Silurian graptolite biostratigraphy of Portugal:  an overview with notes on the Portuguese “Sardic” fauna  

José Manuel Piçarra¹  

¹Instituto Geológico e Mineiro, Ap. 104, 7802–902 Beja, Portugal. E–mail: jose.picarra@igm.pt

Key words: Graptolites. Biostratigraphy. "Sardic" fauna. Silurian. Portugal.

Silurian rocks are widely distributed within the Central Iberian and Ossa Morena Zones of Portugal (Figure 1). They are known in two major tectono–stratigraphic units of the "Hesperian Massif" that form the south–western part of the European Hercynian Fold Belt.

In southern Portugal, Silurian rocks occur in small scattered and tectonically disturbed outcrops within the Ossa Morena Zone (OMZ), except for the Barrancos area where the precise stratigraphy of the complete and condensed euxinic succession has been recently established. In the southern part of the Central Iberian Zone (CIZ), i.e., in the Dornes area and the Portalegre syncline, the upper part of Silurian successions contain more terrigenous deposits, hence the difficulty in its subdivision. In the northern part of the CIZ (the Valongo syncline and Trás–os–Montes area), the Silurian succession has never been studied in detail and the structural complexity and the high grade of metamorphism make stratigraphical studies difficult.

The black shales are the most common and characteristic rocks in all the Portuguese Silurian sequences. They contain abundant graptolites allowing precise regional biostratigraphic subdivision and correlation. The first graptolite species were listed by Delgado (1908). Important additional collections were made mainly by Romariz (1962, 1969), who established several new taxa which were considered typical for the so called "sardic" fauna. In the last twenty years, the Silurian succession has been studied more in detail in many localities within Portugal resulting in new lithological and biostratigraphical data (Gutiérrez–Marco et al., 1998), including taxonomic revision of some old graptolite collections (Figure 3).

The present paper aims at summary of state of art of the Silurian biostratigraphy in Portugal, including the most recent data.

The best documented Silurian succession in Portugal is known in the Barrancos area (Figure 2, log A), in the south–eastern Portuguese part of the OMZ (Robardet et al., 1998). It is a condensed succession up to 80 m thick, where 19 graptolite biozones have been recognized (Figure 3). Among other fossils orthoceratids, bivalves, brachiopods, crinoids and sponges also occur (Delgado, 1908; Rigby et al., 1997). The first graptolite data were given by Delgado, who defined several distinct lithostratigraphical units, all of them being used today. Later on, the graptolites were studied by Perdigão (1961) and Romariz (1962), but their publications contain some inaccurate taxonomic and biostratigraphical interpretations.

Figure 1. Distribution of the Ordovician to Lower Devonian rocks (stippled) in Portugal, including the Silurian materials. A, B, C, D, E1 and E2 correspond to the Silurian sucessions shown in figure 2. CIZ, Central Iberian Zone; OMZ, Ossa Morena Zone.

The lowermost part of the Silurian succession assigned to the basal Rhuddanian Parakidograptus acuminatus Biozone corresponds to the boundary beds of the Colorada (C) and "Xistos com Nódulos" (XN) formations. Graptolite bearing rocks consist of quartzitic beds and black shales alternating with lydites. It is followed by strongly weathered black shales, 20–25 m thick, with rare lydite layers, that constitute the dominant lithofacies of the "Xistos com Nódulos" Formation. This formation extends from the lower Llandovery up to the lower Ludlow. The graptolite units recognized in this interval are: the Rhuddanian Cystograptus vesiculosus and Coronograptus cyphus biozones, the Aeronian Demirastrites triangulatus, D. convolutus and Stimulograptus sedgwickii biozones, the Telychian Spirograptus guerichi, S. turriculatus–Streptograptus crispus, Monoclimacis griestoniensis and Oktavites spiralis biozones, the Sheinwoodian Cyrtograptus murchisoni and C. rigidus biozones, the Homerian C. lundgreni, Pristiograptus parvus–Gothograptus nassa and Colonograptus? ludensis biozones, and the Gorstian Neodiversograptus nilssoni and Lobograptus scanicus biozones.

The uppermost Silurian strata are represented by the lower part of the "Xistos Raiados" Formation, about 20–30 m thick. It consists of dark siltstones alternating with shales yielding graptolites of the Pridoli Neocolonograptus parultimus and Monograptus bouceki biozones.

The most complete Silurian succession in the CIZ is that of the Dornes area (Figure 2, log B) (Oliveira et al., 2000). This succession begins with the quartzites and sandstones of the 10–25 m thick Vale da Ursa Formation (VU). It contains poorly preserved biserial graptolites (?Normalograptus and ?Glyptograptus) indicating either early Silurian or late Ordovician age. The Vale da Ursa Formation is conformably overlain by the 100 to 200 m thick black shales of the Foz da Sertã Formation (FS) bearing frequent Llandovery and Wenlock graptolites, rare orthoceratids, bivalves and brachiopods. In this formation, the Aeronian Demirastrites triangulatus Biozone, Telychian Spirograptus turriculatus Biozone, Sheinwoodian Monograptus belophorus Biozone and the Homerian Gothograptus nassa graptolite biozones are recognized (Figure 3). There are also graptolite assemblages which can be indicative of the Sheinwoodian Cyrtograptus centrifugus and the Homerian Colonograptus? ludensis biozones. The most recent studies (Piçarra in Oliveira et al., 2000) revealed graptolite assemblage of the Homerian Cyrtograptus lundgreni Biozone and rare specimens of Saetograptus fritschi linearis, indicating early Ludfordian age.

Figure 2. Correlation of the main Silurian sucessions of Portugal. C, Colorada Fm.; FS, Foz da Sertã Fm.; S, Sobrado Fm.; SM, Serra Mendeira Fm.; Serra de Luação Fm.; Sy L, “Scyphocrinoid limestone”; VS, Vale de Serrão Fm.; VU, Vale da Ursa Fm.; XCi, “Xistos Carbonosos inferiores”; XCs, “Xistos Carbonosos superiores”; XN, Xistos com Nódulos Fm.; XR, Xistos Raiados Fm. Graptolites are shown as open circles for Llandovery, open tringles for Wenlock, open squares for Ludlow and black circles for Pridoli. Black triangles indicate other fossil groups.

The remaining part of the Silurian succession (ca 220 m thick) comprises terrigenous rocks of the Vale do Serrão (VS), Serra Mendeira (SM) and Serra do Luação (VL) formations bearing microfossils of Ludlow to Lochkovian age.

The Silurian succession of the Portalegre syncline (Figure 2, log C) in the southern border of the CIZ, is not clearly defined, though it may be similar to that of the Dornes area (Piçarra and Oliveira, 2002). It comprises the following members: 1) the 20–25 m thick dark and pyrite–rich quartzites (VU), 2) the 30–40 m thick black shales (FS), locally micaceous and rich in fossiliferous nodules bearing scarce graptolites of late Llandovery to late Wenlock age, and 3) shales and siltstones , 80–100 m thick, that include also sandstone beds becoming more frequent upwards (VS+SM+SL). The latter member may correspond to the uppermost Silurian and possibly to the lowermost Devonian.

Results of re–evaluation of old graptolite collections and study of the new material (Piçarra and Oliveira, 2002) confirm the presence of the Sheinwoodian Cyrtograptus murchisoni, Monograptus belophorus and Cyrtograptus rigidus biozones, as well as the Homerian Cyrtograptus lundgreni Biozone (Figure 3). Poorly preserved Aeronian graptolites also occur in the dark quartzites.

Figure 3. Graptolite biozones identified in the Silurian successions of Portugal (after several authors listed 1 to 11).

In the Valongo syncline the Silurian succession (Figure 2, log D) comprises the folloving members, in ascending order: 1) black shales of the "Xistos Carbonosos Inferiores" (XCi), approximately 100 m thick, 2) dark shales with intercalations of black siliceous layers and alternating siltstones and shales of the "Xistos Carbonosos Superiores" (XCs) of uncertain thickness, and 3) unfossiliferous sandstones and siltstones of the Sobrado Formation (S), about 800m thick.. The biostratigraphical control of the Silurian sequence remains imprecise (Figure 3). The first palaeontological data were given by Delgado (1908), who discovered numerous graptolites. They were later revised by Romariz (1962, 1969), who introduced some inaccurate identifications and described some new species and subspecies of the so–called "sardic" fauna.

More recently, Couto et al. (1997) have reviewed the previous biostratigraphical data and concluded that the Aeronian Demirastrites triangulatus, D. convolutus and Stimulograptus sedgwickii biozones and the Telychian Spirograptus guerichi Biozone (Figure 3) can be identified within the "Xistos Carbonosos Inferiores". The Wenlock assemblages cited from the "Xistos Carbonosos Superiores" are not enough diverse for biostratigraphic subdivision except for recognition of the Sheinwoodian Cyrtograptus rigidus Biozone (Figure 3).

The Silurian sequence in the Trás–os–Montes area known from the autochtonous and allochtonous rock units is composed of three lithological members/formations (Meireles, 2000) with a total estimated thickness of 300 m to 1100 m. The lower member ("Formação Infraquartzítica") comprises quartzites and black shales with intercalations of lydites. This is followed by a sandstone lenticular barren unit ("Formação Quartzítica") that is overlain by a sequence of brown shales ("Formação Supraquartzítica") with intercalations of hematitic siltstones, quartzites, lydites, volcanics, and limestones in the upper part. The stratigraphy is based on graptolites (Delgado, 1908; Romariz,, 1962, 1969) occuring in black shales and lydites. Among graptolites four news "sardic" taxa were recognized.

Recent studies in the southern (Moncorvo syncline and Lagoaça region) and northeastern (Guadramil region) parts of the Trás–os–Montes area show that the Silurian is represented here by the condensed succession similar to that of the OMZ (Sarmiento et al., 1999; Piçarra and Meireles, 2003) and of the Sil and Peñalba synclines of the West Asturian–Leonese Zone in Spain (Rábano et al., 1993). The Silurian succession of the Moncorvo syncline (Figure 2, log E1) is dominated by graptolitic black shales bearing some carbonaceous nodules. In the upper part it includes the "Scyphocrinoid limestone" (SyL) yielding Ludlow and Pridoli conodonts. Based on the studies of the old graptolite collections the Llandovery Demirastrites convolutus and Rastrites linnaei and the Wenlock Cyrtograptus rigidus–perneri and Cyrtograptus lundgreni biozones (Figure 3) are identified (Sarmiento et al., 1999).

In the Guadramil and Lagoaça regions the Silurian succession (Figure 2, log E2) includes Llandovery graptolitic lydite beds overlain by black and dark shales. In the first named region the black shales yield graptolite faunas of the Wenlock age and of the Gorstian Lobograptus scanicus Biozone (Piçarra and Meireles, 2003) (Figure 2).

The first references to graptolites of the "Sardic type" were made by Delgado (1908), who mentioned a presence of "giant species" in the Silurian rocks of several regions of Portugal, though he did not make formal taxonomic designations. Later on, new data were presented by some authors resulting from studies of similar faunas in several regions of south–western and central Europe. Late Wenlock "sardic" taxa were mentioned from the Barrancos area (Perdigão, 1961) and 24 new taxa (most of them of Wenlock age) were listed by Romariz (1962, 1969) from several areas of Portugal. The new Llandovery taxa were also cited from the Valongo syncline (Waterlot, 1965).

More recently, a re–evaluation of the Romariz material has shown that the definition of new taxa was based on strongly deformed graptolites, which can not be properly identified (Piçarra and Gutiérrez–Marco, 2001). This justifies serious doubts expressed by several graptolite workers regarding the necessity of differentiation of the so called "Sardic fauna" (Jaeger, 1976; Rickards et al., 1990).

Despite the scarcity of data in some areas, the lithological and biostratigraphical analyses allow a recognition of two types of Silurian sedimentary successions in Portugal (Gutiérrez–Marco et al., 1998).

The first sedimentary type is characterized by the condensed black shale sequence, without any important clastic influx, whose thickness does not exceed 80–100 m. This is exemplified by the Silurian succession of the Barrancos area, similar to that of the Valle syncline of the Spanish Ossa Morena Zone (except for the "Scyphocrinoid limestone"). A closely similar succession occurs also in the Moncorvo syncline (with a limestone unit in the upper part) and most probably in the northeastern part of the Trás–os–Montes area.

The other type of succession begins with a sandstone unit that is overlain by black shales ranging from late Llandovery to early Ludlow in age. It ends with the Ludlow and Pridoli sandstone–siltstone sequence. These coarser terrigenous lithofacies, more than 100 m thick, occur in most areas of the Portuguese Central Iberian Zone, except for the Trás–os–Montes regions. The biostratigraphical control in the upper part of the succession is poor and, up to now, no good fossil markers were recorded. This is also the case for the neighbouring Spanish Central Iberian areas that show similar lithological development.

It is rather difficult to give a comprehensive paleogeographical reconstruction for different regions of the Iberian Peninsula during the Silurian. However, based on the interregional correlations (Gutiérrez–Marco et al., 1998) the following suggestions can be made concerning a position of particular areas within the North Gondwanan shelf. The shallow water inner–shelf environments existed in the northwestern and southern Portuguese Central Iberian Zone and in other shallow platform areas of the Iberian Peninsula (Iberian Cordillera, Guadarrama area) and North Africa (Moroccan Sahara). Deeper outer–shelf conditions were characteristic for the Barrancos area and Spanish part of the Ossa Morena Zone, also for the Moncorvo syncline and northeastern part of the Central Iberian Zone. In these regions the sections are closely similar to those of the Catalonian Coastal Ranges, the Pyrenees, the southern part of the West Asturian–Leonese Zone, and also to those of other parts of southwestern Europe, as, for example, Sardinia (Italy) and Thuringia (Germany).

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Received: February 15, 2003

Accepted: June 15, 2003