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  • Do the morphological characteristics of Chilean gravel-bed rivers exhibit latitudinal patterns?
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-12
    H. Ulloa; A. Iroumé; L. Picco; G. Vergara; T. Sitzia; L. Mao; B. Mazzorana

    Gravel-bed rivers exhibit remarkable dynamics and distinctive morphological patterns which influence the biological processes, as well as the river ecosystem resilience to human impacts. Classifications of river ecosystems into groups with morphologically distinct patterns are therefore of paramount importance for a better understanding of their dynamics, their management and conservation. Investigations on fluvial morphology and the related dynamics in Chilean gravel-bed rivers are still scarce, and, so far, no efforts have been made to identify latitudinal patterns of their morphological characteristics that may serve as a basis for further classification purposes. In fact, the great variability in bio-geographic , climatic, geologic and geomorphologic settings of the country offer an almost unique opportunity to study how the morphological characteristics of gravel-bed rivers may vary within extremely heterogeneous environments. We performed a regional analysis of nineteen Chilean gravel-bed rivers, from semi-arid Mediterranean to rainy temperate conditions (latitude ranging from 32º S to 39º S). Planform characteristics within these reaches were identified through interpretation of satellite images. Topographic features of the river basins were derived from open source digital elevation models and water discharge values were obtained from available national databases. We addressed the following research questions: a) can these 19 rivers be grouped into clusters considering their distinct planform characteristics of the river channel and basin features, as related to latitudinal gradients?, b) do river morphological characteristics vary along the analyzed latitudinal gradient?, c) does the distribution of specific geomorphic units (i.e. islands) follow a longitudinal pattern?, and finally, d) are these units associated to specific hidrogeomorfic characteristics? We generated a clustering which reflects latitudinal and morphological gradients in Chilean gravel-bed rivers, with the northernmost and southernmost rivers grouped according to their distinctive hydro-morphological characteristics and geomorphic units. We also observed a general tendency of an increase in the proportion of fluvial islands relative to the active channel area from the 33 ° Latitude in southward direction.

  • FTIR spectroscopic features of the pteridosperm Ruflorinia orlandoi and host rock (Springhill Formation, Lower Cretaceous, Argentina)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-11
    Maiten A. Lafuente Diaz; José A. D’Angelo; Georgina M. Del Fueyo; Martín A. Carrizo

    Ruflorinia orlandoi (Pteridospermophyta) fronds are chemically analyzed for the first time by semi-quantitative Fourier Transform Infrared (FTIR) spectroscopy. This analysis allows the chemical characterization of the mesophyll and cuticle revealing the functional groups preserved in different frond parts (pinnae and rachis). The specimens collected in the Springhill Formation at the Río Correntoso locality (Lower Cretaceous, Santa Cruz province, Argentina) are compressions with very well-preserved cuticular features. The R. orlandoi remains (pinnae and rachis) are spectroscopically analyzed into two samples: compressions (Cp) and cuticles (Ct). Additionally, a third sample form from the host rock and named associated coal (V) is spectroscopically analyzed. Semi-quantitative data derived from Cp, Ct, and V spectra are evaluated by principal component analysis. The results indicate that Cp samples have a similar chemical composition whereas Ct samples show a greater variability. The latter could be related to intraspecific variability of foliar characters (e.g., trichomes and cuticular striations). Furthermore, Ct samples exhibit high contents of aromatic carbon groups suggesting a distinctive composition, likely including cutin/cutan biomacropolymers and/or phenolic compounds. Considering each specimen, the rachis shows a higher aromatic carbon content than pinnae as a consequence of the presence of more lignified tissues in the former. The V samples have the lowest relative intensity of aliphatic groups. On the other hand, the functional-group composition of R. orlandoi remains and V samples are compared with kerogen types and coal macerals showing a general chemical composition similar to type II kerogen. The latter is related to cuticles, spores, pollen grains, and resins.

    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-07
    Ricardo Barragán; Rafael López-Martínez; Bruno Chávez-Vergara; Fernando Núñez-Useche; Hugo Salgado-Garrido; Agustín Merino

    A multiproxy study of a 29 m-thick carbonate succession in the Apulco area, Puebla State in central Mexico including pyrite framboids, δ13Ccarb, δ18Ocarb, Total Inorganic Carbon (TIC), Total Organic Carbon (TOC) and Organic Matter (OM), reveals minor geochemical changes across the Jurassic/Cretaceous boundary. Anoxic bottom conditions during the Tithonian were registered by high values of TOC and OM, and low values of δ13Ccarb and TIC. Near the Jurassic/Cretaceous boundary, signatures of all measured proxies changed drastically due to processes related with the organic matter preservation in the basin. These changes can be associated with the late Tithonian sea level drop and paleoclimatic variations passing from dry to semi-arid conditions. Otherwise, on late early to late Berriasian facies, values of all proxies tend to the stabilization indicating more oxygenated bottom conditions. The δ13Ccarb goes to positive values while δ18Ocarb and TIC remain constant, the organic matter and total organic carbon decrease to the lowest values. The geochemical behavior of the Apulco section is similar to those reported from other Tethyan sections as Brodno and DSDP 534A and can be preliminary correlated by its chemostratigraphic signal. Additionally, a central age of 141.65 Ma (87Sr/86Sr) in the very base of the calpionellid Elliptica Subzone strengthens the calpionellid biostratigraphic framework for Mexican sections.

  • Mesozoic Biochronostratigraphy and Paleoenvironment of the South Atlantic: A revised framework based on 20 DSDP and ODP deep-water sites
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-07
    Eiichi Setoyama; Sudeep Kanungo

    The South American and West African margins of the South Atlantic and the Southern Ocean are characterized by Mesozoic black shales that serve as source rocks of known and unknown petroleum systems. The knowledge of regional chronostratigraphy and paleoenvironment enables the prediction of the stratigraphic and geographic distribution of black shales with precision as well as to understand local heterogeneity in regional- or global-scale oceanographic changes, such as oceanic anoxic events. This study provides a high-resolution Mesozoic chronostratigraphic and paleoenvironmental framework with the help of new micropaleontological data generated from 17 DSDP-ODP sites (out of 20 study sites), ranging from the Equatorial South Atlantic to the Southern South Atlantic, through sample re-analyses. A series of eight regional Mesozoic unconformities (based on 20 sites) are proposed and related to previously known unconformities or seismic reflectors. The regional paleoenvironment interpretation, which utilizes the standardized chronostratigraphic framework, is herein accomplished for four time-horizons, namely, Middle Jurassic–Valanginian, Hauterivian–Albian, Cenomanian–Turonian, and Coniacian–Maastrichtian. Chronostratigraphic cross-sections and reconstruction maps for the time-horizons show that significant changes in paleoenvironmental parameters, such as bottom-water oxygenation and allochthonous sedimentation, were governed by the tectonic evolution of the South Atlantic margins.

  • Phytoliths indicate environmental changes correlated with facies analysis in a paleo island-lake, Upper Paraná River, Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-07
    Giliane Gessica Rasbold; José Cândido Stevaux; Mauro Parolin; Isabel Teresinha Leli; Leandro Domingos Luz; Hermes Dias Brito

    Phytolith analysis has become a useful tool for paleoenvironmental studies. Phytoliths are produced by plants and are composed of biogenic silica, giving them higher resistance to diagenetic and oxidation processes that commonly occur in tropical sediments. Here, we analyzed the preserved phytolith assemblages in a sedimentary core recovered from an island in the Upper Paraná River. Deposition and preservation of distinctive phytoliths of different vegetation types in the sedimentary facies have provided evidence of the formation of the fluvial island and the floral succession since the Late Pleistocene. Three distinctive phases were described in the island formation. The sandy base (Sr facies) of the core (240 cm) was dated to 14,620 cal yr BP. Most phytoliths in this phase were characteristic of the Podostemaceae family -- aquatic plants adapted to intense water flow. At 97 cm depth, organic sedimentary facies (Fm facies) were dated to 7,382 cal yr BP. At this phase, the phytolith assemblage is composed mainly of parallelepipedal bulliform, cuneiform bulliform, and elongate psilate; such morphotypes are primarily produced by grasses. Towards the core top is a layer of massive fine sediments (Vm facies) with greater abundance of plant fragments. The assemblage is composed of short cells, predominantly the bilobate morphotype, which is characteristic of the Panicoideae subfamily (Poaceae) and indicative of soil humidity.

  • Middle Eocene mixed carbonate-siliciclastic systems in the southern Caribbean (NW Colombian margin)
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-05
    Edward A. Salazar-Ortiz; Daniel Rincón-Martínez; Liliana A. Páez; Sandra M. Restrepo; Sofía Barragán

    Middle Eocene carbonates and mixed carbonate-siliciclastic deposits occur in disconnected outcrops in the San Jacinto Fold Belt in northern Colombia. Three lithofacies were distinguished: A) claystones and siltstones with planktonic components; B) packstones / rudstones with red algae and claystone-siltstone rip-up clasts in medium to thick plane-parallel, plane-convex, and channel-shaped beds; and C) poorly-bedded rhodolith rudstones with minor larger benthic foraminifers. The latter are interpreted as rhodolith beds accumulated in middle-ramp settings. Lithofacies B consists of bioclasts from the middle ramp redeposited as sediment gravity flows in individual beds and in small channel-lobe systems in the outer ramp and slope. The autochthonous sedimentation basinward from the middle ramp was dominated by hemipelagic claystones and siltstones (Lithofacies A). In outcrops with exposure of vertical successions, these facies are arranged in a transgressive pattern. The limited continuity of outcrops prevents any assessment of the original extension of ramps and lateral dimensions of channel-lobe systems. Comparison with modern tropical rhodolith beds suggests that the middle Eocene carbonate factories in the southern Caribbean developed on ramps under mesotrophic conditions.

  • 更新日期:2020-02-06
  • Aliphatic and aromatic biomarkers in source rocks from Pimenteiras Formation of devonian age: Distribution and paleoenvironment significance
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-04
    Alek A.C. de Sousa; Edymilaís S. Sousa; Márcio S. Rocha; Gustavo R. Sousa Junior; Igor V.A.F. de Souza; Ailton da Silva Brito; Alexandre Araújo de Souza; José A.D. Lopes; Afonso C.R. Nogueira; Sidney G. de Lima

    Previous geochemical studies carried out in the Parnaíba Basin (Brazil) show that natural gas deposits, whose source rock is formed by the shales of the Pimenteiras Formation (PF; Devonian), stimulated new research to understand the petroleum system. Despite previous studies of PF, it's characterization with emphasis on the biomarker content is still incipient. In this work, the organic matter (OM) of outcrop samples of the Pimenteiras Formation outcrop samples was investigated through diagnostic biomarkers, to evaluate paleoenvironmental conditions of deposition, the thermal evolution, and the organic matter type. The general distribution of hydrocarbons suggested that the samples present low thermal evolution, marine depositional paleoenvironment, and dysoxic conditions. The molecular parameters based on steranes, hopanes, Pr/C17 and Ph/C18, presence of n-C25, n-C27 and n-C29 n-alkanes showed strong evidence of marine origin OM with higher plant input. The data also suggested that OM was not affected by igneous intrusions, being below the oil window. Also, the isomers 4α(methyl)- and 4β(methyl)-24-ethylcholestane and 24-n-propylcholestane have been detected, which are indicative of a marine depositional system. Aromatic carotenoids and derivatives suggest that euxinic conditions have reached the photic zone, an environment conducive to the preservation of organic matter.

  • Uca maracoani (Crustacea, Decapoda, Ocypodidae) from a Miocene paleomangrove in Brazil: a case of evolutionary stasis among tropical American fiddler crabs
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-03
    Daniel Lima; Marcos Tavares; Ricardo T. Lopes; Olga M. Oliveira de Araújo; Orangel Aguilera

    The Miocene fiddler crab Uca antiqua Brito, 1972, from the Pirabas Formation (Brazil), has been recurrently claimed to be a valid species and the oldest fossil record of the brachyuran crab family Ocypodidae. However, morphological investigations with the aid of microCT scan images of a large number of topotypic specimens and re-examination of the type material of U. antiqua revealed that it should actually be merged into the synonymy of Uca (Acanthoplax) maracoani (Latreille, 1802). Therefore, U. maracoani, the morphology and ecological traits of which have persisted essentially unchanged for at least 16 Ma (Burdigalian to present), is actually a long-lived crab species inhabiting an estuarine, sheltered mudflat environment that, likewise, has essentially remained unchanged, as suggested by palynological evidence. Burgeoning evidence suggests that evolutionary stasis may not be uncommon among mangrove crabs, as revealed by Afruca tangeri (Eydoux, 1835) from the middle Miocene in northeastern Spain and U. ornata (Smith, 1870) from the Plio-Pleistocene in Honduras. Uca antiqua is redescribed and illustrated based on 3D volume reconstruction and several topotypic specimens. The microCT analysis revealed a considerable number of fossil concretions of U. maracoani, with both males and females likely sharing the same burrow. This is the first fossil record of a male fiddler crab with a female inside a burrow, where mating usually takes place. This mating style is common to almost all extant American fiddler crabs.

    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-02-03
    Paulo Victor Luiz Gomes da Costa Pereira; Ingrid Martins Machado Garcia Veiga; Theo Baptista Ribeiro; Ryan Henrique Bezerra Cardozo; Carlos Roberto dos Anjos Candeiro; Lilian Paglarelli Bergqvist

    The Rebbachisauridae (Diplodocoidea) comprises small to medium-sized sauropods that diversified at the end of the Early Cretaceous–beginning of the Late Cretaceous in Africa, South America and Europe. The goal of this paper is to review the distribution of fossils assigned to Rebbachisauridae at Early-Late Cretaceous and describe the first occurence of this group in the Açu Formation, Potiguar Basin, Brazil. The material consists of five isolated caudal vertebrae. The specimens have the combination of some characteristics shared with rebbachisaurid taxa, such as Limaysaurus, Amazonsaurus and Demandasaurus: (i) the anterior and posterior articular facets of the centrum are sub-hexagonal to sub-quadrangular; (ii) they bear a wide base; (iii) the centrum presents an amphicoelic condition; (iv) the centrum lacks pneumatization in both lateral and ventral surfaces; (v) an anterior position of the neural arch is implied by the anteriorly located centroneural suture of the centrum (Salgado et al., 1997; Wilson & Sereno, 1998; Canudo et al., 2018). The rebbachisaurids have been largely used for biogeographical interpretations due to its presence in both Gondwanan and Laurasian continents; the material presented here is a further evidence of this relationship once the specimens show similarities with both European and Argentinean forms. Furthermore, this occurrence enlarges the presence of the group in Brazil and South America, and is important for a better understanding of the faunistic context of the Northeast of South America and its relationship with other Early-Late Cretaceous faunas.

    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-31
    Rosana Silveira Resende; Carlos Alberto Tello Sáenz; Elton Luiz Dantas; Peter Christian Hackspacher; César Anthony Chavez Machaca; Ulrich A. Glasmacher

    Combined zircon fission-track (FT) and U-Pb dating were used to obtain correlated cooling ages and tectonic processes in the eastern border of the Phanerozoic Paraná Basin, a large cratonic sedimentary basin in South America deposited over Precambrian basement, which is represented mainly by the Neoproterozoic Brasília Orogenic Belt. Zircon ages obtained for clastic sediments suggest that the provenance history of the Paraná Basin was derived chiefly from the proximal basement sources. Twelve zircon samples were collected, from which 194 grains were analyzed and simultaneously dated by the FT and U-Pb methods, and 146 zircon grains were additionally dated only through the fission-track method (FT). The standard sample of Fish Canyon Tuff (FCT) was also analyzed through FT and U-Pb, and the age results of 26.6 ± 1.1 Ma, and 28.33 ± 0.68 Ma, respectively, are consistent with the literature data. The fission-track ages, obtained for the Araxá (∼ 330 Ma) and Canastra (∼ 500 Ma) Groups, and Piumhi Massif (∼ 700 Ma) pertaining to the Brasília Belt, in average increase towards the São Francisco Craton. This fission-track age pattern can be linked to the temperature and exhumation of each one of these units. The U-Pb detrital zircon ages obtained for the basement units range from 1.6 to 3.2 Ga and indicate different provenance sources in the evolution of the Brasília Belt. However, it is clear that the main sources of metasedimentary rocks are derived from the São Francisco-Congo Craton. For samples collected in the NNE border of the Paraná Basin, FT ages were obtained for 127 zircon grains with ages ranging between 100 and 600 Ma. Forty of these grains dated simultaneously with FT and U-Pb bear mean age populations of 0.6 and 1.0 Ga, and subordinately between 1.5 and 2.5 Ga. These results suggest that the units that represent the São Francisco Craton’s Neoproterozoic passive margin were being formed with depositional sediments 1.0 Ga ago and their recycling was the main source for the zircon derivation recorded in the Paraná Basin sediments. An important methodological result of this work is that U-Pb radiometric dating was done in the same areas of single zircon crystals employed to determine the FT ages, which improved the concordance in the U-Pb ages. As such, U-Pb dating on zircon micro areas that have undergone leaching, metamictization or another type of lattice damage was avoided.

  • Magmatic activity period and estimation of P-T metamorphic conditions of Pre-Collisional Opx-Metatonalite from Araçuaí-Ribeira orogens boundary, SE Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-31
    Lucas Pequeno Gouvêa; Silvia de Medeiros; Julio Cezar Mendes; Caroline Cibele Vieira Soares; Rodson Marques; Marilane Melo

    The southern region of the Espírito Santo State is located in the Araçuaí-Ribeira Orogen System (AROS) limit, where neoproterozoic orthoderived rocks undergone metamorphism at high-grade condition exhibiting partial melting and deformation features. This study focuses on the Pre-collisional Opx-granitoids Unit that shows varied proportions of orthopyroxene and garnet. Petrographic, geochemical, geothermobarometric and U-Pb in zircon studies were carried out. We distinguished two mineral assemblages: (i) consisting of plagioclase, quartz, biotite, amphibole, orthopyroxene and ilmenite; and (ii) with plagioclase, quartz, orthoclase, biotite, amphibole, garnet and ilmenite. The former is related to the opx-metatonalite protolith and the latter to metamorphic paragenesis. The geochemical analyses of the Pre-collisional Opx-granitoids Unit reveal an arc-related environment, similar to the Rio Doce and Rio Negro - Serra da Prata Arcs defined in the Araçuaí and Ribeira orogens, respectively. The temperature conditions obtained for assemblage (ii) were 670 to 860 °C (granulite facies) and pressure from 5.24 to 10.2 kbar. Four different intervals of concordant ages were yielded in the studied samples. The oldest age of 716 Ma is interpreted as related to the pluton emplacement (E1). The ages of 604 Ma and 595 Ma are associated with the same magmatic event (E2), and the magmatic age of 586 Ma (E3) indicates another magma injection event. The age of 574 Ma (E4) is related to a high-grade metamorphic event whose P-T conditions were obtained in assemblage (ii) from geothermobarometric calculations. These results confirm the extension of Rio Doce magmatic arc toward the south and establish a connection between Araçuaí and Ribeira orogens.

  • Source Identification of Polycyclic Aromatic Hydrocarbons in Terrestrial Soils in Chile
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-31
    Woranuch Deelaman; Siwatt Pongpiachan; Danai Tipmanee; Chomsri Choochuay; Natthapong Iadtem; Oramas Suttinun; Qiyuan Wang; Li Xing; Guohui Li; Yongming Han; Muhammad Zaffar Hashmi; Junji Cao

    In this study, a combination of the diagnostic binary ratios of PAHs and multivariate descriptive statistics was applied to identify the sources of PAHs in Chilean terrestrial soils. A total of 15 PAHs from the terrestrial soil of 28 locations in three cities of Chile were chemically characterized using gas chromatography mass spectrometry (GC-MS). The total concentrations of twelve likely carcinogenic PAHs were defined as the sum of Phe, An, Fluo, Pyr, B[a]A, Chry, B[b]F, B[k]F, B[a]P, Ind, D[a, h]A and B[g, h, i]P and ranged from 0.0215 to 4.37 μg g-1 with an arithmetic mean of 0.618 ± 0.911 μg g-1. The levels of these PAHs were classified as moderate to high compared to World Soils (WS). All sampling stations were dominated by high molecular weight PAHs, four-ring (39.1%) and five-ring (29.6%) PAHs were the most abundant groups in the terrestrial soils of Chile. The PAH diagnostic ratios suggested that PAHs are primarily of pyrogenic origin. Further multivariate descriptive statistics (i.e., hierarchical cluster analysis (HCA) and principal components analysis (PCA)) identified pyrogenic combustion as the main emission source of PAH contamination in Chilean terrestrial soils.

  • Reply to “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 : 2020-01-30
    Alexandre de Oliveira Chaves; Raphael Martins Coelho

    Unlike the question brought by Massonne (2020), mica-quartz schist as a source of Córrego do Peixoto granite from Itaguara Sequence is clearly confirmed by geochemical new data. It is a peraluminous (S-type) syn-collisional granite generated by partial melting of the schist as a result of orogen exhumation during Paleoproterozoic continental collision, i.e during the decompression part of clockwise P-T-time path. Preserved in mica-quartz schist, garnet-chloritoid-phengite-kyanite mineral assemblage, typical of high-pressure metapsammopelites, is in equilibrium close to the metamorphic peak suggested by de Oliveira Chaves et al. (2019), according to new pseudosection of this rock. The new discovery of retroeclogite in the IS (Chaves and Porcher, in review) close to schist and granite is sufficient to oppose the comment of Massonne (2020), which sustains that mica-quartz schist from IS has never reached conditions of the eclogite-facies. The geotectonic setting proposed by de Oliveira Chaves et al. (2019) for Itaguara Sequence seems to indicate a reliable Paleoproterozoic accretionary prism in a paleosubduction zone, which can correspond to one of the earliest records of subduction process followed by continental collision in world.

  • A Cambrian age for the upper Bambuí Group, Brazil, supported by the first U-Pb dating of volcaniclastic bed
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-30
    Débora Silvano Moreira; Alexandre Uhlein; Ivo Antônio Dussin; Gabriel Jubé Uhlein; Ana Maria Pimentel Misuzaki
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-30
    Patricia Sruoga; Martín Gozalvez; Cintia Marquetti; Mariela P. Etcheverría; José F. Mescua; Angel Jara; Noelia Iannizzotto; Bradley S. Singer; Brian R. Jicha

    For the first time, the eastern flank at Cordón Santa Elena, within the Valle Hermoso district, has been mapped and described in detail. Based on key field observations, petrographic, geochemical and geochronological results, a new Miocene volcanic stratigraphy is proposed for this particular area. The Cordón del Burrero Group (Arroyo del Perdido, Puesto Gutiérrez, Bolchiquito and Santa Elena Formations) has been defined for the ∼18-13 Ma timespan, whereas the Huincan Formation has been constrained as younger than ∼11 Ma. According to La/Yb ratios, Cordón del Burrero volcanics have been emplaced in a relatively thin crust compared to the Huincan dacites, suggesting that the main phase of crust thickening and tectonic deformation took place at ∼12-11 Ma. The studies carried out at Cordón Santa Elena revealed a spatial and genetic relationship between the ∼13 Ma dioritic subvolcanic pluton and a widespread calcic-sodic hydrothermal alteration. The areal distribution of the hydrothermal alteration shows a core of calcic-sodic alteration accompanied by propylitic alteration, cut by quartz-pyrite veins spatially associated with phyllic alteration and restricted potassic alteration. This hydrothermal system may be considered as the oldest pulse at this latitude.

  • Moult intensity in blue petrels and a key moult site off West Antarctica
    Antarct. Sci. (IF 1.653) Pub Date : 2019-11-14
    Peter G. Ryan; Jasmine R. Lee; Fabrice Le Bouard

    Blue petrels (Halobaena caerulea Gmelin) rapidly moult their flight feathers in Antarctic waters in February–April, immediately following the breeding season, yet the behaviour of moulting birds at sea has not been described. We observed large numbers of moulting blue petrels off West Antarctica from 67–71°S and 78–119°W in mid-February 2017. Most of these birds probably breed at the Diego Ramirez archipelago, southwest of Cape Horn, which is the closest colony to this area. Moulting petrels often sit on the water in dense flocks, just outside the marginal ice zone, at sea temperatures of -0.7 to 0.9°C. Wing moult is intense, with 7–8 inner primaries (62–75% of primary length and 55–69% of primary mass), their corresponding primary coverts and all greater secondary coverts being grown at the same time. Moulting petrels need a reliable food source during this energetically demanding period, so the waters off West Antarctica are probably crucial for the Diego Ramirez population, which makes up more than half of the world's blue petrels.

  • Hybridization and cuckoldry between black-browed and grey-headed albatrosses
    Antarct. Sci. (IF 1.653) Pub Date : 2019-12-12
    M.G.W. Jones; N.M.S. Techow; M.M. Risi; C.W. Jones; Q.A. Hagens; F. Taylor; P.G. Ryan

    A vagrant black-browed albatross Thalassarche melanophris bred with a grey-headed albatross T. chrysostoma on Marion Island at least four times between 2000 and 2009 (and continued to return to the colony until at least 2019). The eggs failed to hatch in three breeding attempts, but the pair fledged a chick in the 2006/07 breeding season. Genetic sexing identified the black-browed albatross as female and she shared all eight sampled microsatellite alleles with the chick, whereas the grey-headed albatross social parent did not match the chick. The fledgling was banded and re-sighted in its natal breeding colony in 2016 and 2018, when it displayed an intermediate black-browed x grey-headed albatross phenotype, similar to a putative hybrid photographed at sea off Australia. These results suggest that the black-browed albatross cuckolded its social mate with another grey-headed albatross in 2006/07. The failures of the other three breeding attempts at the egg stage possibly indicate genetic incompatibility with the social partner.

  • Serological and molecular surveys of influenza A viruses in Antarctic and sub-Antarctic wild birds
    Antarct. Sci. (IF 1.653) Pub Date : 2019-12-03
    Oliver Gittins; Llorenç Grau-Roma; Rosa Valle; Francesc Xavier Abad; Miquel Nofrarías; Peter G. Ryan; Jacob González-Solís; Natàlia Majó

    To evaluate how avian influenza virus (AIV) circulates among the avifauna of the Antarctic and sub-Antarctic islands, we surveyed 14 species of birds from Marion, Livingston and Gough islands. A competitive enzyme-linked immunosorbent assay was carried out on the sera of 147 birds. Quantitative reverse transcription polymerase chain reaction was used to detect the AIV genome from 113 oropharyngeal and 122 cloacal swabs from these birds. The overall seroprevalence to AIV infection was 4.8%, with the only positive results coming from brown skuas (Catharacta antarctica) (4 out of 18, 22%) and southern giant petrels (Macronectes giganteus) (3 out of 24, 13%). Avian influenza virus antibodies were detected in birds sampled from Marion and Gough islands, with a higher seroprevalence on Marion Island (P = 0.014) and a risk ratio of 11.29 (95% confidence interval: 1.40–91.28) compared to Gough Island. The AIV genome was not detected in any of the birds sampled. These results confirm that AIV strains are uncommon among Antarctic and sub-Antarctic predatory seabirds, but they may suggest that scavenging seabirds are the main avian reservoirs and spreaders of this virus in the Southern Ocean. Further studies are necessary to determine the precise role of these species in the epidemiology of AIV.

  • Pygoscelis penguin diets on King George Island, South Shetland Islands, with a special focus on the krill Euphausia superba
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-14
    Anna Panasiuk; Justyna Wawrzynek-Borejko; Aleksandra Musiał; Małgorzata Korczak-Abshire

    In the krill-based ecosystem of the Antarctic, fluctuations in the distribution and abundance of Euphausia superba may have strong impacts on predator populations; thus, it is crucial to observe the feeding ecology of Antarctic predators, especially in the light of climate change and increasing human pressure. We determined the composition of euphausiid species in diet samples collected from Adélie (Pygoscelis adeliae), chinstrap (Pygoscelis antarcticus) and gentoo (Pygoscelis papua) penguins on King George Island (South Shetlands Islands) during a breeding season. For all three penguin species, euphausiids (mainly E. superba) represented almost the entirety of researched stomach samples (i.e. 99.9% in the case of Adélie and chinstrap penguins), while gentoo penguins also proved to feed on fish (99.4% krill; 0.5% fish). Analysed material differed in the size of eaten E. superba specimens, with the smallest crustaceans consumed by Adélie penguins. Furthermore, we found differences in the ratio of consumed krill and krill size. Such disparities may be a result of sex-based differences and slight differences in feeding areas between the birds. Additionally, we noted some fragments of plastic debris in the investigated penguin diet samples.

  • Physics and geochemistry of lakes in Vestfjella, Dronning Maud Land
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-29
    Matti Leppäranta; Arto Luttinen; Lauri Arvola

    Shallow Antarctic surface lakes belong to the most extreme aquatic environments on the Earth. In Vestfjella, proglacial surface lakes and ponds are characterized by a 2–5 month long period with liquid water and depths < 2 m. We give a detailed description of nine seasonal lakes and ponds situating at three nunataqs (Basen, Plogen and Fossilryggen) in western Dronning Maud Land. Their physical and geochemical properties are provided based on observations in four summers. Three main ‘lake categories‘ were found: 1) supraglacial lakes, 2) epiglacial ponds and 3) nunataq ponds. Category 3 lakes can be divided into two subgroups with regards to whether the meltwater source is glacial or just seasonal snow patches. Supraglacial lakes are ultra-oligotrophic (electrical conductivity < 10 μS cm−1, pH < 7), while in epiglacial ponds the concentrations of dissolved and suspended matter and trophic status vary over a wide range (electrical conductivity 20–110 μS cm−1, pH 6–9). In nunataq ponds, the maxima were an electrical conductivity of 1042 μS cm−1 and a pH of 10.1, and water temperature may have wide diurnal and day-to-day fluctuations (maximum 9.3°C) because snowfall, snow drift and sublimation influence the net solar irradiance.

  • Intertidal foraging by gentoo penguins in a macroalgal raft
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-29
    James B. McClintock; Charles D. Amsler; Margaret O. Amsler; William R. Fraser


  • The impact of the Pan-African-aged tectonothermal event on high-grade rocks at Mount Brown, East Antarctica
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-29
    Xiaochun Liu; Bin Fu; Qiuli Li; Yue Zhao; Jian Liu; Hong Chen

    This study presents monazite and rutile U–Pb and hornblende and biotite 40Ar/39Ar geochronological data for high-grade rocks of the eastern Grenville-aged Rayner orogen at Mount Brown in order to analyse the extent and degree of Pan-African-aged reworking. Monazite from paragneiss yields U–Pb ages of 910 Ma for larger granular grains and 670–630 Ma for smaller globular beads around garnet porphyroblasts or hosted by symplectites. Rutile from leucogneiss yields U–Pb ages of 520–515 Ma. Hornblende and biotite from different rock types yield 40Ar/39Ar plateau ages of 744 and 520–505 Ma, respectively. Combining these results with published zircon U–Pb age data suggests that granulite facies metamorphism occurred at 910 Ma, with a local low-temperature fluid flow event at 670–630 Ma and thermal reworking at 520–505 Ma. The older age of 744 Ma may reflect cooling or partial resetting of the hornblende 40Ar/39Ar system, indicating that Pan-African-aged reworking did not exceed temperatures much higher than the hornblende Ar closure temperature. These data also suggest that the complete isotopic resetting of some minerals may occur without the growth of new mineral phases, providing an example of the style of reworking that is likely to occur in polymetamorphic terranes.

  • Design and construction of a bespoke system for the detection of buried, iron-rich meteorites in Antarctica
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-22
    John W. Wilson; Liam A. Marsh; Wouter Van Verre; Michael C. Rose; Geoffrey Evatt; Andrew R.D. Smedley; Anthony J. Peyton

    Iron-rich meteorites are significantly underrepresented in collection statistics from Antarctica. This has led to a hypothesis that there is a sparse layer of iron-rich meteorites hidden below the surface of the ice, thereby explaining the apparent shortfall. As standard Antarctic meteorite collecting techniques rely upon a visual surface search approach, the need has thus arisen to develop a system that can detect iron objects under a few tens of centimetres of ice, where the expected number density is of the order one per square kilometre. To help answer this hypothesis, a large-scale pulse induction metal detector array has been constructed for deployment in Antarctica. The metal detector array is 6 m wide, able to travel at 15 km h-1 and can scan 1 km2 in ~11 hours. This paper details the construction of the metal detector system with respect to design criteria, notably the ruggedization of the system for Antarctic deployment. Some preliminary results from UK and Antarctic testing are presented. We show that the system performs as specified and should reach the pre-agreed target of the detection of a 100 g iron meteorite at 300 mm when deployed in Antarctica.

  • ANS volume 32 issue 1 Cover and Front matter
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-29


  • ANS volume 32 issue 1 Cover and Back matter
    Antarct. Sci. (IF 1.653) Pub Date : 2020-01-29


  • Fluidized sand effusion over successive basalt flows of the northwestern Paraná volcanic province
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-23
    Sandro Kucera Duarte; Léo Afraneo Hartmann; Sérgio Benjamin Baggio

    A remarkable sand injectite complex (1 million km2) is hosted in Paraná volcanic province, South America, and is here described. Named Novo Hamburgo Complex, these sand injectites originated by the interaction of several major geological processes, as presently described in Campo Grande and Serra de Maracaju cuesta, Mato Grosso do Sul, Brazil. The geologically unique relationship of a major continental flood basalt province with the underlying large freshwater Guarani aquifer – hosted in paleodune sands of the largest known paleoerg – resulted in a network of sand injectites where each basalt lava flow was injected and then covered by sand extrudite. The basaltic lavas were individualized by chemical characteristics and correlated (with cover extrudite) along the escarpment of the cuesta for 10-30 km. After cooling, each volcanic unit underwent prolonged hydrothermal alteration. Porosity vanished by sealing with chalcedony during continued percolation of hot water. The process culminated in the explosive injection and effusion of fluidized sand, resulting in the covering of each lava flow with a sand extrudite. Lavas were sealed in sequence above the underlying aquifer, resulting in the explosive injection and effusion of fluidized sand. The interplay of three major geological features was required in the Cretaceous Paraná Basin to build the geology of Serra de Maracaju and Novo Hamburgo Complex: (1) a huge continental flood basalt province, (2) a huge underlying paleoerg, and (3) a large, underlying Guarani aquifer. An infinite volume of hot water and its vapor was available for the alteration, injection, and effusion processes of sand extrudites. Injection was triggered by residual volcanic heat and possibly earthquakes.

  • LA-ICP-MS U-Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt, SE Brazil
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-22
    Author Statement; Maristella Moreira Santos; Cristiano Lana; Ricardo Scholz; Ian S. Buick; Sandra L. Kamo; Fernando Corfu; Gláucia Queiroga
  • Additional Pelagornithidae remains from Seymour Island, Antarctica
    J. South Am. Earth Sci. (IF 1.655) Pub Date : 2020-01-20
    Carolina Acosta Hospitaleche; Marcelo Reguero

    Two incomplete mandibles are assigned to Pelagornithidae given the presence of a well marked neurovascular furrow and the unique bony projections, or “pseudo-teeth”, along the crista tomialis. Specimens IAA-Pv 175 from Ypresian levels of La Meseta Alloformation (Cucullaea I Allomember), and IAA-Pv 823 from Bartonian beds of the Submeseta Alloformation, in Seymour Island (Antarctic Peninsula, West Antarctica) corresponds to the morpho-type 1, previously proposed for Antarctic pseudo-tooth birds. The intermediate condition of the pseudo-teeth of these specimens reinforces the idea that diet changed from piscivory to molluscivory along the evolutive history of the group.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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
  • 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.

  • 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.

  • 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.

  • 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.

  • 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
  • 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.

  • 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.

  • 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
  • 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.

  • 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.

  • 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.

  • 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.

  • 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
  • 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.

  • 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.

  • 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.

  • 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
  • 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.

  • 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
  • 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.

  • 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.

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