Geomorphological correlation of the tectonically displaced Tejo River terraces (Gavião–Chamusca area, central Portugal) supported by luminescence dating

Abstract

A suite of fluvial terraces (T1-T6, from top to bottom of the staircase) occur along a 70 km stretch of the Tejo River in central Portugal, between the small towns of Gavião and Chamusca. Terrace correlation was based upon the following: (a) aerial photograph analysis, geomorphological mapping and field topographic survey; (b) sedimentology of the terrace deposits, namely the maximum particle size, clast composition and lithofacies identification; and (c) K-feldspar infrared stimulated luminescence (IRSL) dating of the three lower terraces levels, as quartz optically stimulated (OSL) signal of the samples from these terraces was too close to saturation for all but two samples. The two upper terraces (T1 and T2) lack suitable materials for luminescence dating (e.g. sands/silts), but also their probable ages are beyond the upper range of the dating method. Faults affecting terraces and older deposits have been reported. The luminescence dating results suggest that some assignments of local terrace remnants were incorrect because of fault-related vertical displacements. The luminescence dating procedure also included a correction for anomalous fading in order to obtain more reliable estimates of the burial ages. The fading rate was identical for all samples, so for correlation of the terraces anomalous fading of the feldspar IRSL signal is considered not to be a significant problem. The T5 terrace has corrected ages of 42-99 ka, the T4 from 107 to 222 ka and the T3 terrace has a minimal age of 300 ka. Fluvial incision appears to have been principally controlled by regional uplift but also by localised movements along fault structures. Using the corrected ages of the T4 surface a time-averaged incision rate can be quantified as follows: (1) 0.29 m/ka for reach III, (2) 0.53 m/ka in the Chamusca area (east side on reach IV) and (3) 0.13 m/ka in the Mato Miranda area (west side on reach IV, the less uplifted block according to the geomorphic framework). r

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).…”

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

“…190 m in the NE region of the basin and decreasing in height towards the SW. The regional uplifting that occurred since the Lower Pleistocene lead to entrenchment of the present fluvial network and the generation of a staircase of fluvial terraces (Martins 1999, Martins et al 20092010, Cunha et al 2016. Along the present alluvial plain of the Tagus River occurs a thick (up to 70 m) Upper Pleistocene to Holocene alluvial cover.…”

Techniques for analysing Late Cenozoic river terrace sequences

“…Direct evidence of deformation through folding and faulting of terrace landforms is commonly absent or where present, is typically localised leading to complications in terrace stratigraphy and correlation difficulties (e.g. Martins et al, 2009). Many examples of research have used river terraces as geomorphic markers to quantify fluvial incision amounts and rates in tectonically active areas, and in some instances, it has even been argued that fluvial incision can be used as a proxy for quantifying uplift rates (e.g.…”

“…The number of terrace levels can differ significantly along the length of a river system but generally terraces are absent or limited in number in areas where rivers cut across resistant bedrock, especially where river gorges or deeply dissected bedrock reaches occur (e.g. Martins et al, 2009). Such river terrace sequences typically span time periods of several tens to hundreds of thousands of years or longer (Bridgland and Westaway, 2008a;Gibbard and Lewin, 2009;Westaway et al, 2009).…”