Dating the Tejo river lower terraces in the Ródão area (Portugal) to assess the role of tectonics and uplift

Abstract

The Tejo river is one of the major drainages in Iberian Peninsula; it is a long-lived system (ca. 3.4 Ma) and provides an archive of long-term landscape development and environmental change controlled by tectonics, climate and eustasy. The most upstream Portuguese reach of the Tejo river, ∼ 200 km from the Atlantic coast, shows evidence for five fluvial terraces (T1 to T5) with elevations reaching more than 120 m above the modern river bed. A chronological framework for these terraces is established here by integrating geomorphological, stratigraphical and archaeological information with ages from luminescence dating. Optically stimulated luminescence dating of K-feldspar, (involving the correction for anomalous fading of the luminescence signal), indicates that the younger terraces have a probable age range of: T5 -31 to 40 ka; and T4 -100 to ∼ 280 ka. We deduce that the related major fluvial changes are likely to have been as follows:∼10 m of aggradation from ∼280 to 100 ka (0.06 m/ka); 14 m of incision from 100 to 40 ka (0.23 m/ka); 8 m of aggradation from 40 to 31 ka (0.89 m/ka); 16 m of incision during the last 31 ka (0.52 m/ka). These values indicate that the duration and rate of both aggradation and river downcutting episodes were variable. There is widespread evidence for neotectonic activity in this intraplate region. Neither eustatic nor climatic changes during the Quaternary provide clear trends that might explain the observed pattern of valley incision, thus we conclude that this tectonic activity is the most likely driving mechanism. In the study area, the probable age of the Tejo river sediments deposited before the beginning of valley incision allows the calculation of a time-averaged incision rate of ∼0.07 to 0.10 m/ka over the last ∼2.6 Ma. This long-term incision was probably determined by an increase in the relative uplift rate, resulting from the intensification of intraplate compressive stress. During the late Cenozoic fluvial incision stage, the Ródão depression underwent less uplift than the adjacent areas along the river, in which the Tejo has incised a narrow valley into basement rock, with almost no terrace development. Terrace formation was also promoted by soft bedrock (Tertiary arkoses) and by impoundment of alluvium behind resistant barriers crossed by the river. Geomorphological evidence for terrace tectonic offset was also supported by luminescence dating.

Fluvial terraces of the northwest Iberian lower Miño River

“…This and other examples on the map suggest that local block movement was also an important factor for terrace formation and preservation. Differential uplift of blocks has been observed in other river systems, for instance in southern Iberia (Cunha, Martins, Huot, Murray, & Raposo, 2008;Martins et al, 2009), Germany (Peters & Van Balen, 2007), and the Netherlands (Veldkamp & Van den Berg, 1993), but always on a larger spatial scale or over longer time periods (e.g. Bridgland et al, 2012;Demir et al, 2012).…”

Quartzite pebble technology in the final Middle Pleistocene of the Ribeira da Ponte da Pedra site (High Ribatejo, Central Portugal)

“…Also in the Tagus valley, in the region of Vila Velha de Rodão, recently published detailed geomorphologic studies provide a more precise age for the archaeological materials from the site of Monte Famaco, recently considered to represent the T4 terrace (middle terrace) of a suite of six terraces identified at the Lower Tagus Basin and dated (IRSL) between 280-136 ka (Cunha et al 2008;Martins et al 2009). Two series of lithic industries were identified at Monte do Famaco (GEPP 1977;Raposo 1987;1993).One comes from the T4 terrace, which is made up of a 1-m thick clast-supported gravel-boulder conglomerate, with poor sorting (Cunha et al 2008).…”

“…Two series of lithic industries were identified at Monte do Famaco (GEPP 1977;Raposo 1987;1993).One comes from the T4 terrace, which is made up of a 1-m thick clast-supported gravel-boulder conglomerate, with poor sorting (Cunha et al 2008). The assemblage is composed of thirty-four worn-out quartzite artifacts tentatively ''attributed to the early Middle Acheulean (Lower Palaeolithic)'' (Cunha et al 2008: 47;Raposo 1987).…”

Use wear analysis of quartzite lithic implements from the Middle Palaeolithic site of Lagoa do Bando (Central Portugal)

“…51 ka ago (Zilhão 2001). The late MIS 3 human occupation is testified by the open air site of Foz do Enxarrique U-Th dated between 32,938 ±1005 BP and 34,093 ±920 BP (Raposo & Cardoso 1998) and more recently by OSL to between 38.5 ±1.6 ka and 31.6 ±1.3 ka (Cunha et al 2008). The Oliveira cave burnt bone of level 8 is dated by 14C-AMS to between 38,552 BP to 36,835 BP and by U-Th from 40,580 BP to 35,770 BP (Richter 2014).…”