156 research outputs found
Sismostratigrafia dos contornitos da margem do Algarve : eventos deposicionais, tectónicos e hidrológicos no Pliocénico Superior-Plistocénico
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
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,
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Surveying the Alentejo continental shelf for minerals and Quaternary environmental changes: preliminary results of the MINEPLAT project survey
The tectonic uplift of South Portugal in the last 5 Million years (My) was firstly identified on the basis of
morphologic criteria by Mariano Feio (1952, “The evolution of the relief of Baixo Alentejo and Algarve”, transl.).
However, the assessment of continental vertical movements off Portugal and its relation with tectonics was only
initiated in the 1990-ies. This work was carried out in the framework of FP6 and FP7 in the domains of Natural
Hazards funded by the European Community. The swath bathymetry cartography of the southwest part of the
Iberian Peninsula resulted from the effort of European and national projects, of 19 oceanographic surveys, a total
of 200 ship time days executed from 2000 to 2006, involving 14 research institutions from 7 European countries.
As a result of this effort together with acquisition and interpretation of thousands of km of seismic reflection
profiles, the Pliocene-Quaternary uplift of the Alentejo continental margin (SW Portugal) is now widely accepted
by the scientific community. This uplift has not been yet quantified but it is possible that can have contributed to
erosion and deposition of metallic ores as placers in the continental shelf.
This argues in favor of the potential existence of placers in the continental shelf and the need for the detailed
investigation that will allow determination of ideal location for placers deposition in the past Pliocene-Quaternary
(5 My). The source for metals can arguably be associated to the Iberian Pyrite Belt ores hosted in the Alentejo
Paleozoic formations and to the hyper-alkaline intrusions of Sines and Monchique of Late Cretaceous age.
Artificial renourishment of beaches with offshore sand has not been assessed for the Alentejo littoral, despite that
the coast located to the south of the Sines segment shows high susceptibility to erosion. This has been observed
on a regular basis as the beaches are frequently devoid of sand, thus jeopardizing their touristic potential. The
detailed assessment of good quality sand deposits between approximately 30m and 50m below sea level aims at
minimizing this problem that affects the economy of littoral of Alentejo.
The MINEPLAT project (Assessment of the mineral resources potential in the continental shelf of Alentejo
and of the environmental conditions caused by the tectonic uplift in the Pliocene-Quaternary) aims at assessing
the existence of mineral deposits off the Alentejo. The MINEPLAT-1 geophysical survey acquired very high
resolution multi-channel reflection seismics, swath bathymetry, backscatter and magnetic data in a test area.
The preliminary results show: previously unknown NNW-SSE aligned magnetic anomalies, possibly corresponding
to buried Late Cretaceous alkaline rocks; faults affecting recent sediments; well defined patches of sediments
associated to morphologic traps.
Future work will involve ground-truthing to characterize the shallow deposits.Alentejo 2020 contract ALT20-03-0145-FEDER-00001
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)
Deltaic sedimentary structure interpreted from high-resolution seismic data : Sado Estuary, Portugal
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