A Framework Approach for the Modelling and Interpretation of Complex Strike Slip Basins: Douglas Coal Field, Midland Valley, Scotland

Masters Thesis

Adam C Marsh BSc (Hons)

A Framework approach

for the Modelling and Interpretation

of Complex Strike Slip Basins

The Douglas Coalfield, Midland Valley, Scotland

School of Earth and Environment,University of Leeds,

Leeds,United Kingdom,

LS2 9JT

August 20, 2014Word Count: 10000

Abstract

This project extends the existing 3D model of the Douglas Coale ld, as initially proposed by the

British Geological Survey, in both terms of geographical extent a nd elevation (with horizons now

projected above and below topography). The New Regional Model shows a structurally complex

strike slip system exhibiting multiple phases of transpressiv e and transtensional deformation within

the Carboniferous stratigraphy. Into this regional framework three sm aller, higher resolution models

of the basin have been integrated. These include a southern portion of the basin (as proposed by

Craven (2013)), the Gasswater Mine (as proposed by Monks (2014)) and the Mainshill Wood Mine

(interpreted and modelled here).

From this, a framework has been proposed for further study of the Dougla s Coaleld as more

data becomes available from industry. This has then be extended into a more generalized framework

for the exploration and study of structurally complex basins bas ed on a four order fault system.

University of LeedsDepartment of Earth Sciences

Declaration of Academic Integrity

To be attached to any essay, Dissertation, or project worksubmitted as part of a University examination.

I have read the University regulations on Cheating and Plagiarism, and I state that thispiece of work is my own, and it does not contain any unacknowledged work from anyother sources.

Name: Adam C Marsh

Signed:

Date:

Programme of Study:

MSc Structural Geology with Geophysics

Acknowledgments

Acknowledgements and thanks must be extended to the British G eological Survey and Hargreaves

Surface Mining who collated and provided the data for this project. Specic thanks go Graham

Leslie and his colleagues who were very generous with their timeand I have valued their guidance

and support. Thanks must also go to Andrew McCaig, Taija Torvela, Ben Craven and Toby Dalton

at the University of Leeds for their academic and software assistance during the project. My nal

thanks must go to my colleagues and friends on the MSc course who provided support throughout

the project and to anyone who was silly enough to agree to proof read this report.

Contents

List of Figures 2

1 Section I: Introduction 9

1.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.2 REVIEW OF CURRENT LITERATURE . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.1 Geological History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.2 Mainshill Wood Specic Information . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.2.3 Structural Examples and Models Used. . . . . . . . . . . . . . . . . . . . . . . . . 18

2 Section II: Regional Model 21

2.1 DOUGLAS COALFIELD REGIONAL MODEL . . . . . . . . . . . . . . . . . . . . . . . 22

2.1.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.1.2 Key Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.1.3 Validity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.1.4 Model Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.2 COMPARISION TO EXISTING MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2.1 British Geological Survey Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2.2 Fault Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.2.3 Horizons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.2.4 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3 Section III: Mainshill Wood 44

3.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.2 MAINSHILL WOOD MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2.1 Available Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2.2 Cross Section Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2.3 Model Extrapolated into 3D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.3 INTEGRATION WITHIN REGIONAL MODEL . . . . . . . . . . . . . . . . . . . . . . . 57

3.3.1 Comparison with Regional Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.3.2 Eects on Regional Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

0

4 Section IV: External Model Integrations 60

4.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2 GASSWATER MINE MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.2.1 Presentation of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.2.2 Integration with Regional Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.3 CRAVEN (2013) MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.3.1 Presentation of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.3.2 Integration of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.4 CONVERTING BETWEEN SCALES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.5 INDUSTRY IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.6 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.6.1 Suggested Framework for the Douglas Coaleld. . . . . . . . . . . . . . . . . . . . 72

References 73

A SUGGESTED WORKFLOWS 75

A.1 WORKFLOW FOR DEVELOPING A REGIONAL MODEL . . . . . . . . . . . . . . . . 76

A.2 WORKFLOW FOR MODEL INTEGRATION . . . . . . . . . . . . . . . . . . . . . . . . 85

B CROSS SECTIONS 90

C MAINSHILL WOOD LOG 108

D GEOLOGICAL MAP 116

E STRATIGRAPHIC COLUMN 119

List of Figures

1.1 Map showing the sediments of the Carboniferous, by age. See inset for general location within

UK. Douglas Coaleld Outlier highlighted in red. Modied after Waters et al. (2012). . . . . 10

1.2 Diagram showing the relationship between exploration action and level of investment required. 11

1.3 Map of the two bounding faults of the Midland Valley in relation to Glasgow and Edinburgh.

Modied after Smith (1995). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.4 Progresive schematic diagrams of the closing of the Iapetus through theSilurian. Shows

relative locations of Avalonia, Lauratia and Baltica with other structural d etail. Modied

after Vaughan and Johnston(1992). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.5 Map showing the opening of normal faults oblique to the main strike slip trend on the north

side of the Southern Upland Fault (SUF). Modied after Smith (1995). . . . . . . . . . . . . 14

1.6 Schematic diagram showing the orientation of continental convergenceand direction of continental

escape between the Variscan (nee Avalonia) and Baltica plates during the Devonian. Modied

after Coward (1993). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


escape between the Variscan (nee Avalonia) and Baltica plates during the Early Carboniferous.

Modied after Coward (1993). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16


escape between the Variscan (nee Avalonia) and Baltica plates during the Late Carboniferous.

Modied after Coward (1993). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.9 Labelled 1:10,000 British Geological Survey, geological map of the area surounding the Mainshill

Wood site. Open cast site marked on underlying Ordnance Survey map(also 1:10,000 scale). 18

1.10 Diagram summerising the various styles of strike slip geometries possible (From Cunningham

and Mann (2007)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.11 Diagrams showing the eect of wet (ductile) or dry (brittle) cla y (lithology) on faulting styles

within strike slip systems. D for displacement. From Dooley and Schreurs(2012). . . . . . . 19

1.12 Diagrams showing pre-existing fabric on faulting styles withinstrike slip systems. d for

displacement. From Zahasky and Hudleston(2014). . . . . . . . . . . . . . . . . . . . . . . . 20

2

List of Figures 3

2.1 Petrel 3D view of the Fault system. Faults colour coded. Red: Strike slip and Normal Fault

geometries. Blue: Thrust Fault geometries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2 Petrel 3D view of a portion of the Fault system with the Base of the Coal Measures Horizon

added for reference. Faults colour coded as before. Dextral Slip sense along the through going

faults indicated. Direction of slip on selected faults indicated bythe blue arrows showing the

terracing created by the normal faults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24


added for reference. Faults colour coded as before. Sinistral Slip sense on of the through

going faults indicated. Antiform/Synform pair axis indicated. Black arro w indicates possible

direction of thrusting generated within the fault block to create t he small true thrust. . . . 25


added for reference. Faults colour coded as before. Sinistral Slip sense along one of the through

going faults indicated. Indicated original Horst Block bound either side by Normal Fault

Geometries (note easterly fault has been inverted). Black arrow indicates internal direction

of compression to invert the normal fault to create the current thurst geometry . . . . . . . . 26

2.5 Petrel 2D Maps of the six horizons showing their elevations inmetres and colour coded

to the spectrum shown. 1. Base of the Middle Coal Measures, 2. Base of theLower Coal

Measures, 3. Base of the Upper Limestone Formation, 4. Base of the Lower Limestone

Formation, 5. Base of the Strathclyde Group, 6. Base of the Inverclyde Group. . . . . . . . . 27

2.6 Petrel 2D Maps of the thickness (in metres) between horizons. Using the numbering from

Figure 2.5: 1. Thickness between 1 and 2, 2. Thickness between 2 and 3, 3.Thickness

between 3 and 4, 4. Thickness between 4 and 5, 5. Thickness between 5 and 6. Colouring to

spectrum shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.7 2D map from Petrel, showing the restorational cross sections relative to the fault system.

Cross Section 1, northeast-southwest. Section 2, northwest-southeast. . . . . . . . . . . . . . 29

2.8 Line length restoration of Cross Section 1 (as shown in Figure 3.1). a) Unrestored, b)

Restored. Horizons from base of secton, base of the Inverclyde Group (darkpurple), base

of the Strathclyde Group (pink), base of the Lower Limestone Formation (light pink), base of

the Lower Coal Measuresl (light green), base of the Middle Coal Measures (light blue), length

of unrestored section (black). Topography shown as white in cross section. . . . . . . . . . . . 30

2.9 Line length restoration of Section 2 (as shown in Figure 3.1). a) Unrestored, b) Restored.

Horizons from base of secton, base of the Inverclyde Group (dark purple),base of the Strathclyde

Group (pink), base of the Lower Limestone Formation (light pink), base of the Lower Coal

Measures (light green), base of the Middle Coal Measures (light blue),length of unrestored

section (black). Cross marks intersection with Section 1. Topography shown in dark green in

cross section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.10 2D Move cross sections along line 10 comparing the original inputs (top) against the surfaces

generated by the algorithm (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

List of Figures 4

2.11 2D Move cross sections along line 30 comparing the original inputs (top) against the surfaces

generated by the algorithm (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.12 Petrel 3D view of the Fault system, focusing on the Southwestern portion of the model. Base

of the Lower Limestone Formation horizon is added for reference. Note boxed, ungeological,

fault. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.13 Map of the Base of the Lower Coal Measures Horizon (as seen in Figure 2.5) showing the

location of three modelling errors, highlighted on the both the map and in the inset. . . . . . 35

2.14 3D model of the Douglas Coaleld, as generated by the British Geological Survey. After

Monaghan (2012). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.15 3D Fault model of the Douglas Coaleld, as generated by the British Geological Survey. After

Monaghan (2012). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.16 Closer view of the fault model proposed in the New Regional Model (shown in Figure 2.1)

in the Petrel 3D view. Labels link to text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.17 3D view of the base of the Lower Coal Measures and the base of the Lower Limestone

Formation horizons as modelled by the BGS. Labels link to text. After Monaghan (2012). . . 40

2.18 Maps showing the extent of the Base of the Lower Coal Measures (green)and Base of the

Lower Limestone Formation (pink) below topography in the New Regional Model. . . . . . 41

2.19 Side-on 3D view of both models from the same angle comparing the dierent proles taken by

the base of the Lower Coal Measures and the base of the Lower Limestone Formation. BGS

Model on top, New Regional Model below. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.1 Anotated panoramic photograph of the south-western pit wall of the Mainshill Wood mine.

Bedding highlighted in white. Main coal seams in blue (and labelled).Edge of ower structure

(red). Truck highlighted in green for scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.2 Anotated photograph of internal defomation shown within the Manson Coal seam. Red, shear

planes (with sense of shear indicated). Blue, deformed competent layers. Green dashed, folding

in ner material. A4 sheet of paper for scale. Exposed face is oriented northwest-southeast. . 47

3.3 Anotated photograph of the complex structural feature in the southern corner of the pit.

Yellow, truncations. White, highlighting change in lithology. Youngin g arrow based on

stratigraphy seen to the northwest (as described in Figure 3.1). Section is approximately

30m in height. Modied after Leslie (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.4 Anotated photograph showing the interpretation of the features shownin Figure 3.3. After

Leslie (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.5 Conceptual sketches showing how stratigraphy could be manipulated to a near vertical orientation

next to a positively inverted fault.x marks the location of the Mai nshill site if F1 was the

Kennox Fault. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.6 Close scale (1:10,000 scale) British Geological Survey geological map of the Mainshill Wood

Site (labelled as in Figure 1.9. Underlaying map, 1:10,000 Ordnance Survey. Fault Colouration

as discussed in text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

List of Figures 5

3.7 Conceptual sketches showing how the Mainshill ower structure could form from the structures

generated in Figure 3.5. Kinematics indicated. . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.8 Conceptual sketch of the larger ower structure around the Mainshill Wood Mine. Fk; Kennox

Fault. Other Fs numbered in order of formation. Box indicates sectionobserved in the pit wall. 52

3.9 3D Move model of the Mainshill Wood seams and ower structure. Index Coal Seam shown

in Blue. Topography with the pit at its maximum excavation also shown. . . . . . . . . . . . 54

3.10 3D Move model of the fault system as shown in 2.18. Magent faults; Kennox Fault and Main

Splay. Red faults are internal faults and shears as seen in the cross section in Figure 3.8. Pit

plan surface of maximum excavation shown with Index Coal seam for reference. . . . . . . . . 54

3.11 3D Move model of the Mainshill Wood ower structure extrapolat ed towards the Southwest.

Purple faults are unconstrained. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.12 Sketch of a vertical step over structure for the Kennox Fault. . . . . . . . . . . . . . . . . . . 56

3.13 Conceptual 3D diagram, generated in Move, of the Mainshill Site as a vertical step over fault. 56

3.14 3D view of the New Regional Regional fault model with the Mainshill Wood ower structure

faults added. Base of the Lower Limestone Formation horizon also shown.. . . . . . . . . . . 57

3.15 Elevation Maps of the base of the Lower Limestone Formation. 1) Original Surface from the

New Regional Model. 2) Altered Surface after the intergration of the Mainshill Wood Model.

Note the change in horizon geometry on the north side of the Kennox Fault.. . . . . . . . . . 58

3.16 3D Move view of the regional fault system with the bounding faultsmarked in red. . . . . . 59

4.1 3D Petrel view of the regional fault system showing the location ofthe other sub models to

be integrated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2 3D Move views from various angles of the Gasswater Mine Model as produced byMonks

(2014). Horizons in White, Faults in Red. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.3 3D Move view of the Gasswater Mine Model sitting within the Regional Fault Model. Cross

Section lines shown for Figure 4.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.4 Cross sections of the Gasswater Mine Model in comparison to the New Regional Model.

White horizons and red Faults are from the Gasswater Model. Horizons are coloured in

Section 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.5 3D Move view of the Craven Model. Red and Blue Faults are considered through going

faults. Yellow and Green, inter fault block faults. After Craven (2013). . . . . . . . . . . . . . 66

4.6 3D Move view of the Craven Model with the New Regional Modelfault system. Colouration

as before. Note through going faults with consistent strike across both models while inter fault

block faults change from north to south. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.7 Branching Vein System Diagram showing how the fault order systemalso applies to other

vien hosted deposits. Diagram modied afterDalradian Resources(2012). . . . . . . . . . . . 70

4.8 Scale of interest for a mining prospect relative to cost (as shown in Figure 4.7) also showing

the points at which each Fault Order becomes important. . . . . . . . . . . . . . . . . . . . . 71

List of Figures 6

A.1 3D view in Move showing imported 1:50,000 British Geological Surveymap of the Douglas

Coaleld, geo-referenced and sitting at 0m (sea level). . . . . . . . . . . . . . . . . . . . . . . 76

A.2 3D view in Move of fault picks from geological map projected onto the terrain model to give

true Z value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

A.3 3D view in Move of extrapolated fault model of picks shown in A.3 . . . . . . . . . . . . . . 78

A.4 2D Map view in Move of the 31 cross sections used to construct the initial surfaces for the

horizons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

A.5 3D view within Petrel showing the fault model generated from the inputs taken from Move 80

A.6 3D view in Move of the point cloud for the Base of the Coal Measures created from the cross

sections shown in Figure A.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

A.7 3D view within Petrel showing the Base of the Coal Measures surface generated from the

inputs taken from point cloud shown in Figure A.6. . . . . . . . . . . . . . . . . . . . . . . . . 82

A.8 3D view in Petrel of the borehole data before it has been QCed. Fault model shown for

reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

A.9 3D view in Petrel of the nal surfaces after being adjusted with the borehole data. . . . . . 83

A.10 3D view in Petrel of all the nal Surfaces after the extra horizon s are infered from the

borehole data.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.11 Table outlining the key steps for generating the New Regional Model. . . . . . . . . . . . . . 84

A.12 3D view in Move of the two fault models. White from the New Regional Model, pink from

the Mainshill Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

A.13 3D view in Move of the two fault models in the correct relative positions after performing

the transform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

A.14 3D view in Petrel of the intergrated fault models. Note the bulge along the northern side

of the Kennox Fault. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

A.15 3D Petrel view of the integrated point cloud of the base of the Lower Limestone Formation

horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

A.16 3D view in Petrel of the nal intergrated horizon for the base of the Lower Limestone

Formation taking acount of the faults from the Mainshill Wood model. . . . . . . . . . . . . . 88

A.17 Step by Step summary of integrating a small, high resolution model into a coarser New

Regional Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

B.1 Map showing the location of the cross sections used to generate theNew Regional Model

surfaces. British Geological Survey 1:50,000 Geological Map for Reference.. . . . . . . . . . . 91

B.2 Cross sections 1 and 2 for New Regional Model. See Figure B.1 for lines of section. Horizons

coloured as in Section 2.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92





List of Figures 7





B.7 Cross sections 11 and 12 for New Regional Model. See Figure B.1 for lines of section.

Horizons coloured as in Section 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97



















B.17 Cross section 31 for New Regional Model. See Figure B.1 for lines of section. Horizons


C.1 Graphical sedimentary log of the Mainshill Wood Site. Key found in Figure C.7. After

Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

C.2 Continued graphical sedimentary log of the Mainshill Wood Site. Key found in Figure C.7.

After Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110


After Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111


After Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112


After Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113


After Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

List of Figures 8

C.7 Continued graphical sedimentary log of the Mainshill Wood Site. Key included. After

Callaghan (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

D.1 British Geological Survey 1:50,000 (1:50,000 Ordnance Survey base map). Key found in Figure

D.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

D.2 Key for Figure D.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

E.1 Summary Stratigraphic Column including relevent ages and Geologicaleras. Lithology names

and key tectonic events. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

1

Section I: Introduction

9

1. SECTION I: INTRODUCTION 10

1.1 INTRODUCTION

The major issue with interpreting structurally complex areas, regardless of setting, is that many facets of

geology can aect the style and extent of the deformation. For instance, deformation at the metre scale

is heavily inuenced by the competency of the material that is being deformed. This is largely due to the

lithology which is dictated by the depositional/intrusion material foll owed by its post burial/intrusion

history, such as level of lithication or its cooling rate. If we assumethe material is sedimentary (as with

most basin material), the depositional material is controlled by climate, proximity to source material

and topography. All of this only dictates how a rock will break, this is before the local stress regime is

taken into account which will dictate the style of deformation (extensional, compressional or strike slip).

Beyond this, all of these styles are possible within virtually any tectonic setting or basin.

Structural geology is a jigsaw of pieces of information which, when amalgamated, give a fascinating,

detailed and varied picture lled with the dierent hues and satur ations of geometries that are possible

within one complex system. To side step the gaps in knowledge, either from lack of understanding or

incomplete data, we model the earth by making assumptions. The more weknow and can put into the

model, the better it will reect the real world, which ultimatel y is both the aim of academia and industry.

The Douglas Coaleld in Scotland is one such structurally complex area. Sitting within the Midland

Valley of Scotland (see map in Figure 1.1), it, along with the other coal elds of Ayrshire, produced

approximately a third of all coal extracted in the UK between 2004 and 2012 (East Ayrshire Council

(2013)). From this extensive exploitation, a large volume of geological data has been produced over the

last 200 years as the sites have been excavated. However, as the industry has started to decline, this data

is becoming available to academia for study and research.

Figure 1.1: Map showing the sediments of the Carboniferous, by age. See inset for general location within UK.Douglas Coaleld Outlier highlighted in red. Modied after Waters et al. (2012).


The aim of this project is rstly to evaluate the existing available data from the Douglas Coaleld to

see whether modelling with the lates computer software brings newinsights above and beyond the current

model. This will then be followed by the interpretation, modell ing and evaluation of a portion of the

newly available data (the Mainshill Wood site) before integrating it into the New Regional Model. The

methodology devised from this will then be applied to two other models which are subsets of the Douglas

Coaleld. This will all be done with a view to creating a framework for approaching the integration of

further data into the Douglas Coaleld, and eventually how this may app ly to other regional models.

By creating this framework, it should then be possible to add more detail to the regional model as it

becomes available, hopefully allowing greater understanding of complex, strike slip basins. A by-product

of this should be a more generalized frameworked approach for any structurally complex basin which

could be applied by academia or industry.

This is, at its most basic level, how industry should approach prospecting basin scale targets (for

minerals or petroleum). The broad pattern of taking a regional model (such as can be derived from large

scale maps) and then inserting within it smaller models of an area of interest at a higher resolution,

should correlate with the desired level of nancial investment required to generate models of value for

industry (this is summarised in Figure 4.7).

Figure 1.2: Diagram showing the relationship between exploratio n action and level of investment required.


1.2 REVIEW OF CURRENT LITERATURE

As would be expected for such a thoroughly exploited area of the UK, many academic papers have

been written about the Douglas Coaleld (not to mention the countless technical reports and data sets

developed by industry for their various exploration and extraction targets). However, as is often the case

in such areas, industry reports are focused purely on the resource in question (here coal) while academic

papers are limited by the amount of non-proprietary data available for analysis. This leads to signicant

gaps in our collective understanding.

1.2.1 Geological History

The Midland Valley of Scotland sits between the Highland Boundary Fault (to the North) and the

Southern Upland Fault (to the south), creating what can be broadly described as a graben structure

(see map in Figure1.3). These faults follow the pre-existing lineaments generated by the closure of the

Iapetus Ocean during the Lower Palaeozic (Cameron and Stephenson(1985)). The Douglas Coaleld lies

in the Southwestern portion of the Midland Valley, close to the Southern Upland Fault. This, while not

actually within the coaleld, is the main controlling structure for t he fault system that is present.

Figure 1.3: Map of the two bounding faults of the Midland Valley in relation to Glasgow and Edinburgh. Modiedafter Smith (1995).


While it would be useful to have a clearer picture of the pre-closure lithologies, this stratigraphy never

outcrops at the surface so most of the data that is avalible has been based onconjecture derived from

seismic surveys. The predominant theories for these Caledonian aged rocks are either the accretionary

prism generated by the subduction of Avalonia under Laurentia or an IslandArc complex (Bluck (1985)).

Given that the scope of this report is mainly concerned with the Carboniferous stratigraphy, the actual

material (whether it be metamorphosed sediment or igneous material) islargely irrelevant and can just

be considered as homogeneous Caledonian Basement as supported by geophysical studies of the area

(Bamford (1979)).

The Caledonian material itself plays a very small role in the later development of the basin. However,

the fault system which was generated during the collision, will become the controlling structure for the

next 450Ma up until the present. This underlying fault system is heavily inuenced by the nature of

the collision between Laurentia, Avalonia and Baltica and its individual eccentricities. This is shown

schematically within Figure 1.4. The complexity arises from the proximity of the Baltica plate to th e

other participating bodies which, coupled with the oblique angle ofapproach, starts to impress a strike-slip

component into the system. Initially Avalonia collided softly wit h Laurentia during the Llandovery (see

Appendix E for Stratigraphic Column) to create a foreland basin on what would now be the south side

of the Southern Upland Fault.

Figure 1.4: Progresive schematic diagrams of the closing of the Iapetus through the Silurian. Shows relativelocations of Avalonia, Lauratia and Baltica with other structu ral detail. Modied after Vaughan and Johnston(1992).


This collision creates a point around which Avalonia starts to pivot anti-clockwise. This is in contrast

to the relative clockwise rotation still being experienced by the Laurentia plate. In turn this generates a

sinistral sense of slip coupled with the compression (Soper et al. (1992)). From this point onwards the

Iapetus is essentially closed as supported by geochemical analysis of greywacke lithologies on both sides

of the sutra (Stone et al. (1993)). Through the next 30 million years this collision continues until B altica

and Avalonia also collide, to the south, during the early Devonian generating a transpressional sinistral

strike slip boundary along the margin. This has the eect of reversingthe relative oblique nature of the

of Avalonias contact with Laurentia, creating a dextral, strike slip cont act.

Beyond the rather simplistic rotating blocks model described above, it is worth considering the

internal deformation that each of the plates would undergo during this 50million year period. Parallel

to the main sutra lines, internal thrust faults are generated absorbing a portion of the potential energy

generated by the two plates colliding. Most of these will show at least some level of oblique slip, generated

by the rotation explained above. This is before the reactivation in theopposite direction during the early

Devonian period, creating transtension. An interesting side eect of this oblique slip is that it is possible

to generate normal faults at an inclined angle to the strike slip surfaces (Smith (1995)) as shown in Figure

1.5. Therefore, while being in an overall compressional setting (at theplate scale), what will become the

Midland Valley is actually undergoing transtension and generating a basin. However, when the system

has the sinistral movement switched to dextral, some of the faults gointo compression. Whilst all these

faults are in stratigraphy that is signicantly below the Carboniferous , the Midland Valley is built upon

and deforms to this earlier fabric.

Figure 1.5: Map showing the opening of normal faults oblique to th e main strike slip trend on the north side ofthe Southern Upland Fault (SUF). Modied after Smith (1995).


The Devonian is marked by a rare period of tectonic calm in the southernMidland Valley ( Coward

(1993)). The North Sea-Baltic Block continues to move away from Lauratia and Avalonia generating a

slight transpression leaving the Midland Valley as a basin relative to the neighbouring uplifted blocks

(see Figure1.6). This does, however, eect the style of deposition radically as the environment becomes

predominantly arid or semi-arid, creating thick sandstone units (Cameron and Stephenson(1985)). These

are the Old Red Sandstone formations which are seen across much of Scotland.

Figure 1.6: Schematic diagram showing the orientation of conti nental convergence and direction of continentalescape between the Variscan (nee Avalonia) and Baltica plates during the Devonian. Modied after Coward(1993).

The very end of the Devonian is marked by a severe climatic shift backtowards humid equatorial

conditions leading to large uvio-deltaic systems being deposited ona continental scale within the basin

(Cameron and Stephenson(1985)). This is coupled with a slight change in the tectonics as the North

Sea-Baltic Block continues to be expelled away from the other two plates altering the forces imparted

on Avalonia and Lauratia enough to start restart transtensional movement within the basin (Coward

(1993)). This heralds the start of the Carboniferous (see Figure1.7).


Figure 1.7: Schematic diagram showing the orientation of conti nental convergence and direction of continentalescape between the Variscan (nee Avalonia) and Baltica plates during the Early Carboniferous. Modied afterCoward (1993).

With the basin nearly at, or around, sea level the environment uctuates between lush forest growth

of the delta tops (which eventually become the coal seams) to the occasional ood surfaces created

by a transgressive sea, causing the deposition of thin limestones and calcareous muds (Cameron and

Stephenson(1985)). These make up Inverclyde Group, Strathclyde Group, ClackmannanGroup and

Scotish Coal Measure Group that we see within the Douglas Coaleld stratigraphy (see Appendix E).

Within the context of this project, the actual sedimentology of the beds is largely irrelevant at the scale

of the Douglas Coaleld. Whilst the beds vary greatly in thickness, they are usually within the same

magnitude and are laterally continuous and conformable. Any variation is thought to be due to the

irregularity of the topography left by the Silurian fault system which is creating localised depositional

centres (Cameron and Stephenson(1985)).

As the Carboniferous drew to a close (the Stephanian) the North Sea-Baltic Block went into reverse

(see Figure1.8), once again switching the sense of movement through the Midland Valley (Coward (1993)).

This creates a relative positive inversion within the Carboniferous units along some of the faults.


Figure 1.8: Schematic diagram showing the orientation of conti nental convergence and direction of continentalescape between the Variscan (nee Avalonia) and Baltica plates during the Late Carboniferous. Modied afterCoward (1993).

This, stratigraphically speaking, is the top of the Douglas Coaleld sediments. After the Carboniferous

there is believed to have been a hiatus of deposition (Cameron and Stephenson(1985)), which caused the

reddening of some of the uppermost sediments of the sequence due tothe exposure to terestial conditions.

This is before the deposition of the Permian sandstones that are seen elsewhere across the Midland Valley.

These Permian sediments show a return to more arid conditions, creating the New Red Sandstones, as

seen across a large portion of Scotland.

The nal point of note in terms of the coalelds geological history, is the signicant dyke swarm

and intrusion emplacement seen across a large portion of the basin (see geological map in Appendix D).

This is currently undened in terms of age or composition (British Geological Survey), however it is most

likely linked to the Tertiary volcanic centres of Mull and Arran ( Cameron and Stephenson(1985)).

1.2.2 Mainshill Wood Specic Information

The Mainshill Wood mine sits within the stratigraphy of the Clackman nan Group, being comprised

of mostly sandstones, shales and coals. A sedimentary log of the southwestern pit wall was taken by

Callaghan (2014) as the pit nished its operations. This log can be seen in Appendix C.Structurally, the

site sits up against the footwall of the Kennox Fault, which in turn, f orms the southern boundary fault

for the northern end of the Douglas Coaleld Basin, as shown in Figure1.9.


Figure 1.9: Labelled 1:10,000 British Geological Survey, geological map of the area surounding the MainshillWood site. Open cast site marked on underlying Ordnance Survey map (also 1:10,000 scale).

1.2.3 Structural Examples and Models Used

Strike slip systems have become a signicant area of research over the past couple of decades with many

papers being published and then expanded upon. Collectively these form what can be described as

the hypothetical model for how this type of system is formed, grows and interacts with itself and the

deposited sediments.

Broadly speaking, strike slip structures fall into a variety a die rent classications (as shown in

Figure 1.10). These are generally controlled by one of two factors; direction of shearrelative to fault

geometry and the oblique forces at play (transpressional or transtensional) if any.


Figure 1.10: Diagram summerising the various styles of strike slip geometries possible (From Cunningham andMann (2007)).

This work has been substantially extended by Dooley and various colleagues to account for more

and more variables. In his 2012 review of variations on the Reidel Experiment and Distributed Strike

Slip Shear experiments, he comprehensively covers the factors that can eect fault geometries; lithology

competency, variable stratigraphy, transpression and transtension, oblique faulting, multiple controlling

faults, step-overs and bends and crustal weakening. Whilst this istoo large of a topic to cover to a

satisfactory level here, the paper can be considered a key reference for this project. Some of the most

relevant work for this project is the eect of lithology on faulting as shown in Figure 1.11.

Figure 1.11: Diagrams showing the eect of wet (ductile) or dry (b rittle) clay (lithology) on faulting styles withinstrike slip systems. D for displacement. From Dooley and Schreurs (2012).


Some of the latest research into strike slip systems concerns the eects of pre-existing fabric on the

fault geometries (Zahasky and Hudleston(2014)). This is best illustrated by Figure 1.12 which is taken

from their work.

Figure 1.12: Diagrams showing pre-existing fabric on faulting styl es within strike slip systems. d for displacement.From Zahasky and Hudleston (2014).

2

Section II: Regional Model

21

2. SECTION II: REGIONAL MODEL 22

2.1 DOUGLAS COALFIELD REGIONAL MODEL

To gain a full and useful understanding of any small scale geological feature, it has to be considered within

its regional context. In most cases a broad understanding of the regional tectonic and stratigraphic factors

at play is sucient. However, the Douglas Coaleld is an outlier, sitti ng within the complex Midland

Valley setting which has undergone several dierent phases of deformation. As such, an intermediary

step between the plate scale and the smaller scale, helps with thisunderstanding.

A full workow for the generation of this model can be seen in Appendix A.1. However, the key

assumptions and thought processes behind them are outlined in this section.

2.1.1 Assumptions

As with any geological model, there are a series of assumptions made to compensate for information

that is unavailable. In this particular example, whilst the input d ata is reasonably comprehensive in a

single 2D slice (the British Geological Survey 1:50,000 Surface Geological Map), very little information is

available for the Z axis (Depth/Elevation). As such, most of the assumptions are related to extrapolating

out of the 2D (surface) plane into 3D.

The rst signicant set of assumptions are related to the fault system. The geological map, whilst

oering dip direction, gives no indication to its magnitude. To circ umvent this issue, it has been assumed

that, since the rst phase of deformation to the Carboniferous aged rockswithin the basin is believed to

have been transtensional, most faults would form with normal fault style geometries, i.e. approximately

at a 60! dip (as described byHecker (1993)). There is no way of deducing which faults formed during

this period, therefore all faults are assumed to be of this type unless they clearly exhibit dip indicators

along both sides of the trace. For these other faults, a vertical dip (90!) has been assigned. It is assumed

that these have been steepened during the second phase of deformation (transpression).

These initial fault planes, based on the fault traces on the geological map, have then been extrapolated

above and below the surface. This obviously fails to consider the curved nature of faults, however, without

any intercepts within the boreholes to constrain the structures at depth, this becomes a factor that the

model struggles to represent from the input parameters available.

The second signicant assumption is that, generally speaking, the stratigraphy is believed to be

largely conformable. In addition, while the thickness of the beds do tend to vary along strike, they are

generally believed to stay within the same magnitude (i.e. if a bedis measured to be 5m at point a,

along strike, the same bed could be 8m or 2m, but is unlikely to be 40m). With this in mind, on the

scale of this model, it is expected that the thickness between the layers should be reasonably consistent.

2.1.2 Key Features

The fault system is dominated by a set of northeast-southwest striking faults which act as the main

through going system (as shown in Figure2.1). These are interconnected by shorter faults which sit


roughly orthogonally to the main faults and generally strike northwest-southeast, although there is far

more variability in this orientation compared to the main faults.

Figure 2.1: Petrel 3D view of the Fault system. Faults colour coded. Red: Strike slip and Normal Faultgeometries. Blue: Thrust Fault geometries.

If the strike slip component is ignored, generally the faults show a normal style geometry with the

descended block being in the hanging wall of the fault. This is bestshown in Figure 2.2, where the

terracing is generated within a larger fault panel. Here the dextral, trantensional, slip puts individual

fault blocks under extension causing them to fan, or fracture completely, creating the terraced appearance

of normal faults. The irregularity of the faulting may be due to deeper structures as outlined in Section

1.2.1.


Figure 2.2: Petrel 3D view of a portion of the Fault system with the Base of the Coal Measures Horizon addedfor reference. Faults colour coded as before. Dextral Slip sense along the through going faults indicated. Directionof slip on selected faults indicated by the blue arrows showing the terracing created by the normal faults.

Along with the normal faulting, there are also several faults which showa compressional style oset.

These are colour coded in Figure2.1. This is most prominent in the central block, detailed in Figure 2.3

where an antiformal structure has developed oblique to the slip direction. If this interpretation is correct,

it is a small step to explain the smaller fault in the middle of the gu re as a true thrust (as opposed to

an inverted normal or strike-slip fault), generated by the antiform being constrained by the bend in the

major fault behind.


Figure 2.3: Petrel 3D view of a portion of the Fault system with the Base of the Coal Measures Horizonadded for reference. Faults colour coded as before. Sinistral Slip sense on of the through going faults indicated.Antiform/Synform pair axis indicated. Black arrow indicates poss ible direction of thrusting generated within thefault block to create the small true thrust.

The inverted fault in the next block to the north (as seen in Figure 2.4) is a good example of

the inverted structures seen across the model. The geometry of thefaults suggests that they formed

under an extensional, dextral phase creating a horst and graben structure. When the whole system goes

into transpression with the sinistral movement (as indicated by the arrows) the blocks undergo relative

compression forcing the easterly block (the graben) up onto the horst, creating a positively inverted fault.


Figure 2.4: Petrel 3D view of a portion of the Fault system with the Base of the Coal Measures Horizonadded for reference. Faults colour coded as before. Sinistral Slip sense along one of the through going faultsindicated. Indicated original Horst Block bound either side by Normal Fault Geometries (note easterly fault hasbeen inverted). Black arrow indicates internal direction of comp ression to invert the normal fault to create thecurrent thurst geometry

While erratic in orientation, most of the faults can be explained as above, either being through going

strike slip faults, normal faults, inverted normal faults or true th rusts with their geometries dictated by

the variations in the idocentracties of the main faults. Unfortunately, this methodology breaks down at

the southern end of the model. This is outlined further below in Model Limitations.

The horizons can be split into two main groups of mapped and inferred. The mapped units (Base

of the Lower Coal Measure (Scotish Coal Measure Group), Base of the Lower Limestone Formation

(Clackmannan Group), Base of the Strathclyde Group and Base of the Inverclyde Group) were generated

from the British Geological Survey 1:50,000 Geology Map of the area and a series ofboreholes. The

inferred horizons (Base of the Middle Coal Measures and Base of the Upper Limestone Formation) were

generated by the layering algorithm within Petrel, which assumes they must lie between two designated

horizons and are tied using boreholes.

In general the horizons are of a consistent elevation across the model, only varying in the major up

thrown or down thrown blocks (as shown in the Horizons Maps in Figure2.5). This is to be expected

within a strike slip setting, where the majority of movement is lateral rather than vertical. Where there

are signicant elevation changes, the dierence tends to be localized on just a few fault blocks.


Figure 2.5: Petrel 2D Maps of the six horizons showing their elevations in metres and colour coded to thespectrum shown. 1. Base of the Middle Coal Measures, 2. Base of the Lower Coal Measures, 3. Base of theUpper Limestone Formation, 4. Base of the Lower Limestone Form ation, 5. Base of the Strathclyde Group, 6.Base of the Inverclyde Group.

The exceptions to this rule are the Strathclyde and Inverclyde Groups which seem to deepen

signicantly to the northern and southern edges of the basin. However,this is more likely to be due

to a broad inversion feature within the middle of the model; the moredramatic feature shown in Figure

2.3 is just an amplied sub-feature of this wider structure.


The thickness maps shown in Figure2.6, can be used to show on lapping stratigraphic relationships.

Red colouration indicates 0m thickness, essentially meaning that theunit is missing. This is most obvious

between the Base Upper Limestone Formation and the Base of the Lower Limestone Formation which

suggests the unit is only present in the northern end of the model. This could be evidence for selective

deposition based upon the relative elevation of individual fault blocks at the time. These then form

sub-basins.

Figure 2.6: Petrel 2D Maps of the thickness (in metres) betw een horizons. Using the numbering from Figure2.5: 1. Thickness between 1 and 2, 2. Thickness between 2 and 3, 3. Thickness between 3 and 4, 4. Thicknessbetween 4 and 5, 5. Thickness between 5 and 6. Colouring to spectrum shown.

Other fault blocks also show distinctive thinning, particularly between the Lower Coal Measures


and Upper Limestone Formation horizons towards the west of the model and within the Strathclyde

Group. This could point towards an isolated high at the time of deposition focused upon the current

horst structure.

2.1.3 Validity

The problem with generating large 3D models (especially with high levels of uncertainty), is that it is

very dicult to perform a series of rigorous restorations to check the models validity. As well as checking

for un-geological structures (outlined within the next section), two cross sectional lines (one for each of

the major axis) were taken from the nished model and restored back byline length (see Figure 2.7 for

the map).

Figure 2.7: 2D map from Petrel, showing the restorational cross sections relative to the fault system. CrossSection 1, northeast-southwest. Section 2, northwest-southeast.

Cross section 1 (shown in Figure2.8) is taken from northeast to southwest, along the strike of the

main through going faults. The base of the Inverclyde Group marks the beginning of the Carboniferous

and can be considered the basement for these sections. It is therefore not surprising that it shows the least

deformation with a line length very similar to the deformed section. Rising up through the sequence the

line length decreases which, with closer inspection of the unrestored cross section, is due to some of the

horizons being absent within certain fault blocks. This is consistent with the general interpretation that,


during this period of deposition, some fault blocks descended quicker than others, creating topography

which in turn inuenced deposition.

Figure 2.8: Line length restoration of Cross Section 1 (as shown in Figure 3.1). a) Unrestored, b) Restored.Horizons from base of secton, base of the Inverclyde Group (dark purple), base of the Strathclyde Group (pink),base of the Lower Limestone Formation (light pink), base of th e Lower Coal Measuresl (light green), base of theMiddle Coal Measures (light blue), length of unrestored sectio n (black). Topography shown as white in crosssection.

The base of the Lower Coal Measures has a signicantly greater length than the unrestored section

which is again consistent with the working model of the area. Depositedduring the end of the extensional

phase where most of the blocks have reached their maximum drop, the basin is at its most topographically

consistent, leading to a widespread deposition. Therefore, when the system reverses into relative compression,

this horizon shows the largest inversion.

The base of the Middle Coal Measures has a dierent characteristic to the others as it was not

deposited over a portion of the basin (see thickness map in Figure2.6). This is consistent with the

restoration shown above.

Cross Section 2 (shown in Figure2.9) has been taken from northwest to the southeast (see Figure

3.1 for line of section). It cuts across, perpendicular, to the main through faults. In this section, the line

length is roughly the same as the unrestored section, which is consistent with strike slip systems. The

only lines to show any real variation, are the two uppermost horizons (Middle Coal Measures and Lower

Coal Measures), which, like Cross Section 1, are longer than the unrestored section. This is probably due

to the same depositional reasons as Cross Section 1.


Figure 2.9: Line length restoration of Section 2 (as shown in Figure 3.1). a) Unrestored, b) Restored. Horizonsfrom base of secton, base of the Inverclyde Group (dark purple), base of the Strathclyde Group (pink), base of theLower Limestone Formation (light pink), base of the Lower Coa l Measures (light green), base of the Middle CoalMeasures (light blue), length of unrestored section (black). C ross marks intersection with Section 1. Topographyshown in dark green in cross section

The second check is against the algorithms used to generate the surfaces(the Convergent Gridder

Algorithm), because these alter and ex the initial inputs to generate smoother (and generally more

realistic) surfaces. This is done by comparing the surfaces generated along two of the cross sections used

derive the inputs. Cross Sections 10 and 30 (lines of section shown inFigure B1) were selected because

between them, they cover the best resolved two thirds of the model (see below for why the southwest is

considered less than ideal) and are taken in dierent orientations.

Cross Section 10 (Figure2.10) shows broadly the same geometry between the two versions, even

keeping the synformal feature 2500m along in the lower beds. However, the displacement along the

northern fault has been reversed into a normal fault. Fortunately this is the bounding fault for the area

of interest, so no input data was placed into the model for the generated surface. It does show, however,

that the software also works on the geometry of the fault, not just the entered points.


Figure 2.10: 2D Move cross sections along line 10 comparing the original inputs (top) against the surfacesgenerated by the algorithm (bottom).

Cross Section 30 (Figure3.5) also shows the same general geometry, capturing the main inversion

structures in the middle of the section to a reasonable level of accuracy. It is interesting that the purely

strike slip faults (as they were percieved during the generation ofthe inputs) have little or no displacement

in the generated version, although the algorithm has determind that there should be a displacement in

the horizons higher up the stratigraphy (base coal and younger). This seems to make the horizons in the

adjacent fault blocks ex less (second to third block from the right in the gure for instance).

Figure 2.11: 2D Move cross sections along line 30 comparing the original inputs (top) against the surfacesgenerated by the algorithm (bottom).

Overall the software has generally honoured the input data where it has been provided, only adding

a level of smoothing between the points so that the surfaces ex less (generally a geologically favourable


situation). It is possible that some of the vergence on the strike sliprelated structures has been lost

though.


2.1.4 Model Limitations

Despite being largely consistent with both the inputs and balancinginternally (as shown above), there

are several problems with the model which are mostly due to the limitations of the software or errors

within the input data.

The most obvious of these is the faulting towards the southwestern end of the model (as shown in

Figure 2.12). The observations and possible mechanisms for faulting described above (in Section 2.1.2)

start to break down here. The orientations become more erratic and throws decrease considerably. This

lack of displacement causes a number of issues for the software which struggles to resolve the surfaces

with the faults leading to the highly ungeological geometries exhibited by some of the faults (such as

the one marked). The broader source of the issue could be the lack of through going faults at this end

of the model, with only the bounding faults (north and south) being present and sitting further apart.

This spreads the force over a larger beam (in the engineering sense) allowing other factors to dictate

the orientation of faults (such as the underlying faults).

Figure 2.12: Petrel 3D view of the Fault system, focusing on th e Southwestern portion of the model. Base ofthe Lower Limestone Formation horizon is added for reference. Note boxed, ungeological, fault.

As noted above, some of the horizons show on lapping relationships and thinning within certain fault

blocks. Whilst the broader versions of this eect (such as seen between the Middle Coal Measures and

the Lower Coal Measures) are likely to be accurate (or at least geologically possible), the individual fault

block example shown by the Strathclyde Group are more likely to be generated by an error of input,


since the geological map (as shown in Appendix D) gives no indication of this.

Figure 2.13: Map of the Base of the Lower Coal Measures Horizon (as seen in Figure2.5) showing the locationof three modelling errors, highlighted on the both the map and in t he inset.

Beyond these more obvious issues, there are a number of smaller corners that the software failed

to cope with, largely linked to how it connects up the faults (as shown in Figure 2.13). Occasionally the

model produces angles or an interaction to which it cannot calculate a reasonable solution. The error

towards the north of the model shows where the software failed to connect the faults in a geologically

viable way, thus creating an overlap. As a consequence a spur of horizonhas been added incorrectly. The

most southerly of the errors marked demonstrates a similar occurrence where four faults meet at slightly

dierent orientations creating a geometry that the software can not generate correctly. This has led to

another spur of horizon coupled with some overlapping faults. Finally, the most westerly of the errors

is highlighted in Figure 2.13. The software is set up to model geological conditions, so the unnatural

geometries that the faults exhibit here generates an additional spur of horizon.


2.2 COMPARISION TO EXISTING MODEL

2.2.1 British Geological Survey Model

In 2012, the British Geological Survey (BGS) started to look in more detail at the Douglas Coaleld

because more data was becoming available from industry. The BGS Model ( Monaghan (2012)) focused

particularly on the northern end of the coaleld, in the area surrounding Douglas and on the two main

upper stratigraphic units (the Lower Coal Measures and Lower Limestone Formation). However the

approach taken within the BGS Model was to only project the horizons below the surface, not above

(as the New Regional Model presented in Section 2.1). An overview of the BGS Model can be seen in

Figure 2.14.

Figure 2.14: 3D model of the Douglas Coaleld, as generated by the British Geological Survey. After Monaghan(2012).


2.2.2 Fault Model

The fault model shown in Figure 2.15, from the BGS Model, has broadly the same geometry as the New

Regional Model proposed in Section 2.1 (shown again in Figure2.16 for comparison), showing a box

shaped set of boundary faults on three sides with an increased complexity to the south. Interestingly the

faults labelled A and B in Figure 2.16 are presented as a complimenting pair of normal faults forming

a graben structure in the BGS Model, whereas the New Regional Model suggests they are parallel

fanning structures. These faults are also signicantly longer in the BGS Model cutting almost half

way across the fault block towards the southeast. This seems to be due to the New Regional Model,

which is based largely on the 1:50,000 scale geology map (as shown in Appendix D),dierentiating to the

Formation level (with the internal layers inferred in afterward s), unlike the BGS model which, being

based on the 1:10,000 scale map, has them marked in initially.


Figure 2.15: 3D Fault model of the Douglas Coaleld, as generated by the British Geological Survey. AfterMonaghan (2012).


Figure 2.16: Closer view of the fault model proposed in the New Regional Model (shown in Figure 2.1) in thePetrel 3D view. Labels link to text.

The triangular fault feature (labelled C) has been broadly modelled in the same way, with the fault

angles at a very similar azimuth. However, the New Regional Model connects these up fully, creating

an isolated fault block, whilest the BGS Model creates relay rampstructures. These could be based on

local knowledge which was not used to generate the New Regional Model, however these structures are

not supported by the current 1:50,000 map. Many of the faults to the south of the area marked C are

missing from the BGS Model, probably due to the layers not beingpresent below ground, whilst they

are projected to intersect in the New Regional Model. This is a result of a dierent scope of interest

between the two models.

Finally, the fault at point D in the New Regional Model is split int o two separate faults in the

BGS Model which meet at an oblique angle. Again this is not present on the 1:50,000 map, so is could

have been derived from an outside source of information.


2.2.3 Horizons

The horizons in the BGS Model are not projected above the surface (see Figure2.17), hence for a

more direct comparison, Figure2.18presents the New Regional Model with the Base of the Lower Coal

Measures and base of the Lower Limestone Formation in the same way (i.e cuto at topography).

Figure 2.17: 3D view of the base of the Lower Coal Measures and the base of the Lower Limestone Formationhorizons as modelled by the BGS. Labels link to text. After Monaghan (2012).


Figure 2.18: Maps showing the extent of the Base of the Lower Coal Measures (green) and Base of the LowerLimestone Formation (pink) below topography in the New Regio nal Model.

The BGS Model and the New Regional Model both show a gentle synformin the main fault block

which is plunging towards the southwest however the BGS Modelextends this with a nger of the

Coal Measures unit projected to the northern boundary fault. This maybe to account for extra data not

available during the creation of the New Regional Model, however, its is more likely that a small sub

basin is located where the X is labelled (in Figure2.17).

The Lower Limestone Formation is similar again, with slight morphology dier ences between the two

models but the general geometries are the same. However, the area aroundlabel Y is notably dierent.

In the New Regional Model it is continues to project the horizons towards the south, observing the the

Strathclyde and Inverclyde Groups, while in the BGS Model it t erminates.


Figure 2.19: Side-on 3D view of both models from the same angle comparing the dierent proles taken by thebase of the Lower Coal Measures and the base of the Lower Limestone Formation. BGS Model on top, NewRegional Model below.

An interesting comparison can be drawn from a side-on view from the south (looking northwest)

as shown in Figure2.19. It allows a direct comparison of the prole taken by the horizons against one

of the main through going faults at the deepest parts of the basin which is covered in the BGS Model

(circled in both models). Without a scale on the BGS Model, it is dicult to quantiably evaluate the

dierence, however it would seem that the BGS Model takes the horizons to a deeper elevation and with

a greater ex in the beds. Interestingly though, both models show some thickening into the deepest part

of the basin.


2.2.4 Evaluation

Generally, the models show the same broad geometry with the main dierences being the variation in

focus, rather than interpretation. The fault system suggested by the New Regoinal Model includes more

faults and suggests greater throw lengths in some locations, however the displacement direction is usually

agreed upon. The horizons are mostly dictated by the focus taken when generating the inputs. Notably

where these two horizons are well constrained by surface intersections the models are very similar.

3

Section III: Mainshill Wood

44

3. SECTION III: MAINSHILL WOOD 45

3.1 INTRODUCTION

In an industry setting, Section 2 could be seen as a desk top analysis of the regional structure used

for applying for licenses or exploration permits. The issue with these style of models is their inherent

lack of resolution. At best, regional models created from a 1:50,000 scale map,will be accurate to

approximately 100m. Whilst in relatively simplistic structural are as this will create accurate geometries

at all scales (within a small tolerance relative to the input), withi n a strike slip system, lateral change is

signicantly more pronounced. These are often the structures of most interest. Fracture networks form

in relation to the local stress regimes which can either become veinhosted (and hence of interest for

mineral exploration) or disrupt bed related deposits (coal or BIFs) or interfere with reservoirs.

With this in mind, being able to predict these structures at the smaller scale from the regional models

is of signicant use to industry. Taking advantage of the data now being made available by Hargreaves

Surface Mining, it is possible to look at some of these more complex substructures within the context

of the regional model. The example shown here is from the Mainshill Wood open cast site (as seen in

Figure 1.9).


3.2 MAINSHILL WOOD MODEL

3.2.1 Available Data

The model shown here is derived from the mine plans currently held by Hargreaves Surface Mining,

which include the outline of maximum excavation (in three axes) and seam plans for all the coal units.

In addition, the Ordinance Survey 1:10,000 surface data was used to createan approximate topography

pre-excavation and to constrain the wider geology. Photographs of the south-western pit wall have been

converted into a cross sectional line along the edge of the pit and are used to add a vertical constraint

on the data.

3.2.2 Cross Section Model

The standard methodology for developing models of a close scale, complexstructural area is often to

examine the area and interpret it on its own merits before then trying to slot it into the wider regional

picture. Using this approach the rst step is to characterise the area. This is best done using the

photographs of the southwest wall of the pit.

The majority of the pit wall (as shown in Figure 3.1) is made up of vertical beds of the Clackmannan

Group younging to the northwest, consisting of interbedded shales,sandstones and coal seams. Ignoring

the southeast the sequence is largely intact with no cross cutting features suggesting that any faulting or

deformation must be taken up within individual beds and hence have avertical orientation.

Figure 3.1: Anotated panoramic photograph of the south-western pit wall of the Mainshill Wood mine. Beddinghighlighted in white. Main coal seams in blue (and labelled) . Edge of ower structure (red). Truck highlightedin green for scale.


Figure 3.2: Anotated photograph of internal defomation shown wi thin the Manson Coal seam. Red, shear planes(with sense of shear indicated). Blue, deformed competent layers. Green dashed, folding in ner material. A4sheet of paper for scale. Exposed face is oriented northwest-southeast.

The internal deformation (as shown in Figure 3.2) is taken up within the more shaley and coal rich

units, and in terms of geometry starts to behave in a manner approachingductile. The shortening in the

beds suggests a compression event at least at a local scale.


The feature at the far south-eastern end of the section is shown in greater detail in Figure 3.3. As

can be clearly seen, a signicant cross cutting feature truncates the vertical stratigraphy with a more

deformed package plunging approximately 45! towards the southeast. The package includes several lenses

of more competent material (possibly sandstone). This sheared material, which at this scale looks largely

uniform (charcoal grey), at outcrop scale shows complex folding, picked out by thin, more competent

layers.

Figure 3.3: Anotated photograph of the complex structural feature in the southern corner of the pit. Yellow,truncations. White, highlighting change in lithology. Youn ging arrow based on stratigraphy seen to the northwest(as described in Figure 3.1). Section is approximately 30m in height. Modied after Leslie (2014).

This package is juxtaposed against a large, saucer-shaped lens of white material, possibly from the

unit on the extreme right of the gure. The contact between these two packages is noted by a slight

change in colour (from charcoal grey to a lighter grey), which appears to runparallel to the truncation

noted above.

Beyond the saucer-shaped package, the lithologies become far more changeable, varying with

signicantly more regularity and showing several lens of lighter material. Here, however, the units are

dipping towards the northwest at approximately 60!, creating a V with the rst two shear zones. This

package continues to the edge of the pit, where the wall turns the corner to make the southeastern edge

to run along strike with bedding. From these observations the following interpretation can be made for

this end of the pit (see Figure3.4).


Figure 3.4: Anotated photograph showing the interpretation of t he features shown in Figure 3.3. After Leslie(2014).

In broad terms, the feature has been interpreted as a positive owerstructure. These are generally

conceived as shear zones (or faults) splitting from a single root into several branches before reconverging

into a single fault again higher up (as illustrated in Section 1.2.3). Interestingly this is also how the

structure varies in the y-axis, spreading from a single fault, to many, and then back to one (usually as a

step over structure between two oset faults). In which case, the exposed section here at Mainshill Wood

is in the middle of the structure in the y-axis and near the root in the z-axis.

If the structure has formed purely as a positve owstructure, the overall geometry would resemble (a

very distorted) antiform, doming towards the middle of the structu re. Because the beds dip towards the

root system (forming a synform), this would imply that the feature was originally a negative structure

which has then been inverted and tightened, with the hanging wall blocks rotating towards the centre (as

marked) placing them into a more vertical alignment. This suggests at least two phases of deformation.

This, however, does not explain the vertical beds o to the north of the section. Fortunately if we

account for the large Kennox Fault which is the bounding fault for thi s portion of the basin (as shown

in Section 2) a progression of events, as outlined Figure3.5, could generate the observed geometries.


Figure 3.5: Conceptual sketches showing how stratigraphy could be manipulated to a near vertical orientationnext to a positively inverted fault.x marks the location of th e Mainshill site if F1 was the Kennox Fault.

By taking the Kennox Fault to be a transtension strike slip fault du ring the majority of the

Carboniferous (as shown in the literature), the Douglas Basin will be forming against it concurrent

with deposition. This causes a aring of the beds into the fault, in eect making the horizons steeper

in close proximately to the fault (diagram 3). If a splay was then generated from depth out to the

north (F2), this could cause a dislocation of the beds, allowing them to reach an even steeper dip prior

to inversion (as indicated in diagram 5). This is a potential interpretation of the other fault marked

in red in Figure 3.6. Another factor in the steeping of the stratigraphy could be the tightening of the

pre-existing synform by the compresional inversion.


Figure 3.6: Close scale (1:10,000 scale) British Geological Survey geological map of the Mainshill Wood Site(labelled as in Figure 1.9. Underlaying map, 1:10,000 Ordnance Survey. Fault Colouration as discussed in text.

From this general structural outline, it is now possible to create a concept model of this particular

feature. If we consider it as a ower structure related the Kennoxfault, which has undergone transtension

before progressively more and more transpression and make the competence variation between the beds

stark, it is possible to derive a similar structure to the one observed (see Figure3.7).

Figure 3.7: Conceptual sketches showing how the Mainshill ow er structure could form from the structuresgenerated in Figure 3.5. Kinematics indicated.

First a splay, similar to the one shown in Figure 3.5, forms, possibly due to a need to step over


between faults vertically (in a similar way to how they behave in the y-axis) or to accommodate a

lateral variation along the length of the Kennox Fault in either geometry or lithology. As the system

is, at this point in time, in transtension, the central block rotates anticlockwise towards a more at

lying orientation compared to the surrounding stratigraphy (Figure 3.7, diagram 1). When the system

develops into transpression, this block is then rotated to almost at lying while also steepening the faults

against the structure (Figure 3.7, diagram 2). This also steepens the outer splay fault. Concurrently,

the majority of the movement is in the strike slip orientation creat ing signicant shears through most of

the weaker layers, creating signicantly more deformation without moving the blocks any great distance

in the y-axis. As the transpression continues, the block trapped against the fault becomes more and more

deformed, leading to almost ductile style deformation in the lithode rock which has not experienced

metamorphosism. This leads to a lens of more competent material with the shears running through the

ductile shales and coal seams. As the central blocks starts to lock upa new splay forms outside the

existing fault to release some of the stress (Figure3.7, diagram 3).

Therefore the slightly larger conceptual sketch of the site would looksimilar to Figure 3.8 with

geometrically similar structures at dierent scales. The smaller example (as seen in the pit wall at

Mainshill), being closer to the fault and hence more deformed.

Figure 3.8: Conceptual sketch of the larger ower structure around the Mainshill Wood Mine. Fk; Kennox Fault.Other Fs numbered in order of formation. Box indicates section obs erved in the pit wall.

This style of deformation is only possible because of the nature of the stratigraphy coupled with

the large amount of deformation concentrated into a relatively small volume of rock. If the dierence

in competency was smaller, the rocks would act in a more consistent nature, meaning the deformation


would be more likely to concentrate on just a few fault planes. It is also worth taking note of the ratio

of lithologies. If the stratigraphy had been shale dominated then the deformation zone would be sheared

to a much higher degree, allowing it to focus onto a smaller area. Due to of the lens of sandstone (the

more competent material) the deformation has to spread enough to accommodate the largest lenses of

material (such as the saucer-shaped lens) which has a major impact onthe width of the structure.

It is interesting that these behaviours and interactions of ductile and more competent material are

possible at a fairly shallow depth with little or no heat, creating geometries more comonly found in high

grade gneisses.

3.2.3 Model Extrapolated into 3D

As interesting as the 2D interpretation of the pit wall is, it is only of an y use if it can be extrapolated out

into 3D, so it can be used as a predictive tool. Fortunately, strike slip structures tend to be reasonably

similar in all three dimensions when it comes to the boundaries of thestructure. This is largely because

they have a tendency to form within the orientation of least resistance (in essence streamlining into the

controlling fault where possible).

This, coupled with the observations made from the seam plans providedby Hargreaves Surface

Mining, have allowed the generation of a 3D conceptual model of the structure (see Figure3.9). This is

based on knowing that the seams are intact and not displaced (as shown in the seam plan) and from the

observed dips from the cross section. The fault system can be seen inmore detail in Figure 3.10. Here

the faults between the two major splays are not particularly long but are more localized, taking up the

deformation within the larger structure, quickly feeding back int o the bounding faults.


Figure 3.9: 3D Move model of the Mainshill Wood seams and ow er structure. Index Coal Seam shown in Blue.Topography with the pit at its maximum excavation also shown.

Figure 3.10: 3D Move model of the fault system as shown in 2.18. Magent faults; Kennox Fault and Main Splay.Red faults are internal faults and shears as seen in the cross sectionin Figure 3.8. Pit plan surface of maximumexcavation shown with Index Coal seam for reference.


Extrapolating out towards the southwest is signicantly more dicult because of the absence of any

data to constrain the faults or seams. This essentially leads to two options; either the structure splays out

and the faults diverge away from each other to innity (at least at this s cale), or the structure returns

towards the controlling fault (the Kennox Fault) to create a lens of material caught between the two

faults. If the main splay fault was to keep diverging away from the Kennox Fault, it would eventually

have to create a signicant disturbance in the surrounding stratigraphy and show up on the geological

maps (such as the 1:10,000 map of the site shown in Figure3.6). Since it does not appear, this would

suggest that it returns towards the main fault, making a structure small enough to go unnoticed on the

1:10,000 survey. An example of how this might look is shown in Figure3.11 (extrapolated faults shown

in purple).

Figure 3.11: 3D Move model of the Mainshill Wood ower struct ure extrapolated towards the Southwest. Purplefaults are unconstrained.

Geometrically speaking this creates an issue. To create a structure along the strike of a strike slip

fault, a restraining or releasing bend is required (as shown in Section 1.2). However, at the Mainshill

Wood site, the structure has formed on the outside of the bend of the Kennox Fault. The local stress

regime usually associated with this geometry is not present at this location. Note that the fault marked

in purple rather than red in Figure 3.6 seems to be a later structure, judgin

Documents

A Framework Approach for the Modelling and Interpretation of Complex Strike Slip Basins: Douglas Coal Field, Midland Valley, Scotland