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Este estudio presenta una actualización del registro isotópico de la Formación de Tamengo, parte superior del Grupo Corumbá, que se encuentra en la parte meridional del Cinturón Paraguayo. Este trabajo incluye cortes estratigráficos detallados enlazados con la geoquímica de alta resolución δ13C y δ18O de dos yacimientos mineros cercanos a Corumbá (Mato Grosso do Sul, Brasil).

El Ediacárico terminal inlcuye una serie de cambios drásticos en los ciclos biogeoquímicos, muchos de los cuales se asocian con modificaciones evolutivas en los registros fósiles correspondientes. Entre ellos destacan las condiciones redox, las cuales pueden haber causado un impacto profundo en la evolución animal temprana. Este trabajo destaca el significado de la lito-, bio- y quimioestratigrafía en la investigación geobiológica del tiempo profundo.

Con el propósito de llevar a cabo un estudio quimioestratigráfico integrado, orgánico e inorgánico, en la sección de la Formación Barco (Paleoceno) que aflora en el Río Lobaterita, San Pedro del Río, Edo. Táchira, Venezuela, fueron realizados varios análisis, los cuales incluyeron levantamiento y descripción litoestratigráfica de la sección, determinación de las concentraciones de carbono total (culombimétrico), elementos mayoritarios y minoritarios mediante ICP-OES y activación de neutrones, extracción de bitumen, separación SARA y análisis de las fracciones saturadas y aromáticas mediante cromatografía de gases acoplada a espectrometría de masas. Los resultados indican que la unidad bajo estudio es pobre en Rev. Téc. Ing. Univ. Zulia. Vol. 30, Edición Especial, 2007 materia orgánica y térmicamente sobremadura, y que la sección se depositó sobre una plataforma interna somera en la que se reflejan cambios eustáticos con características episódicas y posiblemente cíclicas. En la plataforma ocurrieron dos grandes variaciones en la hidrodinámica del medio sedimentario: un periodo regresivo y uno transgresivo. El tipo de materia orgánica varía de terrestre en la parte basal de la sección hacia mixta y marina en las zonas intermedias para volver a ser terrestre hacia el tope de la misma. La unidad está compuesta por una secuencia estratigráfica completa y una parcial, donde ocurrieron al menos dos cambios en el nivel del mar.In order to achieve an integrated organic-inorganic chemostratigraphic study in Paleocene Barco Formation in the Lobaterita River, San Pedro del Río, Edo. Táchira, Venezuela, several analyses were carried out, those including lithostratigraphic description, bulk rock sampling, columbymetric COT determination, major and minor elements concentration by ICP-OES and neutron activation, bitumen extraction, SARA isolation, as well as saturated and aromatic fractions analysis by gas chromatography coupled to mass spectrometry. Results indicate that the unit under study is poor in organic matter and thermally overmature, and was deposited in internal shallow shelf environment. Section shows eustatic changes evidences, which occurred with episodic and possibly cyclical characteristics. Two major changes were inferred in hydrodynamics of the sedimentary environment: a regressive event and a transgressive one. The organic matter is terrestrial in the basal part of the section changing towards mixed/marine in the intermediate zones, finally terrestrial in the upper part of the section. The unit is composed by a complete stratigraphic sequence and partial one, where at least two changes occurred in the level of the sea.

C-isotope and 87Sr/86Sr values for five carbonate successions from the São Caetano Complex, northeastern Brazil, were used to constrain their depositional age and to determine large variations in the C- and Sr-isotopic composition of seawater under the framework of global tectonic events. Three C-isotope stages were identified from base to top in a composed chemostratigraphic section: (1) stage in which delta13C values vary from +2 to +3.7‰ PDB and average 3‰ PDB, (2) stage with delta13C values displaying stronger oscillations (from -2‰ to +‰ PDB), and (3) stage with an isotopic plateau with values around +3.7‰ PDB. Constant 87Sr/86Sr values (~ 0.70600) characterize C-isotope stage 1, whereas slightly fluctuating values (from 0.70600 to 0.70700) characterize C-isotope stage 2. Finally, 87Sr/86Sr values averaging 0.70600 characterize C-isotope stage 3. The C- and Sr- chemostratigraphic pathways permit to state: (a) the C- and Sr-isotope secular curves registered primary fluctuations of the isotope composition of seawater during late Mesoproterozoic- early Neoproterozoic transition in the Borborema Province, and (b) onset of the Cariris Velhos/Greenville cycle, widespread oceanic rifting, continental magmatic arc formation and onset of the agglutination of Rodinia supercontinent, mostly controlled the C- and Sr-isotope composition of seawater during the C-isotope stages 1, 2 and 3.Valores de isótopos de C e 87Sr/86Sr de cinco seqüências de carbonatos do Complexo São Caetano, nordeste do Brasil; foram usados para estimar a sua idade de deposição e relacionar variações da composição isotópica na água do mar com eventos tectônicos globais. Três estágios de variação de isótopos de carbono foram identificados de base para o topo numa seção quimioestratigráfica composta: (1) estágio em que delta13C varia de +2 a +3.7‰PDB (media 3‰PDB), (2) estágio no qual delta13C varia consideravelmente (de -2 a +3‰PDB) e (3) estágio apresentando valores constantes de cerca de +3.7‰PDB. Valores de 87Sr/86Sr ~ 0.70600 caracterizam o estágio isotópico 1, tornando-se mais variáveis no estágio isotópico 2 (entre ~ 0.70600 e ~ 0.70700). Finalmente, valores em torno de 0.70600 caracterizam o estágio isotópico 3. A quimioestratigrafía de C e Sr permite concluir que: (a) as curvas de variação seculares dos carbonatos do Complexo São Caetano registram flutuações primárias na composição isotópica de C e Sr da água do mar durante a passagem Meso-Neoproterozóico na Província Borborema; (b) o ciclo orogênico Cariris velhos/Greenville, a ocorrência de rifteamento e formação de arcos magmáticos continentais, e a formação do supercontinente Rodinia, controlaram a composição isotópica de C e Sr da água do mar durante os estágios isotópicos 1, 2, 3.

Con el propósito de llevar a cabo un estudio quimioestratigráfico integrado, orgánico e inorgánico, en la sección de la Formación Barco (Paleoceno) que aflora en el Río Lobaterita, San Pedro del Río, Edo. Táchira, Venezuela, fueron realizados varios análisis, los cuales incluyeron levantamiento y descripción litoestratigráfica de la sección, determinación de las concentraciones de carbono total (culombimétrico), elementos mayoritarios y minoritarios mediante ICP-OES y activación de neutrones, extracción de bitumen, separación SARA y análisis de las fracciones saturadas y aromáticas mediante cromatografía de gases acoplada a espectrometría de masas. Los resultados indican que la unidad bajo estudio es pobre en Rev. Téc. Ing. Univ. Zulia. Vol. 30, Edición Especial, 2007 materia orgánica y térmicamente sobremadura, y que la sección se depositó sobre una plataforma interna somera en la que se reflejan cambios eustáticos con características episódicas y posiblemente cíclicas. En la plataforma ocurrieron dos grandes variaciones en la hidrodinámica del medio sedimentario: un periodo regresivo y uno transgresivo. El tipo de materia orgánica varía de terrestre en la parte basal de la sección hacia mixta y marina en las zonas intermedias para volver a ser terrestre hacia el tope de la misma. La unidad está compuesta por una secuencia estratigráfica completa y una parcial, donde ocurrieron al menos dos cambios en el nivel del mar.

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.