NE Mexico Research Articles

Deposition and age of Chicxulub impact spherules on Gorgonilla Island, Colombia

AbstractThe end-Cretaceous mass extinction (66 Ma) has long been associated with the Chicxulub impact on the Yucatan Peninsula. However, consensus on the age of this impact has remained controversial because of differing interpretations on the stratigraphic position of Chicxulub impact spherules relative to the mass extinction horizon. One side argues that the impact occurred precisely at the Cretaceous-Paleogene boundary, thus coinciding with the mass extinction; the other side argues that the impact predated the Cretaceous-Paleogene boundary, based on the discovery of primary impact spherules deposits in NE Mexico and Texas near the base of planktic foraminiferal zone CF1, dated at 170 k.y. before the Cretaceous-Paleogene boundary. A recent study of the most pristine Chicxulub impact spherules discovered on Gorgonilla Island, Colombia, suggested that they represent a primary impact deposit with an absolute age indistinguishable from the Cretaceous-Paleogene boundary. Here, we report on the Gorgonilla section with the main objective of evaluating the nature of deposition and age of the spherule-rich layer relative to the Cretaceous-Paleogene boundary.The Gorgonilla section consists of light gray-yellow calcareous siliceous mudstones (pelagic deposits) alternating with dark olive-brown litharenites (turbidites). A 3-cm-thick dark olive-green spherule-rich layer overlies an erosional surface separating Maastrichtian and Danian sediments. This layer consists of a clast-supported, normally graded litharenite, with abundant Chicxulub impact glass spherules, lithics (mostly volcanic), and Maastrichtian as well as Danian microfossils, which transitions to a calcareous mudstone as particle size decreases. Mineralogical analysis shows that this layer is dominated by phyllosilicates, similar to the litharenites (turbidites) that characterize the section. Based on these results, the spherule-rich layer is interpreted as a reworked early Danian deposit associated with turbiditic currents. A major hiatus (>250 k.y.) spanning the Cretaceous-Paleogene boundary and the earliest Danian is recorded at the base of the spherule-rich layer, based on planktic foraminiferal and radiolarian biostratigraphy and carbon stable isotopes. Erosion across the Cretaceous-Paleogene boundary has been recorded worldwide and is generally attributed to rapid climate changes, enhanced bottom-water circulation during global cooling, sea-level fluctuations, and/or intensified tectonic activity. Chicxulub impact spherules are commonly reworked and redeposited into younger sediments overlying a Cretaceous-Paleogene boundary hiatus of variable extent in the Caribbean, Central America, and North Atlantic, while primary deposits are rare and only known from NE Mexico and Texas. Because of their reworked nature, Gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred.

MVT-Like Fluorite Deposits and Oligocene Magmatic-Hydrothermal Fluorite–Be–U–Mo–P–V Overprints in Northern Coahuila, Mexico

The formation of most fluorite deposits in northern Coahuila (NE Mexico) is explained by MVT models, and is a part of the metallogenic province of northeastern Mexico. However, fluorite skarn deposits also occur in the same region, and there is evidence for late hydrothermal manifestations with no clear origin and evolution. The latter are the main focus of this study; in particular, F–Be–U–Mo–V–P stringers in the Aguachile-Cuatro Palmas area that overprint preexisting fluorite mantos. The region experienced the emplacement of several intrusives during the Eocene and the Oligocene that are collectively grouped into the East Mexico Alkaline Province (EMAP) and postdate MVT-like deposits. Some of these intrusives have associated skarn deposits; most of them are polymetallic, but the unusual El Pilote deposit contains fluorite mineralisation that was remobilised from MVT-like deposits. The formation of the Aguachile deposit (and, collectively, part of the Cuatro Palmas deposit) has been attributed to a shallow retrograde skarn model. The Cuatro Palmas and Las Alicias fluorite deposits consist of MVT-like deposits overprinted by late hydrothermal fluorite mineralisation rich in Be–U–Mo–V–P, and the Aguachile deposit consists entirely of the latter type. The systematic fluid inclusion study of MVT-like, skarn, and late hydrothermal fluorite deposits reveals a very different distribution of temperature and salinity data that allows the discrimination of mineralising fluids for the type of deposit. MVT-like deposits were formed by fluids with temperatures of homogenisation that range between 50 °C and 152 °C and salinities between 5 and 15.5 wt.% NaCl equivalent. The El Pilote fluorite skarn was formed by fluids with temperatures of homogenisation that range between 78 °C and 394 °C and salinities between 5 and 34 wt.% NaCl equivalent, and include CaCl2-rich brines with salinities that range between 24.5 and 29.1 wt.% CaCl2. Late shallow fluorite–Be–U–Mo–V–P hydrothermal deposits were formed by fluids with temperatures of homogenisation that range between 70 °C and 180 °C and salinities between 0.9 and 3.4 wt.% NaCl equivalent; the sole exception to the above is the La Fácil deposit, with salinities that range between 7.9 and 8.8 wt.% NaCl equivalent. While temperatures of homogenisation are similar between MVT-like and late hydrothermal deposits, and both even have hydrocarbon-rich fluid inclusion associations, the salinity of late deposits is similar to that of retrograde skarn fluids, although further diluted. However, homogenisation temperatures tend to be higher in late hydrothermal than in MVT-like deposits, thus making them more similar to retrograde skarn fluids. Although this characteristic cannot solely establish a genetic link between a retrograde skarn model and late hydrothermal deposits in the study area, the characteristics of fluids associated with the latter separate these deposits from those ascribed to an MVT-like model. Assuming that mineralising fluids for late fluorite–Be–U–Mo–V–P hydrothermal deposits may correspond to a retrograde skarn (or “epithermal”) deposit, the source for fluorine may be either from (A) the dissolution of earlier formed MVT-like deposits, (B) the entrainment of remaining F-rich basinal brines, or (C) hydrothermal fluids exsolved from highly evolved magmas. Possibilities A and B are feasible due to a hypothetical situation similar to the El Pilote skarn, and due to the occurrence of hydrocarbon-rich fluid inclusions at the La Fácil deposit. Possibility C is feasible because intrusive bodies related to highly evolved magmas would have provided other highly lithophile elements like Be, U and Mo upon the exsolution of their hydrothermal fluids. Such intrusive bodies occur in both study areas, and are particularly conspicuous at the Aguachile collapse structure.

Additions to continental gastropods from the Late Cretaceous and Paleocene of NE Mexico

espanolSe ofrecen adiciones a la diversa fauna de gastropodos estuarinos, dulceacuicolas y terrestres de las unidades litoestratigraficas del Cretacico Superior (Campaniano) y Paleoceno del Grupo Difunta, NE de Mexico (Coahuila y Nuevo Leon). Diecinueve especies de gastropodos continentales (cinco de ellas nuevas) son reportadas e ilustradas para la Formacion Cerro del Pueblo del Campaniano superior (localidad Las Aguilas/Porvenir de Jalpa) y la Formacion Las Encinas, del Paleoceno (localidad La Leona), porcion norte de la Cuenca de Parras, Grupo Difunta, Coahuila. El primer registro de gasteropodos pulmonados para el Grupo Difunta, se basa en varios ejemplares de diferentes especies. Esta es la tercera contribucion que trata sobre caracoles continentales de la region y se presenta una lista revisada de especies. Aunque seguimos la mas reciente clasificacion de gasteropodos continentales, tambien basamos la identificacion en sistematica paleontologica, concientes de que algunas caracteristicas diagnosticas importantes no fueron preservadas. Por lo tanto, no intentamos proponer inferencias evolutivas o paleobiogeograficas. Nuestro proposito principal es presentar la paleodiversidad conocida para el area de estudio, en espera de que pueda ser de utilidad para estudios mas profundos a futuro. EnglishAdditions are offered to the diverse estuarine, freshwater and terrestrial gastropod fauna of the Upper Cretaceous (Campanian) and Paleocene lithostratigraphic units of the Difunta Group, NE Mexico (Coahuila and Nuevo Leon). Nineteen species of continental gastropods (five of them new) are reported and illustrated from the upper Campanian Cerro del Pueblo Formation (Las Aguilas/Porvenir de Jalpa locality) and the Paleocene Las Encinas Formation (La Leona locality), northern Parras Basin, Difunta Group, Coahuila. The first record of pulmonate gastropods from the Difunta Group is based on several specimens of different species. This is the third contribution dealing with these continental gastropods from the study region and a revised list of reported species is presented. Although we follow the most recent classification of continental gastropods, we also base the identification on paleontological systematics, knowing that some important diagnostic features were not preserved. Thus, we do not intend to propose paleobiogeographic or evolutionary inferences. Our main goal is to present the paleodiversity known from the study area, in hope that these data would be useful for more detailed studies in the near future.

Novel vegetation and establishment of Chihuahuan Desert communities in response to late Pleistocene moisture availability in the Cuatrociénegas Basin, NE Mexico

With over 200 pools, lakes and rivers supporting over 70 species of endemic flora and fauna, the Cuatrociénegas Basin (CCB), Coahuila, NE Mexico, is an extremely important area for conservation studies. However, the palaeoenvironment of this unique area has been relatively neglected. Here, pollen data are presented alongside U-series dating and 14C AMS dating techniques from a 15-m sediment core taken from Poza Tierra Blanca in the CCB. These data suggest the CCB contains palaeoenvironmental information spanning at least the late Pleistocene (84.5 ka BP) to the present and has undergone extensive environmental change, possibly controlled by stadial–interstadial cycles. The CCB is currently functioning as a hydrologically closed system, established around 4 ka BP synchronously with regional drying of the Chihuahuan Desert. Pollen data suggest similar closed hydrology conditions from ~33 to 23.13 ka BP – before the onset of full glacial conditions at the LGM. Hydrologically open system characteristics with a dominance of wetter, winter monsoon climate punctuate the long-term record. The wetter conditions observed in these units appear to have facilitated the downslope movement of montane taxa and the expansion of wetland taxa. These data illustrate that novel vegetation assemblages are not just products of deglaciation but represent the interaction of the individualistic response of taxa with the unique climate spaces formed by millennial-scale variability during both glacial and interglacial times.

Concurrence of folding and remagnetization events in the Monterrey Salient (NE Mexico)

Carbonate formations from many locations reveal magnetizations that are secondary. Attaching a numerical age to these remagnetizations would provide information on orogenic processes. To contribute to the effort of assigning an age to remagnetization events, Lower Cretaceous formations from the Monterrey Salient in northeast Mexico were sampled. Individual fold tests reveal eight site-pair synfolding remagnetizations (set A) of Eocene age. The remanence is a chemical remanent magnetization (CRM) carried by magnetite. The magnetization of set B is possibly due to an early carbonate alteration that produced a remanence significantly older than folding. A complexity of set B is that the site pairs reveal k-versus-unfolding percentage diagrams with maximum unfolding peaks at 100 to 125%. An explanation of multiple deformation phases with an earlier horizontal folding phase around a non-plunging horizontal axis and a later vertical axis rotation is suggested for the past 100% peaks of the B sites. The latter deformation, rotation about a vertical axes, is revealed by a declination-strike test in set A sites and site means of the full collection that deviate counterclockwise from the reference declination. By combining synfolding results with published 40Ar/39Ar illite ages of folding (Fitz-Diaz et al., 2016), remagnetization ages in set A sites of 48–52Ma are obtained. The magnetization in set B sites is likely older, though not well constrained. Interestingly, prior results from the central Sierra Madre Oriental, to the south, show two remagnetization events, Late Cretaceous and Early Eocene in age, in a succession from W to E (Nemkin et al., 2015). The age of folding and remagnetization acquisition in set A sites along the Monterrey Salient are concurrent with the frontal, remagnetized folds in the central Sierra Madre Oriental. Thus, the timing of major remagnetization in the Monterrey Salient occurred in the Eocene.

Oldest record of Mathildellidae (Crustacea: Decapoda: Goneplacoidea) associated with Retroplumidae from the Upper Cretaceous of NE Mexico

A new genus and species of the Mathildellidae Prebranchioplax cretacica (Crustacea: Decapoda: Goneplacoidea) is reported from shallow marine sediments of the upper Campanian Parras Shale and Cerro del Pueblo Formation (Parras Basin), Coahuila, NE Mexico. Prebranchioplax cretacica was collected from siliceous concretions associated with more abundant specimens of the retroplumid Costacopluma mexicana Vega and Perrilliat, 1989. P. cretacica bears similarities to Eocene species of Branchioplax from Japan, USA (Alaska and Washington), England, Hungary and Tajikistan as well as with Eogeryonidae (Portunoidea) species from the Upper Cretaceous of Spain and Marocarcinidae (Styracocarcinus) from Morocco. However, clear differences in the carapace frontal shape places P. cretacica in the Mathildellidae. This record represents the oldest known Mathildellidae, and along with the Retroplumidae, appear to have originated during the Late Cretaceous in ancient seas of Mexico, with a wide distribution during Paleogene times becoming restricted today to deep waters of the Indo-Pacific region and Atlantic Ocean. Comments on preservation and morphology of Costacopluma mexicana are also included. Crab specimens preserved in siliceous concretions from one locality (Entronque) show peculiar desiccation marks, and a possible model of taphonomy.

Observed trends in ground-level O<sub>3</sub> in Monterrey, Mexico, during 1993–2014: comparison with Mexico City and Guadalajara

Abstract. Here, we present an assessment of long-term trends in O3 and odd oxygen (O3 + NO2) at the industrial Monterrey metropolitan area (MMA) in NE Mexico. Diurnal amplitudes in Ox (AVd) are used as a proxy for net O3 production, which is influenced by the NO2 photolysis rate. No significant differences in the AVd are observed between weekends and weekdays, although the largest AVd values are observed at sites downwind of industrial areas. The highest O3 mixing ratios are observed in spring, with minimum values in winter. The largest annual variations in O3 are typically observed downwind of the MMA, with the lowest variations generally recorded in highly populated areas and close to industrial areas. A wind sector analysis of mixing ratios of O3 precursors revealed that the dominant sources of emissions are located in the industrial regions within the MMA and surrounding area. Significant increasing trends in O3 in spring, summer, and autumn are observed depending on site location, with trends in annual averages ranging between 0.19 and 0.33 ppb yr−1. Overall, from 1993 to 2014, within the MMA, O3 has increased at an average rate of 0.22 ppb yr−1 (p < 0. 01), which is in marked contrast with the decline of 1.15 ppb yr−1 (p < 0. 001) observed in the Mexico City metropolitan area (MCMA) for the same period. No clear trend is observed from 1996 to 2014 within the Guadalajara metropolitan area (GMA).

Volcanic sequence in Late Triassic – Jurassic siliciclastic and evaporitic rocks from Galeana, NE Mexico

In northeastern Mexico, volcanic rocks interbedded with Late Triassic–Jurassic siliciclastic and evaporitic strata have been linked to magmatic arcs developed in the Pangea western margin during its initial phase of fragmentation. This work provides new petrographic and geochemical data for volcanism included in the El Alamar and Minas Viejas formations outcropping in the Galeana region. Andesitic dykes and sills (n= 10) in the El Alamar redbeds show SiO2= 47.5–59.1% and MgO= 1.2–4.2%, as well as a geochemical affinity to island arc magmas. This work represents the first report of this tectonic setting in the region. Geological and petrographic evidence suggest that this arc system likely developed after ~220 and before ~193Ma. Trachy-andesitic and rhyodacitic domes (n= 20) associated with the Minas Viejas gypsum-carbonates sequence show SiO2= 61.8–82.7% and MgO= 0.1–4.0% with a tectonic affinity to continental arc. A rhyodacite sample from this region has been dated by U-Pb in zircon, yielding an age of 149.4 ± 1.2Ma (n= 21), being the youngest age related to this arc. Finally, we propose a threestep model to explain the tectonic evolution from Late Triassic island arc to Jurassic continental arc system in the northeastern Mexico.

The Upper Campanian – lower Maastrichtian cephalopod fauna of Botellos, Nuevo León: a key to understand faunal turnover across the Campanian–Maastrichtian boundary in NE Mexico

Abstract A new cephalopod collection from the Campanian-Maastrichtian boundary interval of NE Mexico, consisting of 1076 individuals assigned to 29 species and 22 genera is presented. This collection is a mix of ammonoids, one coleoid and one nautilid, which originate from at least three ammonoid biozones: The upper Campanian Exiteloceras jenneyi and Nostoceras (Nostoceras) hyatti zones, and the lower Maastrichtian Pachydiscus (Pachydiscus) neubergicus Zone. The age of the collection is thus middle late Campanian to late early Maastrichtian, and it closes a stratigraphic gap between faunas described formerly from this region. The specimens are nuclei collected from the desert pavement. The abundance of specimens allows for a comparison to other Campanian-Maastrichtian ammonoid records from Mexico, North America and Europe.

Use of Combined Observational- and Model-Derived Photochemical Indicators to Assess the O3-NOx-VOC System Sensitivity in Urban Areas

Tropospheric levels of O3 have historically exceeded the official annual Mexican standards within the Monterrey Metropolitan Area (MMA) in NE Mexico. High-frequency and high-precision measurements of tropospheric O3, NOy, NO2, NO, CO, SO2, PM10 and PM2.5 were made at the Obispado monitoring site near the downtown MMA from September 2012 to August 2013. The seasonal cycles of O3 and NOy are driven by changes in meteorology and to a lesser extent by variations in primary emissions. The NOy levels were positively correlated with O3 precursors and inversely correlated with O3 and wind speed. Recorded data were used to assess the O3-Volatile Organic Compounds (VOC)-NOx system’s sensitivity through an observational-based approach. The photochemical indicator O3/NOy was derived from measured data during the enhanced O3 production period (12:00–18:00 Central Daylight Time (CDT), GMT-0500). The O3/NOy ratios calculated for this time period showed that the O3 production within the MMA is VOC sensitive. A box model simulation of production rates of HNO3 (PHNO3) and total peroxides (Pperox) carried out for O3 episodes in fall and spring confirmed the VOC sensitivity within the MMA environment. No significant differences were observed in O3/NOy from weekdays to weekends or for PHNO3/Pperox ratios, confirming the limiting role of VOCs in O3 production within the MMA. The ratified photochemical regime observed may allow the environmental authorities to revise and verify the current policies for air quality control within the MMA.

Paleocene decapod Crustacea from northeastern Mexico: Additions to biostratigraphy and diversity

New decapod specimens from mid-Paleocene shallow marine deposits of NE Mexico represents an important addition to the diversity, paleobiogeography and evolution of the Crustacea record. In this work, we describe additions to the decapod assemblage from the Paleocene (Selandian) Rancho Nuevo Formation (Difunta Group, Parras Basin, Coahuila). Due to the evident size differences with other decapod assemblages, we compare the new assemblage with those from the Lower Paleocene (Danian) Mexia Clay Member of the Wills Point Formation, Texas, and the Lower Eocene (Ypresian) El Bosque Formation in Chiapas. Species reported from the mid-Paleocene (Selandian) assemblage of the Porters Creek Formation (Alabama), are correlatable to the decapod species from NE Mexico in age, size and systematic composition. The erymid lobster Enoploclytia gardnerae (Rathbun, 1935) is represented by several carapaces and chelae remains. One isolated palm of Callianassidae is included. Numerous carapaces of Linuparus wilcoxensis Rathbun, 1935 are described, representing the most abundant lobster. A new record for the raninid Notopoides sp., and presence of Quasilaeviranina sp. cf. arzignagnensis and Quasilaeviranina ovalis are here reported. New raninids, Claudioranina latacantha sp. nov. and Claudioranina sp. (Cyrtorhininae) are also part of this assemblage. Paraverrucoides alabamensis (Rathbun, 1935), and Tehuacana americana (Rathbun, 1935) are represented by several carapaces exhibiting intraspecific morphological variation. Different sizes among the Early and Middle Paleocene and Early Eocene decapod populations suggests a possible effect of variation in seawater temperatures and/or a Lilliput effect after the K/Pg event.

Biogeography and ecology of the genus Turbinicarpus (Cactaceae): environmental controls of taxa richness and morphology

The biogeography and the ecology of the genus Turbinicarpus are scarcely known and the taxonomic segregation of the genus Rapicactus from Turbinicarpus has been debated. The present study aims to (i) evaluate the distribution of all taxa of Turbinicarpus in the full distributional range of the genus (NE Mexico); (ii) investigate the potential ecological divergence between Rapicactus and Turbinicarpus; and (iii) analyse the influence of environmental variables on plant morphology. The distribution pattern of all taxa belonging to the genus Turbinicarpus s.l. (Turbinicarpus + Rapicactus) was mapped using information from 12 herbaria, as well as unpublished databases. The study area was divided into cells of 25 km × 25 km. The relationships between taxa richness, sampling effort and environmental variables were tested by using grid data, via spatially explicit models. The elevational distribution of populations and the effects of climate, latitude and elevation on the mean steam diameter of the populations were separately investigated for Turbinicarpus and Rapicactus. Turbinicarpus s.l. taxa richness is highest within 100°W and 101°W longitude, about 2500 km north of the equator, with maximum diversity in areas with a mean annual temperature of 17–19 °C and a mean annual rainfall of 550–650 mm. A significant relationship was observed between taxa richness and sampling effort, and between taxa richness and longitude, mean annual rainfall and calcium-rich substrate. Rapicactus taxa were found to grow at higher elevations compared with Turbinicarpus. Body size of the plants showed a negative relationship with latitude and elevation in Turbinicarpus, and a positive one with latitude in Rapicactus. The results showed that the genus Turbinicarpus s.l. presents diversity hotspots in the floristic regions of Chihuahuan Desert and Sierra Madre Oriental. The responses of Rapicactus and Turbinicarpus to latitudinal and elevational gradients were different, supporting the segregation of Rapicactus from Turbinicarpus.

The metallogenic evolution in Mexico during the Mesozoic, and its bearing in the Cordillera of Western North America

The Mesozoic ore and industrial mineral deposits in Mexico can be classified into four supergroups of deposits or ‘realms’: oceanic magmatic, continental magmatic, sedimentary-diagenetic, and orogenic-metamorphic. The oceanic magmatic realm comprises volcanogenic massive sulfide and sulfate deposits (VMS) and mineral deposits in ultramafic–mafic massifs that formed in oceanic arcs, largely between the Upper Jurassic and the early Upper Cretaceous. The continental magmatic realm comprises porphyry-type, non-iron oxide skarns, and magmatic-hydrothermal iron oxide deposits (MHIO) that formed in continental arcs, basically between the late Lower Cretaceous onward into the Cenozoic. Both magmatic realms are dominant along the Pacific margin of Mexico and extended up to several hundreds of km inland. The sedimentary-diagenetic realm comprises Mississippi Valley-type (MVT) and associated deposits, sedimentary-exhalative (SEDEX), red-bed or Kupferschiefer-type, and sedimentary phosphorite deposits that started forming in the basins of eastern Mexico since the latest Middle Jurassic, and formed mostly between the late Lower Cretaceous and the Eocene, although their ages remain largely undetermined. The orogenic-metamorphic realm is known only to comprise orogenic gold deposits, which formed between the Upper Cretaceous and the Eocene in western and southwestern Mexico.Three main metallogenic epochs for the Mesozoic are recognized in Mexico: (I) Triassic to Middle Jurassic, (II) Upper Jurassic to Lower Cretaceous, or Oxfordian to Albian-Cenomanian, and (III) Upper Cretaceous to the Cretaceous-Cenozoic limit, or Albian-Cenomanian to Maastrichtian, which connects with the Cenozoic epochs. The Triassic to Middle Jurassic epoch basically comprises deposits in ultramafic–mafic complexes and VMS deposits in the Vizcaíno terrane. The metallogenic belts or provinces for Upper Jurassic to Lower Cretaceous epoch occur mostly in the Guerrero composite terrane (GCT) and in the Baja California Peninsula. These are: (1) Late Jurassic to earliest Cretaceous province of sedimentary and sedimentary-exhalative deposits in the domain of the Gulf of Mexico, (2) Late Jurassic to earliest Cretaceous province of VMS deposits in the southern GCT, (3) Early Cretaceous province of VMS deposits in the Alisitos arc terrane, (4) Early Cretaceous province of deposits in ultramafic–mafic complexes on the southeastern border of the GCT, (5) Early Cretaceous province of generic MHIO deposits, and other deposits associated with calc-alkaline magmatism in the Peninsular Range batholithic belt (Baja California), along with (6) a few similar deposits in the coastal GCT, and (7) Cretaceous province of deposits in ultramafic–mafic complexes in the northern GCT. The Upper Cretaceous provinces are: (a) orogenic gold belts in NW Sonora and the southern Baja California Peninsula, which possibly constitute different belts, (b) the beginning of the metallogeny associated with the Sierra Madre Occidental silicic large igneous province (SLIP) during the Late Cretaceous to the Paleocene with generic MHIO deposits and deposits associated with calc-alkaline magmatism in Sonora and Chihuahua, (c) a trail of Late Cretaceous non-iron oxide skarn deposits along the northeastern and eastern (?) terrane boundaries of the GCT, (d) Late Cretaceous to Cenozoic province of magmatic-hydrothermal iron oxide deposits in the southern portion of the Pacific coast of Mexico, and (e) Late Cretaceous to Paleocene province of generic MHIO deposits and non-iron oxide skarn deposits in northern Baja California.The above configuration of metallogenic events and provinces occurred in response to drastic changes in the general tectonomagmatic regime: from magmatism associated with Jurassic to Albian synvolcanic extensional unroofing (magmatic oceanic realm for ore deposits) to a contractional continental volcanic arc thereafter (magmatic continental-subaerial realm for ore deposits). The transition from these metallogenic magmatic realms occurred between the mid-Aptian and mid-Cenomanian, and Mexico represents the last region in the Cordillera of western North America in which such transition occurred—it was predated by similar processes in Alaska and Canada between the Lower Jurassic and the Middle-Upper Jurassic, and in the southwestern USA between the Middle-Upper Jurassic and the Lower Cretaceous. Such southward and cratonward (eastward) progression was accompanied by orogenic pulses, which overlap in NE Mexico as thin-skinned Sevier structures were followed by thick-skinned Laramide structures, and were largely controlled by flattening of the subducted slabs. Laramide orogenic pulses played a key role in the formation of orogenic gold deposits, but are also responsible for the mobilization of basinal brines that ultimately led to the formation of MVT and red-bed deposits, although several of such deposits might have formed earlier, and others are post-orogenic.

Age and depositional environment of the “dinosaur graveyard” at Las Águilas, southern Coahuila, NE Mexico

Here we provide a detailed description of the upper Campanian sediment succession at Las Águilas, southern Coahuila, northeastern Mexico, including the first absolute age dating for this interval, paleoenvironmental reconstructions and taphonomic observations on the abundant dinosaur remains at the locality. Stratigraphic investigations of the dinosaur-bearing succession at the Las Águilas vertebrate fossil area near Porvenir de Jalpa reveal a diverse vertebrate assemblage, including dinosaurs, crocodilians and turtles. New findings in adjacent sites include eusuchian crocodylomorphs, four different kinds of turtles, dromaeosaurids, lambeosaurines, pterosaurs and elasmosaurid plesiosaurs. Strontium isotope measurements on fossil oyster shells provide an absolute age of 73±1Ma for the lower part of the Las Águilas section. The locality is thus of late Campanian age. The vertebrate, invertebrate and plant materials as well as the sediment structures observed in a 50m thick predominantly siliciclastic succession of the Cerro del Pueblo Formation suggest deposition in an extensive delta plain environment. The facies succession indicates a short-termed cyclicity of limnic, brackish and shallow marine environments during the late Campanian–early Maastrichtian Cerro del Pueblo Formation with numerous layers containing dinosaur fossil remains. ResumenAquí damos una descripción detallada de la sucesión de sedimentos del Campaniano Superior en Las Águilas, sur de Coahuila, noreste de México, incluyendo la primera datación absoluta para este intervalo, reconstrucciones paleoambientales y observaciones taxonómicas sobre la abundancia de restos de dinosaurios en la localidad. Investigaciones estratigráficas de los estratos con dinosaurios en el área fosilífera de Las Águilas cerca de Porvenir de Jalpa revelan un ensamblaje vertebrado diverso, incluyendo dinosaurios, cocodrilos y tortugas. Nuevos descubrimientos en sitios adyacentes incluyen crocodylomorfos eusuquios, cuatro diferentes tipos de tortugas, dromaeosauridos, lambeosaurinos, pterosaurios, y plesiosaurios elasmosauridos. Mediciones de isótopos de estroncio en conchas de ostiones fósiles proveen una edad absoluta de 73±1 millones de años para la parte inferior de la sección de Las Águilas. La localidad es por lo tanto de edad Campaniana tardía. El material de vertebrados, invertebrados y de plantas así como las estructuras sedimentarias observadas en una sucesión predominantemente siliciclástica de 50m de la Formación Cerro del Pueblo sugieren una depositación en un ambiente de una extensa planicie deltáica. La sucesión de facies indica una ciclicidad de periodo corto de ambientes limnicos, estuarinos y marinos someros durante el Campaniano tardío-Maastrichtiano temprano de la Formación Cerro del Pueblo con numerosos estratos conteniendo fósiles de dinosaurios.Palabras Clave: Las Águilas, sucesión de dinosaurios, Campaniano superior, ambientes deposicionales, Formación Cerro del Pueblo, Coahuila.

Fracturing and fluid flow in a sub-décollement sandstone; or, a leak in the basement

Crack-seal texture within fracture cements in the Triassic El Alamar Formation, NE Mexico, shows that the fractures opened during precipitation of quartz cements; later, overlapping calcite cements further occluded pore space. Previous workers defined four systematic fracture sets, A (oldest) to D (youngest), with relative timing constrained by crosscutting relationships. Quartz fluid inclusion homogenization temperatures are higher within Set B (148 ± 20°C) than in Set C (105 ± 12°C). These data and previous burial history modelling are consistent with Set C forming during exhumation. Fluid inclusions in Set C quartz have higher salinity than those in Set B (22.9 v. 14.2 wt% NaCl equivalent, respectively), and Set C quartz cement is more enriched in 18 O (20.2 v. 18.7‰ VSMOW). Under most assumptions about the true temperature during fracture opening, the burial duration, the amount of cement precipitated and fluid-flow patterns, it appears that the fracture fluid became depleted in 18 O and enriched in 13 C. This isotopic evolution, combined with increasing salinity, suggests that throughout fracture opening there was a gravity-driven influx of fluid from upsection Jurassic evaporites, which form a regional décollement. Fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading, rather than increases in fluid pressure.

High spatial resolution geochemistry and textural characteristics of ‘microtektite’ glass spherules in proximal Cretaceous–Paleogene sections: Insights into glass alteration patterns and precursor melt lithologies

Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we have conducted spatially resolved trace element analysis on fresh, unaltered microtektite glasses linked to the Cretaceous–Paleogene (K–Pg) boundary Chicxulub crater and on their surrounding alteration phases. This unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process. In addition, two-dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling. Glasses from two localities in opposite quadrants from the source crater were studied. At the Beloc locality (Haiti), the glass population is dominated by the presence of yellow high-Ca glass and black andesitic glass formed by admixture of carbonate/dolomite/anhydrite platform lithologies with crystalline basement. These glasses alter according to the well-established hydration–palagonitization model postulated for mafic volcanic glasses. REEs become progressively leached from the glass to below the detection limit for the applied spot size, while immobile Zr, Hf, Nb, and Ta passively accumulate in the process exhibiting both inter-element ratios and absolute concentrations similar to those for the original glass. In contrast, The Arroyo El Mimbral locality (NE Mexico) is characterized by abundant green glass fragments high in Si, Al and alkalis, and low in Mg, Ca, Fe. Low Si black glass is less abundant though similar in composition to the black glass variety at Beloc. The alteration pattern of high-Si, Al green glass at the Mimbral locality is more complex, including numerous competing reaction processes (ion-exchange, hydration, dissolution, and secondary mineral precipitation) generally controlled by the pH and composition of the surrounding fluid. All green, high-Si, Al glasses are hydrated and variably enriched in Sr, Ba, and Cs, indicating preferred adsorption from seawater during hydration. Despite the onset of ion-exchange reactions, which only seem to have affected the alkalis, the trace element composition of the green high-Si, Al glass is still largely representative of the original melt composition. Refining the geochemical signature of (altered) melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation.

Bottom topography, recent sedimentation and water volume of the Cerro Prieto Dam, NE Mexico

The Cerro Prieto dam, a relatively small water reservoir in NE Mexico, is one of the main resources of potable water for Monterrey, a city with a population of about fourmillion inhabitants. A high resolution seismic study using non-linear parametric sub-bottom echo-sounder SES-2000 was carried out in this water reservoir. High resolution acoustic data interpretation shows that the thickness of recent sediments due to siltation of the reservoir reaches 3.5-4.0m. It shows a high recent sedimentation rate (1-2 up to 14cm/ year). Based on the echo-sounder data, the first bathymetric map and a digital model of recent sediment thickness were designed. A significant (5-12%) difference between the volume capacity value used by National Commission of Water (CNA) and acoustic survey results was revealed. Differences between the CNA and acoustic data indicate storage losses from 12-17 up to 30million cubic meters. The results obtained through study such as this one, could be useful to improve a water resources management.

Holocene-aged human footprints from the Cuatrociénegas Basin, NE Mexico

Two sets of well-preserved human footprints have been found in tufa sediments in the Cuatrociénegas Basin, NE Mexico, and here we present their U-series dates of 10.55 ± 0.03 ka and 7.24 ± 0.13 ka. The former are the oldest known footprints in Mexico, although their exact location is unknown, the latter form part of a trackway with eleven in situ human footprints. Oxygen (and to a lesser extent) carbon isotope data from the sediments suggest that the tufa with in situ footprints formed during a transition to a wetter (less arid) period, while pollen evidence indicates the basin floor presence of pecan (Carya) and willow (Salix sp.) before the onset of regional Chihuahuan Desert aridity. These footprints confirm the presence of humans, possibly nomadic hunter–gatherer groups, which persisted until the 18th Century AD.

Assessment of sources and fate of nitrate in shallow groundwater of an agricultural area by using a multi-tracer approach

Nitrate isotopic values are often used as a tool to understand sources of contamination in order to effectively manage groundwater quality. However, recent literature describes that biogeochemical reactions may modify these values. Therefore, data interpretation is difficult and often vague. We provide a discussion on this topic and complement the study using halides as comparative tracers assessing an aquifer underneath a sub-humid to humid region in NE Mexico. Hydrogeological information and stable water isotopes indicate that active groundwater recharge occurs in the 8000km2 study area under present-day climatic and hydrologic conditions. Nitrate isotopes and halide ratios indicate a diverse mix of nitrate sources and transformations. Nitrate sources include organic waste and wastewater, synthetic fertilizers and soil processes. Animal manure and sewage from septic tanks were the causes of groundwater nitrate pollution within orchards and vegetable agriculture. Dairy activities within a radius of 1000m from a sampling point significantly contributed to nitrate pollution. Leachates from septic tanks caused nitrate pollution in residential areas. Soil nitrogen and animal waste were the sources of nitrate in groundwater under shrubland and grassland. Partial denitrification processes helped to attenuate nitrate concentration underneath agricultural lands and grassland, especially during summer months.

Chicxulub impact spherules in the North Atlantic and Caribbean: age constraints and Cretaceous–Tertiary boundary hiatus

AbstractThe Chicxulub impact is commonly believed to have caused the Cretaceous–Tertiary boundary mass extinction and a thin impact spherule layer in the North Atlantic and Caribbean is frequently cited as proof. We evaluated this claim in the seven best North Atlantic and Caribbean Cretaceous–Tertiary boundary sequences based on high-resolution biostratigraphy, quantitative faunal analyses and stable isotopes. Results reveal a major Cretaceous–Tertiary boundary unconformity spanning most of Danian subzone P1a(1) and Maastrichtian zones CF1–CF2 (~400 ka) in the NW Atlantic Bass River core, ODP Sites 1049A, 1049C and 1050C. In the Caribbean ODP Sites 999B and 1001B the unconformity spans from the early Danian zone P1a(1) through to zones CF1–CF4 (~3 Ma). Only in the Demerara Rise ODP Site 1259B is erosion relatively minor and restricted to the earliest Danian zone P0 and most of subzone P1a(1) (~150 ka). In all sites examined, Chicxulub impact spherules are reworked into the early Danian subzone P1a(1) about 150–200 ka after the mass extinction. A similar pattern of erosion and redeposition of impact spherules in Danian sediments has previously been documented from Cuba, Haiti, Belize, Guatemala, south and central Mexico. This pattern can be explained by intensified Gulf stream circulation at times of climate cooling and sea level changes. The age of the Chicxulub impact cannot be determined from these reworked impact spherule layers, but can be evaluated based on the stratigraphically oldest spherule layer in NE Mexico and Texas, which indicates that this impact predates the Cretaceous–Tertiary boundary by about 130–150 ka.