Kerogen variation in a Devonian half graben system

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UNIVERSITY OFSOUTHAMPTON

Kerogen

variation in

a

Devonian

half

graben

system

Reuben Guthrie

Speed BSc,

MSc.

Doctor

of

Philosophy

Department

of

Geology

UNIVERSITY OF SOUTHAMPTON ABSTRACT

FACULTYOF SCIENCE GEOLOGY Doctor of

Philosophy

KEROGEN VARIATION INADEVONIANHALF GRABEN SYSTEM

by

Reuben Guthrie

Speed

The Middle Devonian

Rousay Flagstone

_{Formation of}

Orkney

is a 200m thick lacustrine

succession that containsabundant

preserved organic

_{matter. Itwas}

deposited

_{intoaseries of half}

graben

formed

by

the

collapse

of over-thickened Caledonian crust. The 14 lake

cycles

that

comprise

the

Rousay Flagstone

Formation

(RFF)

werecorrelatedacross

Orkney during

6months of fieldwork. Two of thelake

cycles

were

sampled

in detail for

geochemical

analysis.

This work has enabled an

understanding

of the

sedimentary

_{and tectonic processes that controlled the} distribution of facies and the

quality

ofsourcerocks within the Orcadian Basin

during

this time. The processes and environments

present

in

Orkney during

this time may be

compared

to the Horton

Group

in Nova Scotia

(Hamblin

& Rust

1989).

The main

finding

_{of this research has been the extent to which the East}

Scapa

Fault

(ESF)

caused variation in sedimentation.

By

slowly extending throughout

the

RFF,

the half

graben

bounding

fault caused certainareasof

Orkney

to

experience

continued relative

uplift.

Two

aspects

ofthe structure of the half

graben

were of

greatest

influence.

Firstly

the

uplifted

footwall of the half

graben

provided

an environment away from the influence of

inflowing

sediment and oxygen rich

turbidity

currents. The

quiescent

environment in this areaallowedthe

greatest amount of laminite facies accumulation. It was found that

high

TOC

(total

organic

carbon),

H/C

(hydrogen/carbon ratio)

_{and spore numbers} were associated with these areas of

enhanced laminite

deposition.

The secondarea wasatransferzone locatedatthe northern

splay

of the ESF. The zoneactedas a

linkage

zonebetween the ESF andahalf

graben

tothe north. Because of itslocationbetween

adjacent

half

graben depocentres,

sedimentation was affected

by

therelative

uplift

of the areain amannersimilartothe

uplifted

footwallareatothewestof theESF.

The main

agents

thatweredetrimental totheformationofsourcerocks were

turbidity

currents.

Thesecurrents

originated

from thethreemain areas ofalluvial fan

input

in the basin.

Turbidity

currents

carrying

sediment and

oxygenated

water from these fans would

bypass

the shallower andmore

uplifted

areasand

preferentially

deposit

in themoredistal anddownthrownareas. The area

immediately

tothe eastof the ESF wasthe main location to have

experienced

reductionin

kerogen quality

(TOC,

H/C and spore

numbers)

because ofturbidite

deposition.

The

preserved organic

matteris

predominantly composed

of

amorphous

organic

_matter,

making

the main

kerogen

type

Type

I. About 40% of each

Rousay

Flagstone

Formation lake

cycle

contains measurable

organic

_matter, on _{average about 0.8%.} The amount of

organic

matter is controlled

by

facies

type,

_{with laminite facies}

having

_the

highest

average TOC

(1.55%)

and grey silts

having

least

(0.3%).

Exinite

reflectivity

_{and spore colour variation}

analyses

_{from across}

Orkney

indicate that the thermal

maturity

of the sediments is _{within the thermal range of}

hydrocarbon generation.

Additionally

the uniform

spread

of

maturity

valuesacross

Orkney

indicatesthatfaultmovement

Contents

Page

Abstract _i

Contents _ii

List of

Figures

_v

List of Tables and Plates vii

Acknowledgements

viii

Faim _ix

1. Introduction/Aims _1-1

1.1 Aims _1-1

1.2 Introduction _1-2

1.3

Regional geology

_1-4

2. Astrati

graphic

framework _2-1

2.1 Abrief

history

of the

study

of the

Orkney Flagstone

2-1

Group

2.2 Note on the use of fossil fish in the

stratigraphy

of 2-3

Orkney

2.3

Introducing

the

present

correlation of the

Rousay

2-4

Flagstone

Formation

2.4Fieldwork _2-5

2.5 Pointsto notefrom the

stratigraphic

framework 2-6

3.

Sedimentology

_3-1

3.1 Facies

descriptions

_3-1

Laminites _3-2

Near laminites _3-2

Dark silts _3-3

Sandy

dark silts 3-4

Wick _3-4

Dark Wick 3-6

Mid_{grey silts 3-6}

Blue silts _3-6

Wavy

green sands 3-7

Wavy

greensilts 3-7

Sheetfloods _3-8

Mixed flats 3-8

Channel sands 3-9

Conglomerates

3-9

Summary

oftheenvironment of

deposition

of the

Rousay

3-10

Flagstone

Formation

3.2 Noteonthe

comparison

of facies

types

asdefined

by

3-11 other workers in the Orcadian basin

3.3 Presentation of

sedimentary

data 3-12

Contents continued

4.

Sampling

and methods 4-1

4.1

Sampling

_4-1

4.2

Palynology

_4-2

4.21 Absolute_{spore number}counts 4-3

4.22

Spore

_{diameter variation} 4-5

4.23 Visual _sporecolouridentificationasameasure

'

4-7 of

maturity

4.24 Falsestarts 4-9

4.3 Exinite

reflectivity

_4-10

4.4Elemental

analysis

_4-12

4.41 TOC _4-13

4.42 TOC

profiles

asameasureof

weathering

4-14

4.43 Atomic H/C & O/C ratios 4-15

5.Results _5-1

5.1 Individuallake

profiles

_5-2

5.2

Palynological

andfacies_maps 5-10

5.21

Description

of thelithofacies_maps 5-11 5.22

Description

of the

kerogen

distribution_maps 5-11 5-23

Description

of the

maturity

_{data maps} 5-12 5-24

Description

ofthe

megafacies

distribution 5-14 maps

5.25

Description

of the

palaeocurrent

maps 5-14

5.3

Megafacies graphs

_5-38

5.4Correlation

graphs

_5-42

5.5 Loth

Quarry

TOC

profiles

5-54

6.Discussion

Introduction _6-1

6.1 The effect offaultmovementonsedimentation 6-2 6.11 A

possible

transferzonein the North of 6-5

Orkney

6.12 Differential

compaction

6-6

6.2 Thedistributionof

specific

facies

types

across

Orkney

Introduction 6-6

6.21 The controls on the distribution oflacustrine 6-7 sediments

6.21 iDistributionof laminite andnearlaminite 6-8 facies

6.2 lii Distribution of

sandy

dark silt and

organic

6-11

matterrich facies

6.22

Synthesis

and

explanation

of the

sedimentary

6-14 trendsseeninthe lacustrinesediments of the

Rousay Flagstone

Formation

Contents continued

6.3 The controlsonthedistributionand

preservation

of

organic

matter

Introduction _6-16

6.31 The thermal

maturity

of the

Rousay

Flagstone

6-16 Formation

6.32 TOC and H/Cvariationacross

Orkney

6-18

6.32i TOCand facies

type

6-18

6.32Ü

TOC,

H/C andfacies

type

6-19

6.32iii TOC and H/C

spatial

variation 6-20

6.33

Spore

variation_across

Orkney

_6-22

6.33i

Spore

numbervariation 6-22

6.33Ü

Spore

diameter _6-23

6.33iii

Spore

numbersversusTOC and H/C 6-24

6.33iv Profiles 6-25

6.34

Synthesis

of thedistributionand

preservation

6-27 of

organic

matter

6.4 Distribution

offluvially

dominated sediments 6-29

6.5 A

changing

environment 6-31

6.6Palaeoenvironmentalreconstruction 6-34 7.Conclusions

8. References

Appendix

1. Listof locations

logged during

thesummersof1996 & 1997

Appendix

2.

Palynological

datafrom

cycles

26 and 36

Rousay Flagstone

Formation

Appendix

3. Breakdown offacies

logged

Appendix

4.

Megafacies

data

Listof

Figures

page

1-1

Location,

regional geology

andislandnames 1-8

1 -2 Schematic

stratigraphic

column of theDevonianof

Orkney

1 -9

1-3 Schematic

diagram

of~2lake

cycles

1-9

1 -4Astin's

stratigraphic

framework _1-10

1-5 _{Structure map of}

Orkney

_1-11

1-6 Crosssectionof north

Mainland,

Rousay

and

Eday.

1-11

1-7 Offshore fault_{map 1-12}

2-1

Rousay

Flagstone

Formation

stratigraphic

correlation 2-9

2-2 Location map of

key logged

sections 2-10

3-1

Proposed

model of Wickfacies formation 3-15

3-2 Faciesmodelfor terminal fans 3-16

3-3

Sacquoy

Sandstone Member clast sizedistribution 3-17 3-4

Regional geological

structureof

Orkney

as

interpreted

from 3-18

geophysical

and

geological

data

4-1

Rhabdosporites langii photomicrographs

_4-18

4-2 Schematic

diagram

ofanElemental

Analyzer

4-19

4-3 Van Krevelen

diagram

_4-19

5-1 Lake

profiles-Evie,

Rousay, Noup Head,

Surrigarth

5-5 5-2 Lake

profiles-North

Faray,

Fers

Ness,

Wars

Ness,

Spurs

Ness 5-6 5-3 Lake

profiles-Loth, Stronsay,

Head of

Work, Tankeraess, Burray

5-7

5-4 Lake

profiles-Flotta,

Hoxa

Head,

Halcro Head 5-8

5-5

Summary

_map

showing

the

relationship

betweenabsolute 5-9 spore

numbers,

facies

type

and

position

within the basin

5-6

Key

findings

-lacustrine facies

5-15

5-7

Key findings

-fluvial facies 5-16

5-8

Key findings

-

kerogen

data 5-17

5-9

Cycle

thicknesses

(cm)

_5-18

5-10

Percentage

laminiteandnearlaminite facies 5-19 5-11 Thickness laminiteandnearlaminite facies 5-20 5-12

Percentage organic

matter

containing

facies 5-21

5-13Thickness

organic

matterrichfacies 5-22

5-14

Percentage sandy

dark silt 5-23

5-15

Percentage

sand rich facies 5-24

5-16Thicknessessand richfacies 5-25

5-17

Percentages

of all sheetfloodfacies 5-26

5-18Thicknessesallsheetfloodfacies 5-27

5-19

Percentages

mediumandcoarse

grained

sheetfloods 5-28 5-20 Maximum number of spores per gram of rock:

cycle

26

cycle

36 5-29

5-21

Average

TOC %:

cycle

26, cycle

36 5-30

5-22a

Average

exinite

reflectivity:

5-31

5-22 bMostcommon_{spore colours:} 5-31

5-23 a

Average

H/C values

cycle

26: 5-32

5-23b

Average

Rhabdosporites

langii

diameter: 5-32

List of

figures

continued

5-24

Percentage

lacustrine

megafacies

_5-33

5-25

Percentage

emergent

megafacies

_5-34

5-26

Percentage

fluvial

megafacies

_5-35

5-27

Average

thickness

(cm)

megafacies

_5-36

5-28Palaeocurrent_maps 5-37

5-29 West-east

megafacies

thickness variation

(cm)

5-40 5-30 North-south

megafacies

thickness variation

(cm)

5-40

5-31 West-east

megafacies

percentage

variation 5-41

5-32 North-south

megafacies

percentage

variation 5-41

5-33a TOCvmicrofacies

cycle

26 _5-47

5-33b TOCv

Abspore cycle

26 _5-47

5-33c

Abspore

vmicrofacies

cycle

26 5-47

5-34a TOCvmicrofacies

cycle

36 _5-48

5-34b TOCv

Abspore

cycle

36 5-48

5-34c

Abspore

vmicrofacies

cycle

36 _5-48

5-35

Average

TOC

cycles

26 and 36

compared

_5-49

5-36a

Abspore

vH/C 5-50

5-36b Microfacies vH/C _5-50

S^cTOCvH/C 5-50

5-37

Spore

colourversusexinite

reflectivity

5-51

5-38Exinite

reflectivity

vmicrofacies 5-51

5-39a Van Krevelen

graph

Evie

cycle

26 5-52

5-39b Van Krevelen

graph Surrigarth cycle

26 5-52

5-40a

Surrigarth

O/Cvmicrofacies 5-53

5-40b

Surrigarth

H/Cvmicrofacies 5-53

5-40cEvie O/Cvmicrofacies 5-53

5-40d Evie H/Cvmicrofacies 5-53

5-4la Loth TOC

profiles

#1 and #2 5-56

5-4 lb Loth TOC

profile

#2 with

palynological

components

5-56

6-1 Main trends

relating

tolacustrine

phase

deposition

6-36

6-2 Main trends

relating

to

kerogen

distribution 6-37

6-3 Main trends

relating

tofluvial

phase

deposition

6-38

6-4 Half

graben

terminology diagram

6-39

6-5 Transferzonemodel 6-39

6-6

Splay

model 6-40

6-7 Lake

input

diagram

6-41

6-8 Exiniteversusvitrinitereflectivites 6-41

6-9 Model oflacustrine

phase deposition

6-42

6-10 Model of fluvial

phase

deposition

6-43

List of Tables and Plates

Page

Table 1-1 Thickness revisions of the

Rousay Flagstone

Formation 1-7

Table 2-1 Location_summaryof

logged

sections. 2-7to2-9

Table 3-1

Megafacies

sub-divisions 3-14

Table4-1 Particlesseenin

kerogen

isolate slides 4-5

Table 4-2

Key

characteristics of

Rhabdosporites langii

4-6

Table 4-3

Spore

colour chart 4-8

Table 5-1

Summary

of lake

profile graphs

5-4

Table 5-2 Loth

profile

#1 and #2 TOC results 5-55

Table 5-3 Loth

profile

#2

components

5-55

Table 6-1.

Average

percentages

andthicknesses of

organic

matter 6-8 rich

sediments,

per

cycle,

Rousay

Flagstone

Formation

Table 6-2

Average

TOC from

organic

rich

fades, cycles

26 and 36 6-19

Plate 3-1

Photomicrographs

laminite facies 3-19

Plate 3-2

Photomicrographs

laminite,

Wick and sheetflood facies 3-20 Plate 3-3 Field

photographs

laminite facies and mud cracks 3-21 Plate 3-4 Field

photographs

channel,

mixed flats and sheetfloodfacies 3-22

Acknowledgements

This

study

wouldnothave been

possible

ifnotfor the

previous

work of_my

supervisor

John

Marshall,

and Tim Astin. Both of who

inspired

metolookalittle

deeper

into

Orkney.

Thanks fortheconstant

support

and

encouragement

During

thecourseof the fieldwork for this

project,

I metmorewonderful

people

thanI

thought possible.

These kind souls shared_tents,

hostels, houses,

_{cars,vans,garages,}stone

circles,

food and drinkwithanover-enthusiastic

geologist

anddidn't

complain

much. John

Brown,

geologist

of

Stromness,

_{thank you for your time and vision.}

My

stay

inthesouthwasmade all thewarmer

by

the

friendship

of

Kevin,

Nik, Rob, Robin,

Gavin, Dom, Lawrence,

Big

Dan, Leila, Helen, Elena,

Sylvia,

Nora,

Jenny,

Rosie and

Lynda.

Cath,

I_{couldn't have done it without you.}

Shir Akbariis

gratefully

thankedfor

teaching

mehowtohandle

HF,

and for

processing

above and

beyond

the call of

duty.

Jack Saxon of Scrabster

helped

mefind the allusive Asmusia.

Cheers!

I would also liketothank

Tony

Hewett and Richard

Pawlyn

for their

prompt

and

pragmatic

help

aswellas accesstoKerr McGee's bountiful resources.

Big

thanks alsotoLawson Brown of Alastair BeachAssociates for the

breathing

_spaceto

finish this thesis.

The financial

support

ofaNERCCASE

studentship

with Kerr McGee Oil Ltd is

gratefully

acknowledged.

Faim

Si

j'ai

du

goüt,

ce_{n'est guere}

Que

pour laterreetles

pierres.

Je

dejeune

toujours d'air,

De roc, de

charbons,

de fer.

Mes

faims,

tournez.

Paissez, faims,

Le predessons.

Attirez la

gai

veniri Des liserons.

Mangez

les cailloux

qu'on

brise,

Lesvieilles

pierres d'eglises;

Les

galets

des vieux

deluges,

Painssemesdans lesvallees

grises.

Arthur Rimbaud 1854 -1891.

Dedicated

to

my Mum.

1.1 Aims

Thisdissertation

reports

afield and

laboratory

based

study

of the

sedimentary

facies and

preserved

organic

matter_presentwithin the MiddleDevonian lacustrine rocks of

Orkney.

The main aims of this workareto:

Establishabasin wide

stratigraphy

of the

Rousay Flagstone

Formation,

using

detailed

fieldmeasurement and

lithostratigraphic

correlation.

Describe the

sedimentary

and tectonic

regime

present

in

Orkney

during

the time of

deposition

of the

Rousay

Flagstone

Formation,

with

particular

referencetohow these factors affected the

quality

anddistribution of

sedimentary organic

matter.

Kerogenvariation inaDevonian_{half graben}system Introduction

1.2

Introduction

One is

doubly

blessed whenfieldworkin

Orkney

is undertaken. These islands which lie

immediately

north of Scotlandare oneof themostawe

inspiring places

in whichto

study

geology,

andareinhabited

by

someof themostkind hearted folkonecould wishtomeet.

Thereareabout20inhabited_{islands and numerous, smaller uninhabitedonesin the}

archipelago

(Figure 1-1).

Most ofthe 20 000Orcadians liveonthe

biggest

island

imaginatively

calledMainlandand which will bereferredtoassuch

throughout

this thesis.

The

general topography

is that of

gently rolling

hills which in

Rousay, Westray

and West Mainlandcanexceed

heights

of 250m.

High

seacliffsare

developed

tothewestof_manyof the islands.

Hence,

Orkney

is

ideally

suited for

geological study

becausea

significant proportion

of its coastline offers excellent exposure,

regularly

scoured clean

by

fierce Atlantic storms. In manycasesthe

generally gently dipping

strata_{may be}

easily

followed between

outcrops

both

onthe sameisland and between islands. Thismeansthat the 2500

km2

that the islands occupy, form the ideal

setting

for the

study

of the

large-scale sedimentary

system

that

comprised

thisareain Devonian times.

The

sedimentary

rocks of

Orkney

are

composed

mostly

of the

Orkney Flagstone

Group,

which is mid Devonian in age. This

Group (or

its

equivalent)

extends southto

Caithness and northto Shetland

(Mykura 1976).

Italsooccursoffshoretotheeast

(Duncan

and Buxton

1995)

and

possibly

tothewest of

Orkney

(Coward

&Enfield

1987).

Theareainto which these Devonian sedimentswere

deposited

is

generally

referredtoas

theOrcadian Basin.

These rocks include

organic

matterrich

sediment,

whichwere

deposited

acrossthe Orcadian

Basin,

inone or aseries of

large

stratified lakes. The

Orkney Flagstone Group's

lateral

equivalent

in the

Moray

Firth is

accepted

asthesourceof all

(Bailey

etal.

1990)

or

part

of

(Duncan

& Hamilton

1988)

the Beatrice oil field.

It is

generally accepted

that these sedimentswere

deposited

inasemi-arid

environment

(Donovan 1980, Rogers

& Astin

1991)

andwereassociatedwitha

variety

of

sand-rich

continental-type

facies. The

interplay

between the lake and sand rich sedimentscan

beseentobe the result ofa

periodically changing

environment. When the climatewaswetter lakes

formed,

and when itwas

drier,

sand rich sedimentswere

deposited by

terminalfans that

Kerogenvariation inaDevonian_{half grabensystem}

_

Introduction

transported

sandontothe

dry

lake beds

by

amixtureof processes. Thus the

Orkney Flagstone

Group

canbeseen as aseries of around 80

wet-to-dry sedimentary cycles

formed

by

climate variation

(most

recently

discussed in Marshall 1996 and Astin

1990).

Careful field

observation of these lake

cycles

has allowed_parts of the

Orkney Flagstone Group

tobe reconstructed and correlatedacross

Orkney

(Astin

1990 and this

thesis).

This

stratigraphic

frameworkhasallowed the

sedimentary

_{processes in individual lake}

cycles

tobe studied in detail.

Using

detailed

lithostratigraphic

correlations and labbased

organic

matter

analysis,

a

study

of the distribution and variation of the

kerogen

found in individual lakeunits

throughout

Orkney

provides

avaluable

insight

into the

depositional

_{processes that}were

operating

in the

ancient lake

system.

This information about the

potential

richness of Devoniansourcerocks

in the Orcadian basin will be ofusein future

hydrocarbon exploration

in thearea.

Kerogenvariation inaDevonian_{half graben}system Introduction

1.3

Regional Geology

Stratigraphy

/Sedimentation

The

majority

of

Orkney

is

composed

of continental sediments of

Early

toMid Devonian age, withrareoccurrencesof

exposed

pre-Devonian

metamorphic

basement in the

westof

Mainland,

andsomelate-Middle Devonian volcanic rock. The continentalsediments

were

deposited

ina

gently extending

and

subsiding

basinnow

generally accepted

as

being

caused

by

the

approximately

east-west

collapse

of over-thickened Caledoniancrust

(McClay,

etal..

1986).

Ofthese

sediments,

perhaps

themost

interesting

arethe group of Middle Devonian

lacustrine sediments knownasthe

Orkney

Flagstone

Group, comprised

of the

Upper

and

Lower Stromness and

Rousay

Flagstone

Formations

(Figure

1

-1).

Thesearethe

deposits

of anancient lake

system

that extended from the

Moray

Firthtothe Shetland Islands-an

original

distance ofover500km

(Rogers,

Marshall & Astin

1989).

The

Orkney Flagstone

Group

was

deposited

inadistalareaof this basin and

developed

amarked

cyclicity

caused

by

periodic

waterlevel fluctuations in the lake

(Donovan 1980).

The lake

cycles

represent

changes

fromwettertodrier environments of

deposition suggesting

that the lake

partially

dried up andwas

replenished frequently.

The

cycles

are

mainly

in the

region

of 10to20

metresthick. At

present

athickness ofover 1500m of these sedimentsare

preserved

within

the Orcadian Basin

(Astin 1991)

(Figure

1-2).

The

flagstone cycles

have been

interpreted by

Rogers

& Astin

(1991)

as

consisting

of aninitial

permanent

lake

deposit, forming

thelower

part

of each

cycle

and

containing

laminite

and other fine

grained

sediments

(along

with fish fauna and

stromatolites).

This isfollowed

by

sediment

deposition

inan

arid,

ephemeral

lake environment with associated terminal fan

deposits (Figure

1-3).

The transition between thetwo

depositional styles

is

thought

tohave been

rapid

and caused

by

cyclic

climatic

change

onthe scale of

123,000

_years

(Astin

1991,

Marshall

1996).

Hypersalinity

and anoxia

during

the

deposition

of the

lacustrine-type

facies has resulted in the

widespread preservation

of

organic

matter

(Marshall

etal.

1985,

Parnell

1985).

The

Orkney Flagstone Group

isdividedinto three formations

(Figure

1-2).

The Lower Stromness

Flagstone

Formation is

stratigraphically

the lowest and

onlaps

onto

metamorphic

basement inwest

Orkney.

Abovethisis the

Upper

Stromness

Flagstone

Formation. Thesetwo_{formations occupy}mostofwestMainland

(Figure

1-1).

Kerogenvariation inaDevonianhalfgrabensystem Introduction

The

boundary

between the

Upper

and Lower Stromness

Flagstone

Formations is the Sandwick Fish Bed

Cycle (the Orkney

lateral

equivalent

of the Achanarras fish bed of

Caithness).

This

cycle

is the

unusually

thick

deposit

ofa

laterally

extensive,

deep

lake,

which

persisted

for

longer

than usual and containsadiverse and distinctive fauna of fossil fish

(Wilson

etal.

1935,

Trewin

1986,

Fannin

1970).

Duetothemonotonousand

repetitive

nature of the lake sediments in the

Orkney Flagstone Group,

basinwidecorrelation

depends heavily

onthe

recognition

of this

unique

and distinctive horizon.

The third formation of the

Orkney

Flagstone Group

and the focus of this thesis is the

Rousay

Flagstone

Formation which is of Eifelian/Givetian age. The

Rousay

Flagstone

Formation is themost

widespread

unit in

Orkney,

occurring

overmostof the northern isles of

Orkney,

east

Mainland,

southeast

Hoy

and

parts

of the southern islands-

roughly

1600

km2

(Figure 1-1).

Inthe

past,

the

Geological

Survey

used the firstoccurrenceof the fossil

branchiopod

Asmussia

(formerly

Esther

id)

to

place

the base of the

Rousay

Flagstone

Formation

(Wilson

etal.

1935).

However,

recently

a

lithostratigraphic

framework has been

constructed

by

Astin

(1990),

which redefines the

Orkney Flagstones

intermsof correlatable lake

cycles (Figure

1-4).

Inthis framework the

Rousay

Flagstone

Formation is definedas

occurring

between the 25th lake

cycle

above the Sandwick Fish Bed

Cycle

and the

overlying

Eday Group.

Astin's work has also reduced the thickness of the

Rousay

Formationfroman

estimated 1675m

(Wilson

etal.

1935)

toaround 200 m

(Table 1-1).

The consequences of

Astin's work will be discussed in

Chapter

2.

The

Eday

Group

immediately

overlies the

Rousay Flagstone

Formation. It consists three

large-scale

fluvial and aeolian sandstone units interbedded with 2 units of marl and

flagstone.

The sandstonesare

characteristically

redor

yellow

in colour andso

provide

a

visualmarker

indicating

the

top

of the

predominantly blue-grey

coloured

Rousay

Flagstone

Formation. The

Eday Group

is found

mainly

in

Eday,

east

Mainland,

Hoy

and South

Ronaldsay (Figure

1-1).

General literatureonthe

geology

of

Orkney

(e.g.

Wilsonetal.

1935,

Mykura 1976)

regards

the

Hoy

Sandstoneas

belonging

tothe

Upper

Old Red

Sandstone,

separated by

a

regional unconformity

from the

underlying Eday

Group.

However

increasing

evidence

suggest

that

only

alocal

unconformity

exists between thetwo

units,

and that thetwoare

lateral

equivalents (Rogers

1987 and Astin

unpublished).

Kerogenvariation inaDevonianhalf grabensystem _

Introduction

Structure and tectonics

The broadstructureof

Orkney

is

relatively simple (Figure 1-5).

Mostof the folds that affect the Devonian sediments are_veryopenandmany havea

northerly

trend. Themost

noticeable

regional

foldsarethe northward

plunging Eday Syncline

and the West Mainland

Anticline,

whichare

responsible

for much of the structural variation in

Orkney (Figure

1-6).

The mainfaultson

Orkney

strike

north-northeast,

south-southwest and

east-northeast,

west-southwest

(Figure 1-5).

Bothsetsof faultsarebelievedtohave hadtwomain

phases

of

movement.

Firstly

the faults extended

normally during

the

deposition

of theDevonian sediments andarebelievedtohave controlled sediment distribution

(Astin

1985;

Enfield & Coward

1987;

Astin

1990;

Hippler 1993).

After

this,

_{many of the faults}werereactivated

prior

tothe intrusion ofasuite of Permian

dykes.

Slickensidemeasurements

suggest

a

sinistral,

oblique-slip

motiontothe reactivation

(Hippler

1993).

Commercial

speculative

seismic data

acquired immediately

tothewestof

Orkney

(Enfield

& Coward

1987)

has revealedaseries of

generally

northeast

trending,

east

dipping

faults

(Figure 1-7).

These faultswere

probably

firstactiveasextensional faults in the

Devonian

(Brewer

&

Smythe 1984)

andare

thought

tobe the

graben bounding

faults

delimiting

the

graben

into which the Devonian sedimentswere

deposited

(Enfield

&Coward

1987).

Itis

thought

that faults of similar trendseen on

shore,

suchasthe East

Scapa

Fault

(Figure 1-5),

arethe exhumed remains of other such

graben bounding

faults

(Astin 1990).

Note that duetothenatureof the

outcrop

in

Orkney

(limited

onshoreorcovered in

water),

the

exact trace _{of many of the faults is uncertain. Different workers in this field have tended}to

usedifferent estimations of the

position

of these faults. For

example

Enfield & Coward

(1987)

mark the East

Scapa

Faultas

passing

tothewestof

Shapinsay,

whereas Astin

(1985)

has the East

Scapa

Fault

passing through Shapinsay. Additionally

Astin

(1990)

has noted several short unlinked faults

along

thewestcoastof

Mainland,

whereas Enfield & Coward

(1989)

have

amalgamated

these faults intoa

single,

east

dipping

normal fault

cutting

the entire

length

of thecoast. It isnot_{within the scope of this thesis}to_{argue the}

placing

of these

faults,

since the alternative

position

of these faults donotconflict with_any of the results of this work. For consistencies sake the structural_mapof Coward & Enfield

(1989)

is used

throughout

this thesis

(Figure

1-5).

Note also that the

previously

unnamed faultonthewest coastof Mainland is referredto in this thesisastheWestMainland Fault.

"066 L ujjsy uuojj

peu!poiAi

'dnoj0

euo+sßDH

Aeu>|jo

-siosjagj sseu^oiy;

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40

AjDuuuuns

'[-[

^--^^^j^^

dnoj0

euojsßoij

sseuujojis J3MO1

UU9SG

(S9pAo 172)

LUJ

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ujg ep/Qpeg l|SIJ >)O|AApUDS

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xojddD

(sepAo

t7 L ~

psog

066L uusv pud Sim

uu19-gg 8|OAq

pea Ljsi-i )|O!MpuDS

UU06 L

sseuuuojis

Jeddfi

(9Z6L)

DJn>(A|/M

ui

pe^onb

(0Z6L)ujuuDd

DJJDH

AqouseA

4SS AqousoA

SS9UULIOJ4S J9MO1

peg L|sy

UJ06S

(0Z6L)

sseuuuojis

peg i|sy

sseuuuojjs

Jeddfi

speg

speg

(9C6L)

|D -\.

1

I

Kerosenvariation inaDevonian half grabensystem Introduction 10km

Papa

l Westray

I

Hoy/Eday Group

I

Rousay

Flagstone

Formation

?

Upper/LowerStromness FlagstoneFormation

t

North,

Ronaldsay

North

Rousay _Faray '

IJSancloy

West

Mainland

Figure

1-1. _{Location map}of

Orkney

and

simplified

Devonian

geology.

Kerogenvariation inaDevonianhalf grabensystem Introduction

1500

Upper Eday

Sandstone

EdayMarls MiddleEday

Sandstone

Eday Flagstone

Lower_EdaySst

a

o

Ö

Rousay Flagstone

Formation

Upper

Stromness

FlagstoneFm

Sandwick Fish Bed

Lower Stromness

FlagstoneFm

a

i

0 <D

O

b>

3

u_

I

i

O

Figure

1-2. Schematic

representation

of the Middle Devonian

stratigraphy

of

Orkney

(modified

from Astin

1990).

CD

p

%

>- "r=

o Ö

15m

10m

Fineormedium sands, massive,

currentorwave rippled(sheetfloods) Interlayeredlightsilts and finesands,

mudcracks,currentripples(mixed fiats)

Darksilts, mudcracks, fish remains,stromatolites

Laminite, commonfish remains Darksilts, mudcracks,fish remains

Figure

1-3. Schematic

diagram

of~2

Orkney Flagstone

Formation

cycles,

showing

the

approximate

scale of the

cycle,

and the main

lithologies

present.

Kerosenvariation inaDevonian_{half grabensystem} Introduction

North Coastof

West Mainland Blotchnie South Eday

Fiold TheChapel

Lower_Eday

Sandstone

Sacquoy Sandstone

Rousay FlagstoneFornnation UpperStromness

Flagstone

Formation

Burgar

23

The

Brough

23

Digger

Di99er

Mid Howe

Upper

Stromness

Flagstone

SandwickFish Bed Lower

Stromness Flagstone

Rousay

section /

Sacqouy

Head

"122m

igger Mid Howe Anticline1

Sacquoy

Sandstone

SouthEday Greenigeo Bay

Eday Syncline

South

Eday

Section

Figure

1-4. The

Orkney Flagstone

Group

stratigraphic

framework

proposed

by

Astin

(1990).

Kerogenvariation inaDevonian_{half grabensystem} Introduction

Papa Westray

North

Ronald'say

Stronsay

Line of section in _Figure 1 -6

Syncline

Anticline Inverted fault

\

NormaI fault

Burray

WMF-West Mainland Fault

NSF - North Scapa Fault

BRF - Brims Risa Fault

ESF - East Scapa Fault

South

Ronaldsay

Figure

1-5. _{Structural map of}

Orkney showing principal

faults and folds. Modified from Cowardetal. 1989.

UpperStromness

Flagstone

Formation Sandwick FishBed(notto_scale) LowerStromness

Flagstone

Formation

a LowerEdaySandstone

RousayFlagstoneFormation

West Mainland

Brough Costa

Head Head

Rousay

Egilsay

Eynhallow Blotchnie Fiold

Eday

Eday syncline

i

500m,

0 12 3 4 5km

Figure

1-6. West-eastcrosssection of northern

Mainland, Rousay

and

Eday.

Modified

from Astin 1990. See

Figure

1-5 for location ofcrosssection.

Kerogenvariation inaDevonian halfgrabensystem Introduction

Half_graben

bounding

faults Onshore

Devonian sediments

Figure

1-7.

Map

of half

graben

system

tothewestof

Orkney,

asshownon seismic data.

Modified from Coward etal. 1989.

2 A

stratigraphic

framework

Tomake

possible

the

study

of the

sedimentary

_{processes that}occurredwithin the Devonian

sedimentary

basin,

someform of

stratigraphic

frameworkwasneededtoenable the

correlation of individual lake unitsacross

Orkney.

Inthe

past

the

general

similarity

of both the thicknesses and the sediments contained within the lake

cycles

has made

estimating

thetruethicknessof the

Orkney Flagstone

Group

difficult

(Table

1-1).

The Sandwick Fish Bed

(Achanarras

Fish Bed

equivalent)

is

thought

to

be themostreliable

regional stratigraphic

marker,

and has been used inanumber of worksto

aid correlation.

As

knowledge

and

understanding

of the Middle Devonian

stratigraphy

of

Orkney

has grown,thenames

given

tothe different rockunitshave

changed.

Table1-1 isa_{summary of}

the variousnamesand their thicknesses. Theformationnames

proposed by

Astin

(1990)

are

used in this thesis.

It is

only

with detailed section

measuring

and correlation

(Fannin

1970,

Astin

1990)

thataclearer

picture

of the

stratigraphy

of the

Orkney

Flagstone

Group

has

emerged.

Below isabrief outline of the

development

of the

present

understanding

of Orcadian

stratigraphy.

2.1Abrief

history

ofthe

geological study

ofthe

Orkney

Flagstone Group

J. S. Flett

produced

the first detailed

stratigraphy

of

Orkney

in 1897 where he

recognised

the

Rousay

Bedsas a

separate

formation.

Following

this in

1935,

the

Geological

Survey produced

a

descriptive

memoir of the

geology

of the islands

(Wilson

etal.

1935).

They

estimated about 3100m of Middle Old Red Sandstone lake sediments

(Wilson

etal.

1935).

This thicknesswas

composed

ofatleast 1067m ofLowerStromness

Flagstones

between the Sandwick Fish Bed and the

metamorphic

basement. Above thiswas335m of

Upper

Stromness

Flagstone

and 1700m

Rousay

Flagstones

between the Sandwick Fish Bed and the distinctive sandstone

oftheEday Group (Wilson

etal.

1935).

Their correlation between

outcrops

wasbasedonthe

recognition

of

key

species

of fossil fish and the

branchiopod

Asmusia

membranacea,

with little attention

being given

to

lithological

variations.

Fannin

(1970

and

quoted

in

Mykura

1976)

inan

unpublished

thesis,

brought

a

degree

of accuracytothe

study

of Orcadian

stratigraphy.

Detailed section

measuring

in West

Kerogenvariation inaDevonian_{half graben}system _Stratigraphy

recognised

the Sandwick Fish Bed in manymore

places

than the

Geological Survey,

and thus

wasabletorevise the

stratigraphic

_{thickness of the Lower Stromness}

Flagstones

downward from 1087mto350m. He also

proved

atleast 290m of

Upper

Stromness

Flagstones

above the Sandwick Fish Bed. Workonthe

Rousay

Flagstone

Formationwasoutside of the scope

ofthat thesis.

South of

Orkney,

in

Caithness,

Donovanetal.

(1974)

estimatedatotal ofmorethan

3800m of

cyclic

Middle Old Red Sandstone lake sediments. Thiswas

composed

ofmore

than 2300m of the Lower Caithness

Flagstone

Formation below the Achanarras Fish Bed. Thiswas overlain

by

atleast 1500m of the

Upper

Caithness

Flagstone Group

between the Achanarras Fish Bed and the

mainly

fluvial sedimentsof the John O'Groats

Group.

The John O'Groats

Group

is knowntocorrelatewith the

Eday Group

of

Orkney (Flett

1897,

Houseet

al.

1977,

and Astin

1985).

Donovanetal. didnot

present

detailed correlationsacrossthe

numerousfaults thatcut_{the coastal sequence in Caithness. The presence of}

key

fossil fish

species

and broad

lithological

variations

being

usedto

assign structurally separated

sectionsto

specific stratigraphic positions.

Plimmer

(in

his

unpublished

1974 PhD

thesis) attempted

todefine the

stratigraphy

and

sedimentology

of what he called the

'Rousay

Group'.

The main

assumption

made in this thesiswasthe estimate for the

top

and base of the

Rousay

Group

from the

Orkney

Memoir

(Wilson

etal.

1935)

and henceanestimated 2000m thickness for the unit. Correlationwas

based

mainly

onfossil fishoccurrenceand took littleaccountoffault

repetition.

Inthis thesis the

'Rousay Group'

was subdivided into threelithostrati

graphically

defined

subgroups.

No

attempt

wasmadeto

explain

these

lithological

variations intermsof lateral facies variation rather than vertical facies

change.

Themost recent

attempt

atdetailed correlation within the

Orkney Flagstone Group

was

by

Astin

(1990).

Careful

stratigraphic logging

of central andwestern

Orkney,

alloweda

frameworkof about 45 lake

cycles,

tobe extended from the Sandwick Fish Bed

through

the

Upper

Stromness

Flagstone

Formation and the

Rousay Flagstone

Formationtothe base of the

Eday

Group

(Figure 1-4).

Astin

(1990)

used the Sandwick Fish Bedasthe

key

correlative horizon in his

stratigraphy.

This horizon contains adistinctive fish fauna and has been

mapped by

various

workers

(Wilson

etal.

1935,

Fannin

1970)

atseverallocations inwesternMainland.

Working

upfromthis

horizon,

Astin correlated individual lake

cycles

in the

Orkney Flagstone Group

across Western

Mainland, Rousay

and

Eday.

The

key

features usedtoaid correlationwere:

Kerogenvariation inaDevonianhalf_grabensystem _Stratigraphy

Patterns of

cycle

thickness

Thenatureof individual fish beds

including

their overall thickness The

abundance,

size and

degree

of articulation of fish fossils Presenceorabsence of

stromatolites,

Thenatureof the

ephemeral

_componentsof the

cycles

Inwest_{Mainland the presence of decollement horizons and in the upper}

part

of the

Rousay

and

Eday

sections the

position

offluvialsand bodies with distinctive

palaeocurrent

directions

(the Sacquoy

Sandstone

Member).

From thisframeworkAstinestimatedthat the

Upper

Stromness

Flagstone

Formationwas325m

thick,

which agrees well with Fannin's

(1970)

figure

of about 290m

(Table 1-1).

Astin redefined the

Rousay

Flagstone

Formationas lake

cycles

25to 38 above the Sandwick Fish Bed. Hedefines the base of the

Rousay

Flagstone

Formationas:

'the base ofathick 'fish bed' rich in fossil fish and stromatolites

occurring

at Scara

Taing

in

Rousay,

eastof

Grugar

in N

Mainland,

andat

Greenigeo Bay

in south

Eday...and

starts a

sequence of much thinner

cycles'(Astin

1990).

The correlations

presented

in Astin's work reduce the thickness of the

Rousay Flagstone

Formation from 2000mto

approximately

200m. Astin's framework is the basistothe correlation

panel

presented

in this thesis

(Figure

2-1).

2.2 Noteontheuseof fossil fish inthe

stratigraphy

of

Orkney

The presence of fossil

fish,

preserved

in the lake sediments of

Orkney

has proven

extremely

useful in the

understanding

the

stratigraphy

of the Orcadian Basin. Fish remains

areoften found in the

deeper

water

sedimentary

facies eitherasdisarticulatedcollections of

scalesor asentirecarcasses. The earliestworkersused differences in the distribution offish

species

tocorrelate between the Old RedSandstoneof Caithness and

Orkney (Miller

1849,

1858)

andto differentiate the

Orkney

Flagstone

Group

intodifferent formations

(Murchison

1897).

For

example

two

species

of fossil fish Thursins

pholidotus

and Millerosteusminor

occurin

only

in the

Rousay

Flagstone

Formation

(Flett 1898b),

notin any older formation.

No

attempt

wasmade inthisthesisto

identify

individual fish

species.

Previous workers

(Wilson

etal.

1935,

Fannin

1970,

Marshall 1998

etc)

have

through

useof sporesor

fish_{defined with reasonable accuracy, the}extentof the formations of the

Orkney Flagstone

Group.

A_{summary of the fish}

biostratigraphy

for Caithness and

Orkney

is

given

in Donovan

Kerogenvariation inaDevonian_{half grabensystem Stratigraphy}

etal

(1974), together

withanoutline of the inherent uncertainties in

establishing

the

biostratigraphy.

Saxon

(1975)

_{describes in detail the appearance of the}commonfossil fish

found in the Devonian of the north of Scotland. When fish remainswerefound

during

the

fieldwork for this

thesis,

only

theirabundanceand

degree

of articulationwerenotedtoaid

correlation between lake

cycles.

2.3

Introducing

the

present

correlation of the

Rousay

Flagstone

Formation.

Itwasdecidedto create a

stratigraphic

framework

only

for the

Rousay

Flagstone

Formation,

rather than for the entire

Orkney

Flagstone

Group.

The mainreasonfor thiswas

that the

Rousay

Flagstone

Formation has the

largest

areaof exposure of the three

Orkney

Flagstone Group

formations

(Figure

1-1).

This allowsamoredetailed

study

of

lithological

variation withinadistinct

unit,

rather thanaless

rigorous study

of several units.

As noted

by

Astin

(1990)

the base of the

Rousay Flagstone

Formation isnot

immediately

obvious in the field, hiorderto

place

accurately

the base of the

Rousay

Flagstone

Formation,

the

original

field

logs

used

by

Astin for the

preparation

of his1990

stratigraphy

wereusedto

help

re-log

the

parts

of his

stratigraphy.

Thesewere sections that

linked the

Upper

Stromness

Flagstone

Formationtothe

Rousay

Flagstone

Formation

(the

Evie,

Digger

and South

Eday

sections)

(Figure

1-4).

Having

establishedsectionsof known

stratigraphic position, nearby

sectionswere

logged

and where

possible

matched with the

known sections. Inthis_wayaframework of

logged

sections of known

stratigraphic position

wasextendedacross

Orkney.

For the purpose of this

thesis,

aswith Astin

(1990),

the base of the

Rousay

Flagstone

Formation,

is definedasthe first well

developed

fish bed

(containing

fish remains and

stromatolites), occurring

abovean

especially

thick

cycle containing poorly

developed

lake

faciesin the

Upper

Stromness

Flagstone

Formation. This definition is

adequate

for the north

western

part

of

Orkney,

however further south the character of the base of the

Rousay

Flagstone

Formation

changes.

This isas

expected

since all of the lake

cycles change

dueto

natural variation in faciesacrossthe Orcadian Basin.

The

Rousay

Flagstone

Formationcontainsabout 14 first order lake

cycles,

and ends at

the base of the

distinctive,

yellow

Lower

Eday

Sandstone. The

top

of the

Rousay

Flagstone

Formation is often heralded

by

the

irregular

occurrenceof reddened silts andsandstones. This

increase in subaerial exposure may

signify

increased faultmovementand

uplift

(Astin 1990).

Several other

lithological

featureswereusedtoaid correlation. Of_great

stratigraphic

valuewas adistinctive

pebbly

sandstone unit called the

Sacquoy

Sandstone Member

(Astin

Kerogenvariation inaDevonian_{half graben}system Stratigraphy

1990)

whichoccursclosetothe

top

of the

Rousay Flagstone

Formation.

Although distinctly

fluvial,

the member isnoterosive innatureasinnoinstance does itcutinto the

underlying

dark

silts,

suggesting

that it isnotdiachronous innature. The

Sacquoy

Sandstoneoccursin

cycle

35 and ismost

prominent

in the northwest of

Orkney.

Inshort

sections,

orin sections where the

top

of the

Rousay

Flagstone

Formationwas

not

present,

themost

important

method of

stratigraphic

correlationwas

comparing

patterns

of

cycle

thickness. Thismeansthat when

trying

tolinktwo

locations,

the

patterns

of the relative thicknesses of the

cycles

were

compared

rather than the absolute thicknesses of the

cycles.

In this waymoredistalareas_{may be linked with}

proximal

areas evenwhere absolute thicknesses

of sediment may be different.

Other features used in thecorrelationwerevariations in laminitefacies

thickness,

the

natureof the

ephemeral-type

faciesof the lake

components,

_{and in the upper}

part

of the

Rousay

and

Eday

sections,

the

position

of fluvial sand bodies and

palaeocurrent

data. The

complete

Rousay

Flagstone

Formation correlation chart is

presented

in

Figure

2-1.

2.4 Fieldwork

The detailed

logging required

for this

thesis,

took

place

overtwo,three monthfield

seasons

during

thesummersof 1996 and 1997. Most of the time in the fieldwas

spent

making

detailed

sedimentary logs

from wherever there were

relatively

unfaultedsections of

Rousay

Flagstone

Formationstrata.

Heavily

faulted,

brecciated and

poorly exposed

sections

were

mostly

avoidedduetothe

uncertainty they

would have introducedtothe correlations. In manycasesthe locations visitedwerethose selected sections described

by

Plimmer

(1974)

in his thesis

dealing

with the

sedimentology

and

stratigraphy

of the

Rousay

'Group'.

His workwasinvaluablefor

locating

the best exposures of the

Rousay

Flagstone

Formation.

This thesis

investigates sedimentary

variation within the

Rousay

Flagstone

Formation. However insome cases strataof the

previously

unidentified

underlying Upper

Stromness

Flagstone

Formationwerealso

measured,

as anunavoidable

part

of the

logging

procedure.

The

Upper

Stromness

Flagstone

sectionswerenotstudied.

This research studied processes

operating

on abasin-wide scale. This necessitated the

gathering

ofasmuchdatafromaswidean area as

possible.

Since therewas alimitedamount

of timeto

gather

the

information,

thethinnest

sedimentary

unit thatwasmeasuredwasabout

5cm thick

(apart

from thin but notable

layers

suchasstromatolitesordecollement

surfaces).

Details of the different facies

types

measured in the

field,

and their environmental

interpretations

canbefound in

Chapter

3.

Kerogenvariation inaDevonian_{half grabensystem Stratisraphv}

Forthe_{purpose of this thesis} abase ofalake

cycle

is definedaswhere the first visible

organic

matterrich sediment is seen. The

top

of the

cycle

ends

immediately

before the reappearance of

organic

matterin the

following

cycle

(Figure

1-3).

Intotalmorethan 4.5km of sedimentwas

logged

from 49 sections. Detailed

informationonall of the sections

logged

is

given

in

Appendix

1. Of

these,

19 sections could be

confidently

included in the

stratigraphic

framework

(Figure

2-1).

These sections allowed thecorrelationof the

Rousay

Flagstone

Formationacrossmostof

Orkney.

The sectionsnot

included inthe

stratigraphy

wereeithertooshortor

badly

faultedtobe linked

reliably

with otherareas of

Orkney.

_{Table 2-1 lists the locations and other information about the sections}

used in the

stratigraphic

framework.

Figure

2-2 shows the locations of the section used.

2.5 Pointsto notefrom the

stratigraphic

framework

Although

the

Bight

of Aith section from

Stronsay

is shownonthe correlation

panel

(Figure 2-1),

no

sedimentological

or

palynological

datawasused from this section

because theamountof

faulting

and

fracturing

present

in the section made_any

precise

measurementsunrealistic. For similarreasonsthe

Skerry

of Work section

(east

Mainland)

wasalso shownonthe

stratigraphic

correlation,

butnotexamined in detail.

In two

cycles,

lake conditions didnot

develop fully.

These

semi-developed

lakesare

characterised

by

the

development

of

lighter

_{silts and the presence of}mudcracksrather than

by

laminites and dark silts. Inthe correlation

panel

these

semi-developed

lakesare

denoted

'a,

b,'

etcandoccurin

cycles

31 and 35. The

poorly developed

lakes have

limited lateralextent. Lake 31b is

only

foundeastof the East

Scapa

Fault. Lakes 35b and

carefound

only

in theextremenorthwestof thearea. Astin

(1990)

notessimilar

partly

developed

lakes in his

stratigraphy.

In thecaseof North

Ronaldsay,

the sections

logged

didnot_{match with any}

portion

of the

stratigraphic

framework. It is

suspected

that the island_maybe

composed

of

Upper

or

LowerStromness

Flagstone,

rather than

Rousay

Flagstone

Formation.

Logging

certain

parts

of the

stratigraphic

framework

required

theuseof

climbing

equipment.

The section of cliff

marking

the

resumption

of the

Rousay section,

and severalsectionsatHalcro Headwere

logged

in thisway. Needlessto_say

great

careis

required

when

visiting

these locations.

Kerosenvariation inaDevonian half grabensystem Stratisrayhv Island West Mainland Rousay Westray North_Faray Eday Sanday Stronsay Section Name Evie Rousay Noup Surrigarth North_Faray Wars Ness Fers Ness SpurNess Loth_Quarry HuipNess Bightof Aith Location co-ordinates HY355273 -HY364268 HY363317-HY382350 HY408494-HY395501 HY492452-HY497437 HY528381-HY531378 HY557288 -HY549289 HY529339 -HY534338 HY667332 HY602342 -HY600344 (HY648299) -(HY641308) (HY645241) -(HY647239) Outcrop Quality Moderate intertidal Moderate intertidal& clifftop. Inaccessible 50msection Good intertidal/ clifflop Good above HWM Moderate intertidal Poor intertidal/

good

above HWM Goodnarrow intertidal Moderate intertidal Good_quarry andintertidal Intermittent moderate

-poorly

exposed intertidal Narrow intertidal Stratigraphic Position 1/2 USFF 1/2RFF 1/2 USFF 1/2RFF Upper RFF Lower USF UpperRFF EntireRFF Some USFF Entire RFF EntireRFF UpperUSFF LowerRFF UpperRFF LESM Lower & Middle RFF UpperRFF LESM Notes Sacquoy present Sacquoy present Sacquoy present Sacquoy present Sacquoy present Sacquoy Faulted but nosection missing Extreme brecciation and faulting Lengthof logged section 103m 166m 128m 282m 185m 217m 250m 250m 160m 153m 92m?

Table 2-1. Location of sections used in the

stratigraphic

framework. See

Figure

2-2 for map.

Kerogenvariation inaDevonianhalfgrabensystem Stratigraphy Island East Mainland Burray Flotta South Ronaldsay Section Name Head of Work Skerryof Work Tankerness BurrayNess Flotta HoxaHead North Halcro Head South Halcro Head Location Co-ordinates (HY482140) -(HY476411) HY477122 -HY479130 HY545088 -HY545097 ND507965-ND497965 ND362923 -ND363929 ND406935 -ND415944 ND469861 -ND464863 ND475855 Outcrop Quality Low_lying intertidal Moderate -poorlow intertidal Poor intertidal and clifftop Moderate -poor intertidal Good- low cliffs and intertidal shoreline Good-moderate intertidal Poor-cliff base,tidal Moderate clifftop Stratigraphic position LowerRFF Lower RFF UpperUSFF RFF RFF? LowerRFF EntireRFF UpperRFF UpperUSFF LowerRFF Notes Poor exposure, faulted Poorly developed lake and sheetflood facies Verythick laminites Topsection obscuredby seaweed Treacherous access Logged using climbing gear Lengthof logged section 97m 132m 130m 140m 62m 200m 78m 130m

Table2-1 cont. Location of sections used in the

stratigraphic

framework. See

Figure

2-2 for map of locations.

Figure

2-1.

Llthostratigraphic

correlation of the

Rousay Flagstone

Formation

19 North Halcro

granule/congfomerate N

cHum/tearse sandstone

base

cycle

36

base_cycle26

mixed flats/fine sandstone

iigirtsitt dark

fleh m

stomatolites

laminlte/nearlaminitedeveloped basecycle25

Kerogenvariation inaDevonian_{half grabensystem Stratigraphy}

20 km

Noup

Head

Surrigarth

Loth

Quarry

North Fa ray Fers

Ness1

Wars Ness

Head of Work Tankerness

BurrayNess

North Halcro Head South Halcro Head

Figure

2-2. Location of the sections used in the

lithostratigraphic

framework and

subsequent

lithofaciesand

kerogen analysis.

3

Sedimentology

This section deals with the

description

and

presentation

of the

sedimentary

data collected for the thesis. The first

part

of this section describes the different

sedimentary

rocks of the

Rousay

Flagstone

Formation in

Orkney.

The informationcomesfrom detailedfield

description,

augmented

by

thin section

descriptions

and information from

existing

literature. The second

part

of this section

briefly

_{deals with the various ways in which the facies data}

was

presented

and

analysed.

3.1 Facies

descriptions

During

the six months fieldwork

spent

in

Orkney,

thesame

sedimentary

facies were seento occur

repeatedly throughout

the

Orkney

Flagstone Group.

The

following

facies definitions

represent

the level of detailatwhich rock

types

wererecorded in the

field,

during

the

logging procedure. Although they

arenotasdetailedas

previous

workers

(e.g.

Rogers

& Astin

1991,

Astin &

Rogers

1991), they

arebelievedtobe

sufficiently

accuratefor

abasin-scale

study.

Representative samples

of the

key

faciesweretaken and thin sections weremade for

petrographic

examination. The clastic

samples

camefrom

cycle

25 of the

Rousay

section. Several

organic

matter

containing samples

came from

cycle

26 of the

Flotta and Head of Work sections.

Much has been writtenonthe carbonate

mineralogy

of the Orcadian basin. Further

research into this is outside the scope ofa

study

into

kerogen

variation,

and has been

covered

adequately by

other workers

Kerogenvariation inaDevonianhalfgrabensystem _____^_^__ Sedimentology

Laminite Field

description:

This rock is

composed

of

parallel,

mterlaminated dark and

light

coloured,

very

fine-grained

sediments

(Plate 3-3a).

Fish remains of

varying degrees

of intactnessareoften

present.

Thin section

description:

Sample

location-

cycle

26Head of Work and Flotta.

These showed well defined

sub-millimetre,

dark/light

laminae

(Plate

3-la and

b).

Half of the

light

coloured sediments were

laterally

continuousbands of

angular-subangular,

course

siltto_{very fine}

quartz

sand. The restof the

light

coloured sedimentwas

micrite,

which

tendstoform inmorebulbous

layers

(Plate

3-1

b).

The dark laminaeare

composed

of dark

silts and bronze coloured

organic

matter

(Plate

3-2a).

Generally

the

light

coloured sand bands

grade upward

into the dark sediments

(Plate 3-la).

Inthe

sample

from

Flotta,

the

light/dark layers

wereless well defined than those from the Head of Work.

Environmental

interpretation:

Thelack of_anyevidence ofwave

activity,

the fine lamination and thepresence of

organic

matter

suggest

that this facies

represents

the sub-wavebase

deposits

ofa

permanent

lake

that wasforatleast

part

of the time

chemically

or

thermally

stratified.

It is

suggested

(Donovan

1980)

that the

dark/light couplets

of

organic

matterand micrite formed when

algal

blooms increased the lakewater

pH

causing

the

precipitation

of the carbonate. The

phytoplankton

then died and rained down into the anoxic bottomwaters of the lake

accumulating

as

organic

carbonon

top

of the

recently precipitated

carbonate

layer.

The clastic material of fine sand/coarse silt

grade

maybederivedfrom sourcessuchas

density

flows,

wind

transport

or

post-storm

sediment

re-settling (Rogers

& Astin

1991).

Near laminite Field

description:

This facies is similartothe 'laminite' facies but with less well

developed

laminae,

which_may be thicker and sandier than

simple

laminites. Mud cracks and stromatolites may also be

present.

Fish remains arecommon.

Kerogenvariation inaDevonianhalf grabensystem Sedimentologv

Thin section

description:

Sample

location-

cycle

26Flotta.

The

sample

is

mainly

composed

ofsub-millimetreinterlaminated

pale

and dark brown sediment.

Roughly

half of the

pale

laminations are

angular/sub-angular

coarse siltto

medium sand

grade

quartz

with around 5% muscovite also

present.

The other half of the

pale

sediment is micrite ofabulbous appearance.

The dark

layers

are

composed

ofopaque

clay,

organic

matterand less than 10% muscovite.

Environmental

interpretation:

The environment of

deposition

ofthis facies is_verysimilartothat of the laminite

facies,

except

that several features indicate

slightly

shallower lacustrine conditions. These include the presence of mud cracks

(discussed

later),

stromatolites andanincrease in the

quantity

and

grain

size of the

quartz

sediment.

This faciescanbe

interpreted

as

being

transitional between

permanent

lake conditions and more

ephemeral

conditions.

Dark silt Field

Description:

This rock

generally

_appearsasmassiveor

poorly

laminated,

_verydark silt.

Fish remainsare

present.

Stromatolites identifiableas

thin,

pale

mounded

layers

interlaminated with the siltsarealsocommon.

Ripples

in any formare

extremely

uncommon.

Environmental

interpretation:

Two

possible

interpretations

canbe made of this facies. The first is that the dark silt is in some cases

actually

alaminiteor nearlaminite in which the delicate lamination that

characterise the

facies,

has been obscured

by

_poor

lighting, vegetation

or

weathering.

This

misidentification has

certainly

occurred

before,

when localities had been revisited for the collection of

samples

and itwasnoticed that facies

logged

asdark siltswere,under closer

scrutiny,

laminites.

The second

interpretation

of this facies isindicated

by

the fact it contains abundant

organic

matter

(shown

from TOC

analysis),

but isnotlaminated. This

suggests

that it isafurther

stage

in theincremental

change

from

permanent

stratified laketo

ephemeral

lake conditions

see in all theOrcadian lake

cycles.

Kerogenvariation inaDevonianhalfgrabensystem Sedimentology

Howeverwhat

definitely

canbe said about this fades is that it is fine

grained,

dark in

colour,

was

deposited

insub-wavebase conditions and contains variable amountsof

organic

matter

andis thus akintothe other lacustrine sediments.

Sandy

dark silt. Field

description:

This fades appearsas amassiveor

poorly

laminateddark silt that

commonly

contains coarse

silttomedium sand

grade

quartz

grains

inits matrixorin discrete laminae.

Environmental

interpretation:

As with the 'dark silt' facies

above,

therearetwo

possible

interpretations

of this

facies,

either

as an

incorrectly

identified 'near laminite' faciesor as afurther

stage

between

deep,

permanent

and

ephemeral

lake

systems.

Ineither

interpretation,

the

probable

mechanisms of sand

input

could be

density

flows,

wind

transport

or

post-storm

sediment

re-settling

(as

discussed in

Rogers

& Astin

1991).

Wick

Also knownas

simple flagstone (Rogers

& Astin

1991)

orWick

type

flagstone

because it is

particularly

commonand well

developed

around

Wick,

inCaithness

(pers

comT.

Astin).

Field

description:

This isacommonand distinctive facies

composed

of

finely

interbeddedto

coarsely

interlaminated muds and siliciclastic silts and fine sands

(Plate 3-2b).

From_{every sand}

layer,

lenticular mud cracks pass down into the

underlying

mud

layer

and

usually

nofurther Plate

3-2b).

The

lengths

of the_{cracks vary from less than 2cm}to over 10cm. The crack infillsare

usually

bulbous incross

section,

deformed

by

post-depositional

compaction (Plate

3-2b).

Thin section

description:

Sample

locations -

cycle

25

Rousay

and

cycle

26 Head of Work.

The mudstrataare

usually

dark

blue-grey

tobrown and 1to 15 mmthick.

They

are

micaceous

clayey

siltsor

silty

clays

with variable

proportions

of

pyrite, organic

matterand

carbonate. The calcite ranges from0toover50% of the mudstrata. The silt

grains

contained

in the mudare

mainly

quartz.

The sandstrataare

composed

ofcoarsesilttofine sand

grade,

well sorted

angular

to

subangular

quartz

andare 1 to20mmthick. The sand

layers

also contain around 5-10%

Kerogenvariation inaDevonianhalfgrabensystem Sedimentology

muscoviteand 5% calcite clasts. The

layers

canbe

parallel

or

cross-laminated,

andare

generally laterally persistent.

Mud cracksarecommonin this facies andaredistinctive in thin section

(Plate 3-2b).

The

cracksare filled with

sub-angular

quartz

andare

commonly

deformed

by pre-lithification

compaction.

Environmental

Interpretation:

1.Mud cracks

The

interpretation

of this

particular

facies has been

subject

tomuch discussion. Most of this has centredonthe

origin

of the

ubiquitous

mudcracksseenin this facies. Two schools of

thought

exist-one ofa

subaqueous origin

of the cracks and another ofasubaerial

origin.

Whether the cracks formed underwaterorunder air is crucialtothe

understanding

of the Orcadian environment in Middle Devonian

times,

considering

that about 30% of the lake sediments in the Orcadian Basin exhibit these cracks

(Donovan

&Foster

1972).

The mechanism fora

subaqueous

crack

origin

is summarised inDonovan& Foster

(1972).

They

suggest

that the cracks wereformed

by clay

expansion triggered

by

salinity

changes

in an

extremely quiescent

lacustrine environment.

The other

explanation

of the mudcracks

origin

is summarised in Astin &

Rogers

(1991),

discussed in Astin &

Rogers (1992)

and in Trewin

(1992)

andincontextof theOrcadian Basin in

Rogers

& Astin

(1991).

These authors

suggest

that the mud crackswereformed

by

a

combination of gypsum

crystallisation

and limited subaerial desiccation

(Figure

3-1).

Although

both

arguments

are_very

persuasive

itseemsthat the subaerial mechanism fits better

with themost recent

interpretations

of how the Orcadian Basin worked

during

Middle Devonian times. Infact Astin &

Rogers

(1991)

suggest

that

subaqueously

formed

cracks,

maynotbe

preserved

atall in the

geological

record duetotheir

fragile

nature. Smoot

(1983)

supports

this in his re-evaluation of the Wilkins Peak Member of the Green River Formation.

Like the Orcadian Basin the Wilkins Peak Member has been usedas a

type

example

for the

study

of

'subaqueously'

formed mud cracks. Smoot

(1983)

refutes this and in hisre¬

assessment

suggests

adesiccat