ABSTRACT This study presents the first apatite fission-track results from the Tolimán area, which is located in the western portion of the southern Sierra Madre Oriental, central Mexico. In total, six rock samples from different lithostratigraphic units were dated, adding new results to the thermochronological data set of the Sierra de los Cuarzos–San Joaquín–Tamazunchale transect in the Mexican fold-and-thrust belt. The apatite fission-track ages vary from 84 ± 4 Ma to 52 ± 2 Ma, indicating that the main denudation period of the Tolimán area lasted until the Eocene. Combining our results with previous geological data, we suggest that the western part of the southern Sierra Madre Oriental was uplifted and undergoing erosion during the whole period of development of the Campanian–Ypresian Mexican orogenic system. Therefore, the Tolimán area may be considered as one of the source areas from which clastic materials of the Campanian–Maastrichtian Méndez and Paleocene–Eocene Velasco and Chicontepec Formations were partially derived. Older cooling ages recording the latest Aptian accretion of the Guerrero terrane with the Mexican continental interior were not detected in samples from the Tolimán area.

Abstract The impact of the asteroid (KPg impact event) at Chicxulub is now a well-documented geologic event which took place at the Cretaceous/Paleogene (KPg) boundary ( Schulte et al. , 2010 ). However, the effect of this event is relatively unknown in the Tampico-Misantla Basin, which is only about 900 km to the west of the impact site. In the detailed well reports in the Tampico-Misantla Basin, it is noted that a “brecha” (breccia) is often described at the top of the Cretaceous in many of the wells.. The breccia is described as being gray or white, containing mudstone clasts, having a sandy matrix, recrystallized Globotruncana , and traces of chert, amber, and bentonite (for example, in the Marques-1 well). Early geologists thought the breccia had been deposited in response to the Laramide uplift of the Sierra Madre Oriental. To our knowledge, none of the 100 project wells cored the breccia. The same KPg breccia crops out in the southern part of the Tampico-Misantla Basin to the southwest of the town of Martinez de la Torre ( Figs. 1 and 2 ). Here, the breccia is a clast-supported conglomerate consisting of cobbles and boulders of limestone, sandstone (medium to coarse grained), and quartz. The matrix is a medium- to coarse-grained sandstone. The KPg contact has been documented just to the west of this outcrop (Mark Bitter, personal communication). The limestone clasts are thought to have been derived from the Tuxpan platform to the northeast, and the sandstone clasts are thought to have been derived from the Sierra Madre Oriental to the west by the backwash of the tsunami generated by the impact event. In many of the well reports, the wellsite geologists also note that the “Velasco Formation” overlies this breccia. The Velasco Formation is always described as a shale, red, gray, or brown, and compacted. The Velasco Formation has been cored in the Entabladero-101 well from 1140-1149 m and the core has been described as compacted grey/brown shale ( Fig. 3 ). It is devoid of sand. In this study, the presence and thickness of both the breccia and the Velasco Formation were noted and mapped from the well reports. The wells were drilled between 1936 and 2010 and the early wellsite geologists were probably not always aware of the detailed stratigraphic sequence and certainly not aware of the relevance of the breccia in the basin. The thicknesses of both the breccias and Velasco Formation were estimated from the cuttings descriptions in the wells. The breccia is absent in the northern third of the Chicontepec Basin. The thickness of the breccia deposit varies between 4 m and 38 m, and is generally about 10-15 m thick. The deposit is fan-shaped, the source is interpreted to be from the northeast (Tuxpan platform), and it pinches out to the southwest ( Fig. 4 ). The only carbonate source area that is present to the northeast is the Faja de Oro atoll, an Albian age reef complex. Additionally, the distribution of the Velasco Formation seems to mimic the distribution of the breccia, albeit covering a slightly larger area. The Velasco Formation varies in thickness between 16 m and 145 m, but it is generally in the range of 30 to 40 m thick. It is proposed that the breccia plus the Velasco Formation are actually a “megabed” created by the huge tsunami (estimated by some authors to have been over 300 m high) from the KPg impact event ( Fig. 5 ). Many other megabeds around the world show these same characteristics ( Cossey and Ehrlich, 1979 ). The breccia would represent the basal Bouma “A,” or graded division. The Velasco Formation would represent the muddy top, or Bouma “E” division. A good analog for this megabed is from the Jurassic of northern Tunisia ( Cossey and Ehrlich, 1979 ) where a carbonate megabed up to 90 m thick is exposed. Prior to the KPg impact event, the Tampico-Misantla Basin is primarily a site of carbonate deposition throughout the Cretaceous. Afterwards, the Chicontepec Basin forms as a foredeep in front of the rising Sierra Madre Oriental to the west. The overlying Paleocene Chicontepec Formation consists of turbidite sands composed of over 50% carbonate material ( Bitter, 1993 ) derived from the uplift and erosion of the Sierra Madre Oriental.

Abstract Of all the countries in the world considered to be oil rich, Mexico is the only one that consistently has been losing production and reserves in the last ten years. Even though Mexico has five major producing provinces: two for oil (the Southeast and the Tampico–Misantla basins) and three for gas (the Sabinas, Burgos and Veracruz basins), and has seven more with potential, (California, Gulf of Cortès, Chihuahua, Sierra Madre Oriental, Sierra de Chiapas, Progreso shelf, and the deep Gulf of Mèxico), its output and reserves have declined consistently. Many reasons can be attributed for these results, and as this note proves, least of them is the country’s endowment of oil and gas resources. The problem is that Mexico, since 1938, has had only one oil company responsible for all of its upstream activities and even though Pemex’s performance is comparable with that of most of the majors’ (it is world’s third largest in terms of production), it is impossible that all the remaining potential of the entire country can be found and produced with only one company, no matter how large, wealthy, efficient, technologically advanced, and successful it can be. The good news is that once the country opens up for third-party participation in exploration, which will eventually take place, results are going to be spectacular. So far there has only been a timid opening for development and exploitation opportunities.

Abstract Basement outcrops are rare in northeastern Mexico, but the effect of older basement structures on the current structural plan is clear. Two large structures, the San Marcos fault and the northern edge of the Sierra Madre Oriental, are oriented roughly east-west, and have been postulated as sites of the Mojave-Sonora megashear. Both structures were probably active in the late Jurassic. The San Marcos fault continued to be active into the Cretaceous, while the northern edge of the Sierra Madre Oriental became inactive. Both structures were strongly reactivated during the Laramide orogeny. Potential fields data integrated with regional geologic mapping suggests a series of horsts and grabens were created during Middle-Late Jurassic rifting of the basement, but that relative displacements are far less than those proposed for the Mojave-Sonora megashear system. The northern edge of the Sierra Madre Oriental, with its large left-step along the Monterrey salient, is kinematically more compatible with the postulated left-lateral offset along the Mojave-Sonora megashear, while the San Marcos fault has numerous right-stepping offsets, which is kinematically incom-patible with the Mojave-Sonora megashear hypothesis. In general, however, these data do not support the concept of a large-offset left-lateral megashear in northeastern Mexico.

Mesa Central is an elevated plateau that can be divided into two regions. In the southern region, the topography is higher than 2000 masl, except for the Aguascalientes valley. This region is mostly covered by Cenozoic volcanic rocks. The northern region shows an advanced degree of erosion, and is below 2000 masl. The crust in Mesa Central is ∼32 km thick, and it is bordered by the Sierra Madre Oriental, which has an average crustal thickness of ∼37 km, and the Sierra Madre Occidental, which has an average crustal thickness of ∼40 km. The presence of magmas below the crust is inferred, suggesting an underplating process. The oldest rocks are Triassic marine facies underlain by Jurassic continental rocks. Marine environment prevailed between the Oxfordian and the Cretaceous, forming three distinctive lithological sequences, from E to W: the Valles–San Luis Potosí Platform, the Mesozoic Basin of Central México, and marine volcanosedimentary Mesozoic rocks. All of the above rocks have plicative deformation and inverse faulting, which was produced during the Laramide orogeny. An angular unconformity separates these lithological sequences from the continental Cenozoic rocks. The bottom of the Cenozoic sequence consists of conglomerate with andesitic and rhyolitic volcanic rocks. These were followed by Oligocene topaz-bearing rhyolites, and the uppermost part of the Cenozoic sequence is Miocene-Quaternary alkaline basalt. The boundaries of Mesa Central are the Sector Transversal de Parras and major fault systems active during the Cenozoic to the E, W, and S. A major structure, the San Luis–Tepehuanes fault system, separates the northern and southern regions of Mesa Central. The majority of the mineral deposits found in Mesa Central or in its vicinities, especially epithermal deposits, is located on the traces of the major fault systems described above. The data available suggest that the structures associated with the major fault systems controlled the emplacement of both volcanic-hypabyssal rocks and mineral deposits.

The Trans-Mexican Volcanic Belt has been recognized as a major volcanic arc, which crosses México from the Pacific Coast to the Gulf of México, that has displayed normal faulting and volcanism since the Miocene. In this work we present the deformation events that have been recorded N and S of the belt in order to establish when the crustal discontinuity originated and also to determine the deformation field precursor of the volcanic arc emplacement. In Mesa Central, the post-Laramide deformation occurred in three extensional events during the Eocene, Oligocene, and Miocene-Recent. The three events produced extension in two horizontal directions and shortening in a vertical direction. The direction of the principal extension in the Eocene is not well known. A 20% extension in an ∼ENE-WSW direction is recorded for the Oligocene event. The most recent event, active since the middle Miocene, has developed in the Trans-Mexican Volcanic Belt and along its northern boundary. In the Sierra Madre Oriental, Cenozoic deformation has been minimal. In the Taxco region, there were two post-Laramide deformation events, mainly a result of NW-SE and N-S lateral faults. The first one occurred in the late Eocene with a NNW-SSE horizontal extension direction. The second event was early Oligocene with a maximum extension to the NE-SW. It is concluded that since the Eocene, the deforma tion style has been different in Mesa Central and in the Sierra Madre del Sur, which implies the presence of a detachment zone between these provinces.