-Ì.ótg THE UNIVERSITY OF ADELAIDE Department of Geology and Geophysics South Australia Tertiary uptift and its Implications for the Tectonic Evolution of Sedimentary Basins, Offshore South-'West tlnited Kingdom. Robert John Menpes B.Sc. Hons (Adelaide 1992) Submitted in fulfilment of the requirement for the degree of Doctor of Philosophy September 1997 Contents Abstract vtr Acknowledgments .ix 1. Introduction I 1.1 Project Rationale.... I t.z The Celtic Sea/South-Western Approaches.. I 1.3 The Scope of this Thesis 2 2 . Geological History of the Celtic Sea/South-Western Approaches 5 2.1 Introdu ction 5 2.2 Structural Framework of the Celtic Sea/South-Western Approaches 6 2.2.1 Caledonian and Variscan structural trends.......... 2.2.2 Cornubian batholith 2.2.3 Mesozoic structural elements..... 2.3 Permo-Triassic Basin Formation 2.3.1 Permo-Triassic post-orogenic col1apse........................ 2.3.2 Mid- to Late Triassic rifting......... 2.4 Early-Middle Jurassic Thermal Subsidence.. 2.5 Late Jurassic-Early Cretaceous Rifting and the "Cimmerian" Unconformity 2.6 Late Cretaceous/Tertiary Thermal Subsidence.. 2.1 Tertiary Invelsion..... 2.1 .l Palaeocene exhumation 2.1.2 Eocene-Oligocene subsidence.......... 2.1.3 Oligo-Miocene exhumation.............. 2.8 Internal and External Basin Geometries............ 3. Quantifying Exhumation using Compaction Methodology.. 29 3.1 Introduction 29 3.2 Porosity in Sedimentary Rocks..... 29 3.2.1 Primary and secondary porosity. 29 3.2.2 Intergranular and intragranular porosity... 29 3.2.3 Absolute and effective porosity....... 30 Menpes Table of Contents 3.3 Compaction of Sedimentary Rocks...... .30 3.3. 1 Processes of porosity loss dependent on burial-depth............. .30 3 .3.2 Processes of porosity loss independent of burial-depth... ..... .31 3.3.3 E,xamples of compaction trends.... .31 3.4 Geophysical Logs and Porosity Determination................ .32 3.4.1 Porosity logs........... .32 3.4.2 Selection of porosity log for compaction analysis .34 3.4.3 Determining porosity from the sonic 1og..... .35 3.5 Basic Principles of the Compaction Methodology........... .35 3 .5 . I In troduction .35 3.5.2 Previous studies involving compaction-based quantification of exhumation... .36 3.5.3 The advantages of using multiple stratigraphic units in compaction analyses. .36 3.6 Quantifying Exhumation and Maximum Burial-Depth .37 3.6.1 Quantifying exhumation. .37 3.6.2 Exhumation, apparent exhumation, and maximum burial-depth............... .37 3.6.3 Uplift, erosion, and exhumation............. .39 3.1 Selection of Stlatigraphic Units for Analysis. .40 3.8 Interval Transit Time/Depth Plots and Normal Compaction Relations..... .42 3.8.1 Linear versus exponential normal compaction relations .42 3 .8.2 Normal compaction relations and reference wells .. ... .. .. .42 4. Tertiary Exhumation in the celtic Sea/south-western Approaches 49 4.1 In troduction 49 4.2 Comparison of Apparent Exhumation Results from the Eight Stratigraphic Units Analysed 49 4.2.1 Statistical testing of apparent exhumation results......... 49 4.2.2 Discussion of statistical results...... 54 4.3 Apparent Exhumation in the Celtic Sea/South-Western Approaches . .. .. .. s6 4.4 Exhumation at the Time of Denudation in the celtic sea/South-western Approaches........, 67 4.4.1 Timing of exhumation in the Celtic Sea/South-'Western Approaches.... 67 4.4.2 Magnitude of exhumation in the celtic Sea/south-western Approaches 67 4.5 Maximum B urial-Depth in the Celtic Sea/South-Western Approaches ................. 70 4.6 Comparison of Results with Related Published Studies 13 4.7 Implications of Exhumation Results for Models of Basin Evolution 13 5 Determining Apparent Exhumation from Chalk Outcrop Samples, Cleveland Basin/East Midlands Shelf.......... 71 5.1 Introduction. 77 Menpes lt Table of Contents 5.2 Previous Studies of Apparent Exhumation of the Cleveland BasinÆast Midlands Shelf.. 1l 5.3 Post-Variscan Geological History of the Cleveland BasinÆast Midlands Shelf ................ 79 5.4 Methodology for Quantifying Apparent Exhumation. 80 5.4.1 Selection of normal compaction relation....... 80 5.4.2 Calculation of mean porosities from well data........... 83 5.4.3 Determination of porosity of Chalk samples 83 5.5 Apparent Exhumation in the Cleveland BasinÆast Midlands Shelf.......... 85 5.6 Conclusions 91 6 Determining the Tectonic Subsidence/Uplift History of the Celtic Sea/South-Western 4pproaches................. 93 6.1 Introduction 6.2 Selection and Depth Conversion of SWAT Seismic Profiles....... 6.3 Basic Principles of Sediment Decompaction . .. ... .. .. 6.4 Decompaction of Exhumed Sequences 6.4.1 Exhumation with no later subsidence.............. 6.4.2 Exhumation of greater or equal magnitude to later subsidence 6.4.3 Exhumation of lesser magnitude than later subsidence... 6.4.4 Exhumation and sediment densities. 6.5 Porosity/Depth Relations for Sediment Decompaction............. 6.5.1 Interval transit-time/porosity relationships 6.5.2 Porosity/depth plots and relationships for decompaction 6.6 Backstripping and Decompaction of the Celtic Sea/South-Western Approaches Profile 6.7 Calculation of Tectonic Subsidence.. 6.1.1 Sedirnentloading 6.'1.2 Palaeobathymetry......... 6.7.3 Eustatic sea-level change 6.8 Calculation of Tectonic Uplift .....,... 6.9 Tectonic Subsidence/Uplift History of the Celtic Sea/South-Western Approaches.......... 7 . Tectonic Models of Basin Evolution ...1 15 1.1 Introduction 115 '7.2 Modelling the Subsidence/Uplift History of Sedimentary Basins in One, Two, and Three Dimensions ....1 15 7.3 Pure Shear Models of Lithospheric Deformation............ .....1 16 7.3.1 McKenzie's (1978) pure shear model.......... .....1 l6 7 .3.2 Two-layer pure shear heterogeneous extension ....1 l8 Menpes Iil Table of Contents 7.3.3 Incorporating multiple rifting events into two-layer, heterogeneous extension...................... 7.4 odling's (1992) Method for Modelling Multiple Lithospheric Deformarion Events 7 '4.1 Superimposing two lithospheric deformation events using Odling's (1992) method.... 7.5 Discussion of the Assumptions and Constraints used in Odling's (1992) Method in this Thesis 7 .5.1 Airy versus flexural isostasy 7.5.2 Pure shear versus simple shear ......... 7 .5.3 Instantaneous versus time-dependent extension.................. 7.5.4 Depth of decoupling between upper and lower lithospheric deformation..............,..... 8 Modelling the Tectonic Evolution of the Cettic Sea/ South-Western 4pproaches................. 121 8.1 Introduction 127 8.2 Results of the Two-layer Heterogeneous Model of Lithospheric Deformation ............. 127 8.2.1 An initial analysis of the tectonic evolution of the Celtic Sea./South-Western Approaches: balanced B factors..... 127 8.2.2 A revised analysis of the tectonic evolution of the Celtic Sea,/South-Western Approaches: required B factors..... 130 8.3 Discussion of the Results of the Two-layer Heterogeneous Modelling of Lithospheric Deformation 133 8.3. I Subcrustal deformation distributed from adjacent areas........... 135 8.3.2 Thermal anomalies.... 136 8.4 Comparison of Modelling Results with Other Studies in the Celtic Sea,/South-Western Approaches 131 8.5 Igneous Underplating as a Cause of Tertiary Exhumation 138 9 . Summary and Conclusions 141 9.1 Introduction ..t41 9.2 Tertiary Exhumation, Celtic Sea/South-Vy'estern Approaches ......... ..141 9.3 Tectonic Modelling, Celtic Sea/South-'Western Approaches.. ..143 9.4 Compaction Analysis, Cleveland Basin/East Midlands Shelf.... ..t44 9.5 Unresolved Issues and Recommendations for Future Research........ ..t44 Appendix I r41 Appendix II 153 Appendix III 155 Menpes lv Table of Contents References 157 Menpes Abstract Sonic velocities from the Danian Chalk, Upper two-layer, heterogeneous deformation model. Two- Cretaceous Chalk, Lower Cretaceous Greensand/Gault dimensional, multiple-event modelling of the tectonic Clay, Lower Cretaceous Vy'ealden, Upper Jurassic history has shown a distinct difference in the nature of mudstones, Triassic Mercia Mudstone (above and both crustal and subcrustal deformation between the below salt), and Triassic Sherwood Sandstone have internal basins closest to the Cornubian Platform (St. been used to quantify apparent exhumation (i.e. Mary's, Plymouth Bay, and South Celtic Sea basins), amount of missing section) in the Celtic Sea/South- and the external basins further away from the Western Approaches. The recognised structural Cornubian Platform (North Celtic Sea, Fastnet, inversion axes of the Brittany, South-West Channel, Brittany, and South-West Channel basins). The latter North Celtic Sea, Caernarvon, Cardigan Bay, and Kish basins are best modelled by localised crustal Bank basins are highs in apparent exhumation of up deformation, with minimal associated subcrustal to 2 kilometres. However apparent exhumation of deformation. In contrast, the St. Mary's, Plymouth approximately one kilometre also occurred in the Bay, and South Celtic Sea basins, as well as the uninverted St. Mary's and Plymouth Bay basins, and Cornubian Platform, are best modelled by significant on the margins of the Armorican and Cornubian subcrustal deformation, with little or no associated Platforms and the Pembroke Ridge. Hence Tertiary crustal deformation. The amount of crustal exhumation was regional, and not restricted to deformation in the external basins is not sufficient to inversion axes, and thus it must have had a thick- balance the excess subcrustal deformation in the skinned origin. internal basins and Cornubian