Early Diagenesis of Lower Pliensbachian Sediments from the Algarve Basin (Portugal): Characterisation and Relation with Tectonic Evolution

The Lower Jurassic (Lower Pliensbachian) sedimentary record of the western end of the Algarve Basin (Portugal) is made of decimetric thick layers of limestone and dolomitized limestone with chert nodules and inter-layered chert beds. Most of the observable lithologies are the product of an early diagenetic evolution and the original lithological content of the formation included limestones, marls and calciclastic limestones. In this area the sedimentation was controlled by the tectonic stretching responsible for the evolution of the Algarve Basin, as well as by short-lived events of tectonic inversion. These episodes of tectonic inversion were responsible for the some uplift with the development of unconformities, sometimes with erosional surfaces separating the different sedimentary packages. The early diagenesis affecting the Lower Pliensbachian sediments is characterized by: (i) a mechanical event controlled by the syn-sedimentary stretching, responsible for the development of calciclastic dikes and nodules alignements; (ii) the substitution of the carbonates from the most permeable calciclastic limestone layers by silica, leading to the development of the observed cherts; (iii) the dolomitization of the preserved limestones; and (iv) the infilling of joints and normal fault planes by silica-rich fluids leading to the development of quartz veins. This set of diagenetic transformations took place before the Upper Pliensbachian which lacks the evidences of their occurrence. The whole-rock geochemical data of the carbonate and siliceous sediments of the Lower Pliensbachian revealed some affinities between both lithological types namely the lack of Ce anomalies and the presence of La anomalies in the REE patterns of all samples. The similarities between the carbonate lithologies which diagenetic evolution is marked by the replacement of calcite by dolomite and the siliceous sediments derived from the replacement of calcite by quartz establish a chemical connection between the two sets of rocks. This connection can be interpreted as a testimony of a common primary precursor for both lithological groups or as a testimony of a common character of the diagenetic fluids which interacted with the original sediments. The dolomitization changed the Sr isotopic signature increasing the 87Sr/86Sr ratios from 0.7073 in the unchanged limestone to a maximum value of 0.7113 in the dolomitized samples. This strong increase together with the 13C/12C and 18O/16O values support an external origin for the dolomitizing fluids. A possible origin for the diagenesis is meteoric water coming into the basin after weathering the country rocks. The hydrologic regime in the basin certainly underwent important changes during the uplift events associated to the mentioned tectonic inversion episodes and uplift allowing for input of water from land

The Legisprudential Role of National Parliaments in the European Union

National parliaments’ contribution to the law-making process at European level should focus on the overall rationality of the draft legislative proposals. The Early Warning Mechanism must not be limited to considerations regarding the breach of the principle of subsidiarity, but also encompass the principle of conferral and the principle of proportionality. The Political Dialogue could be enhanced to acknowledge the legisprudential role assigned to national parliaments, encompassing configurations akin to the green or red card without the need to amend the Treaties.info:eu-repo/semantics/publishedVersio

Early Diagenesis of Lower Pliensbachian Sediments from the Algarve Basin (Portugal): Characterisation and Relation with Tectonic Evolution

The Lower Jurassic (Lower Pliensbachian) sedimentary record of the western end of the Algarve Basin (Portugal) is made of decimetric thick layers of limestone and dolomitized limestone with chert nodules and inter-layered chert beds. Most of the observable lithologies are the product of an early diagenetic evolution and the original lithological content of the formation included limestones, marls and calciclastic limestones. In this area the sedimentation was controlled by the tectonic stretching responsible for the evolution of the Algarve Basin, as well as by short-lived events of tectonic inversion. These episodes of tectonic inversion were responsible for the some uplift with the development of unconformities, sometimes with erosional surfaces separating the different sedimentary packages. The early diagenesis affecting the Lower Pliensbachian sediments is characterized by: (i) a mechanical event controlled by the syn-sedimentary stretching, responsible for the development of calciclastic dikes and nodules alignements; (ii) the substitution of the carbonates from the most permeable calciclastic limestone layers by silica, leading to the development of the observed cherts; (iii) the dolomitization of the preserved limestones; and (iv) the infilling of joints and normal fault planes by silica-rich fluids leading to the development of quartz veins. This set of diagenetic transformations took place before the Upper Pliensbachian which lacks the evidences of their occurrence. The whole-rock geochemical data of the carbonate and siliceous sediments of the Lower Pliensbachian revealed some affinities between both lithological types namely the lack of Ce anomalies and the presence of La anomalies in the REE patterns of all samples. The similarities between the carbonate lithologies which diagenetic evolution is marked by the replacement of calcite by dolomite and the siliceous sediments derived from the replacement of calcite by quartz establish a chemical connection between the two sets of rocks. This connection can be interpreted as a testimony of a common primary precursor for both lithological groups or as a testimony of a common character of the diagenetic fluids which interacted with the original sediments. The dolomitization changed the Sr isotopic signature increasing the 87Sr/86Sr ratios from 0.7073 in the unchanged limestone to a maximum value of 0.7113 in the dolomitized samples. This strong increase together with the 13C/12C and 18O/16O values support an external origin for the dolomitizing fluids. A possible origin for the diagenesis is meteoric water coming into the basin after weathering the country rocks. The hydrologic regime in the basin certainly underwent important changes during the uplift events associated to the mentioned tectonic inversion episodes and uplift allowing for input of water from land

Tectonic control and mass-wasting processes along S. Vicente Canyon (SW Iberia) : evidences from multibeam bathymetry and seismic reflection data

European Geosciences Union General Assembly 2009 (EGU 2009), Vienna, Austria, April 19-24, 200

Tsunami hazard induced by a newly discovered landslide in the Tagus delta off Lisbon (Portugal)

Deltas are sedimentary bodies with high sedimentation rates that are very sensitive to changes in environmental conditions. The Tagus delta off Lisbon (Portugal) is characterized by the presence of high energy deposit and erosive structures with an estimated age of approximately 8 ky BP. In this work, we investigate, through numerical modelling, the tsunami hazard induced by a newly discovered underwater landslide at the Tagus delta. The Tagus delta landslide is more than 9 km of length (parallel to the delta front) and more than 3 km of width with a maximum thickness of ∼20 m with a volume of approximately 0.27 km3. To model the landslide downslope movement and its induced tsunami we employed a Bingham visco-plastic model coupled to non-linear shallow water equations. The numerical simulations are performed over a high-resolution paleo-bathymetry that allows evaluating the tsunami hazard at the time of the landslide failure. We constructed the paleo-bathymetric model of the Tagus delta taking into consideration the sedimentation rates, the age of the landslide, and the paleo mean sea level. Additionally, we evaluate the tsunami hazard resulting from the occurrence of a similar-size Tagus landslide at the present-day bathymetric and sea-level conditions. Results show that landslides at the Tagus delta pose a significant tsunami hazard to the surrounding coasts, particularly in Lisbon. Therefore, more attention must be given to shallow water submarine landslides and their induced tsunamis in the marine Geo-hazards assessment of the region. This work is supported by the FCT funded projects: MAGICLAND - MArine Geo-hazards InduCed by underwater LANDslides in the SW Iberian Margin (Ref: PTDC/CTA-GEO/30381/2017), TAGUSGAS - Tsunami hazard assessment of the Lisbon city associated to the Tagus delta landslide (Ref: PTDC 2017 - PTDC/CTAGEO/31885/2017), and TagusDelta - 3D high-resolution seismic stratigraphy of the Tagus Delta – imaging of tsunami and earthquake evidence for natural hazards assessment (Ref: PTDC/MAR/113888/2009)

Improving the Knowledge on Seismogenic Sources in the Lower Tagus Valley for Seismic Hazard Purposes

The Lower Tagus Valley, that includes the metropolitan area of Lisbon, has been struck by several earthquakes which produced significant material damage and loss of lives. Their exact location remains unknown. Our goal is to shed some light into the seismogenic sources in the area using seismic reflection and geological data. In areas with no seismic coverage, potential-field data interpretation was carried out. Seismicity was overlaid to the potential seismogenic structures and high-resolution data was acquired in order to confirm which structures have been active into the Quaternary. Three major fault-zones affecting the Neogene were identified: V. F. Xira, Samora-Alcochete and Pinhal Novo. For the first fault, strong evidences suggest it is active. The other two fault-zones and other structures previously unknown can be correlated with several epicentres. Empirical relationships between maximum moment magnitude and fault area indicate that MW > 6.5 earthquakes can be expected for the larger structures

Geologia de Portugal no contexto da Ibéria

Trata-se de uma síntese sobre a Geologia de Portugal na qual colaboraram cerca de 70 investigadores pertencentes às principais instituições de investigação nacionais