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

Highlights

• Re-analysis of the Vale do Forno archaeology sites, Lower Tejo River, Portugal.

• Evidence for Palaeolithic human occupation derived from the T4 river terrace.

• Artefacts from Lower Gravels unit to Upper Sands unit reveal evolving Acheulian industry.

• New OSL dates suggests Lower Gravels unit to Upper Sands unit time periods span 335–325 and 325–155 ka.

• Suggests human occupation is older than the initial Late Pleistocene interpretation.

Abstract

The Vale do Forno archaeological sites (Alpiarça, central Portugal) document the earliest human occupation in the Lower Tejo River, well established in geomorphological and environmental terms, within the Middle Pleistocene. In a staircase of six fluvial terraces, the Palaeolithic sites were found on the T4 terrace (+24 m, above river bed) which is made of a basal Lower Gravels unit (LG) and an overlying Upper Sands unit (US). Geomorphological mapping, coupled with lithostratigraphy, sedimentology and luminescence dating (quartz-OSL and K-feldspar post-IRIR₂₉₀) were used in this study. The oldest artefacts found in the LG unit show crude bifacial forms that can be attributed to the Acheulian. In contrast, the US unit has archaeological sites stratigraphically documenting successive phases of an evolved Acheulian. Luminescence dating and correlation with the Marine Isotopic Stages suggest that the LG unit has a probable age of ca. 335 to 325 ka and the US unit an age of ca. 325 to 155 ka. This is in contrast to previous interpretations ascribing this terrace (and lithic industries) to the Last Interglacial and early phases of the Last Glacial. The VF3 site (Milharós), containing Micoquian (Final Acheulian) industries (with fine and elaborated bifaces), found in a stratigraphic level located between the T4 terrace deposits and a colluvium associated with Late Pleistocene aeolian sands, is younger than 155 ka but much older than 32 ka.

Introduction

Understanding of the Palaeolithic occupation of Northern Europe has been substantially based on evidence from rivers (e.g. Bridgland et al., 2006). For Western Europe and specifically in Iberia, insights into the Palaeolithic occupation is similarly derived from its major rivers (from north to south: Douro, Tejo/Tagus, Guadiana, Guadalquivir) (e.g. Santonja and Villa, 2006). Within this context, the Vale do Forno archaeological sites (Alpiarça, in central Portugal; Fig. 1) provide the earliest and most well documented human occupation in the Lower Tejo (the Portuguese part of the Tejo basin). Lower Palaeolithic bifacial sites are common, often well represented by lithic industries. Their local stratigraphy and technological characteristics indicate successive phases of what is commonly named as the Acheulian. However, they were chronologically poorly constrained.

From the Spanish border to the Atlantic coast, the Tejo crosses major faults which provide a natural geomorphological subdivision of the river into a series of valley reaches (I to V; Cunha et al., 2005). In Reach I, within the Ródão and Arneiro depressions, the Tejo has cut well-developed staircases of alluvial terraces (Cunha et al., 2008, Cunha et al., 2012), built on a sedimentary bedrock of the Lower Tejo Cenozoic Basin (LTCB; e.g. Pais et al., 2012). Here, the Tejo also flows across a series of highly resistant quartzite ridges. In Reach II, the Tejo flows over Paleozoic basement through a NE–SW orientated valley for some 30 km along which terraces are largely absent. In reach III, the Tejo is routed E–W, crossing three structural depressions with a Tertiary cover (Martins et al., 2009). In reach IV, the Tejo changes to a NNE–SSW trend, with valley sides displaying different degrees of terrace staircase development (Martins et al., 2010a, Martins et al., 2010b) (Fig. 1, Fig. 2). Reach V corresponds to the Tejo estuary which covers a wide area but its final connection to the Atlantic is through a gorge. Here, terraces are not well described. Throughout the Lower Tejo region the best developed terraces, sometimes forming up to six levels, are typically associated with areas of weaker bedrock (e.g. Tertiary sediments in reaches I, III and IV) where the river has been able to enlarge the valley.

The study area is located on the left flank of the middle part of a ∼85 km long NNE-SSW orientated valley-section of reach IV, some ∼100 km from the river mouth. The geomorphological characteristics of this reach are quite different from the Lower Tejo upstream (reaches I, II and III). In reach IV, the Tejo has a wide alluvial plain (up to 10 km wide) with an extensive staircase of six fluvial terraces (T1 to T6) developed along its valley sides (Martins, 1999). Here, the modern Tejo can be considered a low sinuosity, single-channel river with large sand bars, and its alluvial plain connects with a tide-dominated estuary.

The NNE-SSW oriented valley of reach IV is controlled by the NNE-SSW Lower Tejo Valley fault zone (e.g. Choffat, 1907, Bensaúde, 1910, Freire de Andrade, 1933, Cabral et al., 2003, Cabral et al., 2004, Cabral et al., 2013, Cabral and Ribeiro, 1988, Cabral and Ribeiro, 1989, Cabral, 1995, Cabral, 2012, Vilanova and Fonseca, 2004, Carvalho et al., 2008, Carvalho et al., 2014, Martins et al., 2009, Besana-Ostman et al., 2012). This is expressed as a regional scale tectonic lineament concealed by Holocene sediments. This valley is developed in a low uplift area (<100 m during the last ca. 2 Ma) with some subsidence in the present estuary (reach V). The interplay between these regional tectonic characteristics and Pleistocene to Holocene glacio-eustatic sea-level changes has strongly influenced the formation of fluvial terraces and sedimentary valley-fill (eg. Merritts et al., 1994, Blum and Törnqvist, 2000). The high amplitude sea-level changes that characterized the Middle and Late Pleistocene controlled the episodic down-cutting phases of the river during sea-level lowstands; these alternated with flooding and agradation phases in the incised valley during highstands (Cunha et al., 2016). Such climate related eustatic oscillations are superimposed onto a long-term uplift pattern, both phenomena having controlled the river terrace staircase development (Cunha et al., 2005, Cunha et al., 2008, Martins et al., 2010a, Martins et al., 2010b).

In reach IV, several archaeological studies have been undertaken in the last 70 years (e.g. Breuil and Zbyszewski, 1942, Breuil and Zbyszewski, 1945, Zbyszewski, 1946, Raposo, 1995a, Raposo, 1995b, Mozzi et al., 2000). The prehistoric human occupation of this area is of renewed interest because it contains evidence for an extensive Palaeolithic occupation. Related archaeological sites are present on both sides of the river, from the Lisboa area to the vicinity of Torres Novas (Fig. 2). The site of Santo Antão do Tojal (near the estuary) is considered especially important, because dated fossil remains of Palaeoloxodon antiquus and Equus caballus were found in association with lithic industries (Zbyszewski, 1943, Zbyszewski, 1977, Raposo, 1995a). The study of these sites has brought new insights into the technology and the typology of the Lower and Middle Palaeolithic. Further new insights have been achieved by combining archaeological, geomorphological and lithostratigraphic studies. A chronological framework for the lithic industries found in these sites has been attempted, based on their stratigraphic position in the terrace deposits and by thermoluminescence (TL) dating (Mozzi et al., 2000). However, these TL dates from the Vale do Forno area were minimum age estimations. Furthermore, the validity/accuracy of these TL ages was unclear because of a lack of geochronological dating from other archaeological sites within reach IV (e.g. Raposo and Cardoso, 1998).

The main aim of this paper is to develop a new chronostratigraphy for the sedimentary succession of the Lower Tejo terrace T4 in the Vale do Forno area. A new and more robust chronologic framework will allows us to provide better dating of the Palaeolithic industries found here in situ and to estimate the duration of aggradation and down-cutting phases associated with this terrace. It will also allow correlation of these phases with extrinsic fluvial controls (climate, glacio-eustatic sea-level changes and tectonics). Finally, integration of the terrace sedimentology with an improved chronostratigraphic framework will allow a revised assessment of the palaeo-environmental conditions affecting this region of western Iberia during the Middle Pleistocene.