The River Mondego terraces at the Figueira da Foz coastal area (western central Portugal): Geomorphological and sedimentological characterization of a terrace staircase affected by differential uplift and glacio-eustasy

Abstract

A geomorphological and sedimentological characterization of the River Mondego terraces in the Figueira da Foz coastal area, Portugal, is presented. The relief is dominated by a Pliocene a marine sandy unit~10-15 m thick, reaching~250 m a.s.l., that covers a shore platform surface. The River Mondego has incised into the Pliocene relief and its long drainage evolution is recorded within a series of inset river terrace landforms. These river terraces are better preserved on the right-hand (northern) bank, where they form a staircase developed against the uplifted Serra da Boa Viagem structure. A set of five river terraces, represented by sedimentary deposits or erosional straths, are identified, namely: T1 at 128-125 m; T2 at 101-90 m; T3 at 70-60 m; T4 at 29-24 m; T5 at 11-10 m a.s.l. Some luminescence dating ages were obtained from the river terrace deposits: > 390 ka from the middle of T4; > 170 ka from the top of T4;~120 ka from the base of T5;~109 ka from T5 (average age for three samples); and 53 ± 3 ka from a colluvium at 35 m a.s.l. Some differences in altitude of the uppermost sedimentary unit and of the terraces are here interpreted as resulting from vertical displacements of active faults during the Quaternary. The main tectonic structure is the WNW-ESE trending Quiaios fault, responsible for the regional tilting towards SW. Other probably active faults and tectonic lineaments trend NNW-SSE to N-S, NNE-SSW to NE-SW and WNW-ESE. This study reports a long-term uplift rate of 0.004-0.055 m/ka for the last 3.6 Ma, but 0.017-0.118 m/ka for the last~1.8 Ma (using as references, respectively, the base and the surface of the uppermost sedimentary unit). The facies associations that characterize the older terrace deposits (T1 and T2) consist of poorly sorted fluvial sandy-gravels and silts, but also some colluvium at the top of each terrace. The younger terraces (T3, T4 and T5) show better developed sedimentary structures and less sedimentary matrix; the sedimentary features are indicative of both fluvial and coastal environments (estuary and beach, as nowadays). Under conditions of continuous uplift, the episodes of river down-cutting, valley widening and aggradation in the studied area can be attributed to the rise and fall of sea-level in response to the global Quaternary climatic fluctuations. Considering the number of terraces and the dating obtained, it seems that the control of terrace genesis in this coastal area should be dominated by glacio-eustasy whereby episodic valley incision would have been determined by periods of very low sea-level, probably at~460-410 ka (T3/T4; MIS12),~200-125 ka (T4/T5; MIS6) and 100-14 ka (T5/Recent alluvial infill; late MIS5, MIS4, MIS3 and MIS2). Some sandy colluvium deposits on the slopes are probably related with mild-cold and wet climate conditions during the period 60 to 32 ka. The aeolian dunes are younger (cold to temperate dry conditions; MIS2 and MIS1).

New data on the chronology of the Vale do Forno sedimentary sequence (Lower Tejo River terrace staircase) and its relevance as a fluvial archive of the Middle Pleistocene in western Iberia

“…We should also note that the sedimentary controls for the formation of the Lower Tejo staircases are different (mainly glacio-eustasy and differential uplift) and that the Lower Tejo is separated from the Middle and Upper Tejo by a large permanent knick zone of hard basement Silva et al, 2016). The ages already obtained from the terraces of the lower reach of the Portuguese Mondego River (Ramos et al, 2012) seem to correlate well with the Lower Tejo River terraces; both of the river reaches seem to have glacio-eustatic control on their development.…”

Fossils from Quaternary fluvial archives: Sources of biostratigraphical, biogeographical and palaeoclimatic evidence

“…A surge of interest in Iberian fluvial systems followed the successful FLAG Biennial Meeting at Castelo Branco in 2010, particularly with the development of improved chronological frameworks for several fluvial systems in Spain and Portugal (such as the Tagus/Tejo, Minho, Douro/Duero, Mondego and Guadiana rivers), which have been considerably refined through the application of new luminescence techniques (Cunha et al, 2008;Vis et al, 2008;Martins et al, 2010;Antón et al, 2012;Ramos et al, 2012;Viveen et al, 2012Viveen et al, , 2013Carvalhido et al, 2014;Sancho et al, 2016). However, with the exception of a few notable regions previously summarized by Schreve et al (2007), preservation of palaeontological material is generally poor across the Iberian Peninsula and the terraces of these rivers have yielded relatively little new fossil material (e.g.…”

“…However, with the exception of a few notable regions previously summarized by Schreve et al (2007), preservation of palaeontological material is generally poor across the Iberian Peninsula and the terraces of these rivers have yielded relatively little new fossil material (e.g. Cunha et al, 2012). Fluvial archives such as that preserved in the Ter River basin, northeastern Spain, have proved difficult to date due to the absence of fossiliferous deposits (Garcia, 2015).…”

Tectonic and lithological controls on fluvial landscape development in central-eastern Portugal: Insights from long profile tributary stream analyses

“…Sea level markers in modern coastal regions include a regional Pliocene wavecut platform dated as ca. 3.70-3.61 My (Diniz et al, 2016) overlain by a sedimentary unit that has the top surface (the Culminant Fluvial Surface) with a probable age of 1.8 My (Cunha et al, 2016) and some dated Pleistocene marine terraces (e.g., Ramos et al, 2012;Carvalhido et al, 2014). Inland, terrestrial land surface markers include also the Culminant Fluvial Surface (CFS) as well as fluvial terraces along major river valleys.…”