Abstract The deposition of organic carbon–rich sediments during the Late Cretaceous in northern South America was controlled by global and local oceanographic, climatic, and tectonic variables. Key in establishing “source rock” depositional systems across the region were eustatic sea-level rise, warming global sea-surface temperatures, the formation of low-latitude saline bottom waters, and a relatively constant supply of fine-grained hemipelagic sediment (mostly derived from the south and east). Specific paleobathymetric conditions enhanced the development of stagnant water masses from the proto-Caribbean plate to Suriname. Organic-matter preservation was aided by the presence of these water masses across the region. Primary productivity was elevated above “normal” marine levels only in the protocentral Caribbean and along the ancestral Costa Rica/Panama island arc, or during seasonal upwelling in northern South America. Cooler, wetter climatic conditions that began in the late Santonian also were modified by regional and local variables. The development of new intermediate/bottom-water masses, increased polar heat transport caused by improved deep-ocean circulation, and fluctuations in volcanogenic CO 2 provided a background effect for local variables such as bathymetry and topography. The development of oxygenated high-latitude water masses provided a means for ventilation of stagnant, low-oxygen bottom waters across northern South America and the central Caribbean. Stronger seasonal upwelling (increased wind stress caused by better polar heat transport, and northward movement of the South American Plate into the zone of northward Ekman transport), more frequent fluvial outflow and deltaic deposition, and the submergence of key paleobathymetric barriers aided ventilation and subsequently diminished organic-matter preservation. Most of these variables had a positive impact on primary productivity and caused rapid changes in the diversity of planktonic foraminifera through the end of the Cretaceous.

Abstract Tectonostratigraphic data derived from ongoing biostratigraphic, chronostratigraphic, paleobathymetric, paleobiogeographic, and lithostratigraphic investigations in west-central and east-central Mexico suggest that the Gulf of Mexico formed in two phases: Phase 1: Rifting and subsequent sea-floor spreading during the Late Jurassic (middle Oxfordian). All but the southwestern portion of the Gulf of Mexico formed during Phase 1. Phase 2: Northwest-to-southeast tectonic transport of allochthonous San Pedro del Gallo terrane remnants along the west side of Walper Megashear during the Middle Jurassic to Early Cretaceous. Where the stratigraphic successions are complete, megafossil data indicates that the San Pedro del Gallo terrane was situated at Southern Boreal paleolatitudes (>30° N) in the Nevadan back arc domain during the Middle Jurassic (late Bathonian to early Callovian) and was subsequently carried to lower paleolatitudes during the Late Jurassic and Early Cretaceous. For example, in the Huayacocotla remnant, the Boreal ammonite Kepplerites was recovered in the subsurface from the Palo Blanco Formation by Cantú-Chapa. In North America, Kepplerites is known from the Izee terrane (east-central Oregon), Western Interior (Montana and Saskatchewan), and northward to southern Alaska. Radiolarian, calpionellid, ammonite, and bivalve faunal data indicate that the Huayacocotla remnant had been transported to Northern Tethyan paleolatitudes (23° N to 29° N) during the Kimmeridgian and Tithonian and to Central Tethyan paleolatitudes (<23° N) by the beginning of the Early Cretaceous.

Abstract This paper presents a set of paleogeographic maps that illustrate the formation and evolution of the Caribbean from latest Triassic to latest Jurassic. Stratigraphic data and plate-tectonic models indicate that the Caribbean first evolved as a system of latest Triassic–Middle Jurassic rift valleys in west-central Pangea. Probably since the Bajocian, but certainly since the Oxfordian, it became a marine seaway connecting western Tethys with the eastern Pacific. In contrast, abundant paleontological data strongly suggest that the seaway across west-central Pangea opened during the Early Jurassic (Hettangian-Pliensbachian), which data conflict with the stratigraphic data. This contradiction between paleontology (biogeographic interpretations) and stratigraphy (paleogeographic interpretation) reveals our insufficient knowledge about the Mesozoic geology of west-central Pangea. This paper is a contribution to IUGS/UNESCO IGCP Project 433.

Abstract Deeply incised and backfilled paleocanyons in early Paleogene shelf strata along the western and northern Gulf of Mexico margin attest to large relative sea-level fluctuations, but they predate the accepted age for onset of Cenozoic continental glaciation. Using Pleistocene canyons as a crude yardstick, the scale of these paleocanyons suggests relative sea-level changes at least as large as Pleistocene fluctuations. Therefore, we speculate that water level in the Gulf of Mexico was drawn down while the Gulf was isolated from the world’s oceans during the late Paleocene/early Eocene interval. We suggest that the cause for isolation was the progressive collision of the Cuban arc with the Yucatan and Bahamas carbonate platforms, which temporarily closed off the southeastern Gulf of Mexico. In Miocene Mediterranean and Holocene Black Sea examples of marine-basin isolation, evaporation greatly exceeded rainfall and runoff, and our examination of the Gulf of Mexico case suggests that water level may have dropped below the level of the world’s oceans at least once by several hundred meters, and possibly much more. Implications for geology and hydrocarbon exploration in the Gulf may include: • bypass of enormous quantities of coarse detritus into the deep basin • seaward collapse of exposed clastic shelf margins • triggering and/or acceleration of salt evacuation (basinward “squeegee” effect of slumping sediments) • release of gas hydrates from sediments under shallower and warmer water, thereby contributing to the ˜100,000-year-long worldwide Paleocene/Eocene boundary heating event • development of secondary porosity in both platform and deep-water carbonate sections by dissolution and phreatic diagenesis, e.g., in the Golden Lane/Poza Rica area of Mexico • hypersalinity and possible sea-bottom stagnation with source-rock deposition in areas that remained marine • deposition of fine-grained condensed sections (seal and source rock) during flooding period(s) when connection with the world’s oceans was reestablished, creating stratigraphic traps at canyon flanks and turbidite reservoirs in the canyons. Recognition that early Paleogene relative sea-level changes seen in the Gulf may pertain to basin isolation is grounds for treating “eustatic” curves derived for or from the Gulf with suspicion. In addition, catastrophic basinward transfer and collapse of mass near the shelf edges would have caused isostatic unloading (rebound) of shelf margins that was proportional to the mass transfer. In the case of a discreet slumping event, such as the Lavaca “Megaslump” event of south Texas, this effect may have caused uplift of several to a few tens of meters of footwall areas within about 100 km from the slump. Larger downslope movements such as those related to the collective Wilcox fault province would have caused far larger isostatic rebounds on the shelf, perhaps in excess of 100 m if sedimentation did not keep pace with faulting. A body of circumstantial evidence continues to grow in support of this hypothesis; its potential implications, both academic and commercial, merit further investigation. Integration of information from Cuba, Mexico, the United States, and the Bahamas will be required to fully test the hypothesis.

Abstract The southern margin of the Caribbean plate, cropping out in the Venezuela belt, consists of an assemblage of four main terranes: the Dutch-Venezuelan Islands, Margarita Island, Cordillera de la Costa, and Serrania del Interior. These terranes have been located, since the middle Cretaceous, along the transform boundary between the Caribbean and South American plates. On the basis of both new data and the literature, a critical review of the complex and long-lived evolution recorded in different units of these terranes is herein provided in order to highlight the Mesozoic–early Tertiary geodynamic evolution of the southern Caribbean. The analysis of the lithostratigraphic, petrologic, and tectono-metamorphic features of the terranes, as well as their regional correlations, allows us to define the main geotectonic elements (as oceanic basins, magmatic arcs, subduction zones, continental margins, continental microplates, etc.) involved in the evolution of the southern Caribbean margin. The magmatic, tectonic, and metamorphic histories of these elements provide valuable constrains for the evolution of the southern Caribbean, as, for instance, the beginning of the convergence during the Early Cretaceous, the atypical evolution of the suprasubduction system during the middle Cretaceous, the role of the middle Cretaceous strike-slip tectonics, the exhumation histories of the high-pressure/low-temperature (HP-LT) units. The collected data suggests a Middle Jurassic–Early Cretaceous location of these elements in a westernmost, “near mid-America” position, almost at the northwestern corner of the South American plate. Starting from the middle Cretaceous, the elements have been affected by a right-oblique convergence along the transform boundary connecting the two oppositely dipping subduction zones of the Andes and Aves–Lesser Antilles. According to the geologic constraints, three possible geodynamic scenarios can be proposed for the beginning of the convergence during the middle Cretaceous, taking into account the different locations of the transform fault in the geodynamic setting of the southern Caribbean. The collisional belt, resulting from the middle Cretaceous tectonics, has been dissected in different terranes, progressively rotated clockwise, reciprocally juxtaposed, and then eastward displaced. The geodynamic framework was closely related to the progressive eastward motion of the Caribbean plateau which, in turn, was associated with the development of a west-southwest-dipping, intraoceanic subduction of the proto-Caribbean oceanic crust below the plateau, and related island-arc calc-alkaline magmatism, today preserved in the Dutch–Venezuelan Islands and Aves–Lesser Antilles. At that time, the terranes were already emplaced onto the South America continental margin. Northward, the dextral strike-slip tectonics of the Caribbean southern margin increasingly involved the southern part of the magmatic arc, which gradually became inactive and underwent a progressive rotation clockwise. In contrast, the Aves–Lesser Antilles were gradually bent eastward by the oblique convergence occurring at the southern end of the magmatic arc. Since the late Paleocene, the whole marginal belt was already completely identifiable with the large shear zone occurring today at the transform boundary between the Caribbean and South American plates.

Abstract Oceanic plateaus are areas of elevated and anomalously thick oceanic crust that are believed to form by enhanced partial melting in a mantle plume that is hotter than ambient upper asthenosphere. They are regarded as the oceanic equivalent of continental flood-basalt provinces. Because of the continual subduction of oceanic crust, the oldest known oceanic plateaus occurring in situ are Cretaceous in age. In order for oceanic plateaus to be preserved in the geologic record, they must be accreted onto continental margins. This process, involving their preservation as tectonic slices, depends on the fact that oceanic plateaus are more buoyant than normal ocean floor; thus, they are not easily subducted. If these plateaus encounter an oceanic arc, subduction polarity reversal may occur, and/or the locus of subduction may step back behind the trailing edge of the advancing plateau. At a continental subduction zone, only subduction back-step occurs. Geochemical evidence shows that basaltic and picritic rocks exposed in the thickened part of the Caribbean plate and around its margins (including northern South America) are parts of an accreted oceanic plateau that originated in the Pacific Ocean during the middle-to-late Cretaceous. Cretaceous subduction-related rocks also occur around the Caribbean margins and possess geochemical signatures (e.g., lower Nb and Ti) that are distinct from those of the oceanic plateau rocks. This arc material represents the remnants of the subduction-generated rocks with which the plateau collided at 80–90 Ma. Both island arc tholeiite and calc-alkaline magmatism occurred in these Cretaceous arcs, but the changeover between the two types appears to be gradual and cannot be used to determine the timing of subduction polarity reversal. Many Cretaceous tonalitic batholiths around the Caribbean margins appear to have formed during or shortly after accretion of the plateau rocks. In addition to the arc and oceanic plateau assemblages, Jurassic to Early Cretaceous fragments of the preexisting oceanic crust also occur around the region. The environmental impact of oceanic plateau volcanism around the Cenomanian-Turonian boundary and its link to the formation of organic-rich black shales is discussed in this paper.

Abstract Some El Abra Formation carbonate reservoirs produce from fractured and brecciated rocks resulting from paleokarstic events. The western margin of the Gulf of Mexico front of the Sierra Madre Oriental exposes Albian-Cenomanian rocks of the El Abra Formation. In the Actopan Platform, these outcrops allow the study of the paleokarst development in peritidal rocks. Included in the karst are dissolutional, depositional, fracturing, brecciation, and collapse features. Toucasia wackestone beds of restricted-marine lagoonal to intertidal environment overlay the paleokarst. Similar events also occur in subsurface Albian-Cenomanian carbonate rocks around the Gulf of Mexico: Jordan Knoll (eastern Gulf of Mexico) and San Marcos and Cordoba Platforms (northwestern and western Gulf of Mexico). Some of the paleokarsted rock intervals have been studied only by space-core samples or by geophysical methods (Jordan Knoll). The Actopan Platform rocks allow an Albian-Cenomanian model to be developed and to be compared to similar rocks around the Gulf of Mexico.

Abstract Middle Eocene compression resulted in formation of the Sierra Madre Oriental fold and thrust belt and end-early Miocene compression resulted in formation of the Chiapas-Campeche fold and thrust belt. These events mask the importance of other periods of deformation, principally in the Middle–Late Jurassic, Late Cretaceous, and Paleogene. Deformation is represented by folding, thick-skinned thrusting, basin inversion, and development of major angular unconformities. Associated features include development of karstification, production of breccias, onlap, lowstand wedges, seeding of carbonate platforms, entry of siliciclastic sediments into carbonate basins, significant switches in input directions of clastic sedimentary systems, initiation of extensional tectonism basinward of the compressive deformation front and igneous activity. We propose that, during the late Mesozoic and the Cenozoic, Pacific plate-margin compressive deformation often extended eastward into the Gulf of Mexico. Two main belts of deformation are identified, which are linked back to Pacific plate-margin processes by postulated deep-seated faults. The first and outer (easternmost) belt is seen on regional seismic lines as a long-wavelength, easterly facing, monoclonal fold that developed close to the transition of thick into thinned continental crust. The Sierra Madre Oriental is the second belt of which the structural history already has been well described in the literature. Where salt is present at depth, compressional events are expressed only as laterally propagated thin-skinned folds and thrusts. These events are of critical importance in that they contribute many unique geologic features that cumulatively give Mexico a world-class petroleum system.

Abstract The most important red-bed and salt sequences in Mexico are Jurassic and are located in eastern Mexico in or around the Gulf of Mexico. Most of these rocks are Middle Jurassic, and they are overlain almost always by evaporitic sequences that mark the beginning of the Middle Jurassic transgressive sequence. In some places, they overlie pre-Jurassic rocks. Mesozoic red-bed sequences have recently been dated with organic and inorganic components of palynological residues. Information from red-bed sequences in the Los San Pedros Allogroup and Huayacocotla Group (Rhaetic-Liassic age in the Huayacocotla–El Alamar Basin), La Joya Formation (Middle Jurassic age in the Sabinas Subbasin), Rosario and Cahuasas Formations (Middle Jurassic age in the Tampico-Misantla Sub-basin), and Todos Santos Formation (Middle Jurassic age in the Veracruz and Tabasco-Chiapas-Campeche Subbasins) allows us to construct a model for the origin and evolution of the Gulf of Mexico. The model includes three different stages: (1) the formation of one (or two?) Rhaetic-Early Liassic wrench or shear basin(s) (Huayacocotla–El Alamar Basin) related to the evolution of the Pacific convergent system; (2) formation of the Tampico-Misantla Sub-basin during the late Liassic as a result of the southwest displacement of the Huayacocotla and Tlaxiaco Blocks along the Tampico–Lázaro Cárdenas and Teziutlán-Acapulco Megashears; and (3), the origin during the Middle Jurassic of the Gulf of Mexico Basin and the Sabinas, Veracruz, and Tabasco-Chiapas-Campeche Mexican petroleum subbasins as a result of the development of a triple junction. This triple junction allowed the northwestward displacement of the Texas-Louisiana block and the western region of Mexico from the stable Chiapas-Tabasco-Campeche-Yucatán block along the Lewis Clark–Bahamas and Texas-Boquillas-Sabinas lineaments and the Pico de Orizaba–Laguna Inferior Megashear.

Abstract Selectively oil-impregnated limestones from the Upper Cretaceous Guzmantla Formation, outcropping in the Cordoba Platform of eastern Mexico, were studied to determine the factors controlling the porosity and hydrocarbon distribution and to reconstruct the fluid-flow history. In the two exposed upward-coarsening (i.e., upward-shoaling) sequences, three limestone lithotypes were distinguished, based on sedimentary, diagenetic, and oil-impregnation characteristics. Lithotype I is comprised of mud-dominated low-energy deposits, which have been affected strongly by compaction. These strata are oil impregnated only along stylolites. Lithotype II consists of bioclastic wackestones to packstones deposited in an open-platform lagoonal environment. This lithotype is pervasively oil impregnated. The preservation of porosity is explained by the development of framework-stabilizing, interparticular, early diagenetic (marine and meteoric) calcite cements. Furthermore, secondary porosity was created after layer-parallel shortening (LPS), when LPS-related structures were opened during subsequent folding of the strata. Lithotype III consists of bioclastic shoal grainstones that have been cemented pervasively during early-marine and later meteoric diagenesis, occluding primary porosity and thus preventing oil impregnation. However, Lithotype III strata display an important modern macroporosity, related to a telogenetic phase of karst development that postdates oil migration. Due to the lack of driving forces, the oil did not migrate into these karst-related pores. In Lithotype II, the presence of oil reduced the effective porosity and hindered further fluid migration. Lithotype II strata thus were less affected by the telogenetic karstification. Lithotype I was less affected because of the completely compacted matrix. This late-stage (postoil migration) dissolution phase is not important in this specific history, but it may be very important in similar deposits in the subsurface, where it can enhance appreciably the reservoir capacity. Factors controlling porosity-permeability are, first, the sedimentary environment, which influenced early and, thus also, later diagenetic evolution. Furthermore, stylolite development (compactional as well as tectonic), which exerts a negative effect on porosity-permeability because of pressure-dissolution and related matrix cementation, also is an important factor. However, because of tectonic opening of some of the stylolites and channelling of meteoric fluids, with porosity development as a result, these stylolites also may increase permeability and total porosity. Finally, fracturing of the strata, whereby tectonic opening and/or cementation can take place, exerts a major influence on reservoir characteristics.

Abstract The Upper Jurassic (Tithonian) Edzna Formation is considered to be the most important source rock in the southeastern Mexico Campeche shelf, Gulf of Mexico. The formation is penetrated by a number of wells, in which cumulative and test production show that the Edzna Formation also is an important hydrocarbon reservoir. The formation is composed of a condensed section of mudstone and bentonitic shale. Thicknesses vary between 62 m and 120 m in the northeastern area, thickening to 393 m in the central marine area. The structural top commonly reaches depths of 6100 m in the southwestern and western areas, with a minimum depth of 1600 m in the northeast. There is a distance of approximately 120 km between deepest and shallowest burial areas. There are three important structures in the Campeche shelf: the Ceeh-Cantarell anticlinorium in the northeast, the Sinan-Mison syncline in the central area, and the May-Oktan anticlinorium in the southern area of the marine region. The northeast-southwest-trending Edzna Formation reservoir is about 150 km long by 50 km wide. To date, cumulative oil production is about 45 million barrels.

Abstract A thick package of sedimentary carbonate breccias accumulated under deep-water conditions in southeastern Mexico and offshore Campeche in the latest Cretaceous. The origin of these breccias has been linked to the Chicxulub impact event that occurred on the Yucatan Peninsula at the Cretaceous/Tertiary (K/T) boundary. This sedimentary succession was examined at three locations: outcrops at El Guayal and Bochil (in the Sierra de Chiapas) and at the Cantarell field in the Bay of Campeche. The succession fines upward and is composed of (1) a basal very coarse-grained carbonate breccia followed gradationally by (2) a fine-grained carbonate breccia and (3) an ejecta-rich layer. (4) A very thin shaly layer caps the sedimentary sequence in the exposed sections and has not been documented in wells. In some wells, another fine-grained carbonate breccia is found in the ejecta-rich layer. Thickness of the carbonate breccia succession ranges from 50 to 300 m. Areal distribution, stratigraphic architecture, and stratigraphic relationships indicate deposition of the K/T sedimentary succession under deep-water marine conditions. Carbonate microfacies analysis of the lithoclasts that comprise the K/T boundary calcareous breccias was carried out to identify the source. Microfacies identified in the carbonate breccias are typical of three depositional environments: inner platform (lagoon and tidal flats), platform margin, and deep-water settings. Inner-platform microfacies include: (1) miliolid-peloid wackestone and packstone, (2) alveolinid wackestone and packstone, (3) orbitolinid peloidal-skeletal packstone, (4) macroforaminifer-algal packstone, (5) lime mudstone and wackestone with fenestrae and cryptmicrobial laminites, (6) Microcodium -bearing lime mudstone and packstone, (7) dolostone, and (8) dolomitized evaporite. Platform-margin carbonate microfacies include: (1) skeletal-peloidal grainstone, (2) macroforaminifer skeletal grainstone, and (3) rudist fragment microfacies. The deep-water carbonate microfacies consist of pelagic lime mudstone and wackestone with common planktonic foraminifers. Lithoclast composition in the calcareous breccias varies from place to place, but in general the most abundant microfacies types were derived from inner-platform and platform-margin environments. Deeper-water microfacies fragments are less common, mainly in the Cantarell field. The microfacies types provide clear evidence for a high-energy platform-margin facies tract that sheltered a lagoonal setting in which diverse facies were deposited in the western Yucatan Peninsula and the Sierra de Chiapas.

Abstract The configuration of the top Jurassic is based on structural maps derived from oil-well data on the subsurface from the Poza Rica and Tampico regions, eastern Mexico. The maps are based on depths to the contact of the Pimienta and Lower Tamaulipas Formations at the Jurassic-Cretaceous boundary. Lithostratigraphic and chronostratigraphic data are from gamma-ray logs and ammonites, respectively. In the Poza Rica district, the La Mesa syncline was surveyed in the northwest, and the Sultepec homocline in the southeast. The depth to the top Jurassic varies from 2000 to 3700 m in the east and southeast of this region. An elongated area remained emergent during deposition of the Tithonian Pimienta Formation, along the present Gulf of Mexico coast east of Poza Rica, continuing to the southeast of the Tampico area. The Pimienta Formation was eroded in the southeast of Poza Rica along the initial cut of the San Andrés paleocanyon. In the Tampico district, the top of the Pimienta Formation is found from 1000 to 3000 m in depth. Two structures, the Tranquitas anticlinorium and the Tanquian anticlinorium, are observed in the northwest and central part of the Tampico district, respectively. Various areas remained emergent during deposition of this formation to the east and southeast of Tampico. They were part of an ancient continent composed of metamorphic and intrusive rocks, and upper Paleozoic continental origin. In the Bejuco region east of the Tampico district, two structures, the Piedra de Cal anticline and the Jabonera syncline, are observed; depth to the top Jurassic varies from 1400 to 3000 m, respectively. In this same region, two areas, Llano de Bustos and La Aguada, remained emergent during the upper Tithonian.

Abstract The Yucatan Block is a rifted continental microplate covering 450,000 sq km in southern Mexico, northern Guatemala, and Belize. The crystalline basement is mantled by a Late Jurassic through Holocene carbonate/evaporite platform up to six-km thick. While the northern and western edges of the Yucatán Block have been passive margins since the Mesozoic, its southern margin was affected by Late Cretaceous suturing to the Chortis microplate, followed by Miocene to Holocene strike-slip faulting. Its eastern margin was modified by Paleogene strike slip against the Cuban Arc Terrane. The Yucatán Block has received very little terrigenous sedimentation since being isolated from nearby landmasses by the Jurassic separation of North and South America. Major hydrocarbon production exists in Mexico from the area immediately west of the Yucatán Block in the Reforma Trend, Campeche Sound, and the Macuspana Basin. Oil has also been found west and south of the block in the Sierra de Chiapas of Guatemala and Mexico. Only one commercial oil accumulation has been found to date on the stable block itself (Xan field in Guatemala), and mineral exploration without commercial success has been limited to the small area of exposed crystalline basement in the Maya Mountains of Belize. Based on current knowledge, it is the author’s opinion that the economic potential of the Yucatán Block should not be discounted.Hydrocarbon and mineral exploration has been sporadic and generally low-tech, and there is a clear need for high-quality regional seismic data to reveal structural configuration and sedimentary architecture. Among the many geological factors to be understood are: 1) geometry of Triassic-Jurassic rift structures (horsts and grabens); 2) location and geometries of possible Jurassic and Cretaceous intraplatform hydrocarbon source basins, carbonate buildups, and structural traps in the evaporite/carbonate section; 3) paleoheatflow as it affected organic maturation; 4) effects within the block of tectonics along its margins (tilting, mass wasting, and foreland bulging); and 5) possible role of the Chicxulub K/T astrobleme in hydrocarbon and mineral occurrence.

Abstract In the Sonda de Campeche offshore region of Mexico, the Tithonian sedimentary sequence is the most important source of hydrocarbons that occur today in Paleocene traps. The maturity of both source rocks and petroleum in reservoirs is known to increase from northeast to southwest across the region. This is manifested at the molecular level and in bulk petroleum properties such as API gravity and gas-oil ratio (GOR). We have analyzed a selection of source-rock samples from across the area, covering the entire maturity spectrum, by pyrolysis gas chromatography. These data give insights into the GOR of yet-to-be-generated petroleum for each maturity stage. A mass-balance model based on these same data and complementary data from laboratory experiments (MSSV pyrolysis) provided cumulative GOR as a function of generation stage (transformation ratio). Regional field GOR trends are consistent with instantaneous rather than cumulative GOR predictions, thereby supporting the notion of mainly localized vertical migration avenues in association with a late timing of trap formation.

Abstract The latest Cretaceous to Eocene Difunta Group in the Parras, La Popa, and the southern part of the Sabinas Basins in the states of Coahuila and Nuevo León in northeast Mexico once occupied anextensive basin in the foreland to the Sierra Madre Oriental fold and thrust belt. The Difunta foreland basin records a complex history of initial Cretaceous deformation in the Sierra Madre Oriental and subsequent early Tertiary salt withdrawal in the region covered by the salt basin in the western part of the Gulf of Mexico. As a result of rapid facies transitions in the Difunta Group, stratigraphic correlation between the three structural basins is complex. Only one regionally extensive lithostratigraphic unit occurs in the Difunta Group, namely, the Maastrichtian Cañon del Tule Formation in the Parras Basin and the correlative Muerto Formation in La Popa Basin and the southern part of the Sabinas Basin. Detailed sedimentologic and sequence stratigraphic studies of the Cañon del Tule and Muerto formations have led to dramatic revisions in correlations in the Difunta Group. The Difunta Group is now subdivided into five informal “stratigraphic cycles” termed SC1, SC2, SC3, SC4, and SC5, each composed of marine mudstone and sandstone with overlying red fluvial mudstone. Stratigraphic cycles SC1 to SC3 were deposited in the latest Cretaceous in response to tectonic loading by encroaching thrust sheets in the Sierra Madre Oriental. In the southeastern Parras Basin, near the Sierra Madre Oriental frontal zone, the foredeep fill is at least 3677 m thick, thinning to 922 m, 150 km (structurally unrestored) to the north in the southern Sabinas Basin. Sediment dispersal was from west to east along the axis of the Difunta foredeep with a dissected volcanic arc provenance presumed to be the Guerrero composite terrane. Exceptionally high subsidence rates of >1 m/1000 years caused sediment to be “trapped” in the southern part of the foredeep, adjacent to the thrust belt, preventing early deltaic complexes in SC1 and SC2 from prograding eastward. The contemporary Mendez shale in the Tampico-Misantla foredeep, which was connected to the Parras–La Popa Basins across the Monterrey salient, represents a starved, underfilled equivalent of the Difunta foredeep to the southeast. In the distal northern part of the Difunta foredeep, in northern La Popa and southern Sabinas Basins, stratigraphic cycles are characterized by forced regression caused by limited subsidence. By the Paleocene and Eocene, thrusting in the Sierra Madre Oriental and accompanying foreland basin subsidence had ceased. In the region of the Parras Basin, no more sediment accumulated in the Difunta Group. In La Popa Basin and the southern part of the Sabinas Basin, which overlie the western extension of the Gulf of Mexico salt basin, growth of salt diapirs and associated salt withdrawal resulted in accumulation of more than 2300 m of sediment in structural “minibasins.” Large volumes of volcaniclastic detritus continued to be supplied from the west, filling the salt minibasins with fluvial and shallow-marine sediment. These Tertiary sequences represent cycles SC4 and SC5 in the Difunta Group. By the Paleocene, in the Tampico-Misantla portion of the Difunta foredeep, axially derived sands were deposited in the Chicontepec paleochannel. Only limited carbonate clastic input from the Sierra Madre Oriental highland was received in the Tampico-Misantla Basin in the Tertiary, and at no time did the Sierra Madre Oriental supply detritus to the Parras or La Popa Basins. Some time after the Eocene, probably in the early Oligocene, the region covered by the Difunta foredeep was deformed, uplifted, and eroded, leading to the present outcrop pattern of structural basins and highs. This episode of uplift resulted in large volumes of sediment being deposited in the western Gulf of Mexico Basin and led to substantial progradation of the northeast Mexican continental margin and establishment of a large, early Oligocene depocenter.

Abstract The coal deposits of México are located in the states of Sonora, Chihuahua, Coahuila, Hidalgo, Puebla, Oaxaca, and Tamaulipas. Particularly, the Sabinas Basin in Coahuila contains more than 5000 m of Upper Jurassic and Cretaceous rocks. The coal seams are at the top of the Upper Cretaceous section and occur from the present surface to a depth of about 900 m. They are identified on well logs by high resistivity, low density, and by lithology inferred from gamma-ray logs. Interpretation of logs from 12 wells in the Sabinas coal basin and 4 wells in the Río Escondido coal basin, all with thick Upper Cretaceous sections, allow regional identification of the Austin and Upson Formations, four units in the San Miguel Formation, the Olmos Formation, and a thick sandstone in the Escondido Formation. A preliminary interpretation of this study is that several coal seams assigned to the Olmos Formation likely are contained in the lower one-third of the Escondido Formation, although it is possible that coal seams may occur at other stratigraphic levels in other areas. Coal in the Sabinas Basin was deposited in regressive, high-frequency, deltaic sequences characterized by overall lenticular geometries. A concentration of higher plants led to the accumulation of type III kerogen favorable for the in situ generation of methane. Coal in the Sabinas Basin is bituminous, with high-to-medium volatility. It has 45% fixed carbon, 14% volatile material, 40% ash content, and 1% sulfur and other impurities. Its caloric content is 1300 to 1400 Btu with a vitrinite reflectance of about 0.5, which places it between the zones of diagenesis and catagenesis with a maximum burial depth that may have been between 800 and 1200 m. By drilling 60 shallow wells (average depth of 700 m) at a cost of $US 21.6 million, approximately 18 million ft 3 of gas per day could be produced with a payout of the investmentin 18 months calculated at a price of$2.15 per thousand ft 3 . This area could potentially produce coalbed methane commercially for nearly 20 years, considering production increase though time as the coal seams desorb.

Abstract Detailed and extensive studies of Cretaceous (Aptian-Cenomanian) gastropods from 19 localities in Mexico (Baja California, Sonora, Jalisco, Colima, Michoacán, Querétaro, Puebla, Guerrero, and Oaxaca) yielded gastropod species that also have been reported from other regions in the world and, thus, support a paleobiogeographic relation with similar faunas in the United States (New Mexico and Texas), the Caribbean region (Cuba), South America (Peru, Brazil), the Mediterranean region (Spain, France, Italy, Switzerland, Rumania, Syria, Lebanon, Algeria, Morocco, Tunisia, and Somalia), and Japan. The widely distributed taxa imply that an ample marine faunistic province existed, which included parts of the southwestern United States, western and southeastern Mexico, and the Caribbean and Mediterranean regions.

Abstract Three species of lower Eocene ostreids are reported from the Viento Formation of the La Popa Basin, Nuevo León, northeastern Mexico. Two of them are described as new: Ostrea ( Turkostrea ) ventosa new species, and Ostrea ( Turkostrea ) ovata new species. Ostrea sp. is also reported associated with the new forms. The large size of these species, in contrast with their small relatives from equivalent depositional environments in the underlying Adjuntas Formation, suggests that paleoclimate played an important role in their size development. These ostreids are the youngest fossils reported from the La Popa Basin, as the Viento Formation has remained undated prior to this contribution. The age suggested for the Viento Formation corresponds to the early Eocene, probably the upper part of the Ypresian.