Paleoambientes no Modio Tejo desde o Último Màximo Glaciàrio (Middle Tagus paleoenvironments since the Last Glacial).

In order to study the genesis and evolution of the Tagus alluvial plain, the Geotarif1 team presents the results of the analysis of the SEV core, retrieved from the Tagus alluvial plain (8.60 m above msl), North of Santarém, using Shelby samplers driven by hydraulic pressure down to 19.4 m below surface, where a course gravel unit was found and stopped further sampling. The core was sub-sampled in 10 cm intervals for the study of grain-size, organic matter, CaCO3 content and geochemistry; twenty-two samples were processed for palynological study and seven 14C dates of organic matter provided time boundaries. The main conclusions are: (i) Unit I – 19.4 m to 13.5 m – continental fine to medium sand, poorly sorted, without organic matter; it indicates an energetic fluvial environment, with a high gradient incised channel; this unit was deposited during the last glacial and until circa 9 200 cal BP; the available 14C dates suggest a 0.3 mm/yr sedimentation rate; (ii) Unit II – 13.5 m to 2.9 m – a lower sub-unit (13.5-8 m), dated between circa 9 200 cal BP and circa 5 000 cal BP, consists of clayey sediment, organic matter decreasing upward, higher Br and U concentrations indicating marine influence, emphasized by the presence of foraminifera until 10 m; it corresponds to a low energy freshwater environment with occasional marine incursions in a Mediterranean domain, indicated by pollen of Quercus ilex, Vitis vinifera and Olea europea; the upper sub-unit (8-2.9 m), dated between circa 5 000 cal BP and circa 1 800 cal BP, is slightly coarser, the content in Br decreases and the foraminifera are absent; it represents a low energy fluvial environment, similar to present-day analogues; Unit II represents an important period of low energy fluvial plain aggradation, at an average rate of 1.4 mm/yr. (iii) Unit III – 2.9 m to surface, deposited between circa 1 800 cal BP and present-day. Two sub-units have been recognized, between 2.9 m and 0.9 m, with coarser (sandy) sediment, vestigial organic matter, showing a high energy fluvial environment, with avulsion episodes and natural levees; the surface sediments show strong textural oscillations probably related to anthropogenic influence.