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  • Linking rock age and soil cover across four islands on the Galápagos archipelago
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-15
    Franz Zehetner; Martin H. Gerzabek; J.Gregory Shellnutt; Franz Ottner; Christopher Lüthgens; Daniel P. Miggins; Pei-Hao Chen; I Nyoman Candra; Greta Schmidt; Maria V. Rechberger; Tobias Sprafke

    The Galápagos Islands have been a place of preeminent scientific discoveries. The archipelago has formed on a tectonic plate moving over a volcanic hotspot, which has generated a chain of volcanic islands of different ages. This tectonic setting has favored the evolution of a unique flora and fauna on the islands; however, little is known on how it has influenced the formation and evolution of soils and belowground ecosystems. In order to enable the investigation of soil and ecosystem evolution, the aim of this study was to establish a soil chronosequence covering the full age range from the younger islands in the west to the older islands in the east of the archipelago. Six sites in the humid zones of the islands as well as two drier comparison sites were selected for this study. We characterized the geochemistry and mineralogy of the soil parent materials and constrained their ages using 14C (<10 ka) and 40Ar/39Ar dating. The parent materials were scoriaceous with bulk densities ≤1.6 g/cm3. Their major element composition was basaltic, including both alkali basalts and tholeiites. The scorias had amorphous contents between 20 and 85%, and their mineralogical composition was dominated by plagioclase and pyroxene. The numerical ages of the soil parent materials at the humid sites were: 1.45 ± 0.06 ka and 4.29 ± 0.09 ka on Isabela Island, 26 ± 7 ka on Floreana Island, 165.5 ± 11.6 ka on Santa Cruz Island, 825.6 ± 11.2 ka and 1070 ± 10 ka on San Cristóbal Island. Hence, the ages were distributed logarithmically covering a time span of approximately one million years. Along this chronosequence, we observed a non-linear increase in solum thickness, rubification degree and clay content with rock age. Changes were rapid in the initial phase of pedogenesis and more gradual in the later stages. Compared to the humid sites of the chronosequence, the pedogenic development at the dry comparison sites was much less advanced. The established chronosequence shall provide a solid basis for future studies on soil and ecosystem evolution in the unique environment of the Galápagos Islands.

    更新日期:2020-01-15
  • The first mammal assemblages from the Malargüe Group: implications for the Paleogene evolution of the northern Neuquén Basin (Argentina)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-15
    Bárbara Vera; Maisa Tunik; Esperanza Cerdeño

    We present two new Paleogene fossiliferous sites that provide the first mammal remains for the Malargüe Group in the Neuquén basin of southeastern Mendoza Province, Argentina. Identified taxa from Agua de Flores-Agua de Isaac: Kibenikhoria sp., Oldfieldthomasiidae indet., ?Henricosborniidae indet., and a large Notoungulata probably related to the Isotemnidae. Taxa from Liu Malal: cf. Kibenikhoria and Notopithecidae gen. et sp. nov. Based on facies composition and geological distribution, we consider that the Cenozoic deposits from these fossiliferous localities correspond to the already defined Puesto Fortunata Formation. This unit is correlated to the Coihueco Formation, the top of the Malargüe Group in the northern Neuquén basin, and its mammal content supports the proposed hiatus between the ∼40 Ma and 20 Ma based on chronostratigraphy. The presence of Kibenikhoria in the local faunas of southeastern Mendoza extends the geographic range of this genus so far known in Patagonia to central-west areas of Argentina and supports an early-middle Eocene age for the fossil-bearing levels of the Puesto Fortunata Formation.

    更新日期:2020-01-15
  • A new peculiar species of the subterranean rodent Ctenomys (Rodentia, Ctenomyidae) from the Holocene of central Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-15
    Nahuel A. De Santi; Diego H. Verzi; A. Itatí Olivares; Pedro Piñero; Cecilia C. Morgan; Matías E. Medina; Diego E. Rivero; Eduardo P. Tonni

    The subterranean rodent Ctenomys is the single living representative of the family Ctenomyidae, and the most diverse genus within Hystricomorpha. Its fossil record begins in the late Pliocene and shows an increase in diversity since the Pleistocene. Here we describe a new middle-sized Ctenomys species from a large cranial sample collected at the archaeological site Quebrada del Real 1 in the high plains of the Pampa de Achala (Córdoba, Argentina). For this goal, we analyzed the cranial and mandibular shape variation through a geometric morphometric approach, and performed a comparative morphological analysis in the context of the living Ctenomys. The new species is characterized by having a wide rostrum with a deep rostral fossa, strong temporal fossa in the frontal, upper incisors strongly procumbent and with grooved enamel, and mandible with low corpus, long procumbent diastema and descending masseteric crest. A phylogenetic analysis demonstrated its close affinity with Ctenomys osvaldoreigi, an extant species from a nearby area. The new species represents the first notice of an extinct Ctenomys species from the Holocene. Given its peculiar morphology, the extinction of this species would have resulted in significant loss of morphological diversity, thus constraining the current boundaries of variation of the genus. The last record of the new species occurs in a period characterized by marked extinction of small mammals in southern South America mainly due to anthropic causes.

    更新日期:2020-01-15
  • Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation.
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-14
    Thalia Avilés Esquivel; Carlos Flores; Valeria Reyes Ortega; Steven Constable; Enrique Gómez-Treviño; Antonio González-Fernández

    Different geological and geophysical disciplines have suggested the Wagner Basin as a promising location of geothermal resources. As a reconnaissance tool Magnetotelluric (MT) data were measured at 10 sites along a profile over this basin in the northern Gulf of California. The subsurface distribution of electric resistivity was estimated with two-dimensional smooth inversion using the apparent resistivities and phases from both field polarizations. We found a deep conductor underneath the center of the basin, interpreted as a region of partial melt feeding the accretion zone of this incipient oceanic crust. From six MT sites with close-by heat flow measurements, we also found a positive correlation between the interpreted shallow (first 100 m) low resistivities and high heat flows. This shows that the MT method can be used to map submarine heat sources.

    更新日期:2020-01-14
  • Multivariate statistics and geochemical approaches for understanding source-sink relationship - a case study from close-basin lakes in Southeast Amazon
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-07
    Aline Mamede Moraes; Prafulla Kumar Sahoo; José Tasso Felix Guimarães; Alessandro Sabá Leite; Gabriel Negreiros Salomão; Pedro Walfir Martins Souza-Filho; Wilson Nascimento Júniora; Roberto Dall’Agnol

    Multivariate statistical approaches were applied on geochemical data of lake sediments and the catchment basin materials (Al-enriched crusts and soils) from four upland lakes (namely LB1, LB2, LB3, and LB4) in the Serra da Bocaína, Southeast Amazon, to recognize the geochemical signatures of lake sediments and their link with source rocks. This is also compared with Fe-enriched crusts and soils, which collected from Serra Sul de Carajás. The major and trace elements were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and X-ray Fluorescence (XRF), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), respectively. The data closure effect was reduced by centred log ratio (clr) transformation. The upper continental crust (UCC) normalized patterns show that sediments are mainly enriched with Ti, Fe, P, Se, V and Cr, similar to catchment basin crusts and soils. The distribution of elements varied between lakes, which is mainly influenced by the morphology and lithology of the basin. The chemical index of alternation (CIA) values were very high (>90) for both lake sediments and catchment materials, which indicates intense source area weathering. Index of lateritization (IOL) shows that the catchment crusts are strongly lateralized and it identifies samples between extremely weathered and reworked samples that do not meet the criteria for strict in situ formation. The principal components analysis (PCA) and Spearman correlation matrices based on clr-transformed data revealed four geochemical groups in sediments. The major detritic geochemical association of sediments (Group 1: Al–Ti-Ga-Cr-V-Ti-Nb-Sc, which indicative of mafic/metavolcanic rock; and Group 2: Zr-Hf-HREEs, which refers to the heavy mineral fractions that remain stable during lateritization) had a close relationship with catchment lateritic crust, except a few alterations, which possibly influenced by weathering and transport processes. REE pattern normalized to chondrite and the geochemical ratios show the analogies between the lake sediments and the catchment lateritic crusts. The linear discriminant analysis (LDA) and PCA distinguished Al-rich laterites from Fe-rich laterites and indicates that the former (which likely derived from mafic/metavolcanic rocks) is the main source of lake sediments.

    更新日期:2020-01-07
  • Chafalote Metamorphic Suite (Uruguay): Reflections on the evolution of the Punta del Este Terrane
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-16
    Daniel Triboli Vieira; Carla Cristine Porcher; Edinei Koester; Rodrigo Chaves Ramos; Andreia Oliveira Monteiro da Silva Gross; Henri Masquelin; Luis Alberto D’Ávila Fernandes

    The Chafalote Metamorphic Suite, located in the Uruguayan sector of the Punta del Este Terrane, southeasternmost Dom Feliciano Belt, comprises semipelitic, migmatitic metapelites, mafic granulites, and calc-silicate/amphibolitic gneisses. These supracrustal rocks occur as roof pendants in a granodiorite belonging to the Cerro Olivo Complex. The main structure of the Chafalote Metamorphic Suite is a composite S0/S1 banding, related to the main deformation event (D1) developed under upper-amphibolite/granulite facies conditions. This work presents an integrated study on the latter rocks, involving different geochronological methods (monazite U-Pb LA-ICP-MS and zircon U-Pb SHRIMP) and whole rock Sm-Nd isotope data in order to better understand the metamorphic evolution of the Chafalote Metamorphic Suite during the amalgamation of the SW Gondwana paleocontinent in the Neoproterozoic. The studied rocks record a peak metamorphic event (M2) occurred at around 660 Ma (monazite U-Pb ages), which was followed by decompression and partial melting (M3 event) at around 640 Ma (zircon U-Pb ages), probably during the late stages of the D1 event. The retrograde methamorphic event (M4) is related to the transition between the D1 and the development of NE-SW striking, low-grade sinistral shear zones (D2 event) and intrusion of syn-to post-orogenic Brasiliano granitoids of the Aiguá Batholith at around 600 Ma. The metamorphic conditions, together with geochronological and isotopic similarities, allow the correlation between the studied high-grade metasedimentary rocks and those from the Paso del Dragón Complex in Uruguay, and from the Várzea do Capivarita, Arroio Grande, and Telho Complex in Brazil. The latter rocks possibly share similar source areas and represent different portions of the Neoproterozoic fore-arc and/or back-arc basins developed in the Dom Feliciano Belt during the Brasiliano/Pan-African orogenic cycle.

    更新日期:2020-01-04
  • The large American opossum Didelphis (Didelphimorphia, Didelphidae) in the Late Pleistocene of Uruguay, and paleoecological remarks
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-22
    Aldo Manzuetti; Martín Ubilla; Daniel Perea; Raúl I. Vezzosi; Andrés Rinderknecht; Martín Ghizzoni; Washington Jones

    The fossil record of the genus Didelphis, despite its current diversity (6 species, 5 occurring in South America) and wide distribution (from southern Canada to Patagonia, Argentina), is very scarce. The oldest record of the genus in South America comes from Late Miocene sediments and the youngest records from the Late Holocene and historical times. In Uruguay, the only record is stratigraphically questionable because the material has been lost. The present contribution describes the first materials with a precise stratigraphic context assigned to Didelphis cf. D. albiventris, based on comparative morphological and numerical analysis. The materials were unearthed from several outcrops of the Sopas Formation (Late Pleistocene, northern Uruguay) and consist of an associated skull and mandible, right hemimandible, and postcranial material. These findings allow us to discuss several aspects related to the biology and ecology of Didelphis. These animals, which exhibit a mixed diet, were able to feed at intermediate levels of the food chain in highly diverse environments during the Late Pleistocene.

    更新日期:2020-01-04
  • Genesis and variation spatial of Podzol in depressions of the Barreiras Formation, northeastern Espírito Santo State, Brazil, and its implications for Quaternary climate change
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-23
    Jolimar Antonio Schiavo; Luiz Carlos Ruiz Pessenda; Antonio Alvaro Buso Júnior; Marcia Regina Calegari; Mileni Fornari; Mateus Luiz Secretti; Marcos Gervasio Pereira; Francis Edward Mayle
    更新日期:2020-01-04
  • Biomarker signatures of Cretaceous Gondwana amber from Ipubi Formation (Araripe Basin, Brazil) and their palaeobotanical significance
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-16
    Ricardo Pereira; Flaviana Jorge de Lima; Felipe M. Simbras; Sheila Maria Bretas Bittar; Alexander Wilhelm Armin Kellner; Antônio Álamo F. Saraiva; Renan A.M. Bantim; Juliana M. Sayão; Gustavo R. Oliveira

    An amber (UFRPE 5037), from the black shales of the Ipubi Formation, Araripe Basin (Brazil), has been analyzed to understand its organic geochemical characteristics and possible botanical source using gas chromatography–mass spectrometry (GC–MS). The analyses carried out detected monoterpenes, hydrocarbonic sesquiterpenes, and aliphatic diterpanes like abietanes and norabietanes, as well as aromatic diterpanes such as simonellite, norsimonellite, retene, hinokiol, ferruginol and dehydroabietic acid. Such a combination of components is mainly known to have originated from gymnosperms. The individual identification of diterpenoids allowed the suggestion of Cheirolepidiaceae or Podocarpaceae families as possible amber producers, pointing to a flora similar to the underlying Crato Formation. Therefore, UFRPE 5037 consists on the first amber record in the Ipubi Formation, allowing the acquisition of important information regarding the botanical sources associated to new Brazilian amber occurrence described here.

    更新日期:2020-01-04
  • Basic and specialized geophysical well logs to characterize an offshore carbonate reservoir in the Campos Basin, southeast Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-22
    Abel Carrasquilla; Renan Lima

    Assessing the geometry, flow dynamics and, petrophysical properties, of a carbonate reservoir has been a key task for a long time, but the challenges presented by these highly heterogeneous rocks seem endless. Geophysical well logs have been, over this time, a very important tool in this type of evaluation. The applications of logs, in the evaluation of the geological formations, range from the estimation of lithology, porosity, permeability and mechanical properties. Commonly, logs are classified as basic or special. The basic logs, used in this work, are the gamma ray, resistivity, density, neutron and sonic. The special ones, utilized in the study, are the acoustic image and the photoelectric logs. From the analysis of the basic logs, the lithologies were identified, the reservoir was delimited and zoned, the fluids and the contact between them were identified and, petrophysical properties, as porosity and water saturation, were estimated. The total porosity was estimated using the empirical relationships from density, neutron and, sonic logs. The identification of the porosity type, such as fractures, vugs etc., although, was determined from the acoustic image borehole wall, as well as different geological structures, such as lithological contacts, inclined layers, natural and induced fractures, besides the identification of caves. The photoelectric log helped to identify the different lithologies, which were compared with those derived from the basic logs. Thus, the main objective of this work was the joint use of the basic and specialized logs, in the characterization of a carbonate reservoir in the Campos Basin, using real data from the post - salt layer. In the end, the advantages and disadvantages, in the usage of these tools, were indicated, as considerably as the applications and care with the occurrence of no geological structures. The joint use of the data and, the adopted methodology proved effective in the characterization of this reservoir.

    更新日期:2020-01-04
  • Zeolites in acid vitreous rocks, southern Mendoza, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-23
    Francisco Locati; Fernanda Cravero; Silvina Marfil; Leticia Lescano; Lenís Madsen; Pedro Maiza

    A mordenite-rich deposit located in the south of the province of Mendoza (Argentina) was studied. The host rock in this area has not been previously described and corresponds to rhyolitic lavas of high-K calc-alkaline series. Petrographic, mineralogical and geochemical analyses of fresh and altered rocks are shown, and a proposal for the alteration process and the stratigraphic position of the protolith is presented. The main mordenite-rich sectors developed in zones of autobreccias of high permeability, with minor clinoptilolite, smectites, secondary K-feldspars, opal-CT, and colloform silica. The proportion of relict glass vs. alteration minerals is variable and mainly depends on the texture (permeability) of the host rock. The studied rhyolites overlie the Upper Cretaceous sediments of the Neuquén Group and are covered by Tertiary basalts of the Molle Eruptive Cycle and Quaternary sediments. Geochemically, they present similarities with rhyolitic rocks of Cordón del Burrero Volcanic Complex of Lower to Middle Miocene age.

    更新日期:2020-01-04
  • Macroinvertebrates of the Capivari marine bed, late Paleozoic glacial Itararé Group, northeast Paraná Basin, Brazil: Paleoenvironmental and paleogeographic implications
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-23
    Marcello Guimarães Simões; Jacqueline Peixoto Neves; Arturo César Taboada; Maria Alejandra Pagani; Filipe Giovanini Varejão; Mário Luis Assine

    A 2-m-thick silty shale bed within the Taciba Formation, Itararé Group, Paraná Basin, State of São Paulo, southeastern Brazil, records marine sedimentation in a siliciclastic-dominated, low-energy, shelf setting, during a short-lived deglacial event. The bed is located 100–150 m below the base of the lower Permian, post-glacial Tatui Formation. The marine assemblage is dominated by rhynchonelliform brachiopods, with subordinate bivalves, gastropods and crinoids, recording the highest phylum-level diversity so far identified within a given fossil-bearing horizon in the uppermost portion of the Itararé Group. Two new species are described, one brachiopod Biconvexiella saopauloensis and one gastropod Peruvispira brasilensis. Additionally, shells of Lyonia rochacamposi, Rhynchopora grossopunctata, Quinquenella rionegrensis, Phestia tepuelensis, Streblopteria aff. S. lagunensis, Limipecten capivariensis, Praeundulomya cf. subelongata and Mourlonia (Woolnoughia)? sp. are identified. Crinoid columns were assigned to øPentaridica sp. (a genus based on elements of the columnal). This is the first systematic description of members of the Eurydesma-Lyonia fauna in the northeastern part of the Paraná Basin, Brazil. The overwhelming majority of brachiopods belong to Biconvexiella saopauloensis, followed by Rhynchopora grossopunctata. The record of Lyonia rochacamposi closely resembles that of the uppermost part of the Taciba Formation in southern Brazil. Hence, the Capivari marine fauna correlates approximately with that of the upper part of the Taciba Formation. Lyonia rochacamposi also indicates correlation with Permian units of the Sauce Grande-Colorado (Argentina), Huab (Hardap shale of the Dwyka Group), Aranos area (Namibia), southwest Africa, and the Carnavon (Western Australia) basins. These correlations support a latest Asselian-earliest Sakmarian age for the fauna.

    更新日期:2020-01-04
  • Hydro-morphological characteristics and recent changes of a nearly pristine river system in Chilean Patagonia: The Exploradores river network
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-26
    Camila Bañales-Seguel; Alejandro Salazar; Luca Mao

    Fluvial systems provide multiple life-supporting functions, but their values are affected by a range of anthropogenic disturbances. Hydromorphology is used as a conceptual framework for assessing the status of fluvial systems and design river restoration strategies but is rarely applied to nearly pristine environments. This paper presents one of the first assessments of river characteristics and changes in the Aysen Region, an area in southern Chilean Patagonia. The analysis of multitemporal satellite images allowed to define key patterns related to river morphology of the Exploradores river network. The Exploradores basin experienced only limited and recent human disturbances, and fluvial changes are related almost only to natural climatic or geomorphological processes. The river experienced moderate reduction of active channel width and braiding index over the past 70 years. The basin represents a suitable site to study fluvial processes and dynamics in nearly reference conditions, and changes due to the likely increase of human activities and disturbances in the near future.

    更新日期:2020-01-04
  • 更新日期:2020-01-04
  • A new abelisaurid from the Huincul Formation (Cenomanian-Turonian; Upper Cretaceous) of Río Negro province, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-30
    M.A. Cerroni; M.J. Motta; F.L. Agnolín; A.M. Aranciaga Rolando; F. Brissón Egli; F.E. Novas

    The record of abelisaurids in South America, particularly Argentina is relatively abundant. However, abelisaurid diversity is far from being well-known. Huincul Formation has yielded diverse theropod remains, including the abelisaurids Ilokelesia and Skorpiovenator. The aim of the present contribution is to describe a new abelisaurid coming from the Upper Section of the Huincul Formation (Cretaceous; Cenomanian-Turonian) at the Violante Farm fossil site, Río Negro province, northern Patagonia, Argentina. The new taxon described in this contribution is represented by an incomplete specimen including well-preserved maxilla and dorsal vertebrae, and distorted pelvic girdle and caudal and sacral vertebrae. Its size, much smaller than large abelisaurids as Abelisaurus and Carnotaurus indicates that the new taxon probably occupied a different ecological niche within the carnivoran guild.

    更新日期:2020-01-04
  • Morphotectonic analysis of two axial tributary basins of the San Juan river controlled by the Precordillera fold and thrust belt, Central Andes of Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-27
    Pablo A. Blanc; Flavia Tejada; Laura P. Perucca; Karen Espejo; Gabriela Lara; Nicolás Vargas

    Analysis of several morphotectonic tools (longitudinal river profiles, knickpoints, swath profiles, and geomorphic indices) together with geomorphological, lithological, and structural data provided objective and quantifiable elements to improve the knowledge of the Quaternary tectonic activity in a region. In this study, we analyzed and compared two large intermountain basins located in the Precordillera of the Central Andes of Argentina. Local distortions in longitudinal river profiles and river patterns may reflect resistant lithology, an increase in shear stress, or differential surface uplift. Both basins have similar shapes, although one mirrors the other producing a large-scale broom-shaped drainage pattern. The values of the Asymmetry Factor and Transverse Topographic Symmetry Factor show opposite signs and almost identical values, suggesting that both basins have mirror symmetry independently of order and scale. The asymmetry in both basins would be a result of the progressive migration of the Andean orogenic front towards the east. Knickpoints were mainly associated with resistant rocks, faults with the scarp facing upstream, or both. In the reaches where the rivers flow across areas with presumed higher rates of uplift, we observed a reduction in the width of the active channel belt by lateral confinement and river pattern changes. The results presented in this paper constitute one of the first attempts to apply geomorphic indices to large drainage basins in the Precordillera fold-and-thrust belt.

    更新日期:2020-01-04
  • Strike-slip related folding within the Malvinas/Falkland Trough (south-western Atlantic ocean)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-03
    Federico D. Esteban; Juan Pablo Ormazabal; Fermín Palma; Lubin Eric Cayo; Emanuele Lodolo; Alejandro Tassone
    更新日期:2020-01-04
  • Identification of the components of a complex groundwater flow system subjected to intensive exploitation
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-22
    Selene Olea-Olea; Oscar Escolero; Jürgen Mahlknecht; Lucia Ortega; Raúl Silva-Aguilera; Jose Roberto Florez-Peñaloza; Juan Perez-Quezadas; Olivia Zamora-Martinez

    The Mexico City region, like many other densely populated regions in the world, has problems in guaranteeing the supply of drinking water to its inhabitants. Its groundwater flow system is subject to intensive exploitation. To ensure proper water management, it is important to understand the behavior of groundwater flow system components under conditions of intensive exploitation. In this study, we analyzed 169 samples from drinking water supply wells, according to their chemistry. The principal components of the flow system were inferred using exploratory statistical analysis and analysis of the mixture of the end-members of the hydrochemical dataset. Four components were identified: (1) waters of recent infiltration and local circulation, (2) waters with evidence of intermediate chemical evolution, (3) waters that are chemically more evolved and of large flow trajectories, (4) waters that are chemically more evolved and of deep circulation. A relationship between the components, the structural geology, and the recharge zones was established. The mixing proportion of the four components in each water sample extracted from drinking water wells was identified. It was observed that the faults and the design of screened section of the well tubes influenced the mixtures captured by the wells. The impact generated by the intensive exploitation with respect to the distribution of the components within the flow system was evaluated.

    更新日期:2020-01-04
  • Apatite LA-ICP-MS U–Pb and fission-track geochronology of the Caño Viejita gabbro in E-Colombia: Evidence for Grenvillian intraplate rifting and Jurassic exhumation in the NW Amazonian Craton
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-26
    Amed Bonilla; Jose A. Franco; Thomas Cramer; Marc Poujol; Nathan Cogné; Simon Nachtergaele; Johan De Grave

    The 1.80–1.76 Ga crystalline basement in Colombia as part of the W-Amazonian Craton is composed mainly of gneisses, granitoids and migmatites, affected later by several compressive and extensional events resulting for example in A-type granites, but also mafic intrusions and dikes. Here we present, after a revision of main geological features, research results obtained on the NW-SE trending ilmenite-apatite-rich Caño Viejita gabbro in the SW-Vichada department some 500 km east of Bogota. Petrographic and geochemical data hint to a metaluminous continental alkaline gabbro enriched in K, Ti and P, possibly due to continental crust reworking or magma mixing, as also confirmed by trace elements characteristics in the apatites like HREE enrichment (Ce/Yb)cn 12–13, negative Eu-anomaly, and Y, Th, Sr, Mn ratios. LA-ICP-MS U–Pb apatite geochronology suggests an early Neoproterozoic emplacement age between 975 ± 9 and 1002 ± 21 Ma related with rifting triggered by the Amazonia-Baltica-Laurentia collision during the Rodinia Supercontinent assembly and associated Grenvillian events. These events also caused mafic intrusions in other parts of the craton. Apatite fission track thermochronometry and thermal history modelling on one sample suggest the onset of the final exhumation stage during Jurassic (~180 Ma), which brought the rocks slowly to their current outcrop position.

    更新日期:2020-01-04
  • Quaternary non-marine ostracods of Runtuyoc lake, northern Argentina: New taxonomic descriptions and the implication for Holocene paleoenvironment
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-05
    D. Sabina D'Ambrosio; Leandro D. Rojo; Sonia L. Fontana

    We performed an ostracod taxonomic analysis on samples from a shallow lake (Runtuyoc Lake) in north-western Argentina in order to contribute to the understanding of the evolution of this water body through the last 5000 years. The ostracod assemblages consist of 15 species, two species exclusively from modern samples (Herpetocypris helenae and Chlamydotheca incisa), 8 exclusively from Quaternary samples (Limnocythere jujunensis sp. nov., L. titicaca, L. alexanderi, L. aff. staplini, Cyprideis salebrosa, Penthesilenula incae, Cypridopsis vidua, Sarscypridopsis aculeata) and 5 from both, modern and Quaternary samples (Ilyocypris ramirezi, Potamocypris smaragdina, Heterocypris incongruens, Heterocypris similis, Candona sp.). The lake is located in the Miraflores river course and is characterized nowadays by freshwater conditions, but a saline past is evident as it went through different periods of desiccation. From 5500 to 3000 cal a BP, the lake was absent, probably related to extreme dry conditions. Then, water inputs into the lake produced a saline shallow lake until to reach modern setting as a freshwater body basin. Runtuyoc lake records, supports the regional environmental trending change for the Argentinian northwest, with wetter conditions in the early and late Holocene, and arid for the middle Holocene at least from 5500 to 3000/2500 cal a BP.

    更新日期:2020-01-04
  • Geophysical analysis in a Quaternary compressive environment controlling the emplacement of travertine, eastern piedmont of Argentine Precordillera
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-23
    Juan Manuel Alcacer Sanchez; Martín Rothis; Federico Haro; Laura Perucca; Silvia Miranda; Nicolás Vargas

    This work carries out the analysis, through a geophysical (gravity) method, of an area in the eastern Piedmont of Eastern Precordillera, Central Andes of Argentina. Geological evidence shows several neotectonic reverse fault scarps affecting Quaternary alluvial deposits located to the East of the Sierra de Villicum. Topographic features observed in the understudy region suggest an important Holocene historical seismic activity, one of them the 15th January 1944 San Juan earthquake (Mw 7.0), the worst natural disaster in Argentina. Several N–S springs and carbonate deposits (travertine) are found aligned along the traces of these active faults, disappearing abruptly to the north and south, possibly due to a tectonic control. Travertine deposits are located along fault traces and have been traditionally considered to be contemporaneous with active faults. The existence of travertine bodies deposited by springs, and their low preservation potential, has for the understudy region a neotectonic significance. The eastern Piedmont of the Sierra de Villicum is characterized by the ~N–S La Laja west-vergent east-dipping reverse fault and several subparallel faults (e.g., Cantera and Museo faults) that uplift Neogene sedimentary rocks over Quaternary (Late Pleistocene-Holocene) alluvial deposits. Gravity data derived from the global Earth gravity model WGM 1.0 were used in the regional geophysical analysis. The analysis aims to delimit geological structures, which characterized the interest area. Three detailed gravimetric profiles were measured to analyze local anomalies related to the main structures. The gravimetric and geological analysis confirms the existence of north-south faults parallel to La Laja fault related to travertine deposits. On the other hand, a set of southeast-northwest structures that limit and control the travertine bodies both northward and southward are recognized. Employing the gravity method associated with structural analysis, the spatial continuity of these neotectonic structures was defined in those sectors where field evidence is not so clear or blind structures are present. Our study confirms the existence of probable cross strike structures trending NW. We suggest they would represent preexisting crustal fabrics reactivated during the Andean orogeny in the Precordillera province, segmenting this orogen-parallel fold and belt system. One of the main hypotheses is that these fault systems related to basement-involved strike-slip faults perform as control structures in fluid migration during the travertine bodies' emplacement. Thus, our work points to the relevance of travertine location in the analysis of the seismotectonic configuration of a region.

    更新日期:2020-01-04
  • Paleoproterozoic P-T-d-t evolution of a gneiss with quartz-sillimanite nodules from the Azul ranges, Rio de la Plata craton, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-09
    Franco Denis Bianchi; Juan Cruz Martínez; Melisa Angeletti; María Cristina Frisicale; Hans-Joachim Massonne; Jorge Anastasio Dristas
    更新日期:2020-01-04
  • A new fossil talitrid amphipod from the lower early Miocene Chiapas amber documented with microCT scanning
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-10
    Thomas A. Hegna; Eric A. Lazo-Wasem; María de Lourdes Serrano-Sánchez; Ricardo Barragán; Francisco J. Vega

    The blind fossil amphipod, Caecorchestia bousfieldi, Hegna and Lazo-Wasem, new genus and species, is described from the lower Miocene Campo La Granja amber mines in Chiapas, Mexico, based on more than 130 specimens in diverse states of preservation. It is the second amphipod species to be described from Chiapas amber. C. bousfieldi belongs to the Talitridae, a family of amphipods that have successfully colonized terrestrial habitats. Exactly how fully terrestrial Caecorchestia bousfieldi was is unclear, and its blindness likely indicates that it occupied a cryptic habitat. A review of the amphipod fossil record is given which demonstrates that the amphipods first appeared as fossils in the Eocene.

    更新日期:2020-01-04
  • First temnodontosaurid (Ichthyosauria, Parvipelvia) from the Lower Jurassic of the Atacama Desert, northern Chile
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-14
    Rodrigo A. Otero; Patricio Sepúlveda

    Describimos restos craneales fragmentarios de ictiosaurio pertenecientes a un único individuo, recuperado desde estratos marinos del Jurásico Inferior del norte de Chile. Los dientes preservados muestran características distintivas, como una raíz muy larga, robusta y con gruesos pliegues, así como también coronas bajas, triangulares, masivas, labio-lingualmente comprimidas y con carenas. Las características dentales son consistentes con las del género Temnodontosaurus, anteriormente conocido en el Jurásico Inferior de Europa. Este descubrimiento representa el primer registro de un ictiosaurio temnodontosáurido en el hemisferio sur, reforzando un patrón de intercambio faunístico entre el Tetis norte y Panthalassa sur, antes de la separación de Laurasia y Gondwana, y antes del establecimiento completo de la vía marítima del Caribe.

    更新日期:2020-01-04
  • Pleistocene-Holocene sedimentary deposits of the Solimões-Amazonas fluvial system, Western Amazonia
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-09
    Marcel Silva Passos; Emílio Alberto Amaral Soares; Sonia Hatsue Tatumi; Marcio Yee; Juan Carlos Ramirez Mittani; Ericson Hideki Hayakawa; Carlos Alejandro Salazar

    Three levels of Pleistocene-Holocene fluvial terraces, informally referred to as Upper (UT), Intermediate (IT) and Lower (LT) Terraces, are described in the stretch between the Purus and Manacapuru tributaries (Western Amazonia) of the Solimões-Amazonas river system, using remote sensing, sedimentological, stratigraphic and geochronological data. The terrace levels are located at heights between 15 and 75 m, defining bands parallel and symmetrical to the channel system that extends for tens of kilometers, truncated by paleochannel features. Internally, the intercalations of sand and mud (silt and clay) layers form the Inclined Heterolithic Stratification (IHS) pairs that are related to the migration of scrollbars, with ages from 204 to 0.75 ka determined by Optically Stimulated Luminescence (OSL). In this period, the low stability of the channel system was related to the meandering fluvial pattern that originated an extensive alluvial plain area, with symmetrical fluvial terraces levels, consisting of point bars with IHS and paleochannel features. However, multi-temporal data analysis of the last decades shows that the Solimões-Amazonas river system has a high degree of stability, indicated by the low migration rate of the channels, and the presence of muddy and phytostabilized islands and marginal bars that define the current anastomosed-anabranching pattern. The time involved in changing the river pattern and the associated processes has been a source of controversy. Geomorphological, current sedimentary and stratigraphic data indicate that the change occurred in the Pleistocene - Holocene transition, probably due to the paleoclimatic, tectonic and glacio-eustatic changes that affected the sea level rise (North Atlantic), consequently, decreasing the gradient and increasing the sedimentary input that, associated with the increasing humidity (forest vegetation), made the channel system more stable.

    更新日期:2020-01-04
  • Sampling biases and Paleozoic sporomorphs diversity dynamics in Western Gondwana strata
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-10
    Ángeles Beri; Ximena Martínez-Blanco; Luciano Varela; Mercedes di Pasquo; Paulo Alves de Souza
    更新日期:2020-01-04
  • 更新日期:2020-01-04
  • A new constraint on the central Andean rotation pattern from paleomagnetic studies in the southern Subandes of Bolivia
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-17
    Juan M. Calvagno; Leandro C. Gallo; Renata N. Tomezzoli; Ernesto O. Cristallini; Alejandra Dalenz Farjat; Roberto M. Hernández

    New paleomagnetic and anisotropy of magnetic susceptibility (AMS) studies were performed in the southern Subandes of Bolivia in order to assess vertical axis rotations in a poorly studied area. Due to the presence of polarity reversals, it was possible to perform a reversal test that provided evidence of a reliable paleomagnetic record interpreted to be primary. In an area where vertical axis rotations are a principal component of deformation, we have developed a nonparametric method for the determination of tectonic rotations. We calculated a paleomagnetic pole for the Miocene Tariquia Formation (Lat. = 78.4° S, Long. = 113.1° E, A95 = 3.5°, N = 72, K = 23.73, Slat = 22.08°, SLon = 64.06°) and from our results, rotations around vertical axes, commonly sought around the Arica-Santa Cruz bend were ruled out. From the integration of paleomagnetic and AMS results, it was seen that the magnetic lineation lies within the bedding plane, consistent with the early stages of layer parallel shortening. However, it is not parallel to the structural trend, implying material displacement parallel to it. In concomitance with paleomagnetic results, this allows us to argue that a local change in the azimuth of the structures at this latitude could be the consequence of an asymmetric or heterogeneous basement and/or exogenous agents like differences in erosion along the course of the structures during the evolution of the Bolivian fold and thrust belt.

    更新日期:2020-01-04
  • Factors that control the seasonal dynamics of the shallow lakes in the Pampean region, Buenos Aires, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-13
    Maria Florencia Pisano; Gabriela D'Amico; Nicolas Ramos; Nicole Pommarés; Enrique Fucks
    更新日期:2020-01-04
  • Along-strike variations in recent tectonic activity in the Santander Massif: New insights on landscape evolution in the Northern Andes
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-20
    Helbert García-Delgado; Silvia Machuca; Francisco Velandia; Franck Audemard

    The integration of drainage analysis and topographic metrics provides excellent tools to assess the Quaternary tectonic activity modeling landscape evolution in evolving orogens. Furthermore, detecting active structures through the geomorphological analysis of landscapes helps to identify potentially seismogenic structures that could impact the socio-economic conditions in developing countries. Therefore, this paper presents a tectonic geomorphology study conducted on the Santander Massif (SM, Colombian Eastern Cordillera) and surrounding areas. The SM has structures with reported Quaternary activity such as the Bucaramanga, Morro Negro-Las Mercedes, and Chitagá faults, which summed to a poorly-constrained paleosismological history with significant shallow events is the prime incentive of this work. Our study is based on the acquisition of topographic data through swath profiles, local relief, slope variability, filtered topography, minimum bulk erosion, which along with geomorphic indices like the normalized concavity steepness (ksn), the hypsometric integral (HI), the ratio valley floor width to valley height (Vf) and normalized river profiles provided new insights on the recent landscape evolution in this part of the Northern Andes. We compared published apatite fission tracks (AFT) data for the study area with the uplift pattern deciphered from the geomorphic indices to detect the consistency in exhumation and denudation processes both in the long and short-term. Consequently, the central SM, as well as the faulted block between the Bucaramanga and Guamalito faults, evidence a highly-incised landscape with local relief values exceeding the 1500 m, then, responding with a sharp incision to recent surface uplift. Interestingly, this uplift pattern in central SM matches with published AFT data and is related to Neogene exhumation events controlled by secondary structures like the Suratá Fault. We attribute this exhumation event, and accelerated denudation rates within the SM, to the influence exerted by the “collision” between the SM and the Pamplona Indenter, leading to the unroofing of basement rocks at the deformation front (e.g., the Vetas High) and the topographic building of the range. For the upthrown block (hanging wall) of the Bucaramanga Fault, the occurrence of poorly-graded river profiles with an average concavity factor of 12.01, as well as irregular hypsometric curves with convex lower reaches, reinforce the hypothesis that late Cenozoic topographic rejuvenation in the SM was also forced by the tectonic activity of the Bucaramanga Fault. We expect empirical uplift rates >0.08 mm/yr (very high to high tectonic activity) for the mountain front encompassed between La Esperanza and Bucaramanga localities. We state this assumption since the mountain front delimited by the structure mentioned above is quite straight, it preserves remarkable morphostructural features and displays V-shaped valleys with an average Vf index of 0.38. This uplift rate is in good agreement with recent thermochronological exhumation rates (0.1 to > 0.3 mm/yr) published by Siravo et al. (2019). Higher uplift rates associated with the Bucaramanga Fault were also constrained with the erosion proxy index, i.e., the ksn index. High ksn values (above 128) were observed following a linear pattern, which in turn seems controlled by the main trace of the Bucaramanga Fault. For instance, the river system draining the western piedmont of the SM, near Aguachica, records slope-break knickpoints that we interpret as evidence of an incisional wave migrating upstream and adjusting a late Neogene uplift event associated with the Bucaramanga Fault. Conversely, the northern SM presents variable tectonic activity with the inner part, east of the Guamalito Fault, presenting low tectonic activity with little surface uplift and dominant strike-slip kinematics. We recognize the Ocaña-Ábrego zone as a relict landscape with well-preserved non-consolidated deposits that may record the initial exhumation and denudational pulses of the SM during the Andean Orogeny.

    更新日期:2020-01-04
  • Assessing landscape response to tectonics in the Jalisco block and adjacent areas (west-central Mexico) using topographic analysis
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-18
    Arturo Godínez-Tamay; Miguel Castillo; Luca Ferrari; Fernando Ortega-Gutiérrez

    To elucidate how landscapes respond to either tectonic and climatic changes through time, geomorphologists use the analysis of river and basin morphometry. Many studies have evaluated how rivers, and consequently hillslopes, respond to changes in the rate of rock uplift in compressional settings. The case of extensional settings, however, has received less attention. We examine the case of landscape response to tectonics in west-central Mexico to identify zones where there is notable increase in the tectonic activity. The study of west-central Mexico is challenging for the morphometric analysis since there is an interplay between the development of extensional structures and pulses of volcanism, mainly from Eocene to Oligocene and Pliocene to Pleistocene, that have buried large parts of the landscape. For our purpose we use the normalized channel steepness index (ksn) to evaluate how this index correlates with other morphometric parameters such as the hillslope gradient, hillslope convexity and concavity, terrain rugosity and first order stream gradient (Fosg). We use the landform units map as the spatial unit of analysis and from this, we estimate the morphometric properties of the landscape. The results of the morphometric analysis indicate that the topography of Eocene to Oligocene volcanic landscapes are well explained (~70%) by the ksn and in less degree for Pliocene to Pleistocene landscape where ksn accounts for ~55–66% of landscape morphometry. Our results also indicate that the ksn is the best suited among all morphometric variables used in this study and confirm that the landscape of west-central Mexico is dominated by river incision. Landscape incision is particularly high on the northern part of the Jalisco block, mainly along the Ameca river and its tributaries such as Atenguillo river, and north of the Jalisco block, along the Santiago river and on tributaries incising the Sierra Madre Occidental. Finally, the morphometric analysis indicates that even for the case where the volcanic rocks and sediments are filling up large parts of the landscape, river incision resulting from the tectonic activity is capable to cut into lavas, propagating the change imposed by the tectonic activity.

    更新日期:2020-01-04
  • Using Ground Penetrating Radar and attribute analysis for identifying depositional units in a fluvial-aeolian interaction environment: The Guandacol Valley, northwest Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-17
    Peter Zabala Medina; Carlos Limarino; Néstor Bonomo; Salomé Salvó Bernárdez; Ana Osella

    This paper deals with the application of the Ground Penetrating Radar (GPR) method and the analysis of attributes of the GPR data to characterize and interpret a fluvial-aeolian interaction field located in the Guandacol Valley, northwest Argentina. Several profiles over dunes, interdunes, aeolian mesoforms, and fluvial channels have been acquired. Each data section is analyzed by using standard images of the amplitude of the electric field, as well as representations of different attributes of the reflections such as contrast, dip, curvature, parallelism, and RMS frequency. The analysis of attributes improves the interpretation of the subsurface, by quantifying and making evident properties of the reflection patterns that characterize the sedimentary units. The information obtained using the GPR profiles allows defining seven radar packages, which are useful for reconstructing the internal structure of the fluvial-aeolian succession. Packages 1, 2 and 3 illustrate the stratification of different types of low-sinuosity and high-sinuosity aeolian dunes, as well as aeolian mesoforms. Package 4 corresponds to horizontal or low-angle inclined reflectors obtained in both sandy interdunes and upper parts of several aeolian dunes. A muddy bed that covers most of the area (package 5) probably indicates a period of climate amelioration linked to a high level of the water table. The fluvial component of the fluvial-aeolian succession exhibits two different packages; package 6 represents the infill of partially incised fluvial channels with frequent incisions (concave-up bounding surfaces) and bars (convex-up surfaces). Package 7 is composed of the stacking of parallel to subparallel horizontal reflectors, without concave-up surfaces that indicate deep channels. Finally, we propose a conceptual model that relates the principal radar packages with the temporal evolution of the fluvial-aeolian interaction field of Guandacol Valley.

    更新日期:2020-01-04
  • Seismic tectono-stratigraphic analysis of the Aptian pre-salt marginal system of Espírito Santo Basin, Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-24
    Francyne Bochi do Amarante; Juliano Kuchle; David Iacopini; Claiton Marlon dos Santos Scherer; Renata dos Santos Alvarenga; Patrycia Leipnitz Ene; André Basso Schilling

    The Aptian sedimentary sequence of Mucuri Member, lower Mariricu Formation, constitutes the pre-salt of Espírito Santo Basin. Proximal successions are dominantly clastic consisting of fluvial and coastal sediments deposited within the margins of a wide lake, thus corresponding to the marginal deposits of lacustrine carbonate reservoirs located in distal portions of the basin. Economic interest for oil is centered on such distal reservoirs, consequently proximal sections lack detailed studies. The Espírito Santo Basin is located near pre-salt official polygon, with oil exploration active for over six decades. The main objective of this study is the tectono-stratigraphic analysis of the clastic Mucuri Member, Aptian sag succession of the Espírito Santo Basin. Leading methodology is a seismostratigraphic characterization based on 220 2D lines, 1 3D volume, 103 well log data and 1 cored well. Four seismostratigraphic units were identified, delimited by three conformable horizons. Unit-bounding reflectors coincide with gamma ray maxima or minima interpreted to reflect shale or anhydrites deposited in deep water environments, corresponding to lacustrine maximum flooding surfaces. The structural framework is characterized by minor faults in the north onto shallow basement platforms, and in the south by half-graben bounded by normal faults developed during the rift phase. The Mucuri Member records an enlargement in depositional area from its base to top unit witnessing an overall lacustrine base level rise during depositon. The geometry of depositional area was initially influenced by the paleorelief of the preceding rift basins. The seismic units record a decrease in thickness as remnant topography was gradually filled and the upper units extended beyond the rift boundaries, covering a larger area. The Early Cretaceous Mucuri Member composes the first post-rift sequence of Espírito Santo Basin marked by the cessation of fault activity and the onset of thermal subsidence.

    更新日期:2020-01-04
  • Paleocene Las Violetas Fossil Forest: Wood anatomy and paleoclimatology
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-15
    Daniela P. Ruiz; M. Sol Raigemborn; Mariana Brea; Roberto R. Pujana

    Las Violetas Fossil Forest of the Salamanca Formation was a mixed forest of Paleocene age located in the Golfo San Jorge Basin, central Patagonia of Argentina, which includes conifers (previously studied), dicots and palms. In the present work, the dicot fossil woods outcropping in one of the fossiliferous levels (L3) of the Salamanca Formation are described, taxonomically assigned, and their affinities are discussed. They are three new species, including a new genus related to the Myrtaceae and Laurales. The complete fossiliferous assemblage is used to infer the climate and environment of the fossil forest. This assemblage represents a parautochtonous fossil plant association that lived either on the margins of channels or exposed bars in well-drained soils, probably tropical red-soils, close to their depositional setting (tidal channels and bars of an estuary). Warm and humid conditions and porous host-rocks favored the silicification of woods and pigmentation with Fe, resulting in yellow-orange and green fossil woods. We apply growth ring analysis to the conifer woods. For the dicots, wood anatomical characters influenced by the environment were analyzed, and the Vulnerability and Mesomorphy indices were used. These methods, comparisons with extant forests and the sedimentology, suggest that the Las Violetas Fossil Forest was an evergreen forest that developed under uniform growing seasons that ended abruptly, with an abundant water supply and high mean annual temperatures.

    更新日期:2020-01-04
  • New materials and an overview of Cretaceous vertebrates from the Chubut Group of the Golfo San Jorge Basin, central Patagonia, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-09
    Lucio M. Ibiricu; Gabriel A. Casal; Rubén D. Martínez; Bruno N. Alvarez; Stephen F. Poropat

    In this paper, we present an updated revision of fossil vertebrates from the Chubut Group, Golfo San Jorge Basin, while also describing some new remains. Extensive exposures of both Lower and Upper Cretaceous sedimentary sequences are present in central Patagonia. These outcrops have, over the past several decades, yielded a varied vertebrate fauna, including fishes, turtles, crocodyliforms, pterosaurs, and dinosaurs, currently herein characterized and described. Although vertebrate diversity in the Chubut Group in central Patagonia is remarkable, the most abundant vertebrates recovered are dinosaurs. The Matasiete Formation (Hauterivian?–Albian) is markedly less prolific in terms of fossils discoveries than either the Bajo Barreal Formation (Cenomanian–early Turonian) or the recently recognized Lago Colhué Huapi Formation (Coniacian–Maastrichtian). The Bajo Barreal fauna is, at a high level, typical of coeval Gondwanan faunas. However, interestingly, several taxa occupy a basal position within their respective groups. The Lago Colhué Huapi Formation has produced a more derived vertebrate fauna, again similar to those from other Gondwanan regions. Finally, in a broad context, the new materials described augment our understanding of Cretaceous terrestrial vertebrate assemblage of central Patagonia and add to the generally meager record of vertebrate in the Cretaceous of the Southern Hemisphere.

    更新日期:2020-01-04
  • Paleomagnetic evidence of the brittle deformation of the Central Patagonian Batholith at Gastre area (Chubut Province, Argentina)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-11-28
    Víctor Ruiz González; Carla G. Puigdomenech; Claudia B. Zaffarana; Haroldo Vizán; Rubén Somoza

    The Central Patagonian Batholith (CPB) in the Gastre area, central Patagonia, constitutes a set of two I-type Late Triassic plutonic suites, the Gastre superunit with a U-Pb zircon age of 221 ± 2 Ma, and the Lipetrén superunit with a U-Pb SHRIMP age of 215 ± 1 Ma. The source of this calc-alkaline batholith is characterized by crustal and mantle contributions, and it registers NW-SE subvertical magmatic and solid-state fabrics. These features, together with its intraplate position away from the inferred proto-Gondwana margin, have made it the focus of different tectonic interpretations. Early studies have interpreted it as the record of major dextral motions along the transcontinental NW-SE-striking Gastre fault system in Jurassic times. Later interpretations have proposed that the magmatic and tectonic foliations of the CPB were formed during a sinistral transpressive regime which was aided by Late Paleozoic widespread NW-SE subvertical structures in the North Patagonian Massif. Paleomagnetism, a unique tool to detect the presence of tectonic block rotations on vertical axes, was applied in the CPB in order to constrain the timing and type of deformation in the area. A paleomagnetic pole was obtained, which statistical parameters are: N = 45, Lat. = 81.4°S, Long. = 207°E, K = 11.5, A95 = 6.6°. Although the position of this pole does not coincide with equivalent Late Triassic poles, this position can be reconciled with the presence of a NE tilting of about 11° of the sampled block of the batholith. The tilting would have been aided by the NW-SE subvertical structures that affect the area. These paleomagnetic results rule out the possibility of vertical axis rotations from Late Triassic to present times and suggest that the ductile syn- to post-emplacement deformation of the CPB in Gastre area occurred during this period (Late Triassic), being the later brittle deformation triggered by the Andean Orogeny a possible explanation for this tilting.

    更新日期:2020-01-04
  • Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-03
    Fernando Martínez; Juan Kania; Belén Muñoz; Rodrigo Riquelme; Cristopher López

    Thick- and thin-skinned “hybrid” thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Perú, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26°S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.

    更新日期:2020-01-04
  • 更新日期:2020-01-04
  • Insight into the Katari-Lago Menor Basin aquifer, Lake Titicaca-Bolivia, inferred from geophysical (TDEM), hydrogeological and geochemical data
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-03
    Gabriela Patricia Flores Avilés; Marc Descloitres; Céline Duwig; Yvan Rossier; Lorenzo Spadini; Anatoly Legchenko; Álvaro Soruco; Jaime Argollo; Mayra Pérez; Waldo Medinaceli

    The increasing demand for water and irrigation in the semi-arid Bolivian Altiplano requires a better knowledge of the available resources, particularly groundwater. The aim of this study is to provide a first insight into the hydrogeological structure (0–200 m deep) and groundwater dynamics of the Katari-Lago Menor Basin aquifer located between the Eastern Cordillera and Lake Titicaca, Bolivia. This aquifer is studied using geophysical data (a total of 187 Time Domain Electromagnetic (TDEM) soundings), piezometric data (97 groundwater level measurements) and geochemical data (52 groundwater samples), combined with geological, lithological and topographical information. The results allowed identifying stratigraphic models consistent with the Quaternary sediments being hydraulically connected and behaving as a single regional basin-aquifer. This basin-aquifer is delimited by the most ancient lake invasions towards the southern, western and northern sides and by the lower limit of rock glaciers towards the eastern side. A large portion of the aquifer presents an unconfined behaviour varying from 50 to 150 m while the confined portion varies from 100 to 150 m. Groundwater flow within the Katari and Lago Menor Basin aquifer is composed of several interconnected groundwater flow systems. The main groundwater flow system starts in the high mountain ranges of the Eastern Cordillera, follows the topographic Piedmont gradient (NE to SW) and discharges in a series of wetlands. This multidisciplinary approach proved to be an appropriate method to derive a consistent picture of the hydrogeological functioning of the Katari-Lago Menor Basin aquifer.

    更新日期:2020-01-04
  • Volcaniclastic sedimentation influenced by logjam breakups? An example from the Blanco River, Chile
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-03
    Aldo M. Umazano; Ricardo N. Melchor

    The Blanco River drains the southern flank of the Chilean Chaitén volcano toward the Pacific Ocean. During 2008, much of the rhyolitic tephra erupted by the volcano was remobilized by this (and others) fluvial system. In the Blanco River, the sediments were temporarily stored upstream of the valley dammed by logs, epiclastic and pyroclastic detritus. The logjam generation was controlled by a complex combination of several factors including abundant tephra influx, rains, steep topographic gradients, forest vegetation, and widening of the fluvial course. Sediment-laden flows generated by logjam breakup or overflow severely affected the downstream-located Chaitén village. The resulting sedimentary record is remarkable by the important proximal-distal changes in sediment composition, grain-size and types of flow within specific depositional episodes. Exposures of fluvial deposits underlying those of the 2008 eruption in the upland sector of the Blanco River display a similar sedimentary record suggesting a comparable hydro-sedimentary dynamics during deposition. This contribution analyzed the sedimentary succession that underlies the 2008 eruption deposits, in order to: i) document in detail the sedimentary processes occurred in the Blanco River as consequence of the interaction with the Chaitén volcano tephra and analyze if this hydro-sedimentary behavior departs from the expected response in similar situations; and ii) contribute to the dating of generation and rupture of logjams and exploration of the associated volcanic hazard. Basic methodology included facies analysis, recognition and correlation of depositional episodes, and 14C dating of wood and charcoal in the deposits. Four depositional episodes (named A to D) were identified within the proximal and distal fluvial segments. The proximal sector is located along the current fluvial course and the distal sector is a lowland floodplain area. In the proximal fluvial segment, the depositional episodes representing syn-eruptive conditions are composed by volcaniclastic gravels and sands, formed by stream flows (with different sediment concentration) and debris flows; and reworked ash and lapilli strata generated by the dilute flow-debris flow continuum. In this fluvial segment, particularly in the depositional episodes B and C, there are downstream facies and compositional changes that suggest recurrent generation and rupture of logjams. Depositional episode B probably occurred between 1216 and 1294 cal AD and 1391–1438 cal AD as suggested by the radiocarbon dates, and is related with the widespread “Vilcún Tephra”. In the distal fluvial segment the depositional episodes record sedimentation of volcaniclastic mud in ponded zones or temporally flooded areas, with incorporation of abundant organic matter, associated with sediment (sandy)-laden dilute flows and scarce preservation of ash-fall beds. It is further proposed that downstream compositional changes in syn-eruptive fluvial deposits (volcaniclastic sediments that change downstream to pyroclastic sediments) can be indicative of the presence of former logjams in similar fossil fluvial volcaniclastic sequences.

    更新日期:2020-01-04
  • Acuetzpalin carranzai gen et sp. nov. A new ophthalmosauridae (Ichthyosauria) from the Upper Jurassic of Durango, North Mexico
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-03
    Jair Israel Barrientos-Lara; Jesús Alvarado-Ortega; Marta S. Fernández

    The family Ophthalmosauridae gathers the most derived ichthyosaurs from the Middle Jurassic - earliest Late Cretaceous marine deposits of North America, South America, Asia, Europe, and Australia. Although this group has a worldwide distribution, its records from the Caribbean realm, particularly from Mexico, are still comparatively scarce; therefore, the role of these fossils to understand the diversification of the family is negligible. Herein we describe Acuetzpalin carranzai gen. et sp., based on an almost complete specimen from the Kimmeridgian deposits at the Sierra de Palotes, Durango, central Mexico, which belong to the La Casita Formation. This new taxon exhibits a unique combination of features supporting its taxonomical singularity; these include the presence of the parietal foramen entirely surrounded by the parietals, a robust and short supratemporal process on the parietal, a long middle parietal symphysis, the internal carotid foramen exposed on the posterior surface of the basisphenoid, stapedial shaft moderately slender, the opisthotic and stapedial facets of the basioccipital laterally exposed, the intermedium separates the radius from the ulna and has no contact with the humerus, and the nasal lacks the lateral “wing” over the dorsal border of the naris. This fossil, the second ophthalmosaurid species found in Mexico, complements the ichthyosaurs diversity in the western domain of the Tethys Sea, at the southern end of North America. A phylogenetic analysis reveals that Acuetzpalin carranzai is member of the subfamily Platypterygiinae and is located among the most-derived platypterygiines.

    更新日期:2020-01-04
  • Comments to “high-pressure eclogite facies metamorphism and decompression melting recorded in Paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)”
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-02
    Hans-Joachim Massonne

    The comments to the publication by de Oliviera Chaves et al. (2019) refer to the derivation of eclogite-facies conditions for micaschist from the Paleoproterozoic Itaguara Sequence in the southern São Francisco Craton in Brazil. These authors derived peak pressures of 18.5 kbar at 626 °C for this rock using phengite geobarometry. A scrutiny of their analytical data and the applied geothermobarometry led to the conclusion that these pressures were at most 13–14 kbar and possibly even below 10 kbar. Temperatures did not exceed about 600 °C. Thus, melting of the studied micaschist is unlikely. A subduction-related process in the Paleoproterozoic to form the Itaguara Sequence, as suggested by de Oliviera Chaves et al. (2019), must be called in question.

    更新日期:2020-01-04
  • Study of seismic activity at Ceboruco Volcano (Nayarit, Mexico) in the period 2012 to 2014
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-31
    Francisco Javier Núñez-Cornú; Felipe de Jesús Escalona-Alcázar; Diana Núñez; Elizabeth Trejo-Gómez; Carlos Suárez-Plascencia; Norma Rodríguez-Ayala

    Ceboruco volcano is a stratovolcano located in western central Mexico within the state of Nayarit. Ceboruco volcano is one of the active volcanoes of the Trans-Mexican Volcanic Belt and lies within the Tepic-Zacoalco Rift structure. Eruptions at Ceboruco volcano have been reported over the last 1000 years with eruptions every 126 years on average, with the most recent eruption occurring in 1870. Current activity at Ceboruco volcano is primarily fumarolic emissions. Continued population and socio-economic growth around the volcano over the last 20 years, make urgent the need to study Ceboruco volcano. Previous investigations of seismicity classified the recorded earthquakes into four families using waveform and spectral features. In this paper, we present a seismicity study from March 2012 to July 2014 using four portable seismic stations within the context of the regional stress and structures near Ceboruco volcano. Of the 489 volcanic earthquakes recorded during this time period, only 33 could be located using P- and S-wave arrivals registered at least three stations. The P- and S-phase arrival times were obtained using particle motion. The epicentral distribution of these earthquakes is around the volcanic edifice along three structural lineaments with preferred ENE-WSW orientations, roughly perpendicular to Tepic-Zacoalco Rift and, following the youngest deformation pattern. Hypocentral depths locate within first 10 km, indicating the earthquakes are a result of local tectonic stresses, intrusions into the magma chamber, or both.

    更新日期:2020-01-04
  • Taxonomy and paleoenvironmental inferences from fossil vertebrates of Paripiranga Borboletas Cave, Northeastern Bahia, Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-31
    Johnson Sarmento de Oliveira Nascimento; Édison Vicente Oliveira; Jorge Luiz Lopes da Silva

    The fossil mammals that lived in the Brazilian Intertropical Region during the Quaternary are found in different types of deposits (e.g. natural tanks, ponds, caves, rockshelter, and riverbeds) throughout the Brazilian territory. The state of Bahia, located on northeastern Brazil, stands out for the presence of large and important caves, which are characterized by the natural deposition of sediments and fossils. The study aims to identify the fauna found in the Borboletas Cave, and to provide the first paleoecological inferences for the environment during the Late Pleistocene-Holocene. Here we present the first record of Palaeolama major, Hippidion principale and Smilodon populator in Paripiranga, Bahia. Coendou sp., Mazama gouazoubira, and Toxodon platensis, already known to the region, were also collected and identified. Based on the chronological and geographical distribution of the taxa, it is suggested that the faunal content found lived during the Late Pleistocene – Holocene. The presence of the taxa found indicate an open paleoenvironment, similar to savanna, with areas covered with grasses associated with areas with forest cover. This study demonstrates that the Borboletas Cave is the richest paleontological site carrying the mammals in this region.

    更新日期:2020-01-04
  • Genesis and classification of soils from granitic hills in southern Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-30
    Paula Suélen Corrêa de Medeiros; Paulo César do Nascimento; Alberto Vasconcellos Inda; Luís Fernando da Silva

    Southern Brazil abounds with granitic hills capped by intricately distributed soils most of which have been classified as Entisols or Inceptisols in medium- and small-scale soil surveys. However, field observations have revealed substantial texture gradients that are consistent with better-developed soils; also, the physical, chemical and mineralogical properties of the soils suggest that they are only slightly to moderately weathered. In this work, we examined the influence of formation-related factors and processes in soils from two granite toposequences. The eight soil profiles studied for this purpose were shallow, their C horizons occurring at depths of less than 100 cm. The low degree of flocculation and the fine/total clay ratio of the soils suggest that lessivage, which contributed to the development of the texture gradient in six of the profiles, was the main pedogenic process responsible for the formation of these soils. The soils at topographically high points were dystrophic or allitic, whereas those at low points were eutrophic. Based on their Ki values, the soils were weathered to a medium extent, which is in conflict with their low clay activity and cation exchange capacity. On the other hand, the Fed/Fes ratios of the soils were consistent with strong weathering, which in turn was consistent with their pH and exchangeable Al values. The iron oxides contents increased with increasing depth in the profiles as a result of lessivage. Feo/Fed ratios were suggestive of a prevalence of crystalline iron oxides. Increased substitution of Al in Fe oxides in the topographically higher soils further confirmed their increased weathering. Based on the foregoing, the soils were classified as Ultisols/Alfisols, Entisols and Inceptisols, the last series being less frequent than suggested by existing soil surveys. The transitional nature of these soil classes reflected in the presence of Ultisols/Alfisols with a thin B horizon, and also of argilluvic Inceptisols.

    更新日期:2020-01-04
  • Organic facies and palynology from the middle to late Devonian of the Pimenteiras formation, Parnaiba Basin, Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-30
    Consuelo Lima Navarro De Andrade; Tereza Regina Machado Cardoso; Rafael Regueira Santos; Rodolfo Dino; Altair De Jesus Machado

    A detailed study of organic facies and palynological assemblages was carried out on 11 samples of potential hydrocarbon source rocks of the Pimenteiras Formation, an outcropping at the western edge of the Parnaiba Basin, Brazil. The main objectives are to characterize the depositional paleoenvironment and hydrocarbon source potential. The selection of this outcrop occurred in the function of its preservation and accessibility, and the samples collection observed 1 m of vertical spacing. Generally, the organic matter and the palynomorphs showed a good state of preservation and fluorescence. The most notable spores and acritarchs and phycomates species were discussed in light of international biozones, and the chronostratigraphic range defined as Late Eifelian - Latest Frasnian in age. TOC results show ranges from 0.40 to 2.91%, with most samples showing greater than 1%, thereby holding a high potential generation. The S1 values were virtually null, and the S2 peak ranged from 0.9 to 5.33 mg/g. The kerogen 435 °C, and the ICE between 4.0 and 4.5 demonstrating the material immaturity. For the palynofacies associations definition, only 10 out of the 25 total quantified of subgroups of kerogen categories were considered in the counting of 300 particles. A dendrogram constructed based on these results through cluster analysis, revealed four palynofacies, as follows: I constituted mainly by palynomorphs (acritarchs, prasinophytes and sporomorphs); II – Dominance of Translucent organic matter (cuticles, epidermal tissue and translucent phytoclasts with and without structure); III - Dominance of Opaque/non-fluorescent (opaque phytoclasts and non-fluorescent AOM) and IV - Dominance of AOM fluorescent (amorphous organic matter with fluorescence). The palynomorphs are the most abundant constituents of all studied kerogen. Quantitative analysis based on the distribution of palynomorphs groups throughout the section indicates the dominance of sporomorphs in the palynological assemblages from the Middle Devonian aged samples. On the other hand, occurred the predominance of microplankton elements in the Late Devonian aged samples. Thus, the depositional paleoenvironment was established as a distal oxic mud-dominated shelf with the deltaic influence of the Late Eifelian to Late Givetian, and there is a tendency distal (dysoxic-anoxic ‘shelf’) from the Frasnian, which indicates the maximum flooding surface.

    更新日期:2020-01-04
  • Eruptive style and flow dynamics of the pyroclastic density currents related to the Holocene Cerro Blanco eruption (Southern Puna plateau, Argentina)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-30
    W. Báez; E. Bustos; A. Chiodi; F. Reckziegel; M. Arnosio; S. de Silva; G. Giordano; J.G. Viramonte; M.M. Sampietro-Vattuone; J.L. Peña-Monné

    The Pleistocene-Holocene Cerro Blanco Volcanic Complex (CBVC), one of the youngest caldera complexes in the Southern Central Andes, is the source of possibly one of largest Holocene eruptions on Earth, the 4.2 ka, Cerro Blanco eruption. This caldera forming eruption is the younger of two major explosive events from the CBVC. Previous work has estimated the range from VEI 6 to 7, yet to date there is no detailed study of the stratigraphy and volcanology of the proximal deposits and dynamics of the Cerro Blanco eruption. Here we present the first detailed analysis of the eruptive products of the Holocene Cerro Blanco eruption that reveal the eruptive sequence highlighting the flow dynamics of the related pyroclastic density currents (PDCs). The PDCs were mainly inertia-dominated, however, channelization of parental PDCs into deep valleys resulted in the flow transformation to forced convection-dominated flows. In addition, topographic constriction in valleys enhanced the sedimentation rate producing regressive bed forms and ultimately the avulsion of the main path of the PDCs resulting in flooding of secondary valleys. A model is presented whereby simultaneous convective and collapsing eruptive column dynamics were established and sustained throughout the eruption. Towards its end, instabilities of the column occurred in response to the climax of a protracted incremental caldera collapse. This eruptive sequence is similar to those observed in well-documented small collapse calderas. An important unresolved issue for the CB eruption is it volume. The currently estimated volume of 83 km3 (DRE) by Fernando-Turiel et al. (2019) is inconsistent with the size of the Cerro Blanco caldera and to date the over thickening of the distal ash by local rework is poor assessed. Further work is needed to fully evaluate this mismatch and accurately estimate the volume of this important Holocene eruption.

    更新日期:2020-01-04
  • Age constraints on the Paleozoic Yaguarí-Buena vista succession from Uruguay: paleomagnetic and paleontologic information
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-28
    Marcia Ernesto; Pablo Núñez Demarco; Pedro Xavier; Leda Sanchez; Cesar Schultz; Graciela Piñeiro

    The Yaguarí and the Buena Vista formations from Uruguay are historically correlated to the Brazilian Rio do Rasto and Sanga do Cabral formations, respectively, as they have some lithostratigraphic similarities, indicating a Permo-Triassic or even Triassic age of the Yaguarí-Buena Vista succession. However, they differ in the fossil indexes that characterize the faunistic communities present in both countries. A paleomagnetic work was carried out on some sections of the Buena Vista and the Yaguarí formations, as well as on some layers of bentonites, underlying the Buena Vista sediments. The alternating field and thermal demagnetization procedures revealed both normal and reversed magnetization components, but the samples showed evidence of secondary magnetic minerals and possibly remagnetizations. The calculated paleomagnetic pole for the Yaguarí-Buena Vista Formation plots near to the poles for the Choiyoi magmatism that is believed to be responsible for the bentonite accumulation; it is also in agreement with other Permian paleomagnetic poles for South America. Based on the paleomagnetic results, the available radiometric data for the bentonites, and the fossiliferous content, a Late Permian (Lopingian) age is assigned to the Yaguarí-Buena Vista rocks.

    更新日期:2020-01-04
  • Geological evidences as a guide to OSL dating interpretation and northern occurrence of MIS 7e barrier at Southern Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-28
    Carlos Conforti Ferreira Guedes; Mylene Giseli do Nascimento; Rodolfo José Angulo; Maria Cristina de Souza

    The chronology of Pleistocene strandplains remains a challenge to this day. The lack of reliable geochronology methods arises from the fact that ages from the Pleistocene sequences are beyond radiocarbon age limit method and, sometimes, near the saturation limits to luminescence dating on quartz, which may introduce uncertainties in the results. Also, choosing different curve fits for the dose-response curve leads to geological incoherent results. In this paper, we evaluated three approaches on curve fit equations (single saturating exponential (SSE), double saturating exponential (DSE) or the SSE + Linear (E + L). We also compared the three scenarios with geological evidences such as sedimentary structures, like plane-parallel lamination, and the presence Ophiomorpha nodosa ichnofossil at Paranaguá and Superagüi strandplains, Southern Brazil. Since geological evidences indicate deposition under wave-dominated shoreline, the comparison to Quaternary sealevel curves restricts the beach ridges deposition to high relative sealevel intervals. Our results show that, despite being mathematically consistent, DSE and E + L curve fits lead to geological inconsistent results. The luminescence results using SSE curve fit are geologically consistent and suggest the occurrence of MIS 7e barrier on Paranaguá and Superagüi strandplains, the farther north occurrence of the Barreira II (195–220 ky) correspondent to the Rio Grande do Sul strandplain. However, we stress out that luminescence dating results near the saturation limit must be interpreted with caution, and preferably supported by other evidences.

    更新日期:2020-01-04
  • First record of a fossil procyonid (Cyonasua cf. C. pascuali), Mammalia, Procyonidae) in Bolivia, Tariquía Fm., Late Miocene
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-28
    Leopoldo H. Soibelzon; Martín De los Reyes; Juliana Tarquini; David E. Tineo; Daniel G. Poiré; Gloria González; Gustavo D. Vergani

    We present the first record of an extinct procyonid for Bolivia (Tariquía Formation, La Angostura, Santa Cruz de La Sierra; Late Miocene). It corresponds to Cyonasua, the first Carnivore taxon to enter South America from North America before the Paniamian Isthmus raised and as a part of the “proto Great American Biotic Interchange” (GABI). Cyonasua fossil record, comprises the temporal lapse late Miocene to early Pleistocene in several localities of Argentina, Venezuela and Uruguay. This genus encompasses ten formally named species: C. argentina, C. brevirostris, C. longirostris, C. pascuali, C. groeberi, C. lutaria, C. clausa, C. robusta, C. argentines, and C. meranii. The mandible and teeth preserved of the new specimen, YPFBLIT-PAL-020, allows us to assign it to Cyonasua cf. C. pascuali. This record reinforces the paleoenvironment with warm climates and humid environments with abundant water bodies, vegetation and trees proposed previously for Tariquia Fm. Also, the proposed Huayquerian Age for this Formation is supported by this record.

    更新日期:2020-01-04
  • Wave-dominated lacustrine margin, Aptian pre-salt, Mucuri Member, Espírito Santo Basin
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-28
    Camila Eliza Althaus; Claiton Marlon dos Santos Scherer; Juliano Kuchle; Adriano Domingos dos Reis; João Pedro Formolo Ferronatto; Luiz Fernando De Ros; Tatiana Pastro Bardola

    The Espírito Santo Basin, one of the Brazilian marginal basins formed during the Gondwana break-up, has been the subject of many studies due to its important hydrocarbons accumulations, comprising three main depositional units – rift, sag and drift phases (Winter et al., 2007). The Mucuri Member sandstones are the Aptian sag phase related, onshore reservoirs deposited at the margins of the lacustrine system were the pre-salt (pre-evaporitic succession) lacustrine carbonate reservoirs were generated. The main objective of this article focuses on the stratigraphic and sedimentological definition of depositional model from the top of Mucuri Member, in the transition to Itaúnas Member evaporites of Mariricu Formation, since the depositional conditions of this interval until date remain a subject of discussions and can contribute to the understanding of the correlated pre-salt reservoirs. Core description in detailed scale (1:50) associated with gamma-ray logs allowed the identification of nineteen facies, grouped into five facies associations: gravelly fluvial channel, poorly confined fluvial channels, upper shoreface, lower shoreface and offshore. Stacking patterns of facies associations allowed the definition of four cycles: T, T-R, normal R and forced R cycles. Although these cycles are easily recognizable in the cores, their correlation between wells was not possible. Geochemical analyses of 87Sr/86Sr ratio from anhydrites layers interbedded with the Mucuri sandstones confirmed a non-marine composition for the precipitating fluids. The integrated evidence suggests a wave dominated lacustrine environment for the subaqueous Mucuri Member deposits.

    更新日期:2020-01-04
  • Patterns and origins of the porosity in the productive reservoirs of the upper part of the Chubut Group, southern flank of the Golfo de San Jorge Basin, Patagonia Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-27
    C.O. Limarino; S.R. Giordano; R.J. Rodriguez Albertani; P.L. Ciccioli; F. Bodan

    The Mina del Carmen, Bajo Barreal and Cañadón Seco Formations bear the most important Cretaceous reservoirs in the southern flank of the Golfo de San Jorge Basin, in which the porosity is mainly secondary in origin. Primary porosity is about 4% in average and can be divided in intergranular and intercrystalline porosity, the former is better preserved where chlorite rim cement is abundant, and the second occurs among crystals of the clay cement (main kaolinite). Secondary porosity comprises dissolution of feldspar, ductile fragments, clay cements, lithic grains, and carbonate-zeolite cements. Dissolution of feldspar produces the formation of authigenic kaolinite and secondary porosity; this mechanism dominates in the western area of the southern flank due to sandstones show higher amounts of plagioclase grains, which were very reactive to diagenetic dissolution. Ductile fragments, composed of tuffaceous mudstones (probably of intrabasinal origin), were strongly deformed during the compaction reducing the primary porosity and closing the pore system. Nevertheless, in sandstones containing similar proportions of intrabasinal and extrabasinal clasts, some ductile grains suffered massive dissolution increasing the secondary porosity. Another source of secondary porosity comprises the dissolution of tuff fragments, and locally, carbonate and zeolite cements. The secondary porosity includes the types redistributional (RDS) and pervasive (PVS). This division can be related to previous diagenetic model for the Bajo Barreal Formation. In this way, redistributional secondary porosity is linked to the first and the second events of diagenetic dissolution, while the pervasive secondary porosity to the third dissolution event, which likely had a telogenetic origin.

    更新日期:2020-01-04
  • Palaeoenvironment associated with giant tortoises of Toropí/Yupoí Formation (Late Pleistocene 50–28 ka), Corrientes province, Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-24
    L. Fernandez Pacella; G.G. Zacarías; M. Di Pasquo

    Vertebrate faunas of the Mesopotamian region in Argentina provided palaeoclimatic and palaeoenvironmental information from the Pleistocene. Samples collected into a carapace of the turtle Chelonoidis lutzae and associated sediments obtained from the Toropí/Yupoí Formation (Arroyo Toropí, Bella Vista) palynologically studied for the first time. The palynomorph association includes angiosperms (Brassicaceae, Amaranthaceae, Poaceae, Sapotaceae, Fabaceae, Euphorbiaceae, Myrtaceae), Bryophyta, Pteridophyte (Microgramma squamulosa), chlorophytes (Pediastrum tetras, Botryococcus) and fungal spores (Dichomera, Phaeoseptoria sp., Phragmotrichum sp., Agaricus sp.). Palaeoecologic requirements of these botanical groups indicate the predominance of herbaceous palaeocommunities associated with arboreal or shrubby elements and fresh waters bodies. They characterized dynamic scenarios typical of ecotonal zones under arid/semi-arid and humid warm seasonal climates during the Late Pleistocene (Last Maximum Glacial) in the Mesopotamian region.

    更新日期:2020-01-04
  • Early to Middle Jurassic San Andrés-Cedros plutonic suite, western coast of Baja California, Mexico: Geochemical and isotopic evidence for an island arc extending to the central peninsula
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-23
    Xochitl Guadalupe Torres-Carrillo; Luis Alberto Delgado-Argote; Bodo Weber; Manuel Contreras-López

    In the Vizcaíno peninsula and Cedros Island region, west-central part of the Baja California peninsula, México, Upper Triassic ophiolitic sequences are intruded by arc-related Mid-Jurassic plutons. We analyze the ∼150 Ma old San Roque pluton (SRp) in the Vizcaíno peninsula and plutonic rocks (∼165 Ma) of the Punta Norte zone (PN) in the Cedros Island. The modal composition of the almost 10 km2 extending San Roque pluton varies from biotite-hornblende quartz diorite to biotite-hornblende tonalite intruded by granodioritic dykes. Intrusive rocks are subalkaline, calcic/calc-alkaline and metaluminous, akin to island arc granite setting. The primitive initial εNd values of +8.9 and + 9 of SRp quartz diorite and tonalite are indicative of a depleted mantle source without crustal assimilation. The sequence intrudes amphibolites that are interpreted as metavolcanic remnants of the oceanic-type crust of the Vizcaíno peninsula ophiolite. One amphibolite sample is characterized as magmatically poorly evolved, with flat REE patterns, and MORB-type initial εNd values of +10.1. The PN plutonic rocks of Cedros Island crop out at its northern edge. Their modal compositions are hornblende quartz diorite, hornblende leucotonalite, and microtonalitic dykes. These rocks are classified as subalkaline, magnesian, and calcic, typical island arc granites. The REE patterns tend to be subparallel, with progressive enrichments in REE abundances, suggesting fractional crystallization resulting from the evolution of a single magmatic system. Microtonalite dykes and leucotonalite yield initial εNd values of +7.8 to +8, indicating that the PN rocks were produced from magmas associated with a depleted mantle source. These values are in agreement with the correlation of εSr(165 Ma) vs. εNd(165 Ma), where all samples plot in the depleted mantle field. The PN plutonic rocks intruded pillow basalts with basaltic dykes. Their initial εNd and εSr plot in the depleted mantle field indicative of island arc tholeiites. The SRp and PN plutonic rocks are correlated with the coeval El Arco system (∼160 Ma) in the central part of the Baja California peninsula. They show the same magmatic evolution in a primitive oceanic island arc environment derived from a depleted mantle with no crustal assimilation. We suggest that all the Jurassic sequences of the southern Peninsular Ranges batholith belong to the same arc.

    更新日期:2020-01-04
  • Re-interpretation of volcanic units from San Ambrosio Island and Gonzalez Islet, Southeast Pacific, Chile: Using new textural and geochemical data
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-23
    Nicolás Philippi; Cristian Rodrigo

    The San Felix - San Ambrosio Oceanic Volcanic Complex (SSOVOC), located in the Southeast Pacific, 940 km offshore northern continental Chile, corresponds to a case of intraplate volcanism in the Nazca Plate. The complex is comprised of San Felix, San Ambrosio and González islands, which are mainly composed of alkaline lavas and hyaloclastic sequences. San Ambrosio Island (2.86–2.93 Ma) would be made up mainly, in its subaerial section, of two eruptive shield-type events called Carpa Group and Fardelas Group, the latter event being slightly more alkaline than the Carpa Group. The González Islet (1 Ma?) is made up of a Hyaloclastic Unit, submarine and subaerial basanitic lava flows and the Punta Bari Group, the latter described previously on San Felix Island. The Hyaloclastic Unit and basanitic lava flows have textures associated with an underwater deposition environment and a subsequent transition to a subaerial environment. Geochemical data indicate an increase in alkalinity between San Ambrosio Island and González Islet and different degrees of partial melting based on Th/Nd vs Th and Rb/Nd vs Rb data and enrichment in incompatible trace elements suggest a decrease in partial melting between the generating event of San Ambrosio Island and González Islet. The isotopic mixing model of Sr and Nd indicates that this volcanic complex represents a mixing of 35%–45% between DM and EM-I members, although from the San Ambrosio Island to González-San Félix Islands there is an independent trend between both islands towards EM-I, which suggests a cortical contamination of the same volcanic edifice of which the three islands of the complex are part.

    更新日期:2020-01-04
  • Petrology of the Afonso Cláudio Intrusive Complex: New insights for the Cambro-Ordovician post-collisional magmatism in the Araçuaí-West Congo Orogen, Southeast Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-19
    Ramon de Oliveira Aranda; Alexandre de Oliveira Chaves; Edgar Batista de Medeiros Júnior; Ricardo Venturini Junior
    更新日期:2020-01-04
  • Spatio-temporal variations in the Mesozoic sedimentary record, Golfo San Jorge Basin (Patagonia, Argentina): Andean vs. cratonic sources
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-17
    Nicolás Foix; José Oscar Allard; María Leonor Ferreira; Mario Atencio

    The continental succession of the Cretaceous Chubut Group (Aptian-Maastrichtian) in the Golfo San Jorge Basin (GSJB), central extra-Andean Patagonia (Argentina) contains a high proportion of volcaniclastic input derived from explosive volcanism at the Andes Cordillera. Cutting records obtained from regionally distributed oil-wells distanced by hundreds of kilometers show a significant west-to-east decreasing in the proportion of tuffaceous components. These regional lithologic changes currently difficult the recognition of lithostratigraphic units both in the western Río Mayo sub-basin (high tuffaceous content) and in the eastern basin center-offshore (low tuffaceous content). We use seismic sections and cutting records for: a) to study a basement intra-basin paleo-high in the Northern Flank (Río Chico High) and b) to analyze the marginal basin substrate in offshore areas of the Northern Flank. The sedimentary record adjacent to the igneous-metamorphic Río Chico High contains variable amounts of quartz and detrital muscovite, identified in marine, pre-rift sequences of Lower Jurassic age and within the continental units of the Chubut Group. Basement influence in the infill composition decreases both upward and away from the Río Chico High, regarding the gradual burial of the paleo-high. Our data reveal that the igneous-metamorphic basement remained as a positive paleorelief compartmentalizing the basin and providing siliciclastic sediments at least during 100 millions of years, from early Jurassic to late Cretaceous, even before the beginning of the extensional phase that originated the basin in the uppermost Jurassic. Thus, the GSJB displays a spatially variable volcaniclastic record derived from the Andes, distorted by the presence of inherited morpho-structures that supply siliciclastic sediments provided from cratonic sources.

    更新日期:2020-01-04
  • Sandstone petrography and provenance of the Chubut Group (Cretaceous) in the Cañadón Matasiete (Golfo San Jorge Basin, central Patagonia): Implications for basin evolution and alluvial organization
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-16
    Sabrina Ximena Olazábal; Maísa Andrea Tunik; José Matildo Paredes

    Sandstone petrographic data have been used to investigate the provenance of the Matasiete, Pozo D-129, and Castillo formations (Chubut Group) in the Cañadón Matasiete, San Bernardo Fold Belt at Golfo San Jorge Basin, Argentina. Available petrographical data provides evidence of several distinctive volcanic source areas, with minor contributions of metamorphic, plutonic, or sedimentary components. Paleo-volcanic contributions from basic-intermediate (lathwork and microlithic textures) and acidic compositions (felsitic, seriate, granular, and vitric textures) were mainly derived from the Middle-Upper Jurassic rocks of the Lonco Trapial Formation. Pyroclastic components (glass shards, pumice fragments, and eutaxitic textures) derived from coeval volcanic activity in the Andes Cordillera, generated either gradual or abrupt changes in the source of the lithic fragments along the study succession. The observed trends in detrital lithic provenance can be integrated with previous sedimentological and architectural studies of the Matasiete and Castillo formations, revealing that narrow-sheet geometries of the channel fill characterize stratigraphic intervals dominated by pyroclastic components, whereas stratigraphic intervals that contain a low content of pyroclastic materials in the channel fill show ribbon-shaped geometries. Sandstone petrography in volcaniclastic basins can be used to complement other indicators of the evolution of the fluvial systems, providing reliable information about the nature of the changes in the sediment supply within the drainage catchment.

    更新日期:2020-01-04
  • The enigmatic flat-pebble breccia of the Sete Lagoas Formation (Bambuí Group, Brazil): Evidences of seismic-induced deformation in an Ediacaran carbonate platform
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-11
    J. Okubo; L.V. Warren; G.L. Luvizotto; F.G. Varejão; F. Quaglio; G.J. Uhlein; M.L. Assine

    The Sete Lagoas Formation (Ediacaran), located in the central part of the São Francisco Craton (Brazil), consists of limestones and dolostones deposited in very shallow waters in the inner part of a rimmed carbonate platform. Four breccia types occur throughout the stratigraphic succession: evaporitic breccia with tepees, flat-pebble breccia, hydrothermal breccia and brecciated stromatolites. Here we combined a detailed sedimentological and stratigraphic analyses of the flat-pebble breccia in order to determine its origin and the processes and environmental conditions that originated these brecciated facies. The studied interval consists of a 20 m-thick succession of tabular beds composed of flat-pebble breccia interbedded with laminated microbialites. In these breccia beds, the clasts are usually platy or oblate with angular edges and are mainly disposed horizontally within the sedimentary bed, suggesting that they were little transported or reworked. The presence of clasts with sharp edges and vertices in the Sete Lagoas flat-pebble breccia suggests that the lithification process started very early in diagenesis; even the sediments exposed at the bottom were, at least, partially lithified. Some breccia levels show bidirectional imbrication. Clast size analyses reveal that the higher aspect ratio clasts show a NE-SW orientation whereas square clasts tend to fill the space among oriented clasts. Breccia clasts are vertically oriented and show deformation features increasing upwards typically of deforming beds caused by ascendant expulsion of liquefied sediment. These features are found both in modern and ancient deposits of seismic influence, which suggests a similar origin for the Sete Lagoas flat-pebble breccia. Thus, the processes that led to the formation of the studied flat-pebble breccia are interpreted as seismically triggered, since: a) the breccia beds are laterally continuous and extend for several kilometers; b) the breccia beds are restricted to a 20 m-thick stratigraphic interval; c) the interbedding of breccia beds and laminated microbialite beds is recurrent; d) the rock layers are subhorizontal and present irregular upper and lower contacts; e) the presence of liquefaction structures and dyke injection. Thus, this seismic-triggered breccia deposits represent the product of the synsedimentary tectonism occurred within the São Francisco Craton during the terminal Ediacaran and correspond to a very well-defined local stratigraphic marker in the Bambuí basin. The exact location of the epicenter is uncertain, but it could be related to the NW regional faults in the regional Paleoproterozoic basement of the study area, page17which were reactivated during the deposition of the Sete Lagoas Formation.

    更新日期:2020-01-04
  • Fracture network analysis of Yacoraite Formation in the Tres Cruces sub-basin, northwestern Argentina
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2019-12-02
    Clara Correa Luna; Daniel L. Yagupsky; Jeremías Likerman

    The Tres Cruces sub-basin, located in Jujuy province, northwestern Argentina, is characterized by intense N-S folding and faulting. These structures were formed as a result of the Cenozoic shortening that produced the tectonic inversion of the Salta Rift Basin normal faults. Some of the main folds and faults show abrupt trend variations, controlled by NW-SE transverse lineaments. We performed a fracture network analysis over the Maastrichtian-Danian limestones of the Yacoraite Formation, at three folds located in the central sector of the sub-basin. A total of 832 planar fractures were measured in different structural domains. Five main fracture sets were identified, trending NW-SE, NE-SW, ENE-WSW, WNW-ESE and N-S to NNW-SSE. Their relative chronology was established based on the observed abutting relationships. The analysis performed suggests that NW-SE, NE-SW and ENE-WSW trending sets are regionally represented and were formed before the initial stages of folding. Set ENE-WSW is sub-parallel to the convergence direction acting during the Neogene. The other two sets are associated with local perturbations of the far-field signal control by the oblique NW-SE lineaments. The origin of the WNW-ESE set remains unclear; its distribution is locally restricted to the San Bernardo domain. Finally, the N-S to NNW-SSE set shows a synfolding origin generated when the surveyed folds were growing. We then place the various identified fracture sets into a single stress field setting, perturbed by the leading NW-SE transverse lineaments that dominate this region.

    更新日期:2020-01-04
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