I LATE MESOZOIC and CENOZOIC KINEMATIC RECONSTRUCTION

LATE MESOZOIC AND CENOZOIC KINEMATIC RECONSTRUCTION: ASSESSMENT OF LATE CRETACEOUS MAGMATISM AND SOURCE-TO- SINK CONFIGURATION IN THE NORTHWESTERN GULF OF MEXICO

A Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston

______

In Partial Fulfillment of the Requirements for the Degree Master of Science

______

By Jordan Nicole Dickinson August 2017

LATE MESOZOIC AND CENOZOIC KINEMATIC RECONSTRUCTION: ASSESSMENT OF LATE CRETACEOUS MAGMATISM AND SOURCE-TO- SINK CONFIGURATION IN THE NORTHWESTERN GULF OF MEXICO

______Jordan Dickinson

APPROVED:

______Dr. Michael Murphy, Chairman

______Dr. Joel Saylor

______Dr. Ana Krueger

______Dr. Pete Emmet

______Dean, College of Natural Sciences and Mathematics ii

ACKNOWLEDGMENTS

First and foremost, I would like to thank my advisor Dr. Michael Murphy.

Throughout my Master’s Program with UH, Dr. Murphy has instilled an incredible amount of wisdom and guidance into my education. I greatly appreciate his knowledge and energy spent in teaching and advising me throughout my Master’s Program.

Dr. Ana Krueger was invaluable with her assistance and guidance with teaching me the necessary softwares and demonstrating the utmost patience, and I appreciate her time and efforts.

I am very thankful for my thesis committee which consisted of Dr. Joel Saylor, Dr. Pete

Emmet, and Dr. Ana Krueger as well as Dr. Murphy. Throughout my research I conferred with these faculty, previous students, and attended conferences to gain information from all the people who generously provided me with information which made the process rewarding.

Last but not least, I am also thankful to my parents and to Wills for their unconditional love and support.

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LATE MESOZOIC AND CENOZOIC KINEMATIC RECONSTRUCTION: ASSESSMENT OF LATE CRETACEOUS MAGMATISM AND SOURCE-TO- SINK CONFIGURATION IN THE NORTHWESTERN GULF OF MEXICO

An Abstract of a Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston

______

In Partial Fulfillment of the Requirements for the Degree Master of Science

______

By Jordan Nicole Dickinson August 2017

ABSTRACT

It has been demonstrated that emplacement of 92-108 Ma post-breakup alkalic magmatism and attendant structural culminations in Louisiana and Arkansas facilitated mid-Cretaceous surface uplift, development of unconformities in updip areas, and incision of fringinng reefs along the northern Gulf of Mexico thereby promoting the development of depositional fairways that permitted transport of Late Cretaceous course siliclastic sediments into the deep water gulf. To the west, the Balcones Igneous Province, exposed for 397 km from Austin to Carizo Springs, Texas, was emplaced between 83.5 and 72.4 Ma. Should this hypothesis for the mechanism driving incision and siliciclastic transport past the fringing carbonate reef due to the Sabine and Monroe uplifts prove to be valid, it is possible that surface uplift above the Balcones igneous province and Llano uplift could have produced the same effect. Additionally, it has long been acknowledged that sedimentation, such as that from an influx of Paleocene sediments, is a driving factor of deformational extension in the Gulf of Mexico, though there have been no previous attempts to quantify it. This is a process most easily acheived through structural restoration. For this thesis, I fully interpreted fautls, salt bodies, and 16 horizons on two GulfSPAN regional 2D seismic reflection profiles guided by well ties from 48 available wells, seismic attributes, and previous interpretations in proximal areas. I also conducted a full line-length kinematic reconstruction on GulfSPAN line 2450 from the Holocene to the Middle Jurassic using 2D Move restoration software, and calculated extension based on the results. GulfSPAN line 2450 was restored in 12 time steps back to the Middle Jurassic, revealing the paleobathymetric surface at each step and providing insight into paleodepositional settings. Measurements of extension from the Pliocene to the Middle Jurassic revealed rates of extension that show generally low rates in the Cretaceous and Paleocene that increase and peak in the Oligocene with modest rates through the Middle Miocene before tapering off. Finally, 2D seismic interpretation delineated 5 sets of slope canyons fed from Paleocene age delta systems. I interpret this system to have been driven by surface uplift on land 10 to 30 Mya later in the Balcones Igneous province than in the Sabine and Monroe uplifts to the east.

CONTENTS CHAPTER 1: INTRODUCTION ...... 1 1.1 Motivation and Objectives ...... 1 1.2 Study Area ...... 5 1.3 Significance ...... 6 CHAPTER 2: REGIONAL GEOLOGY ...... 9 CHAPTER 3: DATA & METHODOLOGY ...... 12 3.1 Introduction ...... 12 3.2 2D Seismic Interpretation...... 12 3.3 Gravity and Magnetic Data ...... 16 3.4 Kinematic Restoration ...... 17 CHAPTER 4: INTERPRETATIONS ...... 21 4.1 Strikeline 5000 ...... 21 4.2 Dip Line 2450...... 25 4.3 Gravity and Magnetic Data ...... 28 CHAPTER 5: STRUCTURAL RESTORATION ...... 34 5.1 Stage M (Middle Jurassic) ...... 36 5.2 Stage L (Early Cretaceous) ...... 36 5.3 Stage K (Late Cretaceous)...... 37 5.4 Stage J (Early Paleocene) ...... 37 5.5 Stage I (Late Paleocene) ...... 38 5.6 Stage H (Eocene) ...... 40 5.7 Stage G (Oligocene) ...... 41 5.8 Stage F (Early Miocene) ...... 41 5.9 Stage E (Middle Miocene) ...... 43 5.10 Stage D (Late Miocene) ...... 43 5.11 Stage C (Pliocene) ...... 44 5.12 Stage B (Pleistocene) ...... 44

5.13 Stage A (Present) ...... 44 CHAPTER 6: DISCUSSION AND CONCLUSIONS ...... 45 6.1 Paleocene Incision in the Northwestern Gulf of Mexico ...... 45 6.2 Extension in the Gulf of Mexico ...... 47 6.3 Potential Mechanisms for Late Cretaceous Uplift ...... 50 6.4 Conclusions ...... 50 REFERENCES ...... 54 APPENDIX………………………………………………………………………………58

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CHAPTER 1: INTRODUCTION

1.1 Motivation and Objectives

Examples of gravity induced deformation along a salt detachment producing significant extensional and shortening regimes have been observed in several passive margin settings (Withjack et al., 1995; Hudec and Jackson, 2002). The northern Gulf of

Mexico is one of the best places to study this phenomenon because of the varying nature of extensional deformation across the basin. The most basic type of structural style found in the Gulf of Mexico is a linked system of updip extension, downdip shortening, and an intermediate transitional zone (Figure 1). Along strike, magnitudes of these systems may vary based upon local factors such as sediment input (Rowan et al., 2004; Galloway,

2008). Numerous studies have documented the geology of these areas, and it is now generally accepted that deformation and extension was driven by sediment loading aided by the mobile nature of the autochthonous and allochthonous salt.

Figure 1. Schematic from Dooley et al. 2013 demonstrating a passive margin dominated by gravity-induced deformation and how updip extension is thought to match downdip shortening.

It has been demonstrated in one such area that emplacement of 92-108 Ma post- breakup alkalic magmatism and attendant structural culminations in Louisiana and

Arkansas facilitated mid-Cretaceous surface uplift, development of unconformities in updip areas, followed by incision of fringing carbonate shelf along the northern Gulf of

Mexico thereby promoting the development of depositional fairways that permitted transport of Late Cretaceous coarse siliciclastic sediments into the deep water gulf

(Figure 2) (Snedden et al., 2016). This influx of sediments consequently drove local extension and downdip deformation for this time period.

Should this hypothesis of the mechanism for incision and siliciclastic transport past the fringing carbonate reef due to the Sabine and Monroe uplifts prove true, it is possible that the Balcones igneous province and Llano uplift could have produced the same effect. The Llano uplift and the Balcones igneous province, exposed for 397 km from Austin to Carizo Springs, Texas was emplaced between 83.5 and 72.4 Ma. This magmatism and uplift could have resulted in basinward incision and sediments younger than 75 Ma breaching the Cretaceous carbonate shelf. This event would account for the deposition of the Wilcox and Midway Groups.

Figure 2. Schematic from Puga-Bernabéu et al., 2014 demonstrating siliciclastic incision into carbonate reefs past the shelf-edge barrier and transport into the delta plains.

Previous explanations for the driving mechanism of the magmatism involve migration of the North American place over the “Bermuda” hotspot (Cox and Van

Arsdale, 2002; Snedden et al., 2016). However, the ages of samples along the projected track from Nebraska moving eastward do not support the model. Additionally, both the age and location of the Balcones igneous province do not align with the proposed hotspot model (Figure 3).

Figure 3. Map showing proposed progression of Bermuda Hotspot track (Duncan, 1984). Numbers associated with the black triangles are ages dated by K-Ar method. This hypothesis predicts eastward age progression. Average plate motion velocity between North America and the Bermuda hotspot is 53 mm/yr from Kansas kimberlite (KK) to Arkansas carbonite (A), and drops dramatically to 11 mm/yr from Arkansas to Jackson Dome (JD) Mississippi. Then a significant increase from 11 to 43 mm/yr to the east of Mississippi. The Balcones Igneous province in Texas is not considered in this model. Modified from Cox and Van Arsdale, 2002.

This thesis investigates whether depositional fairways similar to those present along the Louisiana coast in the mid-Cretaceous were similarly emplaced along the Texas coast in the Campanian and filled with sediments from the Maastrichtian and Paleocene.

In addition, I investigate the sequence of deformation to better understand the magnitude and spatial extent of the linked extension-shortening system. First, gravity data, magnetic data, and 2D seismic interpretations of GulfSPAN seismic lines 5000 and 2450 will highlight whether or not these features exist. Second, performing a kinematic restoration of a 2D seismic dip line will aid in understanding the paleotopography during this time

4 frame and delineate the timing of subsequent extension driven in part by the sudden influx of Paleocene sediments onto the abyssal plain. A more reasonable mechanism for the post-alkalic magmatism and uplift producing these incisions is presented.

1.2 Study Area

The study area for this thesis is located in the northwestern Gulf of Mexico and consists mainly of the extent of GulfSPAN seismic survey 5000 along the Texas coast and the full extent of GulfSPAN survey 2450 (Figure 4). Seismic line 5000 runs along and parallel to the gulf coast from the Texas-Mexico border to the Louisiana-Mississippi border. However, for the purpose of this study, only the 538 km extent parallel to the

Texas coast is used where it maintains a 17 to 58 km offshore distance from the present- day coastline. GulfSPAN line 2450 is 597 km long and extends from Canyon Lake,

Texas through East Breaks and Alaminos Canyon to just past the Sigsbee Escarpment.

The area is dominated by extensional fault systems gravitationally induced by remarkably thick sedimentary sequences that are translated basinward along salt-floored detachments that has resulted in allochthonous salt bodies, and numerous minibasins whose geometries are influenced by the surrounding salt bodies (Figure 4 and 5) (Diegel et al.,

1995; Peel et al., 1995; Rowan et al., 2004; Hudec et al., 2009; Galloway et al., 2011;

Dooley et al., 2013).

Figure 4. Map of the study area and present day tectonic setting of the northwestern Gulf of Mexico showing the extent of the seismic lines used in this study.

1.3 Significance

Post rift magmatism is prevalent along the coastal plain of the northern Gulf of

Mexico and has been shown to result in localized surface uplift during their emplacement

(Ewing, 2009; Liu et al, in review). This study investigates the influence of this surface uplift on the source-to-sink configuration in the northwestern Gulf of Mexico during the

Late Cretaceous. The results produced from this thesis not only place new constraints on the history of extension and deformation in the basin, but also give a further understanding of the deposition of the Wilcox and Midway Groups (Figure 5). It will

6 also provide geologic insight to analogous passive margins such as the Niger Delta or the

Orange River Basin.

Geologic events within the study area based on geologic observations of present and restored seismic sections. seismic sections. restored and observations based on events area present of Geologic geologic within study the

Figure Figure also context. were for consulted works Previous

CHAPTER 2: REGIONAL GEOLOGY

The GulfSPAN seismic survey is located in the central Gulf of Mexico basin along the southern margin of the North American plate. Igneous rocks and nonmarine sediments such as the Eagle Mills red beds were deposited in the Triassic during the rift- stage prior to seafloor spreading. The initial opening of the Gulf of Mexico began about

210 Ma in the Late Triassic and continued through the Early Jurassic as Pangea rifted apart and the Atlantic Ocean opened up. The Middle Jurassic was marked by attenuation and development of horst and graben systems as the Yucatan block began to rotate counter-clockwise about a pole located near the Florida Keys (Sandwell et al., 2014).

Simultaneously, the Louann salt was deposited in two main depocenters in the central gulf and Desoto Canyon in the northeastern Gulf (Buffler 1991; Galloway, 2008;

Goldwaithe, 1991; Hudec et al., 2013a; Hudec et al., 2013b). Seafloor spreading initiated and the Yucatan block continued to rotate in a counterclockwise direction along an arcuate shaped spreading in the Late Jurassic (Figure 4) (Bird et al., 2005; Sandwell et al., 2014). During the Late Jurassic the northern passive margin experienced a transgression, from the continental deposits of the Norphlet to the Smackover shallow water carbonates to the deltaic Cotton Valley sandstones capped by limestone (Winker and Buffler, 1988).

The Yucatan block ceased rotating in the Early Cretaceous and the basin began to experience subsidence as sea level rose and the Cretaceous inner seaway formed (Bird et al., 2005; Haq, 2014). The Cretaceous was marked by emplacement of 108-92 Ma post- breakup alkalic magmatism and attendant structural culminations in Louisiana and

Arkansas (Ewing, 2009; Griffen, 2008; Griffen et al., 2010; Liu et al., 2015). This led to regional unconformities in the coastal plain and incision of fringing reefs along the northern Gulf of Mexico, thereby promoting the development of depositional fairways permitting transport of Late Cretaceous and Early Paleocene coarse siliciclastic sediments into the deepwater gulf (Snedden et al., 2016). Late Jurassic clastic sediments spatially transitioned into broad carbonate platforms during six depositional episodes in four main depocenters from the Rio Grande embayment in the west to the Apalachicola embayment in the east. The Mid-Cretaceous Unconformity followed this time period (Galloway,

2008). Depocenters in the Late Cretaceous shifted from the shelf margin onto the fore- shelf continental slope and clastic deposition recommenced, broken up only by the Austin

Chalk and some marginal volcanic activity.

The Cenozoic is marked by an overall regression and 18 separate depositional episodes throughout five tectono-climatic eras in eight fluvio-deltaic axes within the

Northern Gulf as specified by Galloway et al. (2000). The main depocenters shifted several times during this time period (Galloway et al., 2011). Additionally, extensional growth faulting extending as far as the continental shelf and was prevalent throughout the

Cenozoic. From oldest to youngest, they are the Wilcox (late Paleocene-early Eocene),

Yegua (late Eocene), Vicksburg (early Oligocene), Frio (late Oligocene), Lunker (late

Oligocene-early Miocene), Clemente-Tomas (late Oligocene-early Miocene), Corsair

(mid-late Miocene), Wanda (mid-late Miocene), Michoud (Oligocene-Holocene), and

Tepate-Baton Rouge (Pleistocene-Holocene) fault systems (Bebout and Gutierrez, 1983;

Ajiboye and Nagihara, 2012; McCulloh and Heinrich, 2013).

Presently, thick transitional crust exists around the margins of the Gulf of Mexico, and allochthonous salt canopies extend past the continent-ocean boundary (Figure 4). A stratigraphic column pertinent to the study area is presented in Figure 6.

Figure 6. Simplified stratigraphic column for the northwestern Gulf of Mexico. Ages are taken from International Commission on Stratigraphy (ICS) chronostratigraphic chart (v2017/02). Q.= Quaternary and Fm. = Formation.

CHAPTER 3: DATA & METHODOLOGY

3.1 Introduction

In collaboration with the Igneous Gulf of Mexico group (iGOM), ION has provided seismic lines from their GulfSPAN dataset. The dataset consists of roughly

27,000 km of deep image, pre-stack depth migrated (PSDM) 2D seismic survey lines across the Northern Gulf of Mexico with long-offset and long-recording times. Offshore data were reprocessed using ION GX Technology’s Reverse Time Migration (RTM) methods. The record time was 18 seconds two-way travel time using a 9 km ocean bottom cable (OBC).

Data were loaded into both Petrel and 2D Move with workflows specific to the

GulfSPAN dataset. All data for this study were in depth. Seismic lines loaded into 2D

Move were specified in a scale of meters and the scalar value was changed to 100.

Seismic loaded into Petrel required editing of the preset parameters. The following parameters were followed: Trace header was the line detection method, the coordinate scale was 1.0, the X coordinate value was 185, the Y coordinate value was 189, the line number was 25, the trace number was 5, the CDP number was 21, and the shot number was 17. Additionally, 48 wells were loaded into Petrel with horizon well ties.

3.2 2D Seismic Interpretation

Seismic data were analyzed for faults, horizons, sequence stratigraphic relationships, and salt structures to gain an understanding of the formation geometry and the geological evolution of the study area. It is noted that reflection strength on the

12 seismic reduces with depth as a result of the loss of high frequencies. More than 200 faults on Line 2450 and more than 130 faults on Line 5000 were picked and used to guide horizon interpretation. Nearly all faults along the dip line are extensional in nature with displacements generally increasing downdip basinward. I picked 16 independent horizons on seismic surveys 2450 and 5000 from water bottom to the Middle Jurassic at a maximum depth of 15,000 meters. The mapped horizons range from Middle Jurassic to

Pleistocene in age and were derived from wells acquired from Drillinginfo and tops from

BOEM (Figure 7). There are 48 wells with tops along the seismic surveys used in this study. Line 2450 has 36 wells and the deepest top is the Upper Cretaceous. Line 5000 has 12 wells along the extent used in this study and the deepest top is Oligocene in age

(Figure 8). Horizon picks – including the autochthonous Louann Salt – not tied to a well were guided by previous literature interpretations and picks from ION’s dataset.

Figure 7. Seismic well tie located at the toe of salt on line 2450 displaying eight (Pleistocene – Paleocene Wilcox Group) of the fourteen picks in this study.

Figure 8. Map of the study area denoting well locations along the seismic surveys used in the study. In some offshore portions of 2450, wells were projected up to 10 km due to lack of data in the area.

Various seismic attributes were applied to each survey to aid in the interpretation of salt structures and stratigraphy. Sequence stratigraphic relationships were used to interpret Paleocene age channels. Seismic attributes are specific quantities of geometric, kinematic, or statistical features derived from seismic data and can be associated to

15 structure (time-based) or associated to stratigraphy and reservoir characterization

(amplitude-based) (Liner, 2004).

Relative acoustic impedance was used as a catch-all to enhance the seismic image by reflecting physical property contrast. This was useful for delineating both horizons and faults, but it notably highlighted stratal relations and carbonates as well. Relative acoustic impedance was also useful in delineating the extent of salt structures – diapirs and sheets – on the seismic data. Finally, sweetness – which is a combination of envelope and instantaneous frequency – was used to identify channels incised within carbonate reefs by highlighting the carbonates within the seismic image (Figure 9).

Figure 9. Example of seismic attributes utilized for seismic interpretation, Relative acoustic impedance (left) and Sweetness (right). Note the high amplitudes visible in the section with Sweetness applied which are interpreted to indicate carbonates.

3.3 Gravity and Magnetic Data

Gravity and magnetic data were interrogated to help identify geologic features both on a regional scale and a small scale. Free air gravity data was used to identify

16 buried volcanic bodies and possible channel incision offshore. Magnetic data was used to confirm the same buried volcanic intrusions. Oasis Montaj software was used to generate all maps.

Gravity data used in this study is from open-file gravity data in the Decade of

North American Geology (DNAG) database. The grids cover the Gulf of Mexico and

North America as a part of the USGS DNAG project and were spaced at 6 km. Maps produced in this study were constructed using Free Air grid data. Onshore gravity anomaly profile data were gathered by aerial surveys while offshore data were gathered along ship tracks across the Gulf of Mexico.

Free Air gravity was used because it does not correct for topography. Correcting for topography when looking for features that might continue from onshore to offshore areas could skew the results. In addition, 7.5, 10, and 12 km filters were applied to provide context for any continuous features by effectively removing the bulk of the overlying sediments.

Magnetic data used in this study is also from DNAG and uses the North American

Magnetic Anomaly Map. Data gathered by aeromagnetic surveys were gridded on a 1 km interval.

3.4 Kinematic Restoration

GulfSPAN line 2450 was restored using Midland Valley’s 2D Move section- restoration program. The line was reconstructed in 12 time steps from Pleistocene through Lower Cretaceous age horizons to create balanced cross sections along the

17 seismic profile. The reconstruction incorporated hanging wall and footwall cutoff relationships to calculate magnitudes of extension and shortening. All stratigraphic horizons were restored to a surface that approximates the slope at their given time period.

After interpretations were completed, a stratigraphic chart that accounted for rock types of the strata and fault block polygons was created before the backstripping process was initiated. The section was duplicated after each step to preserve previous geometries for analysis. Each restoration stage involved creating a paleobathymetry surface and then unfolding the top geologic horizons to that surface. Next the horizon was restored using decompaction, Airy isostasy, salt movement, and fault movement. Decompaction was not applied to the base autochthonous salt layer as the density of salt, and thus the porosity, is constant with depth. The restoration was also used assess deformation patterns of extension in order to characterize the mode of fault formation, such as growth fault and backsliding models (Tearpock and Bischke, 2002).

A basic assumption of 2D restoration is that the displacement on all faults is within the plane of the section and that rock volume is neither created nor destroyed. It also assumes that no material, such as salt, enters or leaves the section plane (Pearson et al., 2012). These criteria were observed with regards to the results. The simple shear algorithm – which is effective for restoring listric faults – was used in this study to model the relationship between fault geometry and hanging wall deformational features

(Withjack and Peterson, 1993). Simple shear relates the geometry of the deformational features found in the hanging wall to the corresponding fault’s shape. According to the

18 principles of the simple shear algorithm, the area of the hanging wall is conserved and the length of pins (shear vectors) are constant (Verrall, 1981; Gibbs, 1983).

Move-on-fault was used to restore fault blocks with significant displacement to the regional paleobathymetry surface of the given time period with the simple shear workflow. The simple shear algorithm is best suited to model extensional tectonic areas where rollover structures have developed on listric normal faults. This workflow geometrically models the relationship between fault geometry and hanging wall geometry and has the added advantage of maintaining the area between the beds and horizons.

This reconstruction used the Sclater-Christie compaction curve for the decompaction workflow, which is the preferred method for sections containing mixed sediments. The Sclater-Christie compaction method calculates the change of rock volume due to porosity loss with increased depth. This method assumes that porosity

−푐푦 maintains an inverse relationship with burial depth which is represented by 푓 = 푓0(푒 ) where 푓 is current porosity, 푓0 is surface porosity, 푐 is the porosity-depth coefficient, and

푦 is depth (Sclater and Christie, 1980). It is important for this study to note that salt is assigned a zero-porosity depth coefficient during decompaction due to it being incompressible.

As is previously mentioned, the decompaction workflow in this study uses Airy isostasy for a submarine load to restore the strata. Airy isostasy assumes a crust of zero vertical shear strength that is supported and able to move on an underlying fluid layer

(Jackson, 2005). Thus, when overburden is removed from the section, the strata respond by isostatically readjusting to as they decompact. This is a local phenomenon and only 19 the material vertically above and below the load is affected. Isostatic readjustment is

푆−(퐻 −퐻 )(휌 −휌 ) represented by 푍 = 1 2 푐 푤 where Z is the amount of subsidence relative to a 휌푚−휌푤 basement reference, 푆 is sediment thickness, 퐻1 is crustal thickness before sediment loading, 퐻2 is crustal thickness after sediment loading, 휌푐 is the density of the crust, 휌푤 is water density, and 휌푚 is the density of the mantle. In the instance of salt, isostatic response is calculated in several vertical one-dimensional lines outside the area of salt mobility, after which a bulk vertical shift is calculated for the entire section based upon the isostatic response outside the area of salt mobility (Burov and Diament, 1992).

CHAPTER 4: INTERPRETATIONS

GulfSPAN lines 5000 and 2450 were interpreted using well control along the full extent of the study area. Interpretations of bed and autochthonous salt geometries were additionally influenced by published examples where well control was not present (eg.

Diegel et al., 1995; Peel et al., 1995). The offshore portion of the seismic data reaches a depth of 25,000 meters while the onshore vintage composites range from 8,000 to 15,000 meters in depth. As previously mentioned, magnetic data and gravity data with 7 km, 10 km, and 12 km filters applied were also analyzed for congruency with the features seen in the seismic survey, especially channel features. This chapter details some key interpretations made in this study.

4.1 Strikeline 5000

Six age picks were interpreted on strike line 5000 along with allochthonous salt and carbonate reefs. The line is shown in uninterpreted (Appendix Figures A1 - A2) and interpreted (Appendix Figures A3 - A4) format. Channel locations are denoted in map view on Figure 17. The following list outlines key features in the seismic interpretation; numbers correspond to those in Figures A3 - A4 in the appendix:

1. Cone-shaped faulting due to salt diapir movement.

2. Carbonate reefs in the upper Cretaceous Anacacho formation identified by

high amplitudes in the seismic image. Note that areas where reefs are not

delineated does not necessarily mean they are absent. Carbonates were

delineated in locations only where they could be interpreted with certainty.

3. Eocene age salt weld and corresponding salt stalks and diapirs. The weld was

identified by its alignment with diapirs and by changes in the character of

former continuous seismic reflectors.

4. Deep-seated fault into Cretaceous strata.

5. Location where strike line 5000 intersects with dip line 2450.

6. Slope canyon system A located in the westernmost portion of the survey is

likely associated with the Rio Grande embayment. This system reaches a

maximum depth of 13,350 meters at the base of incision. The three individual

slope canyons are on average 752.3 meters deep and 9,463 meters wide.

7. Slope canyon system B is located west of where dip line 2450 intersects with

strike line 5000. As demonstrated in Figure 10, there are visible onlapping

stratal relationships characteristic of a canyon depositional environment

present in this area. These canyons are seated at a maximum depth in the

section of 12,270 meters while they average 972 meters deep and 9416 meters

wide. The eastern portion of this system has been affected by local

deformation, probably by the proximal fault cutting into the Cretaceous strata.

8. Slope canyon system C is located east of the intersection of dip line 2450 and

strike line 5000 and is incised into carbonate reefs well defined on the seismic

data when applying the sweetness attribute (Figure 11). The maximum depth

of these incisions is at 11,600 meters below the surface. The three submarine

canyons average 1,281 meters high and 5,768 meters wide.

9. Slope canyon system D is most likely associated with the previously identified

Yoakum-Lavaca channel system located between Houston and Port Lavaca,

Texas (Berman and Rosenfeld, 2007). This system reaches a maximum depth

of 11,260 meters and the individual canyons average 1,324 meters high and

6,513 meters in width.

10. Slope canyon system E is located in the area of the Houston Embayment and

is likely correlated to the Hardin channel system (Berman and Rosenfeld,

2007). This system displays significant reef buildup visible when the

sweetness attribute is applied to the seismic survey. At a maximum depth of

10,630 meters, the submarine canyons average a height of 1,311 meters and

width of 4,141 meters.

Figure 10. Slope canyon system B with canyon channel fill highlighted in yellow. Note the stratal onlap within the system.

Figure 11. Slope canyon system C with sweetness attribute applied for seismic interpretation of carbonate reefs. Red lines delineate carbonate reefs.

4.2 Dip line 2450

Sixteen age picks were interpreted on dip line 2450 as well as allochthonous salt bodies. The line is shown in uninterpreted (Appendix Figures A5 – A7) and interpreted

(Appendix Figures A7 – A10) format. The following list outlines key features in the seismic interpretations where numbers correspond to those in Figures A7 – A10 in the appendix:

1. Updip pinchout of northernmost extent of Jurassic Louann salt onto Paleozoic

basement.

2. Eocene age erosional surface.

3. Landward extent of extension.

4. Wilcox Fault zone. Faults shown in black. Several faults have relatively

minor displacements, however, the main fault in the system demonstrates

significant displacement within the Paleocene to Early Eocene time periods

coinciding with a thickened Wilcox sedimentary sequences. The main fault is

rooted into the autochthonous salt.

5. Absent Cretaceous strata and older that have rafted basinward along the salt

detachment.

6. Possible salt pillow forming in absence of overlying Cretaceous strata.

7. Frio Fault zone. Expansion of the Frio formation in the hanging walls of the

largest faults suggests that the fault system was most active in the Oligocene

during the deposition of the Frio-Vicksburg Groups. This basinward motion

could have been facilitated by the flow of the Louann Salt.

8. Eocene age salt weld that serves as a detachment surface for overlying growth

faults.

9. Salt pillows related to the Eocene salt weld.

10. Clemente-Tomas Fault zone. The system consists of listric growth faults and

growth wedges resulting from significant sedimentation and extension in the

early Cenozoic. These faults root into the Eocene salt weld.

11. Location where strike line 5000 intersects with dip line 2450.

12. Corsair fault zone. The formation of the faults in this system are specifically

characteristic of the Corsair fault zone. Rollover structures, backsliding

geometry, antithetic faults terminating along a master synthetic fault are

common features (Tearpock and Bischke, 2004).

13. Listric faults with significant displacement rooted into the Louann

autochthonous salt.

14. Large salt diapirs and stocks where the stems are likely out of plane of the

section or are detached.

15. Lower Oligocene age salt weld.

16. Lower Miocene dominated minibasins formed due to salt evacuation through

the overlying strata.

17. Possible pinchout of Oligocene strata (Frio-Vicksburg) into allochthonous salt

sheet. There is no proximal well control along this portion of the seismic and

the nearest wells -which are more than 10 km away – list the Lower Miocene

as their deepest chronostratigraphic top.

18. Salt feeder stocks supplying salt to the above sheets and diapirs.

19. Allochthonous salt canopy situated between Oligocene and Lower Miocene

strata.

20. Thrust front and edge of Sigsbee Escarpment. The most updip portion of the

Perdido Fold Belt can be seen in this portion of the seismic.

4.3 Gravity and Magnetic Data

Magnetic intensity and Bouguer gravity maps were constructed to interpret crustal structures of the study area as well as submarine canyons in Upper Cretaceous carbonate reefs. The gravity and magnetic maps do not effectively show structural features in the area; thus, three filtered free-air gravity maps were created using data enhancement methods. Filters of 7.5, 10, and 12 km were applied to ascertain the validity of the features seen in the unfiltered maps.

Magnetic intensity values across the study area range from -455 to 301 nT. The

Llano uplift and Balcones igneous province are characterized by a magnetic intensity range of -34 to 300 nT. These highs in the data indicate possible igneous bodies that are interpreted to be part of the overall system causing updip uplift in the study area (Figure

12).

Figure 12. Magnetic Map of South Central North America with normal faults and seismic line locations overlain. Possible igneous bodies are indicated by white boxes.

Bouguer gravity values across the study area range from -2433.5 to 231.7 mGal.

Variations of the gravity values are a joined effect of variable crustal thicknesses and moho depth, basins and uplifts, and igneous and sedimentary rocks. The Llano uplift is characterized by a gravity range of -318 to -46.4 mGal and the Balcones Igneous

Province is characterized by a gravity range of -318 to 153.4 mGal. These ranges are interpreted to be related to the presence of igneous rocks within the uplifts. As seen in

Figure 13, there are breaks in the gravity signature along the Texas coastline that roughly spatially coincide with the location of incised channels in the seismic data.

Figure 13. Decade of North American Gravity Bouguer Onshore, Bouguer Offshore with normal faults and seismic line locations overlain. This gravity data was filtered to identify buried topographic lows caused by incision and infilling with less dense sediments.

Filtered free-air gravity maps were created. The intent of these maps was to enhance relatively deeper gravity anomalies by attenuating the effect of shallower anomalies. 7.5, 10, and 12 km filters were run on the gravity data, and it was again possible to identify breaks in the gravity signature that approximately align with the incisions mapped on GulfSPAN line 5000 (Figure 14). However, it is observed that filtering the wavelengths at the depths of the interpreted canyons does not yield any

30 significant visible difference between the 7.5, 10, and 12 km filters. This lack of change suggests that the slope canyons are forming over something deeper or larger. It is possible that some sort of crustal topography is drowning out the signal produced by the canyon incisions.

Figure 14. Gravity maps of the study area filtered for depths at A) 7.5 km, B) 10 km, and C) 12 km.

CHAPTER 5: STRUCTURAL RESTORATION

In order to better understand the structural and stratigraphic relationship between the observed incisions and, major structural features, I restored a regional cross section.

The restoration displayed in Figure 15 details 13 time steps corresponding to the major stratigraphic units of the study area where each stratigraphic horizon was restored to a surface that approximates its paleobathymetry at the time of deposition. An important assumption made in the construction of the cross sections is that out-of-plane motion is not accounted for and salt is flowing into and out of the plane, particularly in areas of prevalent allochthonous salt bodies. For this reason, the cross sections are not line-length balanced.

The amount of extension shown in the restoration – from Lower Miocene to

Lower Cretaceous time – is approximate to the values of regional extension associated with migration of sediments towards the Gulf of Mexico. Horizontal extension estimates are calculated from the heave along faults and the horizontal width of salt bodies. I assume that the horizontal width of salt bodies was accommodated by normal faults that were subsequently intruded by the salt body. The panels of the structural restoration in

Figure 15 have been vertically exaggerated by 200% to better show stratigraphic and structural features. The following descriptions describe key features in each time step of the reconstructed section in chronological order.

Figure 15. Structural restoration panels of the seismic interpretation displayed in Appendix Figures A8 - A10. Each panel has been vertically exaggerated by 200%. All panels are shown at the same scale. Extension respective to time steps are shown in red on left.

5.1 Stage M (Middle Jurassic)

The Middle Jurassic unconformity is present across the present day offshore extent of the basin and much of the onshore extent as well. Directly above, the Callovian

Louann Salt exists in a continuous autochthonous layer for more than 140 km along the length of the section. Salt thicknesses range from about 2400 m in the basinward area to where the unit pinches out updip onshore.

In the northwestern part of the section, the Cotton Valley stratigraphic units were deposited directly on top of the Middle Jurassic unconformity but onlap the Louann Salt moving downdip until they pinch out into the salt. Slightly variable thicknesses and negligible extension is present in the Cotton Valley formations likely due to minor movement of the underlying salt. It is possible that a salt weld is present to the north between the Middle Jurassic and Cotton Valley strata.

5.2 Stage L (Early Cretaceous)

During deposition of the Early Cretaceous strata the autochthonous Louann Salt was still mostly invariable across the basin and any translation of salt down the slope was minimal due to the broad expanse of carbonates. A negligible amount of extension in this time occurs in the vicinity of the Clemente-Tomas fault zone, though not a component of it. This suggests that, like the Late Jurassic Cotton Valley strata, the Early Cretaceous units – consisting of the Sligo, Pearsall James, and Glen Rose Formations in this area – were influenced by the underlying salt, although minimally.

5.3 Stage K (Late Cretaceous)

The Late Cretaceous, marked by deposition of the Washita, Eagle Ford, Austin,

Taylor, and Navarro Groups, saw an increase in slip along faults in this section. This time, an extension of 5.17 km occurred over a few landward faults and along an offshore fault developing in a locale where salt began to express its mobile nature in what will later become a feeder for the diapiric province of the study area. Extension in this time period is largely dominated by salt movement. As carbonate deposition continued, the autochthonous Louann Salt influenced the Late Cretaceous units by forming pillows, stalks, and initiating rafting of blocks basinward along a detachment surface.

The paleobathymetric surface of this time was largely flat due to the nature of carbonate deposition, which is likely a component of the still relatively small amount of extension. The thick carbonates deposited near the end of the Late Cretaceous, such as the Anacacho Formation, will experience incision from sediments produced by the denudation of updip structural domes emplaced by 83.5-72.4 Ma magmatism and subsequent uplift.

5.4 Stage J (Early Paleocene)

This stage in the restoration is represented by the deposition of the Midway

Group. As the paleoslope and thickness of the sediment package overlying the autochthonous salt increased, the section began to experience significant deformation. In the central portion of the section, salt began evacuating to the surface, deforming the strata it passed through in the process. Basinward rafting of fault blocks increased

37 significantly, as evidenced by the 3.59 km of extension experienced at this time. A small extent of the Midway Group lies directly on top of a salt pillow that failed to extrude to the surface where the underlying Cretaceous units have rafted away completely.

5.5 Stage I (Middle Paleocene)

During Wilcox deposition, the Wilcox fault zone undergoes significant displacement and nearly all faulting is local to the updip extent of the Cretaceous shelf margin and a slope basin is formed (Figure 16). By the end of the Wilcox time, the middle autochthonous salt is beginning to create feeders and evacuate towards the surface where it will begin to form diapirs and sheets. This horizon also displays an erosional surface in the middle of the time period (Berman and Rosenfeld, 2007). Berman and

Rosenfeld also suggest that there might be a large counter regional fault at the front of the basin, though it would not show up in the present day seismic image due to salt migration through the section and the deformation of overlying strata deposited later on.

The region experiences 3.05 km of extension during this time, primarily due to the formation of the Wilcox slope basin and the Wilcox Fault Province. Downdip folding and shortening of the Wilcox group is negligible in this time because the extension is essentially absorbed into the wedge. Two things must be considered in regard to this observation. First, the limitations of the 2-dimensional data should be considered given that the problem is a 3-dimensional one. This is even more relevant because due to the change in seismic character in the deeper part of this package, it is apparent that salt is involved rather than just sedimentation. It is possible that salt has moved through this plane and affected the strata. Second, the location of this basin is proximal by tens of

38 kilometers to the Balcones Igneous Province and the Llano Uplift which were emplaced between 83.5 and 72.4 Ma in the Late Cretaceous. The surrounding area could now be subsiding and creating accommodation space for Wilcox sediments.

Figure 16. Structural restoration showing a kinematic model for the development of the Wilcox slope basin. Note that this model does not incorporate salt and is based solely upon deformation through gravity driven extension. Modified from Berman and Rosenfeld (2007).

5.6 Stage H (Eocene)

According to Hudec et al. (2013), the Perdido fold belt, present in the furthest downdip extent of the section, initiated shortening about this time in response to ongoing updip extension. As mentioned by in Hudec et al. (2013), this is illustrated by synkinematic strata deposited in the Eocene Claiborne Group and Jackson Formation seen in the seismic. Also occurring at this time is the consolidation of two larger potentially linked allochthonous salt canopies at or near the paleosurface. As salt moved upsection into these allochthonous bodies, the volume of underlying autochthonous salt was greatly reduced. The idea of a Paleogene salt canopy system was originally proposed by Diegel et al. (1995), though he suggested the timing for this event as the

Oligocene. New seismic data and others such as Hudec et al. (2013) now support the

Eocene as the correct timing for canopy formation.

Large normal faults also formed in the Frio fault zone and abundant volumes of salt was emplaced at the paleobathymetric surface which produced a significantly increased magnitude of extension on the order of 16.60 km. Extension also occurred further downdip in the diapiric province, likely as a response to salt movement into the canopy system. These faults could also be a result of a roho detachment system. Finally, the updip portion of the Eocene strata past the salt weld are interpreted to have experienced significant erosion as illustrated by the disconformity visible in the seismic.

5.7 Stage G (Oligocene)

The Oligocene age Frio-Vicksburg Group is characterized by a considerable amount of sedimentation syndepositional with significant halokinetics that do not slow until the Lower Miocene. Extension along multiple listric normal growth faults also prevalent in this time is measured to be 26.92 km. These faults sole into the Eocene salt weld or in the autochthonous Louann Salt and extend from within the range of the

Clemente-Tomas fault zone, through the Corsair fault zone, and into the Wanda fault zone. Updip of the salt canopy, an Oligocene salt weld is created as the last of the allochthonous salt emplaces itself into the canopy or upsection diapiric bodies.

As a result of this process, the top Oligocene horizon just skims over the top of some salt bodies while pinching out near the peak of others. Depending on the locality of the Gulf of Mexico, it is debated whether or not the Oligocene units include a thin section overlaying the main salt canopy or if they simply pinch out against it. For the purposes of this study, the top Oligocene strata is interpreted to pinch out onto the salt canopy.

The nearest wells projected from at least 8 km away do not have Oligocene tops and the reflectors that terminate against the salt are observed as continuations of Oligocene reflectors in the updip portion of the seismic.

5.8 Stage F (Early Miocene)

During deposition of the Early Miocene Oakville Formation, there was still a significant amount of allochthonous salt movement continuing from the Oligocene time period. This halokinetic movement greatly influences the stratigraphic geometry and

41 structural deformation of the Early Miocene strata. The section experienced 13.28 km of extension within the Wanda fault zone at this time as rates decreased from their

Oligocene peak.

At the southernmost portion of the section, Early Miocene stratigraphic units are the first upsection units to not display folded strata within the region of shortening past the thrust front, rather they onlap the underlying folded Oligocene strata. Given that the chronostratigraphic equivalent of this unit has experienced a relatively large amount of updip extension, it should follow that there will be a nearly equal amount of downdip shortening, however this does not appear to be the case. I interpret two possible explanations for this phenomenon: (1) The Early Miocene Oakville Formation consists, at least in part, of soft sediments which might be bypassing the area without contributing to the compressional deformation just past the Sigsbee Escarpment. This would also explain the behavior of the units to onlap the stratigraphically younger folded formations.

(2) We simply cannot see the Lower Miocene shortening because the seismic data used in this study ends here, but shortening continues into the central Gulf of Mexico. The second explanation is favored over the first because updip extension in this time period is observed without subsequent linked downdip compression.

As previously mentioned, sediment deposition in the Early Miocene was heavily influenced by allochthonous salt movement through the section. At present day, there are several relatively large allochthonous salt bodies within and appearing to push up through the Early Miocene beds. Additionally, when the Early Miocene stage is restored to its correspondent paleobathymetric surface, it is observed that while the Miocene bedding

42 has been flattened, steep beds are still prevalent in the Oligocene. This only occurs when a salt weld is present and there is stress partitioning between the different layers. The culmination of these observations leads to the interpretation that there was a salt sheet emplaced further updip than the present-day extent that has since moved out of plane. It is likely that as Early Miocene sedimentation persisted, the allochthonous salt sheet was on or near the paleosurface and evacuating onto the sea floor where it would dissolve.

This process would actively guide the development of the minibasins prevalently seen prior to flattening and restoration of the section.

5.9 Stage E (Middle Miocene)

Middle Miocene sediments of the Lower Fleming Formation marks the geologic transition from substantially more active tectonics and halokinetics to a somewhat calmer basin evolution. The Middle Miocene experienced 4.44 km of extensional deformation within the Wanda fault zone as local tectonism begins to subside. It is important to note the large scale folding at the toe of the salt canopy. The geometry and shallow minibasins of the Lower Fleming unit in relation to the underlying folds suggests that these folds were actively undergoing compression during deposition of Middle Miocene strata. Halokinetics during this time were present but minor.

5.10 Stage D (Late Miocene)

Sedimentation in the Gulf of Mexico became relatively calm in the Middle

Miocene as allochthonous salt slowed movement, producing 1.20 km of extension. All extension at this time was salt affected and located above the salt canopy. This allowed

43 the Late Miocene Upper Fleming Formation to fill the ample amount of leftover accommodation spaces in the salt-influenced minibasins.

5.11 Stage C (Pliocene)

Although negligible amounts of extension and fracturing persist in the Pliocene on the order of 0.44 km, the Goliad-Willis Formation was deposited in relatively uniform bedding. Due to the lack of tectonic and halokinetic activity, the Pliocene strata were deposited in such a way that the front thrust of the salt nappe became more prominent.

5.12 Stage B (Pleistocene)

The sedimentary layers of the Pleistocene stage were deposited in much the same nature as the underlying Pliocene stage. The Pleistocene age Houston Group retains the same depositional geometry as the Pleistocene bedding below and the Holocene age bedding above it.

5.13 Stage A (Present)

Sedimentation into the Gulf of Mexico continues in the present day, although the largest influx of deposition is from the Mississippi fluvial input axis and from the Red fluvial input axis. There is only minimal sedimentation in this time along line 2450.

CHAPTER 6: DISCUSSION AND CONCLUSIONS

6.1 Paleocene Incision in the Northwestern Gulf of Mexico

Wilcox-age channel systems have long been acknowledged in the Gulf of Mexico along the Texas coast, though rarely mapped out with the aid of seismic data (eg.,

McDonnell et al., 2008). Though there is no disagreement on the existence of these incisions, other studies focusing on the Wilcox delta-system only explore one or two channel systems. Interpretation of submarine canyon systems on a single seismic line across the Texas gulf coast and confirmation of these incisions with gravity mapping gives important insight into the depositional environment of the gulf coast as a whole at the time of deposition (Figure 17).

Using the sweetness seismic attribute, the canyon channel incisions and carbonates become clearly visible in the seismic. It is much more difficult to see the infill. Near the time that deposition and channel-canyon infill would have taken place, the Late Cretaceous bolide impact occurred relatively nearby on the Yucatan Peninsula.

The bolide impact would have created a massive shockwave of sorts that would propagate into the study area. One possible effect of this occurrence is that the channel- canyon infill type would consist of a portion of reworked carbonates. On the other hand, the infill could simply be sands sourced from onshore or even a combination of the two, stacked upon each other. It is important to note that if the base of incision infill is in fact reworked carbonates, then it would be very difficult to see them in the seismic image using attributes designed to differentiate carbonates from siliciclastics.

Figure 17. Geologic map showing post-breakup igneous bodies in northern and northwestern Gulf of Mexico. Channels initially incised at the timing of intrusion of the Balcones igneous complex and identified in the seismic are illustrated by white boxes. Channels are filled with Paleocene - Early Eocene sediments (Berman, 2007; McDonnell et al., 2008). Blue dashed polygons show shear wave velocity anomaly or 7- 10% at 15-25km depth (A. Li personal communication). Also shown is Balcones igneous province (83.5-72.4 Ma): alkali basalt, olivine, nephelinite, nepheline, basanite, phonolite, etc. Arkansas and northwest Louisiana (106-64 Ma): Lamproite, carbonatite, alkali basalt, phonolite, nepheline syenite, etc. Offshore Louisiana (77 Ma) alkali olivine basalt. Ages shown for the igneous bodies are U-Pb zircon ages (Liu et al., in revision). Base map is USGS geologic map of United States of America.

Several studies have used detrital zircon analysis to delineate drainage patterns in

North America during the Paleocene (Blum and Pecha, 2014; Xu et al., 2016). Blum and

Pecha, 2014 explains that a continental-scale drainage reorganization occurred between the Early Cretaceous and Paleocene. Uplift and denudation of the Balcones Igneous

Province paired with basinward drainage of eroded sediments from the Laramide

46 province uplifts in the Rocky Mountains and Cordillera provided the source for Wilcox- age sediments. This is supported by my observation of Paleocene channels which suggests that there was increased surface uplift or there was stream reorganization which exploited the Yoakum-Lavaca system during the Paleocene.

In this study, I interpret Late Cretaceous aged surface uplift, subsequent denudation, and downdip incised pathways to have been exploited by Paleocene sediments up to 10 my younger. If this process is valid, we must account for this gap in time before Paleocene sediments fully breached the fringing reefs to be deposited in the

Gulf of Mexico delta plains. I hypothesize a delayed response process involving a sub- basin to explain the 10 my difference. It is likely that there was an intermediate Wilcox sub basin updip of the Cretaceous-aged fringing reefs which trapped sediments with sufficient accommodation space until it filled and spilled, generating the younger canyon- channel system. This hypothesis is additionally supported by the observed paleogeometry and structure of the Late Paleocene aged restoration (Figure 15).

6.2 Extension in the Gulf of Mexico

Extensional regimes play a significant role in the evolution of the Gulf of Mexico to its present-day configuration. With the aid of a kinematic reconstruction, I was able to calculate estimates of extension for the northwestern Gulf of Mexico along a 2-

Dimensional transect. It is important to note that values of extension can vary greatly across the basin due to the factors that drive extension in the gulf such as fluvial input axes and sedimentation rates. Extension in the study area was likely coevally influenced by sedimentation rates and salt mobility in the section as total extensional values were

47 calculated by adding heave to the width of salt assumed to have replaced sedimentation for the given time.

In the vicinity of the northwestern region of the Gulf of Mexico, amounts of extension were generally modest through the Late Cretaceous and Paleocene before they increased significantly in the Eocene (16.6 km) and peaked in the Oligocene with an extension of 26.9 km (Figure 18). There was a gradual decline in the amount of extension after the Oligocene as seen from Lower Miocene (13.3 km) to the Pliocene

(0.44 km). Any extension thereafter is of a negligible amount. Figure 18 also exhibits the movement of the locus of extension from the Mesozoic through the Cenozoic. The locus of extension first moved landward through the Mesozoic and then gradually moves basinward from the Paleocene to the Pliocene. This occurrence can be attributed in part to shifting fluvial axes and their level of influx at different times. Thermal cooling and subsidence is another likely influence.

Figure 18. Map illustrating the magnitudes of extension in each time of deposition along the seismic line 2450. All values were calculated along line 2450, however, arrows for the time which they represent are abutted against the fault system which produced extension for their respective time. Extension calculated along this transect without accounting for salt – purely heave values – is very small. These values range from 0.016 to 4.54 km. This phenomenon can be explained by two factors. First, extensional faults in the study area are very steep so that they only allow for small and localized horizontal extension. Unlike the eastern Gulf of Mexico, much of the extension in this area is vertical. Second, most of the extension is being accommodated by salt emplacement. The salt would have been much thicker along regional extensional faults and the overlying strata subsided as salt moved up and out of the section (Figure 19).

Figure 19. Schematic demonstrating a passive margin where extensional deformation is heavily influenced by salt emplacement along extensional faults. Modified from Dooley et al. 2013.

6.3 Potential Mechanisms for Late Cretaceous Uplift

There are multiple previous models to explain post breakup magmatism and emplacement of igneous rocks in the northern Gulf of Mexico during the Late Cretaceous with a sublithospheric origin. However, none of them succeed in providing an explanation that works for the entire northern gulf. There are four previous competing models: (1) the Bermuda Hotspot model which proposes that magmatism and subsequent uplift in the northern Gulf of Mexico was driven by the movement of the North American plate over the Bermuda hotspot (Cox and Van Arsdale, 2004; Snedden et al., 2016), (2) edge-driven convection which predicts that there was small-scale igneous activity and convection within the oceanic plate where lithospheric thickness can change suddenly

(King and Ritseme, 2000), (3) lithospheric reactivation which proposes that lithospheric reactivation of the Ouachita-Appalachian and Grenville structures resulted in base lithospheric melting and subsequent upwelling due to a pre-existing weakened zone

(Baksi, 1997; Eby and Vasconcelos, 2009; Griffin et al., 2010), and (4) Farallon plate

50 subduction which advocates for deep seated subduction of the Farallon plate underneath the North American plate leading to sublithospheric mantle-derived magmatism.

To replace these rejected models, we present the Stagnant slab tear model which accounts for subduction of the Farallon slab since the Jurassic. This model proposes that as the Farallon slab subducted beneath the North American plate, portions of the slab tore at sublithospheric depths and creating upwelling of asthenospheric materials, followed by decompression melting and magmatic emplacement. The process can be described in three steps: From 110 to 100 Ma (Figure 20A), the Farallon plate subducted beneath the

North American plate at a normal subduction angle until it reaches the transition zone within the mantle beneath present-day Arkansas and begins to tear and produce enriched hydrous, carbonatitic partial mantle melts. From 100 to 90 Ma (Figure 20B), slab tear propagates perpendicular to the trench to Louisiana and Texas, creating the Sabine uplift and Balcones Igneous Province. Finally, from 90 to 80 Ma (Figure 20C), the slab tear continues to propagate west into Texas and igneous activity dies out in Arkansas. The above process generated uplifts which were then denuded and later deposited in the Gulf of Mexico delta plains as the siliciclastic Paleocene strata of the Midway and Wilcox

Groups. Regardless of which model may be most appropriate, the Balcones Igneous

Province is significantly younger than igneous bodies to the west, suggesting that surface uplift occurred later.

Figure 20. Schematic diagrams illustrating the process of the stagnant slab tear model. Liu et al., in revision.

6.4 Conclusions

1. A system of North-northwest trending channel and canyon systems area observed to be incised into the Campanian carbonates and filled with the Midway and

Wilcox formations in the 2D seismic data.

2. These slope canyons are likely sourced from onshore uplifts associated with

Late Cretaceous magmatism in the Balcones Igneous complex.

3. These Late Cretaceous uplifts are driven by stagnant slab tear of the Farallon plate under the North American plate.

4. Gravity signatures display minor lows of a few milligals difference to the surrounding highs, which suggests canyon incisions are present in the sedimentary layer or a shallow basement feature.

5. My preferred hypothesis describes channel incision driven by these magmatically driven uplifts and suggests slope canyon incision in the Campanian youngs westward.

6. Sedimentation driven and salt aided extension show low rates in the

Cretaceous that increase in the Late Paleocene and peak in the Eocene with modest rates through the early Miocene.

7. Low magnitudes of extension since the Cretaceous indicate that strain within the passive margin wedge is dominated by salt movement.

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APPENDIX

Seismic section interpretations

Figure A1. Uninterpreted version of seismic line 5000. Locations of segments X-Y and Y-Z are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A2. Uninterpreted version of seismic line 5000. Locations of segments X-Y and Y-Z are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A3. Interpreted version of seismic line 5000. Numbered features are described in the text. Locations of segments X-Y and Y-Z are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A4. Interpreted version of seismic line 5000. Numbered features are described in the text. Locations of segments X-Y and Y-Z are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A5. Uninterpreted version of seismic line 2450. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A6. Uninterpreted version of seismic line 2450. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A7. Uninterpreted version of seismic line 2450. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A8. Interpreted version of seismic line 2450. Numbered features are described in the text. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A9. Interpreted version of seismic line 2450. Numbered features are described in the text. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.

Figure A10. Interpreted version of seismic line 2450. Numbered features are described in the text. Locations of segments A-B, B-C, and C-D are shown in Figure 8. Seismic data are displayed at 200% vertical exaggeration.