion
. So
uth
Str
om
nes
s C
oas
t Se
ctio
n
Mid
-Dev
on
ian
B
est
sect
ion
in O
rkn
ey t
hro
ugh
th
e C
aith
nes
s Fl
agst
on
e G
rou
p.
Tar
aclif
f Bay
to
New
ark
Bay
M
id-D
evo
nia
n
Th
icke
st, b
est-
exp
ose
d s
ecti
on
of t
he
Ed
ay G
rou
p.
Gre
enan
Nev
Co
ast,
Ed
ay
Mid
-Dev
on
ian
B
est
sect
ion
th
rou
gh t
he
Ed
ay M
arl F
orm
atio
n in
Ork
ney
. So
uth
Fer
snes
s B
ay, E
day
M
id-D
evo
nia
n
Wel
l-exp
ose
d, a
cces
sibl
e se
ctio
n o
f th
e E
day
Gro
up
. Ye
snab
y an
d G
ault
on
Co
ast
Sect
ion
E
arly
Dev
on
ian
Su
per
b se
a-cl
iff s
ecti
on
s o
f th
e Ye
snab
y Sa
nd
sto
ne
Gro
up
, in
clu
din
g u
niq
ue
aeol
ian
faci
es.
Also
sec
tio
ns
in t
he
Low
er S
tro
mn
ess
Flag
sto
ne
Form
atio
n c
on
tain
ing
the
best
str
om
ato
lites
in t
he
Orc
adia
n B
asin
. O
ld M
an o
f Ho
y C
oas
t La
te D
evo
nia
n
Spec
tacu
lar
sea
cliff
s o
f th
e H
oy
San
dst
on
e Fo
rmat
ion
, in
clu
din
g th
e H
oy
Vo
lcan
ic M
embe
r.
Bay
of B
erst
ane
(P)
Mid
-Dev
on
ian
U
niq
ue
on
sho
re
evid
ence
o
f m
arin
e-in
fluen
ced
d
epo
siti
on
in
th
e M
idd
le
Dev
on
ian
E
day
M
arl
Form
atio
n.
Red
Po
int
M
id-D
evo
nia
n
Spec
tacu
lar
lake
-mar
gin
dep
osi
ts a
nd
bas
emen
t-co
ver
top
ogr
aph
y.
Pen
nyl
and
(T
hu
rso
–Scr
abst
er)
Mid
-Dev
on
ian
W
ell-e
xpo
sed
lac
ust
rin
e cy
cles
of
the
Orc
adia
n l
ake
(Up
per
Cai
thn
ess
Flag
sto
ne
Gro
up
).
Also
a f
oss
il fis
h G
CR
sit
e.
Joh
n o
’Gro
ats
(P)
Mid
-Dev
on
ian
Fo
ssil
fish
GC
R s
ite.
Typ
e lo
calit
y o
f Jo
hn
o’G
roat
s Sa
nd
sto
ne
Gro
up
. W
ick
Qu
arri
es
Mid
-Dev
on
ian
Sp
ecta
cula
r ex
po
sure
s o
f fis
h-b
eari
ng
lake
d
epo
sits
an
d
shri
nka
ge
crac
ks
in
the
Low
er
Cai
thn
ess
Flag
sto
ne
Gro
up
in a
n o
ther
wis
e p
oo
rly
exp
ose
d p
art
of t
he
Orc
adia
n B
asin
. Ac
han
arra
s Q
uar
ry (
P)
Mid
-Dev
on
ian
Fo
ssil
fish
GC
R s
ite,
th
e ri
ches
t lo
calit
y in
Gre
at B
rita
in.
Typ
e lo
calit
y o
f Ac
han
arra
s Li
mes
ton
e M
embe
r (F
ish
Bed
), o
f im
po
rtan
ce r
egio
nal
ly a
s a
mar
ker
ho
rizo
n b
etw
een
th
e Lo
wer
Cai
thn
ess
Flag
sto
ne
Gro
up
an
d U
pp
er C
aith
nes
s Fl
agst
on
e G
rou
p.
Sarc
let
(P)
Ear
ly D
evo
nia
n
Sea-
cliff
sec
tio
ns
in t
he
Low
er D
evo
nia
n S
arcl
et G
rou
p.
Tar
bat
Nes
s M
id-D
evo
nia
n a
nd
Lat
e D
evo
nia
n
Co
mp
lete
sec
tio
n o
f th
e B
aln
ago
wn
Gro
up
an
d o
f th
e ap
par
entl
y co
nfo
rmab
le j
un
ctio
n w
ith
th
e St
rath
R
ory
Gro
up
. Lo
ch D
un
telc
hai
g (D
un
Ch
ia H
ill)
Mid
-Dev
on
ian
D
ram
atic
exp
osu
re o
f M
idd
le O
ld R
ed S
and
sto
ne
con
glo
mer
ates
res
tin
g u
nco
nfo
rmab
ly o
n D
alra
dia
n
met
ased
imen
tary
ro
cks
and
late
Cal
edo
nia
n g
ran
ites
in t
he
sou
th-w
est
of t
he
Orc
adia
n B
asin
. T
ynet
Bu
rn (
P)
Mid
-Dev
on
ian
Fo
ssil
fish
GC
R s
ite.
C
lass
ic f
oss
il fis
h l
oca
lity
yiel
din
g w
ho
le,
wel
l-pre
serv
ed s
pec
imen
s, a
nd
im
po
rtan
t ev
iden
ce o
n t
he
nat
ure
of
the
sou
ther
n m
argi
n o
f th
e O
rcad
ian
Bas
in a
t th
e ti
me
of
max
imu
m
(Ach
anar
ras)
lake
ext
ent.
D
en o
f Fin
do
n, G
amri
e B
ay a
nd
New
A
berd
ou
r (P
) M
id-D
evo
nia
n (
also
Ear
ly
Dev
on
ian
) Fo
ssil
fish
GC
R s
ite,
wit
h s
up
erb
coas
t se
ctio
ns
at G
amri
e B
ay,
Pen
nan
(Lo
wer
OR
S–M
idd
le O
RS
un
con
form
ity)
, New
Abe
rdo
ur
and
Qu
arry
Hav
en.
Rh
ynie
(P)
E
arly
Dev
on
ian
Fo
ssil
pla
nt
GC
R s
ite.
W
orl
d r
eno
wn
ed f
lora
l an
d a
rth
rop
od
lag
erst
ätte
. E
xcep
tio
nal
pre
serv
atio
n i
n a
hyd
roth
erm
al s
pri
ng
dep
osi
t.
Mid
lan
d V
all
ey o
f Sco
tla
nd
an
d a
dja
cen
t a
rea
s T
he
To
utt
ies
(P)
Mid
-Silu
rian
Fo
ssil
fish
GC
R s
ite.
O
ldes
t (M
id-S
iluri
an)
Old
Red
San
dst
on
e fa
cies
in
Sco
tlan
d c
on
tain
ing
fish
an
d
imp
ort
ant
arth
rop
od
fau
na.
D
un
no
ttar
Co
ast
Sect
ion
(P)
M
id-S
iluri
an–E
arly
Dev
on
ian
Mag
nifi
cen
t ex
po
sure
s in
dra
mat
ic s
ea c
liffs
of c
on
glo
mer
ates
an
d s
and
sto
nes
. C
raw
ton
Bay
La
te S
iluri
an–E
arly
Dev
on
ian
Fin
e co
asta
l sec
tio
ns
of c
on
glo
mer
ates
an
d v
olc
anic
s. A
lso
a C
aled
on
ian
ign
eou
s ro
cks
GC
R s
ite
No
rth
Esk
Riv
er
late
Ear
ly D
evo
nia
n
Bes
t se
ctio
ns
of t
he
Stra
thm
ore
Gro
up
M
ilto
n N
ess
(P)
Late
Dev
on
ian
–Ear
ly
Car
bon
ifero
us
Sea
cliff
s ex
po
sin
g o
ne
of
the
best
se
ctio
ns
of
mat
ure
ca
lcre
te
dev
elo
pm
ent
in
Sco
tlan
d
in
the
Kin
nes
swo
od
Fo
rmat
ion
. Ab
erle
mn
o Q
uar
ry (
P)
Ear
ly D
evo
nia
n
Foss
il fis
h a
nd
Pal
aeo
zoic
pal
aeo
bota
ny
GC
R s
ite.
Al
so i
mp
ort
ant
for
arth
rop
od
fo
ssils
. I
mp
ort
ant
for
Du
nd
ee F
lags
ton
e an
d S
con
e Sa
nd
sto
ne
form
atio
ns.
T
illyw
han
dla
nd
Qu
arry
(P)
E
arly
Dev
on
ian
Fo
ssil
fish
GC
R s
ite.
U
niq
ue
lacu
stri
ne
faci
es i
n t
he
Mid
lan
d V
alle
y o
f Sc
otl
and
in t
he
Du
nd
ee F
lags
ton
e Fo
rmat
ion
co
nta
inin
g fis
h, a
rth
rop
od
s an
d t
race
foss
ils.
Wh
itin
g N
ess
Ear
ly a
nd
Lat
e D
evo
nia
n
Sea
cliff
s ex
po
sin
g E
arly
Dev
on
ian
san
dst
on
es u
nco
nfo
rmab
ly o
verl
ain
by
Late
Dev
on
ian
san
dst
on
es, t
he
un
its
bein
g se
par
ated
by
a sp
ecta
cula
r u
nco
nfo
rmit
y.
Tab
le 1
.1G
CR
Old
Red
San
dst
on
e si
tes
and
pro
po
sed
sit
es, w
ith
mai
n c
rite
ria
for
thei
r se
lect
ion
. C
on
tin
ued
on
pag
e 7.
Scope
7
Sit
e A
ge
Sel
ecti
on
cri
teri
a T
ay B
ank
Ear
ly D
evo
nia
n
Typ
e lo
calit
y o
f th
e C
amp
sie
Lim
esto
ne
Mem
ber,
in
clu
din
g th
e St
anle
y Li
mes
ton
e, r
epre
sen
tin
g m
atu
re
calc
rete
dev
elo
pm
ent
and
pro
vid
ing
an im
po
rtan
t st
rati
grap
hic
al m
arke
r h
ori
zon
. G
len
Val
e (P
) La
te D
evo
nia
n
Imp
ort
ant
san
dst
on
es (
the
Kn
ox
Pulp
it S
and
sto
ne
Form
atio
n)
of a
eolia
n o
rigi
n.
Wo
lf’s
Ho
le Q
uar
ry (
P)
Ear
ly D
evo
nia
n
Foss
il fis
h G
CR
sit
e ex
po
sin
g th
e h
igh
est
lava
on
th
e so
uth
ern
sid
e o
f th
e St
rath
mo
re B
asin
.Au
chen
sail
Qu
arry
(P)
E
arly
Dev
on
ian
Fo
ssil
pla
nt
GC
R s
ite
yiel
din
g a
wel
l-pre
serv
ed a
ssem
blag
e o
f la
nd
pla
nts
in t
he
Tei
th S
and
sto
ne
Form
atio
n.
Sicc
ar P
oin
t to
Haw
k’s
Heu
gh (
E)
Late
Dev
on
ian
–Ear
ly
Car
bon
ifero
us
Cla
ssic
se
ctio
n
at
Sicc
ar
Poin
t (H
utt
on
’s
Un
con
form
ity)
.
Foss
il fis
h
GC
R
site
at
H
awk’
s H
eugh
. Pr
op
ose
d e
xten
sio
n t
o i
ncl
ud
e th
e in
terv
enin
g su
per
b se
ctio
ns
of
flu
vial
an
d ?
aeo
lian
san
dst
on
es a
t Pe
ase
Bay
. La
rgs
Co
ast,
Ayr
shir
e La
te D
evo
nia
n
Imp
ort
ant
coas
tal e
xpo
sure
s in
san
dst
on
es il
lust
rati
ng
fluvi
al, b
raid
ed r
iver
san
dbo
dy
mo
rph
olo
gies
. N
ort
h N
ewto
n S
ho
re, A
rran
La
te D
evo
nia
n
On
e o
f th
e th
ree
clas
sic
Old
Red
San
dst
on
e u
nco
nfo
rmit
ies
reco
gniz
ed b
y Ja
mes
Hu
tto
n.
So
uth
ern
Sco
tla
nd
an
d t
he L
ak
e D
istr
ict
Palm
ers
Hill
Rai
l Cu
ttin
g La
te D
evo
nia
n
Exp
osu
res
of c
alcr
ete
in t
he
Sco
ttis
h B
ord
er B
asin
. Po
ole
y B
rid
ge
Ear
ly D
evo
nia
n
Bes
t se
ctio
n o
f Old
Red
San
dst
on
e fa
cies
co
ngl
om
erat
es in
no
rth
-wes
t E
ngl
and
. A
nglo
-Wels
h B
asi
n
Port
h-y
-Mo
r
Ear
ly D
evo
nia
n
On
e o
f th
e be
st, m
ost
acc
essi
ble
Old
Red
San
dst
on
e se
ctio
ns
in t
he
An
glo
-Wel
sh B
asin
. D
evil’
s H
ole
(P)
La
te S
iluri
an–E
arly
Dev
on
ian
His
tori
cally
imp
ort
ant
foss
il fis
h G
CR
sit
e, w
ith
imp
ort
ant
‘Do
wn
ton
ian
’–‘D
itto
nia
n’ b
ou
nd
ary
exp
osu
re.
Oak
Din
gle,
Tu
gfo
rd (
P)
Ear
ly D
evo
nia
n
Foss
il fis
h G
CR
sit
e w
ith
wel
l-do
cum
ente
d s
edim
ento
logi
cal a
nal
ysis
. T
he
Scar
La
te S
iluri
an
Go
od
inla
nd
exp
osu
re o
f th
e R
agla
n M
ud
sto
ne
Form
atio
n.
Cu
sop
Din
gle
(P)
Late
Silu
rian
–Ear
ly D
evo
nia
nB
est,
mo
st c
om
ple
te in
lan
d s
ecti
on
th
rou
gh t
op
mo
st P
ído
lí an
d lo
wer
mo
st D
evo
nia
n s
trat
a.
Saw
dd
e G
org
e (E
) La
te S
iluri
an–E
arly
Dev
on
ian
Pro
po
sed
ext
ensi
on
of G
CR
sit
e to
incl
ud
e h
igh
er P
ído
lí an
d b
asal
Dev
on
ian
str
ata.
Pa
nty
mae
s Q
uar
ry (
P)
Ear
ly D
evo
nia
n
Exc
elle
nt
exp
osu
re o
f Lo
chko
vian
ch
ann
el s
and
sto
nes
an
d f
loo
dp
lain
mu
dst
on
e fa
cies
, in
tern
atio
nal
lykn
ow
n fo
r it
s ar
thro
po
d t
rack
way
s.
Heo
l Sen
ni Q
uar
ry
Ear
ly D
evo
nia
n
GC
R fo
ssil
fish
sit
e. A
lso
rep
rese
nta
tive
of t
he
Sen
ni F
orm
atio
n.
Cae
ras
Qu
arry
E
arly
Dev
on
ian
B
est
exp
osu
re o
f lo
cal p
ebbl
y fa
cies
in t
he
Bro
wn
sto
nes
Fo
rmat
ion
. C
raig
-y-F
ro Q
uar
ry (
P)
Ear
ly D
evo
nia
n
Cla
ssic
foss
il p
lan
t G
CR
sit
e. A
lso
imp
ort
ant
for
exp
osu
re o
f th
e Se
nn
i Fo
rmat
ion
. Ab
ercr
iban
Qu
arri
es
Late
Dev
on
ian
–Ear
ly
Car
bon
ifero
us
Typ
e lo
calit
y o
f th
e G
rey
Gri
ts F
orm
atio
n.
Afo
n y
Wae
n (
P)
Late
Dev
on
ian
Po
ten
tial
foss
il fis
h G
CR
sit
e. A
lso
imp
ort
ant
for
exp
osu
re o
f th
e Pl
atea
u B
eds
Form
atio
n.
Du
ffry
n C
raw
no
n (
P)
Late
Dev
on
ian
T
ype
loca
lity
of t
he
Plat
eau
Bed
s Fo
rmat
ion
, in
clu
din
g p
oss
ible
aeo
lian
faci
es.
Cra
ig-y
-cw
m (
P)
Late
Dev
on
ian
–Ear
ly
Car
bon
ifero
us
Rep
rese
nta
tive
sec
tio
n o
f th
e Q
uar
tz C
on
glo
mer
ate
Gro
up
.
Ro
ss-o
n-W
ye, R
oya
l Ho
tel
Ear
ly D
evo
nia
n
Exc
elle
nt,
wel
l-do
cum
ente
d, a
cces
sibl
e se
ctio
n o
f th
e B
row
nst
on
es F
orm
atio
n.
Wild
ern
ess
(Lan
d G
rove
) Q
uar
ry
Ear
ly D
evo
nia
n
Foss
il fis
h G
CR
sit
e w
ith
su
per
b ex
po
sure
of t
he
low
erm
ost
str
ata
of t
he
Bro
wn
sto
nes
Fo
rmat
ion
. Ly
dn
ey
Late
Silu
rian
–Ear
ly D
evo
nia
nFo
ssil
fish
GC
R s
ite
wit
h g
oo
d s
ecti
on
of t
he
Psam
mo
steu
s Li
mes
ton
e h
ori
zon
. Al
bio
n S
and
s an
d G
ateh
olm
Isl
and
(P
)La
te S
iluri
an–E
arly
Dev
on
ian
Mag
nifi
cen
t se
a-cl
iff a
nd
fo
resh
ore
exp
osu
res
of
Wen
lock
mar
ine
stra
ta a
nd
th
e o
verl
yin
g O
ld R
ed
San
dst
on
e.Li
ttle
Cas
tle
Hea
d (
P)
Late
Silu
rian
–Ear
ly D
evo
nia
nR
efer
ence
sec
tio
n o
f th
e P
ído
lí Sa
nd
y H
aven
Fo
rmat
ion
an
d o
f th
e T
ow
nse
nd
Tu
ff B
ed.
Wes
t An
gle
Bay
(N
ort
h)
Late
Silu
rian
–Ear
ly
Car
bon
ifero
us
Co
nti
nu
ou
s se
ctio
n o
f th
e en
tire
Old
Red
San
dst
on
e su
cces
sio
n a
nd
of
the
un
der
lyin
g an
d o
verl
yin
g st
rata
.Fr
esh
wat
er W
est
(P)
Late
Silu
rian
–Ear
ly
Car
bon
ifero
us
Sup
erb,
acc
essi
ble
dip
sec
tio
n e
xpo
sin
g th
e en
tire
Old
Red
San
dst
on
e su
cces
sio
n.
Fres
hw
ater
Eas
t–Sk
rin
kle
Hav
en
(‘T
enby
Clif
fs’)
La
te S
iluri
an–E
arly
C
arbo
nife
rou
s E
xcel
len
t st
rike
sec
tio
n o
f th
e en
tire
Old
Red
San
dst
on
e su
cces
sio
n.
Llan
stef
fan
La
te S
iluri
an–E
arly
Dev
on
ian
Sup
erb
exp
osu
res
of
stac
ked
car
bon
ate
pal
aeo
sols
of
the
Ch
apel
Po
int
Cal
cret
es M
embe
r(P
sam
mo
steu
s Li
mes
ton
e).
Port
ish
ead
E
arly
an
d L
ate
Dev
on
ian
B
est
sect
ion
of t
he O
ld R
ed S
and
sto
ne
succ
essi
on
eas
t o
f Sev
ern
Est
uar
y.
Gle
nth
orn
e
Mid
-Dev
on
ian
B
est
sect
ion
of O
ld R
ed S
and
sto
ne
faci
es s
ou
th o
f th
e B
rist
ol C
han
nel
. P
Pote
nti
al s
ite
(mos
t o
f the
se s
ites
are
con
firm
ed G
CR
sit
es fo
r th
eir
pal
aeon
tolo
gy)
E P
rop
ose
d e
xten
sio
n t
o s
ite
Tab
le 1
.1–
con
tin
ued
.
Introduction to the Old Red Sandstone of Great Britain
8
Site Stratigraphy Criterion Treatment in this volumeFull description FD
Summary description SD Not described ND
Orcadian Basin Westerdale Quarry Mid-Devonian; Eifelian One of oldest fish-bearing
horizons in Orcadian Basin; complete specimens
ND
Achanarras Quarry Mid-Devonian; Eifelian–Givetian boundary
Richest Old Red Sandstone fish site in Britain
FD
Cruaday Quarry Mid-Devonian; Eifelian–Givetian boundary*
Best Old Red Sandstone fish site in Orkney
ND
Black Park, Edderton Mid-Devonian; Eifelian–Givetian boundary*
Fish well preserved in three dimensions
ND
Den of Findon, Gamrie Mid-Devonian; Eifelian–Givetian boundary*
Prolific fish fauna SD
Tynet Burn, Elgin Mid-Devonian* Rich fish fauna and historically important
FD
Melby Mid-Devonian; Eifelian–Givetian boundary*
Northernmost occurrence of Achanarras horizon
FD
Papa Stour Mid-Devonian; Eifelian–Givetian boundary*
Fish in sedimentary rocks in predominantly volcanic sequence
ND
Dipple Brae Mid-Devonian Fish fauna younger than that of the Achanarras horizon
ND
Spittal Quarry Mid-Devonian Rare fish fauna, including only Mid-Devonian cephalaspid
ND
Banniskirk Quarry Mid-Devonian First ORS site to yield fishes ND Holborn Head Quarry Mid-Devonian; mid-
Givetian10–11 fish species, including Osteolepis panderi
ND
Weydale Quarry Mid-Devonian Well-preserved Osteolepis panderi and Dipterus valenciennesi
ND
Pennyland Mid-Devonian; Givetian Many fish specimens from several fish-bearing horizons
FD
John o’Groats Mid-Devonian; late Givetian
Youngest fish fauna in Caithness
SD
The Cletts, Exnaboe Mid-Devonian; late Givetian
Northernmost late Givetian fish site
FD
Sumburgh Head Late Mid-Devonian; late Givetian
Possibly youngest fish fauna of the Orcadian Basin
ND
Midland Valley of Scotland The Toutties Late Wenlock Oldest Old Red Sandstone
facies rocks in Scotland; unique fish fauna
FD
Tillywhandland Quarry Early Devonian One of best Early Devonian fish sites in Scotland
FD
Aberlemno Quarry Early Devonian Best surviving of the famous Turin Hill fish sites; also a fossil plant GCR site (Table 1.3)
FD
Wolf’s Hole Quarry Early Devonian Unique pteraspid fish fauna FD Whitehouse Den Early Devonian Fossil acanthodian fish ND Grampian Highlands Ardmore–Gallanach Late Silurian–Early
DevonianUnique early fish fauna in sediments associated with Lorne lavas
ND
Bogmore, Muckle Burn Earliest Late Devonian (Frasnian)
Diverse fish fauna with over 15 species
ND
Scaat Craig Late Devonian Diverse late Devonian fish fauna and a distinctive tetrapod
ND
* Achannaras Fish Bed horizon
Table 1.2 GCR sites in the Old Red Sandstone described in the fossil fishes GCR volume. After Dineley andMetcalf (1999). Continued on page 9.
Scope
9
Site Stratigraphy Criterion Treatment in this volumeFull description FD
Summary description SD Not described ND
Southern Uplands Oxendean Burn Late Devonian Abundant fragments of
BothriolepisND
Hawk’s Heugh Late Devonian Only British occurrence of Remigolepis
FD
Anglo-Welsh Basin Ludford Lane and Ludford Corner
Silurian; P ídolí Internationally renowned for rich fish fauna; see also Table 1.4
ND
Ledbury cutting Silurian; P ídolí Historical site yielding complete specimens of Auchenaspis and Hemicyclaspis
ND
Temeside, Ludlow Silurian; P ídolí Historical site in Temeside Mudstone Formation yielding a rich fish fauna including Hemicyclaspis murchisoni
ND
Tite’s Point (Purton Passage)
Silurian; Ludlow–P ídolí Thelodus parvidens fish fauna, allowing correlation with Ludlow Bone Bed, and source of Cyathaspis
Lydney Late P ídolí–Early Devonian
Sequence of vertebrate faunas, including specimens of Sabrinacanthus
FD
Downton Castle area (network of 4 sites)
Early P ídolí Several quarries in Downton Castle Sandstone yielding vertebrate remains
ND
Bradnor Hill Quarry Late P ídolí Late P ídolí thelodont fauna
ND
Devil’s Hole P ídolí–Lochkovian Fish fauna straddling Downtonian–Dittonian boundary
SD
Oak Dingle, Tugford Lochkovian (Dittonian) Near-strike section of fish-bearing beds; earliest record of Weigeltaspis
SD
Cwm Mill Lochkovian (Dittonian) Unique preservation of complete cephalaspids, including three new species; also specimens of Rhinopteraspis crouchi
ND
Wayne Herbert Quarry Lochkovian (Dittonian) Well-preserved, diverse fish fauna
ND
Besom Farm Quarry Lochkovian (Dittonian) Rich, diverse fish fauna, including 7 type specimens and sole occurrence of 5 of them
ND
Heol Senni Quarry Lochkovian–Pragian Only occurrence of Althaspis senniensis
FD
Portishead Late Devonian Unique fish fauna, including only British occurrence of Groenlandaspis
FD
Prescott Corner Late Devonian (Frasnian) Extensive Late Devonian fish fauna
ND
Afon y Waen Late Devonian Bothriolepis and Holoptychius in Upper Old Red Sandstone
FD
Table 1.2 – continued.
Introduction to the Old Red Sandstone of Great Britain
10
The GCR sites provide representativelocalities for the entire stratigraphical range ofthe Old Red Sandstone. The initial selection ofGCR sites for the ‘Non-marine Devonian’ GCRBlock included sites in the Anglo-Welsh Basin instrata that extended down from the base of theDevonian System to the Ludlow Bone Bed.These strata, comprising the Downton Group(the former Downtonian Stage), are of Silurian(PÍídolí Series) age and the sites (Table 1.4) aredescribed in the GCR volume on Silurian strati-graphy (Aldridge et al., 2000). The GCR volumeon Caledonian igneous rocks (Stephenson et al.,1999) includes sites (Table 1.5) in which Old
Red Sandstone strata are present in addition tothe contemporaneous igneous rocks for whichthey are cited. All of the Old Red Sandstone sitesdescribed in the other GCR volumes are listed inTables 1.2, 1.3, 1.4 and 1.5, along with the levelof detail in which they are described in thepresent volume. Only some of these ‘over-lapping’ sites are given full descriptions in thepresent volume, which emphasizes the sedi-mentological and lithostratigraphical features.The Freshwater West potential Old RedSandstone GCR site, and part of the FreshwaterEast–Skrinkle Haven GCR site are also Variscanto Alpine structures GCR sites.
Site Stratigraphy Criterion Treatment in this volumeFull description FD
Summary description SD Not described ND
Orcadian Basin Sloagar Mid-Devonian; Late
GivetianOnly occurrence of Svalbardia in Britain
ND
Bay of Skaill Mid-Devonian Important floral assemblage in Sandwick Fish Bed; type locality of Protopteridium thomsonii
ND
Rhynie Early Devonian Renowned Devonian palaeobotanical site; 22 species unique to this site
FD
Midland Valley of ScotlandTurin Hill Early Devonian Best example of
Zosterophyllum Zone flora in world and type locality of Cooksonia caledonica
FD (as ‘Aberlemno Quarry’)
Ballanucater Farm Early Devonian; Emsian Best Emsian floral assemblage in Britain
ND
Auchensail Quarry Early Devonian; Emsian Well-preserved Emsian floral assemblage
FD
Anglo-Welsh Basin Targrove Quarry Early Devonian;
Gedinnian Most diverse rhyniophytoid plant assemblage in world
ND
Capel Horeb Quarry Late Silurian; Ludfordian–P ídolí
Oldest vascular plants in world in Ludlow Series; Long Quarry Formation yielded some rhyniophytoids including Cooksonia
ND
Perton Lane Late Silurian; P ídolí Classic locality and type locality of Cooksonia
ND
Freshwater East Late Silurian; P ídolí Most diverse Silurian flora in the world
FD
Llanover Quarry Early Devonian; Siegenian
Classic locality yielding one of most diverse PsilophytonZone flora in Britain
ND
Craig-y-Fro Quarry Early Devonian Some of best preserved Devonian plants in Britain; locality second only to the Rhynie site in Britain
SD
Table 1.3 GCR sites in the Old Red Sandstone described in the Palaeozoic palaeobotany GCR volume. AfterCleal and Thomas (1995).
History of research
11
Site selection is inevitably subjective, but theaim of the GCR is to identify the minimumnumber and area of sites needed to demonstratethe current understanding of the diversity andrange of features within each GCR ‘Block’. Thepreferred sites are generally those that are leastvulnerable to the potential threat of destruction,are more accessible and are not duplicated else-where (Ellis et al., 1996). The original selectionof sites was made over 20 years ago, and all ofthese sites are included in this volume. Inaddition, a small number of sites were identifiedduring the course of the compilation of thevolume as representing stratigraphical units orunique features not included in the original GCRsite selection, and are described as ‘potential’GCR sites.
HISTORY OF RESEARCH
The name ‘Old Red Sandstone’ appears to havebeen first applied to the red rocks below theMountain (Carboniferous) Limestone in themistaken belief that they were the equivalents of the Permian Rotliegendes of Germany(Jameson, 1821; Simpson, 1959). It was initiallymapped and named in southern Wales and theWelsh Borderland (Phillips, 1818; Conybeareand Phillips, 1822) and included as thelowermost part of the Carboniferous System.Murchison was the first to champion the OldRed Sandstone as a separate geological entity.According to Miller’s account (1841), a visitingforeign geologist advised Murchison that ‘youmust inevitably give up the Old Red Sandstone:
Site Stratigraphy Criterion Treatment in this volumeFull description FD
Summary description SD Not described ND
Anglo-Welsh Basin Marloes Wenlock–P ídolí Classic site showing early
transition from marine to Old Red Sandstone facies
ND (included with report for Albion Sands and
Gateholm Island) Albion Sands and Gateholm Island
Ludlow–P ídolí–Lochkovian
Complete, conformable succession from Ludlow into early Devonian
SD
Freshwater East (South) Wenlock–P ídolí Wenlock marine strata overlain by Old Red Sandstone; faulted/ unconformable relationship
ND
Ludford Lane and Ludford Corner
Ludlow–P ídolí Classic, internationally renowned site traditionally regarded as reference section for Silurian–Devonian boundary; earliest known land animals, early plants (see Table 1.3), unusual arthropods and fish remains in Ludlow Bone Bed
ND
Brewin’s Bridge/Canal Ludlow–P ídolí–Carboniferous
One of few sites in central England exposing marine Silurian–Old Red Sandstone junction, including Ludlow Bone Bed
ND
Capel Horeb Quarry Ludlow–P ídolí Good section of unconformity between Ludlow and P ídolí; internationally important plant site (Table 1.3)
ND
Little Castle Head P ídolí Old Red Sandstone facies rocks; Townsend Tuff Bed
SD
Lower Wallop Quarry Ludlow–P ídolí Marine to Old Red Sandstone transition later here, well into P ídolí
ND
Table 1.4 GCR sites in the Old Red Sandstone described in the Silurian stratigraphy GCR volume. AfterAldridge et al. (2000).
Introduction to the Old Red Sandstone of Great Britain
12
Site Stratigraphy/ radiometric age
GCR selection criteria
Eshaness Coast Mid-Devonian Representative of Eifelian Eshaness volcanic succession, NW Shetland.
Ness of Clousta to the BrigsMid-Devonian Representative of Givetian Clousta volcanic rocks, Walls, Shetland.
Point of Ayre Mid-Devonian Representative of Givetian Deerness Volcanic Member, mainland Orkney.
Too of the Head Mid-Devonian Representative of Givetian Hoy Volcanic Formation, Isle of Hoy, Orkney.
South Kerrera Late Silurian to Early Devonian
Representative of Lorn Plateau Volcanic Formation. Exceptional examples of subaerial lava features and interaction of magma with wet sediment.
Ben Nevis and Allt a’Mhuilinn
Mid-Silurian425 Ma
Representative of Ben Nevis Volcanic Formation. Exceptional intrusive tuffs. Internationally important as example of exhumed root of caldera, and historically fordevelopment of cauldron subsidence theory.
Stob Dearg and Cam Ghleann
Mid-Lochkovian421 ± 4Ma
Representative of succession in eastern part of Glencoe caldera, including basal sedimentary rocks. Exceptional rhyolites, ignimbrites and intra-caldera sediments. Possible international importance for radiometric dating in conjunction with palaeontology close to Silurian–Devonian boundary.
Crawton Bay* Late Silurian–Early Devonian
Representative of Crawton Volcanic Formation.
Scurdie Ness to Usan Harbour
Early Devonian Representative of ‘Ferrydean’ lavas and ‘Usan’ lavas, comprising lower part of Montrose Volcanic Formation.
Black Rock to East Comb Early Devonian Representative of ‘Ethie’ lavas, comprising upper part ofMontrose Volcanic Formation.
Balmerino to Wormit Early Devonian (Lochkovian)410.6 ± 5.6 Ma
Representative of eastern succession of Ochil VolcanicFormation. Possible international importance forradiometric dating in conjunction with palaeontology close to Silurian–Devonian boundary.
Sheriffmuir Road to Menstrie Burn
Early Devonian 416 ± 6.1 Ma
Representative of western succession of Ochil Volcanic Formation. Exceptional topographic expression of Ochil fault-scarp.
Tillycoultrie Early Devonian 415–410 Ma
Representative of diorite stocks, intruded into Ochil Volcanic Formation, surrounded by thermal aureole and cut by radial dyke swarm. Exceptional examples ofdiffuse contacts due to metasomatism and contamination, with ‘ghost’ features inherited from country rock.
Port Schuchan to Dunure Castle
Early Devonian Representative of Carrick Hills volcanic succession. Exceptional features resulting from interaction of magma with wet sediment are of international importance.
Culzean Harbour Early Devonian Representative of inlier of Carrick Hills volcanic succession. Exceptional features resulting from interactionof magma with wet sediment are of international importance.
Turnberry Lighthouse to Port Murray
Early Devonian Representative of most southerly inlier of Carrick Hills volcanic succession. Exceptional features resulting from interaction of magma with wet sediment are ofinternational importance.
Pettico Wick to St Abb’s Harbour
Early Devonian c. 400+ Ma
Representative of volcanic rocks in the SE of the Southern Uplands. Exceptional vent agglomerates, block lavas, flow tops and interflow high-energy volcaniclastic sediments.
* described in this volume
Table 1.5 GCR sites with Old Red Sandstone sedimentary rocks described in the Caledonian igneous rocksvolume. After Stephenson et al. (1999).
Old Red Sandstone palaeogeography
13
it is a mere local deposit, a doubtfulaccumulation huddled up in a corner, and hasno type or representative abroad.’
‘I would willingly give it up if Nature would,’replied Murchison, ‘but it assuredly exists, and Icannot’. Compared to the richly fossiliferousrocks of the Silurian System below and theCarboniferous System above, the Old RedSandstone seemed relatively barren to the earlyVictorian workers, but as the remains of earlyfishes were discovered, first in Scotland by theyoung Swiss naturalist Louis Agassiz, and later insouth Wales and the Welsh Borderland, interestgradually increased.
The Devonian System was established bySedgwick and Murchison (1839) for the pre-Carboniferous marine rocks of Devon. Theserocks were readily correlated with the Rhenishnearshore rocks and the Bohemian deep-waterrocks of mainland Europe (House, 1977). Withthe recognition of large tracts of Old RedSandstone in North America, Norway, Siberia,Poland and Russia (the last containing many ofthe same fish species as Great Britain), the strataassumed a new importance (Geikie, 1879). Atthe same time, Murchison (1839), impressed bythe great thickness of Old Red Sandstone stratain the Welsh Borderland, and the differencebetween them and the overlying Carboniferousrocks, with which they had hitherto beenmerged, applied the status of ‘system’ to the OldRed Sandstone. This situation held, in the UK atleast, for over 130 years, with the term used in aquasi-chronostratigraphical sense for rocks ofcontinental facies and Devonian age. However,with the advent of more precise stratigraphicalprocedures and classification, and, in 1972, thenew definition of the base of the DevonianSystem at a higher level, equivalent to a horizonwithin the Old Red Sandstone (see below), theterm is now no longer used in a quasi-chronostratigraphical sense. Biostratigrapherstend not to use the term at all; the glossary in the companion GCR volume on Silurian strati-graphy, for example (Aldridge et al., 2000),defines the Old Red Sandstone as ‘a classic termstill applied to the terrestrial, largely clasticfacies of the late Silurian to earliestCarboniferous in Britain’. Sedimentologistsretain the name as a facies (or magnafacies) termfor all the terrestrial red beds and lacustrinedeposits of Silurian to early Carboniferous
(but predominantly Devonian) age (e.g. Friendand Williams, 2000). The term ‘Old Red Sandstone’ is also applied in an informal litho-stratigraphical sense. The three subdivisions of the Old Red Sandstone recognized byMurchison – Lower, Middle and Upper – aresimilarly retained as informal, but long-established lithostratigraphical terms onshore inthe United Kingdom and as formal groupsoffshore in the North Sea.
OLD RED SANDSTONE PALAEOGEOGRAPHY
The Old Red Sandstone represents a periodwhen ocean closure and continental collisionsresulted in a world geography vastly different to that of much of early Palaeozoic times. Thedrift of the early Palaeozoic continents and theirrelative positions can be estimated from thecorrelation of geological successions and theirfaunas, with palaeomagnetism providing data on palaeolatitudes. The Iapetus Ocean, whichseparated the northern (Laurentian) and southern(Gondwanan) continents, closed throughout theOrdovician and Silurian periods as the smallercontinent of Avalonia fragmented from Gondwanaand drifted northwards (Figure 1.3). As theIapetus Ocean closed north of Avalonia, theRheic Ocean opened behind it. To the east, thecontinent of Baltica also drifted northwards andeastwards and the Tornquist Sea, an arm of theIapetus Ocean, slowly closed. The timing andnature of the convergence of the three compo-nents that were to make up the Old RedSandstone continent remain matters of debate.Trench and Torsvik (1992) considered thatBaltica and the eastern part of the Avaloniamicrocontinent collided first, in late Ordoviciantimes, moving northwards together to make firstcontact with Laurentia by late Silurian time atabout 420 Ma (Torsvik et al., 1996; see Deweyand Strachan, 2003, fig. 1). However, Dewey andStrachan (2003) interpret the Scandian Orogenyas the result of collision, by sinistral transpres-sion, of Baltica and Laurentia from about 435 Mato 425 Ma, with a soft collision between Avaloniaand Laurentia/Baltica (Laurussia) at about425 Ma (Soper and Woodcock, 2003). By lateSilurian (Ludlow) time, the continents had fullydocked, with the Iapetus Ocean closed along the
Introduction to the Old Red Sandstone of Great Britain
14
Old Red Sandstone palaeogeography
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line of subduction (the Iapetus Suture) underthe Southern Uplands. Thus, the Caledonian–Appalachian Orogen (or North AtlanticCaledonides) and the newly amalgamated OldRed Sandstone continent (Laurussia orEuramerica) were formed. Continuingcompression and shortening of the continentalcrust resulted in the filling to sea level of theSilurian marine basins, their inversion to uplandareas and the establishment of terrestrialconditions in newly developing basins.
Palaeogeographical reconstructions (e.g.Scotese, 2001) suggest that the continent lay intropical to sub-tropical latitudes from theequator to about 30°S, with the Anglo-Welsh Basin lying approximately 5°S to 15°S.Palaeomagnetic data from the Lower Old Red Sandstone in southern Wales suggest a latitude of 17 ± 5°S (Channell et al., 1992).Sedimentological studies of the Old RedSandstone, and particularly of its fossilcarbonate soils (calcretes) confirm, by analogywith modern calcretes, a warm to hot, semi-aridtropical to sub-tropical setting (e.g. Allen, 1986)with rainfall confined to wet seasons (e.g.Marriott and Wright, 1993). Uplift of the orogenmay have caused broad variations in the rainfallpattern, producing periods of wetter and drierclimate.
Woodcock (2000a), Friend et al. (2000),Dewey and Strachan (2003) and Soper andWoodcock (2003) presented recent overviews ofthe tectonics and kinematics of Old RedSandstone basin formation. Superimposed onthe broadly compressive stresses associated withconvergence of the Avalonian and Laurentiancontinental margins, the oblique angle ofclosure produced strike-slip transpressive andtranstensional movements. The nature and
extent of these movements remain the matter ofdebate, largely centred on whether there was amajor, orogen-wide sinistral megashear orwhether basins were controlled by strike-slipmovements of different sense and at differenttimes during the Caledonian orogenic cycle.Another debate concerns the possible role ofgravitational collapse of the uplifted, granite-buoyed Caledonian Orogen in the formation ofsome at least of the internal basins (e.g.Woodcock, 2000a). Dewey and Strachan (2003)conclude that the diachronous closure ofIapetus, and subsequent deformation and basin formation were controlled by sinistrallydominated relative movement between theLaurentian and Avalonia–Baltica plates. The OldRed Sandstone basins probably formed as aresult of sinistral transtension, with an estimated1200 km of strike-slip movement betweenLaurentia and Baltica. Rheic convergence in the Emsian Age (late Early Devonian) from400 Ma to 390 Ma resulted in the AcadianOrogeny, which affected the basins south of theHighland Boundary Fault (Soper and Woodcock,2003).
The Old Red Sandstone basins were formerlydivided into two main groups on the basis oftheir positions relative to the Caledonian Orogen(e.g. Allen, 1977; Woodcock, 2000a). Thosewithin it (internal or intramontane basins)include the Orcadian Basin, the basins of theMidland Valley of Scotland (but see below) andthe Scottish Border Basin. The Anglo-WelshBasin was regarded as an external, or extra-montane basin, open to the sea to the south.However, the recent models, invoking majororogen-parallel, sinistral movement and threeseparate, temporally discrete collision events(Grampian, Scandian and Acadian) collectivelymaking up the Caledonides have revised theformer view of a continuously progradingCaledonian mountain front.
The recent models (Dewey and Strachan,2003; Soper and Woodcock, 2003) envisage that the highly oblique, sinistral closure of the Iapetus Ocean resulted in, sequentially,transpression, strike-slip and transtension. Thearea of maximum uplift in the Scandian Orogenwas to the north of Britain, in Scandinavia, in an orogen of Himalayan proportions (Deweyand Strachan, 2003). The compression in theLaurentian crust, of which the Scottish
==Figure 1.3 Sketch maps showing the movements and amalgamation of the early Palaeozoic continentsthat produced the Old Red Sandstone (Laurussia)continent. (a) and (b) are global views to illustratethe fragmentation of Avalonia from Gondwana and itsdrift northwards as the Iapetus Ocean closed(adapted from Torsvik et al., 1992, by Trench andTorsvik, 1992). (c), (d) and (e) show the later stagesof the Caledonian Orogeny. Sinistral strike-slip move-ments in relation to the Laurentian marginculminated in the Acadian Orogeny in late EarlyDevonian (Emsian) times (after Stephenson et al.,1999, adapted from Soper et al., 1992).
Introduction to the Old Red Sandstone of Great Britain
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Highlands were part, caused thrusting alongmajor NE-trending faults, granitic intrusion,andesitic volcanicity and low-grade metamor-phism in northern Britain (Stephenson et al.,1999). The volcanic rocks were probablyextensive, their eroded remnants being seen atBen Nevis, Glen Coe, Lorn and just north of theHighland Boundary Fault. Volcanic rocks alsooccur extensively within the Midland Valley ofScotland, at Montrose, in the Sidlaw, Ochil andPentland hills, and in Ayrshire. They also occurmore locally in the Southern Uplands, wheregranitic intrusions such as the Cheviot wereemplaced.
During the transcurrent and transtensionalphases, much of the orogen-parallel, sinistralmovement appears to have been taken up by theGreat Glen Fault and its north-east continuation,with at least 700 km of displacement (Deweyand Strachan, 2003). The formation of the LateSilurian–Early Devonian Old Red Sandstonebasins is also attributed to sinistral transtension(Dewey and Strachan, 2003; Soper andWoodcock, 2003). The Acadian Orogeny endedthis phase of basin formation and causedtranspressive shortening of the early Palaeozoicbasins flanking the Midland Microcraton, as well as the inversion and erosion of the Old Red Sandstone rocks not underlain by the micro-craton (Soper and Woodcock, 2003). The causeof the Acadian event was probably the collisionof a Gondwana-derived continental fragment(Soper and Woodcock, 2003) with the MidlandMicrocraton segment of the amalgamatedLaurussian continent. The evidence for theterrane boundary in the vicinity of the BristolChannel is now confined to the Lizard mafic–ultramafic complex, interpreted as an ophioliteand a fragment of the Rheic suture (e.g. Soperand Woodcock, 2003).
The Orcadian Basin was a large Mid-Devonianintramontane lake basin, totally unconnected to the open sea, apart perhaps from a briefperiod. Its formation was probably due to acombination of both gravitational extension, andtranstensional movements on basin-marginfaults. The Midland Valley of Scotland was not asingle discrete basin in the Devonian Period.Weakened by a long history of igneous activity,internal, transtensional fault movements openedpull-apart basins and transpressive movementssubsequently inverted them, resulting in therecycling of the basin-fills and providing weak
points for continuing volcanic extrusion (e.g.Bluck, 2000). The preserved sequences thusrepresent the deposits of separate pull-apartbasins, formed and brought together in a strike-slip faulted collage. The Stonehaven Basin inthe north-east is the earliest, its sedimentary filldating perhaps from the Wenlock Epoch(Marshall, 1991). It and its larger successorbasins, the Crawton and Strathmore basins,formed by sinistral strike-slip along the HighlandBoundary Fault. The southerly Lanark Basinformed along the Southern Upland Fault. Largevolumes of arc-related volcanic rocks wereextruded along the central axis of the MidlandValley, on lines weakened by the transtensionalstresses (e.g. Bluck, 2000). The late DevonianScottish Border Basin formed after Acadianinversion in Mid-Devonian times and extendedinto the Midland Valley, Northumberland andthe Solway Firth.
The Anglo-Welsh Basin was formerlyinterpreted primarily as the product of load-generated flexural subsidence of the Caledonianforeland (James, 1987; King, 1994; Friend et al.,2000). Dewey and Strachan (2003) and Soperand Woodcock (2003) prefer a transtensionalmechanism for its formation. Transtensionalmovement on faults produced variations in thebasin-fill in Pembrokeshire (e.g. Marshall,2000a,b) and introduced coarse, clastic, detritusfarther north (Tunbridge, 1980a). The isolatedsuccession in Anglesey was probably depositedcontiguously with the PÍídolí–Pragian sequencesto the south, with which there are markedsimilarities, although the initial coarseconglomerates are unique and of localderivation, and lacustrine deposits suggestinternal or impeded drainage.
STRATIGRAPHICAL FRAMEWORKFOR THE OLD RED SANDSTONE
Stratigraphical classification of the rocks in thegeological record has traditionally fallen into two broad categories – lithostratigraphical and chronostratigraphical. Lithostratigraphicalclassification is based on the physicalcharacteristics of a rock body, such as colour,rock type (lithofacies) and mode of formation.Chronostratigraphical classification is based onthe relative age of a rock body, determined by itsfossil content (biostratigraphy) as correlated with
Stratigraphical framework for the Old Red Sandstone
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standard, defined and internationally agreedgeological marker horizons (the ‘golden spikes’),and in the case of igneous rocks, by radiometricage dating. Biostratigraphical classification isachieved by the study of component fossil andmicrofossil groups, with subdivisions based onmarker species or assemblages of species. Thus,in the Devonian System, there are biostrati-graphical zonal schemes for graptolites,ammonoids, brachiopods, fish, conodonts,microvertebrates and miospores.
The chronostratigraphical subdivisions of theUpper Silurian and Devonian (Figure 1.4) areinternationally agreed and defined in fossili-ferous marine strata in continental Europe. TheUpper Silurian PÍídolí Series (not yet dividedinto stages) and Lower Devonian Lochkovianand Pragian stages are defined in the deep-water,graptolite-bearing succession of the PragueBasin in the Czech Republic. The last tworeplace the previously used, but not completelyequivalent Gedinnian and Siegenian stagesdefined in the nearshore succession of theGerman Rhenish Basin. The highest LowerDevonian stage is the Emsian, defined inBelgium. The Middle Devonian stages (Eifelian
and Givetian) are defined in Germany, the UpperDevonian stages (Frasnian and Famennian) arenamed from the carbonate-bearing marinesuccession of southern Belgium.
The problems of classification and correlationof the Old Red Sandstone of Great Britain areinherent in its terrestrial origins and the patchypreservation of its non-marine fossils. Thefossils that are present indicate that theDevonian Period was a time of profoundchanges in the evolutionary record, with the firstsignificant colonization of terrestrial habitats byvascular plants (Edwards, 1979a), the rapidexpansion of the first aquatic vertebrates, andtheir emergence onto land. However, no directcorrelations can be made with the Europeanmarine successions and the internationallyagreed stages. Because of this, a series of looselydefined local stages (Downtonian, Dittonian,Breconian and Farlovian) were erected for theAnglo-Welsh Basin, but now have been largelysubsumed into the international stages as aresult of increasing refinement in correlation.Figure 1.4 (based on House et al., 1977; andMarshall and House, 2000) shows the stages andtheir correlation.
Figure 1.4 Major subdivisions of the Old Red Sandstone and its chronostratigraphical classification. Ages fromWilliams et al. (2000).
Introduction to the Old Red Sandstone of Great Britain
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The principal macrofossils are fish fragments.A biozonal scheme was erected for the Old RedSandstone in the Anglo-Welsh Basin (see Figure5.3, Chapter 5) and was extended to continentalEurope. Refinement of the scheme continues(Blieck and Janvier, 1989; Blieck and Cloutier,2000), but the occurrence of fish remains ispatchy and of limited use in high-resolutioncorrelation. Miospore classifications (e.g.Richardson et al., 2000; Streel et al., 2000) andmicrovertebrate classification (Vergoossen,2000) also aid correlation and stratigraphicalresolution. However, the problem of detailedcorrelation of the terrestrial Old Red Sandstonesuccession with the Bohemian, German andBelgian marine stages, in which miospores are
rare, remains. Progress is, however, being made by chains of correlation involvingmiospores that are common to the Old RedSandstone and the Rhenish marine succession,the latter then being correlated with theBohemian stages. For example, the recognitionof the Breconensis-zavallatus Zone in theArdennes allows correlation of the Anglo-Welshand Rhenish Gedinnian–Siegenian successions(Richardson et al., 2000) (Figure 5.3, Chapter5). A widespread volcanic ash deposit (theTownsend Tuff Bed) and a basin-wide calcrete(the Psammosteus Limestone) are valuablemarker horizons in the Anglo-Welsh Basin,providing lithostratigraphical correlation of thesuccession.