Salt-Influenced Normal Faulting Related to Salt-Dissolution And

Home , Geology of the North Sea, Salt deformation, Salt dome, Salt tectonics

SALT-INFLUENCED NORMAL FAULTING RELATED TO SALT-DISSOLUTION AND

EXTENSIONAL TECTONICS: 3D SEISMIC ANALYSIS AND 2D NUMERICAL

MODELING OF THE SALT VALLEY SALT WALL, UTAH

AND DANISH CENTRAL GRABEN, NORTH SEA

by Mohammad Naqi

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

Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy

(Geology).

Golden, Colorado

Date ______

Signed: ______Mohammad Naqi

Signed: ______Dr. Bruce Trudgill Thesis Advisor

Golden, Colorado

Date ______

Signed: ______Dr. M. Stephen Enders Professor and Department Head Department of Geology and Geological Engineering

ABSTRACT

The northeastern Paradox basin is characterized by a series of salt cored anticlines (salt walls) trending NW-SE. The crestal areas of the salt cored anticlines are breached and cut by faults, forming downthrown valleys, created by the subsidence of the crestal overburden. There has been a prolonged debate on the causative mechanism of subsidence of the anticline crests.

Some researchers, based on field observations favor salt-dissolution of the salt forming the core of the anticlines as the main mechanism for the crestal subsidence. Others based on physical modeling favor extensional tectonics as the main mechanism that triggered subsidence.

A wider variety of salt structural styles are present in the Danish Salt Dome Province of the Danish Central Graben, ranging from salt diapirs that penetrate their overlying sedimentary cover, to gentle, non-penetrating salt-cored anticlines. Salt structures present as pillows (e.g. the

Kraka salt pillow), diapirs (e.g. Skjold salt structure) and wall-and-sill complexes (e.g. Dan salt structure). The mechanism for the development of supra-salt faults above salt structures in the

Danish Central Graben, North Sea is related to pure extensional tectonics that took place in Late

Jurassic.

A 3D seismic data volume covering the northern end of Salt Valley Salt Wall in the northeastern part of the Paradox Basin presents a unique opportunity to investigate and provide a new perspective on the origin of collapse structures. In addition, a 3D seismic covering the

Danish Central Graben was utilized to analyze the supra-salt faults. The analysis of the supra- faults in the Danish Salt Province was performed in order to compare/contrast with supra-salt faults in the Paradox Basin.

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Interpretation of the Salt Valley seismic data shows a graben system that formed in the crestal area of the salt anticline. The graben system has a maximum displacement in the NW and decreases to the SE. The maximum displacement occurs where the salt crest is at deeper level and the minimum displacement, where the salt crest is higher. The data suggest that displacement on the graben system might be attributed to salt migration within the salt wall from where the overburden is thickest (higher head pressure) toward the SE of the salt wall where available seismic data suggest the overburden is thinnest. However, extensional fractures might also help in increasing the groundwater circulations in the areas where the salt anticline is closer to the surface. This might have enhanced salt dissolution in those areas triggering salt to flow from deeper parts along the strike of the salt wall. Several 2D numerical forward modeling was performed to test the hypothesis of salt-dissolution/withdrawal and extensional tectonics.

This study suggests that the mechanism for the formation of the salt valleys might be attributed to extensional forces, salt dissolution and internal salt internal withdrawal rather that favoring a single mechanism for the formation of the salt valleys in the northeastern Paradox

Basin. Also, fault analysis results obtained in this study reveal a slight distinction between the salt-dissolution normal faults and tectonic extensional normal faults. The dmax/L ratio of the salt- dissolution normal faults are slightly higher than the expected values based on the global fault database.

TABLE OF CONTENTS

ABSTRACT……………………………………………………………………………………...iii

LIST OF FIGURES……………………………………………………………………….…….viii

LIST OF TABLES…………………………………………………………………………..…..xvi

ACKNOWLEDGMENTS……………………………………………………..……………….xvii

CHAPTER 1 INTRODUCTION ...... 1

1.1 BACKGROUND ...... 2

1.2 PROBLEM AND OBJECTIVES ...... 5

CHAPTER 2 BACKGROUND ...... 7

2.1 GEOLOGICAL SETTING OF THE PARADOX BASIN ...... 8 2.1.1 Colorado Plateau ...... 8 2.1.2 Ancestral Rocky Mountains Evolution ...... 10 2.1.3 Uncompahgre Uplift and Paradox Basin ...... 13 2.1.4 Stratigraphic Framework of Paradox Basin ...... 15 2.1.5 Salt Structures ...... 16 2.1.6 Fault systems in the Paradox Basin ...... 18 2.1.7 Dissolution and subsidence ...... 20

2.2 NORTH SEA GEOLOGICAL SETTING...... 25 2.2.1 Evolution of the Permian (Zechstein) Basin ...... 25 2.2.2 Central Graben Rift System ...... 27 2.2.3 Danish Central Graben and Danish Salt Dome Province ...... 30 2.2.4 Salt Structures ...... 32 2.2.5 Regional Stratigraphy of the Danish Central Graben ...... 34 2.2.6 Fault Systems in The Danish Salt Dome Province ...... 36

CHAPTER 3 DATA AND METHODS ...... 39

3.1 DATA ...... 39 3.1.1 Salt Valley Dataset ...... 39 3.1.2 Salt Dome Province Dataset ...... 43

3.2 METHODOLOGY ...... 43 3.2.1 Seismic Interpretation ...... 46 3.2.2 Danish Central Graben ...... 48 3.2.3 Seismic Attributes ...... 49 3.2.4 Isochron and Time Structure Maps ...... 50 3.2.5 Throw-Distance Analysis...... 50 3.2.6 Finite Element Method ...... 50

CHAPTER 4 RESULTS ...... 59

4.1 NORTH END OF SALT VALLEY SALT WALL, UTAH ...... 59 4.1.1 Salt Geometry ...... 59 4.1.2 Stratal Geometry ...... 63 4.1.3 Fault Systems Analysis ...... 64

4.2 FORWARD NUMERICAL MODELING ...... 83

4.3 DANISH CENTRAL GRABEN ...... 102 4.3.1 Distribution and Geometry of the Zechstein Salt ...... 102 4.3.2 Stratal Geometry ...... 106 4.3.3 Fault System Analysis...... 107

CHAPTER 5 DISCUSSION ...... 119

5.1 SALT-DISSOLUTION IN THE PARADOX BASIN ...... 119

5.2 HOW SALT-DISSOLUTION AFFECTS FAULT DISPLACEMENT ...... 121

5.3 EVOLUTION OF CRESTAL NORMAL FAULTS IN NORTHERN-END OF SALT

VALLEY SALT WALL ...... 123

5.4 TECTONIC VERSUS SALT-DISSOLUTION FAULTS ...... 124 5.4.1 2D Numerical Modeling ...... 124

5.5 FAULT SCALING OF DISSOLUTION AND TECTONIC INDUCED FAULTS ...... 127

5.6 SALT-DISSOLUTION ACCOMMODATION AND THE POTENTIAL OF

RESERVOIR DEVELOPMENT ...... 128

5.7 DISSOLUTION/WITHDRAWAL FAULT SYSTEMS AND TRAPPING GEOMETRIES .. 132

CHAPTER 6 SUMMARY AND CONCLUSIONS ...... 134

REFERENCES ...... 136

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LIST OF FIGURES

Figure 1.1 Geographic distribution of salt anticlines and salt valleys in the Paradox fold and fault belt of the Colorado Plateau (Utah and Colorado) (after Gutiérrez, 2004) ...... 3

Figure 2.1 Map of the Colorado Plateau (black outline) and the upper Colorado River drainage basin (gray outline) with its major bedrock terrains (after Bursztyn et al, 2015)...... 9

Figure 2.2 Paleogeographic map of southwestern North America (United States) showing the Pennsylvanian–Permian Ancestral Rocky Mountains uplifts and basins (after Soreghan et al. (2012))...... 11

Figure 2.3 Subsurface profile across the Uncompahgre front in Utah, showing Permian onlap onto Precambrian basement (modified from Moore et al., 2008). The Uncompahgre Uplift (to the right) was the source for the thick Cutler Formation. SL—sea level (after Soreghan et al. (2012))...... 12

Figure 2.4 Regional map of the Paradox Basin and associated Uncompahgre and San Luis uplifts, showing the location of the salt wall structures the Paradox Fold and Fault Belt, areal limit of salt tectonics and the depositional limit of evaporite facies (after Trudgill, 2011)...... 14

Figure 2.5 Structural cross section along the SE of Paradox Basin showing the various types of salt structures. In the minibasins salt is welded. Notice the association between the location of the salt structures and the sub-salt faults. The cross section also shows the salt namakiers on top of Pine Ridge diapir (after Rasmussen, 2014)...... 21

Figure 2.6 Tectono-stratigraphic chart of the Northern Paradox Basin. Eight key horizons were interpreted and used in this study. Regional and salt tectonic events are marked...... 22

Figure 2.7 Maps of the contiguous United States. Above; evaporite basins and districts. Below; distribution of gypsum/anhydrite and salt deposits, and areas of evaporite karst and subsidence features (after Johnson (2005))...... 24

Figure 2.8 Sketch map of Permian sedimentary basins in northwest Europe showing the maximum extent of the Northern and Southern Permian basins (after Evans et al., 2003)...... 26

Figure 2.9 Stratigraphic framework, showing the ages and representative lithologies of the formations present in the Danish Central Graben, along with the major tectonic events and megasequence boundaries (modified from Duffy et al., 2012)...... 28

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Figure 2.10 Location map of the Danish Central Graben in respect to the Central Graben of the North Sea (modified from Rank-Friend et al., 2004)...... 32

Figure 2.11 Palinspastic map for the Early Jurassic showing the distribution of active structures and sediment facies (After Evans et al., 2003). The stratigraphic pattern suggests concentric to elliptical area of uplift and erosion. This uplift was a result of thermal doming that occurred during Mid-Jurassic...... 33

Figure 2.12 Location map and structural elements of the Danish Central Graben. (after Sundsbø & Megson, 1993)...... 35

Figure 2.13 Schematic cross sections showing the influence of the ductile salt layer on the rift structural style and stratal geometry style during rifting (after Duffy et al., 2012)...... 38

Figure 3.1 Location map of the seismic survey. The dataset is located across the northern end of the plunging Salt Valley salt wall in Paradox Basin, SE Utah. (b.) Time slice at (-1000) TWT showing the location of the available wells...... 40

Figure 3.2 Crossline and inline profiles from the Salt Valley seismic survey. (a.) Crossline seismic section from the seismic survey along the western minibasin. (b.) Inline seismic section from the seismic survey perpendicular to the salt wall. The two seismic profile show the overall excellent quality of the seismic data...... 41

Figure 3.3 Location map of the seismic survey. The dataset located offshore of Denmark in the southern portion of the Central Graben. (b.) Time slice at (-4000) TWT showing the location of the available wells...... 44

Figure 3.4 (a.) Inline seismic section from the seismic survey across the southern portion of the seismic survey. (b.) Crossline seismic section along the seismic survey. The two seismic sections show the overall excellent quality of the seismic data ...... 45

Figure 3.5 Seismic attribute (Chaos attribute) section (left) and time slice (right). (a.) Seismic section across the Salt Valley Salt Wall, Utah. (b.) Variance slice (-2750 TWT) from the Danish Salt Dome Province. The attribute section and map show clearly the discontinuity in the data resembling faults. They also highlight the chaotic seismic facies representing salt bodies...... 51

Figure 3.6 Depth-Time relationship derived from 16 wellbore check-shots data form Salt Valley salt wall. Time was converted to depth by using the equation in the upper right corner...... 52

Figure 3.7 Depth-Time relationship derived from 9 wellbore check-shots data from Danish Central Graben. Time was converted to depth by using the equation in the upper right corner...... 52

Figure 3.8 The general workflow of forward modeling in Elfen...... 53

Figure 3.9 Cross section across the Paradox Basin oriented NE-SW. The cross section was used to estimate the geometry parameters that will be used to design the 2D models geometry. (Modified after Paz 2006) ...... 55

Figure 3.10 Geometry for the numerical modeling. Compose two layers, salt and brittle (overburden) layer...... 55

Figure 3.11 Flowchart for the series of numerical forward modeling performed in this study...... 56

Figure 4.1 (a) 3D perspective view of the interpreted top of salt horizon. Three variance attribute profiles are plotted in the figure showing the discontinuities (faults) above the salt wall. Notice the change in the structural style along the strike of the salt wall. Half-graben geometry in the NE changes to graben style in the SE. (b) Salt Valley salt wall profile showing the top salt variation along the strike of the salt wall. Note plunging of the salt wall to the NW ...... 60

Figure 4.2 Seismic (left) and geoseismic (right) cross-sections illustrating the structural style and stratal geometries associated with Salt Valley salt wall. All the profiles are perpendicular to the salt wall. The location of the seismic profiles is shown in figure 4.1...... 61

Figure 4.3 (a) Time structure map of top pre-salt (top Leadville Formation).The fault system underlies the salt structure. In the upper right corner of (a) is a rose diagram for the sub-salt faults showing the strike trending NW-SW. The top of Leadville Formation dips toward the NE as it is indicated by the map. (b) seismic profile from the 3D seismic reflection volume...... 65

Figure 4.4 Time thickness maps of (a) Permian Cutler Group, (b) Triassic (Moenkopi and Chinle Formations) and upper Jurassic (Navajo to Dakota Formations). Notice the highly variable thickness of the Permian and Triassic and the uniform thickness of the Upper Jurassic successions...... 66

Figure 4.5 Time structure maps for the key horizons used to analyzed the crestal faults system. (a) Top Dakota Silt Formation, (b) Top Slick Rock Formation, (c) Top Navajo Formation...... 67

Figure 4.6 Map view of all faults mapped using the Salt Valley 3D seismic survey. White dashed polygon represents the extent of 3D seismic data...... 71

Figure 4.7 Time-structure map and corresponding fault polygon map for seismic horizon Dakota Silt (contour interval = 30 ms)...... 72

Figure 4.8 Rose diagrams of faults strike for entire fault arrays (E and W fault arrays)...... 73

Figure 4.9 3D perspective view of the mapped faults in the northern end of Salt Valley anticline, Utah. The faults are color coded based on their dip direction. The E fault array dip orientation is about 225o (SW) and the W fault array dip orientation is about 50o (NE). Interpreted horizons of top salt and top of pre-salt are also displayed. The salt wall plunge to the northwest...... 74

Figure 4.10 3D perspective view of the mapped faults in the northern end of Salt Valley anticline. (a) Western faults array (W). (b) Eastern faults array (E). The faults are color coded based on their dip angle. The eastern array is significantly steeper than the western faults. The western array shows listric geometry at depth...... 75

Figure 4.11 3D perspective view of edge-detection map of Dakota Silt horizon. This map show the fault surfaces and the location of the developed relay ramps that formed along the overlapped fault segments...... 76

Figure 4.12 Throw-distance (T-X) plots for segments (E1-5) top and (W1-5) bottom of the supra-salt fault system based on Dakota Silt horizon. Time structure map is shown for reference...... 77

Figure 4.13 (a) Uninterpreted seismic profile of inline #508. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of W1 and W2 faults. (c) Is T-z crossplot of W1 and W2 faults...... 79

Figure 4.14 (a) Uninterpreted seismic profile of inline #328. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of W1 fault. (c) Is T-z crossplot of W1 fault...... 79

Figure 4.15 (a) Uninterpreted seismic profile of inline #268. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of W1 and W3 faults. (c) Is T-z crossplot of W1 and W3 faults...... 81

Figure 4.16 (a) Uninterpreted seismic profile of inline #268. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of W4 and W5 faults. (c) Is T-z crossplot of W4 and W5 faults...... 81

Figure 4.17 (a) Uninterpreted seismic profile of inline #418. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of E2 fault. (c) Is T-z crossplot of E2 fault...... 85

Figure 4.18 (a) Uninterpreted seismic profile of inline #358. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of E2 and E3 faults. (c) Is T-z crossplot of E2 and E3 faults...... 85

Figure 4.19 (a) Uninterpreted seismic profile of inline #358. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of E4 fault. (c) Is T-z crossplot of E4 fault...... 86

Figure 4.20 (a) Uninterpreted seismic profile of inline #358. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of E4 and E5 faults. (c) Is T-z crossplot of E4 and E5 faults...... 86

Figure 4.21 Cross section of Ge’s experimental apparatus and model before deformation. The model consists of silicon diapir, syndiapiric and postdiapiric layers. Holes are presented at the base of the silicon diapir to allow silicon withdrawal. The sides of the model are fixed and no lateral movement occur (Modified after Ge, 1996)...... 89

Figure 4.22 Ge’s physical models of salt withdrawal for 2 different geometries (a.) semicircular and (b.) triangle. EZ, extensional zone; CZ, contractional zone. Dashed lines are the initial salt top (after Ge, 1996). Both models produced inner contractional zone and outer extensional zone regardless of the initial salt body geometry...... 90

Figure 4.23 Structural evolution model of salt dissolution/withdrawal. (a.) Initial 2D numerical model geometry. The model consists of salt layer (semicircular) and pre-kinematic homogenous overburden. (b.) Effective strain of the final deformation stage. Extensional zone (EZ) and contractional zone (CZ) formed due to the complete salt dissolution/withdrawal. Notice the deformational similarities between the 2D numerical model and the analog model in Figure 4.2 (a.)...... 91

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Figure 4.24 Structural evolution model of salt dissolution/withdrawal. (a.) Initial 2D numerical model geometry. The model consists of salt layer (triangle) and pre-kinematic homogenous overburden. (b.) Effective strain of the final deformation stage. Extensional zone (EZ) and contractional zone (CZ) formed due to the complete salt dissolution/withdrawal. Notice the deformation similarities between the 2D numerical model and the analog model in Figure 4.2 (b.)...... 92

Figure 4.25 Cross section and serial cross sections of salt dissolution of smooth symmetrical salt dome model. (a) Cross section of the initial configuration of the dissolution 2D model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 93

Figure 4.26 Cross section and serial cross sections of salt dissolution of irregular symmetrical salt dome. (a) Cross section of the initial configuration of the dissolution 2D model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 94

Figure 4.27 Cross section and serial cross sections of salt dissolution of smooth asymmetrical salt dome. (a) Cross section of the initial configuration of the dissolution 2D model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 96

Figure 4.28 Cross section and serial cross sections of salt dissolution of irregular asymmetrical salt dome. (a) Cross section of the initial configuration of the dissolution 2D model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 97

Figure 4.29 Cross section of 2D bending model showing the developed extensional zone over the arched area. Normal faults developed in the central area where stretching is maximum. This process was used to create extensional forces over the salt diapirs for the extensional model...... 99

Figure 4.30 Cross section and serial cross sections of salt extension of smooth symmetrical salt dome. (a) Cross section of the initial configuration of the 2D extensional model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 100

Figure 4.31 Cross section and serial cross sections of salt extension of irregular symmetrical salt dome. (a) Cross section of the initial configuration of the 2D extensional model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 101

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Figure 4.32 Cross section and serial cross sections of salt extensional smooth asymmetrical salt dome. (a) Cross section of the initial configuration of the 2D extensional model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 103

Figure 4.33 Cross section and serial cross sections of salt extensional irregular symmetrical salt dome. (a) Cross section of the initial configuration of the 2D extensional model. (b) Cross section of final effective strain after 1 Ma of dissolution. (c) Serial sections of strain rate evolution...... 104

Figure 4.34 Time structure map of Top Pre-Zechstein overlaid by contoured thickness map of the Zechstein Salt highlighting the location of the salt structures (Kraka, Dan, Skjold and Gorm salt structures) in respect to the sub-salt faults. The dashed polygon in the area of interest that is used for further fault analyzes. The dashed black line is the location of Figure 4.35...... 105

Figure 4.35 (a) Uninterpreted seismic and (b) interpreted seismic cross-sections illustrating the structural style and stratal geometries associated with Danish Salt Dome Province. All the profiles are perpendicular to the salt structures. The location of the seismic profiles is shown in figure 4.34...... 108

Figure 4.36 Time thickness maps of (a) Cretaceous , (b) Middle and Upper Jurassic ...... 110

Figure 4.37 Map view of the mapped supra-salt fault arrays underlain by time structure map of top salt (contour lines spaced 100 ms). (a) The faults are color coded based on their dip direction. The faults over the salt wall in the SW are mainly dipping to SW with few smaller faults dipping to the NE (antithetic). Faults over Kraka salt pillow display a wider range of dip directions. However, they can be based on their strike. The WNW striking faults are dipping to the NNE. The NNW oriented faults dip to the ENE. Faults oriented to the NE are dipping to the NW. (b) The faults are color coded based on their dip angle. Generally, the faults arrays show planar geometries with an average dip angle of 50 degrees...... 114

Figure 4.38 Time Structure map of top Mid-Jurassic with interpreted supra-salt fault arrays. The white box to the top is Figure 4.39 and the lower white box is Figure 4.40...... 116

Figure 4.39 Throw-distance (T-x) plots for segments (K1-6) of the supra-salt fault system based on top Mid-Jurassic surface. Time structure map of Mid-Jurassic surface is shown for reference...... 117

Figure 4.40 Throw-distance (T-x) plots for segments G1-8 of the supra-salt fault system based on top Mid-Jurassic surface. Time structure map of Mid-Jurassic surface is shown for reference...... 118

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Figure 5.1 Perspective view of the northern-end of Salt Valley (from Google Earth). The seismic data-set is located to the NW of this image. The red dashed lines are the boundary of the Salt Valley. Note the intermittent (seasonal) river channels crossing the valley perpendicularly and along the axis of the valley. Few scattered outcrop of cap-rock present to the south of the study area...... 122

Figure 5.3 Structural models for the structural styles associated with salt- dissolution/withdrawal or extensional forces. (a) Symmetrical salt wall and the developed crestal graben (symmetrical graben) associated with either salt-dissolution/withdrawal or extension...... 129

Figure 5.4 Fault throw - segment length cross-plot. Data from the interpreted faults from the seismic data used in this study are plotted against the global fault database along with salt-dissolution faults from Spanish Valley (Guerrero, 2014) and Zenzano fault from Iberian Chain, Spain (Carbonel, et al., 2013)...... 130

LIST OF TABLES

Table 3-1 Rock properties used for the numerical modeling...... 57

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ACKNOWLEDGMENTS

I would like to express my deep gratitude to my advisor Dr. Bruce Trudgill for all the help he provided along the course of my Ph.D. journey; for his patience, his willingness to provide insights and gaudiness whenever I sent him an email or knocked his office door.

My committee members, Dr. Rick Sarg, Dr. Dag Nummedal and Dr. Hazim Abbas, thank you for supporting me during my research. I was lucky enough to have top notch names in my

Ph.D. committee.

This work was not possible without the generous funding of Kuwait University. BP for introducing me with the forward numerical modeling (ELFEN). Special thanks for Ryan Thigpen for teaching me the software. Rockfield for granting me a free license for their numerical forward modeling software ELFEN, especially John Cane for his undoubtedly kindness.

Dr. Chuck Kluth was part of my Ph.D. committee but unfortunately he passed away. He was very supportive. I never took a class with him and I was introduced to him by Bruce. I remember the first day I met him he told me “at some point you in your Ph.D you will be so frustrated and fed-up from your Ph.D. research at that time please make sure to come to me and

I’ll make sure to help you in those moments”. He was such a humane. May he R.I.P.

During my stay in Colorado I had the chance to meet a lot of good people that without them my Ph.D. journey and me being away of home and family would a nightmare. Hamideh

Etemeadnia, Taher Alghazal, Mustafa Almubarak, Mustafa Alibrahim, Fuad Alsultan. From my deep heart thank you!

My parents, it was really hard to be far from you for all those years. I wish if I could pay you back for all the care and thought you sent me while I was away from home.

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1 CHAPTER 1 INTRODUCTION

Although rock salt (halite, NaCl) is rarely exposed at the surface, it is widespread both in stratigraphic record and in terms of its distribution worldwide (Kozary et al., 1968; Warren,

2006; Hudec and Jackson, 2007). Rock salt is unique in terms of its physical and chemical properties: (1) low density and compressibility; (2) plastic rheology and low mechanical strength; (3) exceptionally high solubility, which has important geological and environmental implications. It has been well established that rock salt has a major influence on the structural styles in sedimentary packages (overburden) above salt structures and even as an undeformed salt layer (e.g., Hudec and Jackson, 2007).

Salt tectonics has been studied for more than a century, yet the last 3 decades have greatly advanced the understanding of the geometry and the evolution of salt bodies, especially by the advent of new technologies in seismic data acquisition and processing (e.g., Ratcliff,

1993). The utilization of analog modeling (e.g., Vendeville & Jackson, 1991, 1992a, b; Ge,

1996), numerical modeling (e.g., Walsh, 2000; Schultz-Ela and Walsh, 2002; Walsh and Schultz-

Ela, 2003; Allken et al., 2013), structural restoration (e.g., Rowan, 1993; Hossack, 1995) and field-based studies (Lawton et al., 2001; Giles & Lawton, 2002; Rowan et al., 2003; Aschoff &

Giles, 2005; Hearon et al., 2015; Kernen et al, 2012) have also helped in developing a more sophisticated understanding of salt evolution.

Many of the world’s great hydrocarbon provinces are found in salt basins (e.g., Gulf of

Mexico, Persian Gulf, North Sea, Campos Basin, and Pricaspian Basin). This makes the study of salt tectonics of prime interest to the hydrocarbon exploration and production industry. In fact, petroleum systems are highly affected by the presence of salt. Not only by the ability of salt to create structure closures, but also by playing a role in reservoir distribution, heat conduction and

1 acting as a seal to fluid migration. Consequently, it is critical to establish a better understanding of salt tectonics and its role in near salt deformation and salt-sediment interaction.

1.1 Background

Crestal grabens over salt structures, in general, have been studied extensively in the past few decades (Cater, 1970; Lohmann, 1979; Bacoccoli et al, 1980; Stokes, 1982; Jenyon, 1984,

1986, 1988b; Doelling, 1985, 1988; Baars and Doelling, 1987; Chenoweth, 1987; Mart and Ross,

1987), specifically in the Paradox Basin (Baker, 1933; Stokes, 1948; Cater, 1970; Kitcho, 1981;

Doelling, 1988, 2000; Ge and Jackson, 1994, 1998; Ge et al, 1995, 1996; Gutierrez, 2004) and in the North Sea (e.g. Davison et al. 2000; Rank-Friend et al., 2004; Stewart, 2006, 2007 and

Carruthers et al., 2013). Many studies have established the association of the location of graben structures with their underlying salt walls. However, there is a prolonged argument about the origin of those graben structures in the Paradox Basin; whether are due to extensional stresses or due to more local salt dissolution processes.

Crestal grabens in the Paradox Basin have been studied extensively (e.g., Cater, 1970;

Ge, 1996; Doelling, 2000; Gutiérrez, 2004; Guerrero, et al., 2014). Those crestal grabens occupy the crestal part (hence the name “crestal”) of the salt wall anticlines in the Paradox Basin. Deep,

NW-SE striking depressions created along the axis of the salt anticlines may exceed 50 km in length, and may reach several hundreds of meters deep (Figure 1.1). From NE to SW, these discordant valleys are Salt-Cache Valley, Fisher Valley, and Sinbad Valley on the same structural trend; Castle Valley and Paradox Valley on the next anticline; Spanish-Moab Valley and Gypsum Valley and the southwestern most anticline is Lisbon Valley (Figure 1.1).

Salt dissolution was the traditional interpretation of the mechanism responsible for these crestal graben in the Paradox Basin (Cater, 1970; Huntoon, 1982; Doelling, 1985, 1988;

Oviatt,1988; Baars and Doelling, 1987). Through the advancement understandings of salt tectonics, extensional processes became a more favorable interpretation, especially due to the use of analog modeling in studying the deformation of salt and its overburden (Vendeville and

Jackson, 1992a, 1992b; Jackson and Vendeville, 1994; and Ge, 1996).

Figure 1.1 Geographic distribution of salt anticlines and salt valleys in the Paradox fold and fault belt of the Colorado Plateau (Utah and Colorado) (after Gutiérrez, 2004)

Ge (1996) showed by utilizing analogue models that crestal grabens are better explained by regional extension, local stretching or subsidence of salt bodies by salt withdrawal due to regional extension and that salt dissolution is only of a minor importance in the formation of

3 crestal grabens. However, since it is impossible to model salt dissolution with the current physical modeling techniques, Ge (1996) modeled salt dissolution/withdrawal by draining the model salt (silicone) from the base, which is not quite accurately mimicking the dissolution process in the nature. However, several studies showed that many of the crestal grabens faulting in the Paradox Basin and elsewhere are most likely to be related to salt dissolution (Gutierrez,

1996, 2004; Gutierrez et al, 2014; Guerrero et al, 2014). Therefore, it is important to revisit and to elucidate this discrepancy between the dissolution and the extension models of the crestal graben faults, and to further investigate the role of salt-dissolution as a key factor in the formation of crestal grabens.

The North Sea Basin is another salt basin floored with a thick sequence of evaporites

(Zechstein Salt). This basin is characterized by many salt structures distributed across the basin.

The North Sea Basin has a long history of crustal extension during Triassic and extensively during Late-Jurassic times (Ziegler and Horn, 1989; Zeigler, 1992; Evans, 2003). This complex and active geological history played a major role in shaping and defining the geometry of the basin, as well as triggering the Zechstein Salt to deform and generating many salt structures. In the Danish sector of the Central Graben (one of the three main structural elements in the North

Sea Basin), several salt structural styles are present in the Danish Salt Dome Province, ranging from salt diapirs that penetrate their sedimentary cover, to gentle, non-penetrating salt-cored pillows. Salt structures are present in the southern domain of the Danish Central Graben as pillows (e.g. Kraka salt pillow), diapir (e.g. Skjold salt structure) and wall-and- sill complexes

(e.g. Dan salt structure). The supra-salt roof of those salt structures are heavily faulted, forming crestal grabens. However, these crestal grabens are not related to any salt-dissolution but rather

4 purely related to the extensional tectonics that took place in the Late-Jurassic (Rank-Friend and

Elders, 2004).

Since both Paradox and North Sea Basin are salt basins however, with different geological history and tectonic activities, and both are characterized by the presence of salt structure that their overburden is highly faulted. This creates an exceptional opportunity to study the supra-salt fault system in both basins and investigate similarities and differences between them in order to unravel their geological history and their development. Specifically, to characterize the difference between extensional tectonic related salt-influenced fault system and salt-dissolution related salt-influenced fault systems.

1.2 Problem and objectives

The different available interpretations for the mechanism and origin of salt wall crestal faults in the northern Paradox Basin raise a question of how the origin and the development history can be narrowed down and come up with more robust answer, given the notion that these faults system are often used as an analogue for subsurface salt-related normal faults elsewhere.

Salt-influenced normal faults at outcrop in the Paradox Basin are heavily studied and used as an analogue for subsurface salt-influenced normal faults in the North Sea (Clarke et al., 2005; Jolley et al., 2007; Komoróczi, 2014) although, they have different evolution history. Therefore, it is quite important to understand the development history of both fault systems and how this might affect our understanding of the evolution of both fault systems.

The project objectives are: 1) Produce a 3D seismic interpretation of the northern end of Salt Valley salt wall and the

Salt Dome province in the Danish Central Graben using the available seismic data.

2) Characterize and analyze the fault systems over the northern end of Salt Valley salt wall,

Utah and in the Danish Central Graben.

3) Identify geometric similarities and differences between supra-salt fault systems in the two

different salt basins; Paradox, Utah and Danish Central Graben, offshore Denmark.

4) Numerically test the plausibility of salt dissolution to generate normal faults above salt

structures.

2 CHAPTER 2 BACKGROUND

The Paradox and the Central North Sea Basins are both floored by thick salt deposits of

Pennsylvanian and Permian age respectively. While the two basins are characterized by a high degree of salt deformation (halokinesis), where most of the autochthonous salt has deformed and generated different styles of salt structures, each basin has a different geological and tectonic history. The dominant deformational control of post-salt deformation in the Paradox Basin is differential loading from Late Paleozoic through Jurassic time with short pulses of uplift and contractional tectonics followed by a short extensional phase in Paleogene-Neogene time

(Doelling, 1988; Peterson, 1989; Trudgill, 2011). The North Sea Basin was dominated by extensional tectonism, mainly during the Mesozoic time, followed by a short pulse of inversion during the Late Cretaceous and Early Tertiary (Ziegler and Horn, 1989; Zeigler, 1992; Stewart et al, 1996; Stewart and Clark, 1999; Evans, 2003).

In both basins, most of the salt structures are associated with fault systems. The mechanism of faulting in the supra-salt successions in the North Sea Basin is believed to be controlled by extensional forces (Rank-Friend and Elders, 2004). In the Paradox Basin, based on field observations and physical modeling, the origin of faulting is thought to be due to either salt dissolution, withdrawal or extensional stresses.

This chapter will discuss the geological history of the two basins. Discussing the deformational history of the two salt basin and the stratigraphic architecture and emphasizing on the evolution of the salt structures and their supra-salt fault systems.

2.1 Geological Setting of the Paradox Basin

2.1.1 Colorado Plateau

The Colorado Plateau is a relatively undeformed ~ 350,00 km2 province in the North

American Cordillera (Figure 2.1) with an average topographic elevation of approximately 2000 meters (Pederson, 2002). It is bounded to the south and west by Basin and Range extensional province, and to the east and north by elements of the compressional Rocky Mountain system.

Although the western US has experienced a complex tectonic history involving numerous phases during the Phanerozoic, the Colorado Plateau remained a relatively stable cratonic area

(Wernicke, 1992) and has remained largely undeformed by tectonics throughout this period

(Barbeau, 2003). There has been a prolonged debate about the timing and mechanism that led to the evolution of the Colorado Plateau. A number of conflicting lines of evidence make constraints on the timing of Colorado Plateau uplift very controversial. The Upper Cretaceous marine sediments of the Mesaverde Group in east central Utah undoubtedly pre-date the uplift of the plateau and paleobotanical data (fossil leaves) from the Eocene Green River Formation of the northern part of the Colorado Plateau province suggest that the elevation in the Mid to Late

Eocene was about 1.5-3 km (Gregory and Chase, 1992; Wolfe et al, 1998). Spencer (1996) has suggested that the bulk of Colorado Plateau uplift took place during the Laramide Orogeny due to a thermal uplift. Therefore, it is suggested the Colorado Plateau uplift took place in Early

Paleogene.

However, many studies (Longwell, 1946; Lucchitta, 1972; Spencer et al, 2001 and

Ranney, 2005) of the modern drainage network of the Colorado Plateau indicate Neogene rather than Paleogene uplift. Radiometric dating of volcanic rocks interbedded with sediments indicates that the Colorado River drainage did not exist before 6.0 Ma (Spencer et al, 2001) but was

8 definitely established by 4.4 Ma (Faulds et al, 2001). An older (pre-6.0 Ma) drainage system existed and flowed north and north-east of the Laramide-generated highlands (Young and

McKee, 1978; and Young, 2001). This is indicated by Early to Late Neogene gravel deposits in channel cuts across the tops of Colorado Plateau in northwest Arizona.

Figure 2.1 Map of the Colorado Plateau (black outline) and the upper Colorado River drainage basin (gray outline) with its major bedrock terrains (after Bursztyn et al, 2015).

2.1.2 Ancestral Rocky Mountains Evolution

The Ancestral Rocky Mountains (ARM) formed a system of highlands and adjacent basins stretching from southern Idaho to central Texas (Figure 2.2), composed of about 20 basement-involved uplifts and associated sedimentary basins. Some of the largest uplifts and associated depocenters formed in what are now Utah, Colorado and New Mexico including the

Uncompahgre Uplift and the Paradox basin. The ARM formed during the Late Mississippian-

Permian time (Kluth, 1998; Ye et al., 1998; Dickenson and Lawton, 2003). However, the main structural pulse that developed the ARM was culminated during Pennsylvanian time (Kluth and

Coney, 1981; Kluth, 1986; Kluth, 1998; Ye et al, 1998). The special evolution of the ARM is well studied, however causal mechanisms are poorly understood because the intercontinental deformation occupied no discrete or coherent belt that can be related to contemporary plate margins (Dickinson and Lawton, 2003). Kluth and Coney (1981), believe that the development of the ARM is related to the diachronous closure of the Ouachita Orogeny and the collision of

North America with South America-Africa. This continental collision produced the Ouachita-

Marathon orogeny. Based on the pattern of sequential subduction along the suture zone,

Dickinson and Lawton (2003) postulated that the interior of the continental block was forced to absorb torsional strain, forming the ARM deformation. The ARM structures overlap spatially with older structures of the Precambrian-Cambrian rifting of Rodinian supercontinent (Ham et al., 1964; Perry, 1989). Many of the ARM uplifts are bounded by high-angle Pennsylvanian age faults with significant dip-slip offset of several kilometers (e.g., McConnell, 1989; Thomas,

2007). The crystalline basement-cored highs shed thick sequences of sediment into the adjacent basins in western equatorial Pangea (Figure 2.3).

Figure 2.2 Paleogeographic map of southwestern North America (United States) showing the Pennsylvanian–Permian Ancestral Rocky Mountains uplifts and basins (after Soreghan et al. (2012)).

2.1.3 Uncompahgre Uplift and Paradox Basin

The Paradox Basin (PB) is located in the Four Corners Region of Colorado, Utah, New

Mexico and Arizona (Figure 2.4). The basin is considered to be moderately large in size (158 miles by 250 miles). It is one of a series of intracratonic basins developed in the present day western United States during middle Pennsylvanian time, associated with the thick-skinned, basement-cored uplifts known as the Ancestral Rocky Mountains (ARM). The basin is an asymmetrical basin where the thickest deposits occur in the northeastern portion and is also marked by widespread salt movement and referred to as the Deep Fold and Fault Belt (Doelling,

1983). The basin deposits thin toward the west and the south west. The PB trends NW-SE and is located adjacent to the Uncompahgre Uplift to the northeast (Figure 2.4).

The Uncompahgre Uplift is bounded to the southwest by a 200-300 km long fault zone.

The southwestern bounding fault system is moderately NE dipping, with ~10 km of NE-SW shortening (Frahme and Vaughn, 1983; White & Jacobson, 1983).

The formation of the Paradox Basin has always been a subject of debate, being interpreted in a wide variety of ways e.g., as a result of normal faulting (Mallory, 1972) to a feature that formed as a pull-apart zone (Stevenson, 1986). Also a number of researchers referred the boundary between the basin and the Uncompahgre Uplift as a zone of thrust faulting (White and Jacob, 1983; Frahme and Vaughn, 1983; Huffman and Potter, 1993 and Huffman and

Taylor, 1994). Lemke (1985) and Barbeau (2003) interpreted the Paradox Basin to be a flexural foreland basin developed as a result of the loading of the Ancestral Rocky Mountains

Uncompahgre Uplift. However, Kluth and DuChene (2009) proposed a different timing and geometry for the Uncompahgre evolution and also suggested that the deposition of the Paradox

13 and Eagle Valley Evaporites were a single continuous basin that existed at the same location as the later Uncompahgre Uplift.

2.1.4 Stratigraphic Framework of Paradox Basin

In the northern Paradox Basin, the exposed rock units range in age from Mid-

Pennsylvanian to Late Cretaceous (300 to 90 Ma) (Doelling, 1988). Pre Mid-Pennsylvanian rocks are only found in the subsurface and only observed from boreholes. Precambrian basement rocks not penetrated by boreholes however; they are expose in the Uncompahgre Uplift. They consist of gneiss, schist, quartzite, and lamprophyre (Cater, 1970) and are dated at 1700 to 1400

Ma (Hedge et al, 1968).

The Paradox Formation of Pennsylvanian age comprises mainly thick halite deposits (70-

80%) interbedded with shale and carbonate. The Paradox Formation is the thickest and most significant unit in the Paradox Basin (Figure 2.6). It represents a heterogeneous unit, composed mainly of salt, anhydrite and dolomite with some amount of detrital deposits and organic matter

(Doelling, 1988). The conditions under which the Paradox Formation was deposited is repetitive and cyclic (29 cycle) between flooding and desiccation of the basin during glacio-eustatic sea level change (Hite and Buckner, 1981). Due to the salt mobilization and also near surface dissolution, the thickness of Paradox Formation is difficult to estimate. However, an overall thickness is estimated at 1500 to 1800 m in the Salt Valley anticline area, and in the salt walls the maximum salt thickness reaches about 4000 m (Doelling, 1988).

Rock units from Permian through Triassic are predominantly composed of conglomeratic deposits (Cutler Group) shed from the Uncompahgre Uplift. The Triassic section (Moenkopi and

Chinle Formations) comprises mixed marine/terrestrial deposits. From the Upper Pennsylvanian

Honaker Trail Formation through the Upper Triassic Chinle Formation (Figure 2.6), these stratigraphic units were controlled by salt tectonics resulting in significant variation in both

15 thickness and facies across the basin, especially adjacent to the flanks of the steep- sided salt walls (Doelling, 1988; Trudgill et al., 2004).

The Jurassic and Cretaceous units are mainly composed of sandstones and shale respectively. Generally, the Jurassic and Cretaceous successions don’t show any thickness variation indicating cessation of the halokinesis processes by this time.

2.1.5 Salt Structures

Salt structures in the Paradox Basin are restricted to the Paradox Fold and Fault Belt in the northeastern part of the basin (Kelley, 1955; Doelling, 1988) (Figure 2.4). Various salt structural styles are displayed across the basin (Figure 2.5), ranging from salt walls that are exposed at the surface, to deeply buried non-piercing salt pillows (Doelling et al., 2002; Trudgill et al., 2004). The major trend of salt structures in the Paradox Basin is NW-SE, dictated by deep- seated normal faults in the pre-salt section, which acted as a buttress to the flowing salt, nucleating the salt structures above them (Baars, 1966; Ge, 1996; Doelling, 2001; Kluth and

DuChene, 2009; Trudgill and Paz, 2009; Trudgill, 2011).

The evaporites of the Paradox Formation started to deform shortly after or more likely by the Honaker Trail time driven by sediment loading. Salt growth continued through Chinle (upper

Triassic) deposition, reaching a climax during deposition of Cutler Group, while the basin infilled with the deposition of the sediments shed from the rising Uncompahgre Uplift (Doelling,

1988; Trudgill and Paz, 2009; Banbury, 2005). However, tectonic activity during Jurassic and

Cretaceous (Bromley, 1991; Trudgill and Paz, 2009) and during Paleogene (Laramide) compression also subtly affected salt movement and slightly deformed the salt bodies (Trudgill et al., 2004).

As the salt was affected and mobilized by progradation of the sediment shed from the

Uncompahgre Uplift (Ge, 1996; Trudgill, 2011), the salt structures controlled the sediment distribution and stratigraphic architecture within the basin from Honaker Trail through the

Triassic Chinle deposition (Figure 2.6). These stratigraphic units show substantial variation in thickness and facies across the basin especially near the flanks of the salt bodies (Trudgill and

Paz, 2009; Doelling, 1988)

Growing as passive structures, the salt diapirs in the Paradox Basin were at or very close to the surface during their long growth phase (Lawton and Buck, 2006; Kluth and DuChene,

2009; Trudgill, 2011). However, Rasmussen (2014) argues that there is no direct evidence has been found in outcrops or from subsurface data that any of the salt diapirs were breached at the surface before the Early–Permian time. Venus et al. (2015) noted the common presence of intraformational clasts (sandstone and siltstone) in the Undifferentiated Cutler strata on the flanks of Onion Creek diapir and the absence of gypsum clasts. This supports the notion that the salt diapir was close to the surface rather than breached and exposed. Rasmussen (2014) suggest that salt extrusion took place due to the regional erosional event (Leonardian unconformity), which marks the Early Permian Organ Rock Formation and the top of Hermosa Group. This extensive erosion unconformity dug deep enough over the crest of the diapirs that made the pressurized salt to flow on the surface.

Present day, several salt diapirs in the Paradox Basin are exposed at the surface including

Sinbad Valley, Paradox Valley, Gypsum Valley of southeastern Colorado, and Onion Creek,

Castle Valley, Moab Valley and Salt Valley of southeastern Utah (Figure 2.4).

2.1.6 Fault systems in the Paradox Basin

Many regional studies were carried out for the Battelle Office of Nuclear Waste Isolation

(a prime contractor to the U.S. Department of Energy) to investigate the feasibility of siting an underground nuclear waste repository in the Paradox Basin back in the 1980s. Also, many studies were conducted because of major mineral and petroleum exploration during the 1950s.

This included many studies on fault systems in the Paradox Basin (e.g., Kitcho, 1981; Olig et al.,

1996; Wong et al., 1996). There is a clear relationship between the location of the fault systems and salt structures. Nevertheless, there is a lack of a complete understanding of the relationship between the fault systems and salt structure. Faults are present in the sub and supra- salt sections in the Paradox Basin. They are typically normal faults (with few exceptions) and oriented parallel or sub-parallel to the Uncompahgre uplift (NW-SE).

2.1.6.1 Sub-Salt Faults

Many sub-salt faults have been interpreted from seismic or cross sections across the

Paradox Basin and it is widely accepted that salt walls in the Paradox Basin are aligned with sub- salt normal faults (Shoemaker et al., 1958; Cater and Elston, 1963; Joesting et. al, 1960; Baars,

1966; Ge, 1996; Doelling, 2001; Trudgill, 2011). In some cases, sub-salt faults are present, but with no salt structure associated with them. This might be due to the lack of mobile salt in those areas (Paz, 2006). The sub-salt faults might be related to flexure associated with Uncompahgre uplift as inferred from their parallel orientation with respect to the Uncompahgre front.

Doeling and Ross (1998) inferred sub-salt faulting from well data. Also, a map of the sub-salt Top Mississippian surface was generated from well data for the northeastern part of the

Paradox Basin by Banbury (2005). The idea was to investigate if there was any high gradient in the sub-salt structural topography that would indicate to faulting. However, because the lack of

18 close spaced well data, nothing meaningful was identified from the map. The map showed a flexural basin, which supports the flexural model of Barbeau (2003) rather than that of Stevenson and Baars (1986). Salt plays a significant role in decoupling the supra-salt faulting from the sub- salt faulting. This is shown in the majority of the published cross sections of the basin whether based on seismic data or not.

2.1.6.2 Supra-Salt faults

The supra-salt faults are almost exclusively restricted to the crestal parts of salt structures, trending parallel or sub-parallel to the underlying salt structures. These fault systems can be categorized into two families. (1) Large isolated basinward-dipping (NE) normal faults with significant offset and (2) smaller faults (less offset) forming crestal graben systems.

The large basinward dipping normal faults have extensively described, e.g., the Moab

Fault (Doeling et al., 2002, Foxford et al. 1996, Olig et al. (1996), and Lisbon Faults (Ge, et al.

1996, Olig, et al. 1996). The Moab Fault system is considered the largest fault system in the northern Paradox Basin, extending for 31 km and having a range of vertical separation from 100 to 960 meters (Foxford et al, 1998). This fault system still remains a subject of debate regarding the driving mechanism behind, and the timing of its formation. The system is oriented NW-SE and dips toward the NE. The system consists of two main segments; the northern Blue Hills segment and the southern Moab segment. The two segments are hard linked by the complex Mill

Canyon linkage zone.

Crestal graben systems have been mapped extensively for most of the salt structures in the Paradox Basin including Salt Valley-Cache Valley structure (Doelling et al., 1988; Gutiérrez,

2004), Moab-Spanish Valley structure (Doelling et al., 2002; Gutiérrez, 2004; Guerrero et al.,

2014) and Onion Creek structure (Hudec, 1995; Boyers, 2000; Doelling, 2002). These normal

19 faults which cut to the surface are suggested to be either Laramide (Late Cretaceous to Eocene) or older (Kitcho, 1981), or as the result of Neogene salt dissolution and collapse (Cater, 1955c,

1970; Doelling, 1981, 1983, 1988; Ross, 1998; Gutiérrez, 2004). However, Ge (1996) and Ge et al. (1995, 1996) proposed that those fault are a result of Neogene regional extension. Others

(Cater, 1955c, 1970; Doelling, 1981, 1983, 1985,1988; Gutiérrez, 2004: Lorenz and Cooper,

2009) suggest that those faults may have simply resulted from shallow dissolution and collapse of the diapirs walls.

At the end of most salt structures in the Paradox Basin a fault system is developed, whether forming a graben system as in Salt Valley and Castle Valley salt walls or as a larger scale normal fault with significant amount of offset as in Moab Fault.

2.1.7 Dissolution and subsidence

Evaporites are characterized as the most soluble of the common rock types throughout the world. Similar to limestones and dolomites, gypsum and salt dissolve readily to form karst and subsidence features. The only difference is that salt and gypsum dissolve much more rapidly, on a time frame of days, weeks and months, where as in carbonates it takes years, decades and centuries to dissolve. The dissolution of salt can form features like sinkholes, caves, disappearing streams and dramatic or catastrophic subsidence. Volume loss as a result of salt dissolution disturbs the stress state within the rock, behind or above the salt-dissolution front. Depending on rock strength and stress distribution, overlying rocks can fail by either incremental crack-seal processes producing gypsum filled veins (Collins and Luneau, 1986) or collapse that produces breccia (Hovorka, 2000).

Figure 2.6 Tectono-stratigraphic chart of the Northern Paradox Basin. Eight key horizons were interpreted and used in this study. Regional and salt tectonic events are marked.

Johnson (2005) suggested four basic requirements for dissolution of salt or gypsum.

When all four of the requirements are met, dissolution can be quite rapid. The four requirements are: (1) a deposit of gypsum or salt through which, water can flow; (2) a supply of water unsaturated with CaSO4 (or NaCl); (3) an outlet whereby the resulting solution (or brine) can escape; and (4) energy (such as a hydrostatic head or density gradient) to cause the flow of water through the system.

The rate of salt dissolution is controlled and dictated by many factors, e.g. temperature, direction and volume of crossflowing undersaturated pore waters (Hovoroka, 2000). Solubility of chloride salts increases consistently with increasing temperature (prograde solubility). Sulphates and sodium carbonate solubility initially increases with temperature; however, it can then decrease at higher temperatures (retrograde solubility). Typically, dissolution occur first at a shallow depth as the edges of salt beds flushed by meteoric or marine waters and continues deeper in the subsurface as the salt bed edges are flushed by undersaturated basinal brines.

Soluble crystals of more saline salts (typically halite and bitterns) dissolve and leave behind residues of less saline salts (typically gypsum-anhydrite) as noted in many caprocks (Warren,

2006). Caprocks are good indicator for dissolution processes and the former presence of now leached evaporites.

Evaporites are widely distributed worldwide and most salt basins are characterized by dissolution features. In the United States alone, evaporites are present in 32 of the 48 contiguous states, and they underlie about 35–40% of the land area (Johnson, 2005). Dissolution and karst features are known from almost all the areas underlain by evaporites (Figure 2.7)..

Deep seated dissolution of Permian salt in the Delaware Basin (southwest of the United

States) has generated collapse basins up to 150 km long filled with Neogene and Quaternary

23 continental sediments up to 460 m thick (Gutiérrez, 2004). Also, in the Rocky Mountains of

Colorado, the Carbondale collapse center originated by subsidence due to subjacent karstification of Pennsylvanian salt-bearing evaporites and displaced Mio-Pliocene volcanic rocks by greater than 800 m (Kirkham et al, 2002).

2.2 North Sea Geological Setting

The present-day configuration of the North Sea structural framework was shaped by multiple extensional phases that commenced in the Triassic and persisted into the early

Cretaceous (Evans et al, 2003). However, the area has also undergone earlier structural episodes that span from the pre-Devonian, Caledonian and the Variscan (Hercynian) orogenies Permian extension (Evans et al, 2003).

A detailed discussion of the evolution of the North Sea is documented by many workers

(Zeigler, 1992; Evans et al, 2003; Zeigler and Hoorn, 1989; Erratt et al., 1999). However, for the purpose of this paper only a summary of that evolution will be presented here, focused on the deformational episodes that shaped the current configuration of the North Sea.

2.2.1 Evolution of the Permian (Zechstein) Basin

From The Late Devonian to End of Permian (380 to 250 Ma) a gigantic mountain range

(Variscan mountain) evolved through the suturing of Gondwanaland and Laurasia resulting in a new super continent, Pangaea.

In Permian time, two interconnected basins developed across much of NW Europe

(Zeigler, 1981; Gatliff et al. 1994, Glennie et al., 2003, Jenyon et al., 1984). The two basins oriented E-W were termed the Southern and Northern Permian basins and extended across the present day Southern North Sea and Central North Sea respectively (Figure 2.8). The two basins were separated by E-W trending Mid North Sea High and Ringkøbing–Fyn High (Ziegler, 1981).

The development of the Permian basins is attributed to several processes, most of which are related to the late orogenic collapse of Variscan mountain belt and continental rifting with

Figure 2.8 Sketch map of Permian sedimentary basins in northwest Europe showing the maximum extent of the Northern and Southern Permian basins (after Evans et al., 2003).

26 thermal subsidence following the Carboniferous extension (Glennie & Buller 1983; Doré &

Gage 1987; Coward 1995).

These basins are floored by thick Aeolian sediments (Rotliegend Group) of Early

Permian age (Figure 2.9). In the Late Permian, the two basin were flooded and thick salt formation deposited (Zechstein Group) (Figure 2.9). Salt mobility and dissolution processes made salt thickness estimation challenging. Glennie et al. (2003) suggests the thickness of

Zechstein sedimentary rocks possibly exceed 1500 m in thickness. The flooding of the Permian basin probably was probably due to the worldwide rise in sea level, coinciding with the end of

Permian glaciation (Glennie, 1998). The water flooded from the Boreal ocean (Zeigler, 1982), somewhere between Norway and Greenland, along the preexisting fracture system of the Proto-

Atlantic and North Sea fracture systems. The route by which the Zechstein water entered the

Southern Permian Basin is disputed. Both Northern and Southern Permian Basins were connected by graben systems (e.g. Horn Graben and Central Graben). These grabens formed an active passageway between Northern and Southern Permian Basins (Glennie and Buller, 1983).

Ziegler (1990a) believes that the Central-Viking graben system did not develop until the Triassic and therefore has suggested a different route for the flooding. The presence of thick Rotliegend

Aeolian sabkha sand in the southern Viking Graben along with thick Zechstein halite (thick enough to behave diapirically) suggest however that an incipient graben already existed in the time of Zechstein flooding (Glennie, 1998).

2.2.2 Central Graben Rift System

The Central Graben (Figure 2.10) is the southern arm of the North Sea rift system and trends NNW-SSE. From its intersection with the Outer Moray Firth Basin and Viking Graben, it extends southwards c.500 km, through the UK, Norwegian, Danish, west German and Dutch

28 sectors of the North Sea. The development of the Central Graben commenced in the Early

Triassic (Ziegler, 1988a). The newly formed structural features of the Triassic clearly cross-cut the Caledonian basement trends e.g. the Viking and Central graben. However, pre-Triassic structural grain locally (e.g. northern Viking graben) influenced the structural framework of the consecutive rifting development (Triassic and Jurassic) (Johnson and Dingwall, 1981; Ziegler and Horn, 1989;).

Deformation in the North Sea during Triassic time was dominated by extensional rifting

(Ziegler and Horn, 1989; Zeigler, 1992) and mobilization of evaporites (Stewart et al, 1996;

Stewart and Clark, 1999). The Triassic rift system on the North Sea shows a very low level of volcanicity indicating that the onset of Triassic rifting was not associated with major lithospheric thermal perturbances (Ziegler and Horn, 1989; Zeigler, 1992). The Triassic rift system cross-cuts most of the Caledonian structural trend (Permian Basin and Mid North Sea Highs), developing new graben systems such as the Viking and Central grabens (Ziegler, 1988a). Permian-

Carboniferous fracture system were also reactivated (e.g. the Horn Graben).

The Triassic rift system produced deep half grabens in the eastern North Sea (Evans,

2003). Thick sedimentation in the half graben initiated differential loading that caused salt flowage and most of the salt zones started to deform producing salt structures (Hodgson et al.,

1992; Erratt, 1993; Sundsbø & Megson, 1993; Stewart et al., 1997; Stewart & Clark, 1999).

During the Mid-Jurassic the central North Sea was thermally uplifted and a broad dome formed; The Mid North Sea Dome (Ziegler and Horn, 1989; Ziegler, 1992; Underhill &

Partington, 1993) (Figure 2.11). This dome spanned the Central Graben and extended in an E-W direction from Denmark to southern Scotland and in a N-S direction from the southern Viking

Graben into the southern North Sea. Thermal doming led to volcanic activity in the area of the

29 triple junction between the Viking, Central and the Moray Firth grabens. The chemical composition of these volcanics shows a bimodal mafic-felsic alkaline chemistry typical of intracratonic rifts (Dixon and Fitton, 1981). The thermal doming resulted in a structural relief of

1500 to 2500m (Ziegler and Horn, 1989; Ziegler, 1992). As a result of the uplift of the Mid

North Sea Dome Early Jurassic, Triassic and even Permian rocks were deeply eroded and sedimentation ceased in the Central Graben (Figure 2.9).

After the collapse of the Mid North Sea Dome, the Central Graben underwent a new phase of rifting in the Mid-Jurassic to Early Cretaceous (Ziegler, 1990; Cartwright, 1991; Møller

& Rasmussen, 2003). In the Late Jurassic, three extension pulses occurred. The first pulse was in

Late Aalenian to the Early Oxfordian, characterized by the development of NNW-SSE to N-S striking faults, followed by a second rifting episode in Late Kimmeridgian to Early Volgian, preserved in thick deposits in the hanging wall of the NNW-SSE trending faults (Andsbjerg &

Dybkjær, 2003; Møller & Rasmussen, 2003). The third pulse commenced during the Ryazanian and reactivated the NNW-SSE striking faults (Møller & Rasmussen, 2003).

2.2.3 Danish Central Graben and Danish Salt Dome Province

The Danish Central Graben is a major east-dipping half graben system (Figure 2.10), part of the Central Graben where the rift system changes trend from striking NW-SE in the UK and

Norwegian sectors to N-S in the Dutch sector. The graben is bounded to the east by the Coffee

Soil Fault, which separates the Central Graben from a segment of the Ringkøbing-Fyn High to the east. The Mid-North Sea High forms the western limit of the Danish Central Graben. The

Danish part of the Central Graben is subdivided into sub-basins separated by intra-basinal highs.

Most of the Salt Dome Province is considered part of the Southern Permian Basin (Figure 2.12).

The evolution of the Danish Central Graben system occurred between the middle Jurassic and early Cretaceous, under the influence of a combination of normal faulting and mobilization of Zechstein and Triassic salt. However, in the Early Jurassic the area underwent uplift and erosion associated with the mantle plume related to Mid North Sea Dome (North Sea thermal dome), resulting in a regional unconformity at the base of the Mid-Jurassic succession (Figure

2.9).

The Danish Central Graben experienced a rifting phase represented in multiple tectonic pulses from the Mid-Jurassic to Early-Cretaceous after the collapse of the Mid North Sea Dome

(Zeigler, 1990; Gregersen & Rasmussen 2000; Møller & Rasmussen, 2003). The first pulse marked the middle Jurassic, characterized by subsidence on NNW-SSE to N-S- striking fault segments with minor salt movement occurred and the development of salt pillows (Møller &

Rasmussen, 2003). In the late Jurassic, the second pulse took place and thick sequences were deposited in the hanging-walls of the NNW-SSE trending fault systems. The accumulation of a thick sedimentary load led to salt movement, forming salt bodies (salt diapirs) (Møller &

Rasmussen, 2003). The third and last tectonic pulse is represented by the Early Cretaceous reactivation of the NNW-SSE trending faults accompanied with minor thrusting inversion and halokinesis (Møller & Rasmussen, 2003).

The presence of the Zechstein deposits is restricted to the northern and the southern part of the Danish Central Graben (e.g., Søgne Basin and Salt Dome Province, respectively) (Møller

& Rasmussen, 2003; Tanveer & Korstgård, 2009). The deformation of the salt is recorded as starting in Triassic time, influencing the distribution of the Triassic deposits.

Figure 2.10 Location map of the Danish Central Graben in respect to the Central Graben of the North Sea (modified from Rank-Friend et al., 2004)

2.2.4 Salt Structures

The salt of Zechstein Group is widespread, yet it is restricted to the northern and southern

Permian Basins (Figure 2.8). It is widely interpreted that salt deformation took place in Triassic time (Brunstrom & Walmsley 1969; Gatliff et al. 1994; Griffiths et al. 1995; Buchanan et al.

1996). However, Hodgson et al. (1992) and Clark et al. (1998) suggest that halokinesis/salt tectonics (gravity creep) started in Permian time (syn-Zechstein). Figure 2.12 shows the location of the salt structures in the Central Graben. Below is a detailed description of the salt structures in the Danish Central Graben will be discoursed.

The thickness of the Zechstein salt varies in the Danish Central Graben. In general salt thickness decreases northward. Salt thickness is highest in the Danish Salt Dome Province, in the southern part of the graben, and thins to the north and west where it is seismically not resolvable

(Duffy et al., 2012). However, in the northern part of the graben in the Sogne Basin, salt thickness increases again, where several salt structures developed (Figure 2.12). Around most of the salt structures, Zechstein salt thins and in some locations is welded (Duffy et al., 2012).

Several salt structural styles are present in the in the Danish Salt Dome Province, ranging from salt diapirs that penetrate their sedimentary cover, to gentle, non-penetrating salt-cored anticlines. Salt structures (Figure 2.12) are present in the southern domain as pillows (e.g. Kraka salt pillow), diapir (e.g. Skjold salt structure) and wall-and-sill complexes (e.g. Dan salt structure). Most of the salt structure developed by the flowage of Zechstein salt into them.

However, the Dan Salt structure is an exception. Jørgensen (1992) based on 2D seismic data interpreted the upper part of the structure as consisting of entirely Triassic Salt (Dudgeon

Saliferous Formation), while the lower part is Zechstein salt. However, Sundsbø & Megson

(1993) suggested that the upper part structure (intra-Triassic) is composed of intruded Zechstein salt along Dan Transverse Zone. The salt structures location coincides with the location and the orientations of the major faults in the Pre-Zechstein successions (Figure 2.12) (Rank-Friend &

Elders, 2004; Duffy et al., 2012). Complex normal fault arrays are developed atop the salt structures. A greater description of the overburden faults will be conveyed below.

2.2.5 Regional Stratigraphy of the Danish Central Graben

Wells in the Danish Central Graben locally penetrate a Caledonian basement of greenschists. The oldest sedimentary deposits encountered by wells in the Danish Central Graben are of Carboniferous age (Michelsen, 1982). The lower Permian (Figure 2.9) is dominated by

Figure 2.12 Location map and structural elements of the Danish Central Graben. (after Sundsbø & Megson, 1993).

35 volcanic rocks, succeeded by aeolian deposits and sabkha sediments of the Rotliegend Group

(Jacobsen & Larsen 1982; Stemmerik et al. 2000). The upper Permian (Figure 2.9) succession comprises of carbonates and evaporites referred to as the Zechstein Group which were deposited as a result of transgression event. Triassic deposition was controlled by basinal areas inherited from the Permian and withdrawal of salt during the Triassic (Evans et al., 2003). Triassic deposits (Figure 2.9) are dominated by non-marine sandstone and shale with subordinate evaporites (Jacobsen, 1982). During the Early Jurassic a relatively uniform succession of marine shales (Figure 2.9) is believed to have been deposited over most of the Danish North Sea region

(Koch et al. 1982; Michelsen et al. 2003). However, most of this was eroded in the middle

Jurassic due to the North Sea thermal dome (Møller & Rasmussen, 2003). Deposition of the middle Jurassic successions (Figure 2.9) was dominated by fluvial, lacustrine and near-shore sandstone and capped by shales of the late Jurassic (Møller & Rasmussen, 2003). Early

Cretaceous deposition (Figure 2.9) was a continuation of the upper Jurassic shale and sandstone.

The upper Cretaceous sediments (Figure 2.9) are composed of carbonates of the Chalk Group, considered to be one of the main reservoirs in this part of the North Sea.

2.2.6 Fault Systems in The Danish Salt Dome Province

The major structural element in the Danish Central Graben is the Coffee Soil Fault system (Cartwright, 1991). It marks the eastern boundary of the Tail End Graben, a major Late

Paleozoic to Late Mesozoic depocenter that strikes NNW-SSE and separates the prominent E-W trending Mid North Sea and Ringkøbing-Fyn Highs (Figure 2.12). The Tail End Graben is asymmetrical in cross section manifesting rotational thickening to the east, toward the Coffee

Soil Fault. Based on seismic reflection data, Cartwright (1991) suggested that rifting along the

Coffee Soil Fault commenced in two distinct phases, a non-rotational Triassic phase, resulting in

36 parallel reflection configuration. The second phase is a rotational Mid-Late Jurassic phase, resulting in divergent reflection configuration. The Coffee Soil Fault comprise at least five major linear segments (Cartwright, 1991). The stratigraphic configuration in the Tail End Graben shows stratigraphic segmentation that points directly to a partitioning in the motion history of the

Coffee Soil Fault system (Cartwright, 1991).

Another structural feature in the Danish Central Graben is the Dan Transverse Zone. It was originally described by Cartwright (1987) and he suggested the origin of the structural feature to be related to Tornquist lineament, which was reactivated through time. Cartwright

(1987) mapped the Dan Transverse Zone with approximately WNW-ESE direction but later with better seismic data it was redefined to be more NW-SE. Three of the salt structures (Gorm,

Skjold and Dan) in the Danish Salt Dome Province aligned above the Dan Transverse Zone striking NW-SE.

The Zechstein Salt plays a significant role in decoupling sub-Zechstein and supra-

Zechstein (cover) normal fault arrays in the Danish Salt Dome Province (Duffy et al.,2012).

Fault arrays are present in the Danish Salt Dome Province in both sub- and supra-Zechstein sections. Where the salt layer is absent, rooted faults in the pre-Zechstein (basement faults) propagate upward, displacing the overburden successions forming thick-skinned normal faults.

In the regions where the Zechstein Salt is thick (southern domain of Duffy et al., 2012) pre-

Zechstein faults die out at the base of salt. These faults trigger the salt to deform and develop salt structures (pillow and diapir). Above these salt structures, high amplitude – short wave length folding developed. This folding, along with regional thick-skinned extension and local thin- skinned faulting driven by fault- or load induced salt mobilization and lead to the development of high density of fault arrays in the cover (Duffy et al., 2012) (Figure 2.13).

Figure 2.13 Schematic cross sections showing the influence of the ductile salt layer on the rift structural style and stratal geometry style during rifting (after Duffy et al., 2012).

3 CHAPTER 3 DATA AND METHODS

3.1 Data

To achieve the objectives of this study and address the proposed questions, three- dimensional (3D) seismic data were utilized from the Paradox Basin, Utah, and Danish Salt

Dome Province, North Sea Basin. In the following sub-sections; a detailed description of the data will be discussed.

3.1.1 Salt Valley Dataset

The Salt Valley seismic dataset is a grid of 3D time migrated seismic reflection data covering an area ca. 190 km2. The survey has an inline and crossline spacing of 110 m; consists of 541 inlines, oriented NE-SW (approximately perpendicular to the present day structures) and

328 crosslines trending NW-SE (approximately parallel to the major faults). The survey has a sampling interval of 2 ms and covers a two-way travel time (TWT) of 3000 ms.

The dataset covers the northern end of the Salt Valley salt wall (Figure 3.1) and the two flanking minibasins on the eastern and western side of the salt wall. Also, the data image a graben system that occupies the crestal area of the salt wall (Figure 3.2). The quality of the seismic data in general is excellent. However, over the crestal area the quality is relatively low due to the high amount of fracturing and faulting (Figure 3.2). The imaging of the sub-salt section is good and sub-salt faults cut-offs can be easily determined. Petrophysical data from 21 vertical/deviated wells are available, along with formation tops to calibrate and constrain horizon interpretation. The majority of the wells are located in the eastern minibasin and in the crestal area. Only two wells are located in the western flanking minibasin. This made the correlation between the two minibasin challenging.

3.1.2 Salt Dome Province Dataset

The dataset is a grid of 3D seismic reflection time migrated, zero phase and European polarity, covering an area ca. 4114 km2 from the Danish Central Graben and extends from the southern portion of the Tail-End Graben into the Salt Dome Province (Figure 3.3). The 3D grid has an inline and crossline spacing of 25 m and consists of 2998 inlines (trending E-W) and 4796 crosslines (trending N-S) and 4 ms vertical sampling interval and covers a two-way travel time

(TWT) of 8000 ms.

The overall quality of the data is excellent with lower quality below 5000 ms (Figure

3.4). The data set images in the southern portion several large salt structures with minibasins that surround the structures and in some places a layer of undeformed salt. No salt was imaged in the northern part of the dataset. The major structural feature in the dataset is the Coffee Soil Fault bounding the data to the east and north, separating the Ringkøbing-Fin High (to the east) from the Tail end Graben (to the west). The sub-salt section is dissected with several faults where most of the structures developed above them.

86 vertical/deviated wells are available along with formation tops to confine the surface interpretation. The wells have a suite of petrophysical data covering different depths. The wells are well distributed over the survey area (Figure 3.3), which made the correlation of the interpreted surfaces between different minibasins more straightforward.

3.2 Methodology

Several techniques and methods were used in order to address and answer the questions of this study:

(1) Seismic interpretation of key horizons and faults.

(2) Structure maps for the interpreted surfaces were performed.

(3) Isochron maps for different sedimentary packages were produced to evaluate the change in thickness spatially and temporally.

(4) Seismic attributes were also generated and used to complement the seismic interpretation.

(5) Fault throw analyses (throw-distance) for several faults were calculated.

(6) Numerical forward modeling was used to investigate and test the plausibility of fault formation atop of salt wall in the Paradox Basin. Two different mechanisms were tested; dissolution and extensional forces. Each of the used methods will be elaborated in the below sub- sections.

3.2.1 Seismic Interpretation

Seismic interpretation is the core method in this study. The interpretation of 3D datasets was carried for both study areas; Paradox and the North Sea Basins. Petrel™ seismic interpretation software has been used to interpret the seismic data, to create time structural and time thickness maps (isochron maps) to analyze the evolution of salt structures with respect to the regional deformation.

3.2.1.1 Paradox Basin

It was decided to interpret key horizons to unravel the evolution of the northern end of

Salt Valley salt wall. Based on the provided formation tops, eight horizons were interpreted across the entire dataset. Since the data quality is good, seeded 2D autotracking was used to interpret most of the horizons. Autotracking is a tool in Petrel™ uses a seedpoint on a particular

46 seismic profile and automatically (based on criteria assigned by the interpreter) interpret the horizon. However, in most of the crestal areas manual picking was used due to the lower quality data and the abundance of faults. Top Leadville Formation, the top of the sub-salt unit, was not penetrated by any of the wells. It was interpreted based on the stratigraphic position, structural configuration and the seismic facies. The wells in the crestal areas penetrated the salt, so they were used to delineate top of the salt (Paradox Formation). Since salt is usually imaged in the

Paradox Basin as chaotic seismic facies thus, seismic facies were also used along with formation tops to interpret and delineate the salt body. The Cutler Group top is a high amplitude surface and was easily interpreted over the entire seismic survey. The surface was also constrained by few wells that penetrated the Cutler Group. The Triassic successions consist of Moenkopi and

Chinle formations and both tops were penetrated by several wells in the eastern minibasin, but only two wells were provided in the western minibasin which made the interpretation more challenging. Cutler Group, Moenkopi and Chinle formations were deposited while the salt was deforming and rising as is indicated from thickness variation along the strike and across the salt wall adding more complexity to the interpretation. In the Jurassic section, only one formation top was interpreted; Navajo Formation, constrained by several wells. However, over the salt wall crestal area the interpretation was not straightforward due to the chaotic seismic signature and low reflection continuity. The same scenario prevails for the Cretaceous formation tops, Dakota

Sandstone and Silt. However, in the northern portion of the dataset where salt is deeper, both formation tops have high amplitude and continuous reflection, except where faults are present.

Several faults were interpreted based on horizons cut-offs, especially the Dakota

Sandstone and Navajo horizons. Seismic attributes were generated to highlight the discontinuities in seismic reflections to better interpret faults. The most helpful attributes were

47 the Chaos and Variance seismic attributes. They showed clearly the discontinuities due to faulting (Figure 3.5). The majority of faults are concentrated above the salt wall in the crestal area. Three fault arrays are represented as master faults, with many others as antithetic and synthetic faults with less displacement. The amounts of displacement generally decreasing southward along the crest of the salt wall.

3.2.2 Danish Central Graben

To unravel the tectono-stratigraphic evolution of the study area, several key horizons were interpreted. The interpretation of the horizons was constrained by many wells across the entire study area. The deepest interpreted horizon is the top of sub-salt (top of Rotliegende

Group), although none of the provided wells have penetrated the top sub-salt Rotliegende Group.

Therefore, the interpretation of this horizon was solely based on stratigraphic position. This horizon is dissected by several basement-involved faults, that controlled salt structure location.

Due to the high reflection coefficient between the salt unit (Zechstein Group) and the supra-salt, the top of the Zechstein Group is imaged as a high amplitude reflection and the seismic facies of the salt appeared chaotic, predominantly low amplitude and low continuity. Generally, the interpretation of the top of salt was therefore straightforward, except where the salt has formed tall stocks with bulb and stem geometry. The image below the bulb structure is of lower quality making the interpretation more challenging. The interpretation of top salt was performed by using the MultiZ tool in Petrel™. This tool is designed to map complex surfaces where a horizon can have a multi Z values. In general, salt thickness thins northward in the seismic survey to reach a thickness which is not imaged by the seismic data resolution.

The supra-salt section was deposited in minibasins surrounding the salt structures or in a fault controlled half-graben depocenters. The Triassic section is imaged as a thick, layered unit

48 penetrated by several wells. In the area close to the salt structures where the sedimentary succession is highly deformed, manual picking was utilized, otherwise, 2D seeded autotracking was used.

One surface was interpreted in the Jurassic section; the Top of Mid-Jurassic constrained by wells and by the stratal terminations. The Base Cretaceous Unconformity (BCU) is a regional surface that is widespread over the North Sea Basin (Kyrkjebø et al., 2004). Even though a formation top is available to define this surface, stratal terminations also helped in delineating this surface. The two youngest interpreted horizons are Top of Cretaceous (Ekofisk Formation) and Mid-Miocene Unconformity (MMU). They are of moderate-to-high amplitude and high- continuity across the entire dataset.

The upper Cretaceous to Cenozoic successions is characterized by high density and small faults (polygonal faults). Since they are out of the scope of this study, those faults were not interpreted. The interpreted faults lie mainly in the Triassic – Jurassic section, with small to large displacement. Many of the faults are concentrated above salt structures.

3.2.3 Seismic Attributes

Seismic attribute analysis is one of the powerful aids for seismic interpretation. Utilizing this method can greatly influence the quality of the interpretation. Exploiting the various types of seismic attributes, supplemented in Petrel™, helps define many structural elements and discontinuities in the seismic data such as faults and salt bodies (Figure 3.5). Thus various types of seismic attributes were produced to help and improve the seismic interpretation of this study.

3.2.4 Isochron and Time Structure Maps

Isochron and structural maps were generated for each of the interpreted surfaces in the study. The generated maps (isochron and structure maps) are in time domain using Petrel™.

3.2.5 Throw-Distance Analysis

Throw distance analysis is a tool that used to understand fault development and evolution and how throw changes along the strike of a fault. This analysis was performed for many faults in the study area by utilizing the Fault Analysis tool in Move ™ software and manual picking of horizon cut off. Excel was used to plot throw versus faults length. The Two-way travel time was converted to depth by using depth-velocity relationship from the available well logs for both datasets; Salt Valley, Utah and Danish Central Graben, offshore Denmark (Figure 3.6 and

Figure 3.7).

3.2.6 Finite Element Method

3.2.6.1 Overview

The finite element method (FEM) is a numerical technique which solves partial differential equations (PDE) approximately (Zienkiewicz & Taylor, 1989). The FEM and its practical analysis are used to solve a wide spectrum of problems from structural, thermal and stress analysis (Fish & Belytschko, 2007).

Elfen is a commercial software package for FEM. It is developed by Rockfield Ltd. The

Elfen Forward Modeling (FM) Module, enables both expert and neophyte users to create, modify and analyze structural geological finite element simulation at any level from basic to complex.

The forward modeling has several steps that follow a very simple and straightforward workflow

(Figure 3.8).

3000 m

Figure 3.6 Depth-Time relationship derived from 16 wellbore check-shots data form Salt Valley salt wall. Time was converted to depth by using the equation in the upper right corner.

Figure 3.7 Depth-Time relationship derived from 9 wellbore check-shots data from Danish Central Graben. Time was converted to depth by using the equation in the upper right corner.

Figure 3.8 The general workflow of forward modeling in Elfen.

Elfen FM has been applied to simulate and understand a range of complex events such as the development of turtle structures, formation of regional and counter regional systems and the evolution of the many structural features associated with passive margins (e.g. Nikolinakou et al., 2014).

In this study, numerical forward modeling was performed using Elfen™ software. The aim of performing numerical modeling is to investigate the plausibility of salt dissolution and withdrawal on the formation of collapse salt structures in the Paradox Basin especially in the northern end of the Salt Valley. Elfen has been used as the fundamental tool to design and perform numerical modeling based on finite element method (FEM).

3.2.6.2 Model geometry

The collapsed structures in the northern end of Salt Valley were modeled using 2D numerical modeling. Geometries of the 2D models were obtained from this study seismic interpretation as well as published seismic and field data of the Paradox Basin Salt Walls (Figure

3.9). The geometry of the models has a length of 20 km wide and 3 km in height. The geometry of the salt has a base width of 4 km and narrows down in upward (Figure 3.10). The modeling was designed to capture two mechanisms; Dissolution and extension (Figure 3.11). For each scenario, symmetrical and asymmetrical geometries were modeled. And for each geometry a smooth top and irregular top was designed and modeled. A total of eight models were generated to investigate the formation of collapsed structures atop salt wall.

3.2.6.3 Rock Properties

One of the important aspects for performing a realistic numerical modeling is using rock properties that correspond to the properties of the rocks in the study area. Few studies have used numerical modeling in Canyonlands in the Paradox Basin (Walsh and Schultz-Ela, 2003;

Horizontal scale = Vertical scale

Figure 3.10 Geometry for the numerical modeling. Compose two layers, salt and brittle (overburden) layer.

Figure 3.11 Flowchart for the series of numerical forward modeling performed in this study.

Schultz-Ela and Walsh, 2002; and Walsh, 2000; Allken et al., 2013). Schultz-Ela and Walsh

(2002) described the numerical modeling and the choices for material properties that represent the rocks in the Paradox Basin (summarized in Table 3-1).

Table 3-1 Rock properties used for the numerical modeling.

Brittle layer (overburden) Salt layer Young’s Density Viscosity Density modulus Poisson’s Young’s modulus ratio (kg/m3) (GPa) (Pa.s) (kg/m3) (GPa) 25 0.25 2400 4.1 1018 2200

3.2.6.3.1 Salt

The Paradox Formation is comprising primarily of halite (70 to 80%) interbedded with minor cyclic anhydrite, black shale and limestone and dolomite (Trudgill, 2011). Thus the modeling was designed to capture the mechanical behavior of the Paradox Formation. An average density of 2200 kg/m3 (slightly higher than pure halite) was chosen to account for the impurities within the Paradox Formation, which are denser than halite.

Salt rock of moderate to fine grain size typically deforms as a Newtonian fluid that has no yield strength (Senseny et al., 1992, Van Keken et al., 1993 and Weijermars et al., 1993). The salt was modeled as a solid viscoplastic material, using a reduced form of the Munson and

Dawson formulation (Munson and Dawson, 1979).

3.2.6.3.2 Overburden Layer

The successions, in the study area, above the Paradox Formation vary from alluvial fans to shallow marine to fluvial and aeolian deposits. In our model, they are assumed to represent one unit and their material properties were assumed as a bulk value. Those values were obtained

57 from Schultz-Ela and Walsh (2002) and are summarized here in Table 1. The abundance of faulting in the study area indicates the overburden deforms by brittle behavior. Thus the overburden in the numerical modeling was modeled as elastoplastic material using the SR3 constitutive model from the Elfen material library (Rockfield, 2010). SR3 is a critical state formulation that is based on the principles of Modified Cam Clay (MuirWood, 1990).

4 CHAPTER 4 RESULTS

4.1 North End of Salt Valley Salt Wall, Utah

4.1.1 Salt Geometry

The available seismic data cover the northern end of Salt Valley salt wall. The salt wall strikes NW-SE and extends across the entire seismic dataset. Interpreting top salt is always a challenge. However, many techniques have been used to help in the interpretation and delineation of top of salt such as seismic facies, seismic attributes (Variance) and horizon cut- offs. Salt Valley salt wall in the study area plunges to the northwest (NW), where top of salt is deepest at around -1300 (ms) TWT (Figure 4.1). To the southeast (SE) the salt shallows and reaches around -700 TWT (Figure 4.1b). The top of salt wall is flanked on its both sides with minibasins filled with thick sedimentary sequences of Permian to Cretaceous age. To the SE of the study area, the Paradox salt is welded on both sides of the salt wall (Figure 4.2). Toward the

NW however, a thin salt layer separates the pre-salt from the post-salt sections (Figure 4.2). The salt wall varies in geometry along its strike from symmetrical in the SE to asymmetrical in the

NW (Figure 4.2). Faults are present above and below the salt wall. Both fault systems (sub- and supra-salt) have normal sense of displacement. The salt wall trend follows the sub-salt faults which strike NW-SE. The supra-salt fault arrays can be categorized into multiple fault families.

The interpretation of the supra-salt faults reveals relatively more throw toward NW where the salt is deepest and less throw toward the SW where the salt wall becomes shallower. The structural style of the supra-salt faults seems to be affected by the geometry of the salt wall.

Where salt wall is asymmetrical (e.g. NW part of the salt wall), supra-salt faults appear to form a half-graben (Figure 4.2); and where salt is symmetrical, the supra-salt faults form asymmetrical graben (Figure 4.2).

Figure 4.1 (a) 3D perspective view of the interpreted top of salt horizon. Three variance attribute profiles are plotted in the figure showing the discontinuities (faults) above the salt wall. Notice the change in the structural style along the strike of the salt wall. Half- graben geometry in the NE changes to graben style in the SE. (b) Salt Valley salt wall profile showing the top salt variation along the strike of the salt wall. Note plunging of the salt wall to the NW

4.1.2 Stratal Geometry

The pre-salt succession is seismically well imaged down to the depth of -2250 ms (Figure

4.2). Figure 4.4 is a time structure map of top pre-salt along with interpreted faults. The map shows the general dipping trend of the pre-salt unit to the NE. This interpretation matches the interpreted regional dipping trend of the pre-salt successions based on regional 2D seismic data

(Barbeau, 2003; Trudgill, 2011).

Geometrically, the supra-salt successions can be grouped into two groups; lower and upper. The lower group (Permian Cutler Group up to the upper Triassic Chinle Formation) is highly variable in thickness along the strike of the salt wall and between the eastern and western flanking minibasins (Figure 4.3). This thickness variations are due to salt mobility during the

Permian and Triassic (Paz, 2006; Trudgill, 2011). The lower Cutler strata dip and converge toward the salt wall (Figure 4.2) and their thickness decreases toward the salt wall (Figure 4.3).

While the upper Cutler and Triassic strata dip away from the salt wall (Figure 4.2); and their thickness increases toward the salt wall (Figure 4.3). This stratal geometry is interpreted to be due to the interplay between salt movement and sedimentary influx (Trudgill, 2011; Giles and

Rowan, 2012).

On the other hand, the Jurassic and Cretaceous sections are almost uniform in thickness in both flanking basins and along the strike of the salt wall (Figure 4.3). This indicates the quiescence of salt mobility during the time of deposition. The post-Triassic strata form an anticlinal geometry where the hinge of the fold is primarily atop the salt wall and the eastern and western limbs dip away from the salt wall; to the NE and SW respectively (Figure 4.2). The crestal area of the anticline is dominated by faults arrays forming graben and half-graben structures resulting in collapsed structure (Figure 4.2).

The thickness of the Jurassic and the Cretaceous strata in the crestal area above the salt wall exhibit a uniform thickness as well (Figure 4.2). The quality of the seismic data in the crestal area is relatively poor due to the high grade of fracturing however, with careful examination no growth strata have been observed next to the crestal graben faults. This implies that the faulting postdates the youngest strata in the seismic dataset (e.g. post- Mancos shale).

4.1.3 Fault Systems Analysis

The interpretation of the seismic dataset reveals two fault systems; in the sub-salt and in the supra-salt sections. The two systems are completely decoupled by the thick Paradox Salt; there are no signs of any linkage (hard- or soft-linkage) between the two systems. For the purpose of this study only a brief discussion will be conveyed for the sub-salt fault system and as a more detailed analysis of the supra-salt fault system was performed and it will be discussed in the following section.

Three horizons were interpreted to assess the deformation in the crestal area above northern end of Salt Valley salt wall. Navajo Formation of the Lower Jurassic, Slick Rock

Formation of the Middle Jurassic and Dakota Formation of the Lower Cretaceous were chosen to demonstrate the deepest, middle and the shallowest horizons which have been offset by supra- salt faults (Figure 4.5). The time structure maps in (Figure 4.5) show (1) the crestal graben is narrowing downward. (2) the Dakota Silt stratigraphic surface is structurally deepest in the northwest portion of the study area and become shallower in the southeast portion.

Figure 4.5 Time structure maps for the key horizons used to analyzed the crestal faults system. (a) Top Dakota Silt Formation, (b) Top Slick Rock Formation, (c) Top Navajo Formation.

4.1.3.1 Sub-Salt Fault System

The sub-salt of the northern end of Salt Valley salt wall is well-imaged in the seismic reflection dataset used in this study (Figure 4.3). Time structure map of pre-salt (e.g. Top

Leadville) displays two major fault systems which underlie the Salt Valley salt wall that strikes

NW-SE (Figure 4.3). Two major faults were interpreted; the one to the east dips SW and the one to the west dips to the NE forming a graben geometry. The peak of the salt wall is located approximately above the center of this graben (Figure 4.2).

The two faults extend from end to end across the seismic dataset and might continue to the rest of the Salt Valley salt wall as was captured by other 2D seismic profiles across the Salt

Valley salt wall (Paz, 2006; Trudgill, 2011). Throw on the faults is about 100 ms or less; the low estimated throw might be due to the pull-up effect of the salt wall above it. The upper tips of the two faults decouple at the evaporite-bearing Paradox formation (base of the salt wall) (Figure

4.4).

4.1.3.2 Supra-Salt Fault System

4.1.3.2.1 Faults Geometry

A total of 10 normal faults were interpreted in the supra-salt fault system; composed of two fault arrays located to the east and west of the northern end of Salt Valley salt wall (Figure

4.6); best illustrated with a time-structure map of Dakota Silt horizon (Figure 4.7). The fault arrays comprise a series of five right-stepping overlapping fault segments with lengths up to 10 km long (Figure 4.7). For descriptive purposes, the fault array has been divided into 2 primary groups, defined by their geographical location above the salt wall. They are labeled here as (E) and (W) for the eastern and western fault arrays, respectively. The strike orientation of the fault arrays is predominantly NW-SE (Figure 4.8). However, they dip in opposite direction. The E

68 fault array dips to the SW and the W fault array dips to the NE (Figure 4.9). The E faults generally have steeper dips, while the W faults dip shallow with depth and have a strongly listric fault geometry specially in fault segments W1 and W5 (Figure 4.10).

The supra-salt fault system led to the development of a series of grabens and half-grabens that trend subparallel to the salt wall beneath them; with throws of hundreds of meters (Figure

4.12). One striking observation of the crestal graben is the variation in structural style along the axis of the graben system. The crestal graben geometry changes from asymmetric, to the NW, to symmetric, in the SE. The change in geometry from asymmetric to symmetric seems to follow the geometry of the top of the salt structure (Figure 4.2). Where the top of salt has an asymmetrical geometry, the graben system tend to form half-graben geometry bounded by a listric master fault on one side with smaller antithetic faults on the other side (Figure 4.2a).

Where the top of salt geometry has a symmetrical geometry, the graben system is symmetrical graben, bounded on both side with planar faults (Figure 4.2b).

In each fault array, several faults overlap and relay zones are formed in the overlap zones.

This can be seen between fault segments E2, E3 and E4 (Figure 4.7). And between the western fault segments W1, W2 and E3 (Figure 4.7).

4.1.3.2.2 Lateral throw Distribution of the Supra-Salt Fault Array

In order to investigate the growth history of the supra-salt fault arrays in the northern end of Salt Valley, faults throw was calculated and throw profiles were generated to examine the throw distribution throughout the system. Throw was calculated along all the faults at one stratigraphic level (Dakota Silt) (Figure 4.12). The maximum throw recorded was on the western fault array on fault segment (W1), toward its center with throw amount of 554 m (Figure 4.12).

On the eastern fault array a maximum fault throw was recorded on fault segment (E4) (Figure

4.12).

The eastern fault array (E) is composed of five south-west-dipping fault segments: E1-5.

Fault segment E1 is the shortest fault segment in the E fault array. Its throw profile displays a bell-type geometry, where the maximum throw (94 m) is located at the center of the 1 km long fault segment (Figure 4.12). The southern portion of the fault overlaps fault segment E2 which has a length of about 7 km. The throw profile of E2 is similar to a half-ellipse geometry, where the maximum throw (151 m) is toward the NW tip of the fault and decreases southward toward its southern tip (Figure 4.12). The entire length of fault segment E3 is overlapped by fault segment E2. The throw geometry of E3 represent a skewed bell-shape. The throw reaches its maximum (108 m) where fault segment E2 starts to decrease in throw (Figure 4.12). Fault segment E4 is the longest in this fault array. It reaches a length of about 9 km. The geometry of the throw profile exhibits a plateau with irregular top. The maximum throw (440 m) is located somewhere close to the center of the fault and throw decreases toward the fault segment tips

(Figure 4.12).

The western fault array (W) is composed of five north-east-dipping fault segments: W1-

5. The cumulative throw profile for the western fault array (W) has a broadly flat-top T-x profile.

Fault segment W1 is the longest segment in this fault array. It extends for about 12 km across the study area. The throw profile exhibits an almost flat top in the middle with sharp drop in throw value toward its tips (Figure 4.12). The maximum throw recorded is 554 m, located towards the center of the fault segment. At both ends of this fault segment, shorter fault segments overlap with fault segment W1. Faults (W2 and W3) both start where the throw of W1 is steeply dropping (Figure 4.12). Two relay ramps exist in the overlapping zones between E1 and E2 and

70 between E1 and W3 (Figure 4.12). Both the E2 and E3 fault segments display similar throw geometry (half-ellipse geometry), where the maximum throw is located toward the northwestern tip. The maximum throw is 301 m and 432 m on W2 and W3 fault segments, respectively.

Figure 4.6 Map view of all faults mapped using the Salt Valley 3D seismic survey. White dashed polygon represents the extent of 3D seismic data.

Figure 4.7 Time-structure map and corresponding fault polygon map for seismic horizon Dakota Silt (contour interval = 30 ms).

Figure 4.8 Rose diagrams of faults strike for entire fault arrays (E and W fault arrays).

4.1.3.2.3 Vertical throw distribution of the Supra-Salt Fault Array

Throw-depth (T-z) plots were constructed on eight seismic inlines to investigate and capture the vertical variation in throw gradient on the supra-salt fault system. The plots were constructed at the relay-zones for the eastern and western faults and at the maximum throw of the main fault segment. The throw was calculated for three different horizons Dakota Silt, Slick

Rock and Navajo Formations, from youngest to oldest, respectively. Four seismic inlines were analyzed to study the western fault array (Figure 4.13, Figure 4.14, Figure 4.15 and Figure 4.16) and the eastern fault array (Figure 4.17, Figure 4.18, Figure 4.19 and Figure 4.20). In each figure,

Uninterpreted seismic profile, interpreted profile and throw vs. depth plot (T-z) is shown. The T- z plots are plotted for each fault presented in the seismic profile for three seismic horizons

(Dakota Silt = yellow, Slick Rock = orange and Navajo = red, Formations). Faults of interest are drawn in bold solid-black polylines and other faults with thin dashed-black polylines. 73

Figure 4.12 Throw-distance (T-X) plots for segments (E1-5) top and (W1-5) bottom of the supra- salt fault system based on Dakota Silt horizon. Time structure map is shown for reference.

The north-most profile is inline #508 (Figure 4.13). In this seismic profile, fault segments

W1 and W2 are captured. Both faults have a listric geometry, where the fault trace is planar in the upper part and shallows with depth. In the footwall and hanging-wall of W1, the strata are planar. However, in the hangingwall of the W2 fault segment, the strata display a rollover geometry with slightly upturning toward W2. The throw of the three horizons is almost constant on both faults W1-2. The average throw on W2 is about 310 m and higher than fault segment W1 where the average throw is about 80 m. The area between W1 and W2 fault segments represents a relay zone, with throw on W1 transferring to W2 through this relay zone.

The seismic profile of inline # 328 is the second profile analyzed from the north. This profile displays fault segment W1 only (Figure 4.14). In a same manner as Inline # 508, the geometry of W1 fault is displaying a strong listric geometry (master fault) and the three eastern fault segments dip toward W1 (antithetic faults) forming a half-graben geometry. The three seismic horizons in the hanging-wall dip toward the W1 fault plane forming a reollover geometry. The strata in the footwall of W1 and in the far distance of W1, in the hanging-wall, exhibit a planar geometry. Notice the asymmetry of the salt wall beneth the half-graben system and how the W1 fault tapers on to the contact of top of salt wall. The T-z crossplot of W1 shows that the throw on W1 (avarage of 520 m) for the three horizons is almost constant where the T-z curve is almost straight.

Inline # 268 in Figure 4.15 shows a consistantly planar footwall geometry similar to the other profiles. Fault segments W1 and W3 are captured in this seismic profile. Both fault planes display almost a planar polyline geometry, where both faults are diminishing on the top of salt contact. The average throw on W3 (315 m) is much higher than on W1 (75 m).

Both fault planes display almost planar polyline geometry where both faults are dying at the top of salt contact. The average throw on W3 (315 m) is much higher than on W1 (75 m). However, both T-z crossplots have almost straight line, indicating similar throw for the three horizons on each of the fault surfaces. The geometry of the crestal graben still represents more of a half- graben but with less asymmetry than observed in the previous seismic profile (e.g. inline 508 and

328). The relay zone between the two faults W4 and W5 seems to be responsible for throw transfer between the two faults as it is clearly captured in the seismic profile (Figure 4.15).

Although, the salt wall geometry is interpreted to be symmetrical, with a careful investigation of the top of salt contact, a slight asymmetry can be obsereved.

Figure 4.16 displays inline # 108. This seismic profile is located in the most southern part of the seismic data. The striking observation in this profile is the drastic change of the crestal graben geometry. Contrasting the previous half-graben geometry that was present in both Inlines

# 508 and 328, the crestal graben geometry here is more symmetrical, bounded on both sides by planar faults. Fault segment W4 and 5 are captured in this seismic profile. The average throw

(265 m) on W4 is much higher than the average throw (30 m) on W5. The strata in the center of the crestal graben display a concave down geometry as oppose of the rollover geometry that is present on inline # 508 and 328. No upturn of the three strata is observed close to the W 4 and 5 fault segments. The T-z crossplot for W4 and 5 display an almost straight polyline indicate a similar throw for the three horizons.

For the eastern fault array, the northern-most profile is inline # 418 (Figure 4.17). In this seismic profile, fault segment E2 is captured. The fault surface displays a planar geometry and cross-cuts the three interpreted horizons. In the footwall of the E2 fault, the strata are planar.

However, the strata display a rollover geometry in the crestal graben area. The developed crestal graben is a half-graben and the E2 fault segment is the eastern bounding fault (antithetic). The throw of the three horizons decrease slightly up- and down-ward of the Slick Rock horizon as the

T-z plot is indicates (Figure 4.17c). The average throw of the three horizons on E2 fault segment is about 163 m. Notice the asymmetry of the top of salt geometry.

Figure 4.18 displays inline # 358. This profile is located south of inline #418 and the overall geometry of this profile is very similar to #418 except that fault segment E3 is captured here. The footwall of fault segment E2 still shows a planar geometry and strata in the crestal graben area still show a rollover geometry, forming a half-graben geometry. The vertical throw distribution on fault segment E2 is highest in the middle (Slick Rock horizon) and decreases slightly up- and down-ward (bi-directionally). The average throw is about 182 m. E3 fault segment is crosscut the D horizon only, therefore T-z cross-plot is not complete for this fault.

The throw of D horizon on E3 is almost half of the throw on E2. Both E2 and E3 fault segments display a planar geometry and are antithetic to the master fault in the western side of the graben.

The area between the two fault segment resembles a relay ramp structure.

Inline # 218 (Figure 4.19) shows consistently planar foot-wall geometry. However, one striking observation in this profile is the geometry of the crestal graben. In a contrasting manner to the two previous profiles (Figure 4.17 and Figure 4.18), the crestal graben in this profile exhibits more of a symmetrical graben geometry bounded by planar fault segments on both sides.

This coincides with symmetrical geometry of the top of salt. The vertical throw distribution on

E4 fault segment displays throw increasing downward (Figure 4.19c).

The last profile to investigate the throw distribution is inline # 78. This profile is the most southerly profile in the study area. In this profile, the geometry of the crestal graben goes back to

82 a half-graben geometry, similar to the northern profiles (Figure 4.17 and Figure 4.18). The strata in the crestal graben display a rollover geometry. Two faults are presented in this profile, E4 and

E5. Both are planar in geometry forming a relay ramp between them. E4 cross-cuts the three horizons (Dakota, Slick Rock and Navajo). The throw is minimum in the central horizon (Slick

Rock) and increase slightly up- and down-ward with an average throw of 87 m. Fault segment

E5 is cross-cutting Dakota horizon. The throw of Dakota on E5 fault segment is lower than the throw of D on E4.

4.2 Forward Numerical Modeling

Crestal grabens over salt structures, in general, have been studied extensively in the past few decades (Cater, 1970; Lohmann, 1979; Bacoccoli et al, 1980; Stokes, 1982; Jenyon, 1984,

1986, 1988b; Doelling, 1985, 1988; Baars and Doelling, 1987; Chenoweth, 1987; Mart and Ross,

1987) and specifically in the Paradox Basin (Baker, 1933; Stokes, 1948; Cater, 1970; Kitcho,

1981; Doelling, 1988, 2000; Ge and Jackson, 1994, 1998; Ge et al, 1995, 1996; Gutierrez, 2004).

However, the origin behind the formation of the crestal graben is still not well understood. Two main interpretations of the crestal graben origin are suggested in the literature, (1) salt dissolution and (2) extensional origin.

Salt dissolution was the traditional interpretation for the formation of crestal graben in the

Paradox Basin (Cater, 1970; Doelling, 1985, 1988; Baars and Doelling, 1987) but with the advancement in the understandings of salt tectonics, an extensional process became the more favorable interpretation, especially due to the use of analog modeling in studying the deformation of salt and its overburden (Vendeville and Jackson, 1992a, 1992b; Jackson and

Vendeville, 1994; and Ge, 1996). Ge (1996) showed that crestal grabens are better explained by regional extension, local stretching or subsidence of salt bodies by salt withdrawal due to

83 regional extension and salt dissolution is only of a minor role in a crestal graben formation.

However, several studies showed that many of the faults in the Paradox basin and elsewhere are most likely to be related to salt dissolution (Gutierrez, 1996, 2004; Gutierrez et al, 2014;

Guerrero et al, 2014).

The work of Ge (1996) was a breakthrough in the origin of crestal grabens in the Paradox

Basin. His models strongly linked the formation of crestal grabens to extensional forces rather than salt dissolution/withdrawal. However, since it is impossible to realistically model salt dissolution using current physical modeling techniques, he modeled salt dissolution/withdrawal by draining the model salt (silicone) from the base (Figure 4.21) which is not quite mimicking the dissolution process in the nature.

Therefore, it is important to revisit and to elucidate this discrepancy between the dissolution and the extension models of the crestal graben faults. Here, several numerical models were produced to examine the plausibility of crestal graben formation due to salt dissolution/withdrawal and extensional forces.

I started my modeling by reproducing Ge (1996) dissolution/withdrawal models by using

2D forward numerical modeling via Elfen software. This step was performed to test the ability of the numerical modeling to mimic the previous analog models and to correlate results with the analog model results. Ge, (1996) findings were that regardless of the modeled salt geometry, two deformational zones formed, an outer extensional zone and an inner contractional zone (Figure

4.22).

Figure 4.18(a) Uninterpreted seismic profile of inline #358. (b) Interpreted seismic profile showing the three key stratigraphic horizons (D=Dakota Silt, S=Slick Rock and N=Navajo) and the trace of E2 and E3 faults. (c) Is T-z crossplot of E2 and E3 faults.

Two numerical models were generated. (1) Has semicircular salt (Figure 4.23) and (2) a triangular salt geometry (Figure 4.24). The numerical models produced almost identical results comparing to Ge (1996) results. These results granted the numerical modeling credibility to proceed.

Dissolution Models:

In this series of models, the roof sediments were added before deformation (pre- kinematic) and salt dissolution was modeled by moving the top of salt contact downward. The sequential evolution of the structures is illustrated by serial cross sections showing the strain rate.

The final deformation is shown in cross section showing the effective strain. The model geometries vary in salt symmetry and salt top roundness.

(1) Salt dissolution of smooth symmetrical salt dome

This model depicts a symmetrical salt diapir with a smooth top (Figure 4.25). In the initial stages and as the salt top moves downward, modeling dissolution, normal faults form and concentrate in the central part, over the salt diapir and diminish toward the salt. Less fault deformation occurs in the outer zone away from the salt diapir. As the model evolves, the outer zone deforms, more drastically and well defined faults form. By the end of the modeling a set of normal faults (graben system) form above the salt diapir in two distinctive families (outer and inner zone). The faults in the inner zone dip toward the right side of the model and the outer zone faults form antithetic-synthetic geometries with antithetic faults dipping toward the salt body.

The generated graben system is symmetrical in shape and it is almost a mirror image of the salt diapir.

(2) Salt dissolution of irregular symmetrical salt dome

This model resembles a more complex scenario where the symmetrical salt diapir has more complexity. The salt top has irregularities consisting of many cusps (Figure 4.26). This

This geometry of top salt has been observed in some salt outcrops and seismic images (e.g. Giles and Rowan, 2012). This complexity will test how much the geometry of the salt top controls the resultant deformation. In the early deformation stages, normal faults develop in outer zones away from the center of the salt diapir. As the model progresses, more faults develop in the inner part above the center of the salt diapir. The outer zone faults are larger in size than the inner zone faults. These large faults in the outer zone detach on the salt surface and dip towards the salt body. Faults in the inner zone are smaller and dip toward the salt diapir. The geometry of the outer zone faults resembles antithetic-synthetic sets of faults. The most noticeable observation is that many of the faults nucleate above those cusps.

(1) Salt dissolution of smooth asymmetrical salt dome

In this experiment, the geometry of the salt diapir is asymmetrical with smooth salt surface (Figure 4.27). In the early stages of the model, a large fault develops above the left flank of the salt diapir and starts to detach on the salt surface. As the model evolves through time, smaller faults start to develop on top of the salt diapir. One of the faults has a longer length forming a master fault. To the right of the master fault, smaller faults form and dip toward the master faults forming an antithetic-synthetic geometry. The final crestal normal faults possess an asymmetrical geometry (half graben), where there is one major fault (on the left) and smaller faults (antithetic) dipping toward the master fault. Strain also accumulated in the right side of the model forming a smaller graben. This strain buildup, in both flanks of the salt diapir is related to the movement of both sides of the diapir.

a. b.

(2) Salt dissolution of irregular asymmetrical salt dome

This model has more complexity where dents were added to the top salt surface (Figure

4.28). In the early stages, cusps in the salt top surface play as a focal point for strain buildup in the inner zone. In the outer zone, few faults start to develop. The formation of these faults is related to the movement of the salt diapir flanks. As the model evolves through time, a master fault fully develops, with smaller antithetic faults. There are two areas where strain develops in the outer zone forming two different graben systems. The final graben geometry in the inner zone has asymmetrical geometry.

Extensional Models:

In this series of experiments, extension was modeled as bending the section upward rather than stretching the sides of the model (Figure 4.29). The upward arching over the middle of the model produced an extensional zone that is required to model the effect of extensional forces on the salt diapir and its over burden. The overburden is modeled as a pre-kinematic. The progressive evolution is illustrated by serial cross sections showing the strain rate in each stage

(every 0.2 Ma). The final effective strain is shown in a separate cross section. The series of the model geometries differ in salt diapir symmetry and salt top roundness (Figure 4.30, Figure 4.31,

Figure 4.32, Figure 4.33).

(1) Extensional deformation of smooth symmetrical salt dome

In this experiment, extensional forces are applied to a 2D model of symmetrical salt diapir with a smooth top (Figure 4.30). The model starts to develop strain (faulting) in the inner zone above the salt diapir. Then, strain accumulation begins to develop outward in an outer zone above the flanks of the salt diapir. The inner zone faults are smaller in size with respect to the

97 outer zone faults. The inner zone shows symmetry in fault distribution and faults do not extend to the salt top by the end of modeling time (1 Ma). Faults in the outer zone also form a symmetrical graben and they cease before reaching the salt top.

(2) Extensional deformation of irregular symmetrical salt dome

This model consists of symmetrical salt diapir with irregular salt top (Figure 4.31). Most of the strain is developed in the outer zone and minimum strain is accumulated in the inner zone.

The outer zone faults are symmetrical, forming symmetrical grabens and in some places faults detach on the top salt surface. The inner zone faults are much smaller in size and they diminish within the overburden section above the salt diapir.

(3) Extensional deformation of smooth asymmetrical salt dome

The evolution of structures above the extended asymmetrical salt diapir with smooth salt top is shown in Figure 4.32. The first deformation appears to start in the inner zone above the salt diapir. The overburden of the inner zone is deformed by multiple faults that detach on the top salt surface. These faults dip in the same direction (synthetic) in a half graben style. Then, the outer zone starts to deform and many normal faults develop. Those faults form a graben systems above the salt diapir flanks.

(4) Extensional deformation of irregular asymmetrical salt dome

The salt diapir has an asymmetrical geometry and its top surface is irregular (Figure

4.33). At an early stage, the extensional normal faults start to develop in the inner zone above the salt diapir. The resultant faults nucleated above the cusps of the top salt and extend all the way to the surface. In later stage, the faults grow and detached on the top salt. Faults also developed in the outer zone and they appear to be larger in size.

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4.3 Danish Central Graben

4.3.1 Distribution and Geometry of the Zechstein Salt

The Zechstein Salt shows an overall southward increase in thickness (Figure 4.34), ranging from not seismically resolvable deposits on the Poul Plateau in the north, to major evaporite pillows, diapirs and wall-and-sill complexes in the south of the 3D seismic survey. The salt structures in the southern domain varies in shapes between salt pillows (e.g. Kraka salt pillow) to tall salt diapir (e.g. Skjold). However, the majority of the structures are characterized by tall salt stocks except for few of them (Figure 4.34). In the eastern part of the study area, the salt is absent on the footwall of the Coffee Soil fault. The salt structures are typically surrounded by areas where the Zechstein Salt is significantly thinned, welded or even absent. The thick sedimentary sequences in the minibasins around the salt structure indicate the presence of the salt before the deposition of those sequences and that the depositional accommodation space was created by the lateral migration of the Zechstein Salt (most likely toward the present time salt structures) or salt-dissolution. The location of the salt structures coincides with the location and orientation of the major sub-salt fault systems (Figure 4.34). Estimating the original depositional thickness of Zechstein Salt in the southern domain is very challenging. Due to the potential of salt to laterally migrate, in addition to given the nature of salt to lose its volume by dissolution.

However, Duffy et al., (2012) proposed the thickness of Zechstein Salt across the southern domain to be around 200 to 500 ms TWT. They based their estimation by qualitatively redistributing the mass of evaporites contained within the salt structures, as well as accounting for stratigraphic thicknesses in the surrounding minibasins.

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4.3.2 Stratal Geometry

The pre-salt succession is seismically well imaged down to the depth of -5000 ms (Figure

4.35). The top of pre-Zechstein surface in the study area range in depth from – 4500 ms in the hanging wall of Salt Dome province, to -2500 ms in the footwall of Coffee Soil fault (Figure

4.34). Figure 4.34 is a time structure map of top pre-salt along with interpreted faults and overlain by contour map of top of salt of the Zechstein salt. The pre-Zechstein is dissected by several basement faults striking N-NW (Figure 4.34). Many of the salt structures location are coincide and evolved above the basement faults (Figure 4.34).

Generally, the Triassic succession exhibit tabular geometry with few local thickness variations that seem to be controlled by fault displacement and salt evacuation (Figure 4.36).

This is clear in the southern part of the study area. Generally, Triassic sedimentary package thins northward and thickens southward where is seems controlled by the N-S segment of the Coffee

Soil Fault (Figure 4.36).

The accommodation of the Jurassic succession was highly influenced by salt movement.

The thickness of the Jurassic is thin over the current salt structures and thickens in the minibasin adjacent to the salt structures (Figure 4.36). Accommodation was also controlled locally by the displacement on basement fault (e.g. Coffee Soil Fault).

Thickness of the Cretaceous succession display thickness variation however, of less magnitude compared to the Jurassic sedimentary package. The accommodation is most likely controlled by salt evacuation underneath the minibasins. The subtle thickness variation of the

Cretaceous sedimentary package might indicate that the salt has welded in many places around salt structures.

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The Cenozoic section display a slight thickening above salt minibasin and dramatically thins over the salt structures (e.g. Skjold). The minor control of the salt evacuation in the minibasin and the thinning of the Cenozoic strata above salt structure suggest that salt structure might continue rising by passive diapirism or due to contractional forces associated with the inversion episode that took place in both Paleogene and Neogene (Cartwright, 1989; Vejbæk &

Andersen, 2002; Rank-Friend and Elders, 2004; Rasmussen, 2009).

4.3.3 Fault System Analysis

The interpretation of the Danish Central Graben seismic data reveals two fault systems; sub- (basement) and supra-salt (cover) fault systems. Only one of the basement fault segment

(Coffee Soil Fault) penetrate the supra-salt succession however, the other basement faults are decoupled by the thick Zechstein salt. The cover faults (supra-salt) system are generally restricted above salt structures and might be a result of basement soft-linkage (Rank-Friend and

Elders, 2004 and Duffy et al., 2012). For the purpose of this study only a brief discussion will be conveyed for the sub-salt fault system and more detailed analysis of the supra-salt fault system was performed and it will be discussed in the following section. The cut-offs of top of pre-

Zechstein were used to interpret the basement faults (Figure 4.35). For the cover faults interpretation, cut-offs of top of Mid-Jurassic were utilized (Figure 4.35).

4.3.3.1 Sub-Zechstein Fault systems

The pre-Zechstein surface is dominated by N-S to NNW-SSE striking and W- to WSW- dipping faults (Figure 4.34 and Figure 4.35). The most significant faults in the study area are

Coffee Soil Fault and those who are underlining salt structures (e.g. Kraka and Skjold) (Figure

4.34 and Figure 4.35). The Coffee Soil fault is a major fault that change in strike from N-S in the

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Figure 4.36 Time thickness maps of (a) Cretaceous , (b) Middle and Upper Jurassic And (c) Triassic. Notice how thickness varies above the salt structures and in the minibasin around them. In the Cretaceous time, accommodation was controlled by sub-Zechstein fault in the central part of the map and thins above the salt structures. In the Jurassic time, the deposition was concentrated in the hanging wall of the Coffee Soil Fault and the other sub-Zechstein faults. In the Triassic time, sediments were deposited and thickened in the minibasins around the salt structures.

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111 south to NNW in the north dipping to the west. It shows an average vertical displacement of about 2000 ms TWT (Figure 4.34). The other major fault is located to the west of Coffee Soil

Fault (Figure 4.34). It strikes N-S and dips to the west. Several salt structure are located above this fault. There are two smaller (shorter) faults that underlain Kraka and Skjold salt structure.

Both are striking NW however, Kraka fault is dipping to the west and Skjold fault dips to the E.

4.3.3.2 Supra-Zechstein Fault Systems

A total of 52 faults were interpreted in the supra-salt fault system above two salt structures in the study area (Figure 4.37). The interpreted fault arrays in the supra-salt of Kraka salt pillow strike in a wide range of azimuths. Therefore, only 6 fault segments were chosen for further fault analysis. They are all located to the north-west of Kraka salt pillow. They all strike to the WNW and dip to the NNE (Figure 4.37). Another fault family were interpreted above salt structure to the SW of Kraka pillow (Figure 4.37). This fault family comprise of 8 fault segments. They strike NW-SE and dip to the SW and NE forming graben geometry (Figure

4.38).

4.3.3.2.1 Lateral Throw distribution of the supra-salt fault arrays

In order to examine fault throw distributions, faults throw was calculated and throw profiles were generated for the two fault families (Figure 4.39 and Figure 4.40). Throw was calculated along all the faults at one stratigraphic level (top Mid-Jurassic) (Figure 4.39 and Figure

4.40). The maximum throw recorded for the Kraka supra-faults on fault segment (K1) is 660 m

(Figure 4.39).

The Kraka supra-salt fault array is comprised of six NE dipping fault segments: K1-6.

Fault segment K1 is the longest (12 km) with maximum recorded throw of about 660 m (Figure

4.39). Its throw profile displays a skewed bell-shape geometry with maximum throw toward SE

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(Figure 4.39). Fault segments K2, K3 and K5 have more or less the same length (10 km). Their throw profile is similar too and display a plateau geometry with maximum throw of about 220,

270 and 180 m, respectively (Figure 4.39). Fault segment K4 and K6 are the shortest and their throw profile exhibit roughly a half-ellipse geometry (Figure 4.39) with maximum throw of about 140 and 200 m respectively.

Eight fault segments (G1-8) were interpreted in the supra-salt above the salt structure that is located to the SW of Kraka pillow (Figure 4.38). Fault segment G1 and G3 are the longest segments in this fault array (about 12 km). Throw profile of G1 display an almost flat top with sharp drop in throw value toward its tips (Figure 4.40) with maximum recorded throw of 780m.

On contrary, G3 throw profile increase in throw value toward it tips (Figure 4.40). G2 and G5 are the shortest fault segments and their throw profile display a bell-shape geometry with maximum recorded throw of 280 and 240 m, respectively (Figure 4.40). G4 fault segment exhibits half- ellipse geometry where the maximum throw is on its NW tip (810 m) and minimum throw is on its SE tip (330 m) (Figure 4.40). Throw profile of fault segments G6 display a shallow plateau geometry. Fault segment G7 throw profile displays bell-shape geometry with a maximum throw of 190 m (Figure 4.40). G8 fault segment show a bi-modal geometry with a maximum throw of about 290 m (Figure 4.40).

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Figure 4.38 Time Structure map of top Mid-Jurassic with interpreted supra-salt fault arrays. The white box to the top is Figure 4.39 and the lower white box is Figure 4.40.

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Figure 4.39 Throw-distance (T-x) plots for segments (K1-6) of the supra-salt fault system based on top Mid-Jurassic surface. Time structure map of Mid-Jurassic surface is shown for reference.

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Figure 4.40 Throw-distance (T-x) plots for segments G1-8 of the supra-salt fault system based on top Mid-Jurassic surface. Time structure map of Mid-Jurassic surface is shown for reference.

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5 CHAPTER 5 DISCUSSION

5.1 Salt-Dissolution in The Paradox Basin

In central parts of many of the salt valleys in the northern Paradox Basin (e.g., Salt

Valley, Paradox Valley, Cache Valley and Onion Creek), the Paradox Formation salt is outcropped and present on the surface (Rasmussen, 2014). Evaporites are exposed in discontinuous outcrops (Figure 5.1) within the breached core of the salt wall and consists of folded and faulted Pennsylvanian strata and evaporites, with much of the halite component removed by dissolution after reaching shallow depths (e.g., Doelling, 1985, 1988). Salt- dissolution has occurred during many time periods (Rasmussen, 2014), indicated by the presence of Pennsylvanian Paradox Formation clasts in younger rocks. However, the latest episode of salt- dissolution began as the Colorado Plateau uplifted, in Late-Neogene times, and was subjected to erosion and dissolution. Radiometric dating of volcanic rocks interbedded with sediments indicates that the Colorado River drainage did not exist before 6.0 Ma (Spencer et al, 2001), but was definitely established by 4.4 Ma (Faulds et al, 2001). Although the Colorado River

Paradoxically flows crossing perpendicularly to the salt valleys in the Paradox Basin, many other small (seasonal?) tributaries flow along strike in the central parts of the salt valleys (Figure 5.1).

This tributary system might also play a role in the salt-dissolution of the Paradox Formation.

Google Earth satellite imagery also shows a tributary channel that passes perpendicularly through the southern portion of the study area and enters the Green River to the south-west

(Figure 5.1).

Water reaches the shallow salt either by percolation through the unconsolidated materials or through prominent open joins and fractures (Reitman et al., 2014). A thick karstic residuum

(>100 m) present at the top of the salt walls indicates a large volume of soluble material (mainly

119 halite) that has been removed by dissolution due to groundwater circulation (Doelling, 1988;

Gutiérrez, 2004). Gutiérrez (2004) suggests if the original proportion of salt to other rocks in the

Paradox Formation is about 70% (Shoemaker et al., 1958), then about 200 m thick cap-rock above the salt wall would involve removal of 460 m of salt by subsequent dissolution. The dissolution of that thick (460 m) salt column by ground-water implies a subsidence of a similar magnitude in the overburden sediments above the Paradox salt. This demonstrates the importance of salt-dissolution and the subsidence in the generation of salt valleys and normal faults.

Salt flow and dissolution rates can vary drastically temporally and spatially. Since salt can behave as a fluid over geological timescales, it can flow for quite a long distance (rise upward or flow laterally) over geological time scale. Salt flow rates are not constant and vary depending on different factors (e.g. viscosity, wet or dry salt and amount of effective strain). Salt glaciers (namakiers) in Iran, where salt at the free surface, always flow faster after precipitation; the Dashti glacier at Kuh-e-Namak flows at a rate up to 0.5 m/day during the wet season

(Warren, 2006). On the other hand, salt flows much more slowly in the subsurface. Jackson and

Vendeville (1994) proposed a rate of internal salt flow of 10 m to 2 km per million years (0.01 to

2 mm/yr).

In this study, no cap rock was observed and interpreted in the Salt Valley seismic data.

This might be due to low quality data in the areas above the salt structure. However, 2 km to the south-east of the seismic survey (Figure 5.1), Thorne (2016) mapped cap-rock that is exposed in the central parts of the Salt Valley. These cap-rock outcrops are clearly observable on satellite imagery (Google Earth) and they are distributed as small mounds and ridges on the valley floor

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(Figure 5.1). This is a strong evidence that dissolution was active and probably is still active above the Salt Valley salt wall.

5.2 How Salt-Dissolution Affects Fault Displacement

Since most of the salt valleys are crossed and superimposed by the main flowing rivers, like the Dolores River where it crosses the Paradox Valley and Gypsum Valley, and the

Colorado River where it crosses the Salt Valley and Moab Valley, salt dissolution processes might be highly enhanced by the flowing water of those river systems. However, a groundwater modeling study south of the Needles District in Gypsum Canyon (Reitman et al., 2014) shows that on average only 10-5-10-3 mm of vertical displacement per year can be attributed to dissolution of groundwater flow. However, their model shows variable rates of solute inflow at model nodes, indicating that localized dissolution might be larger than the average and may cause volume loss that can result in fault slip or ground subsidence locally. Another important finding of this study is that fracture permeability plays a significant role in controlling groundwater flow in their study area.

Many studies have focused on faults slip rates in the Paradox Basin (e.g., Furuya et al.,

2007; Guerrero et al., 2014), Guerrero et al. (2014) based on trenching data concluded a slip rate for salt-dissolution faults to be >3.07 mm/yr, which is between 2 to 25 times greater than the range estimated for the Wasatch tectonic fault. Furuya et al. (2007) published results, based on

InSAR data, deducing a maximum slip rate of 2-3 mm/yr.

This discrepancy, between dissolution rates and faults slip rates in the Paradox Basin might be due to the nature of the dissolution processes. Since the rate of dissolution is dependent

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122 on many factors such as water flow rate etc. Water flow rate can vary significantly between wet and dry periods during geological history. Another factor which might also affect this discrepancy is salt evacuation as Cater (1970) suggested, and both salt-dissolution and salt evacuation can therefore be reasons behind the deformation of the crestal areas of salt walls. The ability of salt to deform in a ductile manner, groundwater can enhance and increase internal salt flow by hydrologic weakening (Reitman et al., 2014). Crestal arching above salt walls might be another factor which leads to this discrepancy. The Neogene salt movements have uplifted the salt overburden rocks thus folding the overburden and creating extensional zone. These extensional forces might also have added to the fault slip rate. Therefore, fault slip rates in the

Paradox Basin might be controlled by many factors rather than just salt-dissolution rate alone.

5.3 Evolution of Crestal Normal Faults in Northern-End of Salt Valley Salt Wall

Here I propose a model for the origin of the crestal graben in the northern end of Salt

Valley salt wall explaining the role of salt-dissolution and internal salt flow in the formation of crestal normal faults. This model was built based on the seismic dataset used in this study for the northern end of Salt Valley salt wall (Figure 5.2). During the Laramide Orogeny (Late

Cretaceous-Early Paleogene), the Paradox Basin underwent compressional stresses, the salt structures deformed and raised up-ward creating localized arching over the crest of the salt wall

(Baars and Stevenson, 1981; Pevear et al., 1997). Moreover, these extensional forces created a fracture system over the crestal areas of the salt walls; especially where the overburden is thin

(Figure 5.2- stage 2). This fracture sets helped induced surface and ground-water circulation which led to salt-dissolution in those areas (Figure 5.2- stage 3). Salt-dissolution in the areas where salt was at shallow depths disturbed the pressure equilibrium with the salt wall system.

This led to the migration of the salt from the higher, thicker overburden in the NW to lower

123 pressure, thinner overburden in the SE areas. The continuation of salt-dissolution and salt migration with the salt wall created subsidence across the northern end of the salt wall. This gravitational subsidence was accommodated along normal faults.

5.4 Tectonic Versus Salt-Dissolution Faults

5.4.1 2D Numerical Modeling

The interpretation of the large number of crestal faults in the Paradox basin was traditionally attributed to salt-dissolution process of the Paradox Formation in the salt walls

(Baker, 1933; Stokes, 1948; Cater, 1970; Kitcho, 1981; Doelling, 1985, 1988; Baars and

Doelling, 1987). Cater (1970) however, also suggested that the flowage of salt toward areas of reduced overburden has also played a role in the collapse structure and the formation of the crestal faults. However, Ge et al., (1996) proposed another interpretation based on scaled physical models and field observations. They attributed the crestal faulting over the salt walls to regional extension and suggested that salt-dissolution had only a minor role on the development of those structures. Their salt-dissolution physical models produced an inner contractional zone, with reverse faulting and buckle folds, and an outer extensional zone with normal faults. Their field observations however, showed a lack of convincing contractional structures in the salt valleys. Therefore, they refuted the traditional explanation of the crestal normal faulting due to salt-dissolution. However, many other recent studies advocate the salt-dissolution model for explaining the crestal normal faulting in the Paradox Basin and elsewhere (Gutierrez, 1996,

2004; Carbonel, et al., 2013; Gutierrez et al, 2012, 2014; Guerrero et al, 2014).

Ge and Jackson (1994) and (1998); Ge et al., (1995), (1996) modeled salt dissolution/withdrawal in a very simplistic way by draining the model salt (silicone) from the base of the model (Figure 4.21), which is not reproducing the salt dissolution and

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Figure 5.2 Evolution model explaining the role of salt-dissolution in fault formation. In (2) fractures form due to salt raising (arching effect) by the Laramide compressional stresses. (3) these fractures help in surface and ground-water circulation increasing the chance for salt-dissolution. Salt-dissolution disturbs the pressure equilibrium within the salt structure which lead the salt to migrate from higher (in NW) to lower pressure zones (in the SE). (4) the continuation of salt dissolution and migration will eventually increase the fault displacement across the NW end of the salt wall. 125 karstification in a realistic way. The presence of the cap-rock at the top of the salt walls indicates that dissolution is preferentially taking place at the top of the salt wall, not at its base as it was modeled. Also, Gutierrez (2004) argued that studies dealing with natural evaporite dissolution and subsidence agree on that sub-vertical gravity faults are the most common subsidence mechanism in lithified sediments (e.g. Christiansen, 1967, 1971; Sugiura and Kitcho, 1981;

Gutierrez, 1996; Klimchouk et al., 1996; Black, 1997; Ford, 1997; Johnson, 1997; Baars, 2000;

Gutierrez et al., 2001; Gutierrez and Cooper, 2002; Kirkham et al., 2002).

In this study, the physical models of Ge et al., (1996) were regenerated utilizing 2D numerical modeling. The results were very similar to Ge et al., (1996) documented in their work

(Figure 4.22, Figure 4.23 and Figure 4.24). So the 2D numerical modeling is indicating, that if the modeled salt was drained completely from the salt wall (as in Ge, et al., (1996)) reverse faulting (contractional zone) will be generated.

The results obtained in this study by the 2D numerical forward modeling came out in favor of salt-dissolution and withdrawal. All four salt-dissolution and withdrawal models showed after (dissolving/withdrawal salt for 200 m) that only normal faults were generated in the supra- salt section above the salt walls forming crestal graben (Figure 4.25, Figure 4.26, Figure 4.27 and

Figure 4.28). The only difference between the four models is the structural styles of the produced normal faults and crestal grabens. The structural styles seem to be dictated by the top salt surface geometry. The asymmetrical salt wall in the dissolution models (Figure 4.27 and Figure 4.28) favored development of a listric normal fault along with few antithetic normal faults dipping toward the main listric master fault. The master listric fault detaches on the salt surface while the smaller antithetic faults diminish within the supra-salt section. On the other hand, salt-

126 dissolution/withdrawal models with symmetrical salt wall (Figure 4.25 and Figure 4.26) resulted in the formation of symmetrical crestal graben bounded on both side with planar normal faults.

The extensional models of this study also produced normal faulting above the extended salt wall (Figure 4.30, Figure 4.31, Figure 4.32 and Figure 4.33).

Both the dissolution/withdrawal and extensional model share the same result which is: the crestal graben pattern is controlled by the initial salt wall geometry (Figure 5.3). Salt dissolution/withdrawal and extension of symmetrical salt wall produced symmetrical crestal graben bounded on both sides with planar normal faults. however, salt dissolution/withdrawal and extension of asymmetrical salt wall models produced crestal graben (half-graben) bounded from one side by a master listric fault and on the other side by smaller planar antithetic faults.

5.5 Fault Scaling of Dissolution and Tectonic Induced Faults

The relationship between maximum displacement (dmax) and fault length (L) has been studied extensively (Watterson, 1986; Walsh and Watterson, 1989; Marrett and Allmendinger,

1991; Gillespie et al., 1992; Cowie and Scholz, 1992a, b; Dawers et al., 1993; Scholz et al.,

1993; Clark and Cox, 1996; Schlische et al., 1996), mainly in attempts to understand how fault geometry varies over different length scales. An empirical relationship exists between maximum displacement (dmax) and fault length (L) over many orders of magnitude (Kim et al., 2005).

Generally, displacement increase on a particular fault by the increase in fault length. However, when comparing dmax/L ratios for different fault types, strike-slip faults have slightly higher dmax/L ratio than dip-slip faults (normal and thrust faults) (Figure 5.4). To date there is no database for salt-dissolution influenced normal faults. Although, there are some attempts to examine the relationship (if any) between the length of salt-dissolution influenced normal faults and their maximum displacement (Carbonel, et al., 2013; Guerrero, et al., 2014). The two studies

127 have concluded that the ratios of dmax/L are comparable with those reported for extensional tectonic faults. Thorne (2016) also obtained dmax/L ratios from subsurface seismic data and she found that the faults are over-displaced meaning that faults have more throw than what they are expected to have based on the empirical relationship. She related the over-displacement to two mechanisms: (1) enhancement of fault throw and (2) an element that can inhibit lateral fault tip propagation.

The dmax/L ratios of the faults interpreted in Salt Valley (this study) display the same relationship (Figure 5.4). Most of E faults plot in the normal fault region on the T-x cross-plot however, the W fault segments are on the higher-end of the GFD normal fault cluster (Figure

5.4) indicating over-displacement. Faults from the Danish Central Graben also display a dmax/L ratio similar the GFD normal faults indicating that tectonics are the main mechanism for their formation. Salt-dissolution might be a key factor for this over-displacement in the Salt Valley fault segments. Given the nature of salt dissolution process and that dissolution rate can be significantly higher than tectonic extensional rates in the northern Paradox Basin. Guerrero, et al.

(2014) suggested that the average coseismic slip to fault length of the crestal faults in Spanish

Valley, Utah are 8 to 42 higher than expected values for tectonic faults. The heterogeneity in the top salt geometry can play a role in strain localization which can results in fault segmentations.

5.6 Salt-Dissolution Accommodation and The Potential of Reservoir Development

Accommodation generated by salt-dissolution and the formation of collapsed crestal graben can strongly affect reservoir development. As described earlier in this study, the size of the crestal grabens in the Paradox Basin, Utah and in the North Sea can be on order of one to tens of kilometer long, potentially significant in terms of reservoir development.

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Figure 5.3 Structural models for the structural styles associated with salt-dissolution/withdrawal or extensional forces. (a) Symmetrical salt wall and the developed crestal graben (symmetrical graben) associated with either salt-dissolution/withdrawal or extension. 129

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The resultant depression from dissolution of the underlying salt could provide a focus for surficial fluvial systems draining the surrounding areas and therefore deposition of reworked sediments from the salt valley side-walls, as well as sediments transported by the transgression of the sea water. This type of deposits can form a high quality reservoir rock with good porosity and permeability. The distribution of the facies will be somewhat confined along the collapsed depression, however it will have good lateral continuity along the strike of the collapsed salt valley. A good analogue for this hypothesis can be obtained from subsurface data of Skagerrak

(Triassic) and Fulmar (Late Jurassic) Formations from the West Central Shelf, to the west of the

Central Graben, North Sea (Smith et al., 1993; Stewart, 2007).

The salt walls on the Western Central Graben (WCG) were exhumed and differentially eroded during Mid- to Late-Triassic (Wakefield et al.1993; Stewart, 2007). The dissolution of the

Zechstein salt occurred on salt walls crest. This is indicated by the well penetration of the

Zechstein in the Central North Sea where thick anhydritic residuum (tens to hundreds of feet) overlying the salt structures (Smith et al., 1993). Salt dissolution created accommodation above the salt crest for the subsequent depositional sequences (Triassic and Jurassic). Smith et al.

(1993) studied the control of salt on Triassic reservoir distribution in the UK Central Graben.

They concluded that one of the best quality reservoir potential occurred in the collapsed salt valleys.

Therefore, the salt valleys of the northern Paradox Basin and elsewhere (e.g. in the

Middle East) offer excellent geological sites for studying the stratigraphic architecture and sedimentation patterns to be used as an analog for subsurface data.

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5.7 Dissolution/Withdrawal Fault Systems and Trapping Geometries

Trapping related to salt-dissolution and extensional tectonics fault systems share some similarities and also contrast each other. In both systems, the geometry of the resultant graben form an elongated shape depressions bordered on both sides with normal faults. However, the dimensions of the extensional faults (fault length, amount of slip and the width of the resultant graben) most likely to be significantly larger due to the nature of the tectonic processes involved in extension.

Extensional faults also contrast with salt-dissolution faults in terms of footwall uplift

(Jackson and McKenzie, 1983; Barnett, et al, 1987; Schlische, et al., 1996). Footwall uplift will influence the topography surrounding the fault and is considered to be an important source of clastic material. Hence, uplifted footwalls were a major factor in the distribution of Upper

Jurassic reservoirs in the North Sea Basin (e.g. Fulmar Sand Formation). Footwall uplift may act to redirect sediments supply away from the generated graben (in a single isolated graben) however, in case of a multiple adjacent graben systems (e.g. Canyonlands National Park and

North Sea), the uplift of the footwall might be a source for sediment supply. It should also be mentioned that the proximal location of sediments source areas will directly affect in a negative manner on the quality of the deposited sediments (sorting and roundness of the sediments).

The change of top salt geometry along the strike of a salt wall most likely creates strain localization which leads to diverse fault geometries. This can cause an early linkup of the fault segments and results in shorter faults with larger displacements (over-displaced faults). This can be a criteria of salt-influenced fault system which can have implications for trapping geometries.

Smaller (shorter) faults can dissect the reservoir and create compartmentalization in the reservoir. Since more fault displacement generally produces finer grain sediments (cataclasis)

132 along the fault zone (Gibson, 1994; Harris et al., 2002; Yielding, 2002; Clarke et al., 2005) which most likely makes the faults act as seal rather than a conduit, resulting in low connectivity across the faults while good connectivity in a direction subparallel to the faults strike.

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6 CHAPTER 6 SUMMARY AND CONCLUSIONS

This study was the first to numerically model the dissolution and extensional hypotheses for the evolution of the crestal graben faults above the Salt Valley salt wall. The following conclusions can be drawn based on the results obtained:

1- The interpretation of 3D seismic data on the northern end of Salt Valley salt wall reveals:

a NW plunging salt wall overlain by along strike crestal graben. These crestal graben

alternate in geometry between half-graben bounded on one side by a listric master fault

(with smaller antithetic/synthetic normal faults) to a symmetrical graben geometry that is

bounded on both sides by planar normal faults.

2- The 3D seismic data across the Salt Valley salt wall display a relationship between the

symmetry/asymmetry of crestal graben and top of salt wall geometry. Half-graben in the

northern end of Salt Valley are associated with asymmetrical top salt geometry.

Symmetrical top salt geometry tends to be overlain by more symmetrical graben.

3- Both the dissolution/withdrawal and extensional numerical models share the same results

which are: the crestal graben pattern is controlled by the initial salt wall geometry. Salt

dissolution/withdrawal and extension of symmetrical salt wall produce symmetrical

crestal graben bounded on both sides with planar normal faults. On the other hand, salt

dissolution/withdrawal and extension of symmetrical salt wall models produce crestal

graben (half-graben) bounded from one side by a master listric fault and on the other side

by smaller planar antithetic faults.

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4- Salt-dissolution can be on a magnitude of couple of hundreds of feet therefore, can play a

significant role in creating accommodation to focus deposition of new deposits. The

dissolution of the top salt structure can create enough stress to nucleate and enhance fault

displacement in the supra-salt sequences.

5- Fault scaling, (dmax/L ratio) for the interpreted faults in Salt Valley salt wall, Utah are

characterized by over-displacement. The dmax/L ratios are plotted slightly on higher end

of the normal fault cluster of global fault database. Indicating a higher throw per a

particular fault length. On the other hand, the salt-related faults in the Danish Central

Graben are following the general trend of tectonic normal fault database.

6- Although crestal grabens related to salt-dissolution are similar in many ways with graben

formed due to tectonic extensional faults, both can vary in few aspects:

i. The dimension of the crestal grabens related to salt-dissolution are most likely to be

smaller than extensional tectonic graben due to the nature of each processes.

ii. Salt-dissolution faults can have higher dmax/L ratio which can make the fault

surfaces to be characterized by impermeable to fluid flow across the fault and acting

as a seal rather than a conduit. iii. Accommodation generated by salt-dissolution and the formation of collapsed crestal

graben can strongly affect reservoir development. The resultant depression from

dissolution of the underlain salt could provide a focus for surficial fluvial systems

draining the surrounding areas and therefore deposition of reworked sediments to

form high quality reservoir rock with good porosity and permeability.

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