CHARACTERIZATION AND RESTORATION OF INVERTED FEATURES ACROSS THE SOUTHERN TARANAKI BASIN

by

Wesley Shayne Bucker A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of

Mines in partial fulillment of the requirements for the degree of Master of Science (Geology).

Golden, Colorado

Date:______

Signed:______Wesley Bucker

Signed:______Dr. Bruce Trudgill Thesis Advisor

Golden, Colorado

Date ______

Signed:______Dr. M. Stephen Enders Professor and Department Head

Department of Geology and Geological Engineering

ii ABSTRACT The Southern Taranaki Basin is a predominantly offshore basin located between the two

islands of New Zealand. The basin has a complex structural history due to its current location along an unconventionally shaped plate boundary and changes in a tectonic regime that have affected the basin over the last ~80 m.y. The focus of this study is the restoration and strain analysis of the Southern Taranaki Basin, which consists of the Central Graben and the Southern Inversion Zone. 2D and 3D seismic data sets are interpreted across the Southern Taranaki Basin using Schlumberger’s Petrel to create seven horizons that represent boundaries between signiicant structural events. The horizons and transecting fault surfaces are transferred to cross sections perpendicular to the major structures, which are each restored sequentially in 2D using Midland Valley’s Move. The restorations incorporate unfaulting, unfolding, and decompaction to measure the strain within each section and assess the overall distribution of strain within the

Southern Taranaki Basin. Cretaceous extension associated with rifting and break-up of Eastern Gondwana shows a range of 8.1% - 0.8% extension along the cross sections. Localized basin extension in the Eocene (tied to the initiation of spreading between the Paciic and Australian plates in the southwest) resulted in a 1.9 - 0% extension. During the Oligocene-Miocene, Paciic Plate subduction beneath the North Island began to the northeast, resulting in 6.1 - 0.1% shortening. Within the last 7 m.y., as shortening continued in the south of the Southern Taranaki Basin, back-arc extension migrated from the north, resulting in up to 1.8% extension in the northern sections, up to 0.5% shortening across the southern end of the study area, and no strain in sections in the middle of the study area. Across the Southern Taranaki Basin, the net change along the lengths of individual sections range between 7.8% shortening and 2.7% extension. This study shows that the latitudinal transition of strain over time across Southern Taranaki Basin

is not as uniform as previously assumed.

iii TABLE OF CONTENTS ABSTRACT ...... iii

LIST OF FIGURES ...... vii

LIST OF TABLES ...... xiv

ACKNOWLEDGEMENTS ...... xv

CHAPTER 1: INTRODUCTION ...... 1

1.1 Introduction ...... 1

CHAPTER 2: BACKGROUND GEOLOGY ...... 7

2.1 Basin Overview ...... 7

2.2 Early Basin History (>65 Ma)...... 8

2.3 Middle Basin History (65-30 Ma) ...... 10

2.4 Late Basin History (30 Ma - present) ...... 14

2.5 Petroleum Signiicance ...... 17

2.6 Brief History of Petroleum Interests and Early Seismic Surveys ...... 18

2.7 Petroleum System ...... 18

CHAPTER 3: DATA SELECTION ...... 20

3.1 Data ...... 20

3.2 Extent of Study Area ...... 22

3.3 Cross Section Placement ...... 24

CHAPTER 4: SEISMIC INTERPRETATION & MODELING ...... 25

4.1 Seismic Horizon Convention ...... 25

4.2 Interpretation Methodology ...... 29

iv 4.3 2D Model Generation ...... 41

4.4 Depth Conversion ...... 48

CHAPTER 5: RESTORATIONS ...... 53

5.1 Restoration Methodology ...... 53

5.2 Compaction and Isostasy Variables...... 55

5.3 Restorations ...... 57

5.4 Data Trend Analysis ...... 69

5.5 Decompaction Control Group ...... 78

CHAPTER 6: DISCUSSION ...... 81

6.1 Review ...... 81

6.2 Plate Margin Signiicance ...... 81

6.3 Petroleum Signiicance ...... 85

6.4 Resolution of Smaller Faults ...... 88

6.5 Signiicance of Folding in Strain Measurements ...... 88

6.6 Interpretation Discrepancies ...... 94

6.7 Further Compartmentalization of Study Packages ...... 95

6.8 Signiicance of Sub-Seismic Faulting ...... 97

6.9 Signiicance of Section Placement ...... 97

CHAPTER 7: CONCLUSIONS ...... 99

7.1 Summary ...... 99

7.2 Summary of Southern Taranaki Basin Strain ...... 100

7.3 Future Work ...... 101

v REFERENCES ...... 102

vi LIST OF FIGURES Figure 1.1: Map of New Zealand showing basins, rises, and plate boundaries relevant to this study. Base map is modiied from ArcGIS World Ocean Base. Alpine Fault location is simpliied from the New Zealand Active Faults Database available through GNS. Plate movement vectors are compared to a static Australian Plate and are taken from Douglas, et al., (2005)...... 1 Figure 1.2: The Taranaki Basin split into sub-basins. Labels show the Western Stable Platform and the Eastern Mobile Belt, split into the North Graben, Central Graben, Tarata Thrust Belt, and Southern Inversion Zone. Pink represents offshore volcanism. Modiied from King and Thrasher, 1996...... 2 Figure 1.3: Map of faults and wells within the Southern Taranaki Basin. The red polygon is the study boundary. Numbered dark blue lines are the locations of the restored cross sections. Light grey lines across the map show the location of available 2D seismic lines. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2 since Published papers show these faults without names, yet they are referenced in this study in some instances The six wells used to calibrate the depth conversion are illed with red. Modiied from Reilly et al. (2015) ...3 Figure 1.4: Petroleum ields across the entire Taranaki Basin, showing large faults and sediment Thickness. Reverse faults are shown with triangles instead of sticks for dip direction. The boundary for this study is shown in red. Modiied from MBIE, (2014) ...... 5 Figure 2.1: New Zealand 72 Ma. Edited from King & Thrasher (1996) ...... 9 Figure 2.2: Stratigraphic column for the Southern Taranaki Basin. The “Seismic Horizon” column shows the horizons that are interpreted and used to reconstruct the cross sections in this study. This column compares the age of the formations to the thickness of these formations commonly observed in seismic surveys within the STB. The columns on the right show the general depositional environment and structural regime over the life of the Taranaki Basin. Modiied from Reilly, et al. 2014...... 11 Figure 2.3: Active faults throughout the life of the basin. Coastlines shown are present- day. Black lines represent normal faulting, red lines represent reverse faulting. The dashed line represents the boundary between compressional and extensional strain in the basin, which migrated south over time. Taken from Reilly et al. (2015) ...... 13 Figure 2.4: Evolution of the proto-New Zealand island and subduction zone. For reference, the timing of the horizons shown in Figure 2.2 (described in Section 4.1) are, U_P50 ~30 Ma, N15 ~19 Ma, U_N50 ~7 Ma, and N82 ~2 Ma. Modiied from King and Thrasher, 1996...... 14 Figure 2.5: Producing oil and gas ields of the Southern Taranaki Basin and Taranaki Peninsula in relation to study cross sections. Field locations are from GNS’s New Zealand Exploration Map. Modiied from Reilly et al. (2015) ...... 17

vii Figure 3.1: Data prevalence compared to cross sections (dark blue) and study boundary (red). Available 2D lines shown in light grey, lines used to interpret surfaces in black. 3D volumes used in this study are shown as boxes. Pink areas are places within the project boundary where structures are not adequately imaged with present survey density. Major faults are included. Modiied from Reilly et al. (2015) ...... 20 Figure 3.2: 2D seismic line: BO_p116-81-23-2304, displaying the northern end of the Manaia Fault. This is a good quality seismic line that beneits from being realized and structurally smoothed. A: un-edited seismic line. B: line after basic amplitude range adjustments and structural smoothing...... 21 Figure 3.3: Edited print screen in 3D with 7.5x vertical exaggeration. This is the lower eastern boundary of the study area. The combination of the gap between surveys (the thin lines represent available seismic lines, different colors indicate different seismic surveys) and the Manaia Fault overthrusting basement (lower left end of the seismic lines shown) led to the decision to bound the survey on the thicker pink line at the top of the igure. Arrow points north...... 23 Figure 3.4: Detailed interpretation of the Taranaki Fault with emphasis on Paleogene units. Modiied from Strogen et al., (2014a) ...... 23 Figure 4.1: Stratigraphic column showing horizons used in this (far right column) and recent studies. Age dates are determined by biostratigraphic data in order to constrain the horizons across Zealandia, since many basins around New Zealand do not have the resolution of lithostratigraphy available in the Taranaki Basin. To compare horizons to their appropriate position in the basin lithology, refer to Figure 2.2. Modiied from Strogen & King, (2014). . . 26 Figure 4.2: A: Composite line across the basin showing interpreted horizons and faults. Major faults are labeled, (1) is the Manaia Fault, and (2) is the Taranaki Fault. B: Yellow line shows the composite line location across the basin ...... 27 Figure 4.3: N82 surface topography in time, with prominent fault offsets shown in black. Some offset occurs through this horizon and overlying package bounded by the sea loor, indicating modern slip along fault surfaces Cross sections shown as dotted black lines. Arrow points north...... 29 Figure 4.4: U_N50 surface topography in time, with prominent fault offsets shown in black. This is the best surface to see the fault geometry that creates the Central Graben. Cross sections shown as dotted black lines. Arrow points north...... 31 Figure 4.5: Seismic line along the Manaia hanging wall (location shown in yellow on right, study boundary in pink, seismic lines used in blue). U_N50 horizon is shown in light orange truncating N15 (dark orange) and P50 (blue) horizons. To the south of this section (outside of the study area), the U_N50 horizon directly overlies basement. K96 is green and P00 is shown in pink to be visible against the seismic, notice the bright relections that alternate around K96 are not observed in the surrounding horizons, indicating either sedimentary features or faults contained within the Cretaceous sediments. Interpretations are less accurate to the south because they are outside of the study boundary...... 32

viii Figure 4.6: N15 surface topography in time with prominent fault offsets shown in black. The Cape Egmont and surrounding faults within the Central Graben are normal faults, while the other labeled major faults are inverted faults. The horizon boundary that is restricted due to truncation by the U_N50 horizon in the hanging wall of the Manaia Fault is indicated by a dashed line. Study cross sections are shown as dotted black lines. Arrow points north...... 33 Figure 4.7: U_P50 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault, its splays, and the faults between the Maari Fault and the Cape Egmont Fault are normal faults, while the other labeled major faults, including the Whitiki Fault are inverted faults. The horizon boundary that is restricted due to truncation by the U_N50 horizon in the hanging wall of the Manaia Fault is indicated by a dashed line. Cross sections are shown as dotted black lines. Arrow points north...... 34 Figure 4.8: P00 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults as well as unnamed basement faults, while the other labeled major faults, including the Whitiki Fault are inverted faults. The horizon is truncation by the U_P50 horizon in the hanging wall of the Manaia Fault, indicated by a dashed line. The horizon onlaps the basement horizon any place where red basement is visible away from the edge of the study area. Cross sections are shown as dotted black lines. Arrow points north...... 35 Figure 4.9: K96 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults, as well as unnamed basement faults. The S.I.F.2 changes from being inverted in the south to normal displacement in the north around where the K96 horizon begins to contact the fault. Labeled major faults, including their splays (e.g. the splay at the southern end of the Motumate Fault) are inverted faults. The horizon is truncation by the U_P50 horizon in the hanging wall of the Manaia Fault, indicated by a dashed line. The horizon onlaps the basement horizon any place where red basement is visible away from the edge of the study area. Cross sections are shown as dotted black lines. Arrow points north...... 37 Figure 4.10: Basement Surface surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults, as well as unnamed basement faults. The S.I.F.2 changes from being inverted in the south to normal displacement in the north around where the western hanging wall of the fault becomes deeper than the eastern footwall. Labeled major faults, including the their splays (e.g. the splay at the southern end of the Motumate Fault) are inverted faults. Cross sections are shown as dotted black lines. Arrow points north...... 39 Figure 4.11: Surfaces in Petrel, Cropped along Section 2 in order to demonstrate what the 2D cross sections represent. This igure shows the surfaces in their respective colors and faults in white, with no vertical exaggeration. Section is 101.2 km long which coastline polygon shown in yellow near the top of the igure. Arrow points north and shows the perspective of camera...... 42

ix Figure 4.12: Surfaces in Petrel, cropped along section 2. This igure shows sections vertically exaggerated by 7.5 times. Faults are not shown, but displacement of a surface along a fault is shown by smooth surfaces. Section is 101.2 km long which coastline polygon shown in yellow near the top of the igure. Arrow points north and shows the perspective of camera...... 42 Figure 4.13: Section 1 cross section. Major faults and the Maui-2 well are labeled. The extent of the Maui-A gas ield and the rough extent of the Central Graben is shown above the section. The unnamed basement fault within the Central Graben mentioned in the text is labeled ‘A’...... 44 Figure 4.14: Section 2 cross section. Major faults and the two projected wells used for interpretation and depth correlation are labeled. The extent of the Maui-B and Kupe ields is shown above the section, with red representing the extent of gas in the ield and green representing the extent of oil in the ield. The rough extent of the Central Graben is also shown. The general undeformed area in the hanging wall of the Motumate Fault is referred to as the Opunake High The unnamed basement fault within the Central Graben mentioned in the text is labeled ‘B’...... 44 Figure 4.15: Section 3 cross section. Major faults and the Kea-1 and Tahi-1 wells are labeled, and the rough extent of the Central Graben is shown above the section. The general undeformed area in the hanging wall of the Motumate Fault is referred to as the Opunake High...... 47 Figure 4.16: Section 4 cross section. Major faults and the Maui-4 well are labeled. The lateral extent of the Maui-4 oil ield is shown above the section. Notice the folding caused by the slip of the fault east of the Maari Fault. S.I.F.2 stands for Southern Inverted Fault 2...... 47 Figure 4.17: Section 5 cross section. Major faults are labeled. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2...... 49 Figure 4.18: Section 6 cross section. Major faults are labeled. The Tasman-1 well is labeled, but it does not have public well picks. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2...... 49 Figure 4.19: Comparison between the packages used in this study and the Packages used in Hill & Milner, (2012) Time scale from Strogen and King, (2014)...... 51 Figure 5.1: Section 1 restoration. The western half of this section is in the Central Graben (Figure 1.2), characterized by late Miocene - present extension. The Cape Egmont Fault is an inverted fault that reactivated with this recent extension, though the amount of inversion is reduced due to the decompaction of sediments in the hanging wall (described in detail in Chapter 5.5). The Manaia Fault in the eastern end of the section is one of the early major thrust faults of the basin. Extension occurred into the Paleogene in this section in the sub-basin in the Manaia hanging wall (Figure 4.13). ....59

x Figure 5.2: Section 2 restoration. This cross section encounters the same major faults as Section 1, with the exception of the Whitiki Fault in the western end of the section. This section also cuts through the Central Graben (Figure 1.2) in the western end of the section, while the faults in the eastern end of the section remain inverted. Extension occurred into the Paleogene in this section in the sub-basin in the Manaia hanging wall (Figure 4.14)...... 61 Figure 5.3: Section 3 restoration. This section cuts though the lowest extent of the Central Graben (Figure 1.2) shown in the sections of this study. Neither the basement fault that is part of the Cape Egmont fault and the basement fault that is part of Manaia Fault show inversion (which is seen in the sections to the north and south respectively, though the recent extension is also above the Manaia Fault, indicating at least some amount of reactivation, even if the offset is not apparent in the older packages below the N15 Horizon. Extension occurred into the Paleogene in the Manaia hanging wall, but not to the same extent as Sections 1 & 2...... 64 Figure 5.4: Section 4 restoration. This cross section is completely within the Southern Inversion Zone, below the Central Graben (Figure 1.2). Here there is inverted offset of the Maari Fault, and the hanging wall of the Maari Fault is the rough western boundary of the late Miocene uplift in this end of the basin...... 67 Figure 5.5: Section 5 restoration. This cross section, similar to Section 3, is between major faults, and thus does not show any inverted offset in the packages above N15, just folding. Like Section 4, the Maari is to be the major boundary for uplift, but in this section, a small amount of uplift is above S.I.F.1 ...... 70 Figure 5.6: Section 6 restoration. In the southern end of the basin, this is the section with the largest amount of Paleocene extension, indicating that the Wakamarama Fault is another sub-basin active during that time. Also, the extend of late Miocene uplift is centered around the Wakamarama Fault rather than ending abruptly at any particular structure, as observed in the sections to the north. . . 72 Figure 5.7: Quick comparison of different measurements for each section. Section 1 is the furthest north. The total amount of shortening along the manaia fault for sections 1, 2, and 3, is 2.4 km, 2.0 km, and 3.2 km respectively. The total amount of extension along the fault is 5.5 km, 3.5 km, and, 3.8 km respectively...... 74 Figure 5.8: A: Visualization of the percent extension and shortening shown between restorations in Figure 5.1 - Figure 5.6 B: Same values as the to igure, but calculated to exclude any shortening or extension associated with the Manaia Fault, and with the addition of small scale fault displacements...... 75 Figure 5.9: These graphs show the relative change in section length over time. The left two graphs show the relationship based on each restoration performed, while the right show the same data, but with appropriate scale for time. The bottom two sections, show the results when the displacement of the Manaia Fault is not included in Sections 1-3 and when the minor faults are included in sections 1-4...... 78

xi Figure 5.10: Change in the strain ratio between each restoration in Section 2 in order to compare the difference between using and not using decompaction in structural reconstructions ...... 79 Figure 5.11: Change in length over time of Section 2 comparing the signiicance of incorporating decompaction in structural restorations...... 79 Figure 6.1: This igure shows the entire plate margin at present-day, with cross sections showing how the plate margin changes at varying latitudes. Cross section B is north of this study’s boundary and cross section C is south of this study’s boundary. Taken from Stagpoole & Nicol, (2008) ...... 82 Figure 6.2: Present-day crustal motion vectors above the Hikurangi plate margin and North Island of New Zealand. Each color represents a block in the model that contains vectors of similar character. The pole of rotation for each crustal block is shown with a rate of rotation in the same color as the block it represents. The Southern Taranaki Basin lies within the green block. Taken from Wallace et al., (2004) ...... 83 Figure 6.3: Schematic block rotation model with a southward migrating of the pole of rotation. This model is designed to explain the late Miocene-Recent structural evolution of the Northern Graben (Figure 1.2). “The eastern crustal block rotates clockwise relative to a ixed wester stable platform. The boundary between areas dominated by extension and shortening is indicated by the dashed lines.” Taken from Giba et al., (2010)...... 84 Figure 6.4: This map shows one example of published interpretations for the extent of Cretaceous sediments in the Taranaki Basin (the third color, which is not shown in legend means both formations are present). The boundary of this study and major faults of this study are projected onto the map. Modiied from (MBIE, 2015) ...... 87 Figure 6.5: Scaled-down surface maps of the K96 and P00 horizons for quick comparison to Figure 6.4. A: Figure 4.9, showing the K96 horizon which represents the extent of the Rakopi Fm. B: Figure 4.8, showing the P00 horizon which represent the extent of the North Cape Fm. In the hanging wall of the Manaia Fault the erosional surface is where the U_P50 unconformity truncates the P00 horizon, which represents the top of the North Cape Fm, yet the formation is still present below the unconformity even though no color is shown...... 87 Figure 6.6: This chart shows the amount of shortening or extension experienced across the basin through faulting. Each color represents the amount of strain for a particular time interval, with the red line representing the cumulative strain across the life of the basin accommodated through faulting, the B cross section is on the left border of the igure and the A cross section is the vertical red line near the center of the igure. Taken from Reilly et al., (2015)...... 89 Figure 6.7: These two cross sections follow seismic lines as shown in Figure 6.8. Section A is located around Section 1 & 2 from this study, and Section B is located south of Section 6 in this study the seismic. Taken from Reilly et al., (2015)...... 89

xii Figure 6.8: Location of igures referred to in Chapter 6. Fault placement and bathymetry taken from MBIE, (2014) ...... 90 Figure 6.9: Restoration across the Taranaki Basin extending far enough to the northwest to show the entire modern shelf of the basin. The restoration begins at the initiation of post-rift sedimentation, Taken from Roncaglia et al., (2010) .....91 Figure 6.10: True-scale model showing the development of the Taranaki Fault. For reference, in the STB, the U_P50 horizon represents an unconformity between ~34-30 Ma and the N15 horizon represents ~19 Ma. Taken from Tipathi & Kamp, 2008...... 92 Figure 6.11: Two interpretations of the Wakamarama Fault at 5x vertical exaggeration with the black arrow in each representing the thickness of sediment interpreted between basement and the U_N50 Horizon and. Strogen et al., (2014b) Transect 1 (A), this study (C), and uninterpreted seismic (B) for comparison. The difference between the two interpretations is mainly the footwall thickness of packages below P50 (blue). The brown horizon (base of brown polygon) shown on the left is the pink horizon on the right. Location of the seismic is shown in Figure 6.8 ...... 94 Figure 6.12: A) Transgression resulting in the deposition of shelf muds across the Taranaki basin before P50. B) Uplift and peneplanation across the basin results causes the P50 unconformity. C) Continued transgression combined with the renewal of basin subsidence in the footwall of the Taranaki Basin, likely due to an increase in plate convergence. D) Foredeep development causing subsidence adjacent to the major faults bounding the eastern edge of the basin, with rates of deposition that matched rates of subsidence. E) Maximum transgression, with submergence of the islands reducing clastic source resulting in carbonate development. F) Increase in fault displacement, particularly with the Manaia Fault, combined with regression resulting in marls, with the end of this sequence, the rapid accumulation of clastic material, marking the Top of the Taimana Formation which is the N15 horizon. Modiied from Strogen et al., (2014a) The shape of proto-New Zealand during this time can be seen in Figure 2.4 (b) & (c) ...... 96

xiii LIST OF TABLES Table 4.1: Table summarizing seismic horizons ...... 28

Table 4.2: Depth conversion values used for each section, divided by package. Values that don’t match at least one other section are shaded in grey...... 50

Table 5.1: Table showing inal decompaction values (Right), and the lithological values (bottom) and lithology ratios (Left), used to calculate the inal values ...... 55

Table 5.2: Strain measurement chart for Section 1. Items in red are measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 60

Table 5.3: Strain measurement chart for Section 2. Items in red were measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 62

Table 5.4: Strain measurement chart for Section 3. Items in red were measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 65

Table 5.5: Strain measurement chart for Section 4. Items in red were measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 68

Table 5.6: Strain measurement chart for Section 5 Items in red were measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 71

Table 5.7: Strain measurement chart for Section 6. Items in red were measured, but not included in calculations because the measurement performed is not realistic for that time. The Total change in line length is boxed in bold...... 73

xiv ACKNOWLEDGEMENTS

There are many people I wish to thank for their support throughout this project:

My parents for the quick syntax edits and the constant support

Bruce, for the volume of red ink he was willing to donate in the pursuit of improving my scientiic writing

My Comittee members Yvette Kuiper and Rick Sarg

Mrs. Burch who assisted me immensely through the Robert L Burch Scholarship

Mary Carr for her sane, chuckling, voice of reason

The Department of Geology and Geological Engineering, particularly Debbie and Cheryl

My fellow Orediggers, everyone in the Tectonostratigraphy Research Group, and particularly

Leo Mercado, Bulut Tortopoglu, Mehmet Hazar, Lilly Horne, Steve Schwarz , Anna Hissem,

Sarah King, & Cheryl Fountain who were willing to give me their second, third, and fourth opinions when I needed it

Lastly, but probably most importantly, I want to thank Mitch Weller for smoothing out my Move learning curve to something less than an overhanging ledge.

xv ChApTER 1 INTRODUCTION

1.1 Introduction

Zealandia, or the New Zealand continent, is presently a microcontinent (a large isolated fragment of continental crust that is not large enough to be considered its own continent), with the Taranaki Basin roughly in the center, dividing the two main islands (Figure 1.1). The Taranaki Basin is mainly offshore, mostly west of the North Island and between the two New

New Caledonia Basin

West Norfolk Ridge

Lord Howe Rise

Figure 1.2

Hikorangi Trough North Challenger Island 47 mm/yr Plateau Taranaki Basin

AUSTRALIAN Subcontinent Boundary 41 mm/yr PLATE 38 mm/yr Alpine Fault Chatham Rise

Tasman Basin 37 mm/yr South Island PACIFIC PLATE

Puysegur Margin

Campbell Plateau New Zealand 400 km N 200 mi

Figure 1.1: Map of New Zealand showing basins, rises, and plate boundaries relevant to this study. Base map is modified from ArcGIS World Ocean Base. Alpine Fault location is simplified from the New Zealand Active Faults Database available through GNS. Plate movement vectors are compared to a static Australian Plate and are taken from Douglas, et al., (2005).

1 Figure 1.3

Figure 1.2: The Taranaki Basin split into sub-basins. Labels show the Western Stable Plat- form and the Eastern Mobile Belt, split into the North Graben, Central Graben, Tarata Thrust Belt, and Southern Inversion Zone. Pink represents offshore volcanism. Modified from King and Thrasher, 1996.

2 Zealand islands, with the Taranaki Peninsula also a part of the basin (Figure 1.2). This study focuses on the northern extent of the Southern Inversion Zone and the Central Graben, which are offshore, directly south and southeast respectively, of the Taranaki Peninsula (Figure 1.2). This area is collectively referred to as the Southern Taranaki Basin (STB) (Reilly, 2014). The boundary of the study is shown in Figure 1.3 and covers an area of approximately 12,500 km2.

The purpose of this study is to perform a structural analysis and characterize strain across different latitudes within the Southern Taranaki Basin. Previous studies in the Southern Taranaki

AUSTRALIAN Taranaki Peninsula PLATE

Southern Taranaki Basin 1

2 Cape Egmont Fault

PACIFIC PLATE

Whitiki Fault 400 km 3 Opunake High

Manaia Fault

Rua Fault 4

5

Taranaki Fault Maari Fault

S.I.F.2 6 S.I.F.1 Motumate Fault

Kahurangi Fault N

Surville Fault Wakamarama Fault 50 km

Waimea-Flaxmore Fault System Well Reverse Fault Normal Fault

Figure 1.3: Map of faults and wells within the Southern Taranaki Basin. The red polygon is the study boundary. Numbered dark blue lines are the locations of the restored cross sec- tions. Light grey lines across the map show the location of available 2D seismic lines. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2 since Published papers show these faults without names, yet they are referenced in this study in some instances The six wells used to calibrate the depth conversion are filled with red. Modified from Reilly et al. (2015)

3 Basin with structural emphasis have included: Individual cross sections and restorations across either the northern or southern part of the Southern Taranaki Basin (e.g. Roncaglia et al., 2010); Detailed analysis of fault slip for the Taranaki Basin (Reilly et al., 2015); Plate margin-scale cross sections, with emphasis on the kinematics and history of the Taranaki Fault (e.g. Stagpoole & Nicole, 2008); Interpretations across the Taranaki Basin with descriptions of major structures and their role in the basin history (e.g. King & Thrasher, 1996; Strogen & King, 2014; Fohrmann et al., 2012).

This study measures the longitudinal change in strain from both faulting and folding for cross sections that intersect the major structures in the Southern Taranaki Basin. The cross sections represent a similar interval (10-20 km) to previous work that characterizes the strain across the Northern and Central Grabens (Figure 1.2) (Giba et al., 2010). The goal of the strain analysis is to identify latitudinal trends within the basin, and compare them to the regional basin history detailed in previous studies. At present, the Southern Taranaki Basin contains the transition from back-arc extension (North Graben and Central Graben shown in Figure 1.2) to compression associated with the transpressional Alpine Fault to the South (Figure 1.1), and this study visualizes and quantiies the extent of strain within this transition.

A secondary objective of the study is to identify possible trends in geometry relating to petroleum plays in the basin, particularly in the southern half of the study area (Figure 1.4). The southern half has been less studied due to dry exploratory wells and the absence of many lithological sequences present in other parts of the basin, due to basement highs until late Eocene and uplift in the late Miocene. Offshore wells in the Southern Taranaki Basin have been drilled on the major structures in the basin, and successful wells have resulted in further development of some structures, yet certain areas of the Southern Taranaki Basin, such as the southern half of this study area, have not produced any economic wells, discouraging further exploration. The timing and visualization of structures in undrilled sections of the basin, combined with independent interpretations of the seismic data, may introduce new petroleum leads within the syn-rift Cretaceous plays of the Southern Taranaki Basin. The time frame for the three main

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Maui ¹ Toru Rimu/Kauri

CENTRAL ¹

GRABEN ¹ ¹ ¹

Maari Kupe t

S°04

S°04 ¹

¹ Manaia

¹

¹

¹ ¹

aranaki Faul ¹

T

Study ¹

SOUTHERN INVERSION ZONE ¹

¹ ¹

Boundary ¹ ¹ ¹ Wanganui

Basin

200

¹

¹

¹ ¹ 050 km

¹ ¹

¹

¹

¹

¹

¹ ¹ 171°E 172°E 173°E

Legend Sediment Thickness

Oil fields Gas-condensate fields 0 - 2 km 2 - 4 km

Subsurface fault Basement outcrop 4 - 6 km 6 + km Figure 1.4: Petroleum fields across the entire Taranaki Basin, showing large faults and sedi- ment Thickness. Reverse faults are shown with triangles instead of sticks for dip direction. The boundary for this study is shown in red. Modified from MBIE, (2014)

5 phases of Taranaki structural development are as follows: rift-related extension from mid-late Cretaceous to Paleocene, development of a passive margin with subsidence from Eocene to early Oligocene, and an active margin basin from Oligocene to present (King & Thrasher, 1996).

6 ChApTER 2 BACKGROUND GEOLOGY

2.1 Basin Overview

This study aims to identify how the strain within the Southern Taranaki Basin changes during signiicant structural events, since the Southern Taranaki Basin represents a signiicant location during many of these events. During Cretaceous rifting, the Taranaki Basin was the Southernmost extent of New Caledonia Basin (Figure 2.1) (King & Thrasher, 1996). During initial compression caused by the initiation of subduction to the east, the Southern Taranaki Basin contained the southernmost extent of the Taranaki and Manaia Fault. Since the late Miocene, the STB has contained the boundary between northern extension and southern compression that has caused the diversity of sub-basin kinematics in the Eastern Mobile Belt (Figure 1.2).

The boundary polygon shown in Figure 1.3 shows the extent of this study, covering an area approximately 12,500 km2 in size. Basement lithologies within the basin vary, but consist of crystalline rocks originally intruded along the convergent Gondwana margin from the Permian to the early Cretaceous (Bradshaw, 1989). The oldest overlying sedimentary rocks in the basin are Cretaceous in age (e.g. King & Thrasher, 1996; Reilly et al., 2014; Strogen et al., 2014b).

The modern Taranaki Basin shown in Figure 1.2 can be divided into two sections, the Western Stable Platform and the Eastern Mobile Belt (King & Thrasher, 1996). Both of these areas experienced extension between the late Cretaceous and early Paleocene, but the Western Stable Platform has remained tectonically inactive since then (King & Thrasher, 1996). The Eastern Mobile Belt is so named because it has been affected by subsequent tectonism. The Eastern Mobile Belt can be divided into two different sectors. The northern sector includes the Northern Graben and the Central Graben, and the southern sector includes the Tarata Thrust Zone and the Southern Inversion Zone (Figure 1.2) (King & Thrasher, 1996). When the Paciic Plate began subducting beneath the North Island in the early Miocene, the Central Graben and the Southern Inversion Zone experienced shortening, and reverse slip developed along the preexisting normal fault planes (King & Thrasher, 1996). Within the last 15-10 Ma, extension

7 began in the north of the Taranaki Basin while shortening began in the south of the Taranaki Basin. The transition between the shortening and extension migrated southward over time as plate boundary geometry developed and slab roll-back initiated (King & Thrasher, 1996). Plate boundary development involved the southern migration, and later the clockwise rotation of the subduction zone (Giba et. al., 2010). In addition to slab rollback, mechanisms that could have contributed to these changes in plate boundary geometry include movement of a Rayleigh & Taylor mantle instability (Stern et al., 2006) or erosion of continental crust by mantle corner low (Reyners et al., 2007). As the extension in the north continued migrating south, reactivation of previously inverted faults occured in the Central Graben (King & Thrasher, 1996). The focus of this study is the Central Graben and the Southern Inversion Zone, which collectively has been referred to as the Southern Taranaki Basin (STB) (Reilly, 2014).

2.2 Early Basin history (>65 Ma)

Before the Cretaceous, what is now New Zealand was part of the continental margin of Gondwana. Until 110-105 Ma, the Gondwana margin was subject to a compressional regime due to subduction. This was followed by crustal extension that led to the separation of proto- New Zealand from Gondwana (e.g. Bradshaw, 1989; King & Thrasher, 1996; Laird, 2004). The Taranaki Basin was originally the furthest southern extent of the New Caledonia Basin (Figure 2.1) (King & Thrasher, 1996). From magnetic surveys, Uruski & Wood (1991) link the creation of the New Caledonia Basin with rifting before or during the late Cretaceous. The NE-SW trend of the Paciic-Australian plate boundary at that time is still visible in the orientation of the modern New Caledonia Basin (Uruski & Wood,1991) (Figure 1.1). After the separation from Gondwana, the proto-island of New Zealand was originally part of the Paciic Plate (Figure 2.1), compared to present day where the Paciic Plate is now subducting beneath the North Island (King & Thrasher, 1996). The oldest sediments found within the Taranaki Basin make up the Taniwha Formation (Figure 2.2). What is preserved of the Taniwha Formation is unconformable on the basement and lies below the Rakopi formation (Figure 2.2). The Taniwha Formation

8 Figure 2.1: New Zealand 72 Ma. Edited from King & Thrasher (1996) represents a passive margin sequence with sediments eroded from exposed basement rocks, uplifted 100-95 Ma, coinciding with continental extension in Antarctic and the spreading of the Antarctic-Australian plate boundary (King & Thrasher, 1996). The western dip of the Taniwha Formation with thicker intervals to the west can be explained by uplift of the underlying basement to the east, and could indicate that the Taranaki Fault irst activated in an extensional sense as early as 110 Ma (King & Thrasher, 1996).

Renewed extension of the New Caledonia Basin occurred synonymously with the opening of the Tasman Sea (80-55.5 Ma) (King & Thrasher, 1996). The southern termination of the New Caledonia Basin aligns with the left-lateral Taranaki Rift (Figure 2.1), evident through magnetic anomalies (Weissel & Hayes, 1977). The Taranaki Rift, as described by Thrasher (1992), initiated ~80 Ma as a left-lateral, obliquely extensional zone, which transferred movement from the New Caledonia Basin to the Tasman Sea spreading centre (Figure 2.1), terminating in or before the Taranaki Basin. The en echelon extensional basins associated with the Taranaki Rift underlie much of the Taranaki Basin (Thrasher, 1992). Unlike previous extension of the New Caledonia Basin associated with rifting, the extension of the basin 80 Ma merely accommodates different rates of the Tasman spreading centre, with the Lord Howe Rise

9 moving away from the spreading centre at a slower rate than the Campbell Plateau and the rest of the proto-New Zealand Island (Figure 2.1) (King & Thrasher, 1996). The formations deposited within the sub-basins created by the North-Northeast trending normal faults during this period of extension are the Rakopi and North Cape Formations (Figure 2.2). The Rakopi Formation is composed of terrestrial swamp and loodplain deposits and the North Cape Formation marks the initiation of a marine transgression, with shallow marine and coastal facies in addition to luvial deposits. Both formations onlap basement highs which remained as isolated islands and peninsulas. Both formations contain coal measures, but they are more common in the Rakopi Formation (King & Thrasher, 1996).

2.3 Middle Basin history (65-30 Ma)

After the cessation of rifting in the Tasman Sea, the Taranaki Basin became a passive margin, with deposition occurring within the bay formed between the Challenger Plateau (Figure 1.1) and the proto-New Zealand Island (King & Thrasher, 1996). Post-rifting lithostatic cooling and thermal subsidence continued until the late Eocene, resulting in a relative transgression across the basin (King & Thrasher, 1996). During the Paleocene, the Farewell, Kaimiro, Mangahewa, and McKee formations (Figure 2.2) made up the shoreface facies in the southeastern landward extent of passive margin sequences. These sequences grade laterally into iner grained marine mudstones to the NW. The mudstones of these sequences are lithologically indistinguishable, so they are grouped together as the Turi Formation. This formation ranges from early Paleocene to Eocene in the NW of the Taranaki Basin, but in the study area, only interingers the Kaimiro and Mangahewa Formations until the marine transgression in late Eocene which covered the entire basin, including remaining basement highs, with bathyal mudstones (Figure 2.2).

In the late Eocene, several asymmetric sub-basins formed in and around the Taranaki Basin developing coal measures, often with no break between Eocene and Oligocene deposition (King & Thrasher, 1996). The sense of fault throw on the faults bounding the sub-basins is

10 Figure 2.2: Stratigraphic column for the Southern Taranaki Basin. The “Seismic Horizon” column shows the horizons that are interpreted and used to reconstruct the cross sections in this study. This column compares the age of the formations to the thickness of these forma- tions commonly observed in seismic surveys within the STB. The columns on the right show the general depositional environment and structural regime over the life of the Taranaki Basin. Modified from Reilly, et al. 2014.

11 unclear due to subsequent displacement. King & Thrasher (1996) state that the initial movement of these faults is generally agreed to be normal, but could also be an initial stage of compression. There may also be evidence of normal slip along the Taranaki Fault, based on thickened Eocene strata near the fault (King & Thrasher, 1996). The extensional stress across the basin in the late Eocene-Oligocene are the result of Australian-Paciic Plate spreading that initiated ~45 Ma (Carter & Norris, 1976; King & Thrasher, 1996; King, 2000). The Eocene-Oligocene Taranaki sub-basins occurred to the north of a series of sub-basins that cut across the New Zealand subcontinent and eventually merged to become the modern plate boundary (King & Thrasher, 1996; King, 2000). The initiation of Australian and Paciic Plate spreading represents the irst phase in the development of the modern plate boundary of New Zealand (King, 2000).

Subsidence of the basin slowed near the end of the Eocene along with lithostatic cooling, resulting in a hiatus of sedimentation as peneplanation of the remaining land above sea level occurred (King & Thrasher, 1996). This resulted in an angular unconformity between the Eocene and Oligocene, with a larger gap in time at the southern end of the basin, likely due more to a lack of deposition as a result of less accommodation rather than weathering in much of the basin (King & Thrasher, 1996). By earliest Oligocene, relative transgression continued, resulting in the island’s submergence and a transition into a shallow marine environment. This environment had few sources of clastic input into the basin and no deposition apart from turbidites in the northern end of the basin, off the shelf (King & Thrasher, 1996).

Until 32 Ma, subsidence occurred at rates commonly found in passive margins, but in the Oligocene, subsidence accelerated (Stern & Davey, 1990), representing the transition from a passive margin to a compressive regime (King & Thrasher, 1996). The increase in subsidence began slowly, primarily adjacent to the Taranaki Fault (King & Thrasher, 1996), but the rate of regional subsidence accelerated, migrating from the north and affecting the Southern Taranaki Basin between 25 and 22 Ma (Stern and Davie, 1990), primarily through displacement (westward downthrow) along the Taranaki Fault (Figure 2.3 C) (King, 2000). The rapid subsidence rates within the basin can be attributed to a lexural foredeep trough that migrated ahead of a

12 Figure 2.3: Active faults throughout the life of the basin. Coastlines shown are present-day. Black lines represent normal faulting, red lines represent reverse faulting. The dashed line represents the boundary between compressional and extensional strain in the basin, which migrated south over time. Taken from Reilly et al. (2015) westward moving thrust front east of the Taranaki Basin (Stern and Davie, 1990), leaving all but the southern extent (which only subsided down to shelf depths) of the basin at bathyal depths (King & Thrasher, 1996). This subsidence resulted in much of proto-New Zealand becoming submerged and sediment starved, reaching maximum inundation around 23 Ma, allowing for the generation and accumulation of carbonate facies (Figure 2.2) between Oligocene and earliest Miocene (e. g. Nelson, 1994, King & Thrasher, 1996). On the tectonic scale, the late Oligocene represents a time of proto-plate boundary development (Nelson, 1994), with the dextral relationship between the Paciic and Australian Plates newly initiated after a southern migration of the pole of rotation (King, 2000). Compression and overthrusting were present, but only mildly so (Nelson, 1994). The southward migration of subsidence that occurred through

13 the Oligocene correlates with the southward migration of the developing subduction zone. The subduction zone reached the developing plate boundary through proto-New Zealand by the earliest Miocene (King, 2000).

2.4 Late Basin history (30 Ma - present)

By Neogene time, subduction of the Paciic Plate was located east of the Taranaki Basin and had a more southerly-dipping orientation than it has at present (Figure 2.4) (King and Thrasher, 1996). The early Miocene is characterized by an inlux of terrigenous material and the cessation of limestone accumulation, caused by active basement overthrusting associated with the development of the modern plate boundary and the further development and localization of the modern Alpine Fault (Figure 1.1) (King and Thrasher, 1996; Nelson, 1994). The irst major overthrust in the early Miocene was the Taranaki Fault, on the eastern edge of the basin (King &

Figure 2.4: Evolution of the proto-New Zealand island and subduction zone. For reference, the timing of the horizons shown in Figure 2.2 (described in Section 4.1) are, U_P50 ~30 Ma, N15 ~19 Ma, U_N50 ~7 Ma, and N82 ~2 Ma. Modified from King and Thrasher, 1996.

14 Thrasher, 1996). The over-thrust character of the fault began to develop in the mid-Oligocene ~29 Ma (Tripathi and Kamp, 2008), The earliest compressive motion recorded along the fault is approximately the Waitakian-Otaian (early Miocene) boundary (~22-21 Ma) (King and Thrasher, 1996). This provided a source of clastic sediments from the east of the fault as well as renewing subsidence to the west of the fault, over-encumbering the carbonates with clastic debris. This transition from carbonate to clastic sediments deines the Taimana Marl (Figure 2.2), representing the irst inlux of clastic sediment. The abrupt contact of the Taimana Formation with the overlying Manganui Mudstone (Figure 2.2) in the southeast of the basin represents a rapid increase in the volume of terrigenous sediments between 21-22 Ma. This increase in sedimentation is tied with an increase in the rate of overthrusting (King and Thrasher, 1996). The age of Taimana and Manganui vary with location, with later and more gradational facies boundaries further basinward (King and Thrasher, 1996). The subsidence of the basin in the Miocene is likely a result of lithostatic loading, and only occurred east of the modern boundary between the Western Stable Platform and the Eastern Mobile Belt (Figure 1.2) (King and Thrasher, 1996).

Inversion of faults within the basin initiated in the early Miocene (King and Thrasher, 1996) and continued into the Pliocene (Figure 2.3 E) (King and Thrasher, 1996). Between 15 and 10 Ma the southern extent of the Hikurangi Subduction Zone (Figure 1.1) reached its present position (e.g. Walcott, 1984, King.& Thrasher, 1996). This migration coincided with the change from shortening to extension in northern Taranaki Basin and renewed shortening in the south of the basin (Southern Inversion Zone) between 13-11 Ma (King & Thrasher, 1996). Though the stresses within the basin are overall rotational, there is no evidence of wrenching in individual structures (King and Thrasher, 1996), implying dip-slip motion along faults, rather than oblique- slip, possibly due to the character of the reactivated normal faults and basement fabrics (King and Thrasher, 1996). The extension in the north initiated the formation of the Northern Graben and Mohakatino volcanism (Figure 2.2), both of which migrated south over time with the trend of extension (Figure 2.3) (King and Thrasher, 1996). As the subduction zone continued to

15 rotate into its present orientation and approach the basin, uplifting and inversion in the Southern Inversion Zone continued to migrate southward as well (King and Thrasher, 1996). This compression is the cause for the late-Miocene unconformity throughout much of the Southern Taranaki Basin. Shortening ceased in the north and northeastern areas of the Southern Inversion Zone by the late Miocene but continued in the southern extent of the basin into the Pliocene (Figure 2.3 F) (King and Thrasher, 1996).

The renewal of clastic sedimentation, associated with overthrusting in the Miocene, corresponds with the trend of regression that is still present today. Initially, subsidence rates exceeded sedimentation rates (King & Thrasher, 1996), but when subsidence rates slowed, the basin became overilled, and progradation of the shelf occurred rapidly. The relative regression of sea level in the region was accentuated drastically by the rapid rates of sedimentation (Holt and Stern, 1994; King & Thrasher,1996), with the current shelf actively prograding over the Western Stable Platform. The southern end of the basin, which was still experiencing compression in the Pliocene (King & Thrasher, 1996) was more elevated, resulting in less deposition. Because of this, where there were topographic highs, the southern end of the basin is missing Pliocene and Pleistocene sequences present in other parts of the basin. In the northwest of the basin, the Pliocene-Recent sediments can make up more than half of the stratigraphic thickness (Strogen et al., 2014). At present, the environment is considered a passive margin, but with signiicantly higher deposition rates than the sediment starved passive margin of the Paleogene (King & Thrasher, 1996).

Volcanism took place as early as 18-19 Ma, but primarily occurred from 14 Ma to present, moving consistently south with the extension of the North Graben (Figure 1.2) (King & Thrasher, 1996). The most recent volcano is presently Mt. Taranaki, which makes up the Taranaki Peninsula (Figure 1.2).

When exactly New Zealand became the two islands that we now recognize is complicated. Different studies (Carter & Norris, 1976; Weissel et al. 1977; Walcott, 1984; King,

16 2000) have modeled the evolution of New Zealand and the tectonic plate boundary to varying degrees. However, due to the number of variables, including varying ocean depths, ductile deformation, and volcanism, the most that models can accurately predict with any accuracy is how the present crustal blocks of New Zealand have changed with relation to one another.

2.5 Petroleum Signiicance The Taranaki Basin is the only basin in New Zealand that is currently producing oil or gas commercially (MBIE, 2014) and of the producing ields, the majority of production is from a few very large ields. At present, the basin’s reserves are distributed between 19 ields, with 56% of remaining oil and condensate reserves in the Maari and Pohokura ields (Figure 2.5) and 38% of remaining gas reserves in Pohokura ield (gas production in the Maari Field is not reported) (MBIE, 2014). However, due to the lack of data compared to other ields around the world, it is

Pohokura AUSTRALIAN PLATE

Southern Moturoa Taranaki Basin

PACIFIC PLATE Pateke 400 km

Amokura Tui Kapuni 1

2 Maui A Toru Maui B 3 Kupe

4 Maari

5 Manaia 50 km

Wells in study 6 Oil Field N Gas Field

Figure 2.5: Producing oil and gas fields of the Southern Taranaki Basin and Taranaki Penin- sula in relation to study cross sections. Field locations are from GNS’s New Zealand Explora- tion Map. Modified from Reilly et al. (2015)

17 hard to predict the remaining reserves within the basin with accuracy.

All of the large anticlinal features within the basin have been drilled (Figure 1.4), and it is very unlikely that any large ields remain, but given the size of the moderate and small ields, it is very likely that there are still smaller ields to be discovered. More small-scale ields have been discovered onshore than offshore, but this is likely do to a hesitancy to drill anything but the largest off-shore structures. (King & Thrasher, 1996)

2.6 Brief history of petroleum Interests and Early Seismic Surveys

The irst well in the Taranaki Basin was drilled in 1865, with the irst oil boom in the area beginning after a successful onshore well in 1906 that produced from the now-depleted Moturoa ield (Figure 2.5) (King & Thrasher, 1996). The 1937 Petroleum Act designated all oil and gas-bearing subsurface as Crown property, encouraging foreign investors, but the next major ield (the Kapuni Field) was not discovered until 1959 (King & Thrasher, 1996). The well that found the ield was funded by the Shell, BP, and Todd consortium and was the irst well in the basin to be drilled using seismic data. The irst offshore seismic surveys were conducted in the 1960’s with the irst offshore wells drilled in 1968 (King & Thrasher, 1996). The Maui Field was discovered in the third offshore well. Throughout the next thirty years, different play concepts continued to be tested, the youngest being Pliocene and the oldest Paleocene in age. The largest increase in new wells occurred in the mid-80’s after several large seismic surveys were acquired, but by 1982 the largest discoveries had already been found and few subsequent discoveries have signiicantly added to cumulative reserves (King & Thrasher, 1996). Between 1966, when the irst survey was conducted, and 2014, there have been over 180 seismic surveys in offshore Taranaki Basin. The irst 3D survey was collected in 1987, with a total of 29 3D surveys by 2013 (NZPAM).

2.7 petroleum System

Hydrocarbons have been produced out of all chronostratigraphic levels in the basin except for the Cretaceous, but even Cretaceous formations are considered potential reservoirs

18 due to their high porosities (e.g. King & Thrasher, 1996). In general, Paleogene reservoirs contain gas, and Neogene reservoirs contain oil accumulations, with little to no gas found above 2 km below sea level (King & Thrasher, 1996). Generation of hydrocarbons in the basin occurs at depths below 4 km (King & Thrasher, 1996), but due to compaction in the depths at which hydrocarbons occur, the source rocks are rarely good reservoirs, unless secondary porosity is present (e.g. King and Thrasher, 1996).

19 ChApTER 3 DATA SELECTION

3.1 Data

The data used are publicly available through New Zealand Petroleum & Minerals and include four 3D seismic volumes, 233 2D seismic lines, and 25 wells with varying resolutions of biostratigraphic and lithostratigraphic data. Published interpretations used for reference in this study include Fohrmann, (2012), regional transects from Strogen et al., (2014b), and well reports available through New Zealand Petroleum and Minerals (NZPAM) and GNS Science. The data were viewed and interpreted using Schlumberger’s Petrel (2015). Figure 3.1 shows all interpreted seismic lines and 3D seismic data sets, as well as the location of all available 2D seismic lines in the Southern Taranaki Basin. A 3D seismic data set is a cube or volume of closely spaced seismic data. The average spacing within 3D volumes in this study is 23 meters,

Taranaki Peninsula Opunake 3D AUSTRALIAN PLATE Maui 3D Southern Taranaki Basin 1

Taranaki Fault

Manaia Fault 2

PACIFIC PLATE Cape Egmont Fault

400 km 3

4 Maari 3D

5 Kerry 3D

6

N 50 km Figure 3.1: Data prevalence compared to cross sections (dark blue) and study boundary (red). Available 2D lines shown in light grey, lines used to interpret surfaces in black. 3D volumes used in this study are shown as boxes. Pink areas are places within the project boundary where structures are not adequately imaged with present survey density. Major faults are included. Modified from Reilly et al. (2015)

20 while in this study, the closest spacing of 2D lines in a survey is 600 m, and more common spacings are between 2-6 km.

Before any regional interpretations were made, each 2D seismic line used was quality controlled and balanced vertically when necessary, using wells and 3D volumes as initial correlation points. To quality control a poorer quality 2D line, a combination of realizing the original data (in SEG-Y ile format) and structural smoothing was typically required. Realizing a line involves saving a new line and editing its standard amplitude range, which is needed when the range is dramatically skewed from the rest of the data. Structural smoothing is a process available through Petrel that applies Gaussian smoothing to seismic relections parallel to the principal structural orientation (Figure 3.2), which assists particularly with deeper, more chaotic relections. Only a handful of surveys required any balancing, but those that did were compared to either 3D volumes, or intersecting 2D lines that had already been proven to line up with the 3D data. A B

Figure 3.2: 2D seismic line: BO_p116-81-23-2304, displaying the northern end of the Ma- naia Fault. This is a good quality seismic line that benefits from being realized and structur- ally smoothed. A: un-edited seismic line. B: line after basic amplitude range adjustments and structural smoothing.

21 3.2 Extent of Study Area The project boundary was determined after interpretation based on the criteria of seismic data availability and structural interest. Many interpretations for individual horizons continued past the designated polygon (Figure 3.1), particularly those with high conidence in Table 4.1, but surfaces extending past the polygon were cropped in order to have the full sequence available throughout the study area. Within the boundary there are still areas with a low density of 2D surveys. These are shown in Figure 3.1 in pink, and are by far the least conidently interpreted because they involve complicated geometries that are not imaged in enough detail.

The western boundary of the study is over the Western Stable Platform. While there are rift geometries present in the basement, the relatively recent changes within the Eastern Mobile Belt are the main focus of this study, so the surfaces continue past the Western Stable Platform Boundary, but stop before the modern shelf edge, since that area is structurally unaltered.

The eastern boundaries of the study were chosen speciically to be the footwall of the overthrusted basement faults bounding the eastern edge of the Taranaki Basin. Just to the south of the Taranaki Peninsula, this bounding fault is the Taranaki Fault (Figure 3.1). In the southern end of this study, the bounding fault is the Manaia Fault (Figure 3.1). The boundary is positioned either where the youngest horizon encounters the fault, or in the case of the more southern sections, where seismic resolution of the lower horizons is inconsistent below the fault shadow of the overthrusted basement. The boundary is switched from the Taranaki Fault to the Manaia Fault when the seismic surveys started spreading and the quality of data lowers (Figure 3.1 & Figure 3.3). A detailed interpretation of the Taranaki Fault just north of the boundary of this study can be seen in Figure 3.4 from Strogen et al., (2014a). Further reason for this boundary is in regards to the restorations. These faults experienced extensive overthrusting, resulting in hanging wall basement to be directly overlain by late Miocene sediments. In order to avoid overreaching assumptions, these fault displacements were chosen to not be restored. Tripathi and Kamp, (2008) discusses the timing and geometries of the Taranaki Fault both in more detail than the scale of this study could replicate. The northeastern end of the study is limited by the

22 Manaia Fault

Overthrusted Basement Horizon

Basement Horizon Manaia Fault

Motumate Fault Splay

Study Boundary

Figure 3.3: Edited print screen in 3D with 7.5x vertical exaggeration. This is the lower east- ern boundary of the study area. The combination of the gap between surveys (the thin lines represent available seismic lines, different colors indicate different seismic surveys) and the Manaia Fault overthrusting basement (lower left end of the seismic lines shown) led to the de- cision to bound the survey on the thicker pink line at the top of the figure. Arrow points north.

Figure 3.4: Detailed interpretation of the Taranaki Fault with emphasis on Paleogene units. Modified from Strogen et al., (2014a)

23 coastline and the lack of seismic data, and the northwestern extent stops with the Maui and Opunake 3D volumes where structures are either non-compressional or onshore with poorer seismic resolution. The southern and southwestern boundary for the data is due primarily to a dramatic drop in seismic survey quality combined with a larger spacing between surveys.

3.3 Cross Section placement

The section locations are shown in Figure 3.1. Section 1 is positioned at the northernmost extent of the Manaia Fault that can be interpreted conidently all the way to the basement. It intersects the Cape Egmont Fault just below the where it splays in the northern end of the study area. Section 2 is positioned to intersect the Whitiki Fault in the footwall of the Cape Egmont Fault, and the Rua Fault in the hanging wall of the Manaia Fault (Figure 1.3) while also passing through two well-dense areas at the same time. Section 4 is positioned in order to intersect the location of maximum displacement along the Maari Fault. Section 6 is positioned at a diagnostic position in relation to the Wakamarama Fault. Section 3 passes through the lowest conidently interpreted extent of the Manaia Fault, while also passing directly below the Tahi- 1 well. On the western side of the section, it passes through the transition from the North Cape to the Maari Fault. Section 5 is positioned between sections 4 and 6 in order to show the area between the Wakamarama and the Maari Fault.

24 ChApTER 4 SEISMIC INTERpRETATION & MODELING

4.1 Seismic horizon Convention

In studies and professional reports in the Taranaki Basin and across New Zealand, there have been a large number of naming conventions. Seismic horizon names that were chosen for a speciic purpose were usually determined with little correlation between one another, particularly from one basin to the next. The naming and color conventions for seismic horizons used in this study are based on Strogen & King (2014) (Figure 4.1), which attempts to create a standardized naming scheme, based on the age of the units, that can be used in basins across Zealandia. The age of a unit is determined through biostratigraphy, though, in the Taranaki Basin, these horizons often represent important lithologic contacts as well. Horizon names are designated based on the age of the horizon, and within each age, speciic numbers are designated, e.g. P00 is the lowest Paleogene horizon and N50 is a horizon roughly in the middle of the Neogene across New Zealand. The goal of using horizons instead of designated lithostratigraphic boundaries is to have chronostratigraphic surfaces that are universally named throughout the basins of New Zealand. When a horizon is represented by an unconformity across a large majority of the Southern Taranaki Basin, then it is designated with a U in this study, e.g. U_P50. Not including the seabed, 7 horizons are interpreted in this area of the Southern Taranaki Basin (Figure 4.2). The horizons chosen for this study (Figure 4.2 & Table 4.1) are chosen as diagnostic boundaries between packages of sediment that represent key structural events in the basin. The timing of structures and sequences within the chosen packages could be narrowed by subdividing each sequence further, yet there is a point of diminishing returns since these signiicant boundaries are relatively simple to interpret across the basin, yet lesser deined horizons have less well control and can be more dificult to distinguish in seismic data. Even with these primary horizons, some are much more dificult to distinguish than others, and well ties are only highly detailed for the formations with hydrocarbon potential.

25 Figure 4.1: Stratigraphic column showing horizons used in this (far right column) and recent studies. Age dates are determined by biostratigraphic data in order to constrain the hori- zons across Zealandia, since many basins around New Zealand do not have the resolution of lithostratigraphy available in the Taranaki Basin. To compare horizons to their appropriate position in the basin lithology, refer to Figure 2.2. Modified from Strogen & King, (2014).

26 (2) (1)

Maari Fault Motumate Fault 7.5x Vertical Exaggeration A B

Figure 4.2: A: Composite line across the basin showing interpreted horizons and faults. Major faults are labeled, (1) is the Manaia Fault, and (2) is the Taranaki Fault. B: Yellow line shows the composite line location across the basin

27 Table 4.1: Table summarizing seismic horizons * Each well has varying resolution of bio-samples used for age dating ** These ages are recorded, but the resolution of the data at this depth is often too low to be useful *** Wells are often not deep enough to encounter this boundary

28 4.2 Interpretation Methodology

The majority of seismic surveys used in this study are SEG reverse, meaning increases in acoustic impedance are troughs and decreases in acoustic impedance are peaks. In igures showing seismic, peaks are shown in red or black, and troughs are shown in blue or white. The horizons interpreted in this study were identiied through a combination of bio and lithostratigraphic well ties, with reference to published interpretations. Table 4.1 shows these horizons and their designated colors, as well as the common phase, a quick summary of the geological signiicance, and the quality of the relections associated with each respective horizon.

Taranaki Peninsula

Cape Egmont Fault

600 ms

200 ms

400 ms

0 50 km

Contour interval 200 milliseconds Figure 4.3: N82 surface topography in time, with prominent fault offsets shown in black. Some offset occurs through this horizon and overlying package bounded by the sea floor, indicating modern slip along fault surfaces Cross sections shown as dotted black lines. Arrow points north.

29 4.2.1 U_N82 horizon - Figure 4.3

The U_N82 horizon (Table 4.1) represents a mid-Pleistocene angular unconformity across much of the Southern Taranaki Basin. The horizon represents top Whenuakura Formation (Figure 2.2), which is incised by recently deposited, shelf-dominated, cyclothemic sediments (Fohrmann et al., 2012). This horizon is the shallowest in this study and has no well control, so picks are based solely on relection geometry. For most of the basin, this horizon is picked on a peak, though around the Kerry 3D volume (Figure 3.1), the horizon is picked as a trough, which delineates the unconformity in that area. While the horizon is unconformable in the east side of the basin, it is conformable to the west as a boundary within the foresets on the passive margin shelf. Due to the multiple incisions deining the unconformities of this horizon and its overlying sediment package, different unconformities are more apparent in seismic lines of different orientations, however the scale of the discrepancies is minor. With no well control, the Strogen et al., (2014b) transects are the primary control, and even those do not tie perfectly between one another.

4.2.2 U_N50 horizon - Figure 4.4

The U_N50 horizon (Table 4.1) represents a major late-Miocene angular unconformity in the Southern Taranaki Basin (Figure 2.2). The unconformity is associated with the ~6 Ma rotation of the Hikurangi Trough (Figure 1.1) to roughly its current orientation (Figure 2.4), causing dramatic uplifting in the Southern Taranaki Basin (Fohrmann et al., 2012). The extent of uplifting was greater in the south of the Southern Taranaki Basin, thus more of the pre-N50 stratigraphy is absent further south, as can be seen in Figure 4.5. The N50 stratigraphic package bounded by the U_N50 and U_N82 horizons contains a marine looding surface dividing the lower units of the package, shelf sands, muds, and shell beds, from the upper units of the package, northward prograding shelf sets composed of cyclothems, sands, muds and shell beds (Fohrmann et al., 2012).

This horizon is picked primarily on a trough with exception between the Motumate and Taranaki Faults (Figure 1.3). This horizon becomes conformable in the western and northwestern edges of the study area as part of the prograding passive margin shelf. The

30 Taranaki Peninsula

2000 ms

Cape Egmont Fault

1000 ms

1000 ms

0 50 km

Contour interval 200 milliseconds Figure 4.4: U_N50 surface topography in time, with prominent fault offsets shown in black. This is the best surface to see the fault geometry that creates the Central Graben. Cross sec- tions shown as dotted black lines. Arrow points north. well control for this horizon is consistent, and it is typically an easily identiied relection above visible truncations, making it one of the more reliably interpreted horizons in the basin. Conformable sequences containing this horizon are the least reliable to interpret, but little emphasis is placed on differentiating these areas since they are above the Western Stable Platform.

4.2.3 N15 horizon - Figure 4.6

In the Southern Taranaki Basin, particularly in areas proximal to the Taranaki Fault, the N15 horizon (Table 4.1) represents the lithologic boundary between the Taimana and Lower

31 Figure 4.5: Seismic line along the Manaia hanging wall (location shown in yellow on right, study boundary in pink, seismic lines used in blue). U_N50 horizon is shown in light orange truncating N15 (dark orange) and P50 (blue) horizons. To the south of this section (outside of the study area), the U_N50 horizon directly overlies basement. K96 is green and P00 is shown in pink to be visible against the seismic, notice the bright reflections that alternate around K96 are not observed in the surrounding horizons, indicating either sedimentary fea- tures or faults contained within the Cretaceous sediments. Interpretations are less accurate to the south because they are outside of the study boundary.

Manganui Formation (Figure 2.2). This is an important horizon in regards to the timing of shortening in the basin and is often named N10 in publications before 2014. The contact between the Taimana and Manganui formations is the sharp contact representing the onset of rapid clastic sedimentation indicating the increase in the rate of the overthrusting major faults on the eastern edge of the basin between 21-22 Ma (King and Thrasher, 1996). The N15 sedimentary package between the N15 and U_N50 horizons contains the thin transitional sequence between Paleogene carbonate-rich sediments and Neogene clastic-dominated sediments, overlain by thick sequences of shelfal to bathyal mudstones (Strogen et al., 2014b).

The horizon is primarily picked on a peak, though in the area surrounding the Kerry 3D volume (Figure 3.1), it is picked as a trough to correlate to that area’s well control. In the Wakamarama footwall (Figure 1.3), it is unclear whether this horizon was truncated or not due to inconsistencies between intersecting 2D lines, so layer-cake geometry is assumed. The same assumption is made in relation to the U_P50 horizon in the grabens of the Opunake High and

32 Taranaki Peninsula

Motumate Fault

Cape Egmont Fault

3000 ms Manaia Fault

2000 ms

Maari Fault Truncated Horizon S.I.F.2

S.I.F.1 2000 ms

Fault Wakamarama

0 50 km

Contour interval 200 milliseconds Figure 4.6: N15 surface topography in time with prominent fault offsets shown in black. The Cape Egmont and surrounding faults within the Central Graben are normal faults, while the other labeled major faults are inverted faults. The horizon boundary that is restricted due to truncation by the U_N50 horizon in the hanging wall of the Manaia Fault is indicated by a dashed line. Study cross sections are shown as dotted black lines. Arrow points north. the northern end of the Cape Egmont Fault splay (Figure 1.3), since the fault shadows hide this horizon.

4.2.4 U_p50 horizon - Figure 4.7

The U_P50 horizon (Table 4.1) represents an unconformity across a large area of the Southern Taranaki Basin in the early Oligocene (Figure 2.2). The unconformity is presumed to represent a period of ~4 m.y. in the east of the basin adjacent to the major basin bounding faults (Fohrmann et al., 2012). In other areas of the basin, the horizon represents a hiatus in deposition

33 caused by a rise in sea level that resulted in peneplanation of the existing bodies of land at the time. The P50 sequence bounded by the U_P50 and N15 horizons represents a condensed Oligocene to early Miocene clastic sequence, overlain by the carbonate-rich Otaraoa and Tikorangi Formations deposited during the maximum looding of the continent. These carbonate facies vary in thickness and lithology within the Southern Taranaki Basin and are condensed further basinward. The top of the sequence is covered by the Taimana Marl, which represents the return of clastic inluence in the basin.

Taranaki Peninsula

Motumate Fault

Cape Egmont Fault

Whitiki Fault

3000 ms Manaia Fault 3000 ms

Maari Fault Truncated Horizon S.I.F.2

2000 ms S.I.F.1

Fault Wakamarama

0 50 km

Contour interval 200 milliseconds Figure 4.7: U_P50 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault, its splays, and the faults between the Maari Fault and the Cape Egmont Fault are normal faults, while the other labeled major faults, including the Whitiki Fault are inverted faults. The horizon boundary that is restricted due to truncation by the U_N50 horizon in the hanging wall of the Manaia Fault is indicated by a dashed line. Cross sections are shown as dotted black lines. Arrow points north.

34 The U_P50 horizon is an easily observed angular unconformity throughout much of the study area and it has well correlations in every well with logs, thus it is the most conident horizon used in the study. It is picked primarily on a peak, but varies in several small areas across the basin, determined by well ties and truncations.

4.2.5 p00 horizon - Figure 4.8

The P00 horizon (Table 4.1) represents the Cretaceous-Paleogene boundary, which divides the active syn-rift sediments from post-rift sediments (Figure 2.2). This horizon

Taranaki Peninsula

Whitiki Fault

Onlap

Onlap Motumate Fault Cape Egmont Fault

Edge of Interpretation

3000 ms Manaia Fault 3000 ms

Onlapping Horizon Maari Fault

Truncated Horizon

2000 ms

S.I.F.1

Fault S.I.F.2 Wakamarama Basement

0 50 km

Contour interval 200 milliseconds Figure 4.8: P00 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults as well as unnamed basement faults, while the other labeled major faults, including the Whitiki Fault are inverted faults. The horizon is truncation by the U_P50 horizon in the hanging wall of the Manaia Fault, indicated by a dashed line. The horizon onlaps the basement horizon any place where red basement is visible away from the edge of the study area. Cross sections are shown as dotted black lines. Arrow points north.

35 represents the top of the shallow marine-terrestrial North Cape Formation, which onlaps basement highs across the basin (Figure 4.8), and is truncated by the P50 unconformity in the Manaia hanging wall (Figure 4.8). The P00 lithological package between the P00 and U_P50 horizons represents an overall transgression from non-marine conglomerates to lower shoreface mudstones over 3 transgressive sequences (Figure 2.2). Active faulting continued in isolated sub-basin throughout the Paleocene, but ended before the Eocene (Strogen et al., 2014b). The Eocene sequences represents non-marine to shallow marine systems with estuarine sands at the base of each section, assigned to transgressive system tracts, and coal measures at the top of each sequence that are interpreted to represent highstand system tracts (King & Thrasher, 1996). The top of this package is characterized by the Turi Formation, a marine mudstone, which through the combination of continued subsidence and transgression covered the basin in the late Eocene, burying the Eocene shelfal deposits and remaining basement highs.

The P00 horizon is picked on a peak and has well control in six wells, though the biostratigraphy used to identify this horizon can have as low as +/- 100m resolution, resulting in few accurate well ties. The low resolution is likely because the horizon is not a high petroleum priority. The interpretations for this horizon vary from published interpretations in the Opunake High and the footwall of the Wakamarama Fault (Figure 1.3). Both of these differences are due to a lower basement interpretation, which adjusts the extent of the P00 horizon before it onlaps basement. This horizon is the irst main example of a surface with an uneven texture that is not necessarily a result of structural deformation. When restorations are created, this irregularity is taken into account in order to not create any artiicial strain from unnecessary unfolding.

4.2.6 K96 horizon - Figure 4.9

The K96 horizon (Table 4.1) is commonly referred to as K90 in publications before 2014 and represents the onset of transgression that continued until the early Miocene (Figure 2.2). This horizon is the boundary between the terrestrial Rakopi Formation and the terrestrial- marginal marine North Cape Formation, both of which are syn-rift. The K96 package between the K96 and P00 horizons contains the North Cape Formation. The package between the K96

36 Taranaki Peninsula

Onlapping Horizon

4000 ms Motumate Fault Cape Egmont Fault

Whitiki Fault

Manaia Fault

3000 ms

Onlapping Horizon

Maari Fault 3000 ms

S.I.F.1

Fault S.I.F.2 Wakamarama

Basement

0 50 km

Contour interval 200 milliseconds Figure 4.9: K96 surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults, as well as unnamed basement faults. The S.I.F.2 changes from being inverted in the south to normal displacement in the north around where the K96 horizon begins to contact the fault. Labeled major faults, including their splays (e.g. the splay at the southern end of the Motumate Fault) are inverted faults. The horizon is truncation by the U_P50 horizon in the hanging wall of the Manaia Fault, indicated by a dashed line. The horizon onlaps the basement horizon any place where red basement is visible away from the edge of the study area. Cross sections are shown as dotted black lines. Arrow points north. horizon and basement is called PreK96 (Table 4.1), and contains the Rakopi Formation and possibly the Taniwha Formation (Figure 2.2) in some sub-basins, though these formations are never differentiated.

The K96 horizon is present in two wells as a contact, and in three other wells where the basement and North Cape contacts are documented, it can be inferred that the horizon onlaps

37 onto basement downdip. Both formations surrounding this horizon contain coal measures, which can explain the inconsistent bright relections observed in some areas. There is no well control in the Opunake High, and the closest well control south of Kerry 3D (Figure 3.1) is 20 km away linked by one 2D seismic line. In the area southeast of Kerry 3D, shown in Figure 4.5, bright relections jump around in depth, but the surrounding horizons are not offset, so this is interpreted as a stratigraphic artifact of the North Cape Formation. In the splay on the southern end of the Manaia Fault in this study (Figure 1.3),

4.2.7 Basement horizon - Figure 4.10

The basement horizon (Table 4.1) in this study represents seismic basement. The lithologic basement is composed of multiple rock types, including “metasedimentary greywacke and argillite facies, volcanics, and intermediate and silicic plutonic bodies, varying in age from Early Paleozoic through to late Early Cretaceous” (Strogen et al., 2014b) (Figure 2.2). Due to the vertical resolution of the 3D volumes, most basement is only interpreted on 2D lines and only with the assistance of ive wells within the study area, all in the eastern part of the basin (Figure 1.3). The resolution of basement is already low due to its depth, and areas below coal measures or near large overthrusted basement blocks are even harder to conidently interpret. Because of these limitations, basement is identiied using three criteria, in this order of priority; the lowest angular unconformity or onlap, the “deepest relatively continuous seismic relector” (Fohrmann, 2012) or the shallowest deep relection not deemed a multiple, and the relections must be below any geometries interpreted as growth packages. In the southern part of the basin, where Section 6 (Figure 1.3) crosses the footwall of the Wakamarama Fault, (Figure 1.3), the interpretation is much lower than the regional transects (Strogen et al., 2014b) show, which as mentioned above, affects the thickness of P00 and other horizons below U_P50. In the northern extent of the Manaia hanging wall in this study area, basement is not interpreted because it is deeper than most 2D surveys, and those with suficient record length no longer have any obvious relections that stand out from the rest. The restriction of interpreting the basement here is why Section 1 (Figure 1.3) is no further north.

38 Taranaki Peninsula

Motumate Fault Cape Egmont Fault

Whitiki Fault 4000 ms

Manaia Fault

4000 ms

Maari Fault

3000 ms

S.I.F.1

S.I.F.2 Fault Wakamarama

0 50 km

Contour interval 200 milliseconds Figure 4.10: Basement Surface surface topography in time, with prominent fault offsets shown in black. The Cape Egmont Fault and its splays are normal faults, as well as unnamed basement faults. The S.I.F.2 changes from being inverted in the south to normal displace- ment in the north around where the western hanging wall of the fault becomes deeper than the eastern footwall. Labeled major faults, including the their splays (e.g. the splay at the south- ern end of the Motumate Fault) are inverted faults. Cross sections are shown as dotted black lines. Arrow points north.

While the Maui-4 well and Fohrmann (2012) paper indicate the basement as a peak, it is picked as a trough in this study (both SEG reverse), because even if the overlying lithology is predominantly coal, which has a higher velocity (~2.4 km/s) than average clays and sands (1.5- 3.5 km/s), any igneous or metamorphic rock in the basement would still have a positive kick with velocities between 4.4 and 6 km/s.

All faults observed cutting the basement were picked, but most are not laterally consistent

39 enough for the 3D model since they are not observed in more than one or two 2D lines. This is why the 3D surface for the basement is so irregular, since the displacement of the fault is only observed at one point along the surface. In 3D volumes like Maari 3D and the hanging wall of the Maui 3D volume (Figure 1.3), a good resolution is possible, but the Wakamarama footwall and areas closer to the west and southeast of the study area did not have enough 2D seismic surveys to interpret the detail of the basement accurately. When sections were constructed, close basement faults were projected onto the cross sections in order to compensate for this. This should not affect the strain measurements drastically because no displacement was added.

Not all available 2D lines needed to be interpreted in order to make accurate 3D horizon surfaces. The 3D volumes with well control were interpreted irst and connected by longer regional lines, followed by 2D lines with well control. 2D lines were then added to the study as needed in order to enhance the resolution of structures. Because of this, all 2D lines available are typically in use around structurally complex areas, while structurally simple areas such as the eastern ends of Sections 4-6 (Figure 1.3) did not need to use every 2D line to accurately depict the geometry for a 3D model.

The basement is a prime example where this methodology is used. Basement within Maari 3D is complex and shallow enough to be interpreted, so it is interpreted on a small interval perpendicular to the faults within the volume. Maui 3D is only deep enough to show basement relections in the footwall of the Cape Egmont Fault, but the geometry is simple, so while the basement is picked at a higher interval close to the fault, away from the fault, the wide spacing is adequate to fully characterize basement geometry. Similar methodology is used with 2D lines for the geometry of each individual horizon. When a structure is adequately depicted, then no more lines need to be interpreted, but if a structure, requires more information to be characterized, any available 2D lines are balanced and added to the study.

When two 2D lines are properly positioned yet do not have matching relections, the position of relections in the middle of 2D lines are trusted over the position of relections closer

40 to the edge of a line, and lines closer to the well control or 3D volume are trusted over those farther away. This occurs for most horizons at one point or another, particularly in the areas south of Maari 3D (Figure 3.1). If both lines in question are isolated from reliable controls, as in the Opunake High (Figure 1.3), then the line perpendicular to the structures of the area is interpreted more faithfully.

4.3 2D Model Generation

In order to accurately represent the structures within the basin, the cross sections need to be oriented orthogonally to the strike of the structures, which for the most part does not coincide with the orientation of the seismic surveys. To overcome this, surfaces were created in Petrel to ill the gaps between the interpreted seismic lines. The surfaces and faults were then imported into Midland Valley’s Move (2016) software. Figure 4.11 and Figure 4.12 show the surfaces in Petrel cut along Section 2 in order to show what the surface lines in each cross section represents. Diagnostic cross sections were chosen to partition the basin while remaining perpendicular to the major structures. The intersections of surfaces and faults were then projected onto each cross section as lines. These lines are used as the boundaries for 2D polygons which represent packages of sediment that can be assigned appropriate variables. Each Package is named and colored after its lower bounding horizon, with the exception of PreK96 (Table 4.1).

4.3.1 Section 1 - Figure 4.13 The position of this cross section is as far north within the interpreted horizons as possible while still remaining perpendicular to the Manaia Fault and avoiding a large bend in the section. The Cape Egmont Fault splay is directly north of this cross section (Figure 1.3). The normal faults that make up the Central Graben are present in the hanging wall of the Cape Egmont Fault in this section. Also, faults large enough to be shown in the cross section that make up the Central Graben are all between the Cape Egmont Fault and the basement fault denoted in Figure 4.13 as fault A. The Cretaceous sediments in the hanging wall and the offset

41 1x V.E. Figure 4.11: Surfaces in Petrel, Cropped along Section 2 in order to demonstrate what the 2D cross sections represent. This figure shows the surfaces in their respective colors and faults in white, with no vertical exaggeration. Section is 101.2 km long which coastline polygon shown in yellow near the top of the figure. Arrow points north and shows the perspective of camera.

7.5x V.E. Figure 4.12: Surfaces in Petrel, cropped along section 2. This figure shows sections vertically exaggerated by 7.5 times. Faults are not shown, but displacement of a surface along a fault is shown by smooth surfaces. Section is 101.2 km long which coastline polygon shown in yellow near the top of the figure. Arrow points north and shows the perspective of camera.

42 of Neogene horizons show that Fault A is a Cretaceous fault that reactivated during the Pliocene- present extension. The graben in the Manaia hanging wall is a sub-basin that formed during Paleocene extension after Cretaceous rifting in the basin had ceased. The P00 stratigraphic package in this sub-basin thickens to the north of this section, and this trend can be observed in the thinning of the package in Figure 4.14, and the absence of the graben in Figure 4.15. Note the thickness of the N50 and N82 sequences. During the Pliocene and Pleistocene, there is adequate accommodation in the northern part of the Southern Taranaki Basin, which is why the N50 and N82 packages are the thickest in this cross section. There is Pleistocene faulting concentrated above the Manaia Fault, which could be due to uneven decompaction of the sediments within the Manaia Fold (the fold above the Manaia Fault) as extensional strain migrated south, as can be seen in Figure 2.3.

While the cross section was chosen to be perpendicular to the largest basin structures, this is not possible without introducing a large bend in the cross section. Figure 4.6 shows the topography of the N15 horizon in time, which demonstrates that the this section is not perfectly perpendicular to several faults, meaning the strain measured from these minor faults is slightly less than it should be. The Motumate Fault in particular is more out of plane than the other faults represented in the section.

4.3.2 Section 2 - Figure 4.14

This cross section is positioned to intersect the Whitiki, Manaia, and Motumate Faults and be near the two wells (Figure 1.3) used for depth conversion. This section intersects a large portion of the Central Graben. The easternmost fault within the Central Graben shown in Figure 4.14 is in line with fault A shown in Figure 4.13. This is most apparent in Figure 4.4. Fault B in Figure 4.14, like fault A, shows offset from the basement up into recent sediments (N82 Package), but unlike Fault A, there is no geometrical evidence for reactivation, due to the consistent thickness of Cretaceous sediments (Green) on both sides of the fault. The Whitiki Fault is signiicant because it is one of the few faults in the study that shows clear signs of folding rather than faulting due to the inversion of Cretaceous faults in the Miocene. The graben

43 Maui-A WNW Field Central Graben Section 1 ESE

Maui-2 Well

TD: 3566 m (Basement)

Manaia Hanging A Wall Graben Cape Egmont Fault Motumate Fault Manaia Fault

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.13: Section 1 cross section. Major faults and the Maui-2 well are labeled. The extent of the Maui-A gas field and the rough extent of the Central Graben is shown above the section. The unnamed basement fault within the Central Graben mentioned in the text is labeled ‘A’. Maui-B Kupe WNW Field Central Graben Section 2 Field ESE

Kupe South-4 Well Maui-1 Well

TD: 3433 m (North Cape Fm) TD: 3800 m (North Cape Fm) Opunake High Manaia B Whitiki Fault Hanging Wall Graben

Cape Egmont Fault Motumate Fault Manaia Fault

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.14: Section 2 cross section. Major faults and the two projected wells used for interpretation and depth correlation are la- beled. The extent of the Maui-B and Kupe fields is shown above the section, with red representing the extent of gas in the field and green representing the extent of oil in the field. The rough extent of the Central Graben is also shown. The general undeformed area in the hanging wall of the Motumate Fault is referred to as the Opunake High The unnamed basement fault within the Central Graben mentioned in the text is labeled ‘B’.

44 in the Manaia hanging wall of this section has more normal faults within the graben than it does in Section 1, and the P00 stratigraphic package is thinner here, as mentioned previously. This indicates that either the sub-basin was initially shallower at this position in the basin, there was more uplift at this position in the basin when the P50 unconformity (which is bounding the top of the package) occurred, or a combination of the two. To the south of this section, the U_P50 horizon truncates the P00 horizon, but this may be due to channelization since the unconformity occurs in a low of the U_P50 horizon and the P00 horizon is present below the unconformity further south in Section 3 (Figure 4.15). However, this horizon relationship is in a gap in seismic survey density bellow the Kerry 3D volume (Figure 3.1), so the relationship is only partially imaged. The Pleistocene faults above the Manaia Fault in Section 1 are also present in this cross section.

4.3.3 Section 3 - Figure 4.15 This cross section is positioned to capture the furthest seismic line south within the study boundary that encounters the Manaia Fault and intersects a well (Figure 1.3) to aid in depth conversion. The western end of this cross section lies between the Maari and Cape Egmont Faults. The southern extent of the Central Graben displacement is more prominent above the Maari Fault than the Cape Egmont Fault in this cross section. The Cape Egmont Fault offset of Cenozoic sediments dies out right at this cross section, even though basement offset of the Cape Egmont Fault is still present. This geometry is visualized well by comparing Figure 4.4, which shows the U_N50 horizon, to Figure 4.10, which shows the basement horizon. This is the only cross section that shows the Motumate and Manaia splays. Uplift as the result of late Miocene compression is more apparent in this cross section then the more northern cross sections. It is most apparent in the Manaia hanging wall, where the N50 unconformity truncates the older packages below it. To the south of this section this trend continues until the N50 horizon directly overlies basement in the Manaia hanging wall. In the rest of the cross section, the late Miocene uplift is represented by how thin the overlying N50 and N82 Packages are compared to Sections 1 & 2. The extension above the Opunake High is similar to the extension above the Manaia Fold

45 in Sections 1 & 2, where extensional strain is accommodated above preexisting weaknesses, which in this case could be the underlying rift structures that may have reactivated to some extent, but are not visible on this scale

Section 4 - Figure 4.16

This cross section is positioned through the maximum offset of the Maari Fault. There are more rift geometries in this section than in the sections to the north, yet there are only two faults in this section that are inverted. Of the two inverted faults, the fault to the right of Maari Fault shows no displacement in this part of the basin, but is roughly in line with the S.I.F.1 to the south (Figure 4.10). The S.I.F.2 in this section only shows normal displacement without any reactivation or inversion. The normal faults in the Neogene sediments are above uninverted rift geometries in the basement that end right before this section but are apparent in the surface map of the basement horizon (Figure 4.10). The Motumate fault is present in the eastern end of the section, but shows no offset, only folding. The thickness of Pliocene and Pleistocene sediments in this section is again due to late Miocene uplift, which affects the N50 package signiicantly in this section from a lack of accommodation at the time of deposition. The wavy nature of the N50 horizon in the west of this cross section is due to this cross section cutting across the northward prograding cross sections that the conformable end of this horizon represents.

4.3.4 Section 5 - Figure 4.17

This cross section is positioned to be between the Maari and the Wakamarama Fault. It has the largest number of rifting structures out of the different cross sections and also has the most folded structure, likely due to the lack of Cenozoic faulting. There is a slight fold over the S.I.F.1, but without any discernible offset along the fault. S.I.F.2 still shows no evidence of shortening in this section. Reverse slip may have occurred along the faults similar to the Whitiki fault in Figure 4.14, but there is no clear displacement to prove such a relationship in this section. The Motumate Fault is present, but again shows no offset of the basement this far south. The Pliocene and Pleistocene packages exhibit similar thicknesses to Section 4.

46 Central Graben WNW Section 3 Tahi-1 Well ESE

Kea-1 Well TD: 1776 m (Rakopi Fm)

TD: 3135 m (Kapuni Group) Opunake High Cape Egmont Fault

Maari Fault Motumate Fault Splay Manaia Fault Splay

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.15: Section 3 cross section. Major faults and the Kea-1 and Tahi-1 wells are labeled, and the rough extent of the Central Graben is shown above the section. The general undeformed area in the hanging wall of the Motumate Fault is referred to as the Opunake High.

Maari WNW Field Section 4 ESE Maui-4 Well

TD: 3920 m (Basement)

S.I.F.2

Maari Fault Motumate Fault

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.16: Section 4 cross section. Major faults and the Maui-4 well are labeled. The lateral extent of the Maui-4 oil field is shown above the section. Notice the folding caused by the slip of the fault east of the Maari Fault. S.I.F.2 stands for Southern Inverted Fault 2.

47 4.3.5 Section 6 - Figure 4.18

This cross section is positioned in the southernmost extent of the study in order to include the Wakamarama Fault and S.I.F.2. This position in the basin shows less rifting geometry than the two sections to the north, but the Wakamarama Fault shows much more syn-rift sediment than the faults in sections 4 and 5. The lack of syn-rift packages in the eastern end of this section indicates that is was likely exposed until the Eocene transgression. This section shows the thinnest of both the N50 and the N82 Packages, which its the trend of late Miocene uplift affecting the southern end of the basin more dramatically, and that the recent extension in the north of the basin has not extended to the south of the basin yet. The Wakamarama Fault and S.I.F.2 both exhibit more offset to the south of this cross section, while S.I.F.1 exhibits the largest amount of offset between Section 5 and Section 6

4.4 Depth Conversion

Depth conversion was calculated in 2D for each cross section individually in Move, and is based on the equation: (ekt -1) Z= V 0 k

Where Z is the thickness of the layer in meters, V0 is initial velocity of the package (m/s), k (m/s per m) is the rate of change in the velocity with increasing depth, and t (s) is the one way travel time for layer thickness. Thus each package in a cross section is assigned values for V0 and k (Table 4.2). For values to be calculated, wells with checkshots are projected onto cross sections to verify and calibrate the depth conversion values when available. The wells used for this are indicated in Figure 1.3 as the well symbols illed red. Four different sets of values were are used to assign geophysical properties to sections 1, 2, 3, and 4. Sections 5 has no well control and while the Tasman-1 well intersecting section 6 has checkshots and the total depth is known, it does not have any litho or biostratigraphic ties to calibrate the depth conversion. The values are calculated from well checkshots which measure the velocity of various intervals within a speciic well. To calculate the values for a speciic package, the checkshot intervals within that package

48 WNW Section 5 ESE

S.I.F.2 S.I.F.1

Southern Extent of Maari Fault Motumate Fault

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.17: Section 5 cross section. Major faults are labeled. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2.

WNW Section 6 ESE Tasman-1 Well TD: 1629 m

S.I.F.1 S.I.F.2

Wakamarama Fault

0 10 20 30 40 50 BpreK96 K96 p00 p50 N15 N50 N82 2x Vertical Exaggeration 95 68 66 30 19 7 2 present (km) (Ma) Figure 4.18: Section 6 cross section. Major faults are labeled. The Tasman-1 well is labeled, but it does not have public well picks. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2.

49 Table 4.2: Depth conversion values used for each section, divided by package. Values that don’t match at least one other section are shaded in grey.

are grouped together and the values for V0 and k which best encompass the range and average of velocities in that package are identiied. These values are edited when the known lithology and horizon picks in the depth converted cross section do not line up with the picks in the well. An acceptable range of these edits is taken from the Hill & Milner (2012) study in the Kupe region (Figure 2.5). This study is useful because it also simpliies the changes in velocity within packages to the same variables. However, checkshot values are still used to estimate the starting values because the packages used in this study have different bounding horizons than the packages used in Hill & Milner (2012) (Figure 4.19). Hill & Milner (2012) packages are a particularly useful starting point for wells with low checkshot resolution, since some wells have more than eighty checkshot measurements, but others have as few as ten. When Hill & Milner

(2012) have two different k values for the same package, V0 and k values that best encompass the velocity range represented in that package are used for this study. An example of a package boundary that differs is P00 - U_P50 (Figure 4.19). Hill & Milner (2012) break the package into two groups, one with a constant velocity (k=0), the other with a high then low k causing a steeper increase in velocity followed by a shallower increase when below a speciic depth, however, Move does not allow conditional parameters to be used. To counter this, the package used in this

50 hill and Milner, 2012This Study Seabed Seabed N82 N70

N52 N50 N50 N45

N15

P60

P50 P50

P20

P10

P00 K96

K90

Basement Basement

Figure 4.19: Comparison between the packages used in this study and the Packages used in Hill & Milner, (2012) Time scale from Strogen and King, (2014).

51 study has the same initial velocity as the Hill & Milner (2012) package with the static velocity value and uses the lower k from their package with two k values. The resulting velocity curve of the package in this study thus remains the same as theirs while also matching the average velocity measured in the projected well in that section. Before converting to depth, a rough bathymetry surface was created using a 50 m resolution topographic map to include the water column in the depth conversion process. The bathymetry of the basin is very gradual, which is why a 50 meter resolution map is accurate enough for the scale of these reconstructions. The only signiicant assumption changed from Hill & Milner (2012) is for the package between K96 and basement (labeled “preK96” in Table 4.2). The values assigned in Hill & Milner (2012) were the same as the basement, but in this study, the preK96 V0 is lowered to allow the package depth to match lithologic picks in the wells, since checkshot data at the depth of this package is either absent or incomplete.

52 ChApTER 5 RESTORATIONS

5.1 Restoration Methodology

Reconstructions were performed using Midland Valley’s Move (2016) software to calculate the rate and extent of strain for a particular section, while simultaneously proving if the interpretation is structurally balanced. “Balanced cross sections honor all available data and are constructed and analyzed to ensure they are geometrically possible and geologically admissible” (AAPG Wiki). The worklow to reconstruct each stratigraphic package involves irst undoing the slip of any faults that offset the horizon and basement, or measuring the maximum heave of any fault that does not continue through to the basement. Once the top horizon is continuous, the horizon is unfolded to its appropriate shape at the time of deposition. This shape can vary depending on whether the horizon represents an unconformity, a passive margin shelf or a combination of the two across the section. The last step is to remove the topmost package, letting the software calculate the decompaction of underlying packages and the isostatic rebound of the basin as a whole.

For unfaulting, Fault Parallel Flow, Simple Shear, and Trishear methods are all suitable for restorations and are each used when appropriate. Simple shear (Move 2016, algorithm incorporates the work of Verrall [1981], Gibbs [1983], and Withjack and Peterson [1993]) maintains the area between beds, and is used commonly in extensional tectonic regimes where the thickness of beds may vary. Fault Parallel Flow (Move 2016, algorithm developed in collaboration with Kane et al., [1997] and Egan et al., [1997]) is best suited for modeling hanging wall movement in thrust faults because it is based on the principal of Particulate Laminar Flow. This unfaulting method does not translate or deform the footwall of the fault, and line lengths in the hanging wall are preserved in parallel systems. Trishear method (Move 2016, algorithm developed in collaboration with Erslev, [1991]) is used when restoring fault-related folds, preserving the area of formations in a series of triangular shear zones above the fault and performing fault parallel low for hanging wall deformation outside of the designated shear zone.

53 Faults within a package were not restored using any of these methods, but their maximum heave was measured when the rest of the package was unfaulted and recorded as minor faulting.

For unfolding, the Flexural Slip method (Move 2016) is used. This unfolding method assumes a slip-system parallel to the unfolded target horizon, where slip for each point in a package that is affected by the unfolding occurs parallel to the horizon that is being unfolded. This method may overly simplify and not accurately represent the ductile strain in this basin, however, it is used since the other option for unfolding available is simple shear, which does not preserve horizon line length or package area. When performing unfolding, minor irregularities deemed unrelated to folding were ignored. This is accomplished by making a line that followed the horizon which is then smoothed across irregularities in the horizon. The new line is then unfolded, with any irregularities in the horizon around the smoothed section of line preserved. For shallow horizons, this method was most commonly used to avoid unfolding sedimentary geometries such as incised channels or passive margin shelf geometries. In the deeper horizons, seismic quality was often poor, causing irregular surfaces that do not represent the actual geometry of the deeper relectors in the basin.

For decompaction, the Sclater-Christie (1980) method (Move 2016) is used, since the other methods available are primarily for systems with much larger ratios of mud. Isostatic relief calculations were included in this step of the reconstruction using the Flexural Isostasy method (Move 2016). The Basement was set as the base, meaning it is affected by the isostatic rebound, but remained unaffected by decompaction during the removal of overlying sediments. Due to the way decompaction calculations worked, the amount of decompaction was inconsistent around prominent structures, which needed to be corrected manually after decompaction was performed.

More information regarding the theories behind the unfaulting, unfolding, decompaction, lex isostasy, and other available methods is available through the Online Help Manual included with Move 2016.

54 5.2 Compaction and Isostasy Variables

The inputs needed to calculate isostasy in Move are mantle density, elastic thickness and Young’s Modulus, with optional inputs for lateral load extension. An elastic thickness of 30 km is used in this project based on Flexural Modeling performed by Holt & Stern (1994). A mantle density of 3350 kg/m3 is used (Holt & Stern, 1994), and the default Young’s Modulus of 70 GPa provided by Move is used since there was no literature found that suggested otherwise.

The formula for computing decompaction is: -cy f = f0(e )

Where f is present day porosity, f0 is the porosity at the surface, c is the porosity-depth coeficient (km-1), and y is depth in meters. Each sedimentary package needs its own set of variables, and these were determined for pure sand, pure mud, and pure limestone. Then, well logs were used to assign ratios of lithology to each package (Table 5.1). The compaction of coal is not calculated independently since the most signiicant compaction for coal occurs within the top ten meters of burial and below that depth the compaction rates are determined by the surrounding lithology (Nadon, 1998). Because of this, a unit of sand with interbedded coal units

Table 5.1: Table showing final decompaction values (Right), and the lithological values (bot- tom) and lithology ratios (Left), used to calculate the final values

55 is modeled as a unit of sand. Secondary porosity was ignored and not used when assigning porosity values.

The f0 and c for shale are determined using Armstrong (1998). For sand, Matthews & Mills, (2001) gave two values, which when the initial porosity of sand of is 0.49, determines c. For limestone, default Sclater-Christie values from Move (2016) are used.

Armstrong (1998) states that porosity and permeability trends do not appear to break over unconformities, meaning erosion in the wells they tested occurred at depths shallower than present burial depth. This simpliies calculations since the amount of compaction each formation has been exposed to is at its highest at present, allowing the equation above to be accurate.

Further compartmentalization could add better resolution, but most sedimentary packages used in the study are relatively consistent in lithology vertically. There are some differences in lithology laterally, so the lithologies from four wells across the basin were quantiied and averaged, with an emphasis on the thicker sequences, since that is where decompaction makes the most impact, and the thicker sequences are also commonly bounding major structures.

5.2.1 Worklow Variances between Sections Due to the inal cross sections passing through parts of the surfaces away from seismic lines, several of the surfaces required manual edits after the surface intersection was plotted on the section. These edits were done with the surrounding seismic open in order to assure the highest amount of accuracy possible. In many cases it was as simple as making two horizons on the limb of a fold parallel to one another as they are in the surrounding 2D lines. Anomalously wavy horizons were smoothed if the surrounding horizons did not show the same geometry. Unless the cross section directly overlapped with a fault, the horizons around faults often were just a curved line, rather than being displaced, so they were corrected, again with input from the surrounding seismic.

The amount of extension for K96 and Basement could be skewed, since any faulted

56 basement was unfaulted as soon as the last of the syn-rift sediment was removed. The error is possible because faulting could have occurred with the rifting of the rest of the basement, and sediment younger than that interpreted to overlie the basement could have been eroded before the next sequence was deposited.

No unfolding was done for K96 and Basement. Despite the irregularity of the surfaces, there would have been no shortening of the sediments through folding at that point in time and any unfolding applied to the horizons would artiicially decrease the longitudinal extension within each respected period of time. These geometries are either the result of wavy interpretations, low seismic quality, or the artifact of improper geometries when unfolding overlying horizons. The P00 horizon is unfolded to help see the pre and syn-rift sediments better, but the change in length caused by this is not included in the calculations

5.2.2 Erosion Compensation The manual addition of sediments was used any time there were obvious truncations in contact with a horizon. This is most notably performed for the mid-Neogene unconformity (U_ N50). The worklow for this involved looking at the seismic to determine which parts of the horizon were conformable, which parts were truncated, and identifying the minimum possible thickness for truncated packages. The modern surface in included within the restored package after this is done (best example shown in Figure 5.3 at 7 Ma). One discrepancy is that erosion over the top of large faults takes away any evidence of displacement from the top of the eroded package.

5.3 Restorations

All six restorations are shown in Figures 5.1-5.6, with labels depicting the approximate age of each step in the restoration as well as the ratio of extension or shortening between each step and the ratio over each period of extension or shortening. Each unfolding, unfaulting, or decompaction step was performed separately, however the igures show a horizon after all steps have been performed for that horizon. The detailed measurements between individual steps

57 are shown in Tables Table 5.2 -Table 5.7. A key point to remember is that the Taranaki Fault is not included in any of the six cross sections due to the ambiguity surrounding the evidence for timing of the fault movement. Also, the Manaia Fault (Figure 1.3) which is present in Sections 1-3 is absent in Sections 4-6 due to the truncation of pre-U_N50 horizons in the hanging wall of the fault in the south of the basin. This truncated geometry can be seen in Figure 4.5.

5.3.1 Section 1

Each step of the Section 1 restoration is shown in Figure 5.1, with detailed strain measurements in Table 5.2. In Figure 5.1 D, the normal fault present in the hanging wall of the Manaia Fault is a graben that deepens to the north, however the basement geometry in this section is not perfectly balanced since it requires a cross cutting relationship to be possible if only rigid deformation is used. In Figure 5.1 F, the geometry of the P50-N15 Package (blue) requires overthrusting of the Manaia Fault by 19 Ma and does not necessarily indicate any inversion of the Cape Egmont Fault. In Figure 5.1 F, compensation for erosion is required above U_N50, but it is on a scale that is too small to be seen in the igures. Fault A reactivated during the recent extension in the basin and is the eastern boundary for extension, on this scale, within the Central Graben. This fault and the Cape Egmont Fault seem to accommodate all the Pliocene-present extension within the basement and older packages in this section, while minor faulting has accommodated much of the extensional strain in the younger, less-consolidated sediments.

5.3.2 Section 2

Each step of the Section 2 restoration is shown in Figure 5.2, with detailed strain measurements in Table 5.3. In Figure 5.2 D, the same graben as Section 1, illed with the P00 package, is present but shallower. The smaller faults within the graben only extend through the packages above the basement, showing extension that was accommodated within the packages and not through to the basement. To the east of this graben, the U_P50 unconformity cuts in to the K96 package directly. There may have been P00 package sediments above the unconformity, but there is no way to estimate their thickness, and since the P50 unconformity is during a time

58 WNW Section 1 ESE h present

G ~2 Ma 0.33% Extension

1.44% Extension 1.77% Extension F ~7 Ma

E ~19 Ma 1.36% Shortening

2.57% Shortening

D ~30 Ma 3.86% Shortening

1.92% Extension C 66 Ma

2.36% Extension B ~68 Ma 10.16% Extension

5.59% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.1: Section 1 restoration. The western half of this section is in the Central Graben (Figure 1.2), characterized by late Miocene - present extension. The Cape Egmont Fault is an inverted fault that reactivated with this recent extension, though the amount of inversion is re- duced due to the decompaction of sediments in the hanging wall (described in detail in Chap- ter 5.5). The Manaia Fault in the eastern end of the section is one of the early major thrust faults of the basin. Extension occurred into the Paleogene in this section in the sub-basin in the Manaia hanging wall (Figure 4.13).

59 Table 5.2: Strain measurement chart for Section 1. Items in red are measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

60 WNW Section 2 ESE h present

G ~2 Ma 0.14% Extension

0.82% Extension F ~7 Ma 0.96% Extension

3.53% Shortening E ~19 Ma

1.14% Shortening

D ~30 Ma 4.63% Shortening

5.39% Extension C 66 Ma

4.50% Extension B ~68 Ma 5.39% Extension

3.90% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.2: Section 2 restoration. This cross section encounters the same major faults as Sec- tion 1, with the exception of the Whitiki Fault in the western end of the section. This section also cuts through the Central Graben (Figure 1.2) in the western end of the section, while the faults in the eastern end of the section remain inverted. Extension occurred into the Paleogene in this section in the sub-basin in the Manaia hanging wall (Figure 4.14).

61 Table 5.3: Strain measurement chart for Section 2. Items in red were measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

62 of peneplanation, any presence of the P00 package sediments would have been very thin. In Figure 5.2 F, the amount of sediment added above the N50 unconformity is thicker than in Section 1, but drastically thinner than the amount of sediment added in sections to the south, correlating with a reduced amount of uplift in this latitude of the basin in the late Miocene. The fault in the footwall of the Cape Egmont Fault only shows normal displacement, however the folding shown in Figure 5.2 F may have cause the weakness that later resulted in normal faulting (or it could just be the artifact of interpreting and decompacting in a fault shadow). In Figure 5.2 G, fault B is connected by a splay to fault A in Section 1, however in this case, there is no evidence of movement before the Pliocene. It is likely that there is structural weakness in the basement caused by either basement lithology or preexisting minor faults, but as extensional strain migrated south, this was the point that accommodated the strain in the basement. Unlike sections to the north, the small scale faults in this section occur further away from the Cape Egmont Fault than the basement fault within the Central Graben (Fault B), occurring in line with fault A (Figure 4.4). The small-scale faults above the Manaia Fold (the fold above the Manaia Fault) show the extent of extension across western end of this section in the last few million years, however, restorations performed at this scale give no more insight as to the structural weakness causing this faulting.

5.3.3 Section 3

Each step of the Section 3 restoration is shown in Figure 5.3, with detailed strain measurements in Table 5.4. The structures present in the middle of this section represent the transition between the Maari and Cape Egmont Faults. Due to this section’s location between two major faults, strain is accommodated through folding rather than faulting. The large amount of folding between Figure 5.3 E and F is easiest to identify by looking at the present N50 placement within the N15 package shown in Figure 5.3 E. This section is the only section that encounters the Motumate and Manaia splays, which is a possible reason why the amount of shortening in this section is anomalously high, even when the slip from the Manaia Fault is not considered. The minor faulting of the U_N50 horizon above the Maari Fault is included with

63 WNW Section 3 ESE h present

G ~2 Ma No Change in Length

0.31% Shortening F ~7 Ma

3.92% Shortening E ~19 Ma

2.22% Shortening

D ~30 Ma 6.05% Shortening

0.32% Extension C 66 Ma

0.28% Extension B ~68 Ma 3.85% Extension

3.24% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.3: Section 3 restoration. This section cuts though the lowest extent of the Central Graben (Figure 1.2) shown in the sections of this study. Neither the basement fault that is part of the Cape Egmont fault and the basement fault that is part of Manaia Fault show in- version (which is seen in the sections to the north and south respectively, though the recent extension is also above the Manaia Fault, indicating at least some amount of reactivation, even if the offset is not apparent in the older packages below the N15 Horizon. Extension occurred into the Paleogene in the Manaia hanging wall, but not to the same extent as Sections 1 & 2.

64 Table 5.4: Strain measurement chart for Section 3. Items in red were measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

65 the other minor faults, and it is clear from the surface map of U_N50 (Figure 4.4) that these faults are the lowest extension of the Central Graben, however it is possible that the faulting was also inluenced by the relative extension of the package as the fold was generated. Across the Opunake High, west of the Motumate Fault, the P00 horizon is shallowly truncated by U_ P50. This is likely due to channelization, considering the U_P50 unconformity is not associated with compressional structures that would cause structural highs in this part of the basin. The truncations were used to estimate the amount of eroded sediment, but similar to U_N50 in Section 1, the change in geometry is too small to show up in this scale of igure. The geologic interpretations along the eastern end of this section above the Manaia Fault were strongly assisted by the restorations. Multiple restorations were tested before an interpretation was robust enough to conidently match the seismic while also proving structurally balanced.

5.3.4 Section 4

Each step of the Section 4 restoration is shown in Figure 5.4, with detailed strain measurements in Table 5.5. When proceeding from the north, this is the irst section that does not extend past the Manaia Fault (Figure 1.3). This, as mentioned before, is because the Manaia Fault at this latitude has the same reconstruction issues as the Taranaki Fault in the northern part of the basin. This however, means that the longitudinal strain accommodated by the Manaia Fault in Sections 1-3 is not represented in this section or Sections 4 and 5. The Motumate Fault is present in this cross section, but while the fault is apparent in seismic, there is no slip interpreted across the fault, merely folding (Figure 3.3), which is why the fault is only shown in Figure 5.4 H, and why it is dashed. U_N50 was not unfaulted, only unfolded very sharply when restored to match the dip of the truncations, as shown by the present N50 boundary in Figure 5.4 F. There likely is faulting that occurred simultaneously with folding, but the truncations in the footwall of the fold gave no indication of slip, so all of the N15 horizon unfaulting is performed in Figure 5.4 E, meaning the amount of shortening likely should be lower in E and higher in F, but summed together, the measured amount of strain should be accurate.

66 WNW Section 4 ESE h present

No change in length G ~2 Ma

No change in length F ~7 Ma

0.66% Shortening E ~19 Ma

0.09% Shortening 0.74% Shortening D ~30 Ma

No change in length C 66 Ma

0.02% Extension B ~68 Ma

0.80% Extension 0.82% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.4: Section 4 restoration. This cross section is completely within the Southern Inver- sion Zone, below the Central Graben (Figure 1.2). Here there is inverted offset of the Maari Fault, and the hanging wall of the Maari Fault is the rough western boundary of the late Mio- cene uplift in this end of the basin.

67 Table 5.5: Strain measurement chart for Section 4. Items in red were measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

68 5.3.5 Section 5

Each step of the Section 5 restoration is shown in Figure 5.5, with detailed strain measurements in Table 5.6. This Section, like Section 3, is between to major structures, the Maari Fault and the Wakamarama Fault. With major strain accommodation to the north and south in those structures, this is another section where there is no faulting in the Neogene, just folding. This section is strongly affected by the shortening in the late Miocene expressed through uplift that can be seen in Figure 5.5 F, but with continued compression into the Pliocene resulting in maintained uplift and much thinner Pliocene and Pleistocene packages. The Motumate Fault is also present in this section, but only shown in Figure 5.5 H because no displacement is interpreted across the fault, only folding.

5.3.6 Section 6

Each step of the Section 6 restoration is shown in Figure 5.6, with detailed strain measurements in Table 5.7. This is the only section that intersects the Wakamarama Fault, yet this section does not cross the fault at its maximum offset, meaning the Wakamarama Fault is not adequately represented by this study. Sediments were added above the N50 unconformity as they were for the other sections, but unlike the other sections, faulting at this time is unlikely. In this section the Wakamarama Fault does not offset the U_N50 horizon, but south of the study the horizon extends up to and is truncated by the unconformity, showing that the slip of the fault not passing through U_N50 in this section is only due to the laterally offset position along the fault. This section, like section 5, is missing much of the Pliocene and Pleistocene sequences due to continued compression. This results in a structural high with a lack of accommodation, causing sedimentary bypassing to the modern shelf above the Western Stable Platform, northwest of this study.

5.4 Data Trend Analysis

This section shows the trends of shortening and extension measured in each section, and compares the strain measurements of sections with and without compaction.

69 WNW Section 5 ESE h present

No change in length G ~2 Ma

0.45% Shortening F ~7 Ma

0.06% Shortening E ~19 Ma 0.59% Shortening

0.08% Shortening D ~30 Ma

0.28% Extension C 66 Ma

0.2% Extension B ~68 Ma 1.79% Extension

1.30% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.5: Section 5 restoration. This cross section, similar to Section 3, is between major faults, and thus does not show any inverted offset in the packages above N15, just folding. Like Section 4, the Maari is to be the major boundary for uplift, but in this section, a small amount of uplift is above S.I.F.1

70 Table 5.6: Strain measurement chart for Section 5 Items in red were measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

71 WNW Section 6 ESE h present

G ~2 Ma No change in Length

F ~7 Ma 0.07% Shortening

E ~19 Ma 1.50% Shortening 1.65% Shortening

D ~30 Ma 0.08% Shortening

0.27% Extension C 66 Ma

0.22% Extension B ~68 Ma 1.54% Extension

1.05% Extension A >95 Ma

BpreK96 K96 p00 p50 N15 N50 N82 95 68 66 30 19 7 2 present (Ma) Figure 5.6: Section 6 restoration. In the southern end of the basin, this is the section with the largest amount of Paleocene extension, indicating that the Wakamarama Fault is another sub- basin active during that time. Also, the extend of late Miocene uplift is centered around the Wakamarama Fault rather than ending abruptly at any particular structure, as observed in the sections to the north.

72 Table 5.7: Strain measurement chart for Section 6. Items in red were measured, but not included in calculations because the mea- surement performed is not realistic for that time. The Total change in line length is boxed in bold.

73 5.4.1 Extension

As shown in Figure 5.7, the sum of all extension measured in each section decreases to the south, with the exception of Section 4. When the extension caused by the Manaia Fault is removed, however, the overall trend of extension shown in “Extension without Manaia” is consistent with Section 4, which may imply that Section 4 is affected by the clipping of the Manaia Fault in the southern end of the study more than Sections 5 & 6. The jump in the extension in Sections 5 and 6 compared to 4 is due to variations in the extent of rifting (Figure 5.7). Section 5 has the larger number of rifting associated faults, and in Section 6, the Wakamarama Fault has larger than average offset during that time, whereas Section 4 shows only minimal extension after 68 Ma. The lack of extension in Section 4 after 68 Ma may indicate that

Figure 5.7: Quick comparison of different measurements for each section. Section 1 is the furthest north. The total amount of shortening along the manaia fault for sections 1, 2, and 3, is 2.4 km, 2.0 km, and 3.2 km respectively. The total amount of extension along the fault is 5.5 km, 3.5 km, and, 3.8 km respectively.

74 extension occurred elsewhere in line with the section. The trend of all extension before 66 Ma shows no dramatic trends when the Manaia Fault slip is removed (Figure 5.8 B) because the extension is due to basin-wide rifting. The extent of rifting after 68 Ma, when transgression began, is reduced in the graph, but does not necessarily imply a change in the rate of rifting, because the K96 package represents a shorter period of time than the PreK96 package. Between 66-30 Ma, Sections 1 & 2 show signiicantly higher amounts of extension compared to the rest of

Figure 5.8: A: Visualization of the percent extension and shortening shown between restora- tions in Figure 5.1 - Figure 5.6 B: Same values as the to figure, but calculated to exclude any shortening or extension associated with the Manaia Fault, and with the addition of small scale fault displacements.

75 the basin, even without Manaia Fault slip (Figure 5.8). This is likely the result of isolated sub- basins, such as the graben in the hanging wall of the Manaia Fault in Figure 5.1 and Figure 5.2, which occurred in conjunction with the initiation of the Australian and Paciic Plate spreading to the south (Figure 2.4) as referenced in Chapter 2. The amount of extension accommodated by the Manaia Fault compared to the rest of the basin is dramatic, but it is likely that the extensional strain in the southern part of the basin is largely accommodated by the Kuhurangi Fault to the West, and the Manaia and Waimea-Flaxmore Fault Systems to the East (Figure 1.3). Miocene- present extension is only found in the northern sections, which results in consistently more net extension in the northern sections.

Small scale faulting in this study refers to any faults with slip that is contained within packages above basement (e.g. there are several faults within the North Cape hanging wall which to not encounter or offset the basement in Figure 4.14). These faults predominantly exhibit maximum slip across the U_N50 horizon, extending as shallow as the N82 package and as deep as the P00 horizon. Minor fault slips uniformly decrease within each section when moving south, as shown in Figure 5.7, but also only accommodate a minor proportion of the extensional strain within the basin. The increase in these inter-package extensional faults matches the trend of extension seen in the major faults, both of which are the result of the Pliocene-Present migration of extension from the north through reactivation of the Cape Egmont Fault.

5.4.2 Shortening

The sum of shortening in each of the sections (Figure 5.7) shows a trend of increasing shortening in sections 1-3 that does not continue through sections 4-5. This however is not due to the Manaia Fault inclusion in those three sections, since while it does add a relatively high amount of slip, that slip is relatively consistent across the 3 sections (Manaia shortening for sections 1-3 are 2.4 km, 2.03 km, and 3.21 km respectively). This is why even when the Manaia slip is taken out of the equation in Figure 5.7, the trend is still present. The timing of the compression in the basin, while always between 30 and 2 Ma, does not show any particular sequence other than the continued shortening in the south of the basin (ignoring Section 4) until

76 2 Ma. Sections 1- 3 are all affected by shortening on the Manaia Fault (Figure 5.8), with the large amount of shortening shown in section 3 that is not caused by the Manaia Fault due to the Motumate Fault (Figure 1.3). During this time, an increase in subsidence in the footwall of the Taranaki Fault resulted in reverse displacement of the fault and increased accommodation above the U_P50 horizon. This also occurred along other main faults in the basin, including the Manaia Fault, but not nearly to the same extent. Active overthrusting of the basement along the Eastern boundary of the basin began 21-22 Ma, and continued until the Pliocene, renewing between 13 and 11 Ma with the rotation of the subduction zone to its present orientation that also coincides with the initiation of extension in the north of the basin.

Shortening resulting in the uplifting of the southern sections is only measured when there is clear erosion. This means that uplifting resulting in a lack of accommodation that causes sediment bypass is not appropriately compared to other measured evidence of strain. However, the large amount of uplift resulting in the signiicant erosion of N15 package by U_N50 likely counts for some of the shortening experienced. However, since there is nothing to unfold or unfold after that point in time, lengths remain the same, meaning no measure of strain is available after the horizon is unfolded.

5.4.3 Net Movement

In Figure 5.7 there are no obvious trends in the net change in length (the inal difference of all the extension and all the shortening for a speciic section) across all six sections. Section 1 unsurprisingly experienced the largest amount of extension since it is the section most affected by the Central Graben, but the other sections through the Central Graben do not share this trend. There is also no trend of net shortening shown in the south, possibly related to the absence of Manaia displacement, or possibly due to a sense of rotation.

Figure 5.9 shows four different representations of how the length of each section changes through time. The northern three sections have a higher amount of initial extension, followed by a jump in shortening that is much larger than the southern three sections, which

77 remain compressive for longer, but less intensely. The initial jump in extension in the irst three sections is mostly due to the Manaia Fault, but the large increase in shortening is not, as can be seen in the lower two graphs in Figure 5.9. The lack of dramatic shortening in the south and the increase in shortening to the north is in contrast to the amount of sediment removed by the N50 unconformity in the southern sections and the lack of signiicant sediment thickness removed in the northern sections.

5.5 Decompaction Control Group

As a control on the signiicance of including decompaction in large scale Taranaki Basin reconstructions, Section 2 was reconstructed using two different methodologies. The irst reconstruction removed sediment using the decompaction module in the same worklow used with every other section, allowing subsequent packages to decompact, while in the second

Figure 5.9: These graphs show the relative change in section length over time. The left two graphs show the relationship based on each restoration performed, while the right show the same data, but with appropriate scale for time. The bottom two sections, show the results when the displacement of the Manaia Fault is not included in Sections 1-3 and when the mi- nor faults are included in sections 1-4.

78 Figure 5.10: Change in the strain ratio between each restoration in Section 2 in order to com- pare the difference between using and not using decompaction in structural reconstructions

Figure 5.11: Change in length over time of Section 2 comparing the significance of incorpo- rating decompaction in structural restorations.

79 reconstruction the top package is simply deleted before unfaulting and unfolding each horizon. Figure 5.10 and Figure 5.11 show the comparison between the Using and not using decompaction, with Figure 5.10 comparing the difference in the strain percentage between the two methods. The difference in percentage of any speciic step is never more than 0.31%, and the cumulative shortening measured when using decompaction is 4.87 km compared to 4.37 km without and cumulative extension is 6.34 and 6.41 respectfully. So while individual sections have varying rates, Figure 5.11 shows that in a sequence of extension or shortening, the measurements in this section eventually balance. The overall net strain measurements remain relatively constant between larger episodes, yet the rates of strain measured are affected by decompaction. This implies that for this section at least, an overall study to ind the net displacement might not be drastically affected by simplifying the reconstruction worklow, but any restorations aiming to identify the timing of a structure should incorporate decompaction.

80 ChApTER 6 DISCUSSION

6.1 Review

This study uses interpreted 2D and 3D seismic data to construct and then restore six diagnostic cross sections within the Southern Taranaki Basin (Figure 1.3), with the goal of representing the latitudinal variation in strain across the basin. The restorations measure longitudinal strain from both faulting and folding, which is critical for strain analysis of this region. The Northern Graben of the Taranaki Basin (Figure 1.2) is characterized by normal faulting, and shows many characteristics associated with rift basins (King & Thrasher, 1996). Because there is only extension in the Northern Graben, the extent of strain can be accurately measured through fault analysis (Giba et al., 2010; Reilly et al., 2015). However, the inclusion of folding in strain estimates is critical in the southern regions of the Taranaki Basin, particularly between the major inverted faults. The extent of shortening caused by folding between the Wakamarama and Maari Faults in Section 5 (Figure 5.5) is nearly as much as the amount of shortening across the Maari Fault. This pattern of folding accommodating strain between major faults is also observed between the Maari and the Cape Egmont Fault in Section 3 (Figure 5.3). The restorations of this study also restore Cretaceous syn-rift geometries in the Southern Taranaki Basin.

6.2 Plate Margin Signiicance The presently inactive Taranaki Fault is the eastern bounding structure of the Taranaki Basin and the boundary for the northern half of this study (Figure 1.3). On the scale of the plate margin, the Taranaki Fault System, which also includes the Manaia Fault, is a subduction related back thrust for the Hikurangi margin, between the Paciic and Australian Plates (Figure 6.1). The Taranaki Fault was primarily active during the initiation of subduction along the Hikurangi Trough, when the margin was roughly parallel to the present-day Taranaki Fault orientation (Stagpoole & Nicole, 2008). Episodic southward retreat of the fault is tied to the rotation of the Hikurangi Trough and coincides with the extension and compression in the north and south of

81 Figure 6.1: This figure shows the entire plate margin at present-day, with cross sections showing how the plate margin changes at varying latitudes. Cross section B is north of this study’s boundary and cross section C is south of this study’s boundary. Taken from Stagpoole & Nicol, (2008) the Taranaki Basin respectively (Stagpoole & Nicole, 2008). This study quantiies the amount of compression during this transition, to better compare the southern compression to studies providing detailed strain analysis of the northern extension within the last 20 m.y. (Giba et al., 2010; Reilly et al., 2015)

The simultaneous extension and compression in the north and south of the Taranaki Basin since late Miocene is often described using block rotation models (e.g. Giba et al., (2010); King & Thrasher, (1996); Wallace et al., (2004)). Measurements of present-day crustal motion show

82 extension in the Central Graben, and shortening south of the study area, both on and offshore. Simpliied as its own crustal block in models, the Southern Taranaki Basin requires slight clockwise rotation in relation to the Western Stable Platform to it present crustal movement vectors (Wallace et al., 2004) (Figure 6.2). To account for the migration of compression and shortening south along the Taranaki Basin between ~17 Ma and present, Giba et al., (2010) propose a block rotation model with a southward moving rotation axis that coincides with the opening of the North Graben (Figure 6.3). However, for the model to it perfectly, there should be incrementally more shortening further south in each section, and this trend is only observed in

Figure 6.2: Present-day crustal motion vectors above the Hikurangi plate margin and North Island of New Zealand. Each color represents a block in the model that contains vectors of similar character. The pole of rotation for each crustal block is shown with a rate of rotation in the same color as the block it represents. The Southern Taranaki Basin lies within the green block. Taken from Wallace et al., (2004)

83 Figure 6.3: Schematic block rotation model with a southward migrating of the pole of rota- tion. This model is designed to explain the late Miocene-Recent structural evolution of the Northern Graben (Figure 1.2). “The eastern crustal block rotates clockwise relative to a fixed wester stable platform. The boundary between areas dominated by extension and shortening is indicated by the dashed lines.” Taken from Giba et al., (2010). sections 1-3 and not in sections 4-6. There is a deiciency of shortening in Section 5 and a reduced amount of shortening from Section 3 to Section 4, even when the Manaia Fault slip is not included in Section 3, breaking the trend of increasing compression south of the proposed rotation axis. The suggested pole of rotation for the Southern Taranaki Basin at present is roughly at the western end of sections 4 & 5 between 39°S, and 40°S (Figure 1.2), itting the measurements of this study showing no strain in those sections (or section 6) at present (Figure 5.10). However, the anomalously low strain in sections 4 & 5 throughout the history of the basin does not it the model if the pole of rotation migrated to this point from the north. The lack of strain in these sections suggests that this latitude may represent a transition zone between separate strain domains in the north and south of the Southern Taranaki Basin throughout the development of the basin, not just at present as a rotation axis. To more accurately portray the

84 heterogeneous strain measurements observed in this study, further division of the basin into more than one crustal block in models during early development of the subduction margin may be necessary. To accurately detail this strain transition through structural restorations, a smaller spacing between cross sections is needed. Otherwise, the measured strain amounts can it the migrating block model if the increasing strain that is absent in sections with less shortening occurs along major faults outside of this study area. For example, if the Manaia Fault has more offset south of the study boundary, and as the Manaia Fault dies out further south, the strain is accommodated by the Wakamarama, Kahurangi, and the presently active Waimea-Flaxmore Fault (Figure 1.3). An increase in Manaia Fault shortening to the south is possible given the increase in basement offset to the south, and the slight increase is the Manaia Fault shortening in Section 3 compared to sections 1 & 2, as described in the Figure 5.7 caption.

The folding in the Southern Taranaki Basin is the result of three different processes; subsidence, lexure, and ductile thickening, each occurring on different wavelengths (Holt et al., 1994). In this study, the uplift that appears in the restorations (which is unfolded to measure strain) is the consequence of lexure of the crust caused by loading, on the scale of 100-200 km, rather than subsidence (on the scale of >500 km) and ductile thickening of the crust (on the scale of 10-30 km) (Holt et al., 1994), though all three scales are accounted for in the restorations.

6.3 Petroleum Signiicance The Taranaki Basin is the only producing petroleum basin in New Zealand (MBIE, 2015). Coaly Cretaceous rocks are believed to be the source rock for the majority of oil found in the basin (MBIE, 2015). Multiple play types are present, with standard play types including half graben rift structures with Cretaceous reservoirs and folds in inverted faults with Paleogene and Neogene reservoirs. Less standard play types include plays in the Northern Graben where heat from volcanism has anomalously matured relatively shallow petroleum systems (MBIE, 2015). In the Southern Taranaki Basin, due to the success of early wells drilled into inversion structures, wildcat wells have drilled all the major inversion structures and four-way dip closures (i.e. domal

85 geometries), but not all of these wells were successful. However, there are other plays in the basin that have either not been proven yet or have presently been less productive (MBIE, 2015). Out of the play types present in the Southern Taranaki Basin, the syn-rift Cretaceous ill in half- grabens is the only play type that may be better conined by this study. Cretaceous source rocks are coal measures within the Rakopi and North Cape Formations (Figure 2.2) that can be up to 500 m thick (King & Thrasher, 1996), yet the sands within these formations have demonstrated reservoir quality in several wells.t

The interpreted extent of Cretaceous formations from this study differs from previous interpretations, primarily due to a low amount of well control causing many interpretations to rely solely on seismic geometry. Out of the nine wells that encounter Cretaceous sediment in the study area, six have available well reports. Out of those six, Pukeko-1 had shows of gas and oil (i.e. the appearance of oil or gas in cuttings or cores) and Kupe South-4 had gas shows (Figure 6.4). The other four wells were dry. Several other wells are located above Cretaceous sediments, but are too shallow to encounter them (e.g. Maari-1, Te Whatu-2, etc).

The difference in the interpretation of Cretaceous sediments away from wells in this study (Figure 6.5) is due to deeper interpretations of basement, which results in a greater extent of Cretaceous sediments in areas with onlapping geometries, and thicker packages of Cretaceous sediment when the sediments are bounded by fault geometries. These differing basement interpretations are determined based on relection geometries, as mentioned in Chapter 4.

This study did not look into the maturation of hydrocarbons. However, more accurate documentation of the extent and timing of uplift in the southern half of this study area could improve the maturation curve reliability of any potential prospects.

From the data available, no new prospects can be conidently suggested, only that potential remains in the Wakamarama footwall region (Figure 4.8). This is suggested because the structures directly south of the Maari Fault are not particularly well imaged and the thickness of source rocks in that area may be thicker if the interpretations provided in this study are accurate.

86 Cretaceous play fairways

174°E 175°E

S°83

S°93

S°93

Boundary of maps in this study Kupe South-4 Pukeko-1

S°04

S°04 Study Boundary

172°E 173°E

Map data source: Strogen (2011) Well with interval present

Rakopi Fm plays North Cape Fm plays

Figure 6.4: This map shows one example of published interpretations for the extent of Cre- taceous sediments in the Taranaki Basin (the third color, which is not shown in legend means both formations are present). The boundary of this study and major faults of this study are projected onto the map. Modified from (MBIE, 2015)

K96 Horizon Taranaki Peninsula P00 Horizon Taranaki Peninsula (Extent of Rakopi Fm) (Extent of North Cape Fm)

Whitiki Fault

Onlapping Horizon Onlap 4000 ms Motumate Fault Onlap Motumate Fault Cape Egmont Fault Cape Egmont Fault

Whitiki Fault Edge of Interpretation

3000 ms Manaia Fault Manaia Fault 3000 ms

3000 ms

Onlapping Horizon Onlapping Horizon Maari Fault Maari Fault

3000 ms Truncated Horizon

S.I.F.1 2000 ms

S.I.F.1

Fault S.I.F.2 Wakamarama Fault S.I.F.2 Wakamarama Basement Basement

A 0 50 km B 0 50 km Contour interval 200 milliseconds Contour interval 200 milliseconds Figure 6.5: Scaled-down surface maps of the K96 and P00 horizons for quick comparison to Figure 6.4. A: Figure 4.9, showing the K96 horizon which represents the extent of the Rakopi Fm. B: Figure 4.8, showing the P00 horizon which represent the extent of the North Cape Fm. In the hanging wall of the Manaia Fault the erosional surface is where the U_P50 uncon- formity truncates the P00 horizon, which represents the top of the North Cape Fm, yet the formation is still present below the unconformity even though no color is shown..

87 However, only further tests and models can determine if this area proves productive.

6.4 Resolution of Smaller Faults

The fault map showing the temporal changes in faulting within the basin (Figure 2.3) shows a much higher resolution of faulting than the surface maps of this study shown in Chapter 4. This illustrates one difference between the scale of the two studies. In Reilly et al., (2015), each fault’s slip is measured, where in this study, sections focus on large scale faults, only measuring the offset of horizons. Therefore, Reilly et al., (2015) have a much more quantitative analysis of individual faults, but this study focuses on the geometries of the horizons and the overall strain experienced by each horizon across the basin, more accurately detailing the total strain in a particular section. The difference in fault density between the interpretation (Figure 4.2) and restoration steps (Figure 5.2) of this study are due to the placement and offset of the faults. When the imported faults are only within a package or did not offset an intersecting horizon, they were removed from the 2D section before restorations took place. This is why there are no small scale faults in Figure 5.6, despite the faults shown in the 6 Ma snapshot of Figure 2.3.

6.5 Signiicance of Folding in Strain Measurements Reilly et al., (2015) cover the entire Taranaki Basin, including this study area and describe the timing, distribution, and strain of the faults across the basin. Figure 6.6 is comparable to Figure 5.8, and shows the amount of extension or shortening experienced by faults for four different time intervals across the Taranaki Basin, and covers more than four times the latitudinal extent of this study (all sections in this study lie roughly between A and B in Figure 6.6. Sections A and B from Reilly et al., [2015] are shown in Figure 6.7, and the position of the sections in the basin is shown in Figure 6.8). Both igures Figure 6.6 and Figure 5.8 show the net strain within the basin. However, Figure 6.6 does not include any strain accommodated from folding, which results in a less dramatic amount of shortening. Section A (Figure 6.7) crosses both Section 1 and Section 2 from this Study, so their strain measurements are the most

88 B A

Figure 6.6: This chart shows the amount of shortening or extension experienced across the basin through faulting. Each color represents the amount of strain for a particular time inter- val, with the red line representing the cumulative strain across the life of the basin accommo- dated through faulting, the B cross section is on the left border of the figure and the A cross section is the vertical red line near the center of the figure. Taken from Reilly et al., (2015). A A’

B B’

Figure 6.7: These two cross sections follow seismic lines as shown in Figure 6.8. Section A is located around Section 1 & 2 from this study, and Section B is located south of Section 6 in this study the seismic. Taken from Reilly et al., (2015).

89 1500

¹ Figure 6.9

200 ¹

1000 ¹ WESTERN PLATFORM

¹ ¹ ¹

Modern Shelf Edge

¹

Cape Egmont Fault ¹ A ¹

Figure 6.7 ¹

Whitiki ¹

¹

Fault ¹

¹ A’ ¹

¹

¹

¹ ¹ B Figure 6.11¹ Study ¹

Taranaki Fault

¹

¹ ¹

Boundary ¹ Manai Fault ¹ ¹

200

¹ ¹

¹ B’ 0 50 km ¹

¹ ¹ ¹ Figure 6.8: Location of figures referred to in Chapter 6. Fault placement and bathymetry taken from MBIE, (2014) appropriate to compare to Figure 6.6 (despite the omission of the Taranaki Fault in this study). For Sections 1 & 2 in this study, the net shortening is 6.9 km and 1.47 km respectively, while the net shortening across the faults in Section A is less than 0.25 km. This difference demonstrates the impact of folding to strain measurements in this region of the Taranaki Basin. Figure 6.6 demonstrates that the trend of net change in length seen in Sections 1-3 (Figure 5.7) should likely continue, with Sections 4-6 possibly experiencing even more net shortening than Section 3 if entire span of the basin was incorporated.

6.5.1 Roncaglia Restoration

Figure 6.9 shows the reconstruction from Roncaglia et al., (2010). The placement of this section is shown in Figure 6.8 and follows 2D seismic lines, crossing the Taranaki, Manaia, Cape

90 NW SE

Figure 6.9: Restoration across the Taranaki Basin extending far enough to the northwest to show the entire modern shelf of the basin. The restoration begins at the initiation of post-rift sedimentation, Taken from Roncaglia et al., (2010)

91 Egmont, and Whitiki Faults as well as the Western Stable Platform perpendicular to the modern shelf edge (Figure 6.8). The interpretations of Roncaglia et al., (2010), shown in Figure 6.9, include much more of the original basement geometry northwest of this study area, though the basement fault geometry between the Manaia and Cape Egmont faults varies from this study. Roncaglia et al., (2010) also subdivide the basin stratigraphy into 14 different packages instead of 7, but have very similar methodology to this study, including the use of compaction in their restoration. The primary difference between Figure 6.9 and the restorations in this study is the inversion of the Cape Egmont Fault between 5 and 3 Ma, which is a time of extension in this study (Figure 5.1 & Figure 5.8). Also, Roncaglia et al., (2010) do not include quantitative strain measurements between different steps.

Fig. 7. Schematic model to true scale of development of the Taranaki Fault Zone, constrained by its structure and the stratigraphy Figure 6.10: True-scalenorthern King Country model depocentres. showing Note how a thenew Taranaki development Fault Zone develops of at c. the29 Ma, Taranaki the Manganui Fault Fault. develops For refer- ence, in the STB, the U_P50 horizon represents an unconformity between ~34-30 Ma and the N15 horizon represents ~19 Ma. Taken from Tipathi & Kamp, 2008.

92 6.5.2 Taranaki Fault Development

The interpretation of the Taranaki Fault in this study was not performed in detail, and eventually cropped from the study area because 1) the seismic below the overthrown basement is very poor quality 2) restoring the fault without any offset constraints would add more variability than constraint to the restorations, and 3) no ield data were collected, which is what many of the studies detailing the Taranaki Fault history require. Tipathi & Kamp (2008) deine the evolution of the Taranaki Fault and created a model displaying the fault’s evolution over 17 m.y., beginning in the late Eocene (Figure 6.10). The initial reverse movement of the Taranaki Fault is identiied at 29 Ma and is determined “from the relative elevation of the top of basement (on) either side of the fault between the Late Cretaceous and Early Oligocene,” suggesting that, unlike the rest of reverse faults in the Southern Taranaki Basin, the Taranaki Fault originally dipped the opposite direction, to the west, with new fault traces occurring with the onset of overthrusting. This model shows minimal horizontal displacement during the initiation of the fault, and only a few kilometers of displacement throughout the Oligocene. While the net displacement of the Taranaki Fault in uncertain, present-day seismic data show the fault overthrusting up to 10 km over sedimentary strata, with up to 6 km of vertical basement offset (King & Thrasher, 1996). Stagpoole & Nicole, (2008) suggest a displacement estimate of 12–15 km.

6.5.3 Decompaction

Taylor et al., (2008) state “...(when comparing compaction models of growth faults to displacement data from the Cape Egmont Fault) associated losses in displacement on growth faults are typically <20%.“ As explained in Chapter 5.5, the effect of decompaction on the scale of this study is signiicantly less (at most a 0.31% difference in extension across the section). Furthermore, while this study used decompaction when restoring each horizon and studies such as Reilly et al., (2014) did not, a higher number of horizons is key, and results in a more accurate timing of strain along a fault. The difference between the restoration with and without using decompaction is small enough that both methodologies result in strain estimates close to previous estimates across the basin, e.g. Bishop & Buchanan, (1995). While the total amount of

93 shortening and extension were similar with and without decompaction, intermittent steps showed differences in strain that will affect the timing of development for major structures in the section (Figure 5.11). Decompaction also affects the extent of inversion in reactivated inverted faults, such as the Cape Egmont Fault. The hanging wall thicknesses of this fault is more strongly effected by decompaction since there is signiicantly more overburden above the hanging wall than the footwall. This results in a reduction of both normal and reverse offset compared to un- decompacted restorations.

6.6 Interpretation Discrepancies

The location of section B (Figure 6.7) compared to this study’s section 6 (Figure 4.18) is shown in Figure 6.8. The difference between the interpreted basement depths in the footwall of the Wakamarama Fault has a dramatic effect on the thickness of overlying Cretaceous sediments. The basis for this interpretation discrepancy is shown in Figure 6.11. The primary difference in interpretation is the depth to basement. This area is not close to any well control, so the relection geometry of the seismic is the basis for this study’s interpretation. Intersecting

A BC

Figure 6.11: Two interpretations of the Wakamarama Fault at 5x vertical exaggeration with the black arrow in each representing the thickness of sediment interpreted between base- ment and the U_N50 Horizon and. Strogen et al., (2014b) Transect 1 (A), this study (C), and uninterpreted seismic (B) for comparison. The difference between the two interpretations is mainly the footwall thickness of packages below P50 (blue). The brown horizon (base of brown polygon) shown on the left is the pink horizon on the right. Location of the seismic is shown in Figure 6.8

94 relection geometries are interpreted below the published basement, and basement in this study is interpreted below these geometries.

6.7 Further Compartmentalization of Study packages

The Neogene sediments could be more closely analyzed and compartmentalized since the largest amount of uncertainty in this study stems from the N50 unconformity (Figure 2.2), the thick package below it, and the sediment eroded from above the horizon. Late Miocene uplift in the Southern Inversion Zone resulted in the truncation of some sediments and condensed sections of sediment on newly created structural highs. Identifying the difference between areas with missing sediments and those where sediments were never deposited would increase the strain estimate accuracy over this time period. The uncertainty associated with restoring this horizon also affects the slip of the faults when the original package thickness varies across the structure. Dividing the package between N15 and U_N50 (Figure 2.2) further, would help signiicantly with the timing of the major thrust faults that terminate within this package(Wakamarama, Manaia, Maari, etc), as well as the younger normal faults that are entirely contained within this package in this study.

The package bounded by the P50 and N15 Horizons represents a period of time from ~35 Ma to ~19 Ma (Figure 2.2), with several depositional environments resulting in a diverse range of lithologies. In this study, the package is generalized to 36% sandstone, 39% shale, and 25% limestone (Table 5.1) for decompaction purposes. This ratio, like the rest of the packages, is determined from the average lithology proportions of several wells, but this package is the only one with drastically different proportions from one well to the next. Figure 6.12 shows the evolution of the depositional environments that make up the U_P50 package, detailing the lateral changes in lithology and the structural constraints that caused the changes in depositional environment. This is relevant because structural restorations are useful when modeling petroleum ields, and the U_P50 package contains an important reservoir, the Tikorangi limestone (Figure 2.2). Yet while the restoration shows the evolution of structures

95 E F

Figure 6.12: A) Transgression resulting in the deposition of shelf muds across the Taranaki basin before P50. B) Uplift and peneplanation across the basin results causes the P50 uncon- formity. C) Continued transgression combined with the renewal of basin subsidence in the footwall of the Taranaki Basin, likely due to an increase in plate convergence. D) Foredeep development causing subsidence adjacent to the major faults bounding the eastern edge of the basin, with rates of deposition that matched rates of subsidence. E) Maximum transgres- sion, with submergence of the islands reducing clastic source resulting in carbonate develop- ment. F) Increase in fault displacement, particularly with the Manaia Fault, combined with regression resulting in marls, with the end of this sequence, the rapid accumulation of clastic material, marking the Top of the Taimana Formation which is the N15 horizon. Modified from Strogen et al., (2014a) The shape of proto-New Zealand during this time can be seen in Figure 2.4 (b) & (c)

96 containing this reservoir package over time, it does not take into account the lateral extent of the Tikorangi formation or the various submarine fans within this package (formed in proximity to the developing structures on the basin margins and shown in Figure 6.12) that are also reservoir quality. While further identiication of this package’s lithological changes may not be entirely practical in the context of structural restorations, when restorations are performed, an awareness of the heterogeneities within a package is crucial for the restorations to prove useful in understanding reservoir development.

6.8 Signiicance of Sub-Seismic Faulting In the Viking Graben, North Sea, discrepancies are observed when comparing the extension calculated using crustal models or area balancing to restorations that use faults interpreted through seismic (Marret & Allmendinger, 1992). 25%-60% less extension is measured through the restorations, meaning actual extension of the basin could be up to 150% greater than the estimate created from restoring structural cross sections. Marret & Allmendinger (1992) suggest that the difference between methods is due to the accumulation of slip along small faults below seismic resolution. The proportion of faults can be quantitatively estimated by comparing the fault population in wells to the population recorded in seismic, but in this study, no such calculations are made. There is possibly an even larger difference in actual extension in this study, since only horizon offset is recorded, meaning any slip within a package was not included in the calculations. In periods of compression, unfolding may account for some sub-seismic strain accommodation, but in times of extension, unfolding will only reduce the calculated amount of extension further.

6.9 Signiicance of Section Placement To accurately compare strain across the basin from each section in the study to the others, it is important to include the same structures in each section when possible. The signiicantly lower amounts of strain expressed in Sections 4-6 could be due to rotation of the basin, but this is hard to validate since it could also be due to the exclusion of major bounding faults in the

97 southern sections. The exclusion of structures within a study should be consistent if decided necessary. Sections 1-3 show signiicantly more strain through the life of the basin than Sections 4-6, which seems tied to the inclusion of the Cape Egmont and Manaia Faults. Sections 4 & 5 both stop before the Manaia Fault, and section 6 excludes major faults that are not included in any other section (Kahurangi and Waimea-Flaxmore), but align with trends of strain that are in- line with signiicant faults in the northern Southern Taranaki Basin (i.e. The Waimea-Flaxmore Fault with the Manaia Fault and the Kahurangi Fault possibly with the Cape Egmont Fault).

98 ChApTER 7 CONCLUSIONS

7.1 Summary

1) The inclusion of folding is critical in any strain analysis within the Southern Taranaki Basin, particularly between the major inverted faults of the basin.

2) The anomalously low strain measurements in section 4 and 5 throughout the history of the basin suggests a consistent transition zone between separate strain domains in the area presently between 39°S, and 40°S. In a regional sense, this suggests that the Southern Taranaki Basin is more heterogeneous than previously assumed, i.e. not just a simple transition from extension in the north to compression in the south. However, on the scale of plate margin cross sections, this simpliication is still valid.

3) Deeper interpretations of basement in this study result in a differing extent of syn- rift potential source-rock formations within the Southern Taranaki Basin. This may have implications for Cretaceous petroleum leads, particularly in the Wakamarama footwall where there is poor seismic survey density and well control.

4) Using decompaction in structural cross sections of this scale has a very small effect on longitudinal strain (with a present day section length of 101.25 km, the restoration that does not involve decompaction results in only 0.21% more net extension, with inal lengths of 99.78 km with decompaction, 99.57 km without). However, not including decompaction in restorations affects the timing of a major faults that are active multiple restoration steps.

5) Decompaction affects the extent of inversion in reactivated inverted faults. In the Cape Egmont Fault, hanging wall thicknesses are impacted more by decompaction since there is signiicantly more overburden above the hanging wall than the footwall. This results in a reduction of both normal and reverse offset compared to un-decompacted restorations.

6) Nearly all compressional faults in the study are inverted basement-involved faults (with exceptions such as the southern end of the Motumate Fault intersecting Sections 4 & 5

99 [Figure 1.3]), though most basement faults did not invert. The major extensional faults are reactivated, but fault B in Figure 4.14 shows that discernible offset before the Miocene is not required for younger faults to offset basement in the Central Graben.

7) Inversion along faults occurs between 30-7 Ma for all sections, but while shortening continued in the southern extent of the Southern Taranaki Basin until 2 Ma, the shortening in that time is accommodated solely through folding and continued uplift, instead of faulting.

7.2 Summary of Southern Taranaki Basin Strain

1) The total amount of extension across all six sections ranges between 12.0%-1%, with an average of 5.5%. The wide range is due to large amounts of late Miocene-present extension in the north and sections in the middle of the basin with relatively low amounts of rifting compared to the other sections.

2) The total amount of shortening ranges between 6.8%-0.8%, with an average of 3.1%. This range is due to large amounts of inversion and folding over multiple structures in some sections, with others accommodating compressional strain in isolated locations resulting in a large structure without signiicant shortening in the rest of the section (e.g. Section 4 shown in Figure 5.4).

3) The range of net change in overall length is between 7.8% extension and 2.7% shortening, with an average of 1.3% extension. In the northern part of the Southern Taranaki Basin, the amount of Miocene-recent extension is much more signiicant than the earlier shortening. Even in the southern sections with little or no recent extension, the extent of rift geometries and the concentration of compressional strain on speciic inverted faults (rather than across the whole section), results in most sections experiencing more overall extension than shortening.

100 7.3 Future Work

Some of the discontinuities between different compressional and extensional trends may be due to the positions of the chosen sections, particularly in the southern three sections. Sections 3 and 5 are positioned to ill the gap between sections that intersect the primary offset of major faults, and thus Sections 3 & 5 may not properly represent the entire strain expressed across the basin. Furthermore, if the goal is to accurately describe the transition between the largest structures in the basin, a reduced spacing of sections is needed, since there are unexplained jumps in the measurements between sections at this study’s spacing. A lower spacing may also improve crustal block models for the early basin history. If a transition between different strain domains proves to be realistic, then further division of the Southern Taranaki Basin into multiple crustal blocks may provide a better constraint for crustal reconstructions of the basin and surrounding areas.

Sections 4-6 are not far enough south to intersect the maximum slip of the large faults below the study boundary, and Section 6 terminates before encountering the Kahurangi and Waimea-Flaxmore Faults to avoid that problem, and because the seismic quality was low. If similar studies to this are performed in the future, including more southern sections could help to better characterize faults south of this study, but it is critical that sections extend as far as possible within the structurally relevant area, despite any lack of survey coverage or data quality. This will help constrain the extent of strain in the basin without having to make assumptions for the structures that are excluded.

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