The Neogene of the Lower Tagus Basin (Portugal)

Revista Española de Paleontologia 19 (2), 229-242

The Tertiary Lower Tagus Basin (LTB) occupies a large area in Portugal and constitutes a symmetric basin of the oriental huge Upper Tagus Basin, centered at Madrid (Spain). The LTB was an endorreic basin during the Paleogene. Marine connection occurred at the Lower Aquitanian; at the Lisbon – Setúbal Peninsula region the sedimentation was in the ocean/continent interface, with several changes in the coast line. Apparently, the first marine transgression came from South, originating a gulf with a N-S coral reef that attained the Lisbon area. Occidental communication to the sea was established after the Burdigalian.The sedimentary and palaeontologic record of the distal sector of the LTB (Lisbon –Setúbal Peninsula) is related to sea level changes. It is possible to correlate levels with planctic foraminifera and mammals, as well as to get isotopic ages (K/Ar in glauconites; 87Sr/86Sr). Good biostratigraphic data can be obtain from continental and coastal deposits. The fossil richness – dinoflagellates, spores, pollen, plant macrorremains, foraminifera, ostracoda, molluscs, echinids, fishes, reptiles, mammals – and O and C isotopes, constitutes a large source of information for environmental and palaeogeographical reconstitutions, as well as for the establishment of correlations between marine and continental deposits. The analysis and the integration of a large amount of lithological and biostratigraphical data joined with isotopic ages (K/Ar and 87Sr/86Sr), allow the establishement of a chronostratigraphic framework for the distal part of the Miocene of the LTB and the definition of 10 depositional sequences, in part agreeing with the Haq’s 3d order eustatic cycles. Climatic evolution during the Miocene has been also characterized. At continental environments, the faunas and floras point out to an alternation of moist and dry episodes, the dryest one at the Langhian. For the inland, two sectors can be characterized. Near the region directly affected by the sea (Ribatejo and Alto Alentejo) deposits are related with the wanderingth of a Pre-Tagus in a large fluvial plain. During the Lower and Middle Miocene detrital sedimentation is well represented. Some marls with oysters denounce high eustatic sea levels; brackish waters attain regions 150 km far from the extant coast line. Some vertebrate fossils sites allow the establishment of correlations with the Lisbon region, namely during the Middle Miocene and Lower Tortonian. Sedimentary conditions changed at Lower Upper Miocene. At the right bank of the Tagus, limestones accumulate in lacustre and palustre environments; latteraly thick clay deposits strecht out to the left bank. At the proximal areas of the LTB (Beira Baixa) the chronostratigraphical controle is poor. The outcrops are largely discontinuous. It only has been possible to define alostratigraphic units, bounded by regional unconformities, resulting from tectonic events recognized at the Iberia scale. During Lower and Middle Miocene, a sandy braided depositional system was installed, draining the fluvial plain of the Pre-Tagus from NE to SW. Plant macro-remains indicate a warmer and moister climate than today. Hispanotherium matritensis (steppe rhinoceros) is known from East of Castelo Branco (Plasencia, Spain); it is characteristic of MN5 zone (Astaracian) and is also known from Lisbon’s Langhian. Younger deposits (Upper Tortonian and Messinian) correspond to alluvial fan sediments occur near tectonic slopes, at the base of rising mountains (Portuguese central chain); the thickness and granulometry quickly decrease downstream. Pliocene fluvial deposits overlay through an erosion surface the marine Tortonian (Setúbal Peninsula) and the continental limestones and clays of the Vallesian of Ribatejo (inland). During the Zanclean the continental sedimentation carried on only at the proximal part of the basin (Beira Baixa) with the deposition of endorreic alluvial fan conglomerates. Away of the mountain feeder relieves, the sedimentation show a decrease of thickness; alternating conglomerates and sandy lutites were deposited. A temperate mediterranean climate with contrasting seasons prevailled. In the Piacenzian, alluvial fans and braided fluvial systems were installed draining to the Atlantic, preceding the actual hydrographical net. Feldspatic sands were deposited in the Setúbal Peninsula; the coast line was to the west of the actual. A transgression took place over the Occidental littoral, affecting the Lower Tagus (Setúbal Peninsula) and the Mondego Basins. Whithish conglomerates with quartzite and quartz rounded clasts overlay the Piazencian sands of the Setúbal Peninsula. They yielded Pre-Acheulian artifacts allowing correlation with the Plio-Pleistocene boundary. Basin inland conglomerates with iron crusts point out to cold and dry environments.

Project: POCTII32345/CTA/2000