This paper aims to provide insight into human occupation and landscape change during the Pleistocene in a central area of the Lower Tejo basin (Portugal). Detailed geomorphological mapping, coupled with lithostratigraphy, sedimentology and luminescence dating, supports the identification of a complete terrace staircase sequence. It consists of six gravely terraces located below the culminant (Pliocene) basin unit. A chronological framework for the sedimentary sequences and associated human industries is proposed and correlated with marine oxygen isotope stages (MIS): T1 terrace, not dated; T2, not dated; T3, >300 ka; T4, $300-160 ka (MIS8, MIS7 and MIS6); T5, $136-75 ka (MIS5); T6, $62-30 ka (MIS3); colluvium and aeolian sands, $30-14 ka (MIS2); valley fill deposits, $14 ka to present (MIS1). The oldest artefacts were found at the base of the T4 terrace, with the local stratigraphic level dated to !175 AE 6 ka (Middle Pleistocene). The lithic assemblages collected from distinct stratigraphic levels (T4, T5 top, T6 terraces and colluvium) are characterized by the predominance of opportunistic technological choices, a feature that can be attributed partly to the preferential exploitation of the available raw material, dominated by local-sourced quartzites and quartz pebbles. The adaptation to local raw material (texture and volume), together with subsistence patterns and behaviours, could explain the rarity of Acheulian types (handaxes and cleavers) and picks in the T4 terraces of the Tejo tributaries; this is in contrast to the same terrace of the Tejo valley, in which these types are found. Interpretation of the environmental conditions (controlled by climate and glacio-eustatic sea-level changes) affecting the hunter-gatherer human groups is also presented.
Através dos registos geomorfológicos e sedimentares, os rios fornecem relevantes arquivos de mudanças paleoambientais, nomeadamente paleoclimáticas e paleogeográficas. As sucessões sedimentares melhor datadas são as mais importantes, com as idades numéricas dos respetivos dos eventos sedimentares, de fósseis e de materiais arqueológicos, obtidas por uma variedade de técnicas. Os arquivos fluviais do Quaternário fornecidos pelo rio Tejo em Portugal (Baixo Tejo) constituem um importante repositório de dados para estudos da evolução da dinâmica sedimentar e da paisagem, bem como da ocupação humana pré-histórica. O atual estado de conhecimentos resultantes das sucessivas abordagens usando métodos da geomorfologia, litostratigrafia, arqueologia e datação numérica no estudo do Terraço T4 do Baixo Tejo é aqui sintetizado. Este trabalho tem enfoque nos sítios com indústrias do Paleolítico que foram encontradas no Terraço T4, o qual é constituído por uma unidade basal de cascalheiras e uma unidade superior dominada por areias. Os mais antigos artefactos são de rara ocorrência e foram encontrados na unidade de Cascalheiras Inferiores, apresentando formas bifaciais pouco elaboradas que podem ser atribuídas ao Acheulense, com uma idade provável de ca. 340 a 325 ka. Em contraste, os níveis estratigráficos inferiores e médios da unidade de Areias Superiores do T4 apresenta vários sítios arqueológicos que documentam fases sucessivas de um Acheulense evoluído, que foram datados de ca. 325 a 200 ka. Nos níveis estratigráficos dos depósitos do topo do T4 foram encontradas indústrias do Paleolítico Médio e datam, provavelmente, de ca. 165 a 155 ka.
a b s t r a c tArchaeological evidence for the earliest human presence in western Iberia is summarised and discussed. Western Iberia is geologically characterised by magmatic and metamorphic rocks (Hesperian Massif) but also by siliciclastic and carbonate Mesozoic and Cenozoic formations. The geological context affects the distribution of Pleistocene archaeological sites, as the most of the archaeological evidence known today is located in river terrace formations or in karst deposits. Very few sites have been fully investigated; the older ones are tentatively dated as middle/late Middle Pleistocene. Recent results have been obtained using an archaeological and geomorphological approach carried out in the Portuguese region of Alto Ribatejo. Dating of the lithic assemblages found in fluvial terraces or in cave deposits indicates that the first human presence in Portugal is not older than the OIS 8-9. However, it remains difficult to explain the long chronological gap between the archaeological evidences in western Iberia and the older sites in central and eastern Iberia, such as those in the Sierra de Atapuerca and Guadix Baxa areas.
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