The stratigraphy of the Iberian Pyrite Belt (Figure 1) consists in two major units, the Phyllite Quartzite Group (PQ) and the Volcano Sedimentary Complex (VSC).
The PQ Group forms the detritic basement and is composed mostly of phyllites, quartzites, quartzwackes and shales with intercalations of limestone lenses and nodules at the upper part which as a whole were laid down in a marine siliciclastic platform. The thickness is in excess of 200m (base not known). The unit is of late Devonian age given by ammonoids, conodonts and palynomorphs (Boogaard, 1963, Fantinet et al., 1976, Cunha and Oliveira, 1989, Oliveira et al., 1997, Oliveira et al., 2004; Pereira et al., 2004).
The VSC includes several volcanic episodes of, either intrusive and extrusive, volcanic rocks dominated by rhyolites, rhyodacites, basalts and minor andesites, and intercalations of black shales, siltstones, minor quatzwackes, thin bedded volcaniclastics, jaspers and purple shales. The thickness is variable, from few tens of meters to more than 1000m. The age, given by rare conodonts and well preserved assemblages of palynomorphs, ranges from the late Devonian to the late Viséan (Oliveira, 1990; Oliveira et al., 1997, Oliveira et al., 2004, Pereira et al., 2004). The VSC was laid down in a submarine environment.
Above the VSC there is a turbidite succession whose designation depends on its position in the belt geographically divided in branches (Oliveira, 1990). In the north branch the succession is named Freixial Formation and occurs usually tectonically imbricated with the VSC lithologies. In the south branch of the belt, the turbidite succession, known as Mértola Fomation, is composed of thick and thin bedded greywackes, conglomerates and shales, with a total thickness of about 3000m.The age of these units is slightly diachronic, the Feixial Fm. being of early late Viséan age, as shown below, while the Mértola Fm. ranges from late Viséan to early Serpukhovian.
It should be noted that further southwest the Cercal Anticline (Figure 1) constitutes another branch of the Pyrite Belt. The VSC stratigraphic sequence is here overlain also by turbidites, in this case of Sepukhovian age. However, this sequence is far from being well understood and as such is not treated in the present work.
The study samples were collected in drill cores and in a road track (Serra Branca). The logged lithologies are labelled as informal units which are used only for descriptive purposes. Correlations with well known lithostratigraphic units in the belt are established, depending of existing similarities.
Biostratigraphic research was based on palynomorphs and standard palynological laboratory procedures were employed in the extraction and concentration of the palynomorphs from the host sediments (Wood et al., 1996). The slides were examined with transmitted light, per BX40 Olympus microscope equipped with an Olympus C5050 digital camera facility. All samples, residues and slides are stored in the Geological Survey of Portugal (INETI). The miospore biozonal scheme used follows the standard Western Europe Miospore Zonations (after: Clayton et al., 1977; Streel et al., 1987; Higgs et al., 1988; Higgs et al., 2000, Clayton 1996 and Maziane et al., 2002). The choice of alternative schemes was stated by the presence of very consistent local miospore assemblages in South Portugal (Figure 2). Stratigraphically important and typical taxa are illustrated in Plate 1.
Figure 2. Miospore zonation schemes used
Western Europe Zonation (adapted from Clayton et al., 1977; Streel et al., 1987; Higgs et al., 1988; Clayton 1996, Higgs et al., 2000, and Maziane et al., 2002); South Portugal Zonation (Adapted from Pereira, 1999).
Plate 1. Stratigraphically important and typical taxa
Plate captions list the taxonomic name of the figured specimen, followed by borehole number, sample number, slide number, microscopic coordinates and INETI collection number of the specimen.
Grandispora echinata Hacquebard, 1957; Borehole MP 3, Sample 36,60-1, 1244-45, INETI 0501.
Grandispora cornuta Higgs 1975; Borehole MP 3, Sample 36,60-1, 1305-225, INETI 0502.
Samarisporites triangulatus Allen 1965; Borehole MP 3, Sample 36,60-1, 1090-180, INETI 0503.
Gorgonisphaeridium ohioense (Winslow) Wicander, 1974; Borehole MP 3, Sample 36,60-3, 1200-138, INETI 0504.
Gorgonisphaeridium plerispinosum Wicander 1974; Borehole MP 3, Sample 36,60-3, 1190-175, INETI 0505.
Veryhachium downiei Stockmans and Willière, 1962; Borehole MSD 1, Sample 42,60-1, 1120-110, INETI 0506.
Emphanisporites annulatus McGregor, 1961; Borehole MP 3, Sample 36,60-2, 1335-160, INETI 0507.
Rugospora radiata (Jushko) Byvscheva 1985; Borehole MP 3, Sample 36,60-1, 1340-148, INETI 0508.
Retispora lepidophyta (Kedo) Playford, 1976; Borehole MSD 1, Sample 42,60-1, 1245-100, INETI 0509.
Lycospora pusilla (Ibrahim) Schopf, Wilson & Bentall 1944; Borehole MSD 1, Sample 255,30-1, 1155-75, INETI 0510.
Densosporites sp.; Borehole MSD 1, Sample 255,30-1, 1095-135, INETI 0511.
Raistrickia nigra Love, 1960; Borehole MSD 1, Sample 359,40-1, 1380-180, INETI 0512.
This branch extends from Mina de São Domingos (east) until Lagoa Salgada (west), the latter below the Cenozoic sediments of the Sado Basin. The stratigraphic research was carried out in the Albernoa, Serra Branca and Mina de São Domingos antiforms (Figure 1). The units identified in selected boreholes and their corresponding ages based on palynomorphs are shown in Figure 3.
The geological map of this antiform (Figure 4) shows felsic volcanic rocks (rhyodacites) in its core which are overlain by siliceous and purple shales. Phyllites and quartzites ascribed to the Phyllite-Quartzite Group lay geometrically above this sequence.
Figure 4. Detailed geological map
Detailed geological map of Albernoa region (Adapted from Geological Map of Portugal, 1 200 000, Sheet 8).
Boreholes AB1 and 11-1 (Figure 3) put in evidence a lithological succession composed of black shales with carbonate nodules (Xc) followed by felsic volcanic rocks (Va). Unit Xc, which is provisionally correlated with the upper part of the PQ Group, yielded a miospore assemblage that includes Aneurospora goensis, Aneurospora greggsii, Auroraspora sp., Camazonotriletes sp., Crassispora catenata, Densosporites devonicus, Geminospora lemurata, Retusotriletes rugulatus and Verrucosisporites scurrus, indicating an early Famennian age. Younging stratigraphic direction is given by graded bedding, flame structures and load casts in the volcaniclastic sequence immediately above the volcanic rocks, indicating that the stratigraphic sequence corresponds to the upright limb of the antiform (Figure 4).
The lithological succession intersected in borehole TR1 corresponds to the overturned limb of the antiform. The stratigraphic succession consists of felsic rocks (rhyodacites) (Va) overlain by: - purple shales (bv); - black shales (Xn1); - shales, siltstones and tuffites (VS1); - purple shales (bv); - shales, quartzwackes and minor tuffites (VS2).
Unit Xn1 provided miospores of late Famennian VH Biozone. The assemblages include Grandispora echinata, together with Auroraspora sp., Diducites sp., Emphanisporites rotatus, Geminospora lemurata, Grandispora cornuta, Punctatisporites sp., Retusotriletes planus, R. rugulatus and Rugospora radiata. All samples contain abundant acritarchs and prasinophytes.
Unit VS1 yielded miospores assigned to the early Viséan, Pu Biozone. The assemblage yielded the zonal specie Lycospora pusilla in association with Auroraspora macra, Convolutispora sp., Densosporites sp., D. spitsbergensis, D. brevispinosum, Dictyotriletes castaneaeformis, Discernisporites micromanifestus, Knoxisporites cf. triradiatus, Vallatisporites pusillites, V. galearis, V. verrucosus and reworked miospores of late Famennian and Strunian age. This unit marks the entry of quartzwackes (turbidites) in the succession.
About 40m above the second purple shale intercalation (bv) samples of unit VS2 provided poorly preserved miospores of mid Late Viséan NM Biozone. The miospore assemblage contains the zonal specie Raistrickia nigra and taxa such as Anaplanisporites sp., Crassispora sp. Densosporites sp., D. brevispinosum, D. intermedius, Knoxisporites triradiatus, Lycospora pusilla, Microreticulatisporites sp., Vallatisporites vallatus and Waltzispora sp.
The VSC of the Serra Branca Antiform (Figure 5) is mostly composed of several felsic units, mafic rocks (mostly spilites), shales and a purple shale horizon at the top of the volcanic sequence. This sequence is overlain by the Freixial Formation turbidites, with a thickness of 200m in the type section, which is well exposed in a road track on the west side of the Guadiana River.
Figure 5. Detailed geological map of Albernoa region
Detailed geological map of Albernoa region (Adapted from Geological Map of Portugal, 1 200 000, Sheet 8).
Borehole SB 8 (Figure 3) intersected the following units from top to bottom: - Freixial Fm. turbidites (Fr); - a purple shale horizon, about 20m thick (bv); - a pile of felsic volcanic rocks with intercalations of dark shales, tuffites and mafic rocks at the lower part (VS3); - black shales, siltstones and siliceous shales (VS4).
The Freixial Fm., in the type section, yielded miospores of the mid Late Viséan NM Biozone as indicated by the following assemblage: Ahrensosporites sp., Anaplanisporites sp., Crassispora trychera, Densosporites sp., D. brevispinosum, Knoxisporites triradiatus, Kraeuselisporites sp., Leiotriletes tumidus, Lycospora pusilla, Microreticulatisporites sp., Vallatisporites cilliaris, V. vallatus and Waltzispora planiangulata, in association to the nominal specie Raistrickia nigra, and taxa such as Emphanisporites rotatus, Geminospora sp., Retispora lepidophyta, Rugospora radiata and Vallatisporites verrucosus, interpreted as reworked miospores of late Famennian and Strunnian age. These assemblages are similar to that described for the unit VS2 identified in theTR1 borehole, Albernoa.
The black shales of unit VS3 gave miospores assigned to the late Famennian VCo Biozone, as indicated by the presence of the zonal specie Grandispora cornuta and Auroraspora sp., Diducites sp., Emphanisporites rotatus, Geminospora lemurata, Punctatisporites sp. and Rugospora radiata, unit VS4 contains a miospore assemblage assigned to the Pu Biozone, identical to that described for unit VS1 of the TR1 borehole.
The lithological succession is in normal stratigraphic order until the base of the felsic volcanics which are in turn thrusted over the VS4 unit.
In general terms, the stratigraphic sequences of Albernoa and Serra Branca antiforms share several characteristics in common, particularly concerning the lithologic successions and ages.
The stratigraphic sequence of this antiform is composed of felsic rocks (mostly rhyolites), spilites, dolerites minor andesites, shales, tuffites (fine volcaniclastics) and purple shales. The VSC appears to be tectonically overlain by phyllites and quartzites of the PQ Group and is also thrusted by the quartzwackes, siltstones and shales of the Represa Formation, the upper unit of the Pulo do Lobo Antiform, which extends geographically to the north (Figure 6). Two boreholes (MP3 and MSD1) were examined for palynostratigraphic research (Figure 3).
Figure 6. Detailed geological map of Mina São Domingos region
Detailed geological map of Mina São Domingos region (Adapted from Geological Map of Portugal, 1 50 000, Sheet 46-D Mértola).
In the MP3 borehole, four main units were identified, namely: the Represa Fm., silstones and shales (Xn2), felsic (Va) and mafic (Vb) volcanic rocks, black shales (Xn3) and a dolerite.
The Represa Fm. shales yielded rich miospore assemblages of the late Famennian VH Biozone, as indicated by the presence of the taxa Ancyrospora ancyrea, Aneurospora greggsii, Auroraspora sp., A. solisorta, Bauscaudaspora callicula, Crassispora sp., Cymbosporites sp., Diducites poljessicus, D. versabilis, Emphanisporites annulatus, E. rotatus, Geminospora lemurata, Grandispora cornuta, G. echinata, G. famenensis, Punctatisporites irasus, Retusotriletes sp., R. crassus, Rugospora radiata and Samarisporites triangulatus. All samples contain very rich assemblages of acritarchs and prasinophytes.
The black shales intercalated in the felsic rocks (Xn2) and also those intercalated in mafic rocks (Xn3), both gave rich miospore associations of the VH Biozone. This means that at least part of the VSC and the Represa Fm. have the same age. A tectonic breccia that occurs at the base of the Represa Fm. is interpreted in close relationship with a thrust fault which brought the unit over the VSC
Borehole MSD1 intersected the following lithologies, from top to bottom: - sandstones and shales of the PQ Group, which are intruded by a dolerite ; - shales, siltstones and tuffites (VS5); - black shales with interbedded phosphate and carbonate ooliths (VS6); - shales and quartzwackes ascribed to the Freixial Fm.
The shales of the PQ Group provided well preserved miospore assemblages assigned to the LN Biozone of late Strunian age. The assemblages contain Retispora lepidophyta and Verrucosisporites nitidus, the nominal species and the taxa Crassispora sp., Densosporites spitsbergensis, Geminospora lemurata, Punctatisporites sp., Rugospora radiata and Vallatisporites sp. Rare acritarchs and prasinophytes are present.
Unit VS5, besides the typical miospore assemblages of the early Viséan Pu Biozone, also yielded one poorly preserved assemblage representative of the late Tournaisian PC Biozone as shown by the occurrence of the taxa Auroraspora sp., Crassispora sp., Retusotrilites incohatus and Vallatisporites vallatus and the zonal specie Spelaeotriletes pretiosus.
The black shales with phosphate and carbonate ooliths (VS6) and the shales of the Freixial Fm have similar miospore assemblages that indicate the mid LateViséan NM Biozone. This borehole, although located close to the São Domingos mine open pit (Figure 6) failed to find any kind of massive sulphides or even felsic volcanic rocks. This is probably due to the highly complicated tectonic structure of the mine region, which is still under investigation.
This branch refers to the rooted antiforms where the VSC is conformably overlain by the Mértola Formation turbidites. It extends from the Pomarão Anticline (west termination of the Puebla de Guzman Anticline in Spain) through the Neves Corvo-Rosário Antiform to the Lousal region (Figure 1). The stratigraphic successions of the Neves Corvo mine and the Pomarão Anticline are chosen as examples for discussion (Figure 7).
Figure 7. Stratigraphic columns
Stratigraphic columns of selected structures of the South Branch of the Pyrite Belt.
All the lithostratigraphic units recognised in Neves Corvo were palynologically dated which allowed the establishment of the local chronostratigraphic column (Oliveira et al., 2004). On the contrary, the ages of the well defined lithostratigraphic units of the Pomarão Anticline (Boogaard, 1969, Oliveira and Silva, 1990) are still poorly known. Recent palynological research led only to the dating of the upper shales of the Nascedios Formation from where an assemblage of the early Strunian LL Biozone, has been recovered. Limestone lenses and nodules interbedded in the lower part of this unit provided conodonts of middle Famennian age (Boogaard, 1963).
Using the Neves Corvo stratigraphic column as a reference and having in mind the existence of lithological similarities, it seems appropriate to make stratigraphic correlations with the succession of the Pomarão Anticline. These correlations suggest similar depositional settings and tectonostratigraphic reconstructions led Oliveira et al., 2004 to conclude that both regions once belonged to a NW-SE oriented half graben.
It is hopped that research in progress, in Pomarão and other structures, as for instance in Lousal region, will permit the establishment of more stratigraphic correlations across this branch and also the north branch, in order to better understand the paleogeographic and geodynamic evolution of the belt.