| 1. |
- Bagherbandi, Mohammad, et al.
(författare)
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Effect of the lithospheric thermal state on the Moho interface : a case study in South America
- 2017
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Ingår i: Journal of South American Earth Sciences. - : Elsevier BV. - 0895-9811 .- 1873-0647. ; 76, s. 198-207
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Tidskriftsartikel (refereegranskat)abstract
- Gravimetric methods applied for Moho recovery in areas with sparse and irregular distribution of seismic data often assume only a constant crustal density. Results of latest studies, however, indicate that corrections for crustal density heterogeneities could improve the gravimetric result, especially in regions with a complex geologic/tectonic structure. Moreover, the isostatic mass balance reflects also the density structure within the lithosphere. The gravimetric methods should therefore incorporate an additional correction for the lithospheric mantle as well as deeper mantle density heterogeneities. Following this principle, we solve the Vening Meinesz-Moritz (VMM) inverse problem of isostasy constrained by seismic data to determine the Moho depth of the South American tectonic plate including surrounding oceans, while taking into consideration the crustal and mantle density heterogeneities. Our numerical result confirms that contribution of sediments significantly modifies the estimation of the Moho geometry especially along the continental margins with large sediment deposits. To account for the mantle density heterogeneities we develop and apply a method in order to correct the Moho geometry for the contribution of the lithospheric thermal state (i.e., the lithospheric thermal-pressure correction). In addition, the misfit between the isostatic and seismic Moho models, attributed mainly to deep mantle density heterogeneities and other geophysical phenomena, is corrected for by applying the non-isostatic correction. The results reveal that the application of the lithospheric thermal-pressure correction improves the RMS fit of the VMM gravimetric Moho solution to the CRUST1.0 (improves ∼ 1.9 km) and GEMMA (∼1.1 km) models and the point-wise seismic data (∼0.7 km) in South America.
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| 2. |
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| 3. |
- Domínguez-Carretero, Diego, et al.
(författare)
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Ultramafic-hosted volcanogenic massive sulfide deposits from Cuban ophiolites
- 2022
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Ingår i: Journal of South American Earth Sciences. - : Elsevier. - 0895-9811 .- 1873-0647. ; 119
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Tidskriftsartikel (refereegranskat)abstract
- Ultramafic-hosted volcanogenic massive sulfide deposits (UM-VMS) located in the Havana-Matanzas ophiolite (Cuba) are the only known example of this type of mineralization in the Caribbean realm. UM-VMS from Havana-Matanzas are enriched in Cu, Ni, Co, Au, and Ag. The mineralization consists of massive sulfide bodies mostly composed of pyrrhotite and hosted by serpentinized upper mantle peridotites. Chemical composition of unaltered cores in Cr-spinel grains found within the massive sulfide mineralization and in the peridotite host indicates formation in the fore-arc region of the Greater Antilles volcanic arc. A first stage of serpentinization probably took place prior to the sulfide mineralization event. The UM-VMS mineralization formed by the near-complete replacement of the silicate assemblage of partially serpentinized peridotites underneath the seafloor. The sequence of sulfide mineralization has been divided into two stages. The first stage is characterized by a very reduced hydrothermal mineral assemblage consisting of pyrrhotite, Co–Ni–Fe diarsenides, chalcopyrite, Co-rich pentlandite, and electrum. In the second stage, pyrite and Co–Ni–Fe sulfarsenides partially replaced pyrrhotite and diarsenides, respectively, under a more oxidizing regime during the advanced stages of ongoing serpentinization. The proposed conceptual genetic model presented here can be useful for future exploration targeting this type of deposit in the Caribbean region and elsewhere.
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| 4. |
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| 5. |
- Goldberg, K., et al.
(författare)
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Diagenesis of Paleozoic playa-lake and ephemeral-stream deposits from the Pimenta Bueno Formation, Siluro-Devonian (?) of the Parecis Basin, central Brazil
- 2011
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Ingår i: Journal of South American Earth Sciences. - : Elsevier BV. - 0895-9811 .- 1873-0647. ; 32:1, s. 58-74
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Tidskriftsartikel (refereegranskat)abstract
- The Parecis Basin is a large intracratonic rift located in central Brazil and filled with Paleozoic carbonate, evaporite and siliciclastic sediments. The occurrence of gas seeps has recently attracted significant exploration interest by the Brazilian petroleum agency and by Petrobras. The continuously cored PB-01-RO well provided the first opportunity to study the depositional environments, diagenetic evolution and hydrocarbon potential of the largely unknown sedimentary successions of the Parecis Basin. The cored lithologies, belonging to the Siluro-Devonian (?) Pimenta Bueno Formation, are interpreted as deposited in playa-lake and ephemeral-stream environments. The deposits display a strong fades control on the diagenetic mineral assemblages and evolution. Diagenetic minerals in the ephemeral-stream deposits include eogenetic hematite and smectitic clay coats and quartz cement, and the mesogenetic process includes precipitation of sulfates (anhydrite and barite) and carbonates (calcite, dolomite and kutnahorite-ankerite-huntite), followed by partial dissolution of these carbonates and sulfates, and of feldspar grains. Telogenetic processes include the precipitation of hematite and kaolinite within secondary pores, and the replacement of anhydrite by gypsum. A second burial phase and mesodiagenesis is indicated by the precipitation of discrete K-feldspar crystals within moldic pores after dissolved feld-spars, and by the illitization of etched, telogenetic kaolinite. The playa-lake deposits show early diagenetic dolomitization of lime mud, precipitation of anhydrite nodules and extensive silicification. The anhydrite nodules were replaced by gypsum and chalcedony during telodiagenesis. Potential source rocks are locally represented by organic shales. The fluvial sandstones show fair reservoir quality and limited compaction, as indicated by their intergranular volume, suggesting that the succession has undergone moderate burial. Potential seals for hydrocarbon accumulations are provided by the evaporites and playa-lake mudstones. The presence of an active petroleum system in the basin, as indicated by the gas seeps, suggests a good hydrocarbon exploration potential for the Parecis Basin, gas and stresses the importance of studying the depositional and diagenetic evolution of its sedimentary successions.
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| 6. |
- Kulhánek, Ota, et al.
(författare)
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Variations of b-values preceding large earthquakes in the shallow subduction zones of Cocos and Nazca plates
- 2018
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Ingår i: Journal of South American Earth Sciences. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0895-9811 .- 1873-0647. ; 82, s. 207-214
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Tidskriftsartikel (refereegranskat)abstract
- The potential of b-value variations as a medium-term (months, years) precursor was investigated by focusing on the eight largest earthquakes, M-w >= 7.0, occurring between January 2000 and April 2010 in the shallow subduction zones of Cocos and Nazca plates. The available ISC and NEIC lists of events are complete for threshold magnitudes 4.3 (4569 events) and 4.6 (2742 events), respectively. Spatial and temporal perturbations of b were investigated in six regions surrounding the eight largest shocks. A technique of moving spatial- and temporal-windows was applied. Deduced b-values reveal large variations between 0.6 and 2.2. All eight earthquakes took place within regions of low b and were all preceded by significant drops in b-values. Observed correspondence between low b and the occurrence of large earthquakes suggests that b(t) has a potential to be employed in medium-term earthquake predictions in subduction zones of Central and South America.
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| 7. |
- Maraschin, Anderson José, et al.
(författare)
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On the radiometric significance of glauconies in studies of provenance : an example of the Quaternary marine sequences of the Rio Grande Cone (Pelotas Basin, Southern Brazil)
- 2024
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Ingår i: Journal of South American Earth Sciences. - : Elsevier. - 0895-9811 .- 1873-0647. ; 141
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Tidskriftsartikel (refereegranskat)abstract
- The sediments of the Pleistocene-Holocene marine sequences of the Rio Grande Cone (RGC) (offshore Pelotas Basin, southernmost Brazilian Margin) are dominantly composed by muddy facies with thin interspersed sandy layers containing abundant very fine-to coarse green grains, heavy minerals assemblage, shards and planktonic foraminifera. These components contribute for a better understanding of the RGC filling during the Pleistocene-Holocene. However, the aim of this work was focused in the radiometric dating of the green grains as tool for provenance studies in marine sedimentary sequences. Based on Electron Microprobe, Scanning Electron Microprobe, Transmission Electron Microprobe and optical analysis the green grains occur as: (01) yellowish-green grains with smooth surfaces, low K2O and moderate Fe2O3 contents, (02) pale green grains with surfaces slightly fractured with moderate K2O and Fe2O3 contents, and (03) dark green grains characterised by fractured (cracked) surfaces and high of K2O and Fe2O3 contents. These characteristics classified them as glauconitic minerals ranging from glauconitic smectite to glauconitic mica (glauconite) indicating the first (nascent) and intermediate (slightly-evolved and evolved) stages of the glauconitization process. K–Ar radiometric dating indicated ages from 33.2 ± 0.99 to 6.69 ± 0.54 Ma corresponding from Rupelian Stage (Oligocene) to Messinian Stage (Miocene) to these grains. Considering these glauconitic grains within a Quaternary sequence, an allochthonous (detrital) origin was attributed whose probable source areas were older sedimentary sequences exposed on the seafloor along fold-and-thrust belts related to the gravitational collapse of the RGC depocentre. After eroded the glauconitic minerals probably were distributed by countourite currents and deposited within the Quaternary deposits of the RGC.
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| 8. |
- Mariño, E. E., et al.
(författare)
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The occurrence of arsenic and other trace elements in groundwaters of the southwestern Chaco-Pampean plain, Argentina
- 2020
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Ingår i: Journal of South American Earth Sciences. - : Elsevier. - 0895-9811 .- 1873-0647. ; 100, s. 102547-
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Tidskriftsartikel (refereegranskat)abstract
- The Argentinian Chaco-Pampean plain is a large sedimentary basin, which southern part, covering about 1.9 millions of km2, encompasses the provinces of Cordoba, Santa Fe, San Luis, Buenos Aires and La Pampa where water demand is mainly satisfied by groundwater. Its quality for human consumption is significantly impaired by the elevated presence of arsenic (As) and other geogenic trace elements. In this paper we investigated the nature and extent of the occurrence of arsenic and other selected trace elements (Fe, Mo and V) in groundwater in the Northern region of La Pampa. To achieve this aim, analytic results from 164 samples were taken into consideration. These samples were collected from community wells located in the study area. For 42 of these samples, determinations of the arsenic species were available. The hydrochemical characterization indicates that these groundwaters are predominantly of Na-HCO3 and Na-SO4-C1 type with variable salinity and high pH and alkalinity. These properties would favor the As mobilization by desorption from the solid phase. Most of the groundwaters revealed As concentrations over 10 μg/L, while guideline values of iron and molybdenum are exceeded in some cases. Arsenic is predominantly found as As(V) which represents between 92 and 97% of the total arsenic. This is consistent with the oxidizing conditions of the hydrogeochemical system.
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| 9. |
- Nina, Lidia, et al.
(författare)
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Anoxic oceanic conditions during the late Permian mass extinction-evidence from the Chutani formation, Bolivia
- 2020
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Ingår i: Journal of South American Earth Sciences. - : Elsevier. - 0895-9811 .- 1873-0647. ; 103
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Tidskriftsartikel (refereegranskat)abstract
- We analyze diagenesis of carbonate rocks from the Late Permian Chutani Formation of western Bolivia (San Pablo de Tiquina section) in the southern Lake Titicaca zone, which is a sedimentary succession of semiarid tidal flat comprised of mixed carbonate and siliciclastic units. The diagenetic study includes petrographic analysis (conventional petrography and cathodoluminescence) and geochemical analysis (carbon and oxygen isotopes and minor element chemistry). An integrated study of lithofacies and isotope stratigraphy of carbonates shows a succession of five types of depositional environments: tidal barrier, tidal flat, shoal coastal and shoreface. The Chutani Formation was subjected to different diagenetic processes including micritization, cementation, mechanical compaction, dissolution, neomorphism, dolomitization and dedolomitization that occurred during marine to shallow burial stages. Carbon isotope (δ13C) values range between −7 and 2.9‰ (VPDB) with variations linked to stratigraphic changes. The transgressive stage of the basin exhibits an upwards decreasing trend of δ13C values whereas regression is marked by an increase in such values. The oxygen isotope values (δ18O) vary from −16.6 to −1‰ VPDB with lighter values towards the top of the stratigraphy. The transgressive trend may reflect mixing of meteoric water and/or volcanic-hydrothermal fluids with seawater or progressive oxygenation with enhanced circulation conditions. Heavier values during regression may reflect more evaporitic and anoxic conditions towards the Permian-Triassic boundary. Significant variation in isotope values among neighbouring samples is observed, especially during trangression, which may be the result of different diagenetic processes.
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| 10. |
- Nina, Lidia, et al.
(författare)
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Diagenesis of the Pennsylvanian –Lower Permian Copacabana Formation, western Bolivian Altiplano
- 2020
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Ingår i: Journal of South American Earth Sciences. - : Elsevier. - 0895-9811 .- 1873-0647. ; 100
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Tidskriftsartikel (refereegranskat)abstract
- This contribution presents the diagenetic evolution of limestone deposits in the Copacabana Formation that occurs in the northern Altiplano, in the Lake Titicaca area of western Bolivia. The best-exposed stratigraphic succession of the Copacabana Formation occurs in the Yampupata section, and its division into five facies successions is based on petrographic analysis, cathodoluminescence, x-ray fluorescence analysis (chemical composition) and stable isotope data (δ18O and δ13C). The results showed that the carbonate rocks experienced early marine diagenetic processes such as micritization during or after the deposition (eogenesis). The initial burial event (mesogenesis 1), characterized by stabilization of temperature-water carbonates by freshwater, and represented by bladed calcite-cement, equant calcite cement, dissolution, dolomitization, neomorphism, silicification and compaction (physical), occurred in shallow burial conditions. During the second burial episode (mesogenesis 2), in deeper burial environment the processes include: compaction (physical and chemical) and neomorphism. Diagenetic processes have affected reservoir quality in the Copacabana Formation during the mesodiagenesis, and reduced the conditions for development of high-quality conventional hydrocarbon reservoirs. Depleted O and C stable isotope signatures indicate that these carbonate rocks deposits underwent both meteoric and burial diagenesis including moderate water-rock interaction.
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