Romariz, C., C. Almeida, M. O. Silva, (1979) Contributions to the geology of Algarve (Portugal).II - Volcanic structures in Eastern Algarve(Portugal)

Bol. do Museu e Laboratório Mineralógico e Geológico da Fac. de Ciências de Lisboa, vol. 16(1), p. 253-263.

Contributions to the geology of Algarve (Portugal)

II - Volcanic structures in eastern Algarve (Portugal) BY

CARLOS ROMARIZ, C. COSTA ALMEIDA, M. OLIVEIRA SILVA

Up to now little attention has been paid to the volcanic structures in the

eastern and central Algarve, south of Portugal. References in geological literature about those structures are very poor. Gouveia (1938) refers to a dike cutting a Jurassic formation at Monte Figo

(Monte de S. Miguel). The same feature is also referred in several other papers but it hasn’t been maped in the main geological map of Portugal (1/500 000).

Riché (1962), TEIXEIRA (1966) and others refer to volcanic activity in Algarve in a general way and they emphasize the relationship of that activity whith the late Cretaceous volcanic activity in the region of Lisbon.

However, the dikes and a volcanic pipe referred by those authors are mainly located in the western litoral coast of Algarve.

In a recent paper it was called attention to the presence of volcanic pipes and subaereal volcanic detritical rocks in Algarve, specially in the eastern side (ROMARIZ et al., 1976). Those volcanic structures and volcaniclastic associated rocks are located in a fault zone cutting Early Mesozoic rocks, as was referred in that paper.

The volcanic breccias of the pipes contain rock fragments of Triassic age, Lower Jurassic dolostones and also diabases.

Side by side are volcanic breccias representing old volcanic pipes and associated volcaniclastic rocks (volcanic sandstones, tuffs and volcanic aglomerates of small pebbles), very well stratified, wich have been folded together with the Lower Jurassic dolostones.

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Identical stratigraphic relationships were found with similar volcanic structures in the south of Spain (Orti-Cabo et al., 1971, GOMEZ et al., 1976).

Many other volcanic pipes and associated outcrops have been found in Algarve since 1976, wich leads to the assumption that volcanic structures are very common in the east of Algarve. There is a need for detailed geochemical and petrographic studies of the volcanic activity in the south of Portugal.

Fig. 1 - Location of the area reported

1 – HETANGIAN VOLCANIC ACTIVITY

The Trias-hetangian detritical series which contacts with a flysch facies of

carboniferous slates and greywackes, contain levels of diabasic rocks, specially thick in the cast side of Algarve. These rocks haven’t been yet properly studied. They are very much alike everywhere and usually very wheathered.

Relatively fresh rock samples of those igneous rocks were used for petrograp.hic observations (1).

The Hetangian diabase is always at the same level in the stratigraphic sedimentary column and can attain some tens of meters of tickness. Those rocks are synsedirnentary, conformable and occur interlayed in limestones and salt mudstones also of Hetangian age.

Everywhere in eastern Algarve the diabasic rocks are conformable with the adjacent strata and veinlets, even small ones, are absent.

(1) The authors are greatfully acknowledged to Prof. C. A. Matos Alves for the

petrological identification of the igneous rocks referred in this paper.

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Faulting of Triasic structures and diapiric movements are very commun in Algarve and in some places those episodes suggest the presence of two or occasionaly three layers of diabases, wich is not the case.

Some of those features can be seen between Vila Real de Sto. António and Castro Marim and at S. Bartolomeu.

The Hetangian diabasic rocks are most certainly the result on an important E-W fissural volcanic activity, along the south of Portugal, at the begining of Mesozoic time.

The absolute age determined (PORTUGAL FERREIRA & MACEDO, 1977) for the Hetangian diabase at eastern Algarve was 188 b1 2 million years.

This confirms the age that was estimated in the field for those diabasic rocks.

2 - PRE-CRETACEOUS VOLCANISM

Pre-cretaceous volcanism activity is very common in the east of Algarve. There has not been much search in the western side. Volcanic pipes and subaereal associated deposits originated a variety of volcaniclastic rocks (agglomerates, breccias, volcaniclastic sands, lapilli and tuffs), already referred in a previous note (C. ROMARIZ et al., 1976).

Recently we found outcrops that might suggest the volcanic activity consisted also of lava flows deposited in aquatic environment (pillow lay as).

A high number of outcrops of pipe breccias and volcaniclastic sediments was found last year. The following are now added to the list (1) referred (C. ROMARIZ et al., 1976)

10 − 800 m, NW Messines de Baixo 11 − 600 m, NW Soidos de Baixo 12 − 1100 m, NW Penina 13 − Km 56-57 the Salir-Alte road 14 − 3 Km, N Tor 15 − 600 m, NE Querença 19 − Barranco da Nora, 5 Km NW Tavira 20 − Bica, Asseca road, public well 21 − Calçadinha, Km 134,8 EN 125 22 − Srª Saúde, 100 SSW of chapel The authors believe that the Pre-Cretaceous volcanic pipes might well

follow a line close to the fault sistem, that in the central and eastern Algarve (1) The tuffs referred in the geotechnical report for the foundations of the bridge over the

Guadiana river probably belong to the same volcanic period. 256

makes separation of the Meso-Cenozoic terrain from the south border of Carboniferous rocks.

The observed structures are characterized: 1) by the presence of clasts of Triasic sandstones in the volcanic breccias of pipes; 2) by the pipes cutting the Lower Liassic dolostones at Moinho de Arrife or cutting older rocks than those, as it can be seen in central Algarve; 3) the volcanoclastic sediments associated to pipes folded together with the liassic dolostones, both possessing the same dip.

The exposures at Querença have an apreciable thickness and have a diversified lithology. It is found there pipe breccias, volcaniclastites, lava flows and dikes of pegmatitic rocks formed in a final volcanic stage.

At Rocha de Pena the Triasic detritical series and the Liassic dolostones are part of a broad syncline.

Fissural Hetangian extrusive rocks and Post-hetangian extrusive structures are quite notorious. They reflect two phases of volcanic activity.

In fact at Rocha da Pena (Pl I - Fig. 1, P1. II Figs. 1-2) there is a volcanic pipe associated with piroclasts similar to others found in many other places; but a second one contacts with the first by faulting, and has in its clastic elements blocks of tuffites of the first volcanic phase.

Unfortunately the material of the volcanic pipes is very wheathered but it is sufficiently differentiated to presume a destinct petrographic composition.

The pegmatitic dikes cut the structures at Rocha da Pena, Rocha de Soidos an Queren~a corresponding to the stage of the volcanic activity. Once again the volcanic rocks are always very altered. Perhaps some acceptable rock samples will be found if a detailed exploration of the area is carried out.

3 - POST-MIDDLE CRETACEOUS VOLCANIC ACTIVITY

Post Middle Cretaceous volcanic activity is represented in Western

Algarve through the important volcanic pipe of Ferrarias, Praia da Luz - Lagos and through a large number of dikes cutting Cretaceous and

Jurassic rocks. That volcanic pipe is well preserved and rather beautiful (1).

In the eastern side of Algarve, Cretaceous rocks lie unconformably on Jurassic beds northwest of Moncarapacho. The contact is through a fault extending from Bita west of Moncarapacho to Sto. Estevão, with a SW-NE trend. Along the fault there are volcanic breccias, possibly Middle Cretaceous in age (Fig. 2, numbers 16, 17, 18, 23). The volcanic pipes of Moita Redonda (Fig. 2, 17) at the contact of Jurassic 97 Cretaceous rocks include phenoclasts of limestone of those systems.

(1) Maria de Lurdes Henriques (1954).

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The scarce material used in petrographic observations showed that the rocks are of limburgitic and lamprophyric types (2).

The volcanic pipe that cutts Creataceous limestones east of Moncarapacho might well be associated with this volcanic activity (3).

It seems then that the basic rock types so commonly found in the west side of Algarve are also present in the east.

The composition of second volcanic pipe (Fig. 2, 23) is rather interesting: although highly altered, it is found to be a microgranular rock similar to a tinguaitic type and therefore petrologically close to the rocks of the Monchique massif.

According to Matos Alves (4) the rock contains a high precentage of a green piroxene of the aegirine - augite type, almost acicular, with a predominant position in the texture; the plagioclase occurs in elongated crystals and the matrix, although poorly identifiable, seems to contain alkaline feldspars, feldspatoids and/or zeolites.

The following rock from structures can also be included in the above group:

1 − The dike of Monte de São Miguel: trends E − W, cutts Jurassic limestones almost to the top of the anticline and as referred it was mentioned by Gouveia (1938).

2 − The dike that cutts Middle Jurassic units, at 1 Km W of Arrifes - Sta. Catarina.

3 − Some volcanic structures very wheathered at the 134.8 Km mark of the Tavira road. They are difficult to detect but may well be volcanic pipes cutting uper Jurassic units.

4 − Other dikes cutting Jurassic rocks such as the ones found along the road of Gilão (Km 40.600 and Km 39.900), and at the Barracha (2.5 Km SE of S. Braz de Alportel).

5 − The dikes (400 m NE of Quelfes Chappel and 300 SW Larangeiro v.g.) cutting, the Middle Cretaceous (5).

BIBLIOGRAFIA

GOMEZ, J. J., TRELL, A. & P. PEREZ (1976) − Presencia y edad de vulcanitos en el Jurásico del Norte de Valencia (Cordillera Ibérica, España. Acta Geológica Hispanica, Año XI, nº 1, p. 1-7).

(2) Petrographic identification C. Matos Alves. (3) Found by Silvério Prates. (4) Oral comunication. (5) Found by Silvério Prates.

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GOUVEIA, A. M. (1938) − Algarve. Aspectos fisiográficos. 160 p. Lisboa. HENRIQUES, M. L. MARQUES (1954) − Notas de Mineralogia e Petrografia

Portuguesas I. Chaminé monchiquitica da Praia da Senhora da Luz (Algarve Ocidental). Bolet. do Museu e Laboratório Mineralógico e Geológico da Faculdade de Ciências de Lisboa, vol. 22(7). Lisboa.

JULIVERT, M. & allia (1977) − Mapa tectónico de la Península Ibérica y Baleares, Instituto Geologico y Minero de España.

ORTI-CABO, F. & SANFELIU MONTOLIO, T. (1971) − Estudio del vulcanismo Jurásico de Candiel (Castellón) en relación con processos de lateritización, condensación y silicificación de la serie calcaria. Public. Inst. Invest. Geol. de la Diputación de Barcelona, vol. XVI, p. 21.

PORTUGAL FERREIRA, M. & MACEDO, C. A. R. (1977) − Actividade basáltica − Pérmico − Liásica no Território Português. Uma achega para a datação. Memórias e Noticias nº 83, p. 39-52. Publ. Mus. Lab. Min. Geol. Univ. Coimbra.

RICHÉ, P. (1962) − Note sur le bassin Mesozoique et Tertiaire de l’Algarve (Portugal). Relatório inédito. Comp. Pet. Portugal.

ROMARIZ, C., SILVA, M. O., ALMEIDA, C. & PALMA, F. (1976) − Episódios vulcano-sedimentares no Algarve (Nota Prévia), Bolet. do Museu e Laboratório Mineralógico e Geológico da Faculdade de Ciências de Lisboa, vol. 14 (2), p. 373-376. Lisboa.

TEIXEIRA, C. (1966) − A evolução do território português no decurso dos tempos geológicos. Palestra. Revista de Pedagogia e Cultura. Vol. 28, p. 111-157, Lisboa.

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Pl I

Fig. 1 − View of the outcrop located on the road to Sarnadas, showing the contact between two volcanic pipes of different ages. The structure on the left has blocks of piroclasts from the one on the right. (Salir, Rocha da Pena).

Fig. 2 − Volcaniclatic rocks covered by mudstones and dolostones Sinemurian in age. (Messines, Rocha de Messines).

Pl II

Fig. 1 − Detail of Pl. I, fig. 1.

Fig. 2 − Detail of Pl. I, fig. 1.

Pl III

Fig. 1 − Volcanic breccia containing clasts of sedimentary rocks. (Querença).

Fig. 2 − Dike cutting volcanic structures. (Alte, Soidos).