Wrench Tectonics in the Carboniferous Coals of the Midland Valley of Scotland

8/15/2019 Wrench Tectonics in the Carboniferous Coals of the Midland Valley of Scotland

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Abstract

Resolution is often poor when studying the 3D nature of geological structures, as usually seismic isthe only imaging method that can be used. This project aims to change this, by demonstrating highresolution (2-5 m) 3D coal seam data can be used to complement geological mapping and boreholing,producing a well-constrained structural model of key coal-bearing horizons and faults in East Ayrshire,Scotland. By creating a high-resolution model in Midland Valley’s Move software, this method candetermine locations of high coal extraction potential and gives a predictive tool to mining rms forfault trends and styles. The project also uses digital BGS data to determine an evolutionary modelfor the complex Midland Valley, where reversals in stresses have created multi-phase fault structures.These results complement recent studies in the area where The Midland Valley is determined tobe a sinistrally reactivated graben in the Early Carboniferous, then dextrally offset in the LateCarboniferous. Folding in the coals also indicates far-eld N-S Variscan stresses may have penetratedas far as the Scottish Uplands. Applications for these results are numerous in other strike-slipsettings, where often the 3D geometry is poorly conned. Additionally, as the coal-bearing rocks aresandstone/shale dominated, they can be used as an excellent analogue for conventional hydrocarbonreservoirs- particularly those with normal or strike-slip faults.


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University of Leeds

Department 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 pieceof work is my own, and it does not contain any unacknowledged work from any othersources.

Name:

Signed:

Date:

Programme of Study:


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Acknowledgements

Acknowledgements and thanks must be extended to Kier Mining, ATH Resources, and British Coalwho provided data for this project. Specic thanks go David Richardson and Giles Hemmings atKier Mining who gave guidance and support, and had the unenviable job of the initial data validationduring this project. Thanks also to Peter Rourke and Jenny Ellis at Midland Valley for their excellenttraining in the software, and their expertise during the project.

Acknowledgements should also be extended to Taija Torvela, Douglas Paton, Toby Dalton and Geoff Lloyd at the University of Leeds for their academic assistance during the project. Also thanks toSimon Libby for proof reading and help during the write-up.


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Contents

List of Figures 3

1 Introduction 101.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2 UK Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3 Hydrocarbon Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.3.1 Conventionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3.2 Unconventionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.4 Project Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 Geological Background 182.1 Research on The Midland Valley of Scotland . . . . . . . . . . . . . . . . . . . . . . . . . 192.2 Ayrshire in the Carboniferous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.1 Stratigraphy and Tectonic History- Literature Compilation . . . . . . . . . . . . . 272.2.2 Coal Seam Correlation in East Ayrshire . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3 Expected Structural Styles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.1 Normal Faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.2 Strike-Slip Faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.3.3 Inter-Coal Faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3 Methodology 383.1 Data Obtained for the Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2 Modelling Workow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.3 Modelling Workow- Large-Scale Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.4 Modelling Workow- Small-Scale Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.5 BGS Cross Section - New Cumnock 14E . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.6 Completing the 3D Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.7 Additional Modelling: Seam Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.8 Additional Modelling: Petrel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4 Results 594.1 Move Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.2 Unfolding the BGS Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.3 Dip Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664.4 Apparent Dip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.5 Fault Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

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4.6 Fault Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.7 Fault Lengths: 5x5 km Move Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.8 Fault Lengths: 20x25 km Move Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.9 Greenburn Coal Seams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.10 Petrel Isochores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.11 Seam Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.12 Potential Coal Exploration Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.13 Correcting the BGS Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5 Interpretation 895.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905.2 Folding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

5.3 Faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.3.1 Graben Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.3.2 Flower Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

5.4 Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

6 Discussion 1036.1 Folding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046.2 Faulting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1066.3 Underlying Fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

6.3.1 Stepover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116.3.2 Boundary Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

6.4 Inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.5 Implications for other Strike-Slip Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7 Conclusions & Synthesis 1187.1 3D Data Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1197.2 Midland Valley Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1197.3 Faults and Coal Thickness Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217.4 Coal Exploration Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217.5 Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

References 1247.6 Software Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

A Petrel Edge Detection 130

B Full Word Count 134B.1 Full Word Count . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

C Digital Data (CD) 136


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List of Figures

1.1 Map of The Midland Valley in relation to the British Isles, showing the two main boundingfaults. Modied after Google Inc. (2013) and Smith (1995). . . . . . . . . . . . . . . . . . . . 11

1.2 Maps showing location of study area. Area covers 20 km N-S and 25 km E-W. Maps compiledfrom Google Inc. (2013) and The British Geological Survey (1980, 2000). . . . . . . . . . . . 13

1.3 Simplied version of the geological map. 25 km E-W, 20 km N-S. . . . . . . . . . . . . . . . . 141.4 Map of UK showing the main Carboniferous coalelds. Modied after Cleal and Thomas (1996). 151.5 A summary of potential shale gas sources across the whole UK. Modied after Selley (2012) . 161.6 The red transparent colour highlights the erroneous sections of the BGS map identied by

Kier. After ( The British Geological Survey , 1980, 2000) . . . . . . . . . . . . . . . . . . . . . 17

2.1 Model showing possible interactions between normal faults and transfer faults, and the structuresthat may form at the intersections. Modied after Gibbs (1990). . . . . . . . . . . . . . . . . 19

2.2 Map of northern Britain showing the locations of LISBP lines ALPHA , BETA and GAMMA.Note the relatively shallow depth to basement within The Midland Valley. Modied afterBamford (1979). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3 Showing the general structure of the Iapetus suture, with specic reference to the faults thatbound The Midland Valley- the Southern Upland fault and the Highland Boundary Fault.These may provide a similar underlying structure to that shown in Figure 2.19 above. Sectiondrawn from WINCH and LISPB lines, plus ‘other geologic and geophysical data’ (see originalreferences). From (Needham and Knipe , 1986). . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 A new model for stress regime that formed The Midland Valley, from interpreting seismic linesaround the Firth of Forth. HBF: Highland Boundary Fault. SUF: Southern Uplands Fault.LM: Lammermuir Fault. Modied from From Ritchie et al. (2003). . . . . . . . . . . . . . . . 21

2.5 A model for the formation of the Great Glen Fault. Modied from From Kennedy (1946). . . 212.6 Location of seismic line is shown in Figure 2.5, note that it is not N-S across The Midland

Valley but is orientated such that it crosses relevant stratigraphy and structures. Horizons arequoted thus: 1: sea bed; 2: rockhead; 3: ?intra-Upper Coal Measures; 4: intra-MiddleCoal Measures; 5: intra-Lower Coal Measures; 6: top Lower Limestone Formation (topDinantian); 7: intra-Strathclyde Group equivalent; 8: ?intra-Devonian. All horizons areshown in Figure 2.10 on page 27. From Ritchie et al. (2003), as the original data were notavailable for this project, the Figure has been left interpreted as presented in the original paper. 22

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List of Figures 4

2.7 Structural evolution of Scotland, with focus on The Midland Valley (shown in pale green).

Black arrows show plate movements, light arrows show the proposed movement of the NorthernEuropean Block (NEB). a: Dinantian: NE trending sinistral strike-slip. b: Silesian:dextral movement on NE-trending faults and sinistral on NW-trending faults. The strainellipse is similar to that Ritchie et al. (2003) in Figure 2.5, but rotated anti-clockwise by 40 ° .c,d,e,f: Cross sections through a and b . Modied after Caldwell and Young (2013), originallymodied after Coward (1993) and Leeder (1982). . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.8 Map of the distribution and source for Limestone Coal Formation. The structural highs of theHighlands and the Uplands stayed the same during deposition of the Coal Measures. FromRead et al. (2002). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.9 Map of alluvial fan deposits found in The Midland Valley. From Cameron and Stephenson(1985). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.10 Compilation chart relating standard geological ages to the map units. These are then relatedto different interpretations of the palaeogeography and tectonic environment. Compiled fromThe British Geological Survey (1980), Browne et al. (1999), Ritchie et al. (2003), Rohde(2005), Heckel and Clayton (2006), Waters et al. (2012), Walker et al. (2012), and Caldwelland Young (2013). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.11 An updated version of the chart originally made by John Crone from Crouch Mining Ltdin 2000 and used by Kier Mining. The ‘BC Code’ refers to the British Coal codes thatare supposed to relate the various naming schemes across England and Wales. A columndisplaying the BGS coal seam map names has been added, and the overall graphical styleimproved. The Taits Marine Band is “the second Lingula band beneath the Pathhead Coal”

(Cameron and Stephenson , 1985), and is not recognised as widely as the Aegiranum MarineBand which stretches even as far as South Derbyshire ( Waters and Davies , 2006). . . . . . . . 28

2.12 Model showing large-scale deformation of the Earth’s crust through extension and normalfaulting. Upbending of the crust results from displacement in the lower layer. From Bott(1976). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.13 Simple models for extensional faulting showing no 3D along-strike variation. From Fossen(2010). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.14 Evolution of syn-rift extensional depocentres. From Cowie, Gupta and Dawers (2000). . . . . 302.15 Diagram of a normal fault cutting a strong-weak-strong layering of rock beds. The weak layer

is deformed and incorporated into the fault plane. This allows some exure within the strong

bed. From Fossen (2010). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.16 Large 3D block model Modied from (Cunningham and Mann , 2007). . . . . . . . . . . . . . 322.17 4D sandbox model evolution of a dextral spindle-shaped strike-slip transtensional ower

structure. Basal layer is 1.5 cm thick ‘SGM 36’ polymer with similar scaled properties tothe brittle-ductile transition zone. Overlaying this is 7.5 cm of dry quartz sand. Model scalesat roughly 1:100,000 (1 cm = 1 km). From Wu et al. (2009) . . . . . . . . . . . . . . . . . . . 33

2.18 Line drawings of experimental sand box modelling. a) Underlapping stepover. b) Neutralstepover. c) Overlapping stepover. Numbers relate to “master fault segments”. From ( Dooleyand Schreurs , 2012). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.19 Line drawings of dextral strike-slip sandbox models overlying a 90 ° dipping normal fault.[1]: A/B: purely brittle deformation. C/D: 5 cm of sand on 1 cm of silicone (acting as adécollement horizon). E/F: 4 cm of sand on 2 cm of silicone. [2]: A similar dextral system,however here the underlying fabric is unactivated. Compiled from Richard and Krantz (1991). 35


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List of Figures 5

2.20 The effects of simple-shear on a strain ellipse. Demonstrates that normal faults can form

oblique to a dextral strike-slip zones, and if enough deformation occurs, can be made toinvert. From ( Waldron , 2005), originally after Harding (1974). . . . . . . . . . . . . . . . . . 35

2.21 Diagram showing the structures that are associated with a pull-apart basin, such as in Figure2.16. From ( Rodgers , 1980). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.22 Diagram showing some of the features that can be expected in a sheared coal seam. Illustrationof features in the Wu seam of the Pingdingshan coaleld, northern China. Whilst this coalelddoes penetrate mineable Carboniferous coal, the Wu seam is Permian in age. Crosses indicateprimary axes of strain ellipses. Colour added to image from ( Li, 2001). . . . . . . . . . . . . . 37

3.1 Workow summary. Covers both small-scale (upper path) and large-scale (lower path) datasources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2 A Coloured DEM of area. B Draped extent of the scanned BGS solid maps. Note the arrow inthe bottom-right indicating North with the red point. ( The British Geological Survey , 1980,2000) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.3 Limits of map area within ArcMap 10, projected in the British National Grid co-ordinatesystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.4 Depending on which way the fault is dipping, a specic colour is assigned. This producessynthetic pairs of NE-SW trending and NW-SE faults, similar to the research by Caldwelland Young (2013) and Hooper (2013). It is also very similar to a method used by Wilsonet al. (2006) when they looked at strike-slip systems on the Norwegian Margin. . . . . . . . . 42

3.5 BGS-mapped faults are coloured based on their dip direction. Draped on maps from The

British Geological Survey (1980, 2000) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.6 A 3D oblique view of coloured faults. NB: glow effect is added post-process in Photoshop. . . 443.7 3D Extrapolation of all faults. Different colours relate to dip direction. Red and Blue (‘major’)

faults dip at 80 ° to a depth of 800 m. Green and Yellow (‘minor’) faults dip at an angle of 70 ° to a depth of 600 m. Also shown are where faults are clipping through each other, and wherethey have been manually cut to join. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.8 Dips displayed on the map. Maps from The British Geological Survey (1980, 2000). . . . . . 473.9 Location map of coal seam data from ATH Resources (Glenmuckloch, Netherton, Skares);

Kier Mining (Braehead, Greenburn, Grieve Hill, Rough Hill) and British Coal (Lanehead).BGS map is shown for scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.10 Full map and interpreted cross section scanned from the BGS map of New Cumnock, 14E.Note 1.5x vertical exaggeration on section. After The British Geological Survey (1980). . . . 49

3.11 Traced section, with bases of UCMS, MCMS, LCMS and LSC shown. Vertical exaggerationhas been removed. A: Thickness variation between the UCMS and the LCMS. B: No senseof reactivation here where C has clear reactivation. D: Large (˜200) normal offset on this fault. 50

3.12 Maps from The British Geological Survey (1980, 2000) with the location of the 5x5 km studyarea shown bound by vertical surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.13 Modied map showing only the key units from the stratigraphic compilation in Figure 2.10.Sections 01 and 02 are shown crossing through the units of interest (the Coal Measures).Modied from The British Geological Survey (1980, 2000). . . . . . . . . . . . . . . . . . . . 52

3.14 3D Model Bounds. 5000x5000x1100 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.15 3D model of faults in the 5x5 km study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.16 3D model showing the modelling of the volcanic intrusions that penetrate the 5x5 km study

area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54


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List of Figures 6

3.17 3D model with topography shown in green, and a red surface 200 m below this to reect the

maximum depth to which a surface mine can typically reach. . . . . . . . . . . . . . . . . . . 553.18 Oblique view from the South of the Greenburn coal seams 6500-6600-6800. Note that due to

software bugs the seams are not shown in the right vertical order. Map cropped from TheBritish Geological Survey (1980, 2000). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.19 The boundary polygon ts the most amount of data across all 3 seams. Data holes are dueto outcrop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.20 RDR’s edge-detection algorithm applied to a 2 m spaced convergent interpolation surface onthe 6800 seam. Hotter colours indicate edges- e.g. curvier surfaces with steeper dips. Samescale as previous Figure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.1 3D model showing the modelling of the Passage Group within the 5x5 km study area. . . . . 61

4.2 3D model showing the modelling of the Passage Group within the 5x5 km study area, colouredby depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.3 3D model showing the modelling of the Lower Coal Measures (LCMS) within the 5x5 kmstudy area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.4 3D model showing the modelling of the Lower Coal Measures (LCMS) within the 5x5 kmstudy area, coloured by depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4.5 3D model showing the modelling of the Middle Coal Measures (MCMS) within the 5x5 kmstudy area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.6 3D model showing the modelling of the Middle Coal Measures (MCMS) within the 5x5 kmstudy area, coloured by depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.7 3D model showing the modelling of the Upper Coal Measures (MCMS) within the 5x5 kmstudy area. As all exposure UCMS is lost in the North-West of the area, it has not beenmodelled there. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.8 3D model showing the modelling of the Upper Coal Measures (MCMS) within the 5x5 kmstudy area, coloured by depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.9 Unfold whole of LCMS to 0 m datum. Beds keep their parallel shape, however the geometryaround the faults becomes unreasonable. This technique has actually, by restoring all linesegments, produced a move-on-fault restoration. . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.10 Unfold only fold-hinge LCMS to 0 m datum. By only restoring the fold hinges, the fault blocksmaintain their offsets but are moved to a more horizontal position. The technique breaks inthe North (left) where there is no further hinge to ‘pull’ the beds down. Note that the faultsdo not warp considerably. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.11 Stereographical analysis of all dip data from map. From Move 2013 . . . . . . . . . . . . . . . 664.12 Average dip for each unit on the 25x20 km map, plotted against maximum age. Error bars

reach the maximum and minimum dips found on the map. . . . . . . . . . . . . . . . . . . . . 674.13 Diagram showing the effect of dip on recorded seam thickness. A vertical borehole cuts through

a coal seam (grey) and the thickness is recorded ( a ). The true thickness is actually the lengthat line b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.14 Graph showing what effect various dip angles will have on the recorded seam thickness downa vertical borehole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.15 3D model showing a dip attribute map for the prospective sub-surface coal seams, pink ellipses

highlight steeply dipping areas. Note that the Middle Coal Measures surface has had the areathat outcrop above topography removed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70


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List of Figures 7

4.16 Plan view of fault model in Move. Note that NW-SE dipping (Blue & Red) faults appear to

partition off the NE-SW dipping (Yellow & Green) faults. Also note that these fault sets donot abut perpendicular, but rather at a similar acute angle across the area. . . . . . . . . . . 71

4.17 By assigning a value to each node for the fault angle between the PDZ faults and the CBFs, asurface could be created to highlight areas of more acute or right-angled joins. There appears,however, to be no pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.18 Schematic diagram demonstrating the effect of normal faulting on coal exploration. . . . . . . 734.19 Schematic diagram demonstrating the effect of normal faulting on coal exploration, with a

more realistic model invoking fault drag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.20 Fault loss map for Middle Coal Measures in 5x5 km area. . . . . . . . . . . . . . . . . . . . . 744.21 Fault loss map for Lower Coal Measures in 5x5 km area. . . . . . . . . . . . . . . . . . . . . . 754.22 Graph showing normal fault lengths plotted by their dip-direction, against the base-10 logarithm

of their overall length. The average (mean) length is also shown. note how the measureslengths have poor data resolution due to fractal scaling of the faults- the largest and smallestsimply cannot be imaged as well as the ‘medium-sized’ ones. . . . . . . . . . . . . . . . . . . 77

4.23 Feature A in Figure 4.24. Photo taken 2013-05-28 at the Greenburn surface mine owned byKier Mining Ltd. Fault drag is visible in the sandstone beds. Also note that the ‘kinks’ in thefault are due to horizontal benches, rather than actual deviations in the plane. . . . . . . . . 78

4.24 Photo provided by David Richardson from Kier Mining Ltd. Seams from Figure 2.11 onpage 28 have been added. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.25 Petrel model of Greenburn seams, created using the edge detection algorithm outputs such asFigure 3.20. Trace beds of 1 m thick ‘coal’ are shown on the 6500 and 6600 seam, with the

6800 seam at the top. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.26 Cross-section through the Petrel model in Figure 4.25. Some fault reactivation is visible in

the South. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.27 Isochores from Petrel, thickness between 6800 and 6600 seams. Hot = thick, cold = thin. . . 814.28 Isochores from Petrel, thickness between 6600 and 6500 seams. Note different scale from

previous gure. Hot = thick, cold = thin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.29 3D model with topograph shown in green, and a red surface 200 m below this to reect the

maximum depth to which a surface mine can typically reach. . . . . . . . . . . . . . . . . . . 834.30 Plan view of 3D Model, showing the depths to the bases of the prospective coal measures that

are below the surface. A lot of MCMS has been eroded, hence it is only really present in the

South-East. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.31 Plan view of 3D Model with red ’maximum extraction depth’ slice present from Figure 3.17.Note that the volcanic intrusions are still shown for reference. . . . . . . . . . . . . . . . . . . 85

4.32 Depth-to-base for the Prospective Coal Measures. Colour map is only applied between 0 and200 m, as that is the maximum extent of potential coal extraction. . . . . . . . . . . . . . . . 86

4.33 Isochore thickness mapping vertically between the topography and the base of the prospectivecoal measures. Note that the syncline has caused the underlying to become buried too deepto extract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.34 New version of the BGS map created using the 3D Move model. Modied from The BritishGeological Survey (1980, 2000). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.1 Map from Figure 1.3 with the stereonet of all dips from Figure 4.11 overlain. 25 km E-W and20 km N-S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90


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List of Figures 8

5.2 Unit MCMS coloured by depth, with the colour map extending to cover only the extent of

the unit, rather than the whole Z range like in the results. . . . . . . . . . . . . . . . . . . . . 915.3 Average dip of each unit (with maximum/minimum dip error bars) plotted by ages from

Figure 2.10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 925.4 Normal faults are partitioned between strike-slip faults. . . . . . . . . . . . . . . . . . . . . . 935.5 A potential interpretation of a graben structure for the study area. Drawn to 6 km depth, as

suggested by the LISPB survey in Figure 2.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.6 The red and blue faults from Figure 4.16 on page 71. Extension is NW-SE . Note orientation

of North arrow in bottom right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.7 The green and yellow faults from Figure 4.16 on page 71. Extension is NE-SW . Note

orientation of North arrow in bottom right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955.8 A potential interpretation of a ower structure for the study area, using the same cross-section

line as Figure 5.5. Again, base is determined by LISPB from Figure 2.2. . . . . . . . . . . . . 965.9 The location of the Greenburn seams from Figure 3.18 on page 56 within the 5x5 km study

area. Note that the seam is not mined across the major blue/red faults. . . . . . . . . . . . . 975.10 Large fault damage zone in the normal faults seen in the Greenburn mine. Photo taken

2013-05-29 by Ben Craven at location Y on Figure 4.9. . . . . . . . . . . . . . . . . . . . . . 985.11 Schematic diagram demonstrating the effect of normal faulting on coal exploration, with a

more realistic model invoking fault drag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995.12 3D model with topograph shown in green, and a red surface 200 m below this to reect the

maximum depth to which a surface mine can typically reach. . . . . . . . . . . . . . . . . . . 1005.13 Areas of thick productive coal measures. Maps modied from The British Geological Survey

(1980, 2000). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.14 Green ellipses highlight areas that overlap from Figure 5.13. These will have the least risk

during mining. Map modied from The British Geological Survey (1980, 2000). . . . . . . . . 102

6.1 Map of The Midland Valley showing major folds and faults. Study area(s) shown in red;trace of Leven-Midlothian (or Midlothian-Leven) Syncline highlighted in green. Modiedafter Rippon, Read and Park (1996). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

6.2 Folding in an inverted strike-slip regime. From Allen, Alsop and Zhemchuzhnikov (2001). . . 1056.3 Plan view of the 20x25 km area modelled in the project. North is the top-right. . . . . . . . . 1066.4 Two styles of en-́echelon faulting, compressive and extensive. Angle x is 40-45° . Modied

after Ramsay and Huber (1987). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1076.5 Strain ellipse with labelled stress tensors. Modied after Sylvester (1988) with orientations

and labels from Hooper (2013). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1076.6 Modied after Wilcox, Harding and Seely (1981). Sub-gures are as follows: B: Dasht-e Ba˜yaz

(Iran). C: Lake Basin fault zone, Montana (USA). D: Cottage Grove, Illinois (USA). . . . . 1086.7 Figure 6.5 overlying the 3D fault model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096.8 Model showing possible interactions between normal faults and transfer faults, and the structures

that may form at the intersections. Modied after Gibbs (1990). . . . . . . . . . . . . . . . . 1106.9 Line drawings of experimental sand box modelling. a) Underlapping stepover. b) Neutral

stepover. c) Overlapping stepover. Modelled with a 10 cm thick layer of“brittle overburdenof silica sand”. Numbers relate to “master fault segments”. From ( Dooley and Schreurs , 2012). 111


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1

Introduction

10


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1. INTRODUCTION 11

1.1 Introduction

Last year, over forty percent of energy generation in the UK was provided by coal-burning power stations(Gosden , 2013). However this rising proportion of coal-based energy production is occurring against abackground of increasing external coal imports, where 80% of steam-coal hails from abroad ( CoalImp ,2013). This is despite there being, in March 2012, an estimated 728 million tonnes of coal remaining inknown UK reserves ( The Coal Authority , 2012). This contribution from outside is only compoundingthe profound decline of the UK coal industry, causing both industry commentators and insiders to referto the industry as “dying” ( Carrington , 2011).

Between 2004 and 2012, a third of all the UK’s saleable coal was extracted from the East Ayrshiredistrict of Scotland ( East Ayrshire Council , 2012). This area lies on the south west ank of The MidlandValley, a NE-SW trending structure that crosses the width of the country; delineated by the HighlandBoundary Fault on the northern edge, and the Southern Uplands Fault on the other; shown in Figure 1.1:

Figure 1.1: Map of The Midland Valley in relation to the British Isles, showing the two main bounding faults.Modied after Google Inc. (2013) and Smith (1995).

Across the whole of the UK, coal mining rms are struggling against imports of cleaner, cheaper coal.Ayrshire is being particularly badly hit by job losses, however this was compounded in May 2013 whentwo major employers in the area went out of business: rst Scottish Coal went into liquidation ( BBC,2013), quickly followed by ATH Resources ( ATH Resources PLC , 2013). This has meant that Kier Miningis the only coal extraction company left ( Coal Action Scotland , 2012).


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1. INTRODUCTION 13

F i g u r e 1 . 2 : M a p s s h o w i n g l o c a t i o n o f s t u d y a r e a . A r e a c o v e r s 2 0 k m N - S a n d 2 5 k m E - W

. M a p s c o m p i l e d f r o m G o o g l e I n c . ( 2 0 1 3 ) a n d T h e B r i t i s h G e o l o g i c a l

S u r v e y ( 1 9 8 0 , 2 0 0 0 ) .


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1. INTRODUCTION 15

1.2 UK Coal

There are 5 major identied coal elds in Great Britain, described by Cleal and Thomas (1996) below andin shown Figure 1.4. Lithological correlation can be made between them, as shown by Waters and Davies(2006), however the formational structural style is different in The Midland Valley. Variscan deformationmay be comparable across the elds (as shown by ( Coreld and Gawthorpe , 1996)), however it will bemuch less pronounced in the North.

1. Culm Trough: Basin inversion and tectonic deformation earlier than the rest of Britain. Probablymiddle Westphalian.

2. Kent Coaleld: Only known through boreholes and deep mining. No exposure.

3. South-Wales: Progressive inll of shallow marine deltaics. Variscan front basins.

4. Central England: Northerly-derived deltaic deposits lling shallow-marine basin. Variscan deformation.

5. Midland Valley: Coal-bearing deposits in the lower Westphalian, with red beds in the upperformations. Transtensional strike-slip.

Figure 1.4: Map of UK showing the main Carboniferous coalelds. Modied after Cleal and Thomas (1996).


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1. INTRODUCTION 16

1.3 Hydrocarbon Exploration

1.3.1 Conventionals

Although the eastern Midland Valley has a proven hydrocarbon system ( Maddox, Blow and O’Brien ,1997; Smith et al. , 2008; Selley, 2012), Ayrshire in the West contains no hydrocarbon exploration wells:the volume of volcanic intrusion present in the area has replaced the main prospective horizon that isprospective further East- the oil-shales within the Strathclyde facies ( Smith et al. , 2008). Additionally,any hydrocarbons that may have been produced from the Carboniferous sediments have likely beenmostly lost during the erosion of the Permian sediments.

1.3.2 Unconventionals

A report from 2008 stated “no [unconventional hydrocarbons] exploration is currently planned for theAyrshire region” ( Smith et al. , 2008). Although Figure 1.5 demonstrates that the lithologies present inThe Midland Valley would ordinarily be an excellent source rock, it is the same activity associated withthe Permian Volcanics ( Shelton , 1995) that has burnt off volatiles within the Coal Measures, makingprospectivity low. Also, as much of the Upper Coal Measures have been converted to limestone, they tooare almost wholly non-prospective ( Smith et al. , 2008).

Figure 1.5: A summary of potential shale gas sources across the whole UK. Modied after Selley (2012)


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1. INTRODUCTION 17

1.4 Project Aims

This project will focus on the specic problems listed below:

Display Kier’s coal seam data in 3D

Produce a structural evolutionary model for the faulting around the mines and relate this to theevolution of The Midland Valley

Correct the BGS map in the area around Kier’s current mining operations (Figure 1.6, below)

Relate the faults to coal seam thickness changes

Identify any new surface mining targets based on these new interpretations

Figure 1.6: The red transparent colour highlights the erroneous sections of the BGS map identied by Kier. After(The British Geological Survey , 1980, 2000)


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2

Geological Background

18


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2. GEOLOGICAL BACKGROUND 19

2.1 Research on The Midland Valley of Scotland

The Midland valley was recognised as a specic post-Caledonian structural terrane by Kennedy (1958)and George (1960), both invoking what would become known plate tectonics ( Bluck , 2002). George ’smodel for the structural evolution of The Midland Valley of Scotland was described in a recent paper byUnderhill, Monaghan and Browne (2008) as a simple rift valley formed by NW-SE extension. The basinboundaries are the Highland Boundary Fault in the North, and the Southern Uplands Fault in the South.

This extensional model was expanded upon by Leeder (1982) when he proposed that it was Iapetus-closureN-S back-arc extension that had created the rift. This was corresponding to the styles of deformation hehad seen in the other Carboniferous basins across the UK (e.g. Bowland, Northumberland and Solway)(Underhill, Monaghan and Browne , 2008).

However, Haszeldine (1988) and Stedman (1998) didn’t agree with Leeder ’s interpretation of N-Strending structures found across the area, instead both proposing models of E-W extension. This wasbacked with evidence from Gibbs (1984, 1990) who modelled the interaction between normal faults inbasin-forming settings and found a variety of structures that were also present in The Midland Valley(Figure 2.1).

Figure 2.1: Model showing possible interactions between normal faults and transfer faults, and the structuresthat may form at the intersections. Modied after Gibbs (1990).

These newly-named “transfer faults” were also seen by Read (1988), and later by Rippon, Read andPark (1996) in their studies of The Midland Valley. The resultant models for basin evolution werecomplex, invoking both dextral and sinistral slip vectors produced in the overall rifting/extension to

explain the geometries seen.This meant that by the late 90’s the upper-crustal structure of The Midland Valley terrane was

becoming clearer. Linking this to the deep crust had already been attempted, but progress was slow.

The overall structure of The Midland Valley was unknown for a long time due to the lack of seismicinformation available in the area. The best available, and still one of the few deep-crust surveys, wasthe 1974 Lithospheric Seismic Prole in Britain (LISPB), rst analysed by Bamford (1979), and shownin Figure 2.2 on the next page . The survey determined that the crust around the Caledonian Orogenyexhibits “clear... lateral variations in the deep structure,” and that there was a certain asymmetry acrossthe orogenic belt ( Bamford , 1979).


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Figure 2.2: Map of northern Britain showing the locations of LISBP lines ALPHA , BETA and GAMMA . Notethe relatively shallow depth to basement within The Midland Valley. Modied after Bamford (1979).

By combining this LISBP survey with another line (WINCH, interpreted from ( Brewer et al. , 1983)),Needham and Knipe (1986) developed a crustal overview for The Midland Valley, shown in Figure 2.3:

Figure 2.3: Showing the general structure of the Iapetus suture, with specic reference to the faults that boundThe Midland Valley- the Southern Upland fault and the Highland Boundary Fault. These may provide a similarunderlying structure to that shown in Figure 2.19 above. Section drawn from WINCH and LISPB lines, plus‘other geologic and geophysical data’ (see original references). From ( Needham and Knipe , 1986).

There was still reason to further investigate the deep crustal features of the area, so the Midland Valley Investigation by Seismology (MAVIS) survey was produced in the mid 80’s and analysed by Dentith andHall in 1989. The authors proposed that instead of the 3-layer LISPB crustal model shown in Figure 2.2,a 4-layer model was more appropriate: “ 1 Carboniferous and Upper Old Red Sandstone, 2 Lower Old RedSandstone and Lower Palaeozoic, 3 crystalline basement and 4 higher velocity crystalline upper crust”Dentith and Hall (1989). The paper concluded that because the crystalline lower basement is virtuallyplanar the surface structures result from thin-skinned tectonic packages that detach either internally, orupon the Lower Palaeozoic sequence.


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The purely extensional interpretation began to lose support in 2003 with the publication of a new set

of commercially available seismic lines across the Firth of Forth interpreted by Ritchie et al. . An exampleinterpreted line is shown in Figure 2.6. The group found evidence of oblique-slip and strike-slip faulting,leading them to propose an oblique compressive stress eld; shown in Figure 2.5. The displacement onthe Southern Uplands fault was found to be “no more than a few hundred kilometres, possibly muchless” by Owen and Clarkson (1992). These strike-slip faults may be reactivating an underlying suture(Coward , 1993). Also seen were large open synclines, such as the ‘Leven Syncline’ that were interpretedas growth structures ( Ritchie et al. , 2003).

Figure 2.4: A new model for stress regime that formed The Midland Valley, from interpreting seismic lines aroundthe Firth of Forth. HBF: Highland Boundary Fault. SUF: Southern Uplands Fault. LM: Lammermuir Fault.Modied from From Ritchie et al. (2003).

This ‘wrench basin’ model was not foreign to the Scottish Highlands, however, as one of the rst papersto recognise strike-slip motion from outcrop was published by William Kennedy in 1946 regarding theVariscan formation of the Great Glen Fault, less than 100 miles to the North:

Figure 2.5: A model for the formation of the Great Glen Fault. Modied from From Kennedy (1946).


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F i g u r e 2

. 6 : L o c a t i o n o f s e i s m i c l i n e i s s h o w n

i n F i g u r e 2 . 5 , n o t e t h a t i t i s n o t N - S a c r o s s T h e M i d l a n d V a l l e y b u t i s o r i e n t a t e d s u c h t h a t i t c r o s s e s r e l e v a n t

s t r a t i g r a p h y a n d s t r u c t u r e s .

H o r i z o n s a r e q u o t e d t h u s : 1 : s e a b e d ; 2 : r o c k h e a d ; 3 :

? i n t r a - U p p e r C o a l M e a s u r e s ; 4 : i n t r a - M i d d l e C o a l M e a s u r e s ; 5 :

i n t r a - L o w e r

C o a l M e a s u r e s ; 6 : t o p L o w e r L i m e s t o n e F o r m a t i o n ( t o p D i n a n t i a n ) ;

7 :

i n t r a - S t r a t h c l y d e G r o u p e q u i v a l e n t ; 8 :

? i n t r a - D e v o n i a n . A l l h o r i z o n s a r e s h o w

n i n

F i g u r e 2 . 1 0 o n p a g e 2 7

. F r o m R i t c h i e e t a l . ( 2

0 0 3 )

, a s t h e o r i g i n a l d a t a w e r e n o t a v a i l a b l e f o r t h i s p r o j e c t , t h e F i g u r e h a s b e e n l e f t i n t e r p r e t e d a s p r e s e n t e d i n t h e

o r i g i n a l p a p e r .


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The most recent paper to look at The Midland Valley is by Caldwell and Young (2013). After detailed

mapping in the Firth of Clyde, and compiling previous studies, they propose that the main sets of faultsin the area that can be linked by invoking two major tectonic events in the Carboniferous: back-arcsubsidence during proto-Tethys closure, then compression during the closure of the Ural Ocean. Theirmodel is shown in Figure 2.7 below:

Figure 2.7: Structural evolution of Scotland, with focus on The Midland Valley (shown in pale green). Blackarrows show plate movements, light arrows show the proposed movement of the Northern European Block (NEB).a: Dinantian: NE trending sinistral strike-slip. b: Silesian: dextral movement on NE-trending faults andsinistral on NW-trending faults. The strain ellipse is similar to that Ritchie et al. (2003) in Figure 2.5, butrotated anti-clockwise by 40 ° . c,d,e,f: Cross sections through a and b . Modied after Caldwell and Young(2013), originally modied after Coward (1993) and Leeder (1982).

The study thus explains the complexity of The Midland Valley with only two tectonic events. It may

be that the area appears more complicated due to the various overprinting stress regimes that haveaffected the British Isles, resulting in “a complex poly-phase tectonic history of extension, transtension,transpression and inversion” ( Macdonald and Fettes , 2007; Caldwell and Young , 2013).


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2.2 Ayrshire in the Carboniferous

Carboniferous rocks occupy much of The Midland Valley of Scotland, but are commonly obscured at thesurface by Quaternary deposits ( Waters et al. , 2012). During the Carboniferous, The Midland Valley laynear the equator in the closing Iapetus Ocean between the Gondwanan and Laurasian continental plates(Soper et al. , 1992); a wet equatorial climate, with swamps and deltas dominating the geography- idealfor the formation of coal ( Browne et al. , 1999).

The thickest and most complete coal deposits in Scotland are exposed in The Midland Valley, knownas “The Coal Measures”. These are units LCMS to UCMS in Figure 2.10. Cyclic uviodeltaics make upthe Lower Coal Measures, deposited on top of the braided river systems of the latest Passage Formation(PGP) members ( Read et al. , 2002).

Figure 2.8: Map of the distribution and source for Limestone Coal Formation. The structural highs of theHighlands and the Uplands stayed the same during deposition of the Coal Measures. From Read et al. (2002).


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The folding that affects the area is determined to have been caused by the Variscan orogeny, after the

deposition of the Coal Measures ( Coreld and Gawthorpe , 1996; Hooper , 2013). The SW-NE trendingsynclines in the Ayrshire coal elds are a later feature as they show no hinge thickening (Hooper ,2013). This is different to the eastern Midland Valley where compressional NNE-SSW folds such asthe Midlothian Leven Syncline have been shown to be active during the deposition of the passage Group(Underhill, Monaghan and Browne , 2008).

As the majority of the folding follows the overall trend of The Midland Valley, it may be due to theVariscan orogeny re-activating the underlying suture that allowed the Valley to form in the rst place.This movement also likely reactivated some of the faults, but this will be difficult to distinguish from anyinversion that occurred during formational transtension of the area (the idea that compression can occurin transtension was rst explored by De Paola et al. in 2005 and was applied to the Northumberlandbasin, less than 150 km to the East).


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2 . 2 . 1

S t r a t i g r a p h y a n d T e c t o n i c H i s t o r y -

L i t e r a t u r e C o m p i l a t i o n

F i g u r e 2 . 1

0 b e l o w s h o w s a c h a r t c o m p i l e d f r o m v a r i o u s s o u r c e s t o c o m p a r e d i ff e r e n t i n t e r p r e t a t i o n s a n d h i s t o r i e s o f T h e M i d l a n d V a l l e y :

F i g u r e 2 . 1 0 : C o m p i l a t i o n c h a r t r e l a t i n g s t a n d a r d g e o l o g i c a l a g e s t o t h e m a p u n i t s . T h e s e a r e t h e n r e l a t e d t o d i ff e r e n t i n t e r p r e t a t i o n s o f t h e p a l a e o g e o g r a p h y a n d

t e c t o n i c e n v i r o n m e n t . C o m p i l e d f r o m T h e B r i t i s h G e o l o g i c a l S u r v e y ( 1 9 8 0 )

, B r o w n e e t a l . (

1 9 9 9 ) , R i t c h i e e t a l . (

2 0 0 3 ) , R o h d e ( 2 0 0 5 )

, H e c k e l a n d C l a y t o n ( 2 0 0 6 )

,

W a t e r s e t a l . (

2 0 1 2 ) , W a l k e r e t a l . (

2 0 1 2 ) , a n d

C a l d w e l l a n d Y o u n g ( 2 0 1 3 )

.


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2 . 2 . 2

C o a l S e a m C o r r e l a t i o n i n E

a s t A y r s h i r e

C o r r e l a t i o n o f c o a l s e a m s b e t w e e n t h e v a r i o u s c o a l e l d s a c r o s s E a s y A y r s h i r e , t

h e M i d l a n d v a l l e y a n d e v e n S c o t l a n d a n d E n g l a n d h a s b e e n a t t e m p t e d

i n t h e p a s t .

A c o m p i l a t i o n o f t h e E a s t A y r s h i r e c o a l s e a m s i s s h o w n b e l o w i n F i g u r e 2 . 1 1

, p r o d u c e d f o r K i e r M i n i n g ( C r o n e , 2

0 0 0 )

.

F i g u r e 2 . 1 1 : A n u p d a t e d v e r s i o n o f t h e c h a r t o r i g i n a l l y m a d e b y J o h n C r o n e f r o m C r o u c h M i n i n g L t d i n 2 0 0 0 a n d u s e d

b y K i e r M i n i n g . T h e ‘ B C C o d e ’ r e f e r s t o

t h e B r i t i s h C o a l c o d e s t h a t a r e s u p p o s e d t o r e l a t e t h e v a r i o u s n a m i n g s c h e m e s a c r o s s E n g l a n d a n d W a l e s .

A c o l u m n d i s p l a y i n g t h e B G S c o a l s e a m m a p n a m e s h a s

b e e n a d d e d , a

n d t h e o v e r a l l g r a p h i c a l s t y l e i m p r o v e d .

T h e T a i t s M a r i n e B a n d i s “ t h e s e c o n d L i n g u l a b a n d b e n e a t h t h e P a t h h e a d C o a l ” ( C a m e r o n a n d S t e p h e n s o n ,

1 9 8 5 ) , a n d i s n o t r e c o g n i s e d a s w i d e l y a s t h e A e g i r a n u m M a r i n e B a n d w h i c h s t r e t c h e s e v e n a s f a r a s S o u t h D e r b y s h i r e ( W a t e r s a n d D a v i e s , 2 0 0 6 ) .


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2.3 Expected Structural Styles

2.3.1 Normal Faulting

As Section 2.1 explained, it was thought that The Midland Valley of Scotland could be modelled as asimple graben, as in Bott (1976). This would give The Midland Valley an overall shape such as that inFigure 2.12:

Figure 2.12: Model showing large-scale deformation of the Earth’s crust through extension and normal faulting.Upbending of the crust results from displacement in the lower layer. From Bott (1976).

Whilst this theory was later surpassed by a strike-slip interpretation ( Ritchie et al. , 2003; Caldwelland Young , 2013), there is still normal faulting within The Midland Valley. These will grossly followestablished normal fault geometries, such as in Figure 2.13:

Figure 2.13: Simple models for extensional faulting showing no 3D along-strike variation. From Fossen (2010).


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If a normal fault array is viewed in 3D, the linkage between the faults will produce recognisable

patterns such as relay ramps and linked faults. Figure 2.14 shows the evolution of normal faults in apurely extensional setting. This model may well apply to The Midland Valley where normal faulting iscertainly prevalent enough to initially be confused as the main proponent of basin formation ( Underhill,Monaghan and Browne , 2008).

Figure 2.14: Evolution of syn-rift extensional depocentres. From Cowie, Gupta and Dawers (2000).


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This normal faulting will also affect the rocks at a small-scale with, with the interaction between

different lithologies producing interesting results. The coal of the Coal Measures is deposited withincyclothems ( Browne et al. , 1999), dominated by mixtures of shale and sand. This difference in rockcompetency, when faulted, may create structures such as fault smear and fault drag, as seen in Figure2.15:

Figure 2.15: Diagram of a normal fault cutting a strong-weak-strong layering of rock beds. The weak layer isdeformed and incorporated into the fault plane. This allows some exure within the strong bed. From Fossen(2010).


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2 . 3 . 2

S t r i k e - S l i p F a u l t i n g

A s t h e s t r u c t u r a l d i s p l a c e m e n t w i t h i n T h e M i d l a n d V a l l e y i s b e l i e v e d t o h a v e a c o n s i d e r a b l e s t r i k e - s l i p c o m p o n e n t ,

t h e r e m a y b e a n y n u m b e r o f

s t r u c t u r e s a s s o c i a t e d w i t h s u c h s e t t i n g s i n t h e a r e a .

F i g u r e 2 . 1 6 d e m o n s t r a t e s a w i d e v a r i e t y o f s t r u c t u r e s t h a t c a n f o r m i n a s t r i k e - s l i p r e g i m e ,

i n c l u d i n g t r a n s t e n s i o n a l a n d t r a n s p r e s s i o n a l f e a t u r e s :

F i g u r e 2 . 1 6 : L a r g e 3 D b l o c k m o d e l M o d i e d f r o m ( C u n n i n g h a m a n d M a n n , 2 0 0 7 )

.


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F i g u r e

2 . 1 7 f r o m W u e t a l . (

2 0 0 9 ) d e m o n s t r a t e s w h a t c r o s s - s e c t i o n a l f e a t u r e s m a y b e e x p e c t e d i n a n e g a t i v e s t r i k e - s l i p o w e r s t r u c t u r e , s u c h a s t h e

o n e i n t h e b o t t o m - l e

f t o f F i g u r e 2 . 1 6 o n t h e p r e c e d i n g p a g e .

F i g u r e 2 . 1 7 : 4 D s a n d b o x m o d e l e v o l u t i o n o f a d e x t r a l s p i n d l e - s

h a p e d s t r i k e - s l i p t r a n s t e n s i o n a l o w e r s t r u c t u r e . B a s a l l a y e r i s 1 . 5 c m t h i c k ‘ S G M 3 6 ’ p o l y m e r w i t h

s i m i l a r s c a l e d p r o p e r t i e s t o t h e b r i t t l e - d u c t i l e t r a n s i t i o n z o n e .

O v e r l a y i n g t h i s i s 7 . 5 c m o f d r y q u a r t z s a n d .

M o d e l s c a l e s a t r o u g h l y 1 : 1 0 0 , 0 0 0 ( 1 c m = 1

k m ) .

F r o m W u

e t a l . (

2 0 0 9 )


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In 2012, Dooley and Schreurs modelled strike-slip faults in sandboxes with quartz sand to attempt to

create analogue models of such zones during brittle deformation. Figure 2.18 shows the result of one of the experiments. These models have a variable component of underlying basement fabric, with the threecolumns indicting different amount of stepover.

Figure 2.18: Line drawings of experimental sand box modelling. a) Underlapping stepover. b) Neutral stepover.c) Overlapping stepover. Numbers relate to “master fault segments”. From ( Dooley and Schreurs , 2012).

Modelling of strike-slip faults with an underlying fabric has also been done experimentally by Richardand Krantz (1991). However, in their paper the researchers looked at the inuence of reactivation of normal faults before strike-slip motion. Once again, sandbox modelling was used. Figure 2.19 shows twosandbox experiments: [ 1] with a dip-slip normal fault, and [ 2] with an un-activated fabric.


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Figure 2.19: Line drawings of dextral strike-slip sandbox models overlying a 90 ° dipping normal fault. [1]: A/B:purely brittle deformation. C/D: 5 cm of sand on 1 cm of silicone (acting as a décollement horizon). E/F: 4 cmof sand on 2 cm of silicone. [2]: A similar dextral system, however here the underlying fabric is unactivated.Compiled from Richard and Krantz (1991).

Models of faulting styles shows that a variety of fault types can result from a simple shear strike-slip,even including normal extensional faulting. These are present in Figure 2.16 within the “releasing bend”,“transtensional rift”, and “extensional horsetail splay”. Extensional faulting in strike-slip environments

was recently analysed by Waldron (2005), with the results shown in Figure 2.20 below. The diagramshows that as displacement increases on the main transfer fault, normal faults can be made to invert.

Figure 2.20: The effects of simple-shear on a strain ellipse. Demonstrates that normal faults can form obliqueto a dextral strike-slip zones, and if enough deformation occurs, can be made to invert. From ( Waldron , 2005),originally after Harding (1974).


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Many of the features associated with a strike-slip basin are summarised in Figure 2.21 below.

Figure 2.21: Diagram showing the structures that are associated with a pull-apart basin, such as in Figure 2.16.From ( Rodgers , 1980).


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2.3.3 Inter-Coal Faulting

In addition to the gross-scale faulting across The Midland Valley, East Ayrshire, and the area studied inthe project, there may be small-scale faulting within the units. This is not to say those larger featureswill not be visible- the fractal nature of geology ensures that most will be likely evident within even thesmallest scale data available. Examples of these structures are seen in Figure 2.22.

Nevertheless, within the coal beds, predicted structures will be associated with soft-rock deformationin the mudstone sections of the lithological layering, but if the coal is sufficiently metamorphosed it maydevelop more brittle fracturing.

Figure 2.22: Diagram showing some of the features that can be expected in a sheared coal seam. Illustrationof features in the Wu seam of the Pingdingshan coaleld, northern China. Whilst this coaleld does penetratemineable Carboniferous coal, the Wu seam is Permian in age. Crosses indicate primary axes of strain ellipses.Colour added to image from ( Li, 2001).


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3

Methodology

38


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3. METHODOLOGY 40

3 . 2 M

o d e l l i n g W o r k o w

I n o r d e r t o c o r r e c t l y b u i l d a 3 D m o d e l i n M i d l a n d V a l l e y P L C ’ s M o v e s o f t w a r e a w o r k o w w a s d e v i s e d . I t w a s m o d i e d d u r i n g p r o g r e s s i o n ( f o r i n s t a n c e

t h e r e - f o r m a t t i n g o f b o r e h o l e s w a s n o t e x p e c t e d i n i t i a l l y ) b u t d o e s d e m o n s t r a t e a l l t h e p r o c e s s e s n e e d e d t o b u i l d t h e 3 D m o d e l .

F i g u r e 3 . 1 : W o r k o w s u m m a r y . C o v e r s b o t h s m a l l - s c a l e ( u p p e r p a t h ) a n d l a r g e - s c a l e ( l o w e r p a t h ) d a t a s o u r c e s .


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3. METHODOLOGY 41

3.3 Modelling Workow- Large-Scale Data

The rst task was to audit all the available data, particularly the old models from Midland Valley. Oncecomplete, it was realised that additional scans of the BGS maps (particularly the cross-section shown inFigure 3.10) were needed at a higher resolution. These scanned maps (from 1980 in the West and 2000in the East) were then georeferenced against the downloaded data from EDiNA Digimap in ArcMap 10(geology polygons for lithological units and fault lines). The combined image was then draped onto aDEM of the area in Midland Valley’s Move software, allowing a 3D model to begin to take form. Animage of this model is shown in Figure 3.2:

Figure 3.2: A Coloured DEM of area. B Draped extent of the scanned BGS solid maps. Note the arrow in thebottom-right indicating North with the red point. ( The British Geological Survey , 1980, 2000)


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3. METHODOLOGY 42

The map extent chosen was based on the limits of data available, as shown in Figure 3.3.

Figure 3.3: Limits of map area within ArcMap 10, projected in the British National Grid co-ordinate system.

Onto this model could be projected the BGS fault lines from EDiNA Digimap, with their down-throwsides manually assigned. Also projected were dips traced from the scanned maps, and then dips inuncertain areas from structure contours.

For analysis purposes, the faults were coloured based on their dip direction, as shown in Figure 3.4.This was calculated by looking at the ticks shown on the map and by looking at unit offsets. The resultson the full area are shown in Figure 3.5.

These can also be displayed on separate layers, as in Figure 3.6.

Figure 3.4: Depending on which way the fault is dipping, a specic colour is assigned. This produces syntheticpairs of NE-SW trending and NW-SE faults, similar to the research by Caldwell and Young (2013) and Hooper(2013). It is also very similar to a method used by Wilson et al. (2006) when they looked at strike-slip systemson the Norwegian Margin.


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3. METHODOLOGY 44

F i g u r e 3 . 6 : A 3 D

o b l i q u e v i e w o f c o l o u r e d f a u l t s .

N B : g l o w e ff e c t i s a d d e d p o s t - p r o c e s s i n

P h o t o s h o p .


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3. METHODOLOGY 45

The model was then extrapolated into the third dimension in Midland Valley’s Move software. This

required knowledge of the geological history of the area, knowledge of the modelling software, and agood interpretation of the map. The approach also assumed that the British Geological Survey map wasaccurate, which is likely for the regional scale features on the map such as the faults and lithologies.There were some areas known to be inaccurate, highlighted by Kier during mining. These are shown inFigure 1.6 on page 17. Fortunately the local scale coal seam data was highly accurate and supersededthe BGS survey when working at a higher resolution.

Section 2.2 summarised that folding is determined to be later than the deposition of the Coal Measures,however due to software limitations and time constraints, planar faults were used in the model. Figure 4.10on page 65 shows that should not have affected the unfolded fault geometries too greatly. Below, Figure3.7 shows the result of the 3D extrapolation of the faults, with the caption describing the specics.

Figure 3.7: 3D Extrapolation of all faults. Different colours relate to dip direction. Red and Blue (‘major’) faultsdip at 80 ° to a depth of 800 m. Green and Yellow (‘minor’) faults dip at an angle of 70 ° to a depth of 600 m.Also shown are where faults are clipping through each other, and where they have been manually cut to join.

Note: A method for nding the fault dips from the map data was attempted in Midland Valley’s3DMove using the Ribbon Tool : a best-t plane is t through the interaction between the fault lineament

and the topography. This, however, proved fruitless as there was not enough difference in slope betweenthe topography and the intersecting fault lines, forcing the ribbon sticks to vary wildly in dip anddip-direction. The method is only useful in places with more extreme topography variation, as used in


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3. METHODOLOGY 46

the study mentioned in Figure 3.4 where Wilson et al. (2006) performed the method at the Lofoten Ridge

on the Norwegian Margin.

The lithological units were then added to the model. These were obtained in a digital format fromEDiNA Digimap, and were put into Move’s “Stratigraphy” tab after manipulation in Excel and ArcMapas 2D base-horizons. Dip data was then manually added from the map, using the plotted values andstructure contours in sparser areas, shown in Figure 3.8 on the next page .


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3. METHODOLOGY 47

F i g u r e 3 . 8 : D i p s d i s p l a y e d o n t h e m a p .

M a p s f r o m

T h e B r i t i s h G e o l o g i c a l S u r v e y ( 1 9 8 0 , 2 0 0 0 ) .


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3. METHODOLOGY 48

3.4 Modelling Workow- Small-Scale Data

The coal seam data from the companies shown on page 39 had to be converted into surfaces in Move .Equally, borehole data had to be converted using Microsoft Excel to a format that could be imported.The locations of the various small-scale datasets are shown in Figure 3.9 below:

Figure 3.9: Location map of coal seam data from ATH Resources (Glenmuckloch, Netherton, Skares); KierMining (Braehead, Greenburn, Grieve Hill, Rough Hill) and British Coal (Lanehead). BGS map is shown forscale.


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3. METHODOLOGY 50

This cross-section was traced in Move so it could be analysed. The result is shown in Figure 3.11, with

some signicant features labelled:

Figure 3.11: Traced section, with bases of UCMS, MCMS, LCMS and LSC shown. Vertical exaggeration has beenremoved. A: Thickness variation between the UCMS and the LCMS. B: No sense of reactivation here where Chas clear reactivation. D: Large (˜200) normal offset on this fault.


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3. METHODOLOGY 51

3.6 Completing the 3D Model

Having compiled all the data, and seen the rough structural shape of the area in Figure 3.11, a 3D modelcould be constructed. Following the initial aims, the study area was trimmed down to a 5x5 km square-shown in Figure 3.12 below. The area covers the parts of the BGS map known to be incorrect. Unitthicknesses were, however, rst taken from the few places on the map where this could be carried out formultiple units- shown in Figure 3.13.

These thicknesses could then be used to construct multiple cross-sections across the focus area, usingpredictable fault offsets and angles to extrapolate across the features. The small-scale coal seams werealso used, particularly Greenburn, Lanehead and Braehead. These lay directly on the BGS section,immediately providing useful dip data and horizons to ‘phantom’ against, as seen in Figure 3.9 onpage 48.

Figure 3.12: Maps from The British Geological Survey (1980, 2000) with the location of the 5x5 km study areashown bound by vertical surfaces.


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F i g u r e 3 . 1 3 : M o d i e d m a p s h o w i n g o n l y t h e k e y u n i t s f r o m t h e s t r a t i g r a p h i c c o m p i l a t i o n i n F i g u r e 2 . 1 0

. S e c t i o n s 0 1 a n d 0 2 a r e s h o w n c r o s s i n g t h r o u g h t h e u n i t s

o f i n t e r e s t ( t h e C o a l M e a s u r e s ) .

M o d i e d f r o m

T h e B r i t i s h G e o l o g i c a l S u r v e y ( 1 9 8 0 , 2 0 0 0 ) .


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3. METHODOLOGY 53

By covering the small study area with over 50 intersecting cross-sections, a 3D model could be built.

The results follow:Firstly, Figure 3.14 shows a 3D box that contains the 5x5 km study area:

Figure 3.14: 3D Model Bounds. 5000x5000x1100 m.

This box is shown cut away below, with the faults now visible.

Figure 3.15: 3D model of faults in the 5x5 km study area.

The rst unit to be modelled was the orange volcanic unit ‘ABFPV’- “Ayrshire basanitic and foiditicplugs and vents” ( The British Geological Survey , 2012). These are modelled as vertical, due to theextrusion method that Move uses to create surfaces from lines.


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Figure 3.16: 3D model showing the modelling of the volcanic intrusions that penetrate the 5x5 km study area.

The Passage Group Formation was used as the lowermost marker for the units. The base of the LowerCoal Measures was created in light grey above this, overlain by the Middle Coal Measures in dark grey.Lastly, the Upper Coal Measures were mapped for the southern area of the 5x5 km area. They are lost toerosion North of this, this there is no coal prospectivity. From the BGS map and cross-section, there isno evidence for packages thickening across the whole area. The coal seam data from the m

Wrench Tectonics in the Carboniferous Coals of the Midland Valley of Scotland

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