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Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/247 Comunicações Geológicas (2017) 104, 1, XX-XX ISSN: 0873-948X; e-ISSN: 1647-581X

Exploração de recursos minerais na plataforma continental

do Alentejo e alterações ambientais no Plio-Quaternário:

resultados preliminares da campanha MINEPLAT

Exploring the Alentejo continental shelf for minerals and Plio-

Quaternary environmental changes: preliminary results of the

MINEPLAT survey

J. Noiva1*, C. Ribeiro1,2, P. Terrinha3,4, M. Neres3,4, P. Brito3 and the MINEPLAT survey team

© 2017 LNEG – Laboratório Nacional de Energia e Geologia IP

Abstract: This study presents the preliminary results of MINEPLAT

survey, organised by Universidade de Évora in partnership with Instituto

Português do Mar e da Atmosfera). It is based on the analysis of

geophysical data collected during that survey: ultra-high resolution

seismic data, multibeam data (both bathymetry and backscatter data) and

magnetic data. The survey took place on the north part of Alentejo

continental shelf (30 to 200 meters depth) between Tróia and Sines. The

interpretation and integration of the acquired data allowed substantial

improvement in the knowledge of the morphology and geology of both

the surface and subsurface of this area, towards the assessment of the

mineral resources potential in the continental shelf off Alentejo. It also

contributed to improve the understanding of the Pliocene-Quaternary

environmental conditions caused by the tectonic uplift.

Keywords: Alentejo continental shelf, Geophysical survey, Mineral

resources, Pliocene-Quaternary, Tectonics.

Resumo: Este trabalho apresenta os resultados preliminares da campanha

MINEPLAT, organizada pela Universidade de Évora em parceria com o

Instituto Português do Mar e da Atmosfera, baseados na análise integrada

de dados geofísicos, nomeadamente, sísmica de reflexão multicanal de

muito alta resolução, multifeixe, retro dispersão e magnética. A

campanha decorreu na zona norte da plataforma continental do Alentejo

(entre os 30 e os 200 metros de profundidade) entre Tróia e Sines. A

interpretação e a integração dos dados adquiridos constituem uma

melhoria substancial no conhecimento da morfologia e geologia do fundo

marinho e da subsuperfície marinha desta região directamente

relacionados com a avaliação do potencial em recursos minerais na

plataforma continental do Alentejo e as condicionantes naturais impostas

pelo soerguimento da margem continental no Plio-Quaternário.

Palavras-chave: Plataforma continental do Alentejo, Campanha de

geofísica, Recursos minerais, Pliocénico-Quaternário, Tectónica.

1 Universidade de Évora, Departamento de Geociências. 2 Instituto de Ciências da Terra (ICT). 3 Instituto Português do Mar e da Atmosfera, Divisão de Geologia e

Georecursos Marinhos. 4 Instituto Dom Luiz (IDL).

*Autor correspondente/Corresponding author: jgng@uevora.pt

1. Introduction

Alentejo is a region where the extraction of mineral resources has a relevant contribution to the economy since at least Roman

times. Nowadays the raw materials sector has concentrated its activity in the exploration and exploitation of ornamental rocks and massive sulphides ore deposits. There are other types of ore deposits but their economic exploitation is not feasible at the present market conditions (e.g. gold in the Montemor-o-Novo and uranium in the northeast Alentejo).

The first phases of onshore/offshore prospection include geophysical investigation, geological mapping, definition of

targets, physical sampling and mineral and chemical analysis of the sampled materials. Despite the lack of detailed geological studies of the Alentejo continental shelf, the present state of knowledge shows that the potential for the existence of both metallic and non-metallic resources is real and promising.

This is based mainly on the assessment of placers of metallic ores associated with the diminution of hydraulic transport capacity (some recent concentrations of heavy-minerals have been recognised on the littoral of St. Torpes, Sines) and

aggregates for beach artificial re-nourishment. Placer deposits form, for example at the mouth of rivers or in

marginal-littoral environments. The present-day conditions of these zones are fairly well known. However, due to the various glaciations that occurred in the last three million years, the geography of the coast has been very different with the coast line positioned up to tens of kilometers offshore while the sea level changed reaching a minimum value of ~150 m below present-day

sea level (de Boer et al, 2010). The Pliocene-Quaternary uplift of the south of Portugal was

characterized by Feio (1951), and is widely accepted by the scientific community although it remains to be quantified. This uplift probably contributed to the erosion of the Alentejo most superficial metallic ores and the deposition of the eroded material in the continental shelf possibly forming placers.

The joint effect of the uplift and the sea level change argue in

favour of the potential existence of placers in the continental shelf justifying the need for detailed investigation that will allow the identification of ideal locations for placers deposition in the past Pliocene-Quaternary (5 My). Also, the accumulation of loose sands in the Alentejo shelf is a prospect worth investigate with potential as a source of material for the artificial ren-ourishment of the beaches along the southwestern coast of Portugal.

Artigo original Original article

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2 Noiva et al. / Comunicações Geológicas (2017) 104, 1, XX-XX

The main goal of MINEPLAT project is the assessment of the potential of mineral resources of the Alentejo continental shelf (metallic and non-metallic ores) formed during Pliocene-

Quaternary times. This will be achieved by producing detailed maps of selected areas and acoustic characterization of the seafloor. A geophysical survey followed by seafloor sampling plus mineral and chemical analysis of the collected materials is the main strategy in place. This will allow the definition of mineral deposits at selected targets, the creation of a database for future studies, as well as the formation of the human resources involved and development of technical capacity to work at sea.

2. State of Art

The tectonic uplift of South Portugal in the last 5 million years

(Myr) was firstly identified based on morphologic criteria by Mariano Feio (1951).

However, the assessment of continental vertical movements off Portugal and its relation with tectonics was only initiated in the 1990s.

The South West Iberian Margin (SWIM) swath bathymetry compilation (Zitellini et al., 2009) resulted from the effort of several European and national projects, 19 oceanographic

surveys, a total of 200 ship-time-days (from 2000 to 2006) and involved 14 research institutions from 7 European countries. This compilation comprehends the area from the Gibraltar Strait to the parallel of Espichel Cape but only includes the deep sea and not the continental shelf.

The swath bathymetry was complemented with the acquisition of thousands of kilometres of multichannel seismic reflection (MCS), side scan sonar (or backscatter from multibeam

echo-sounder systems), direct visual observation and sampling of seafloor. This vast dataset contributed to the definition of present day active tectonic structures (Terrinha P., et al., 2009), mapping the Africa-Eurasia plate boundary (Zitellini N., et al., 2009) and mud volcanoes in SW Iberia (Pinheiro L., et al., 2003), detection of hydrocarbon seepage and methane hydrates occurrences (Magalhães V., et al., 2012), characterization of Fe-Mn (polymetallic) nodules and crusts (Muiños et al., 2013).

Seismic refraction and monitoring of the deep-sea seismicity (Sallarés et al., 2013) using Ocean Bottom Seismometers (OBSs) allowed characterization of the seismicity and the deep constitution of the oceanic basement. The study of hydrocarbon seepage in the area (Hensen, C., et al., 2015) cast new ideas on the economic potential. Among various outputs from these works the following can be highlighted: i) new scientific knowledge of southwest Iberia that lead to further projects being funded (?) by

the EU, USA and European countries; ii) implementation of early warning tsunami centers in the Mediterranean and Portugal; iii) assessment of the tsunami risk of the Sines industrial center; iv) investigation of the oil potential in the Algarve and Alentejo; v) discovery of potential metallic ore deposits in the Portuguese Exclusive Economic Zone; vi) the relationship between the submarine morphology and the Pliocene-Quaternary tectonics in the deep sea.

Terrinha et al. (2009) evidenced the deformation and uplift of

the Alentejo continental slope and abyssal plain during the Pliocene-Quaternary that was coeval with the exhumation of the Monchique alkaline complex (onshore) as demonstrated by the mineral thermochronology data of Rodrigues (2015). It is also accepted that continental uplift was responsible for the sand cover erosion and denudation of the Alentejo coast to the south of Sines. The eroded sand that can be re-deposited on the upper shelf remains to be assessed, distribution and volumes would be

important information for future beach artificial re-nourishment planning.

The uplift of the margin increases erosion and transport of

materials from the Monchique and Sines alkaline complexes and from the Iberian Pyrite Belt; it is possible that some of these materials formed economic valuable placers in the continental shelf.

The mapping of the mobile sedimentary cover (Instituto Hidrográfico, 2005) initiated in the 1960s and was based on the superficial sampling of the seafloor using a 1 nautical mile grid (~1825 m). Regardless their important value, these maps only

represent the starting point for the further useful evaluation of mineral resources.

The amount of multi-channel seismic reflection profiles (which allows measuring sedimentary thicknesses) offshore south Portugal is quite considerable. However, these data are of very low resolution (usually <10 m) not allowing for the imaging of the thinnest deposits. This results from the fact that the goal of most of these surveys was either deep located hydrocarbon

deposits/sources or neotectonic structures.

3. Data and Methods: the MINEPLAT survey

The imaging of the recent mobile cover can only be ensured by using very high resolution methods, complemented by other acoustic methods such as side scan sonar, by seafloor sampling and sedimentological analysis and these data are still lacking on the Alentejo continental shelf.

Therefore, to obtain geophysical data to produce a detailed geological mapping and characterization of the seafloor of the Alentejo continental shelf and to model the Pliocene-Quaternary

uplift, a multidisciplinary geophysical survey (acoustic and magnetic data), organized by Universidade de Évora in

Fig. 1. Multibeam data and Ultra High Resolution Seismic (UHRS) reflection profiles

acquired during the MINEPLAT marine survey (black lines).

Fig. 1 Dados de batimetria multifeixe e de sísmica de reflexão multicanal de ultra alta

resolução (UHRS) adquiridos durante a campanha de geofísica marinha MINEPLAT

(linhas a preto).

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Preliminary results of the MINEPLAT survey 3

partnership with Instituto Português do Mar e da Atmosfera (IPMA), took place on north area of Alentejo continental shelf between 4th and 15th of October 2016 (Fig. 1).

The acquisition of geophysical data was done using the research vessel NI Noruega of the project partner IPMA. During the MINEPLAT survey 2090 km were navigated and the following data were acquired: 350 km of ultra-high resolution multi-channel seismic data (UHRS), 450 sq. km of multibeam echo-sounder data (MBES bathymetric), 450 sq. km of backscatter (MBES backscatter) data and 507 km of magnetic data profiles.

The UHRS data was acquired using a Geo Marine Survey Systems spread, towed from the vessel aft at starboard, consisted of one 200 tips sparker source, a power supply unit configured to output 400-600 J shooting every 0.5 s, a 24-channel hydrophone chain with 3.125 meters of group interval towed with a slanted geometry. The slanted geometry has several advantages namely: i) as the hydrophone groups are towed underwater they are more protected of the swell noise allowing acquisition data at

maximum of two meters of swell, ii) the ghost reflection removal is easier to be done because the receiver notch differs as the cable depth diverges from the near to the far offset, and iii) it allows improving the recovery of high and low frequencies of the seismic spectrum during the full processing of the data.

The geometry of the acquisition lines was designed to characterize the main regional geological structures (Figure 1). The in lines were acquired perpendicular to the coastline spaced a

nautical mile and the cross lines were acquired perpendicular to the in lines spaced four nautical miles.

The multibeam bathymetric data were collected with a Reson SeaBat T50-P echo-sounder mounted in a pole at middle ship portside. This system produces 512 beams arrayed over a 150º equidistant arc and operates by ensonifying a narrow strip of sea floor across-track, and detecting the bottom echo with narrow, across-track, listening beams. The swath of sea floor imaged on each survey line was about 5 times the water depth. Line spacing

was designed to ensure a minimum 30% of overlap between surveyed lines.

In addition to the bathymetric data, backscatter data were

simultaneously collected by the Reson T50-P echo-sounder. The seafloor backscatter is defined as the amount of acoustic energy received by the sonar after a complex interaction with the seafloor. This information can be used to classify the seafloor constitution. For example, a softer bottom such as mud will return a weaker signal (-10 to -30 dB) than a harder bottom, like rock (-30 to -60 dB). UHRS profiles and swath bathymetry together with seafloor sampling will constrain the seafloor

backscatter maps. The main advantage of simultaneous acquisition of

bathymetry and backscatter is that they are geographically referenced together, ensuring the backscatter snippet data will always be shown in the right place on the seafloor.

Magnetic data (Fig. 2a) were acquired with a total field G-882 magnetometer (Geometrics) at 10 Hz acquisition frequency (average along-line resolution of 0.175 m). The magnetometer

was towed at aft center 200 meters behind the vessel, to avoid its magnetic interference into the data, at average depth of 2.2 m below the sea surface. Total magnetic field data were corrected from diurnal variation using data acquired at a static magnetic station in Alentejo at the S. Teotónio IPMA facilities (Fig. 2b, Fig. 2c) at one measurement per minute rate. Magnetic anomaly data was then retrieved by subtraction of International Geomagnetic Reference Field (IGRF) main field. The anomalous

magnetic field provides information about location of metallic objects (e.g. shipwrecks on the seafloor or in sub-seafloor) or geologic structures such as intrusive bodies or metallic placers.

4. Preliminary Results

The MBES bathymetric and backscatter data were processed to clean the outliers and make the data readable. The URHS data was brute stack processed to make the quality control of the data

Figure 2. a) Lines with acquired magnetic data.; b) Location of the São Teotónio magnetic base station, about 70 km SE of survey area; c) Installation of the base station

magnetometer in the São Teotónio IPMA facilities.

Figura 2. a) Linhas com aquisição de dados magnéticos; b) Localização magnetómetro de referência, em São Teotónio, a cerca de 70 km SE da área de pesquisa.; c) Instalação do

magnetómetro de referência nas instalações do IPMA em São Teotónio.

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4 Noiva et al. / Comunicações Geológicas (2017) 104, 1, XX-XX

and to perceive the main seismic features/structures of the seafloor sub-surface.

From the preliminary analysis of UHRS data of the study

area, morphologic and tectonic-sedimentary features can be highlighted. The UHRS profiles confirm that the studied part of the Alentejo inner and middle continental shelf is apparently sediment starved, as shown by the thin shallowest seismic units that probably contain the sediments deposited during the last high stand stage. In the UHRS inline profiles westwards progradational wedges probably deposited during a low stand stage are shown. These progradational wedges are truncated at

the seafloor, by a thin top unit, suggesting that this area is currently under dominant erosive processes. In the shelf break the UHRS profiles show several shelf-marginal sedimentary wedges related with low stand stages. Landslides near the shelf are also imaged indicating present day local instability. Faulting (e.g., F1 of Fig. 4) and folding of recent deposits sometimes affecting the seafloor indicate recent tectonics in agreement with reported onshore and offshore neotectonics.

Paleochannels (e.g., P1, P2 and P3 of Fig. 3) dissecting at various stratigraphic levels the seismic units that acted as traps for sediments are imaged. Also, various progradant (e.g., unit U5 of Figure 4) and aggradant (e.g., unit U3 and U4 of Figure 4) sedimentary bodies are imaged, probably associated with the high and low stand depositional regimes. These will be investigated in further detail using backscatter data and direct inspection using video system and coring for ground truthing.

The preliminary analysis of the MBES bathymetric model of the study area, shown on Figure 1, revealed a continental shelf with a gentle slope, with a shelf break at 200 m below sea level.

One of the main highlights that can be shown without the full processing of the data is a 15-km long belt of sediment sorted sediments and shallow bedforms in the middle shelf, between 90 m and 120 m of depth (Fig. 5).

The bedforms area corresponds to gentle change of the seabed dip (Fig. 5). The bedforms are crescent shaped and

generally oriented parallel to the N-S direction in the northern part of the study area and NNE-SSW in the southern part of the area indicating a slight change of the seabed currents direction.

In Fig. 6a, the MBES bathymetric shows an example of the crescent shaped bedforms. These bedforms, located in the northern part of the study area, strike N-S with a gentle slope (1-1.5º) to the West and 1 meter of maximum height. The analysis of MBES backscatter model of the study area, for the same area

of the bedforms field, shows a sorted bedform pattern. The acoustic response of the bedforms is characterized by lower values of backscatter intensity. In contrast the acoustic backscatter data of the seabed correspondent to the morphology

Figure 3. Interpreted excerpt of L017 UHRS profile. Paleochannels P1, P2 and P3

dissect at different stratigraphic levels. Stratigraphic cross cut relationships allowed

identification of various phases of channel formation.

Figura 3. Secção interpretada do perfil UHRS L017 (localização na figura 1). Os

paleocanais P1, P2 e P3 cortam vários níveis estratigráficos. A análise estratigráfica das

relações de corte permite a identificação de várias fases de formação dos canais.

Figure 4. Interpreted excerpt of L019 UHRS profile. The aggradant units U1, U2, U3

are faulted and folded. These seismic units are truncated by an erosional surface

corresponding to the base of U4. The progradant unit U5 lies under the top of unit U4

erosional surface. In the western part of the profile all units are folded and the most

recent unit U6 is visible. In this area at the top units, the seismic record displays an

acoustic blanking (AB) probably associated with gas escape.

Figura 4. Secção interpretada do perfil UHRS L019 (localização na figura 1). As

unidades agradantes U1, U2 e U3 estão afetadas pela falha F1 e dobradas. Estas

unidades sísmicas são truncadas pela superfície erosiva correspondente à base da

unidade U4. A unidade progradante U5 sobrepõe-se ao topo da superfície erosiva da

unidade U4. Na área oeste do perfil, onde é visível a unidade sísmica mais recente U6,

todas as unidades estão dobradas. Nesta área o registo sísmico apresenta zonas de

sombra acústica (AB) nas unidades de topo, provavelmente associados a escape de gás.

Figure 5. Excerpt of the belt of bedforms in the middle

shelf shaped after the gentle slope break. In this area,

the bedforms are crescent shaped and generally

oriented parallel to the NNE-SSW direction.

Figura 5. Detalhe do campo de formas de fundo

situado na plataforma continental média, numa área de

quebra de declive. Nesta área, as formas de fundo têm

configuração em crescente e orientação preferencial

NNE-SSW.

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Preliminary results of the MINEPLAT survey 5

shown on Figure 6a. The visible sorted bedforms pattern (where the black colors indicate fine grained sediment and white colors indicate coarse sediment) show a match between the morphology

of the seabed and the lithology. Cross check of the bathymetric and backscatter images with UHRS profile suggests there is a structural control of the bedforms by sub-surface hard strata (cf. Figure 6a and 6b with Fig. 7). In Fig. 6 (a and b) the area where the seismic profile X002 cross cuts longitudinally two bedforms is highlighted. In seismic profile (Figure 7) the bedforms position is highlighted and matches the area where the dipping hard strata provide sedimentation traps for mobile sediments. This suggests

that the edification of the bedforms is structurally conditioned. This preliminary cross-analysis of the different types of data

(MBES bathymetry, MBES backscatter and UHRS) points to a belt of bedforms that corresponds to a sequence of small depressions characterized by coarse-grained sediments alternated with small elongated mounds of finer sand bodies. Ground-truthing (previewed for next year cruises) will allow checking for the nature of the trapped sediments.

Lines for magnetic data survey are represented in Figure 2a. Since the São Teotónio IPMA pole had not been tested before as survey base station, we evaluated its data quality by comparing the acquired data to data from the closest observatory of the INTERMAGNET international network, i.e., the San Pablo – Toledo magnetic observatory (Madrid) (Figure 8). The comparison reveals that the quality of data from São Teotónio and Toledo is similar: the frequency content is equivalent and

there is no record of significant magnetic noise. This supports the use of São Teotónio data for correction of local variation of the external field. In Figure 9 examples of acquired and processed profile data are shown: magnetic anomaly before and after correction of diurnal variation, as well the diurnal variation simultaneously recorded at the base station.

These preliminary magnetic data show the existence of several magnetic anomalies of relatively high amplitude (up to 400 nT), with variable wavelengths (from less than 1 km up to 4

km), in some locations showing high spatial frequency. Given their properties, these magnetic anomalies are mostly likely caused by the presence of magmatic rocks intruded at variable depth and / or lateral extension.

5. Conclusion

The used methods proved useful in revealing the recent

morphologic features that are shaping the starved continental shelf of Alentejo. Further processing and sequential stratigraphic analysis will allow associating sedimentary sequences to high

Figure 6. Excerpt of multibeam data (a), values are in meters and backscatter data (b),

values are in dB. Cross check of the bathymetric, backscatter images and UHRS

profiles (highlighted).

Figura 6. Detalhe dos dados de batimetria multifeixe (a), valores em metros e dos

dados de retro-dispersão do fundo marinho (b), valores em dB. Comparação entre os

dados de batimetria multifeixe, de retro-dispersão do fundo marinho e os perfis UHRS

(em realce).

Figure 7. Section of X002 UHRS profile (see location

in Figure 1). The areas where it cross-cuts

longitudinally the bedforms presented in Figure 6 are

highlighted in yellow.

Figura 7. Secção do perfil UHRS X002 (localização na

figura 1). As áreas onde o perfil X002 atravessa

longitudinalmente as formas de fundo, mostradas na

Figura 6, estão realçadas a amarelo.

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6 Noiva et al. / Comunicações Geológicas (2017) 104, 1, XX-XX

and low stand eustatic periods. Channels and other types of

Figure 8. a) Total magnetic field in in nano tesla (Field_nT) measurements at the São Teotónio’s base station at 4th of October 2016. These data were applied to MINEPLAT

surveyed data as a correction for diurnal variation. b) Total field measurements at the international magnetic observatory San Pablo – Toledo (Madrid) for the same day.

Figura 8. a) Medições do campo magnético total em nano tesla (Field_nT) na estação base de São Teotónio efetuadas a 4 de outubro de 2016. Estes dados foram aplicados aos

dados adquiridos na área MINEPLAT para correção da variação diurna. b) Medições do campo total no observatório magnético internacional San Pablo - Toledo (Madrid) para

o mesmo dia.

Figure 9. Examples of acquired magnetic

profiles (location in the inset). In the same

scale is represented the magnetic anomaly

measured (MagAnom; red) and after the

diurnal variation correction

(MagAnom_DiurCorr; green). In blue, the

respective diurnal variation applied as

correction. Magnetic flux density values are

in nano tesla (nT).

Figura 9. Exemplos de perfis magnéticos

adquiridos (localização no mapa de detalhe).

Na mesma escala é representada a anomalia

magnética medida (MagAnom; vermelho) e

após a correção da variação diurna

(MagAnom_DiurCorr, verde). A azul, a

variação diurna respetiva aplicada como

correção. Os valores de densidade do fluxo

magnético são apresentados em nano tesla

(nT).

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Preliminary results of the MINEPLAT survey 7

and low stand eustatic periods. Channels and other types of sediment traps will also be mapped for further investigation and targets for sampling will be selected. The belt of sea bed

bedforms and sorted bedforms are also promising for research as they can act as trap for heavy minerals. The magnetic survey imaged anomalies apparently related to shallow crustal intrusions that may have fed minerals onto the shelf. Recent faulting was also shown in the data. These will contribute to a better understanding of the present deformation of the crust.

Acknowledgments

To MINEPLAT project ALT20-03-0145-FEDER-000013. To the MINEPLAT survey team: Alessandro Righetti, Débora Duarte, Joana Santos, Marcos Rosa, Manuel Teixeira, Sara Rodrigues,

Vítor Magalhães. To Guilherme Madureira for the acquisition of São Teotónio magnetic base station data. To Estrutura de Missão para Extensão da Plataforma Continental (EMEPC) the magnetometer loan. To Halliburton for providing the seismic interpretation software through the Landmark Universities software grant program. To Oasis Montaj for providing a software educational license to process the magnetic data.

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