Fernandes, Paulo

Lopes, Gilda

Oliveira, José Tomás

Pereira, Zélia

Piçarra, José Manuel

Repositório do LNEG

CIMP Faro09 Abstracts 63SILURIAN TO LOWER DEVONIAN PALYNOMORPHS FROM THE BARRANCOSREGION, OSSAMORENA ZONE, PORTUGAL PRELIMINARY RESULTSGilda Lopes1,2*, Zélia Pereira1, Paulo Fernandes2, J. Manuel Piçarra3 and J. Tomás Oliveira41. LNEG LGM, Unidade Geologia e Cartografia Geológica, Rua da Amieira, Ap.1089, 4466 956 S. Mamede Infesta, Portugal, *correspondingauthor: gilda.lopes@ineti.pt2. CIMA, Centro de Investigação Marinha e Ambiental, Universidade do Algarve, Campus de Gambelas, 8005 139 Faro, Portugal3. LNEG LGM, Unidade Geologia e Cartografia Geológica, Rua Frei Amador Arrais 3, Ap.104, 7801 902 Beja, Portugal4. LNEG LGM, Unidade Geologia e Cartografia Geológica, Estrada Portela, Zambujal, 2720 Alfragide, PortugalSUMMARYNew miospore results were obtained from the Palaeozoic stratigraphic succession present in the Barrancosregion, Portugal. The studied formations comprise the Silurian carbonaceous shales intercalated with chertsand rare lenticular carbonates of the Xistos com Nódulos Formation, as well as, the dark shales that alternateswith thin psammit beds of Late Silurian to Early Devonian age from the Xistos Raiados Formation. Twomiospore biozones were identified: the cf. protophanus verrucatus Miospore Biozone, of the Lower Wenlock(Homerian) (Richardson &McGregor, 1986) and the Verrucosisporites polygonalis (Po) Miospore Biozone of theLower Pragian (Streel et al., 1987). For the first time cryptospores are reported. Rare acritarchs andchitinozoans are also present. This new preliminary data allow correlation with the graptolite biozonationalready established in this region (Piçarra, et al., 1995, 1998; Piçarra, 2000) and completes previouspalynological studies (Pereira et al., 1999). Even so, detailed palynostratigraphic research is needed and iscurrently being undertaken to better understand the complex geology of the region.Keywords: Cryptospores, trilete spores, Silurian to Lower Devonian, Barrancos, Portugal._____________________________________________________________________________________________________INTRODUCTIONThe Palaeozoic geology of the Ossa Morena Zone is well represented in the stratigraphic successionof the Barrancos region, Portugal, and comprises sedimentary rocks from Cambrian to EarlyDevonian age (Delgado, 1908; Perdigão et al., 1982; Oliveira et al., 1991, Pereira et al., 1999, Piçarra,2000) (Figure 1).In this region, the Silurian and Devonian strata are represented by two formations, the Xistos comNódulos and the Xistos Raiados Formations (Delgado, 1908). The Xistos com Nódulos consists ofblack carbonaceous shales intercalated with thin black cherts and rare lenticular carbonates at thetop of the succession. This formation is rich in graptolites, which allow very accurate agedeterminations. Seventeen Silurian graptolite biozones were identified in the Xistos com NódulosFormation (Delgado, 1908; Romariz, 1962; Perdigão et al., 1982; Gutierrez Marco, 1982; Rigby et al,.1997; Piçarra et al., 1995, 1998; Piçarra, 2000).CIMP Faro09 Abstracts 64The Xistos Raiados Formation is composed of dark shales that alternate with thin psammit beds. Thisformation has traditionally been considered Wenlock to Early Ludlow age (Delgado, 1908; Perdigãoet al., 1982). However, more recent works indicate an age ranging from the Late Silurian (LudlowPrídolí) to the Early Devonian (Oliveira et al., 1991; Oliveira et al., 1993; Piçarra et al., 1998; Pereiraet al., 1998, 1999).The Xistos com Nódulos Formation reflects a euxinic sedimentary environment related to the endOrdovician glaciation, whose physical conditions remained stable throughout the Silurian. The XistosRaiados Formation, on the other hand, indicates more shallow and oxygenated waters reflecting thebeginning of a differentiated tectono sedimentary evolution (Perdigão et al., 1982; Oliveira et al.,1991; Araújo et al., 2006).Palynological studies established during the last decade mostly concentrated on the upper part ofthe stratigraphic sequence, e.g. the Lower Devonian Xistos Raiados, Russianas, and Terenaformations (Oliveira et al., 1993; Piçarra et al., 1995, 1998; Pereira et al., 1998, 1999; Piçarra, 2000).This presentation is a preliminary account of the palynological assemblages of miospores(cryptospores and trilete spores) of the Silurian to Lower Devonian strata of the Barrancos region,aiming toward a better understanding of the regional palynostratigraphy.Figure 1. Simplified geological map of the Barrancos region (adapted fromPiçarra et al., 1995; 1998; 2000 and Araújo et al., 2006), showing the positionof the studied trenches.CIMP Faro09 Abstracts 65Produced by primitive land plants from the Ordovician to the Lower Devonian, cryptospores are hereidentified and reported for the first time.Herein, we use the cryptospore definition proposed by Steemans (2000): Alete miospores (nonpollen grains) produced by primate embryophyts. Single grains or monads, permanent dyads andtetrads, and sporomorphs from polyads which may or may not preserve contact areas, are included.PALYNOSTRATIGRAPHYTwenty seven samples, collected in the road cut of EN 258, Km 103,7 near Barrancos and in the EirasAltas Section (Figure 1), were processed for palynological studies.Standard palynological laboratory procedures were employed in the extraction and concentration ofthe palynomorphs from the host sediments (Wood et al., 1996). The slides were examined withtransmitted light, per BX40 and CX41 Olympus microscopes equipped with an Olympus C5050 and aSC20 digital cameras facility. All samples, residues, and slides are stored in the LNEG LGM (GeologicalSurvey of Portugal) at S. Mamede Infesta, Portugal.The general miospore biozonal scheme for the Silurian to Lower Devonian used herein, followsRichardson and McGregor (1986), Burgess and Richardson (1995), Richardson et al. (2001), and Streelet al. (1987).1. Road cut EN 258, Santo Aleixo da Restauração to Barrancos, km 103.7 (Figure 2)This section is located in the road cut between kms 103.3 and 103.7 of the road from Santo Aleixo daRestauração to Barrancos, at the entrance of this village. The trench exposes, from northeast tosouthwest, a sedimentary succession that ranges from the Upper Ordovician to the Lower Devonian.It begins with the impure sandstones of the Colorada Formation (?Upper Ordovician LowerSilurian/Rhuddanian) which is in fault contact with the black cherts of the basal part of the Xistoscom Nódulos Formation. These are followed by black shales (when the weathering is very intense,the shales show pale colours) intercalated with thin black cherts and sporadic siliceous nodules (rarein the upper part), containing graptolites of Llandovery to Late Wenlock age.The upper part of the Xistos com Nódulos Formation had four samples (BA4, 8, 9, 10) that yieldedvery poor to moderately preserved miospores (cryptospore and triletes spores) assigned to the cf.protophanus verrucatus Miospore Biozone of the Lower Wenlock (Homerian) (Richardson &McGregor, 1986).Also present in the assemblage are the cryptospores: Gneudnaspora chibrikovae, Gneudnasporachibrikovae?, Gneudnaspora plicata?, Imperfectotriletes vavrdovae, Pseudodyadospora petasus?andQuadritisporites variabilis.Completing the assemblage are the trilete spores Ambitisporites avitus/dilutus Morphon,Ambitisporites tripapillatus, Amicosporites sp., Aneurospora sp., Archaeozonotriletes chulusMorphon, Emphanisporites multicostatus, Retusotriletes sp., Insolisporites anchistinus, in associationwith the key species Emphanisporites cf. protophanus.CIMP Faro09 Abstracts 66The last meters of the road cut section expose dark shales with thin intercalations of sandstones withrare siliceous and ferruginous nodules that belong to the Xistos Raiados Formation. This part of thesection was dated at the Pragian/Emsian boundary, based on miospores assigned to the upper partof the PE Biozone of Richardson and McGregor (1986), and Dictyotriletes subgranifer (Su) subzone ofStreel et al. (1987).New samples collected (BA12) in the mid part of the section yielded a very poor preserved miosporeassemblage assigned to the Verrucosisporites polygonalis (Po) of Streel et al. (1987), that allowsdating this part of the section as lower Pragian. The most common species present are:Ambitisporites sp., A. tripapillatus, Amicosporites miserabilis?, Emphanisporites multicostatus, andRetusotriletes sp., together with the guide species Verrucosisporites polygonalis. The cryptosporesfound was Gneudnaspora chibrikovae.Figure 2. Scheme of the road cut EN 258, Sto Aleixo da Restauração to Barrancos, km 103,7, with location ofproductive palynostratigraphic samples (adapted from Oliveira, et al., 2000).2. Eiras Altas SectionThis section is located on the road Santo Aleixo da Restauração to Barrancos, at km 102.15, andshows the best known and complete, although condensed, Portuguese Silurian succession. Thesuccession comprises the Xistos com Nódulos Formation and part of the Xistos Raiados Formation,and yielded 13 Silurian graptolite biozones (Piçarra et al., 1995, 1998; Piçarra, 2000). Here, the Xistoscom Nódulos Formation consists of intensively weathered black shales that gradually pass to thebase of the Xistos Raiados Formation, which is marked by the first occurrence of thin bedded darkshales and siltstones.Twelve samples collected from the Xistos com Nódulos Formation were barren of palynomorphs,probably due to the intense weathering of the black shales as indicated by its pale colours. However,because of the importance of the section, a new sampling campaign was undertaken, and thesesamples are currently undergoing analysis.CIMP Faro09 Abstracts 67CONCLUSIONSPalynostratigraphy constitutes an important tool for precisely dating the Silurian to Devoniansequences of the Barrancos region. It is important and necessary that the correlation between thedata obtained from palynology and the graptolite biozonations established in this region (Piçarra, etal., 1995, 1998; Piçarra, 2000).The preliminary results obtained in this study allow the recognition of two miospore biozones in oneof the studied sections (road cut EN 258, Sto Aleixo da Restauração to Barrancos, km 103.7):The first one is the cf. protophanus verrucatus Miospore Biozone of Richardson & McGregor (1986),and was obtained from the Xistos com Nódulos Formation, placing part of the studied section in theupper part of the Wenlock Series (Homerian stage), and confirming previous studies based ongraptolites that assigned this section to the ?lundgreni Graptolite Biozone (Piçarra, 2000).The second one is the Verrucosisporites polygonalis (Po) Miospore Biozone of Streel et al. (1987), andcomes from the Xistos Raiados Formation, and thus is assigned to the Lower Pragian. This new datacompletes previous studies that revealed miospores of the Dictyotriletes subgranifer (Su) subzone ofStreel et al. (1987) of the Pragian/Emsian boundary (Pereira et al., 1999) (Figure 2). All of thisinformation indicates that almost the entire Pragian Stage is represented in Xistos RaiadosFormation, despite the complexity of local tectonism. A more detailed palynostratigraphic study fromthis road cut and other sections of the Barrancos region are currently underway.ACKNOWLEDGMENTSThis study was supported by the Ph.D. grant SFRH/BD/48534/2008 of the Fundação para a Ciência ea Tecnologia (FCT).__________________________________________________________________________________REFERENCESARAÚJO, A., PIÇARRA, J.M., BORREGO, J., PEDRO, J.& OLIVEIRA, J.T., 2006. As regiões central e sul daZona de Ossa Morena. In: Geologia de Portugal noContexto da Ibéria, Dias, R., Araújo, A., Terrinha, P.& Kullberg, J.C. 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Silurian to Lower Devonian palynomorphs from the Barrancos region, Ossa Morena Zone, Portugal : preliminary results

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Silurian to Lower Devonian palynomorphs from the Barrancos region, Ossa Morena Zone, Portugal : preliminary results

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