Upper Jurassic structure and evolution of the Yucatán and Campeche subbasins, southern Gulf of Mexico

ABSTRACT

The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatán and Campeche subbasins, separated by a base-salt high near the nose of the Yucatán platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition.

Portions of the Yucatán subbasin are characterized by large-scale (locally as much as 60 km [37 mi]) downdip translation of salt and suprasalt sediments during the Late Jurassic. This translation produced a major detached extensional province at the updip end of the basin, which is not compensated by observed shortening downdip. We interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening, a process mirrored on the conjugate Florida margin.

The Campeche subbasin, in contrast, shows no evidence of significant Late Jurassic translation detached on salt. No large-scale extensional or contractional provinces of Mesozoic age are evident, although some minor translation did occur. We suggest that salt in the Campeche subbasin was confined at its seaward end, which prevented the seaward salt flow experienced in the Yucatán subbasin. Furthermore, salt at the seaward end of the Campeche subbasin lies 2–3 km (1–2 mi) above oceanic crust, in contrast to salt lying on crust whose top sits at or below the level of oceanic crust at the seaward ends of the Tamaulipas, Yucatán, and Florida margins. The Campeche subbasin thus appears to have been perched relative to other parts of the Gulf of Mexico.

INTRODUCTION

The broad outlines of the opening of the Gulf of Mexico are reasonably clear (Figure 1). Rifting between North America and the Yucatán block began during the Late Triassic or earliest Jurassic (e.g., Freeland and Dietz, 1971; Van der Voo et al., 1976; White, 1980; Salvador, 1987). During these early stages, Yucatán pulled southeastward away from North America around a pole of rotation located in the North Atlantic. During the Middle Jurassic, Yucatán began to rotate strongly counterclockwise around a pole located near the western end of Cuba (e.g., Pindell, 1985; Dunbar and Sawyer, 1987; Pindell et al., 2016). Salt was deposited across the entire basin at approximately the time that counterclockwise rotation of Yucatán began (e.g., Pindell and Kennan, 2001, 2007, 2009; Hudec et al., 2013b; Rowan, 2018). Continued rifting and spreading after the end of salt deposition eventually split the salt basin, leaving the two conjugate halves on opposite sides of the Gulf.

Figure 1. Plate restoration showing opening of the Louann salt basin. (A) At circa 200 Ma, prior to rifting, the Yucatán block lies near the present coast of Texas. (B) Between 200 and 170 Ma, the Yucatán block moved southeast, away from Texas. Salt was deposited circa 170 Ma (A. Pulham, 2017, personal communication) across this rifted and stretched terrane. In the western part of the basin, the BAHA high marked the boundary between the separating halves of the salt basin. (C) Between 170 and 150 Ma, the Yucatán block rotated counterclockwise around a pole of rotation located near western Cuba. The BAHA high grew slightly, and oceanic crust formed in the basin, separating Yucatán and the northern margin of the basin. The salt basin split in two, with oceanic crust separating the Central Louann salt basin in the north from the Isthmian salt basin in the south.

Beyond this outline, however, the details quickly become murky. In fact, for such an intensely studied basin, the opening history of the Gulf of Mexico is surprisingly contentious. Issues remain concerning the timing of continental separation and the limit of oceanic crust (LOC) (e.g., Marton and Buffler, 1994; Pindell and Kennan, 2007; Mickus et al., 2009; Kneller and Johnson, 2011; Hudec et al., 2013b; Curry et al., 2018), the timing of salt deposition (e.g., Salvador, 1991b; Snedden et al., 2018), and the timing of salt deposition with respect to continental breakup (e.g., Buffler et al., 1980, 1981; Hall et al., 1982; Buffler and Sawyer, 1985; Pindell, 1985; Buffler, 1989; Salvador, 1991a; Buffler and Thomas, 1994; Hall, 1995; Imbert and Philippe, 2005; Pindell and Kennan, 2007; Hudec et al., 2013b; Rowan, 2018).

Our poor understanding of the tectonic evolution and crustal structure in the northern Gulf of Mexico makes it difficult to understand regional variations in salt styles or patterns of deformation. For example, Hudec et al. (2013b) divided the Central Louann salt basin in the northern Gulf of Mexico into three subbasins based on crustal depth, distribution of salt provinces, and location of salt-detached fold belts. They hypothesized that these subbasins were related to basement lineaments but were unable to provide details concerning the location, geometry, or origin of the proposed lineaments. Until seismic data improve enough to allow imaging of crustal structure in the northern Gulf of Mexico, our models for the origin and evolution of salt provinces there remain speculative.

An alternative approach to understanding salt provinces in the northern Gulf of Mexico is to examine the conjugate margin (Figure 2). The total thickness of suprasalt sediments is much less in the southern Gulf of Mexico than in the northcentral and northwestern part of the basin, and there is less allochthonous salt, so crustal structure in the south is better imaged and easier to interpret. However, until recently, little has been known about salt provinces in the southern Gulf of Mexico outside of Pemex.

Figure 2. Map of salt subbasins in southern Gulf of Mexico. The Yucatán subbasin features two outer troughs at its downdip end, separated by a horst block. The Campeche subbasin has a much higher density of salt diapirs, suggesting originally thicker salt.

The goals of this paper are to describe the differences in base-salt structure and salt styles of the Yucatán and Campeche subbasins and to infer their evolutionary histories. To isolate the influence of basin architecture on salt tectonics, our focus will be on the earliest phases of postsalt deformation in the Late Jurassic. Our hope is that this analysis will provide a framework for improving understanding of provinces that are less well imaged on the conjugate margin in the northern Gulf of Mexico.

DATA

The primary data used in this study were two-dimensional (2-D) seismic lines from the TGS Gigante survey in the southern Gulf of Mexico. This survey comprises more than 186,000 km (>115,600 mi) of prestack-depth-migrated 2-D seismic data covering the entire southern Gulf of Mexico outside of the continental shelf. Data are processed down to a depth of 20 km (66,000 ft).

This information was supplemented in northeastern Mexico by CGG’s prestack-depth-migrated, wide-azimuth Encontrado three-dimensional seismic survey. This survey is a reprocessing of nine legacy data sets and covers 38,000 km2 (14,700 mi2) on the continental slope, just south of the United States border. Data are processed down to a depth of 18 km (59,000 ft).

YUCATÁN SUBBASIN

Little has been published on the stratigraphic or structural evolution of the Yucatán subbasin. However, seismic sections in the Yucatán subbasin published by several authors (e.g., Lin, 1984; Miranda Madrigal, 2011; Williams-Rojas et al., 2011; Miranda-Peralta et al., 2014; Rowan, 2018) have helped outline the major structural provinces in the basin.

Seismic data show a thick package of northward-dipping reflections below salt in the Yucatán subbasin (Figure 3). This section thickens seaward, occupying most or all of the section above the Moho at the seaward end of the section. No wells penetrate this unit, so interpretations of lithology and origin are speculative. Reflection dips tend to become less steep seaward, in contrast to seaward-steepening dips characteristic of seaward-dipping reflector (SDR) sequences (e.g., Mutter et al., 1982). Furthermore, we interpret a series of basement-involved normal faults offsetting the sequence. We therefore follow Miranda-Peralta et al. (2014), Goswami et al. (2016), and Rowan (2018) in concluding that this package comprises a largely synrift sedimentary sequence rather than a series of stacked volcanic flows composing an SDR province. However, high-amplitude reflections in some parts of the presalt sequence may be volcanic.

Figure 3. Seismic section across the Yucatán subbasin: (A) uninterpreted and (B) interpreted. All postsalt horizon picks except for Upper Jurassic 2 are from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. The Upper Jurassic 2 pick is unconstrained, drawn to illustrate intra-Jurassic structure. A large salt-detached extensional province in the updip half of the margin is not balanced by an equivalent amount of shortening at the downdip end of the salt basin. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

The updip edge of the salt in the Yucatán subbasin lies near the edge of the Yucatán carbonate platform. From the edge of the Yucatán platform, the base of salt dips steadily seaward until descending sharply into a trough (Figure 2, 3). This trough is an elongate structural low near the LOC in which the base of salt drops below the level of both the base of salt on the landward side and the top of oceanic crust on the seaward side. At the outer edge of the trough in the Yucatán subbasin, the base of salt intersects an escarpment that climbs 1–2 km (3300–6600 ft) up onto oceanic crust (Figure 3). The escarpment is equivalent to the landward-dipping basement step of Barker and Mukherjee (2011) and the inner ramp of Hudec et al. (2013b).

The nomenclature of this trough is problematic. Unternehr et al. (2010; their figure 2) labeled an analogous structure in the South Atlantic the “distal basin,” but it is not clear if they were referring to the salt-filled structure or a larger-scale structural low in crystalline basement. Imbert (2005) termed the correlative structure in the northern Gulf of Mexico a “salt trough,” but it is not always filled with salt. In fact, in the Yucatán subbasin, the trough continues eastward beyond the depositional limit of salt, where it is filled entirely with nonevaporitic sediments (Figure 2). Curry et al. (2018) and Rowan (2018) termed it an outer marginal trough, but this invites confusion with the “outer marginal trough” as defined by Pindell et al. (2014), which explicitly refers to the larger-scale and longer-wavelength structural low in crystalline basement. We will refer to the shallow structure laterally continuous with the base of salt as the “outer trough.”

No synrift section has been identified beneath the outer trough, either because it does not exist there (as suggested by Rowan, 2018) or because the data quality is too poor. On the conjugate margin in offshore Florida, Pindell et al. (2014; their figure 5) show seismic data suggesting that synrift sediments may continue beneath the outer trough there.

Figure 4. (A) Enlarged section from Figure 3, showing method of calculating extension magnitude. (B, C) To capture the magnitude of the earliest extension in the basin, we restored to level of first bed above top of salt by looking for areas where the suprasalt section lies parallel to top of salt (heavy black lines in [B]). Joining these together via flexural-slip restoration (heavy black line in [C]) gives us our calculated extension. Structural restoration was created using Midland Valley two-dimensional (2-D) Move© software. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

Mapping shows that there are actually two outer troughs at the seaward end of the Yucatán subbasin, separated by a horst block (Figure 2). Most of the large salt structures in the Yucatán subbasin lie in the western trough. These structures are primarily stocks and walls, many of which deform the seafloor, indicating ongoing growth. Updip from the trough, most salt structures are small salt rollers of Jurassic age.

Figure 5. (A–E) Structural restoration of seismic line in Figure 3. During basin opening, salt flowed downslope into the growing outer trough. This unconfined flow of salt allowed large magnitudes of extension in the updip half of the basin that are uncompensated by equivalent shortening downdip. Structural restoration using Midland Valley two-dimensional Move© software. V.E. = vertical exaggeration.

A salt-detached extensional province of Jurassic age occupies the updip half of much of the Yucatán subbasin (Figures 3, 4A). Whereas Jurassic strata are thick enough for their internal structure to be resolved on seismic data, the extensional province is interpreted to have locally as much as 60 km (37 mi) of extension detached on salt (Figures 4, 5). Since the Jurassic, the basin has been relatively quiescent structurally, except for salt withdrawal around the salt structures in the outer marginal trough.

CAMPECHE SUBBASIN

As a consequence of prolific hydrocarbon discoveries in the Campeche subbasin, a great deal has been written about its evolution. However, most structural studies to date have focused either on local areas (e.g., Ambrose et al., 2003; Mitra et al., 2005, 2006; Gomez-Cabrera and Jackson, 2009a, b; Gonzalez et al., 2009; Moreno et al., 2009) or on regional distribution of Neogene structures (e.g., García-Molina, 1994; Oviedo-Perez, 1996; Pindell and Miranda, 2011). Very little has been published on the regional Mesozoic tectonic history of the subbasin, with the notable exception of a Comisión Nacional de Hidrocarburos (2015) report. Here, we will focus on the base-salt structure and postsalt Jurassic evolution to compare with the basin evolution determined for the adjacent Yucatán subbasin.

Figure 6. Seismic section across the northern end of the Campeche subbasin: the top Mesozoic pick is from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. No large Mesozoic extensional province exists at the updip end of the basin, and no salt has flowed down over the seaward edge of the salt basin. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

The presalt section is poorly imaged over most of the Campeche subbasin because of thick salt and complex diapir shapes. In parts of the basin with less complex salt, the presalt basin is variably developed. In some areas, the presalt is characterized by a relatively thin synrift section in half grabens, overlain by a more tabular sequence (Figure 6). Elsewhere, a basinward-expanding wedge similar to the style seen in the Yucatán subbasin is interpreted (Figure 7A).

Figure 7. (A) Interpreted seismic section across the northern end of the Campeche subbasin. (B) Interpreted seismic section across the central Campeche subbasin. The top Mesozoic picks are from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. Note the increase in diapir size toward the south. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

As it does in the Yucatán subbasin, salt in the Campeche subbasin pinches out near the edge of the Yucatán platform, with the base of salt dipping steadily seaward from the pinchout (Figure 6). However, the seaward end of the Campeche subbasin has no outer trough. Instead, the base of salt at the outer edge of the Campeche subbasin crust ramps 2–3 km (6600–9800 ft) down onto oceanic crust (Figure 6, 7). The lower part of the postsalt Mesozoic sequence onlaps this ramp. The seaward edge of salt in the Campeche subbasin lies near the top of the ramp.

A map of diapir distribution in the Campeche subbasin (Figure 2) shows much more salt than in the Yucatán subbasin, with the thickest salt in the south. Salt diapirs, salt sheets, and canopies are widespread, especially in the south. Most salt structures have been modified by Miocene–Holocene shortening caused by a combination of the Chiapanecan orogeny and gravity-driven deformation (e.g., Mitra et al., 2005, 2006; Pindell and Miranda, 2011; Comisión Nacional de Hidrocarburos, 2015; Figure 6, 7A).

Figure 8. Seismic section showing the lack of detached extensional structures at the updip end of the Campeche subbasin. The top Mesozoic pick is from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

Despite the seaward dip of salt from the base of the Campeche Escarpment, no major salt-detached extensional province of Jurassic age has yet been identified in the Campeche subbasin (Figure 8). Mitra et al. (2005) and Chernikoff et al. (2006) have identified small, detached Jurassic normal faults on the Akal–Reforma horst, but nothing on the scale of the 60 km (37 mi) of detached extension locally interpreted in Yucatán has been observed in the Campeche subbasin. Suprasalt strata at the base of the Yucatán platform on the eastern side of the basin are flat lying and undisturbed, except for some compressional folds associated with Miocene–Holocene shortening. We lack seismic data from the onshore southern end of the Campeche subbasin, but seismic-based cross sections released by the Comisión Nacional de Hidrocarburos (2015) show little Jurassic stretching there.

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ABSTRACT

The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatán and Campeche subbasins, separated by a base-salt high near the nose of the Yucatán platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition.

Portions of the Yucatán subbasin are characterized by large-scale (locally as much as 60 km [37 mi]) downdip translation of salt and suprasalt sediments during the Late Jurassic. This translation produced a major detached extensional province at the updip end of the basin, which is not compensated by observed shortening downdip. We interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening, a process mirrored on the conjugate Florida margin.

The Campeche subbasin, in contrast, shows no evidence of significant Late Jurassic translation detached on salt. No large-scale extensional or contractional provinces of Mesozoic age are evident, although some minor translation did occur. We suggest that salt in the Campeche subbasin was confined at its seaward end, which prevented the seaward salt flow experienced in the Yucatán subbasin. Furthermore, salt at the seaward end of the Campeche subbasin lies 2–3 km (1–2 mi) above oceanic crust, in contrast to salt lying on crust whose top sits at or below the level of oceanic crust at the seaward ends of the Tamaulipas, Yucatán, and Florida margins. The Campeche subbasin thus appears to have been perched relative to other parts of the Gulf of Mexico.

INTRODUCTION

The broad outlines of the opening of the Gulf of Mexico are reasonably clear (Figure 1). Rifting between North America and the Yucatán block began during the Late Triassic or earliest Jurassic (e.g., Freeland and Dietz, 1971; Van der Voo et al., 1976; White, 1980; Salvador, 1987). During these early stages, Yucatán pulled southeastward away from North America around a pole of rotation located in the North Atlantic. During the Middle Jurassic, Yucatán began to rotate strongly counterclockwise around a pole located near the western end of Cuba (e.g., Pindell, 1985; Dunbar and Sawyer, 1987; Pindell et al., 2016). Salt was deposited across the entire basin at approximately the time that counterclockwise rotation of Yucatán began (e.g., Pindell and Kennan, 2001, 2007, 2009; Hudec et al., 2013b; Rowan, 2018). Continued rifting and spreading after the end of salt deposition eventually split the salt basin, leaving the two conjugate halves on opposite sides of the Gulf.

Figure 1. Plate restoration showing opening of the Louann salt basin. (A) At circa 200 Ma, prior to rifting, the Yucatán block lies near the present coast of Texas. (B) Between 200 and 170 Ma, the Yucatán block moved southeast, away from Texas. Salt was deposited circa 170 Ma (A. Pulham, 2017, personal communication) across this rifted and stretched terrane. In the western part of the basin, the BAHA high marked the boundary between the separating halves of the salt basin. (C) Between 170 and 150 Ma, the Yucatán block rotated counterclockwise around a pole of rotation located near western Cuba. The BAHA high grew slightly, and oceanic crust formed in the basin, separating Yucatán and the northern margin of the basin. The salt basin split in two, with oceanic crust separating the Central Louann salt basin in the north from the Isthmian salt basin in the south.

Beyond this outline, however, the details quickly become murky. In fact, for such an intensely studied basin, the opening history of the Gulf of Mexico is surprisingly contentious. Issues remain concerning the timing of continental separation and the limit of oceanic crust (LOC) (e.g., Marton and Buffler, 1994; Pindell and Kennan, 2007; Mickus et al., 2009; Kneller and Johnson, 2011; Hudec et al., 2013b; Curry et al., 2018), the timing of salt deposition (e.g., Salvador, 1991b; Snedden et al., 2018), and the timing of salt deposition with respect to continental breakup (e.g., Buffler et al., 1980, 1981; Hall et al., 1982; Buffler and Sawyer, 1985; Pindell, 1985; Buffler, 1989; Salvador, 1991a; Buffler and Thomas, 1994; Hall, 1995; Imbert and Philippe, 2005; Pindell and Kennan, 2007; Hudec et al., 2013b; Rowan, 2018).

Our poor understanding of the tectonic evolution and crustal structure in the northern Gulf of Mexico makes it difficult to understand regional variations in salt styles or patterns of deformation. For example, Hudec et al. (2013b) divided the Central Louann salt basin in the northern Gulf of Mexico into three subbasins based on crustal depth, distribution of salt provinces, and location of salt-detached fold belts. They hypothesized that these subbasins were related to basement lineaments but were unable to provide details concerning the location, geometry, or origin of the proposed lineaments. Until seismic data improve enough to allow imaging of crustal structure in the northern Gulf of Mexico, our models for the origin and evolution of salt provinces there remain speculative.

An alternative approach to understanding salt provinces in the northern Gulf of Mexico is to examine the conjugate margin (Figure 2). The total thickness of suprasalt sediments is much less in the southern Gulf of Mexico than in the northcentral and northwestern part of the basin, and there is less allochthonous salt, so crustal structure in the south is better imaged and easier to interpret. However, until recently, little has been known about salt provinces in the southern Gulf of Mexico outside of Pemex.

Figure 2. Map of salt subbasins in southern Gulf of Mexico. The Yucatán subbasin features two outer troughs at its downdip end, separated by a horst block. The Campeche subbasin has a much higher density of salt diapirs, suggesting originally thicker salt.

The goals of this paper are to describe the differences in base-salt structure and salt styles of the Yucatán and Campeche subbasins and to infer their evolutionary histories. To isolate the influence of basin architecture on salt tectonics, our focus will be on the earliest phases of postsalt deformation in the Late Jurassic. Our hope is that this analysis will provide a framework for improving understanding of provinces that are less well imaged on the conjugate margin in the northern Gulf of Mexico.

DATA

The primary data used in this study were two-dimensional (2-D) seismic lines from the TGS Gigante survey in the southern Gulf of Mexico. This survey comprises more than 186,000 km (>115,600 mi) of prestack-depth-migrated 2-D seismic data covering the entire southern Gulf of Mexico outside of the continental shelf. Data are processed down to a depth of 20 km (66,000 ft).

This information was supplemented in northeastern Mexico by CGG’s prestack-depth-migrated, wide-azimuth Encontrado three-dimensional seismic survey. This survey is a reprocessing of nine legacy data sets and covers 38,000 km2 (14,700 mi2) on the continental slope, just south of the United States border. Data are processed down to a depth of 18 km (59,000 ft).

YUCATÁN SUBBASIN

Little has been published on the stratigraphic or structural evolution of the Yucatán subbasin. However, seismic sections in the Yucatán subbasin published by several authors (e.g., Lin, 1984; Miranda Madrigal, 2011; Williams-Rojas et al., 2011; Miranda-Peralta et al., 2014; Rowan, 2018) have helped outline the major structural provinces in the basin.

Seismic data show a thick package of northward-dipping reflections below salt in the Yucatán subbasin (Figure 3). This section thickens seaward, occupying most or all of the section above the Moho at the seaward end of the section. No wells penetrate this unit, so interpretations of lithology and origin are speculative. Reflection dips tend to become less steep seaward, in contrast to seaward-steepening dips characteristic of seaward-dipping reflector (SDR) sequences (e.g., Mutter et al., 1982). Furthermore, we interpret a series of basement-involved normal faults offsetting the sequence. We therefore follow Miranda-Peralta et al. (2014), Goswami et al. (2016), and Rowan (2018) in concluding that this package comprises a largely synrift sedimentary sequence rather than a series of stacked volcanic flows composing an SDR province. However, high-amplitude reflections in some parts of the presalt sequence may be volcanic.

Figure 3. Seismic section across the Yucatán subbasin: (A) uninterpreted and (B) interpreted. All postsalt horizon picks except for Upper Jurassic 2 are from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. The Upper Jurassic 2 pick is unconstrained, drawn to illustrate intra-Jurassic structure. A large salt-detached extensional province in the updip half of the margin is not balanced by an equivalent amount of shortening at the downdip end of the salt basin. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

The updip edge of the salt in the Yucatán subbasin lies near the edge of the Yucatán carbonate platform. From the edge of the Yucatán platform, the base of salt dips steadily seaward until descending sharply into a trough (Figure 2, 3). This trough is an elongate structural low near the LOC in which the base of salt drops below the level of both the base of salt on the landward side and the top of oceanic crust on the seaward side. At the outer edge of the trough in the Yucatán subbasin, the base of salt intersects an escarpment that climbs 1–2 km (3300–6600 ft) up onto oceanic crust (Figure 3). The escarpment is equivalent to the landward-dipping basement step of Barker and Mukherjee (2011) and the inner ramp of Hudec et al. (2013b).

The nomenclature of this trough is problematic. Unternehr et al. (2010; their figure 2) labeled an analogous structure in the South Atlantic the “distal basin,” but it is not clear if they were referring to the salt-filled structure or a larger-scale structural low in crystalline basement. Imbert (2005) termed the correlative structure in the northern Gulf of Mexico a “salt trough,” but it is not always filled with salt. In fact, in the Yucatán subbasin, the trough continues eastward beyond the depositional limit of salt, where it is filled entirely with nonevaporitic sediments (Figure 2). Curry et al. (2018) and Rowan (2018) termed it an outer marginal trough, but this invites confusion with the “outer marginal trough” as defined by Pindell et al. (2014), which explicitly refers to the larger-scale and longer-wavelength structural low in crystalline basement. We will refer to the shallow structure laterally continuous with the base of salt as the “outer trough.”

No synrift section has been identified beneath the outer trough, either because it does not exist there (as suggested by Rowan, 2018) or because the data quality is too poor. On the conjugate margin in offshore Florida, Pindell et al. (2014; their figure 5) show seismic data suggesting that synrift sediments may continue beneath the outer trough there.

Figure 4. (A) Enlarged section from Figure 3, showing method of calculating extension magnitude. (B, C) To capture the magnitude of the earliest extension in the basin, we restored to level of first bed above top of salt by looking for areas where the suprasalt section lies parallel to top of salt (heavy black lines in [B]). Joining these together via flexural-slip restoration (heavy black line in [C]) gives us our calculated extension. Structural restoration was created using Midland Valley two-dimensional (2-D) Move© software. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

Mapping shows that there are actually two outer troughs at the seaward end of the Yucatán subbasin, separated by a horst block (Figure 2). Most of the large salt structures in the Yucatán subbasin lie in the western trough. These structures are primarily stocks and walls, many of which deform the seafloor, indicating ongoing growth. Updip from the trough, most salt structures are small salt rollers of Jurassic age.

Figure 5. (A–E) Structural restoration of seismic line in Figure 3. During basin opening, salt flowed downslope into the growing outer trough. This unconfined flow of salt allowed large magnitudes of extension in the updip half of the basin that are uncompensated by equivalent shortening downdip. Structural restoration using Midland Valley two-dimensional Move© software. V.E. = vertical exaggeration.

A salt-detached extensional province of Jurassic age occupies the updip half of much of the Yucatán subbasin (Figures 3, 4A). Whereas Jurassic strata are thick enough for their internal structure to be resolved on seismic data, the extensional province is interpreted to have locally as much as 60 km (37 mi) of extension detached on salt (Figures 4, 5). Since the Jurassic, the basin has been relatively quiescent structurally, except for salt withdrawal around the salt structures in the outer marginal trough.

CAMPECHE SUBBASIN

As a consequence of prolific hydrocarbon discoveries in the Campeche subbasin, a great deal has been written about its evolution. However, most structural studies to date have focused either on local areas (e.g., Ambrose et al., 2003; Mitra et al., 2005, 2006; Gomez-Cabrera and Jackson, 2009a, b; Gonzalez et al., 2009; Moreno et al., 2009) or on regional distribution of Neogene structures (e.g., García-Molina, 1994; Oviedo-Perez, 1996; Pindell and Miranda, 2011). Very little has been published on the regional Mesozoic tectonic history of the subbasin, with the notable exception of a Comisión Nacional de Hidrocarburos (2015) report. Here, we will focus on the base-salt structure and postsalt Jurassic evolution to compare with the basin evolution determined for the adjacent Yucatán subbasin.

Figure 6. Seismic section across the northern end of the Campeche subbasin: the top Mesozoic pick is from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. No large Mesozoic extensional province exists at the updip end of the basin, and no salt has flowed down over the seaward edge of the salt basin. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

The presalt section is poorly imaged over most of the Campeche subbasin because of thick salt and complex diapir shapes. In parts of the basin with less complex salt, the presalt basin is variably developed. In some areas, the presalt is characterized by a relatively thin synrift section in half grabens, overlain by a more tabular sequence (Figure 6). Elsewhere, a basinward-expanding wedge similar to the style seen in the Yucatán subbasin is interpreted (Figure 7A).

Figure 7. (A) Interpreted seismic section across the northern end of the Campeche subbasin. (B) Interpreted seismic section across the central Campeche subbasin. The top Mesozoic picks are from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. Note the increase in diapir size toward the south. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

As it does in the Yucatán subbasin, salt in the Campeche subbasin pinches out near the edge of the Yucatán platform, with the base of salt dipping steadily seaward from the pinchout (Figure 6). However, the seaward end of the Campeche subbasin has no outer trough. Instead, the base of salt at the outer edge of the Campeche subbasin crust ramps 2–3 km (6600–9800 ft) down onto oceanic crust (Figure 6, 7). The lower part of the postsalt Mesozoic sequence onlaps this ramp. The seaward edge of salt in the Campeche subbasin lies near the top of the ramp.

A map of diapir distribution in the Campeche subbasin (Figure 2) shows much more salt than in the Yucatán subbasin, with the thickest salt in the south. Salt diapirs, salt sheets, and canopies are widespread, especially in the south. Most salt structures have been modified by Miocene–Holocene shortening caused by a combination of the Chiapanecan orogeny and gravity-driven deformation (e.g., Mitra et al., 2005, 2006; Pindell and Miranda, 2011; Comisión Nacional de Hidrocarburos, 2015; Figure 6, 7A).

Figure 8. Seismic section showing the lack of detached extensional structures at the updip end of the Campeche subbasin. The top Mesozoic pick is from wells drilled in the northern Gulf of Mexico, correlated across the basin on a continuous grid of two-dimensional (2-D) seismic data. The 2-D prestack-depth-migrated seismic data are courtesy of TGS. V.E. = vertical exaggeration.

Despite the seaward dip of salt from the base of the Campeche Escarpment, no major salt-detached extensional province of Jurassic age has yet been identified in the Campeche subbasin (Figure 8). Mitra et al. (2005) and Chernikoff et al. (2006) have identified small, detached Jurassic normal faults on the Akal–Reforma horst, but nothing on the scale of the 60 km (37 mi) of detached extension locally interpreted in Yucatán has been observed in the Campeche subbasin. Suprasalt strata at the base of the Yucatán platform on the eastern side of the basin are flat lying and undisturbed, except for some compressional folds associated with Miocene–Holocene shortening. We lack seismic data from the onshore southern end of the Campeche subbasin, but seismic-based cross sections released by the Comisión Nacional de Hidrocarburos (2015) show little Jurassic stretching there.

The Campeche subbasin thus differs from the Yucatán subbasin in base-salt structure, salt distribution, and Mesozoic postsalt tectonism. Given that the two subbasins are adjacent along a single passive margin, how can they be so different?

DISCUSSION: JURASSIC EVOLUTION OF THE YUCATÁN AND CAMPECHE SUBBASINS

Yucatán Subbasin: Unconfined Salt Flow during Basin Opening

Our interpretation for the evolution of the Yucatán subbasin is related to the Gulf of Mexico tectonic models proposed by Pindell and Kennan (2007), Hudec et al. (2013b; Figure 5), and Rowan (2018). They suggested that salt deposition was followed by a period of unconfined salt flow during basin opening, during which salt flowed seaward toward the center of the basin (Figure 9). The mechanisms that created space for the salt to flow into remain contentious. Pindell and Kennan (2007) and Norton et al. (2016) suggested that salt flowed seaward onto oceanic crust formed after the time of salt deposition. Hudec et al. (2013b) argued that much of the space for seaward flow was through creation of a wide-zone transitional crust during crustal stretching that followed salt deposition. More recently, Rowan (2018), using improved seismic data, suggested that crustal stretching after salt deposition was limited to widening of the outer trough. In Rowan’s model (Figure 5, 9), updip extension in the Yucatán subbasin was accommodated through widening of the outer trough, contractional thickening of the salt in the trough, and closure of early-formed diapirs. Eventually, continued widening split the salt into separate basins on the conjugate margins.

Figure 9. (A, B) Schematic opening model for parts of the eastern Gulf of Mexico, in which unconfined salt is able to flow downslope into the outer trough. Seaward flow of salt stretches the overburden, creating large-scale salt-detached extension in updip parts of the basin. V.E. = vertical exaggeration.

The depositional geometry of salt in this type of scenario is also contentious. Until recently, the general assumption was that salt in the Gulf of Mexico was deposited with a flat upper surface near sea level so that its presence today below the level of oceanic crust in the outer trough is the result of some sort of postdepositional collapse (e.g., Hudec et al., 2013b; Pindell et al., 2014; Norton et al., 2016). However, more recent work—based on considerations of salt viscosity, presence of deep-water suprasalt facies, and evaporite depositional models—argues that salt may be deposited near sea level at the updip end of the basin but several kilometers deeper at the distal end (e.g., Rowan, 2018). We use this newer hypothesis in our reconstructions of the Yucatán margin (Figure 5, 9).

The point in this history at which seafloor spreading begins is unclear. Hudec et al. (2013b) argued that the LOC occurs at the outer edge of the distal salt trough, which is consistent with the location suggested by Pindell et al. (2011, 2014, 2015, 2016) and Rowan (2018). However, Norton et al. (2016) suggested that in offshore Brazil, the LOC might occur inboard of the distal salt trough so that the trough itself is underlain by intrusive oceanic crust. Other scenarios (e.g., Curry et al., 2018) suggest that the LOC could lie outboard of the outer marginal trough. For the purposes of understanding salt behavior during plate separation, however, the crustal types underlying salt are of only secondary importance.

The effect of this history is to create a very large salt-detached extensional province not balanced by observed shortening downdip. Seaward translation is encouraged by seaward dips of both bathymetry and base of salt. Most extensional structures formed during basin widening have no contractional equivalents, and much of the extension following basin separation is balanced by cryptic shortening of diapirs. These large, uncompensated extensional provinces are observed in both the Yucatán subbasin (Figure 3) and on the conjugate margin of offshore Florida (Imbert and Philippe, 2005; Hudec et al., 2013b; Pilcher et al., 2014).

Campeche Subbasin: Confined Salt Flow during Basin Opening

The most obvious difference between the two subbasins is that the Campeche subbasin experienced dramatic shortening during the Miocene Chiapanecan orogeny, which resulted from Pacific plate interactions (e.g., Mandujano-Velazquez and Keppie, 2009; Pindell and Miranda, 2011; Witt et al., 2012; Comisión Nacional de Hidrocarburos, 2015). This compressional overprint has masked the fact that the Yucatán and Campeche subbasins have been distinct features from the time of their initiation. Base-salt structure, salt thickness, and Jurassic tectonism all differ across the nose of the Yucatán platform.

In terms of Upper Jurassic structure, the most striking difference between the Yucatán and Campeche subbasins is the absence of the large-scale detached extensional province in Campeche. This absence implies that the early seaward translation so important in Yucatán did not occur in Campeche. Consistent with the proposed lack of seaward salt flow, we note that the seaward limit of salt in Campeche lies at the top of a ramp down onto oceanic crust. If the salt had been able to flow seaward, we would expect large volumes of salt to have flowed down this ramp and out onto oceanic crust around the margins of the Campeche subbasin. That this process did not occur suggests that something inhibited seaward flow of salt. In other words, seaward salt flow during basin opening in Campeche was confined, in contrast to the unconfined flow in Yucatán. At least two explanations are possible for the absence of seaward flow in the Campeche subbasin.

The first hypothesis (Rowan, 2018) is based on the fact that the Mesozoic plate boundary in the western Gulf of Mexico, where the Campeche subbasin lies, was dominated by transform faults separated by short ridge segments (Figure 2). With this type of plate boundary, formation of a continuous oceanic trough west of Campeche would have been delayed. This hypothesis suggests that in the absence of a continuous basin to the west, no gravitational head to drive significant seaward salt flow existed. By the time the continuous ocean basin formed, the sedimentary sequence above the salt was thick enough to keep the salt anchored in place.

This model can be evaluated using a plate restoration (Figure 10), which shows that given a reasonable opening model, a continuous ocean basin would have formed seaward of the Campeche subbasin within the first 4 m.y. of seafloor spreading. Based on an overall postsalt (ca. 170–66 Ma) Mesozoic thickness of approximately 500–1500 m (∼1600–4900 ft) in the Campeche subbasin (Figure 6, 7), we would expect approximately 20–60 m (∼66–200 ft) of section to be deposited above salt during this 4-m.y. period. It seems very unlikely that such a thin sedimentary sequence would be strong enough to inhibit salt flow down a 2-km-high (6600-ft-high) ramp, so we reject this hypothesis.

Figure 10. (A–D) Plate restoration of the Gulf of Mexico, showing the development of oceanic crust after creation of a continuous ridge–transform system. The restoration shows a continuous oceanic trough developed seaward of the Campeche salt basin within the first 4 m.y. of seafloor spreading. Spreading was calculated using an assumed rotation rate of 1.98°/m.y. around a Florida Straits pole at 23.47°N, 83.69°W, enough to open the Gulf of Mexico in 15 m.y. (170–155 Ma). This angular rate yields a half-spreading rate of 2.25 cm/yr (0.89 in./yr) at the transform boundary along the western edge of the Gulf of Mexico, decreasing to 1.4 cm/yr (0.55 in./yr) at the northwest corner of Yucatán.

Our preferred hypothesis is that the edge of the Campeche subbasin had a raised rim at the base of salt, preventing salt from flowing from the salt basin down into the adjacent basement low (Figure 11). Salt in the Campeche subbasin would have been ponded behind this rim, isolated from the seaward salt flow affecting other parts of the basin. Some gravity-driven extension and shortening took place within the Campeche subbasin, but this was minor compared with the unconfined gliding occurring elsewhere.

Figure 11. (A, B) Schematic opening model for parts of the western Gulf of Mexico, in which the two halves of the salt basin are separated by a base-salt structural high. Salt in the Campeche subbasin was deposited near sea level, perched with respect to salt on the conjugate northern margin. Salt flow toward the basin center during opening is blocked by the basement high. V.E. = vertical exaggeration.

A raised rim may also explain the greater salt thickness accumulated in the Campeche subbasin, as suggested by the larger number of diapirs formed there (Figure 2). A rim would have formed a closed structural low that could be filled with salt, whose thickness could provide an estimate of the height of the rim. The depositional thickness of salt in the Campeche salt basin cannot be determined directly because of subsequent salt flow into diapirs, but 1–2 km (3300–6600 ft) over much of the basin seems reasonable.

If we think that a rim 1–2 km high (3300–6600 ft high) existed around the Campeche subbasin during the Late Jurassic, why is it no longer present? Perhaps the rim was underlain by thermally elevated crust that subsided rapidly as it cooled. Alternatively, the rim may have had something to do with the high spreading rates or dominant transform motions characteristic of the western Gulf of Mexico. Other possibilities no doubt exist. Without knowledge of the crustal structure underlying our proposed base-salt rim, any origin explanations must remain speculative.

Regardless of the origin and fate of our proposed rim, the Campeche subbasin must have had a very different morphology than that of the Yucatán subbasin. New oceanic crust typically forms 2–4 km (6600–13,100 ft) below sea level (e.g., Parsons and Sclater, 1977; Stein and Stein, 1992; Crosby and McKenzie, 2009; Hoggard et al., 2017). If the base of salt in the Campeche subbasin is 2–3 km (6600–9800 ft) above oceanic crust, and 1–2 km (3300–6600 ft) of salt were deposited, then salt originally must have filled the basin to near sea level. The presence of a flat top of salt and synclinal base of salt (Figure 11) explains the relative stability of salt in the Campeche subbasin during the Jurassic.

The morphology of the Campeche subbasin is thus strikingly different from the sloping depositional salt geometry inferred for the Yucatán subbasin (compare Figure 9 and 11). The relationship between salt and oceanic crust in Campeche suggests that this salt basin was perched relative to the Yucatán, separated by the basement ridge that defines the boundary between the two subbasins.

Interestingly, a pronounced ridge exists along the rim of the conjugate margin in offshore Tamaulipas and Texas (Figures 1, 12). We term this ridge the “BAHA high” after the first well drilled on it. The BAHA-1 well was drilled by a partnership of Shell, Texaco, Mobil, and Amoco in 1996. The acronym “BAHA” was derived from the first letters of the four company prospect names: Brachiosaurus, Alpha Centauri, Hi-C, and Anaconda. The step up onto the northwestern side of this ridge has been noted by several previous scientists in offshore Texas (e.g., Peel et al., 1995; Trudgill et al., 1999; Hudec et al., 2013a, b), but the full extent of the structure has not been appreciated. The BAHA high, which runs for 500 km (310 mi) along the downdip end of the Central Louann salt basin, is 1.5–3 km (5000–9900 ft) high over most of its length and serves as the backstop against which the Perdido fold belt formed. In our plate restoration, it lies along the plate boundary at the time of salt deposition (Figure 1B) and therefore most likely formed then.

Figure 12. Seismic section showing the BAHA high in offshore Tamaulipas, northeastern Mexico. Three-dimensional seismic data courtesy of CGG. V.E. = vertical exaggeration.

Despite the fact that the BAHA high forms a ridge similar to the one we propose for the conjugate margin in Campeche, there are some important differences. First, the BAHA high still exists. Second, the base of salt and the top of oceanic crust are at approximately the same elevation on opposite sides of the BAHA high, without a major change in elevation such as the one that exists at the edge of the Campeche subbasin. Our schematic reconstruction of the western Gulf of Mexico (Figure 11) thus shows a strong asymmetry between salt near sea level in the perched Campeche subbasin and the much deeper salt in the Tamaulipas margin. This asymmetry offers another argument in support of a base-salt high at the seaward end of the Campeche subbasin because there must have been some sort of structure separating the salt in Campeche from salt on the conjugate margin.

Detailed discussion of the evolution of the Tamaulipas margin is outside the scope of this paper, but worth noting is the fact that the Campeche subbasin is perched relative to both the Yucatán subbasin along strike and the conjugate Tamaulipas margin.

CONCLUSIONS

The Yucatán and Campeche subbasins have radically different base-salt geometries, which had a significant impact on their subsequent evolutions. The Yucatán subbasin, like most passive margins bearing late synrift salt, had a continuous, seaward-dipping detachment allowing large-scale translation of the salt and its cover downslope toward the widening ocean basin. This salt-detached translation dominated the early history of the basin, producing a spectacular extensional province locally accommodating 60 km (37 mi) of slip. Salt likely had considerable depositional relief, lying several kilometers below sea level at the seaward end of the basin. We refer to this scenario as “unconfined salt flow during basin opening.”

The Campeche subbasin, by contrast, lacks the early seaward translation characteristic of the Yucatán subbasin. Salt at the seaward end of the basin apparently did not flow seaward despite the existence of a topographic low at the downdip end of the basin into which salt might have flowed. We hypothesize that salt in Campeche was bounded by a basement high along its outer rim, a scenario we refer to as “confined salt flow during basin opening.” The base of salt lies several kilometers above sea level, suggesting that salt in the Campeche subbasin was deposited near sea level, perched relative to both the Yucatán subbasin and the conjugate Tamaulipas margin.

The subbasins observed in the Isthmian salt basin appear to correlate with similar structures on the conjugate Central Louann salt basin. Confined salt flow in the Campeche subbasin is conjugate to the BAHA high in offshore Tamaulipas. Likewise, unconfined salt flow evident in parts of the Yucatán subbasin is conjugate to similar types of structures in offshore Florida. Jurassic salt basin configuration thus appears to vary from east to west in the Gulf of Mexico and to correlate across the basin onto conjugate margins. Investigating the configuration of conjugate structures on the Mexican margin offers intriguing possibilities for improving our understanding of the Texas and Louisiana segments of northern deep-water Gulf of Mexico. These questions will form the nucleus of our future research on Gulf of Mexico tectonics.

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ACKNOWLEDGMENTS

We thank Geosoft for the academic licensing of Oasis montaj software and Midland Valley for the use of Move©. The authors would also like to thank TGS and CGG for permission to use seismic data. Quincy Zhang, Duncan Bate, and Purnima Bhowmik of TGS and Dutch Soth of CGG were especially helpful in obtaining images. Nancy Cottington drafted the diagrams. Cari Breton constructed the geographic information system database. Stephanie Jones edited the text. The paper benefitted materially from helpful reviews by Jim Pindell and Carl Fiduk. Many of the ideas in this paper were originally developed in a 2011 workshop conducted at Pemex offices in Ciudad del Carmen. We thank Pemex geologists Enrique Reyes Tovar, Rafael Clemente Martinez, Alejandro Cárdenas Alvarado, Héctor Lopez Céspedes, Rolando Peterson Rodríguez, Juan Jaime Hernández Peñaloza, Héctor Melo Amaro, Juan Carlos Flores Zamora, and David Barrera Gonzalez for their participation in this workshop.

The project was funded by the Applied Geodynamics Laboratory consortium, comprising the following companies: Anadarko, BHP Billiton, BP, CGG, Chevron, Cobalt, Condor, ConocoPhillips, Ecopetrol, EMGS, Eni, Equinor (formerly Statoil), ExxonMobil, Hess, ION Geophysical, Marathon, Midland Valley, Murphy, Nexen, Noble, Petrobras, Petronas, PGS, Repsol, Rockfield, Saudi Aramco, Shell, Spectrum Geo, Stone, TGS, Total, WesternGeco, and Woodside. Additional funding came from the Jackson School of Geosciences. Publication was authorized by the director of the Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin.

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