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.
253
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.
254
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).
257
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.
258
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.
259
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).