Mexican Ridges Research Articles

Using Large-Size Three-Dimensional Marine Electromagnetic Data for the Efficient Combined Investigation of Natural Hydrogen and Hydrocarbon Gas Reservoirs: A Geologically Consistent and Process-Oriented Approach with Implications for Carbon Footprint Reduction

The recycling or burial of carbon dioxide in depleted petroleum reservoirs and re-imagining exploration strategies that focus on hydrogen reservoirs (with any associated hydrocarbon gas as the upside potential) are a necessity in today’s environmental and geopolitical climate. Given that geologic hydrogen and hydrocarbon gases may occur in the same or different reservoirs, there will be gains in efficiency when searching for both resources together since they share some commonalities, but there is no geophysical workflow available yet for this purpose. Three-dimensional (3D) marine controlled-source electromagnetic (CSEM) and magnetotelluric (MT) methods provide valuable information on rock-and-fluid variations in the subsurface and can be used to investigate hydrogen and hydrocarbon reservoirs, source rocks, and the migration pathways of contrasting resistivity relative to the host rock. In this paper, a process-oriented CSEM-MT workflow is proposed for the efficient combined investigation of reservoir hydrocarbon and hydrogen within a play-based exploration and production framework that emphasizes carbon footprint reduction. It has the following challenging elements: finding the right basin (and block), selecting the right prospect, drilling the right well, and exploiting the opportunities for sustainability and CO2 recycling or burial in the appropriate reservoirs. Recent methodological developments that integrate 3D CSEM-MT imaging into the appropriate structural constraints to derive the geologically robust models necessary for resolving these challenges and their extension to reservoir monitoring are described. Instructive case studies are revisited, showing how 3D CSEM-MT models facilitate the interpretation of resistivity information in terms of the key elements of geological prospect evaluation (presence of source rocks, migration and charge, reservoir rock, and trap and seal) and understanding how deep geological processes control the distribution and charging of potential hydrocarbon, geothermal, and hydrogen reservoirs. In particular, evidence is provided that deep crustal resistivity imaging can map serpentinized ultramafic rocks (possible source rocks for hydrogen) in offshore northwest Borneo and can be combined with seismic reflection data to map vertical fluid migration pathways and their barrier (or seal), as exemplified by the subhorizontal detachment zones in Eocene shale in the Mexican Ridges fold belt of the southwest of the Gulf of Mexico, raising the possibility of using integrated geophysical methods to map hydrogen kitchens in different terrains. The methodological advancements and new combined investigative workflow provide a way for improved resource mapping and monitoring and, hence, a technology that could play a critical role in helping the world reach net-zero emissions by 2050.

Three-dimensional anisotropic inversion and electrostratigraphic imaging of marine magnetotelluric data to understand the control of crustal deformations by pre-existing lithospheric structures in the Mexican Ridges Fold belt, Southwestern Gulf of Mexico

SUMMARY3-D imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broad-band (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3-D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5–8 km depth (coinciding with the regional detachment zone in Eocene shales in 3-D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW–ENE trending, 6–10 km wide, electrically resistive subvertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15–20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30°. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.

Cenozoic structural deformation between the southern Lamprea fold-belt and Salina del Bravo salt province by interacting salt and shale detachments, western Gulf of Mexico

The Lamprea fold-belt is a 300 km-long, 20-60 km-wide, deep-water, passive margin fold-belt located southeast of the Salina del Bravo salt province and trending parallel to the eastern continental margin of Mexico. The Lamprea fold-belt detaches along Eocene shale layers to form a compressional toe of the gravitationally-driven system that forms a transitional domain between the salt province to the north and the shale-detached Mexican Ridges fold-belt to the south. Area-depth strain measurements performed for ten characteristic folds across two regional seismic profiles through the Lamprea fold-belt provide insight into its timing of deformation, depth to detachment, and controls on the pre-growth and syn-growth sedimentary deposits. Miocene-age deformation across the Lamprea fold-belt coincides with a phase of renewed salt canopy emplacement driven by ongoing up-dip sediment loading and extensional deformation west of the Salina del Bravo salt province. These results are consistent with early Miocene fold growth onset controlled primarily by the advancing salt canopy and only minor influence from underlying autochthonous salt. The timing of trap formation, tectonic interaction of salt bodies with shale structures, and the mechanisms for fold-belt formation all control hydrocarbon migration from deep Mesozoic source intervals into overlying Oligocene fold-belt reservoirs.

Gas hydrate identification and distribution in the Mexican Ridges, Mexico

We have mapped gas hydrates, free gas, and bottom-simulating-reflector (BSR) distributions in an area of the Mexican Ridges, central Gulf of Mexico, Mexico, revealing the relationship between these three elements and the tectonostratigraphy. The three elements are more visible when the host rock is a high-porosity sandstone because there is a large seismic impedance contrast between the solid gas hydrates above and the free gas below, which manifests itself on the seismic as a BSR. Gas hydrates are identified in the well as higher resistivity sandstone layers with a strong positive amplitude. When the host rock has a higher shale content with lower porosity, the impedance contrast is lower and the BSR is weak or not visible. The study area in Mexican Ridges is an anticline where gas hydrates and free gas are trapped on the crest after migrating through the dipping layers and faults from synclines that are generated in calcareous shale. The main seal is mass transport complex deposits from the Pliocene; when they are not deposited at the crest of the anticline, there is gas escape to the seafloor in the form of a gas chimney. In this way, we established a complete petroleum system for gas hydrates and free gas on the Mexican Ridges.

Potentially large subsurface gas hydrate bodies in the Mexican Ridges, Southwestern Gulf of Mexico

Because rising ocean temperatures can destabilize gas hydrate, identifying and characterizing large shallow hydrate bodies is increasingly important to understanding their hazard potential. In the southwestern Gulf of Mexico, reanalysis of 3D seismic reflection data reveals evidence for the presence of six potentially large gas hydrate bodies located at shallow depths below the seafloor. We originally interpreted these bodies as salt because they share common visual characteristics on seismic data with shallow allochthonous salt bodies, including high-impedance boundaries and homogeneous interiors with very little acoustic reflectivity. However, when seismic images are constructed using acoustic velocities associated with salt, the resulting images were of poor quality containing excessive moveout in common-reflection point offset image gathers. Further investigation reveals that using lower valued acoustic velocities results in higher quality images with little or no moveout. We believe that these lower acoustic values are representative of gas hydrate and not of salt. Directly underneath these bodies lies a zone of poor reflectivity, which is typical and expected under hydrate. Observations of gas in a nearby well, other indicators of hydrate in the vicinity, and the regional geologic context support the interpretation that these large bodies are composed of hydrate. The total equivalent volume of gas within these bodies is estimated to potentially be as large as 1.5 gigatons or 10.5 trillion cubic feet, considering uncertainty for estimates of porosity and saturation, comparable with the entire proven natural gas reserves of Trinidad and Tobago in 2019.

Control of structural style by large, Paleogene, mass transport deposits in the Mexican Ridges fold-belt and Salina del Bravo, western Gulf of Mexico

Abstract The Mexican Ridges fold-belt (MR) of the western Gulf of Mexico (GOM) is a 600 km long, 120–220 km wide, deep-water, passive margin, fold-belt trending parallel to the eastern continental margin of Mexico. Shortening within the deep-water MR occurs above a primary sub-horizontal, shale detachment linked to an up-dip, 50 km wide zone of listric, normal faults along the continental shelf of eastern Mexico. A 20,000 line-km grid of 2D depth-converted industry seismic data tied to one, deep well was used to map the frequency, location, lateral extent, and thickness distributions of twelve, individual, mass transport deposits (MTDs) of Paleogene age that together form a

Development of Detachment Folds in the Mexican Ridges Foldbelt, Western Gulf of Mexico Basin

AbstractExamples of natural folds growing in a homogenous mechanical stratigraphy of alternating competent and incompetent thin layers of fine‐ and coarse‐grained sediments are examined, and the fold growth process is quantified. Our analysis reveals that the overall response to loading of siliciclastic sequences corresponds to that of flexural flow and parallel‐to‐bedding heterogeneous pure shear. Folds start out as low‐amplitude sinusoidal disturbances that rapidly become finite‐amplitude folds of heterogeneous strain. We also derive the following scaling relations: (i) degree of amplification scales with both the height above the detachment and strain, (ii) wavelength selectivity broadens with increasing strain, and (iii) deposition of syn‐sedimentary geometries is function of strain. These relations are a natural consequence of idealized area‐preserving laws of fold growth. From these results we devise a method to estimate fold strain by means of an amplitude versus depth diagram. We are also able to define a progression of fold shape change as a function of the fundamental parameter strain. Initially, structures grow by limb rotation and the selective amplification of a single dominant wavelength giving rise to sinusoidal folds. When strain reaches ~8%, softening/plastic yielding around hinges results in the development of sharp fold profiles. Limbs lock their dips at 35°–45°, suggesting that growth in this stage is permitted by hinge mobility along ramps and blind faults. Moreover, hinge migration causes fold development to accelerate spontaneously. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.

The interplay between deformation, erosion and sedimentation in the deep‐water Mexican Ridges foldbelt, western Gulf of Mexico basin

AbstractWe analyse a regional 2D seismic section of the Mexican Ridges foldbelt (MRFB), western Gulf of Mexico, and construct excess‐area diagrams for each of the structures comprising the foldbelt to estimate shortening, the onset of folding and the degradation of the folded seafloor. From the chronostratigraphy, we derive rates of tectonic and superficial mass transport and illustrate how they change across the MRFB. The resulting tectonic transport in the MRFB is 11.8 km forming a train of twelve buckle folds above a detachment at a depth of ca. 6 s of two‐way travel time, with an average strain of ca. 10%. The fold train grew at a mean uplift rate of ca. 0.21 mm year−1. Cross‐sectional balancing demonstrates that shortening balances the down‐slip motion of the Quetzalcoatl extensional system (QES), suggesting that horizontal compaction, volume loss and other penetrative deformation mechanisms are negligible. By assuming steady‐state denudation, we are able to distinguish sediments derived locally from sediments transported from distant sources. The constant of mass diffusivity, a parameter controlling the degradation rate, is ca. 0.42 m2 year −1, which is characteristic of rapid, episodic, superficial mass movements. The combined sedimentation rate from both, local and distal sources is ca. 0.23 mm year −1. Those values are not constant; structures proximal to the continental shelf are rising rapidly and are being degraded more intensely than those in the distal part of the MRFB, where sedimentation outweighs tectonic uplift. Our results indicate deformation initiated up to 3 Myr earlier than estimated from stacking patterns. Moreover, we find deformation started synchronously during the Late Miocene throughout the MRFB and not in two episodes as the stacking relations suggest. The discrepancy can be explained by a delay in the sedimentary response to folding. During early fold growth, nearly constant thickness strata are deposited before a progressive unconformity and other converging geometries develop. The development of growth strata is fast in the folds near the QES, which are being uplifted rapidly and degraded vigorously. Under these conditions, the stratigraphic relations give only a broad estimate of the pretectonic/syntectonic limit when compared to the excess‐area method. On the other hand, the development of growth strata took twice as much time for folds near the abyssal plain, which are being uplifted at a slower rate and where degradation is less intense. Consequently, the delay takes more time, and the use of stratigraphic relations introduces an even more pronounced bias towards younger ages in the identification of the onset of folding.

Epibenthic megacrustaceans from the continental margin, slope and abyssal plain of the Southwestern Gulf of Mexico: Factors responsible for variability in species composition and diversity

Abstract The community structure of megacrustaceans (orders Lophogastrida, Isopoda, and Decapoda) collected in trawls on the continental margin, upper slope and abyssal plain of the southern Gulf of Mexico was studied to determine to what extent broad-scale variation in community composition and diversity was influenced by geographic regions environmental variability and depth. Trawls were collected in the Mexican Ridges, the Campeche Bank, and the Sigsbee abyssal plain. There was variability in species composition, density and diversity among geographic regions and along the depth gradient. A total of 106 species were identified and grouped in three orders; five infraorders, 40 families, and 70 genera. This study extends the known geographic ranges of the species Homolodromia monstrosa and Ephyrina benedicti. The largest number of species was recorded in the Mexican Ridges and on the upper continental shelf; lower values were found on the continental margin and in the abyssal plain. The largest densities were recorded on the continental margin in the Mexican Ridges. Megacrustaceans show in general low frequencies and low abundances in trawls, characterizing them as rare components of benthic assemblages. Contrary to an accepted paradigm about deep-sea biodiversity, the highest H′ diversity values were recorded in the Sigsbee abyssal plain, followed by values from the upper continental slope; diversity values were correlated with evenness. Canonical Redundancy analysis results showed a significant affinity to regions for 18 crustacean species; 33 species showed a significant affinity to both regions and depth zones within regions.

Neogene crustal shear zone along the western Gulf of Mexico margin and its implications for gravity sliding processes. Evidences from 2D and 3D multichannel seismic data

AbstractNo significant crustal deformation was registered along the western Gulf of Mexico margin since the late Jurassic except the well known Cenozoic gravity tectonics. This is marked by a major extension across the platform and the upper continental slope compensated downslope by shortening across the Mexican Ridges fold belt.Based on extensive offshore 2D and 3D industrial multichannel seismic reflection data provided to us by PEMEX, we have evidenced significant Neogene deep-rooted deformation below the main décollement level (5 to 7 s-twtt) related to these gravitational processes. The main crustal deformation is outlined by a N170° trending deep-seated reverse fault zone, which flattens downwards near the Moho and merges upwards near the main Oligo-Miocene décollement level, close to the Neogene Mexican Ridges fold belt. This deep seated fabric is interpreted as the result of a dextral strike slip fault zone rather steep and linear into the north and connecting southwards to a N150° trending dextral wrench zone east of the Trans-Mexican volcanic belt.We consider that this Neogene transpressive dextral motion could have triggered gravity sliding along the Mexican Gulf margin. It could be located at the continent-ocean boundary and is probably linked to conjugate dextral slip related to the late Neogene N140° trending left lateral slip along the Veracruz shear zone active since the Late Miocene. We discuss whether this deep thrust wrench zone is related with the eastward migrating Laramide orogeny front dated Paleocene-Eocene.

Updated checklist and identification of areas of endemism of benthic amphipods (Caprellidea and Gammaridea) from offshore habitats in the SW Gulf of Mexico

An updated checklist of the benthic amphipods associated with soft bottoms in the SW Gulf of Mexico has been made and the areas of endemism have been identified. Amphipods were collected in a depth interval of 25 to 3700 m during six cruises from 1994 to 1999. A total number of 56 species were identified, 11 belonging to the suborder Caprellidea and 45 to the suborder Gammaridea. Thirteen species extended their geographical distribution within the Gulf of Mexico and four species were considered new records in the Gulf of Mexico. A biogeographic analysis based on the shared species and the parsimony criterion helped identify three areas of endemism in the SW Gulf of Mexico. They were named: area A, a small area on the continental shelf of the Bay of Campeche; area B, the continental slope of the Bay of Campeche; and area C, the lower continental slope of the Mexican ridges extending into the western Sigsbee abyssal plain. These areas are defined in terms of their complex topography that may help isolate habitats that promote endemicity.

Crustal velocity field of Mexico from continuous GPS measurements, 1993 to June 2001: Implications for the neotectonics of Mexico

We combine velocities for 14 continuous GPS stations spanning Mexico and 173 additional continuous GPS sites on the North American and Pacific plates to study the large‐scale deformation of Mexico. The new station velocities, which are derived from more than 6000 days of previously unused GPS data, provide the first ever view of the crustal velocity field of Mexico. Key results are as follows: (1) Areas north of the Mexican Volcanic Belt, not including Baja California, move with the North American plate interior within the 1–2 mm yr−1 station velocity uncertainties. Station velocities for the Mexican Basin and Range are consistent with no present‐day extension and yield an upper 95% limit of 1–3 mm yr−1 for any regional extension. (2) South of the Mexican Volcanic Belt, five of the six sites move significantly relative to the North American plate. All sites in the Yucatan Peninsula move toward the east at 3–4 mm yr−1, possibly defining an independent Yucatan block. (3) Site velocities are consistent with limits of 0–4 mm yr−1 for present slip across the Mexican Volcanic Belt. (4) Tampico, on the gulf coast, exhibits eastward motion consistent with gravity sliding known to occur in the adjacent Mexican Ridges fold belt. (5) Southeastward motion of La Paz relative to the Pacific plate is consistent with the hypothesis that the Baja Peninsula is not fully attached to the Pacific plate. (6) Residual velocities for 160 North American plate GPS stations outside of Mexico exhibit no coherent regional patterns indicative of internal plate deformation.

Application of hyperbolic stress-strain models for sandstone and shale to fold wavelength in the Mexican Ridges foldbelt

The Mexican Ridges foldbelt on the continental slope east of Mexico has major folds with wavelengths averaging 7 km. Locally, there are also small amplitude, near-surface folds with wavelengths of 2 km. Initial dominant wavelength is modelled by the finite element method using the hyperbolic stress-strain relationship originally derived for soils. Hyperbolic stress-strain models for sandstone and shale are calibrated using field porosity-depth relationships for shale, and laboratory one-dimensional consolidation data for sand and clay. Effective tangent Young's modulus calculated from hyperbolic stress-strain models is generally higher for sandstones than for shales, and for both rock types modulus increases substantially with depth. The finite element models of dominant wavelength duplicate the two orders of folds observed in the Mexican Ridges and match their wavelengths. In the models, fold wavelength is nearly constant for large variations in both sandstone fraction in the folded section and pore pressure magnitude in the overpressured basal shale below the folded section, but fold amplification is increased by high pore pressure in the basal shale and by high sandstone fraction in the folded section. Fold wavelength is affected more by depth distribution of high pore pressure than by pressure magnitude, and on the lower continental slope where grain size may be smaller, a thicker zone of high pressure would partly explain the modest downslope increase in fold wavelength.

Structural Geology and Evolution of the Mississippi Fan Fold Belt, Deep Gulf of Mexico (1)

The Mississippi Fan fold belt, located in the deep Gulf of Mexico beneath the upper and middle Mississippi Fan, is characterized mainly by basinward-verging anticlines and associated thrust faults. The fold belt extends approximately 300 km eastward and is approximately 50 km wide. Based on correlations with deep Gulf of Mexico seismic sequences (Challenger, Campeche, Lower Mexican Ridges, Middle Mexican Ridges, and Upper Mexican Ridges), the folded strata are interpreted to be Upper Jurassic through Miocene. Salt tongues or sheets in the lower slope have overridden and partly masked parts of the fold belt. The approximate minimum age of the folded strata suggests that the major deformation took place mainly during the middle to late Miocene. South of the fold belt, strata downlap onto a major middle Miocene surface (15.5 Ma) at the top of the Middle Mexican Ridges sequence in the deep basin. This downlap represents sediment derived locally from the growing anticlines, and marks the beginning of growth of the fold belt. Within the fold belt, the coeval reflector represents the oldest identifiable onlap surface for turbidites deposited in the developing synclinal basins. Onlap of younger (uppermost Miocene-lower Pleistocene) Mississippi Fan strata onto the anticlines indicates that the fold belt formed major topographic barriers after its formation and influenced the distribution of early fan sediment. The structurally highest folds have experienced at least several hundred meters of truncation by deep-marine erosion. In the northern and eastern part of the fold belt, the entire Challenger sequence (Upper Jurassic-Lower Cretaceous) has been deformed with 2000-3000 m of structural relief. This uplift involves a structural thickening of stratigraphic section in the core of the anticlines, suggesting that the structures are cored with mobilized salt. Some of the younger fan sequences locally decrease in thickness across the folds, indicating recurrent movement and growth of the salt in the core of the folds throughout the Pleistocene. In some places, salt has remobilized and completely penetrated the overlying fan sediments to the surface, thus forming diapirs. Basement rocks were not involved in the deformation. Thrust faults beneath the folds sole out into the lower part of the Mesozoic section (pro ably into Middle Jurassic salt). The folds and thrust faults are most likely the result of compressive stresses caused by updip sediment loading during the development of Miocene shallow-water depocenters to the north. These stresses were transmitted to the salt basin edge along a deep decollement surface or surfaces within, or at the top of, the Jurassic salt section. Thus, the original distribution of salt overlying transitional crust controlled the location of the fold belt, which formed where the salt pinches out and the decollement surface ends.

Sequence Stratigraphy and Facies Variations within Neogene Turbidite Systems, Northern Deep Gulf of Mexico

The Neogene strata of the northern deep Gulf of Mexico are characterized by turbidite systems of different stages of evolution and depositional setting. These Neogene sequences are the basinal, downdip equivalent strata to the petroleum-producing intervals in the continental shelf and slope to the north and northwest. The middle to late Miocene Upper Mexican Ridges sequence (12.5-5.5 Ma) and Cinco de Mayo sequence (5.5-4.5 Ma) are interpreted to consist primarily of unchannelized to partially channelized basinal turbidite systems. Many small cut-and-fill features are present in the sequences; small channel-levee systems may be present only in a few places. The overlying Mississippi Fan (4.5 Ma to present) consists of 17 seismic sequences that have a much larger areal distribution than the two underlying Miocene sequences. Each fan sequence is characterized primarily by sediments deposited in channel, levee, and overbank settings. The differences among these turbidite sequences are caused by both the overall progradation of the continental margin and differences in the magnitude of sea-level fluctuations and volumes of sediment deposited in the deep Gulf of Mexico during the Neogene. The Upper Mexican Ridges and Cinco de Mayo sequences are interpreted to be the first coarse-grained turbidite systems deposited in this areamore » of the deep Gulf of Mexico, with the overlying Mississippi Fan being part of a continuum in deep-water depositional systems. The oldest sequences in the Mississippi Fan were relatively small and approximately the same size as the Upper Mexican Ridges and Cinco de Mayo sequences. The magnitude of sea-level fluctuations, which controlled the timing of deep-water sedimentation, was not as large during the middle to late Miocene as during the Plio-Pleistocene.« less

Orientational data on the state of stress in northeastern Mexico as inferred from stress‐induced borehole elongations

Preferential elongation directions are presented from 46 subvertical wells in northeastern Mexico from depth intervals ranging from 161 to 4,912 m. The measurements document a remarkably consistent west‐east to northwest‐southeast direction of the contemporary least horizontal stress in the upper crust of this intraplate region. In the regional neotectonic framework, the data suggest that Mexico north of the trans‐Mexican volcanic belt is being stretched in west‐east to northwest‐southeast direction. The measurements from the area of the Laramide Chihuahua and Coahuila fold‐thrust belts define the southern extend of the Basin and Range—Rio Grande rift stress province of the southwestern United States into northern Mexico. The least horizontal in situ stress directions recorded in the Gulf Coastal Plain (Burgos and Tampico‐Misantla Basins) are oblique to the continental margin of the Gulf of Mexico, but subparallel to the direction measured in the area of the Basin and Range—Rio Grande rift stress province. This suggests that the stress distribution in these basins is not mainly the result of gravitational loading as in the Gulf Coast stress province of the United States, but is controlled by the same lithospheric stress system that characterizes the Basin and Range—Rio Grande rift stress province. This is also supported by the north‐south trending zone of Oligocene‐Quaternary alkaline volcanism that crosses the Tampico‐Misantla Basin. Furthermore, the data indicate that the active deformation of the Mexican Ridges fold belt, east of the study area in the Gulf of Mexico cannot be caused by a compressional external tectonic load. The direction of the least horizontal in situ stress measured in the area of the Laramide Sierra Madre Oriental fold‐thrust belt in Hidalgo and San Luis Potosí States is consistent with that of the other zones. However, wellbore elongations were recorded less frequently and show a rather large circular variance, which suggests that the minimum and maximum horizontal far‐field stresses are about equal in this area.

Seismic Stratigraphic Framework of Deep Central Gulf of Mexico Basin

The deep Gulf of Mexico basin is underlain by up to 10 km (33,000 ft) of Jurassic(?) to Holocene layered sedimentary rocks. The multichannel reflection seismic record from the deep Gulf of Mexico was divided into six seismic stratigraphic units for study of the geologic history of this accumulation. The basal Challenger unit (Jurassic(?) to middle Cretaceous) is considered coeval with early basin formation. We interpret it as a deep marine sequence overlying oceanic crust in the central basin and as continental and shallow through deep marine rocks, including thick evaporites, over adjacent transitional crust. The next three units, Campeche, Lower Mexican Ridges and Upper Mexican Ridges, indicate that from the Late Cretaceous through middle Miocene the basin filled progre sively from the west and north, most probably with siliceous turbidites interlayered with pelagic deposits. By the late Tertiary, however, salt and shale deformation within thick sedimentary sections along the western and northern margins trapped much of the incoming sediment supply on the shelves and upper slopes. The late Miocene to Pliocene Cinco de Mayo unit, therefore, represents a relatively starved interval. In contrast, the uppermost, or Sigsbee unit, includes the Mississippi Fan, an accumulation up to 3 km (10,000 ft) thick of mainly mass-transported deposits that bypassed the shelf and slope and were deposited directly onto the abyssal plain. In the western and southwestern portions of the deep basin, beyond the fan pinch-out, the Pleistocene section is largely a continuation o the Pliocene suspension deposits.

Seismic Stratigraphy of Veracruz Tongue, Deep Southwestern Gulf of Mexico

The Veracruz Tongue is a region of continental slope and rise sediments bounded bathymetrically by the Mexican Ridges fold belt to the west and the Campeche Knolls salt province to the east. Study of two multichannel seismic lines and single-fold sparker data enables five post-early Miocene seismic stratigraphic sequences to be distinguished in the tongue. Sedimentary processes responsible for deposition of each sequence are inferred on the basis of external geometry, internal reflection configuration, and, where possible, core data. The five sequences are each interpreted to consist of turbidites grading laterally into hemipelagites. These primary deposits may be modified later by downslope creep or sliding and slumping. Because bathymetry is a major control on location of turbidity-current flows, slides, and slumps, the depositional history of the Veracruz Tongue provides indirect evidence of tectonic evolution of the Mexican Ridges fold belt and the Campeche Knolls salt province. Salt domes first formed a barrier to turbidity-current flow from the east at the close of the middle Miocene. The Mexican Ridges fold belt evolved gradually until it formed a complete barrier to turbidity-current flow from the west at the close of the Pliocene. As a result of shift in direction of turbidity-current flow through time, potentially sand-prone Miocene turbidites are overlain by hemipelagic clays and clayey oozes--a situation favorable to hydrocarbon entrapment.

A Three-Dimensional Seismic Study of Challenger Knoll: ABSTRACT

Several intersecting multichannel seismic lines shot by The University of Texas Marine Science Institute in the deep Gulf of Mexico provide a three-dimensional grid in the vicinity of Challenger Knoll. Seismic stratigraphic sequences identified on the lines include the Sigsbee, Cinco de Mayo, Upper and Lower Mexican Ridges, Campeche, and Challenger units. Isotime End_Page 548------------------------------ maps document a detailed growth history of the Challenger diapir. DSDP drill holes 2, 3, and 87 provide age data for the grid. Differences in the style of diapirism between the northern (Louann) salt province and the Sigsbee salt province to the south are due to the influence of the basement. Both the original distribution of evaporites and subsequent diapirism in the vicinity of Challenger Knoll are controlled by basement fault blocks of transitional crust that formed during the initial opening of the Gulf of Mexico. End_of_Article - Last_Page 549------------

Seismic Stratigraphy of Veracruz Tongue, Southwestern Gulf of Mexico: ABSTRACT

The Veracruz tongue is an area of continental slope and rise sediments bounded topographically by the Mexican Ridges foldbelt and the Campeche-Sigsbee salt province. Study of two multichannel lines and single fold sparker data enables five End_Page 900------------------------------ seismic sequences to be distinguished in the tongue. Correlation of reflectors with well data allows ages, ranging from middle Miocene to Holocene, to be assigned to the seismic sequences. Sedimentary processes responsible for deposition of each sequence are determined on the basis of external geometry, internal reflection configuration, and where possible, core data. Primary deposits consist of proximal and distal turbidites, and hemipelagites and pelagites. Sediments may be later modified by creep or slumping and sliding. This seismic stratigraphic study allows the depositional history of the Veracruz tongue to be elucidated and aids in the dating of folding and salt movement at the edges of the tongue. Detailed interpretation of seismic waveforms indicates that gas accumulations are present in the study area. Future seismic stratigraphic studies will allow quantitative evaluation of hydrocarbon potential of the Veracruz tongue and the adjacent foldbelt. End_of_Article - Last_Page 901------------