Comunicações Geológicas
Permanent URI for this community
Browse
Browsing Comunicações Geológicas by Sustainable Development Goals (SDG) "15:Proteger a Vida Terrestre"
Now showing 1 - 10 of 10
Results Per Page
Sort Options
- Análogos geoquímicos portugueses de rochas e solos marcianos (S. Saturnino – Fronteira vs Cratera de Gusev e Meridiani Planum)Publication . Rocha, Carla; Alves, Eduardo IvoRESUMO: Este trabalho resulta de estudos petrográficos e geoquímicos feitos sobre afloramentos de rochas filonianas básicas na região de S. Saturnino (Fronteira, Portalegre, Portugal) como análogos de rochas e solos marcianos das regiões da cratera Gusev e de Meridiani Planum analisados nas missões dos rovers Spirit (MER-A) e Opportunity (MER-B). Mostra-se que este é o primeiro local em Portugal Continental que pode ser reconhecido como análogo dos ambientes geológico, petrográfico e geoquímico marcianos.
- Compositional heterogeneity of late to post-Variscan, mafic subvolcanic dykes from northern Portugal (Central Iberian Zone)Oliveira, António; Martins, Helena; Sant'Ovaia, H.ABSTRACT: This study is dedicated to the presentation and interpretation of whole-rock geochemical data concerning a few mafic subvolcanic dykes located in northern Portugal, related to the late to posttectonic stages of the Variscan orogeny. Overall, the analyzed rocks are reasonably fresh, ultrabasic to intermediate, metaluminous to moderately peraluminous, and calc-alkaline or alkaline. There is no evidence implying the occurrence of crustal contamination, but crystal fractionation is likely to have played either a minor or more meaningful role on the petrogenetic evolution. While most dykes were probably generated from subduction-modified, metasomatically-enriched lithospheric sources, others possibly derived from the asthenosphere. There are also important differences regarding the residual mineralogy of the sources, as well as the melting degrees.
- Effects of application of eucalyptus leaf biomass on soil quality: a field trialPublication . Teixeira, Tiago; Cachada, Anabela; Pato, Pedro; Patinha, CarlaABSTRACT: Approximately 30% of Portuguese forests are occupied by eucalyptus (Eucalyptus globulus). As this is a highly flammable invasive species, it is essential to control its proliferation. Thus, the PEST(bio) CIDE project, based on a circular economy perspective, aims to explore the phytotoxic potential of eucalyptus leaves, using its leaf biomass as an effective and environmentally safe biocide, and its use as a corrective to improve soil properties. It is also expected that the introduction of biomass into the soil will contribute to increase carbon reserves and soil fertility. Thus, it is possible to associate an economic benefit with better management of post-fire regenerated eucalyptus areas. Due to this, a field trial was carried out to test the effect of biomass application on the soil’s physico-chemical properties. The results indicate that incorporating leaves has a positive effect on some soil properties, mainly in terms of increasing organic matter and available macronutrients.
- Granitos estaníferos da região de Santa Comba de Rossas, norte de PortugalPublication . Teixeira, Rui; Gomes, Maria Elisa Preto; Silva, Paulo; Pinto de Meireles, Carlos AugustoRESUMO: Na região de Santa Comba de Rossas aflora um complexo granítico (G1-G3) que se instalou durante os estágios sin-cinemáticos da Orogenia Varisca, há 317,9 ± 8,7 Ma (datação Rb-Sr de rocha total). Os granitos são intrusivos em metassedimentos de idade silúrica, do Domínio Parautóctone da Zona da Galiza – Trás-os-Montes, instalando-se no núcleo de uma antiforma NW-SE, resultante das fases de deformação D1 e D3, apresentando evidências de uma deformação em regime dúctil e frágil. Os dados geoquímicos e isotópicos sugerem que o granito G1, com biotite > moscovite e o granito G3, com biotite ≈ moscovite, correspondem a pulsos magmáticos distintos, que terão resultado da fusão parcial de materiais metassedimentares e sofrido uma diferenciação por cristalização fraccionada. Este processo foi responsável pelo aumento do teor de Sn naqueles granitos e, muito provavelmente, pela ocorrência do granito moscovítico G2, o mais rico em Sn (77 ppm), no exocontacto do plutão granítico G1.
- Heterogeneity of the mixed siliciclastic-carbonate deposits of the upper Cabo Carvoeiro formation (Lower-Middle Jurassic), Lusitanian Basin, PortugalPublication . Barata, Joao; Duarte, Luis; Azerêdo, Ana Cristina; Gomes, J.ABSTRACT: The upper CC5 member of the Cabo Carvoeiro Formation (Toarcian-Aalenian(?)) of the Lusitanian Basin is characterized by grainstone facies with quartz grains, ooids and intraclasts. These facies are observed exclusively at Peniche, contrasting with the hemipelagic marly sediments of coeval outcrop sections in the basin. A detailed facies scheme reveals the relative local heterogeneity of the succession on outcrops, while the analysed subsurface data helps to better understand its regional heterogeneity. Lateral facies variability on outcrop is observed to occur through gradation, interfingering or pinching-out geometries. Coeval sections in the analysed offshore wells are generally characterized by micritic mudstone facies, which might transition laterally into the Peniche grainstones through similar geometric relationships based on the existing conceptual model. The relative heterogeneity of this sedimentary system is defined in this study by integrating outcrop and subsurface data, allowing for its perception at different observation scales.
- Investigating the hydrothermal activity at Fogo volcano (Cape Verde) using geochemical analyses of soil samplesCandeias, Carla; Rocha, Fernando; Dumont, Stéphanie; Maineult, Alexis; Ramalho, Ricardo; Madeira, José; Antunes, JoãoABSTRACT: Characterizing hydrothermal systems at volcanoes is particularly important as they play a crucial role in controlling eruptive dynamics. On the long term, hydrothermal systems participate in the geochemical alteration of both rocks and soils, through hydrolysis, dissolution, mineral precipitation, and secondary mineralization contributing in the gradual destabilization of volcanic structures. Fogo volcano (Cape Verde) is the most active volcano in he eastern Atlantic. It features a fumarolic field and passive degassing in its summit crater. A total of 65 soil samples were collected and analyzed, 4 years after the 2014-2015 eruption, in the area surrounding the volcanic edifice including the last eruptive vent with the main objective of detecting and identifying the geochemical and mineralogical signature of hydrothermal activity. Preliminary results show that soil samples were enriched in Ca, Cu, Co, Fe, Na, P, and Ti, when compared to 2014-2015 lava flows, which is confirmed by principal component analysis. Distribution of geochemical elements clearly reflects the mineral phases and is discussed together with geophysical data to shed the light on hydrothermal processes taking place at Pico do Fogo.
- Is the Rosario Antiform, Iberian Pyrite Belt, a Variscan tectonic window?: a tectono-stratigraphic interpretation based on the geology of the Neves-Corvo mine regionPublication . Oliveira, José TomasABSTRACT: This work suggests a new structural interpretation for the Rosario Antiform, Portuguese Pyrite Belt. This is based on the following successive developments: 1 - Deposition of an upper Visean flysch succession, designated by CMt2, which conformably overlaid the autochthon represented in ascending order by the clastic succession of the Phyllite-Quartzite Formation (PQ) followed by the lower Volcano-Sedimentary Complex (VSC, CVS in the geological map annex) dominated by felsic volcanism and black shale sediments, all of late Devonian age, and the upper VSC composed by shales, volcanoclastic sediments, including cherts and jaspers, and mafic volcanic and igneous rocks of late Tournaisian – late Visean ages; 2 - the reinterpretation of fifteen selected drill hole logs previously carried out by Somincor and LNEG geologists and recovered from the region west of the Neves-Corvo mine proved that below the CMt2 flysch succession the upper VSC is absent in many places and the unit is in direct fault contact with the lower VSC, in a position similar to that recognized in the mine. This abnormal contact is inferred to represent the trace of an extensional fault that locally erased the upper VSC lithologies. A sub-autochthon block composed of remnants of the lower VSC and the overlying CMt2 sediments was then formed above the extensional fault, as discussed below; 3 - a SW directed tangential transport, well documented in the Neves-Corvo mine region, placed a distinct flysch succession (CMt1) and the underlying clastic dominated upper VSC units, all of late Visean age, both forming the allochthon, upon the CMt2 succession. The allochthon is organized as a thinskinned package moving southwestward upon the subautochthon developing a large overthrust. All the package units were folded by a second episode of pervasive regional tectonic compression. 4 – a second tectonic reverse fault at the west limb of the antiform caused the uplifting of the autochthon and the subautochthon which after the regional erosion became a true tectonic window according to this interpretation.
- Opportunities and challenges in a world-class mining district, the Iberian Pyrite BeltPublication . Castelo Branco, José M.ABSTRACT: A summary is made on the exploration work carried out in the Iberian Pyrite Belt (IPB) and an attempt is made to describe the evolution of the exploration methods and strategies applied in the field which led to the discovery of the main deposits. Finally, a reference is made to the good potential for new discoveries in the IPB, giving clues about new exploration models to be pursued. Being undoubtedly a world-class metallogenic province, the IPB naturally deserves a special mention regarding the mining history and on the evolution of the exploration techniques, adjusted to the new paradigms, namely in the need to search at ever greater depths. The most recent cases of successful new discoveries in the IPB are described, largely resulting from the refining of geological, geophysical and geochemical prospecting methodologies, and clues that are expected to lead to the discovery of new concealed deposits are equated.
- Reappraisal of active tectonics of the Porto Alto buried fault zone (Portugal) considering new geophysical shallow studiesPublication . Carvalho, João; Cabral, João; Ghose, Ranajit; Borges, José Fernando; Dias, RubenABSTRACT: The Lower Tagus Valley area (LTV), where Lisbon is located, has been affected by several destructive, M 6+ earthquakes whose sources remain to be determined. The identification of expectable surface or near surface ruptures in the area is a challenging task that requires a multidisciplinary analysis that includes geophysical techniques, as the source faults are mainly buried despite likely to deform Upper Pleistocene to Holocene alluvial cover of the Tagus River. This paper focuses on the characterization of the Porto Alto fault zone for seismic hazard mitigation purposes. The Porto Alto fault zone was recognized in oil-industry P-wave 1980’s seismic reflection data as an important, Miocene reactivated, deep structure in the LTV. Highresolution P-wave seismic reflection data were later acquired in the early 2000’s to investigate related Holocene fault activity, leading to the identification of a shallow fault zone near the surface. However, the vertical resolution of the acquired P-wave seismic reflection data was considered insufficient to corroborate any presumably small vertical offset related to fault rupture in the ca. 50 m thick alluvium cover. Trenching for the recognition and characterization of surface faulting was previously tested in the study region but it proved to be a challenging and poorly efficient methodology due to the very shallow water table (at ~1 m) and low cohesion of the sediments. Due to these constraints, we revisited the former fault study site to acquire higher resolution S-wave seismic and ground penetrating radar (GPR) data. The new seismic profiles show interruption of the reflectors in the stacked sections. Diffracted energy, changes in amplitude/shape of the reflection hyperbolae in the shot gathers and spatially coincident low velocity anomalies, also indicate the presence of several shallow fault strands deeper than 10 m. The GPR profile, overlapping and extending the seismic profiles in 30 m reaches a maximum investigation depth of about 15 m and shows the presence of deformation at three locations, one of which matches with one of the fault strands detected in the high resolution S-wave seismic data. In this profile, sediment disruption was detected extending upwards to a depth as shallow as ca. 3.5 m, corresponding to alluvium with a poorly constrained age of ca. 2,300 yrs. Slip rate, maximum earthquake magnitude and recurrence, and other parameters are also estimated for the Porto Alto fault zone. These recently acquired seismic and GPR datasets indicate that there were at most three to five maximum earthquakes generated by the fault in the last 13,100 years, with an average recurrence of approximately 4,400 to 2,600 years respectively. However, the data show a grouping of these earthquakes in time, the first two in the period 13,100-12,300 years, separated by about 800 years, and the third or the last grouped three having occurred in the past 2,300 years with a similar average recurrence time of ca. 800 years. However, the regional historical and instrumental seismicity does not show an obvious link of any known major earthquake with the Porto Alto fault zone.
- Stream sediments geochemistry in the Valongo anticline, northern Portugal: arsenic as a proxy to outline new exploration targetsCarvalho, Paula; Albuquerque, Teresa; Chambel, Luís; Silva, Manuela; Rocha, FernandoABSTRACT: The Valongo anticline is known for the occurrence of several Sb-Au and As-Au mineral deposits, many of which have been exploited since Roman times. The Sb-Au and As-Au deposits occur in brecciated quartz veins, predominantly trending NE-SW, NNW-SSE, and ENE-WSW, cutting Cambrian to Carboniferous formations. An 801-stream sediment campaign was conducted in this area. Factorial kriging was applied as an interpolation method as it enables the estimation and mapping of the individual spatial components of the variogram. Arsenic (As) was used as a proxy to outline new exploration targets. The results allow the identification of NW-SE and NE-SW alignments as preferential areas for elevated As contents. Future studies will incorporate structural information and the distribution of old mine dumps to achieve higher accuracy in target areas for mining prospection.