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- A associação fossilífera do Devónico da localidade de Serra (Rates, Noroeste de Portugal) revisitada: contributo para a valorização geopatrimonial de uma área clássica do Paleozoico IbéricoPublication . Domingos, Rúben; Correia, Pedro; Legoinha, Paulo; M. Callapez, PedroRESUMO: Neste trabalho efetuou-se o estudo de um sítio paleontológico no Devónico de Rates (Noroeste de Portugal), contendo uma fauna marinha composta por invertebrados típicos deste sistema. O afloramento encontra se 35 km a norte do Porto, inserido no prolongamento de unidades paleozoicas do Anticlinal de Valongo. Os níveis fossilíferos correspondem a xistos argilosos amarelos do “Membro Superior” da “Formação Telheiras” e apresentam uma associação paleontológica rica e diversificada, formada por corais, briozoários, braquiópodes, tentaculites, crinoides e trilobites, correlativa do Pragiano–Emsiano (Devónico inferior). Destaca-se o género Pleurodictyum, aqui representado por espécimes morfologicamente distintos da espécie-tipo P. problematicum, a qual tem vindo a ser identificada frequentemente em estudos anteriores. Com a inacessibilidade da maioria das jazidas clássicas do Devónico Inferior desta região, esta descoberta e revisão do conhecimento pretende valorizar o património geológico de uma das áreas clássicas do Paleozoico Ibérico, complementando o do Parque das Serras do Porto.
- 87Sr/86Sr dating of the Alcácer do Sal Formation (upper Miocene, mainland Portugal)Publication . Ressurreição, Ricardo; Legoinha, Paulo; Dias, Ruben Pereira; Santos, José Francisco; Ribeiro, Sara; Patinha, Carla; Miranda, RuiThe Lower Tagus Basin was subjected to several marine transgressions along the Neogene, related to positive eustatic oscillations and also controlled by the regional tectonic activity. In the Alcácer do SalSines region, previous studies identified an episode of marine sedimentation interpreted as having occurred during the late Serravalian to early Tortonian interval, based on biostratigraphic data. This episode is represented in the Alcácer do Sal Formation. The study of the SMS-12- 01B borehole, drilled for mineral prospection purposes, located about 10 km NNE of Melides, allowed the identification of a 3 m thick layer of sediments containing marine fossils. 87Sr/86Sr determinations of oyster shells point to deposition about 11.5 Ma ago, validating previous proposals for the age of the formation. These data correspond to the first numerical ages obtained for the Alcácer do Sal Formation, allowing a solid correlation with other known marine units in the Lower Tagus and Algarve basins related to the same transgression episode
- New 87Sr/86Sr constraints on the marine Miocene of the Lower Tagus Basin and the Alentejo Coastal Plain (Portugal) [Resumo]Publication . Ressurreição, Ricardo; Legoinha, Paulo; Dias, Ruben; Horta Pacheco dos Santos, José Francisco; Monteiro Ribeiro, Sara; Patinha, CarlaABSTRACT: Sedimentation in the Lower Tagus Basin was strongly controlled by eustatic oscillations, with several marine incursions during the Miocene. Previous studies, based on biostratigraphic and isotopic data, identified ten depositional sequences in the southwestern sector of the basin that correlate with thirdorder eustatic cycles. Some of these transgressions reached considerable amplitudes, with marine influence extending up to ~80 km inland under the present-day configuration of the territory. Preliminary results of 87Sr/86Sr isotopic dating from oyster shells collected in the Lower Tagus Basin (LTB) and the Alentejo Coastal Plain (ACP) are presented (figure 1). At Vila Nova da Rainha, a 15.69 Ma age (correlating with Langhian, V-b unit at Lisbon) anchors the marine-influenced base of the local succession, exposing a ~5 Ma hiatus between these levels and the overlying sands with Hipparion (Vallesian, MN 9). This age corroborates previous bio-chronostratigraphic assignments. Marine and marginal-marine sediments crop out along the eastern margin of the LTB and in the ACP. In the Alcácer do Sal region and along the Melides–Sines coast, these deposits are assigned to the Alcácer do Sal Formation (upper Serravallian–lower Tortonian). However, further north, as far as Lavre (Montemor-o-Novo), geological mapping attributes them to a broader interval corresponding to the Vindobonian (Langhian–Tortonian). Oyster shells collected at two sites along the Marateca stream (Monte da Moinhola and Monte Novo) and at Melides yielded ages of 11.13 Ma, 11.03–7.32 Ma and 8.58 Ma. These values indicate Tortonian deposition and are consistent with the Alcácer do Sal Formation, suggesting that marine Miocene deposits cropping out as far north as Lavre may belong to this unit, representing one of the most landward-penetrating Miocene transgressions. At the base of the sea cliff at Galé beach (Melides), calcareous sandstones and sands with marine fauna have been mapped as part of the Alcácer do Sal Formation. However, 87Sr/86Sr dating of oyster shells yielded ages between 6.79 and 5.80 Ma, indicating Messinian deposition. These sediments can therefore be correlated with the Esbarrondadoiro Formation, a Messinian marine unit of the Alvalade Basin whose connection with the coastal area had not previously been unequivocally identified. This study contributes to the understanding of the marine Miocene of southwestern Portugal and has implications for paleogeographic reconstruction and geological mapping.
- Geomorphological and sedimentary evolution of the Southwestern sector of the Tagus river [Resumo]Publication . Dias, Ruben; Ressurreição, Ricardo; Legoinha, PauloABSTRACT: The present-day Tejo River flows within the Lower Tejo Basin, a Cenozoic tectonic depression formed during the Alpine orogeny at the end of the Cretaceous and during the Paleogene under a NNE–SSW compressional stress. Deformation intensified during the Miocene with NNW–SSE compressional stress, reactivating pre-existing faults that controlled the accommodation space and thickness of the basin’s sedimentary fill. Tectonic inversion led to the uplift of the Lusitanian Basin and the Central Cordillera, which supplied sediments laterally to the Tejo Basin. Sedimentation began in the Paleogene with the development of extensive alluvial fan systems, later evolving into fluvial environments. Throughout the Miocene, the southwestern sector of the basin experienced several marine incursions, leading to the formation of reefs and coastal swamp environments that extended inland to the Santarém–Lavre region. Eustatic fluctuations caused repeated shoreline advances and retreats, controlling the position of the fluvial system. The pre-Tejo River developed as an alluvial plain dominated by a meandering system. At the beginning of the Pliocene, the pre-Tejo was characterized by an extensive alluvial plain transporting and depositing mainly medium- to coarse-grained siliciclastic sands. The river likely received contributions from the Alvalade Basin fluvial system and discharged into an estuary north of the Grândola range. The southern margin of the river can be recognized near Melides. Pliocene dating from Galé beach (Melides) and the Setúbal Peninsula indicates ages of ≈ 4.5 Ma (Ressurreição, 2018) and 3.8 Ma (Azevedo, 2006), respectively, showing progressively younger deposits towards the north. These data suggest that during the Pliocene the river gradually migrated northward, eventually reaching the Setúbal Peninsula. Clay deposits in the Pegões region, with considerable lateral extent and thickness, suggest proximity to the mouth of a major river system. During the Pleistocene, fluvial incision began and terrace levels formed as the valley progressively entrenched. The river continued to migrate northward and eventually established its outlet in the Gargalo do Tejo constriction, possibly due to tectonic activity along its fault and/or river capture. The main clay depocentre shifted to the Mar da Palha. Overall, during the Pliocene and Pleistocene, the estuarine sector of the Tejo fluvial system experienced a progressive northward migration until it became entrenched and stabilized in its present position.