CUNHA, P. PROENÇA; BARBOSA, B. & REIS, R. PENA DOS (1992) - Proposal of synthesis concerning the Upper Pliocene infilling of the Lusitanian Basin, in the region between the paralels of Aveiro and Setúbal (western central Portugal). First Congress R.C.A.N.S. - "Atlantic General Events During Neogene", (Abstracts), Lisboa, 12-15 October, pp. 37-42.

INTRODUTION

The aim of this communication is to present recent results, of the Upper Pliocene on the onshore, between Coimbra and Setúbal parallels (REIS & CUNHA, 1989; BARBOSA & REIS, 1989, 1991; CUNHA & REIS, 1991; CUNHA, 1992). The Upper Pliocene record in the Lusitanian Basin — acumulated in two tertiary basins differentiated since middle Eocene times (the Lower Tagus and Mondego tertiary basins, fig. 1) — is composed, onshore, by continental (conglomerates, coarse sandstones and mudstones), transition (lignites, diatomites, "lumachelles") and shallow marine sediments. The sediments are mainly composed of quartz and quartzite clasts; the clay fraction has predominantly kaolinite, with illite and seldom vermiculite. The faunistic (CACHÃO, 1989) and floristic data (TEIXEIRA, 1979; DINIZ & CACHÃO, 1987) — in the marine and transition sediments — pointed out to a relatively warm and humid climate; in the final Pliocene times colder and dry conditions succeed. The deposits and its substratum show weathering alteration features, including kaolinization and hidromorfization (leaching conditions); this can be compared to the "ocre alteration" of the alluvial fini-neogenic platforms and its substratum, which tops the marginal piedmonts in the occidental border of Douro Basin (MARTIN-SERRANO, 1988). The predominance of the most resistant clasts to the weathering; the intense yellowish clay alteration of the slaty clasts; the kaolinite and illite clay association; the fossil data; the facies associations they all testify the general persistence of water mechanisms; the spatial extension of the alluvial systems suggests a humid climate of warm moderated tendency. The early Piacenzian sediments display a transgressive-regressive evolution that can be correlated with the 3rd order eustasy cycle nº 3.6 of HAQ et al. (1987). The penetrative early piacenzian transgression built a marine abrasion platform situated onshore between 10 and 130m (from west to east) (TEIXEIRA, 1979; BARBOSA, 1983; CACHÃO, 1989). At maximum flooding, the shoreline was situated nearby Pombal - Leiria - Rio Maior - Lisboa - Setúbal (TEIXEIRA & ZBYSZEWSKI, 1954). The following sea-level fall was slow and long in time, and it was followed by an important progradation of the alluvial systems. Near the present shoreline, the SLD13 record contacts, by disconformity, with a thin siliciclastic cover. It is composed of sandstones and conglomerate intercalations, with well rounded quartz and quartzites clasts, fossilizing a large marine abrasion platform slightly plunged to the littoral (about 180-200m) and considered of Pliocene age (1) (TEXEIRA, 1979). It testifies another marine incursion correlated with the 3rd order cycle nº 3.7 (maximum transgressive by 1,8 M.Y.) of HAQ et al. (1987).

Fig. 1 - Simplified geological map of Iberia, showing location of the main Tertiary basins and the location of the studied area (CUNHA, 1992).

Some authors (CUNHA, 1987a, 1987b; REIS & CUNHA, 1989; CUNHA & REIS, 1991; CUNHA, 1992) related this deposits with a diastrofic phase in the beginning of the Piacenzian (Iberomanchega II tectonic phase ?) which emphasized the Portuguese Central Range, Maciço Calcário Estremenho (Estremenho Massif) and Western Mountains uplifts. The upper Pliocene record precedes another compressive reactivation of the Lousã fault (the basement uplifted and thrusted over the Cretaceous and Tertiary sediments); in the littoral platform it is affected by several fault systems (RIBEIRO, 1984). This alluvial siliciclastic sediments caracterized the Hesperian landscape prior to its present (quaternary) fluvial dissection and they have culminated the marginal piedmonts in most Hesperian tertiary basins.

THE MONDEGO TERTIARY BASIN RECORD

In the Coimbra southweastern onshore area, the Piacenzian record (about 40 m thick) presents, from the base to the top (fig. 2A): 1) A conglomerate (some times rich in shells) followed up by yellow and white sandstones (Carnide Sandstones and Roussa Sandstones respectively; BARBOSA, 1983). They were formed in shallow marine environment of weak hydrodynamic energy and they are considered early Piacenzian in age (TEIXEIRA & ZBYSZEWSKI, 1951; ROCHA & MARTINS, 1953; CARDOSO, 1984; MULLER, 1984; CACHÃO, 1989, 1990; CACHÃO & SILVA, 1990);

Fig. 2 - Schematic geological sections (SW-NE), showing the upper Pliocene record in the Mondego ( A ) and Lower Tagus ( B ) Tertiary basins. Explanation: 1 - basement (metamorphosed / granitic); 2 - Mesozoic and Cenozoic substratum; 3 - alluvial conglomerates; 4 - alluvial sandstones; 5 - alluvial siltstones; 6 - marsh sediments (sandstones interbeded with lignits and diatomits); 7 - marine sandstones; 8 - sedimentary discontinuity; 9 - Lousã fault; SV - Senhora da Vitória beach (S. Pedro de Muel); CA - Carnide; PO - Pombal; CE - Cernache (Coimbra); SQ - Santa Quitéria; GO - Góis (Carvalhal and Sacões sections); PS - Setúbal peninsula; CO - Coruche; GA - Gavião; CB - Castelo Branco region; SF - Spanish frontier.

2) Breakish sediments, mainly sandstones interbeded with lignits and diatomits (DINIZ & CACHÃO, 1987); 3) Fluvial sandstones, with whitish and gray mudstone intercalations, that become coarser, heterometric and with rubefaction upwards. The lithostratigraphic sequence, which integrates 2) and 3) units, was defined as Barracão Claystones (BARBOSA, 1983); 4) Sandstones with conglomeratic intercalations, showing well rounded clasts;

At the east of Coimbra meridian, the deposition was in continental environment.exclusively The record — boulder and gravel quartzite conglomerates (Santa Quitéria Conglomerates, 250 m maximum thickness) — corresponds to an alluvial fan, prograding to westwards. At the base, this alluvial facies passes gradually (from west to southwest of Coimbra) to marine deposits (CUNHA, 1992). The huge thickness and the facies present on the piedmont of the Portuguese Central Range, suggest that the uplift and relief erosion of this mountain system keep going on during the final Pliocene sedimentary stage.

THE LOWER TAGUS TERTIARY BASIN RECORD

At the onshore of Lower Tagus tertiary Basin (fig. 2B), on the top of the Lisboa-Setúbal neogenic succession area, two lithostratigrafic major units are identified (ZBYSZEWSKI, 1943a; AZEVEDO, 1982): 1) The lower unit (about 10 m thick) is composed by red conglomerates and sandstones with mollusks and vegetals rests (Alfeite Lower Succession). According to ANTUNES (in RIBEIRO et al., 1979), it should be considered of Lower Pliocene age (Zanclean). AZEVEDO (1982) assign it as a lateral equivalent of Cabanas Formation and corresponding to final Miocene and/or Zanclean. 2) The upper unit (325 m of maximum thickness), includes sandstones, granules and pebbles conglomerates with mudstone intercalations of littoral malacologic fossils (assign to the Piacenzian); passes up to a thick sandy succession (with lignite and diatomite intercalations) and to an uppper conglomerate (Belverde Conglomerate). This unit corresponds to the vestibular part (fluvial regime or fluvial-estuarine) of a drainage system develloping from NE, within a great subsidence zone.

Fig. 3 - Esquematic geological section (SW-NE), showing the upper Pliocene record in the northwestern border of the Lower Tagus Tertiary basin (Tomar-Lavre region). Explanation: 1 - basement (metamorphic / granitic); 2 - Cenozoic substratum; 3 - Serra de Almeirim Conglomerates; 4 - Ulme Sandstones; 5 - sedimentary discontinuity.

In the Lisboa-Setúbal northeastern area, the Piacenzian record is exclusively of continental origin (except Rio Maior area). It can achieve 110 m of thickness (Ilite-kaolinitic Complexe of CARVALHO, 1968). It is composed by coarse sandstones with some siltstone intercalations, and by conglomerates offlaping the sandstones named, respectively, as Ulme Sandstones and Serra de Almeirim Conglomerates (BARBOSA & REIS (1989) (fig. 2B and 3). The northwestern marginal system of coalescent alluvial fans, prograde to SW. From meridional piedmont of the Portuguese Central Range (Vila de Rei, Sarzedas, Monfortinho areas) they supplied a longitudinal atlantic fluvial system (the pre-Tagus river) which captured the upper Tagus Basin drainage (from Spain). At maximum flooding during early Piacenzian the sea overtook the Estremenho Massif reliefs and reached the Rio Maior area, producing a succession (littoral sandstones passing up to diatomites and lignites) similar to the other areas near the present shoreline (Caldas da Rainha, Óbidos; ZBYSZEWSKI, 1943b TEIXEIRA, 1979; DINIZ, 1984). The great Rio Maior piacenzian thickness area demonstrates an importante sin-sedimentary tectonic subsidence.

Towards southeast of Lisboa-Setubal area, in the offshore of the present continental platform, the upper neogenic progradating oblique beds — of Messinian-Zanclean and Piacenzian age respectively — lie erosionaly on the substratum and outcrop as two golfs, close to the Lisboa and the Setúbal submarine canyons (MOUGENOT, 1989).

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