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As unidades estratigraficas que resultaram da evolucao do rio Tejo em Portugal, aqui analisadas em pormenor entre Vila Velha de Rodao e Chamusca, possuem distintas caracteristicas sedimentares e industrias liticas: uma unidade culminante do enchimento sedimentar (o ancestral Tejo, antes do inicio da etapa de incisao fluvial) - SLD13 (+142 a 262 m acima do leito actual; com provavel idade 3,6 a 1,8 Ma), sem industrias identificadas; terraco T1 (+76 a 180 m; ca. 1000? a 900 ka), sem industrias; terraco T2 (+57 a 150 m; idade estimada em ca. 600 ka), sem industrias; terraco T3 (+36 a 113 m; ca. 460 a 360? ka), sem industrias; terraco T4 (+26 a 55 m; ca. 335 a 155 ka), Paleolitico Inferior (Acheulense) em niveis da base e intermedios mas Paleolitico Medio inicial em niveis do topo; terraco T5 (+5 a 34 m; 135 a 73 ka), Paleolitico Medio (com talhe Mustierense, Levallois); terraco T6 (+3 a 14 m; 62 a 32 ka), Paleolitico Medio final (Mustierense final); Areias da Carregueira (areias eolicas) e coluvioes (+3 a ca. 100 m; 32 a 12 ka), Paleolitico Superior a Epipaleolitico; enchimento da planicie aluvial (+0 a 8 m; ca. 12 ka a actual), Mesolitico e industrias mais recentes. As diferencas na elevacao (a.r.b.) das escadarias de terracos resultam de soerguimento diferencial, devido a falhas ativas. Numa dada escadaria datada, a projecao da elevacao da superficie de cada terraco (a.r.b.) versus a sua idade permitiu estimar a idade do topo do terraco T2 (ca. 600 ka) e a provavel idade do inicio da etapa de incisao (ca. 1,8 Ma). Obteve-se a duracao da fase de agradacao dos terracos baixos e medios: T6 – 30 ka; T5 – 62 ka; T4 – ca. 180 ka; T3 – ca. 100? ka. Conclui-se que durante o Plistocenico medio e final, as fases de incisao e alargamento do vale foram curtas (ca. 11-25 ka) e ocorreram durante periodos de nivel do mar muito baixo, alternando com mais longas fases de inundacao e agradacao do vale durante niveis do mar mais altos. Estas oscilacoes eustaticas de causa climatica estao sobrepostas a um contexto de soerguimento de longo termo, controlando o desenvolvimento das escadarias. Calculou-se que para os ultimos ca. 155 ka as taxas de incisao de curto-termo apresentam valores (0,09 a 0,41 m/ka), aproximadamente, duplos dos calculados para o intervalo ca. 155 a 900 ka (0,04 a 0,28 m/ka). Este aumento na taxa de incisao deve estar relacionado com um aumento na taxa de soerguimento por intensificacao da compressao devido a convergencia entre as placas Africana e Eurasiatica. Palavras-chave: terracos fluviais, tectonica, taxa de incisao, levantamento tectonico, rio Tejo, Portugal. The terrace staircases of the Lower Tagus River (Rodao to Chamusca) – characterization and interpretation of the sedimentary, tectonic, climatic and Palaeolithic data The stratigraphic units that record the evolution of the Tagus River in Portugal (study area between Vila Velha de Rodao and Chamusca villages) have different sedimentary characteristics and lithic industries: a culminant sedimentary unit (the ancestral Tagus, before the drainage network entrenchment) – SLD13 (+142 to 262 m above river bed – a.r.b.; with probable age 3.6 to 1.8 Ma), without artefacts; T1 terrace (+76 to 180 m; ca. 1000? to 900 ka), without artefacts; T2 terrace (+57 to 150 m; top deposits with a probable age ca. 600 ka), without artefacts; T3 terrace (+36 to 113 m; ca. 460 to 360? ka), without artefacts; T4 terrace (+26 to 55 m; ca. 335 a 155 ka), Lower Paleolithic (Acheulian) at basal and middle levels but early Middle Paleolithic at top levels; T5 terrace (+5 to 34 m; 135 to 73 ka), Middle Paleolithic (Mousterian; Levallois technique); T6 terrace (+3 to 14 m; 62 to 32 ka), late Middle Paleolithic (late Mousterian); Carregueira Sands (aeolian sands) and colluvium (+3 a ca. 100 m; 32 to 12 ka), Upper Paleolithic to Epipaleolithic; alluvial plain (+0 to 8 m; ca. 12 ka to present), Mesolithic and more recent industries. The differences in elevation (a.r.b.) of the several terrace staircases results from differential uplift due to active faults. The age interval for each aggradation phase of T3 to T6 terraces was obtained: T3 – ca. 100? ka; T4 – ca. 180 ka; T5 – 62 ka; T6 – 30 ka. The intervals of river down-cutting and widening of the valley floor were short (ca. 11-25 ka) and coincided with periods of very low sea-level. The plotting of the elevation (a.r.b.) versus the age of each terrace surface allows to estimate the age of the T2 terrace (ca. 600 ka) and the probable age of the beginning of the incision stage (ca. 1.8 Ma). So, the high amplitude sea-level changes that characterized the Middle and Late Pleistocene strongly determined the episodic down-cutting phases of the river during the low stands sea levels that alternated with the flooding and aggradation phases of the incised valley during highstand sea levels. These climate related eustatic oscillations are superimposed onto a long term uplift pattern, controlling the river terrace staircase development. During the last ca. 155 ka, the short-term incision rates (0.09 a 0.41 m/ka) were twice the values determined for the interval 155 to 900 ka (0.04 to 0.28 m/ka). This increase in incision rate should be related with an increase in uplift rate resulting from an intensification of compression due to the convergence between African - Eurasian plates. Keywords: fluvial terrace, tectonics, incision rate, uplift, Tagus River, Portugal.

estudos do quaternário

As escadarias de terraços do Ródão à Chamusca (Baixo Tejo) – caracterização e interpretação de dados sedimentares, tectónicos, climáticos e do Paleolítico