Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

In Buenos Aires coasts, sedimentary processes were particularly active during the Quaternary owing to eustatic fluctuations in sea level. As a result, during the late Quaternary transgressions, marine and marginal marine sediments were deposited in the coastal plain. In order to interpret these Holocene littoral sequences, we analyzed the distribution, diversity, species composition and taphonomic features of total benthic foraminifera assemblages from modern littoral settings, from the top of the dune to the lower shoreface, in two close but geomorphologically different transects located in the Atlantic coast of Northern Buenos Aires Province (Argentina, South America). Total benthic foraminiferal assemblages from subtidal and supratidal environments are distinguishable in terms of composition, diversity and taphonomic features. In upper shoreface, foreshore, backshore and foredune environments, assemblages are clearly dominated by three species: Buccella peruviana, Ammonia beccarii and Elphidium discoidale (the BAE group). This feature is the result of taphonomic processes that favor the selective preservation of such species. The study of taphonomic modifications of shells in modern assemblages allows a better discrimination between subenvironments than the analysis of taxonomic composition. Although Holocene assemblages have no strict counterparts between total modern assemblages, taxonomic composition and taphonomic modification of shells allow us to infer that the Holocene sequence was deposited between the upper shoreface and the backshore.

A comparative study of equid cranial and dental material, recovered from unnamed fluvio-lacustrine deposits of late Pleistocene age that crop out in southeastern Hidalgo, allowed to assign the fossil sample to Equus conversidens. The record gives additional evidence on the widespread distribution of this species throughout the North American subcontinent during the last two million years. On the other hand, we evaluated the dietary behavior of this Equus population from Hidalgo by the extended mesowear analysis method. The observed mesowear pattern is comparable to that of the recent grazer species Bison bison. These species display a particular combination of low occlusal relief and blunt cusps. This suggests that the horse population from Hidalgo was a grass feeder that incoporated abrasive food items into its diet (grass and/or extrinsic grit). The results provide evidence for the existence of local grazing habitats in central Mexico during the late Pleistocene; furthermore, the associated herbivore fauna indicates wooded areas as well, thus suggesting a heterogeneous habitat.

The Río Fuerte Formation, cropping out in Sinaloa state of northwestern Mexico, has been ascribed to the Middle to Late Ordovician based on its fossil content. U-Pb detrital zircon study in a rock sample of this unit yielded peaks at 521 Ma, 605 Ma and 881 Ma. Relative-age probability plots in the region suggest that the Río Fuerte Formation deposited in a basin located between Laurentia and an inactive peri-Gondwanan arc. Thermobarometric and petrographic studies of the Río Fuerte Formation indicate an initial low P/T or Buchan type metamorphic event. Structural analysis indicates that this event had a ~N-S shortening direction, and may be related to the collision of peri-Gondwanan blocks during the final amalgamation of Pangaea. In the same region, a granitic clast within an andesitic meta-agglomerate of the Topaco Formation yielded a 151 ± 1 Ma age, which predates a second tectono-metamorphic event. This event is ascribed to a Late Jurassic overprint in the El Fuerte region, which may be related to collision of an ophiolite block against the North American plate and is coeval with the Nevadan Orogeny of the North American Cordillera.

Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) is a rapidly growing technique that allows many different types of in-situ microanalyses to be performed in geological materials. One of the most used methodologies is U-Pb isotopic dating of accessory minerals such as zircon. These analyses can be performed at the scale of tenths of micrometers, in a rapid, cost-effective way and with very good precision and accuracy. It is critical, however, to perform the data reduction in a fast, transparent and customizable way that takes into account the specific analytical procedures employed in various laboratories and the outputs of different instruments. UPb.age is a freely available data reduction script, written and developed in R, a free statistical environment. The software can read, correct and reduce U-Pb isotopic data obtained by several LA-ICMPS instruments. Its main strengths are transparency, robustness, speed, and the ability to be readily customized and adapted to specific analytical procedures used in different laboratories.

A new fossil palm trunk Palmoxylon lametaei sp. nov. is described from the Lameta Formation (Upper Cretaceous) of Bhisi area of Nand inland basin, Nagpur District, Maharashtra, India. The stem is well preserved revealing all the anatomical features identifiable to the modern arecoid palm (Phoenix). The fossil plant is characterized by the presence of cortical, dermal, sub-dermal and central zones with profuse roots in the bark region, indicating a basal part of the stem. Presence of fibrous and diminutive bundles only in the outer part of the stem is significant while a gradual transformation from compact to lacunar condition of ground tissue from outer to inner part of the stem suggests that the plants thrived under aquatic environment.

En el artículo indicado arriba hubo un error en la asignación de números de catálogo de la Colección Nacional de Paleontología (Instituto de Geología, UNAM, México), por lo que deben cambiarse de la siguiente manera: DESCARGUE EL ARCHIVO PDF a Artículo original http://satori.geociencias.unam.mx/23-2/(4)Lopez.pdf 

The Quaternary Sierra Chichinautzin volcanic field (SCVF) is located at the volcanic front of the Trans-Mexican Volcanic Belt (TMVB), ~350 km from the Middle American trench where the Cocos plate subducts beneath the North American plate. The SCVF is characterized by more than 200 monogenetic centers, ranging in composition from rare basalts to dacites. Less evolved terms have aphyric to porphyritic textures with phenocrysts of euhedral olivine (± Cr-spinels inclusions) + cpx, whereas evolved terms have porphyritic textures with phenocrysts of pyroxene (cpx>opx), xenocrysts of corroded quartz, and sieve-textured plagioclase from the local basement. Regardless of the degree of magmatic evolution, plagioclase occurs as microphenocrysts or in the groundmass.New geochemical and isotopic data presented in this work are in good agreement with previous analyses; however, a new division for the SCVF mafic rocks (MgO ≥6.0 wt.%) is proposed. In fact, although concentrations of large ion lithophile elements (LILE) and light to medium rare earth elements (REE) are roughly constant in mafic rocks, TiO2 and other high field strength elements (HFSE) such as Nb are scattered and vary from 0.8 to 1.8 wt.% and from 5 to 30 ppm, respectively. These characteristics allow grouping the SCVF rocks in two main magmatic series with different TiO2-HFSE enrichment: high(H)-TiO2 and low(L)-TiO2, with subordinate transitional samples. Additionally, a cinder cone with shoshonitic affinity and extremely high LILE/HFSE ratio is reported for the first time in central Mexico. Compositional variability is also observed among the mineral phases of these mafic rocks (e.g., the Cr# of spinel hosted in olivine phenocrysts), and the 87Sr/86Sr ratios that progressively increase from the H-TiO2 series (0.70307–0.70425) to the L-TiO2 series (0.70365–0.70434), up to the shoshonitic scoria (0.70456).The calc-alkaline affinity, LILE, and Pb positive anomalies of the magmas confirm the occurrence of a subduction-related metasomatised mantle wedge beneath the studied area (particularly for the shoshonitic scoria), but enrichments in TiO2 and other HFSE are controversial. These enrichments are not common in subduction-related magmas because these elements are not easily removed by aqueous fluids from the subducted slab. However, recent works have demonstrated that HFSE can be relatively soluble in high-temperature fluids/melts arising from the slab, and the presence of TiO2-rich Cr-pargasite in lherzolite xenoliths of the Valle de Bravo area prove the existence of a mantle phase with high-HFSE concentrations. Therefore, partial melting of such hydrated peridotite could explain the genesis of H-TiO2 magmas as suggested by the proposed REE model.Moreover, isotopic ratios and variations in major and trace element concentrations of western SCVF rocks indicate that fractional crystallization plus crustal assimilation are the main evolution processes in the studied area.

The Cerro Mercado pluton, emplaced in the Coahuila fold belt, contains magmatic, ductile, and brittle fabrics that suggest local and regional deformation during igneous emplacement, later affected by regional deformation in the area. The Cerro Mercado pluton intruded, deformed and uplifted Upper Cretaceous strata in the southern edge of the central depression of the Sabinas basin. A hornblende mineral separate from the pluton yielded a 40Ar/39Ar plateau age of 44.29 ± 0.19 Ma (1σ), interpreted as an approximation to the crystallization age of the monzonite. A younger biotite total fusion age of 41.23± 0.21 Ma (1σ) from a different rock sample is interpreted to represent slow cooling (~80°C/Ma) of the Cerro Mercado pluton. Paleomagnetic analyses of the pluton yield well defined remanent magnetizations of nearly uniform reverse polarity, with a grand mean of D=178.2° and I=-61.7° (N=9 acceptable sites; k=57.5, α95 = 6.8°). These data are discordant with respect to the Eocene reference direction, indicating apparent clockwise rotation (~8°) and inclination steepening (~21°). The simplest interpretation of the discordance is that northwestward tilting of the pluton occurred during basin inversion within a weakly right lateral transpressive regime with NNW directed contraction. The strain field is consistent with contraction during the Laramide orogeny. Also, remanence acquisition is contemporaneous with both tectonic and magmatic fabrics in the pluton indicating that it is syntectonic. This suggests – in agreement with stratigraphic data – that the Laramide orogeny in central Coahuila ended some time after about 44 Ma. We propose that reactivation of the San Marcos fault and other basement faults in the Sabinas basin represents the last manifestation of shortening produced by the Laramide orogeny. This was a short episode of deformation that ended by the time of emplacement of younger plutons of the Candela- Monclova magmatic belt ca. 41 Ma. This indicates that culmination of the Laramide orogeny in central Coahuila is only slightly younger than in central Chihuahua.

We had not clarified that in the above mentioned paper as well as two later papers [Rev. Mex. Cienc. Geol. 25 (2008): 82-96 and 369-381], the critical values reported for test N14 (skewness or third moment test) were of the one-sided type. They were simulated from the actual value of the statistic TN14 and repetitions of 100,000 in 2006 or 1,000,000 in 2008. Using the absolute value of the statistic TN14, we have recently simulated more precise and accurate two-sided critical values (2,000,000 repetitions) for confidence levels of 70%, 80%, 90%, 95%, 98%, 99%, and 99.5% (or equivalently, significance level α of 0.30, 0.20, 0.10, 0.05, 0.02, 0.01, and 0.005, as well as for sample sizes of 5(1)100(5)200(10)500(20)1000(50)1500(100)2000(500)5000(1000)10000(5000)30000. These two-sided critical values are available along with their individual error estimates as txt or Excel files, to anyone interested on request to any of the authors.

ABSTRACT This paper presents an integrated analysis of data from calcareous nannofossils, foraminifera and geophysical logs from two exploratory wells in the Macuspana basin, southeastern Gulf of Mexico. This combination permits to propose a sequence stratigraphy framework for the studied wells. Biostratigraphic resolution and reliability increase with the integration of results from both fossil groups. On well A, the following species are used: Helicosphaera kamptneri, Reticulofenestra pseudoumbilica, Minylitha convallis, Discoaster hamatus, Globorotalia obesa and Globorotalia mayeri. On well B, the useful species are: Calcidiscus macintyrei, Reticulofenestra pseudoumbilica, Discoaster quinqueramus and Globorotalia mayeri. In well A, three stratigraphic sequence candidates are recognizable, and they are correlatable with the third order eustatic cycles TB-2.6, TB-3.1 and TB-3.5?. In well B, five stratigraphic sequence candidates are recognized, which are correlatable with the third order eustatic cycles TB-2.6, TB-3.1, TB-3.2, TB-3.3 and TB-3.6?. Results indicate that the studied sections were deposited from middle Miocene to Pleistocene. The biostratigraphic results, combined with sequence stratigraphy, will help to propose a sequence stratigraphy model.