Yucatan Platform Research Articles

Sediment accumulation rates and carbonate fluxes of deep-sea sediments in the southern Gulf of Mexico

Here we present the mass and carbonate annual fluxes, collected by two sediment traps at 1000 m depth, located on the western and southern deep-water region of the Gulf of Mexico (GM), and compare them with the total mass and carbonate accumulation rates (CAR) from 46 sediment cores retrieved from continental slopes and the abyssal plain of the southern Gulf of Mexico (sGM). CaCO3 fluxes in the sediment traps located close to 1000 m water depth, range between 15.1 and 22.4 g m−2 yr−1, these values fall on the high-end range of the mean fluxes reported for the global ocean (9–22 g m−2 yr−1). The carbonate fluxes from the water column are remarkably close to the CAR in the lower slope and abyssal sediments of the southern GM, implying that dissolution processes of biogenic carbonate in the sediments below 1000 m are negligible in the sGM. The spatial patterns of CARs in the sediments are mainly linked with surface productivity of calcareous organisms and export to depth over the continental slope and abyssal plain regions, with mean values of 15.7 ± 5.7 g m−2 yr−1 (0.15 ± 0.06 mol m−2 yr−1). However, there are some large areas over the abyssal plain to the north of the Campeche escarpment showing enhanced CAR values most likely as a result from two processes, lateral input from the Yucatan carbonate shelf region and massive collapse events originating at the Campeche escarpment and deposited to the north of the Yucatan Platform. These processes can account for an additional increase in carbonate mass accumulation by approximately 8 g m−2 yr−1 in this area, showing a decreasing gradient towards the N of the escarpment. Lateral transport by turbidity currents of carbonate platform sediments along submarine canyons and probably episodic gravity flows further aid to explain the localized higher carbonate mass accumulation (> 20 g m−2 yr−1) observed at the foot of the continental slopes to the west and south of the southern GM. We estimate total carbonate accumulation estimates for the entire Gulf of Mexico, from the upper slope to the abyssal plain (1000–3750 m), considering the northern deep-water region reported by Giering et al., 2018, of 0.09 to 0.12 Tmol/yr accounts for 0.8 to 1.8% of all carbonate sedimentation in the ocean below 1000 m. Taking into considerations its small areal extent of <0.3% of the global ocean the GM carbonate accumulation accounts for a factor of 3 to 6 its areal importance, implying that changes in the production and export of carbonate in the GM due to surface ocean warming and acidification processes in this inland sea will have a sensible effect in the global carbon cycle.

Oil and gas exploration potential in the exclusive economic zone of the Cuba's sector of the Gulf of Mexico

Based on the latest interpretation results of 2D seismic data, combined with drilling, logging, and geochemical data, the reservoir‐forming conditions of the exclusive economic zone in the Cuba's sector of the Gulf of Mexico (Hereinafter refer to GOM) are studied systematically, and the oil and gas accumulation model in the study area is predicted. The favourable exploration zones are evaluated and optimized. The results show that: ① High quality Upper Jurassic‐Lower Cretaceous and potential Middle‐Lower Jurassic source rocks are developed in the study area. ② A set of Upper Jurassic‐Lower Cretaceous petroleum system and a potential Middle‐Lower Jurassic petroleum system are mainly developed in the study area. ③ At present, the proved petroleum system are found in the Upper Jurassic‐Lower Cretaceous strata. The oil and gas mainly comes from the Upper Jurassic‐Lower Cretaceous source rocks, and the reservoirs are mainly Upper Jurassic‐Lower Cretaceous fractured carbonate. The main seals are inter‐reservoir tight limestone, marl, and Palaeocene mudstone. ④ The thrust structural traps in the footwall of the fold thrust belt on the south side of the Florida platform, the margin zones of the Florida platform and Yucatan platform, and the Middle‐Lower Jurassic structural traps on the south side of the Yucatan platform have superior source‐reservoir‐cap configuration conditions. It is suggested that these areas have good oil and gas exploration potential in GOM.

Red Blood Cell Cytotoxicity Associated to Heavy Metals and Hydrocarbons Exposure in Flounder Fish from Two Regions of the Gulf of Mexico.

In this study, the genotoxic effect of contaminants was assessed through detection of DNA damage using the micronucleus (MNs) test in erythrocytes from 149 flounder fish collected in two regions of the Gulf of Mexico (GoM). The frequency of microcytes (MCs) was also evaluated in the same group of fish collected from the Perdido Foldbelt (PF) and the Yucatan Platform (YP). The MCs frequency was different among locations of the YP (p = 0.011), while MNs frequency varied among locations of PF (p = 0.024). MCs and MNs values correlated with heavy metals from fish muscle, fish species and localities. Mean number, prevalence, and intensity of MCs and MNs correlated with Al, PAHs, depth, and locality. MNs frequency showed a species-specific association (p = 0.004). MNs and MCs were associated with heavy metals and PAHs from fish muscle and sediments, and the MNs frequency was species dependent.

Spatio-temporal patterns of Chlorophyll-a in a wide and low-relief shelf sea of the Gulf of Mexico: Insights of interannual climatic patterns on the phytoplankton biomass varying behavior

Chlorophyll-a concentrations were recorded in three oceanographic surveys (GOMEX-4, GOMEX-5, GOMEX-6) during 2015–2018 throughout the Yucatan shelf, southeast Gulf of Mexico (GoM). Such campaigns encompassed the warm-phase, cold-phase, and neutral-phase of El Niño-Southern Oscillation (ENSO), respectively, as well as a progressively increasing upwelling intensification scenario. We undertook the analysis of the Chl-a concentrations in the Yucatan shelf through the multiyear GOMEX series to understand how the interplay between the coastal upwelling, precipitation regime, and wind patterns influence the distribution, magnitude and timing of the phytoplankton biomass. These results suggest that substantial rainfall late summer during ENSO's warm-phase in GOMEX-4 survey improved the coastward Chl-a (1.69 ± 0.07 mg m−3). This pattern could be associated to the increased cold-front activity usually modulated by the wintertime warm-phase of ENSO. The wind-driven mixing during these cold fronts led to water column mix, which contributed to enhance the distinguishing between coastline and marine Chl-a levels. The coastal upwelling pulses depicted a wide-shelf influence over the Chl-a concentration (0.47 ± 0.01 mg m−3 and 0.53 ± 0.03 mg m−3 average concentrations in the marine and coastal areas, respectively). An onshore excursion of cold water masses intermingled with seasonal precipitation traits sustained high nearshore Chl-a in zones already altered by subaqueous freshwater input in the karstified Yucatan seascape (varying within the interval of 1.50–2.76 mg m−3). These results suggest that, similarly to riverine-influenced areas in the northern GoM, warm-phase ENSO episodes have the potential to alter the phytoplankton biomass also in karstified shelves, as the Yucatan platform. This large-scale pattern overlaps processes occurring over a broad spatial and temporal scale, like seasonal rainfall and upwelling, establishing recurrent environmental gradients driving biogeochemical fronts along the seascape. The relationships found allow a better grasp of sound ecologically processes as phytoplankton blooms along the Yucatan shelf, which might be also sensitive to human-driven disturbances.

Seismic attribute analysis of Chicxulub impact crater

Chicxulub crater formed ~ 66 Ma ago by an asteroid impact on the Yucatan platform in the southern Gulf of Mexico. The crater has a ~ 200 km rim diameter and has been covered by carbonate sediments up to ~ 1.1 km thick in the central zone. Previous studies have identified the structure and major crater units through geophysical models from seismic reflection and potential field data, classified as the central uplift, terrace zone, outer and inner ring fault zones and impactite deposits. Impact produced a deep excavation cavity, with fragmentation and ejection of large volumes of crustal target rocks. Understanding the pre-existing structures, impact-induced deformation and post-impact processes requires high-resolution images of the crater and target zone. For this study, we use complex trace attributes of instantaneous phase, frequency, envelope amplitude and similarity, in an E-W seismic reflection profile crossing the crater in the marine sector. Geophysical logs and borehole lithological columns from the on-land drilling projects are used to constrain the petrophysical analysis. Seismic attributes aid to characterize the radial fault zones and physical property contrasts, revealing asymmetries in the crater structure. The reflector packages in the post-impact sediments and target Cretaceous sequence are identified in the frequency and phase attributes. The bottom crater reflectors, with the basal sediments filling the crater floor topography, are enhanced with the envelope amplitude attribute. A set of high-amplitude reflectors is shown in the similarity attribute, in which the reflector geometry is delineated on the target carbonate sequence. The offsets in the high-amplitude reflectors between the eastern and western sectors are possibly associated to target pre-impact asymmetries, impact deformation and effects of central crater collapse.

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

The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatan and Campeche subbasins, separated by a base-salt high near the nose of the Yucatan platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition. Portions of the Yucatan 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 Yucatan 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, Yucatan, and Florida margins. The Campeche subbasin thus appears to have been perched relative to other parts of the Gulf of Mexico.

Last Glacial Maximum deep water masses in southwestern Gulf of Mexico: Clues from benthic foraminifera

The repercussions of climate change during the Last Glacial Maximun (LGM) associated with changes in the structure and chemistry of abyssal waters in the southwestern Gulf of Mexico were studied by numerical analysis of benthic foraminiferal data from three sediment cores (water depth 1988-2735 m). In this context, to better understand the distribution patterns of modern benthic foraminifera in bathyal and abyssal waters of the area, species data from twelve core-tops (water depth 960-3255 m) were also analyzed. Multivariate Q-mode factor analysis separated the modern foraminiferal faunas of the North Atlantic Deep Water (NADW, deeper than ~2000 m), dominated by Nuttallides decorata, Alabaminella turgida, Ioanella tumidula, and Globocassidulina subglobosa, from those of the shallower Subantarctic Intermediate Water (AAIW) and Caribbean Midwater (CMW), characterized by the association of Bolivina lowmani, Bulimina aculeata, Alabaminella turgida, Globocassidulina subglobosa, Epistominella exigua, and E. vitrea. Although most of the species are found in the entire stratigraphic interval studied, detailed examination shows differences between the dominant LGM and Holocene assemblages, which can be traced to climate-related modifications in world ocean circulation in the LGM, particularly those of NADW. During LGM, Alabaminella turgida (a species sensitive to oxygen depletion) along with a group of dissolution-prone species such as Biloculinella irregularis and Cornuloculina inconstans, thrived in the deepest part of the study area, indicating well-oxygenated waters of glacial NADW (GNADW). As the production of GNADW ceased in the Holocene, the deep areas in the Gulf were occupied by the present NADW that contains less oxygen and more CO2 . This water mass is more corrosive and precluded the presence of dissolution-prone species during the LGM, except in the most calcium carbonate- and oxygen-rich areas next to the Yucatan platform. Conditions in this area allowed the proliferation of Nuttallides decorata during the Holocene, while the N. decorata and Alabaminella turgida populations decreased in the western Gulf.

Impact ejecta and carbonate sequence in the eastern sector of the Chicxulub crater

The Chicxulub 200 km diameter crater located in the Yucatan platform of the Gulf of Mexico formed 65 Myr ago and has since been covered by Tertiary post-impact carbonates. The sediment cover and absence of significant volcanic and tectonic activity in the carbonate platform have protected the crater from erosion and deformation, making Chicxulub the only large multi-ring crater in which ejecta is well preserved. Ejecta deposits have been studied by drilling/coring in the southern crater sector and at outcrops in Belize, Quintana Roo and Campeche; little information is available from other sectors. Here, we report on the drilling/coring of a section of ∼34 m of carbonate breccias at 250 m depth in the Valladolid area (120 km away from crater center), which are interpreted as Chicxulub proximal ejecta deposits. The Valladolid breccias correlate with the carbonate breccias cored in the Peto and Tekax boreholes to the south and at similar radial distance. This constitutes the first report of breccias in the eastern sector close to the crater rim. Thickness of the Valladolid breccias is less than that at the other sites, which may indicate erosion of the ejecta deposits before reestablishment of carbonate deposition. The region east of the crater rim appears different from regions to the south and west, characterized by high density and scattered distribution of sinkholes.

ABSTRACT: Best Technology Practices To Reactivate Macuspana Gas Province, Southern Mexico

Abstract Mexico's gas needs for the beginning of the next century are well acknowledged. Major gas demand will come from the oil industry, the growing number of gas-driven electricity plants, transportation, and other industrial and residential consumers. Aware of this, Pemex is implementing a national strategy focused on gas. Macuspana Basin with its offshore extension is located in the states of Tabasco and Campeche in southern Mexico. It is an elongated Neogene basin formed between the Yucatan Platform to the east and the Reforma Akal Folded Belt to the west. This basin contains a thick sequence of fluvio-deltaic and open marine Neogene sediments cut by numerous growth faults associated with rollover anticlines and amorphous shale bodies. The basin is a prolific gas province, in a mature stage of exploration and production. During its 90 years of exploratory history, over 700 exploratory wells have been drilled and 36 fields discovered; so far only 11 of them are commercially exploited. Among other important issues, technology practices will play an important role in facing the challenges of reactivating this basin. Following, a brief description of technology practices in Macuspana Basin is presented. To properly plan and execute seismic activities it is very important to have a clear picture of the ground surface. The use of seismic orthomaps to honor natural and cultural obstacles during the design and execution of seismic activities is now a common Pemex practice. In addition to orthomaps, in some regions aeromagnetic surveys are needed to accurately identify old but active buried pipelines. Based on geologic targets, access, cost, and time needed for operation, a center-shot cross swath was chosen for seismic acquisition because it increases the range attributes (fold, offset, and azimuth distribution) and affords operational simplicity in the field. The use of AVO as a direct hydrocarbon indicator in gas-filled clastic rocks is widespread along the Gulf Coast. A methodology for high-quality prospect generation and reduced turnarounds was also tested. It consisted of a series of steps that included: synthetic seismograms, seismic attributes extraction, correlation between petrophysical properties and seismic attributes, depth conversions, and intensive training in workstation applications. The fulfillment of exploration and production targets in the Macuspana Basin over the next five years will demand intensive operations that must rely on high-performance leadership and teamwork practices, as well as careful selection, adoption, and application of best technology practices that transform Pemex investments into profitable ventures. End_of_Record - Last_Page 8---------

Cretaceous stratigraphy and palaeoenvironments of the Southern Petén Basin, Guatemala

The Cretaceous deposits of the Southern Petén Basin, an oil-producing province, are located to the south of the Yucatan Platform and to the east of the Chiapas Carbonate Platform of Mexico. The succession in the southern part of this basin has been studied both in wells and at outcrop by microfacies analysis. It is composed of 5000m of shallow marine carbonates and evaporites with a few thin layers of pelagic limestones rich in organic carbon and planktic foraminifera deposited during peak transgressions or maximum flooding. The sedimentation of this thick succession was not continuous, and the section is punctuated by subaerial erosional bounding surfaces and a single hardground which marks the final drowning of the carbonate platform during the late Maastrichtian. New age determinations have been obtained on the basis of planktic and benthic foraminifera from the Aptian to Santonian Cobán Formation, D, C, B and A Members. Cobán C (Albian) and B (Cenomanian) Members are the present oil reservoirs. The rudist and alveolinid limestones of late Campanian and Maastrichtian age referred to the Campur Formation in the Southern Petén Basin are here reassigned to the Angostura Formation, as recognized in the Chiapas area. The Actela Formation is defined here to encompass the limestone breccia deposits that occur at the Cretaceous–Tertiary boundary and extend into the Parvularugoglobigerina eugubina Zone of early Danian age. The D, C, B and A Members of the Cobán Formation and the Angostura Formation represent second order transgressive and regressive trends in a passive margin area where the deposits indicate a succession of various environments, including fluviatile, salinas, shallow marine carbonate platform, outer shelf and intrashelf basin. High subsidence rates, sea-level changes and tectonic uplift controlled the sedimentation.

Exploring the KT source crater: Progress and future prospects

It has been 15 years since an iridium‐enriched clay layer at the Cretaceous‐Tertiary (KT) boundary was discovered, providing the first hard evidence linking the most recent mass extinction event to a comet or asteroid strike [Alvarez et al., 1980]. Now it is widely accepted that the site of this collision is on the Yucatan platform, centered near Progreso, Mexico. The 200–300‐km‐wide crater lies buried beneath 300–1000 m of limestone laid down in the intervening 65 million years, and few clues of its presence remain at the surface, save an arcuate arrangement of water‐filled sinkholes centered approximately on the structure (Figure 1). Yet prominent circular anomalies in gravity and magnetic anomaly maps gained the interest of Petroleos Mexicanos (Pemex), and in the early 1950s they began an exploration campaign that included deep drilling to recover samples of the subsurface rocks. The buried feature became known as the Chicxulub structure. Pemex drilling continued throughout the early 1970s and by that time, three wells near the center had recovered silicate rocks with igneous textures, initially mistaken for volcanic rocks. Other wells, located between 130 km and 210 km from ground zero recovered breccia deposits hundreds of meters thick that showed evidence of catastropic or explosive conditions. By 1980, Antonio Camargo, a geophysicist at Pemex, felt the evidence pointed to impact, although a volcanic origin for the Chicxulub structure could not be ruled out.

La plate-forme du Yucatan (Mexique) : un exemple de karst tropical péninsulaire

The Yucatan platform (Mexico ) : an example of a peninsula tropical karst. The Platform of the Yucatân peninsula, south of Mexico, is constituted by a tertiary carbonated series (Eocene to Pliocene ). The karst landforms are a " Kuppenkarst " whose positive reliefs are more conspicuous in the center because of greater altitude (0 to 400 m). Most caves are developed under the base level. The cenote corresponds to drowned pits which can be 100 m deep or more. The flooded caves, like Nohoch Nah Chich (40 km long), are the longest in the world. The history of karstification began during the Tertiary, between the end of Eocene to Pliocene in relation with uplift. The crypto-dissolution occurs under an alteritic cover which comes from a former silicated detritic cover (south peninsula crystalline massif). Speleogenesis depends on the halocline i.e. mixing corrosion zone (salt water /fresh water) and the fluctuations of the sea level.

CUBAN GEOLOGY: A NEW PLATE‐TECTONIC SYNTHESIS

Cuba is considered here to consist of two separate geological units: afoldbelt and a neoautochthon.The foldbelt can be subdivided into:(i) continental units, comprising Mesozoic Bahamian Platform and slope deposits, which are overlain by a Paleocene‐ Late Eocene foreland basin; and the Cuban SW terranes (Guaniguanico, Pinos and Escambray), which were probably originally attached to the Yucatan Platform;(ii) oceanic units, namely: the northern ophiolite belt; the Cretaceous (?Aptian‐Campanian) volcanic arc, which is overlain by a series of Latest Cretaceous — Late Eocene “piggy‐back” basins; and the Paleocene‐Middle Eocene volcanic arc which is overlain by a late‐Middle — latest Eocene “piggy‐back” basin.The neo‐autochthon is composed of slightly‐deformed, latest Eocene to Recent sedimentary rocks, which unconformably overlie the folded belt.A large number of tectonic models for the Caribbean area have been published in recent years, but rarely include modern data on the geology of Cuba. The Author here presents a plate‐tectonic model for the western Caribbean which is based on the following premises: (i) opening of the Caribbean took place along several parallel rifts‐zones, and a main transform fault located between the entrance of the Gulf of Mexico and the Demarara Plateau; (ii) the Cretaceous Greater Antilles volcanic arc faced the ProtoCaribbean Sea, and essentially northward‐dipping subduction took place; and (Hi) the western Caribbean Paleocene‐Middle Eocene volcanic arc also faced the Caribbean Sea, with subduction dipping towards the NNW.Hydrocarbon production in Cuba comes from oilfields located in both continental and oceanic units. The Northern Oil Province coincides with the Bahamian platform and slope deposits and the Guaniguanico Terrain. The Southern Oil Province is represented by the latest Cretaceous — late Eocene sedimentary basins and the Cretaceous volcanic arc.

Structure and tectonics of the Yucatan Basin, caribbean Sea, as determined from seismic reflection studies

The Yucatan Basin preserves a record of the Late Cretaceous to Paleogene Caribbean‐North American convergent history that is largely unaffected by Neogene strike‐slip tectonics of the current plate boundary. An examination of seismic basement within the Yucatan Basin, based upon available seismic reflection data including extensive multichannel data, shows that the basement comprises nine domains distinguished on the basis of internal reflection character and surface topography. These domains encompass three distinct crustal types or blocks. The first underlies the western flank of the basin and represents the offshore continuation of the adjacent Yucatan platform. The second includes the topographically heterogeneous domains of the eastern two‐thirds of the basin, and is dominated by a subsided volcanic rise or arc (Cayman rise) resting upon probable oceanic crust of pre‐Tertiary age. The eastern edge of the rise and adjacent basins dips northeast beneath the Cuban margin along a sediment filled trench. The third type of crust occupies a rectangular deep within the western third of the basin. Available evidence indicates that this crust is oceanic in character, and represents a large, mature pull‐apart basin set within a wide paleo‐transform zone between the western platform and eastern oceanic basin. This zone defines the northwestern portion of the Caribbean‐North American convergent plate boundary. Paleocene to Middle Eocene transform motion was left‐lateral along north‐south to NNE‐SSW trends, with a displacement of about 350 km. A long Middle Eocene transcurrent fault of about 50 km left‐lateral displacement cuts the basin diagonally from SW to NE and continues onland in Cuba as La Trocha fault. This reconstruction is consistent with known Eocene regional tectonics, but the timing of regional events raises questions about present interpretations of plate geometry in the northwestern Caribbean.

Late Triassic-Jurassic Paleogeography and Origin of Gulf of Mexico Basin: REPLY

The basic structural and stratigraphic framework of the Gulf of Mexico Basin was established during the Late Triassic and the Jurassic. During the Late Triassic and Early Jurassic, as the North American plate started to separate from the South American and African plates, the area of the future basin was part of an extensive landmass broken by tensional grabens that were filled by red beds and volcanics. Marine deposition was restricted to embayments of the Pacific Ocean in northwestern and central Mexico. These marine embayments persisted during the early Middle Jurassic, but seawater did not reach the future Gulf of Mexico Basin until the Callovian. Widespread salt deposits known today from two separate areas of the basin resulted from this initial flooding. During the ate Jurassic, marine conditions progressively extended over increasingly larger parts of the Gulf of Mexico Basin. However, the basin was not connected to the Atlantic Ocean until late in the Jurassic. This paleogeographic reconstruction suggests that the Gulf of Mexico Basin formed as a result of the southward drift of the Yucatan continental block away from the remainder of the North American plate. The separation began in the Late Triassic, continued slowly and sporadically during the Early and Middle Jurassic, and quickened after the Middle Jurassic salt formed. As a result, the salt deposits were split into the two segments known today, and oceanic crust formed in the center of the basin. Early in the Late Jurassic, the Yucatan platform reached its present position and the Gulf of M xico Basin was born.

Late Triassic-Jurassic Paleogeography and Origin of Gulf of Mexico Basin

The basic structural and stratigraphic framework of the Gulf of Mexico Basin was established during the Late Triassic and the Jurassic. During the Late Triassic and Early Jurassic, as the North American plate started to separate from the South American and African plates, the area of the future basin was part of an extensive landmass broken by tensional grabens that were filled by red beds and volcanics. Marine deposition was restricted to embayments of the Pacific Ocean in northwestern and central Mexico. These marine embayments persisted during the early Middle Jurassic, but seawater did not reach the future Gulf of Mexico Basin until the Callovian. Widespread salt deposits known today from two separate areas of the basin resulted from this initial flooding. During the ate Jurassic, marine conditions progressively extended over increasingly larger parts of the Gulf of Mexico Basin. However, the basin was not connected to the Atlantic Ocean until late in the Jurassic. This paleogeographic reconstruction suggests that the Gulf of Mexico Basin formed as a result of the southward drift of the Yucatan continental block away from the remainder of the North American plate. The separation began in the Late Triassic, continued slowly and sporadically during the Early and Middle Jurassic, and quickened after the Middle Jurassic salt formed. As a result, the salt deposits were split into the two segments known today, and oceanic crust formed in the center of the basin. Early in the Late Jurassic, the Yucatan platform reached its present position and the Gulf of M xico Basin was born.

Evolution and Seismic Expression of Mesozoic and Cenozoic Shelf Margins, Gulf of Mexico and Vicinity: ABSTRACT

Mapping and classification of modern and ancient shelf margins provide a basis for a concise post-rifting history of the Gulf of Mexico basin. Many hydrocarbon occurrences in the basin can be related to styles of shelf margins and their associated slope with implications for frontier exploration in deep-water facies. Seven basic types of shelf margins have been recognized. 1. Reef-dominated carbonate margins surrounded the deep basin during the Early and middle Cretaceous. This category can be subdivided into: (a) low-relief, short-lived (ca. 10 m.y.) margins in Louisiana and Texas (Stuart City and Sligo), and (b) high-relief (1 to 3 km; 3,280 to 9,843 ft), long-lived (ca. 40 m.y.) margins elsewhere (Florida, Campeche, and Blake escarpment, Golden Lane, El Abra), which are typically associated with fore-reef talus. 2. Sigmoidal progradational carbonate margins developed landward of drowned mid-Cretaceous margins of the Florida and Yucatan platforms during the Cenozoic. Large-scale gravity slides with rollover structures have occurred along the Yucatan slope contemporaneously with deposition of sigmoidal carbonate margins. 3. Carbonate ramps with little or no seismic expression characterized the Late Jurassic, when a deep marine basin was first established, an the Late Cretaceous, following drowning of mid-Cretaceous carbonate platforms. 4. Stable progradational clastic margins with well-developed, undeformed, large-scale clinoform stratification are relatively rare in the Gulf basin. Modern examples are limited to offshore Alabama and Veracruz. 5. Unstable progradational clastic margins result from gravity sliding of the continental slope, commonly associated with salt and shale diapirism, which obscures large-scale clinoform stratification. Growth faults with major expansion and rollover characterize the shelf margin; folds and/or thrust faults characterize the lower slope. This type of shelf margin has dominated the northwestern Gulf during the Cenozoic, and now extends from offshore Mississippi to offshore Veracruz. Numerous episodes of deltaic progradation to the shelf margin have been identified in Texas and Louisiana; these clastic influxes have prograded the shelf edge as much as 350 km (215 mi). 6. Tectonically active progradational clastic margins are present in Tabasco and eastern Veracruz. There the shelf margin has prograded up to 200 km (125 mi) since the early Miocene in an area of active compression and sinistral wrench faulting. Interaction of basement tectonics with diapirs and probably gravity sliding has created great structural complexity in thick Neogene deltaic sequences. 7. Tectonically active sediment-bypassing margins characterize the early Tertiary. The continental margins of Veracruz and Cuba underwent major compressional deformation and did not develop broad constructional shelves; instead, most sediment was probably bypassed to the deep basin. At the same time, a deep-water foreland basin, probably with a similar type of margin, extended into Chiapas and central Guatemala. End_of_Article - Last_Page 570------------

Seismic Stratigraphy and Geologic History of Campeche Escarpment, Gulf of Mexico: ABSTRACT

Interpretation of multifold seismic data from the Campeche Escarpment northwest of the Yucatan Peninsula shows geologic features that suggest the sequence of events in the evolution of the Gulf of Mexico. Profiles of the surface resemble topography typical of subaerial erosion, that is, a pediment surface. An inferred Jurassic salt layer that covers sediments in a downthrown basement block pinches out against the pediment surface. Updip of the salt pinchout and topographically higher are possible Late Jurassic carbonate banks or reefs which onlap the basement erosional surface. A major unconformity separating the carbonate seismic unit from the overlying slope-front-fill unit probably corresponds to a worldwide middle Cretaceous unconformity. Since Late Cretace us, deep-water turbidites emanating from the east coast of Mexico and northern Gulf Coast have dominated the study area. The scenario revealed at Campeche Escarpment suggests that an early Mesozoic mantle event (plume?) uplifted this once continental area and caused thinning of the crust to near oceanic thickness. Whether and how much of the Gulf of Mexico is underlain by thinned Paleozoic or Mesozoic continental crust, or true oceanic End_Page 487------------------------------ crust, is still uncertain. The central gulf and interior basins subsided after this period of crustal thinning. It was in these basins that the Sigsbee and Louann Salt were deposited. Later, as the basin margins subsided farther, carbonate sediments onlapped and covered these margins, marking 5 km of subsidence by the Cretaceous. Since the Cretaceous, clastics have infilled the northwestern Gulf, causing an additional 3 km of subsidence of the gulf basin, while the central Florida and Yucatan platforms have stabilized above sea level. End_of_Article - Last_Page 488------------

Emplacement of Sierra de Santa Cruz, Eastern Guatemala: GEOLOGIC NOTES

Wilson suggested that the Sierra de Santa Cruz moved to its present position by gravity sliding. The writer agrees and some years ago developed a hypothesis to explain how this mass of ultramafic rock might have been uplifted to a position from which it subsequently could slide northward. The Sierra is about 55 km long by 13 km wide, rises to an elevation of about 1,000 m, and is composed mainly of peridotite, gabbro, and dunite. Airborne magnetometer surveys have indicated that beneath the east end the depth to crystalline basement is 4,000-5,000 m. It is postulated that the uplift of the Sierra de las Minas, from which the Sierra de Santa Cruz is presumed to have moved by gravity sliding, may be analogous to the Troodos massif on the island of Cyprus; i.e., the arcuate band of ultramafic rocks across Guatemala was underthrust by a southward movement of the Yucatan platform and the resulting isostatic uplift raised a mass of peridotite to a height from which it could slide northward to form the Sierra de Santa Cruz.

Discussion of the Occurrence and Origin of Sedimentary Palygorskite-Sepiolite Deposits

AbstractMarine and non-marine palygorskite-sepiolite deposits are found throughout the world and occur interbedded with chert, dolomite, limestone, phosphates and other non-detrital sedimentary rocks. The origin of these high-magnesium clays has long been attributed to either alteration of volcanic ash or the structural transformation of smectite clays. More recently, others have argued origin by direct crystallization (neo-formation). Recent laboratory studies support this latter concept, particularly in environments where the concentration of alumina is low, the silica concentration high, and the pH alkaline. Such an origin is proposed for the Georgia-Florida deposits in southeastern United States, since major obstacles exist against formation by alteration of volcanic ash or by transformation of smectites. Lateritic weathering during the Miocene would have favored direct precipitation of these clays in the shallow, marginal seas. The basinward increase in the MgO: Al2O3 ratio is further support.Deep weathering of crystalline rocks in northern British Honduras and Guatemala would have produced similar high silica, low alumina conditions on the adjacent submerged Yucatan Platform during the late Tertiary. The seaward increase in the MgO: A12O3 ratio, the lack of associated detrital constituents, and the absence of associated smectites strongly indicate a similar origin by direct crystallization of these Yucatan palygorskite-sepiolite clays.Some occurrences of palygorskite and sepiolite may well be related to the alteration of smectite clays or volcanic ash. However, many of the large sedimentary deposits are more probably the result of direct crystallization adjacent to areas undergoing tropical or subtropical weathering.