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

Mesozoic volcanic rocks outcrop in North Patagonian Andes, near Trevelin locality, Chubut province, Argentina. Basaltic andesites and basalts with porphyritic to aphyric textures with calcium plagioclase, clinopyroxene, and olivine phenocrysts are the most common rocks. Their groundmass texture is felty to intergranular. These units are affected by a hydrothermal alteration that replaced their phenocrystals and groundmass and filled veins, veinlets and amygdales. Minerals were analyzed by conventional microscopy, scanning electron microscopy-energy dispersive scan, X- ray diffraction, infrared spectroscopy, and fluid inclusions microthermometry. Albite, prehnite, pumpellyite, epidote, clinozoisite, smectite/chlorite, laumontite, wairakite, mordenite, titanite, quartz, and calcite are the secondary minerals. Textural relationship between secondary minerals and their chemical compositions evidence a very low-grade metamorphism in the high-zeolite facies, transitional to prehnite-pumpellyite facies. P-T conditions of the metamorphic event were <260°C and <1.5 kb. Moreover, feathery-flamboyant quartz, adularia, gold, and electrum fill other veinlets as the result of a low sulphidation epithermal system. Volcanism, emplacement of granitoids and extensional events took place in the volcanic arc located in the western sector. These processes favoured fluid flow and gave a high temperature geothermal gradient. These conditions would be favourable for formation of a low-sulphidation epithermal assemblage and typical mineral assemblages of very low-grade metamorphism, as an alkaline hydrothermal alteration. 

Bed-by-bed sampling of ammonites from clayey to calcareous siltstones of the basal part of the Santiago Formation at the type section in Tamán, San Luis Potosí (Mexico) is first reported. Ammonites belonging to assemblages described by Burckhardt (1912), usually interpreted as Middle to Upper Oxfordian, are restricted to the lower Middle Oxfordian Plicatilis Zone, Antecedens Subzone. Perisphinctes (Dichotomosphinctes) durangensis (Burckhardt, 1912) is revised and its biostratigraphic interpretation constrained by its co-occurrence with Gregoryceras. Evidence for the occurrence of Perisphinctes (Perisphinctes), Perisphinctes (Otosphinctes), and Passendorferia or Sequeirosia is first reported from Mexico within a precise biostratigraphic context. Burckhardt’s Oxfordian ochetoceratins are re-examined and their affinity with Cuban counterparts is approached. From a paleobiogeographical point of view, metapopulation dynamics and potentially involved events (e.g., temporary, ancillary, selective vicariancy and subsequent ecological capture) well apply for understanding the biogeographical significance of the ammonite assemblies described.

An integrated petrographic and geochemical study of the Ahwaz Sandstone Member of Oligocene- Miocene age, Asmari Formation in Zagros, southwest Iran, was carried out to infer their provenance and tectonic setting. This study is based on the analysis of core samples from three subsurface sections (wells Az-85, Az-11, and Az-89) in Ahwaz oil field in the Dezful embayment subzone.On the basis of the framework composition (point counting) and whole-rock geochemistry (major elements), the sandstones are classified as quartzarenite, sublitharenite, and subarkose types. Petro- graphic studies reveal that these sandstones contain quartz, feldspars and fragments of sedimentary and metasedimentary rocks. The modal analysis data of 50 collected (medium size and well sorted) samples, imply a recycled orogen and craton interior tectonic provenance. Moreover, petrographic point count data indicate quartz-rich sedimentary (recycled), middle to high-grade metamorphic, and plutonic par- ent rocks for the Ahwaz Sandstone. Tectonic setting discrimination diagrams based on major elements suggest a quartzose sedimentary provenance in a passive continental margin. As indicated by the CIW´ index (chemical index of weathering) of the Ahwaz Sandstone (average value of 67) their source area underwent “intense” recycling but “moderate” degree of chemical weathering. The petrography and geochemistry results are consistent with a semiarid climate and low-relief highlands.

Petrography, major (including four trace elements), stable isotopes (carbon and oxygen), and 87Sr/86Sr geochemistry of limestones of the Shahabad Formation, Bhima basin, Karnataka, southern India are reported. These limestones show a narrow range of δ13C (~1.34−1.96‰) and δ18O (~ -6.04 to -7.61‰) values. The petrographic study reveals the presence of microsparite and micro- and macrostylolites. The δ13C and 87Sr/86Sr values indicate that these limestones were deposited during the late Neoproterozoic age and the δ18O values also are very similar to the average Proterozoic carbonate values. Mn and Sr concentrations and low Mn/Sr ratio (<1) together with the stable and radiogenic isotope data suggest that the studied samples are well-preserved or scarcely altered limestones and probably have retained their primary isotopic signatures.  

This paper presents the basic principles behind “An Earth Scientist’s Periodic Table of the Elements and Their Ions”, originally published in Railsback, L.B., 2003, An Earth scientist’s periodic table of the elements and their ions, Geology, 31(9): 737-740. In contrast to Mendelejeff ’s periodic table, where all elements are classified according to their ground state (or oxidation state = 0), the Periodic Table of the Elements and Their Ions, classifies elements and ions according to their natural oxidation state. Consequently, some elements are displayed in several positions within the table, and some others have been relocated.The classification of the ions according to their oxidation state allows the visualization of trends based upon intrinsic characteristics of each ion (such as polarizability and ionic potential) that evidence the biogeochemical behavior of the elements and their ions. Many of those trends were only semi-empirically inferred until now. Reaction paths for different ions are deducted from their polarizability, whereas the ionic potential allows to infer the behavior of the ions under diverse geochemical differentiation processes.It is shown that the interaction of different cations with the oxide ion (O2-) plays a pivotal role in most processes of geochemical differentiation, such as aqueous geochemistry, weathering, igneous petrogenesis, among others. Because of the wide range of applications, The Periodic Table of The Elements and Their Ions is a valuable tool for the earth scientist.

The southeastern Gulf of Mexico contains the Cuban Exclusive Economic Zone where search for oil and gas is currently in progress. The geological relationships among the low mountains in western Cuba (Cordillera de Guaniguanico) and the SE Gulf are poorly known. Upper Jurassic – Paleocene sections in the Cordillera belong to the North American paleomargin. The paleomargin rocks are present along northern Cuba and contain its main oil and gas deposits. These sections were thrusted northward during the early Cenozoic Cuban orogeny. In Cordillera de Guaniguanico, the uppermost Paleocene and lowermost Eocene are olistostromes and turbidites, settled in a foreland basin in front of the thrusts. The southern flank of the basin was destroyed and imbricated within the thrust pile during the Late Paleocene – Early Eocene orogeny, whereas the northern mildly deformed part lies in the SE Gulf. Seismic data from the SE Gulf suggest lower Paleogene clastic deposits increasing in thickness toward Cuba. The close correlation among the Mesozoic sections in the southeastern Gulf of Mexico and western Cuba highlands points to an original juxtaposition. Therefore, a palinspastic reconstruction of the Cordillera de Guaniguanico sections becomes key to understand the geological history of the Caribbean – Gulf of Mexico transition.Five major tectonic units, each one with its own stratigraphic sections are distinguished in western Cuba. In the estimation of the horizontal movement for each unit, a multilateral, dialectic approach is applied, with a mutual control among geometric-structural and geological data. The NNW horizontal movement estimate (kilometres) for each main sheet was the following: Sierra de los Organos unit: 12.5–25; Alturas de Pizarras del Sur unit: 40–50; Alturas de Pizarras del Norte: 57.5–67.5; Sierra del Rosario/Esperanza unit: 92.5–111.5 and Pan de Guajaibón unit > 92.5–111.5. The proto-Caribbean lithosphere (ophiolite and volcanic arc sections) is represented by the Bahía Honda unit, with minimum horizontal displacement of 122. 5–141.5 km. A proposal of palinspastic reconstruction is developed and the original position of each tectonic unit restored. The minimum area of the Cordillera de Guaniguanico tectonic pile in the sedimentary basin was between 21,000 and 25,000 square kilometers. The primary thrust sheet volume probably was between 42,000 and 100,000 cubic kilometres.

In this final paper of a series of four, using our well-tested simulation procedure we report new, precise, and accurate critical values or percentage points (with four to eight decimal places) of 15 discordancy tests with 33 test variants, and each with seven signi ficance levels a = 0.30, 0.20, 0.10, 0.05, 0.02, 0.01, and 0.005, for normal samples of very large sizes n from 1,000 to 30,000,viz.,1,000(50)1,500(100)2,000(500)5,000(1,000)10,000(10,00)30,000, i.e., 1,000 (steps of 50) 1,500 (steps of 100) 2,000 (steps of 500) 5,000 (steps of 1,000) 10,000 (steps of 10,000) 30,000. The standard error of the mean is also reported explicitly and individually for each critical value. As a result, the applicability of these discordancy tests is now extended to practically all sample sizes (up to 30,000 observations or even greater). This final set of critical values for very large sample sizes would cover any present or future needs for the application of these discordancy tests in all fields of science and engineering. Because the critical values were simulated for only a few sample sizes between 1,000 and 30,000, six different regression models were evaluated for the interpolation and extrapolation purposes, and a combined natural logarithm-cubic model was shown to be the most appropriate. This is the first time in the literature that a log-transformation of the sample size n before a polynomial fit is shown to perform better than the conventional linear to polynomial regressions hitherto used. We also use 1,402 unpublished datasets from quantitative proteomics to show that our multiple-test method works more efficiently than the MAD_Z robust outlier method used for processing these data and to illustrate thus the usefulness of our final work on these lines.

In two earlier papers (Verma and Quiroz-Ruiz, 2006, Rev. Mex. Cienc. Geol., 23, 133-161, 302-319) precise critical values for normal univariate samples of sizes n up to 100 have been reported. However, for greater n, critical values are available only for a few tests: N1 for n up to 147, N4k2 for n up to 149, N6, N14 and N15 (for the latter three tests, critical values were reported for only n=200, 500, and 1000). This clearly demonstrates the need for proposing new critical values for n>100 through an adequate statistical methodology. Therefore, modifications of our earlier simulation procedure as well as new, precise, and accurate critical values or percentage points (with four to eight decimal places; average standard error of the mean ~0.00000003–0.0039) of 15 discordancy tests with 33 test variants, and each with seven significance levels α = 0.30, 0.20, 0.10, 0.05, 0.02, 0.01, and 0.005, for normal samples of sizes n up to 1000, viz., nmin (1)100(5)200(10)500(20)1000, are reported. For the first time in the literature, the standard error of the mean is also reported explicitly and individually for each critical value. Similarly, a new methodology involving artificial neural network (ANN) was used, for the first time in published literature, to obtain interpolation equations for all 33 discordancy test variants and for each of the seven significance levels. Each equation was fitted using 76 simulated data for n from 100 to 1000 for a given test and significance level. Extremely small sums of squared residuals (~5.5×10-8 – 8.4×10-5; generally <10-5) in the ANN equations fitted for n=100 to 1,000 were obtained. As a result, the applicability of these discordancy tests is now extended up to 1000 observations of a particular parameter in a statistical sample. The new most precise and accurate critical values will result in more reliable applications of these discordancy tests than have been possible so far in various scientific and engineering fields, particularly for quality control in Earth Sciences. The multiple-test method with new critical values was shown to perform better than both the box-and-whisker plot and the “two standard deviation” methods used by some researchers, and is therefore the recommended procedure for handling experimental data.

During the great 3 May 1887 Sonoran earthquake (surface rupture end-to-end length: 101.8 km; MW = 7.5±0.3), an array of three north-south striking Basin and Range Province faults (from north to south Pitáycachi, Teras, and Otates) snapped along the western margin of the Sierra Madre Occidental plateau. My detailed field survey of the Teras fault and the 1887 earthquake rupture zone along this fault included mapping the rupture scarp and measuring surface deformation at 27 sites.The Teras fault is ~20 km long, strikes N12°E, and has an average dip of 62°W; its maximum throw is >1,640 m and its long-term slip rate >0.08 mm/yr. The fault is structurally simple; it is not internally segmented along strike and does not branch. Striation measurements and the style of faulting indicate extensional dip-slip without significant lateral displacement. In the north, a 2.5-km wide, unbreached right step-over separates the Pitáycachi and Teras faults, whereas the southern limit of the Teras fault is a partly breached relay ramp, where the fault trace shows a 60° bend and jogs to the west. Here, the displacement of the Teras fault is transferred onto the Otates fault and the normal faults bounding the Iglesitas horst. The 1887 surface rupture along the Teras fault (end-to-end length 19.9 km) generally coincides with the mapped trace of the Teras Basin and Range Province fault. Based on 27 measurements, the maximum surface offset is 184 cm and the mean offset 112 cm. The along-rupture surface offset distribution is asymmetric, with the maximum near the southern end of the segment. This suggests that the Teras and Otates segments could be part of a single continuous 1887 rupture that stepped across the structurally complex basement ridge between them. A rough estimate of the average recurrence interval of 1887-size earthquakes on the Teras fault can be obtained from the average slip rate since 23 Ma and the amount of slip on this segment during the 1887 earthquake; the resulting values are 15 to 26 kyr.

The Nevado de Toluca is a quiescent volcano located in the central sector of the Trans-Mexican Volcanic Belt, 80 km southwest of Mexico City. The activity began ca. 2.6 Ma ago, with andesitic to dacitic lava flows and domes that lasted until 1.15 Ma. During the last 42 ka, the volcano has been characterized by different eruptive styles, including five dome collapses dated at 37, 32, 28, 26, and 13 ka and five plinian eruptions at 42 ka, 36 ka, 21.7 ka, 12.1 ka and 10.5 ka.The 13 ka dome destruction is the youngest event of this type, and originated a 0.11 km3 block-and-ash flow deposit on the northeastern sector of the volcano, here named El Refugio flow. The deposit consists of two facies: channel-like, up to 10 m thick, monolithologic, that is composed of up to five units, with decimetric dacitic clasts set in a sandy matrix; and a lateral facies that consists of a gray, sandy horizon, up to 4 m thick. A 30 cm-thick surge layer lies down at the base of the sequence. The main component is a dacitic lava, with variable degree of vesciculation, with mineral association of Pl-Hbl-Opx. Stratigraphic and petrographic features indicate that the dome was quickly extruded on the summit of the volcano, and its collapse was accompanied by an explosive component. The magmatic process that probably triggered the eruption was an overheating of the magma chamber that induced a self-mixing mechanism yielding to an overpressurization of the system. Finally, the identification of an explosive component associated with dome destruction events at Nevado de Toluca volcano clearly indicate the high risk that a future event with such characteristics can represent for populated areas around the volcano.