Old Red Sandstone Rocks of Great Britain · 2014. 10. 11. · Introduction to the Old Red Sandstone of Great Britain 4 broadly reflect the original sedimentary basins in which they

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).


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

.


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



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.


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.



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:



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).


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


14

Old Red Sandstone palaeogeography

15

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).


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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).


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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.

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Old Red Sandstone Rocks of Great Britain · 2014. 10. 11. · Introduction to the Old Red Sandstone of Great Britain 4 broadly reflect the original sedimentary basins in which they