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).