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- Evidence for late Pleistocene volcanism at Santa Maria Island, Azores?Publication . Ramalho, Ricardo Dos Santos; Quartau, R.; Höskuldsson, Ármann; Madeira, José; da Cruz, João Ventura; Rodrigues, AuroraSanta Maria Island constitutes the oldest volcanic island within the Azores Archipelago, with no onshore record of eruptions younger than ≈2.8 Ma. A recent high-resolution multibeam bathymetric survey, however, revealed the presence of a seemingly young submerged wide volcanic edifice at approximately −70/−80 m, on the northeastern sector of the island shelf. The outer flanks of this volcanic edifice are partially eroded by marine erosion, but its general morphology is largely preserved, attesting to its relative youth. The edifice's aspect ratio and crater size are typical of a tuff ring formed by very violent surtseyan to taalian eruptions (with water/magma interaction ratios close to 1), implying extrusion at sea level or in very shallow waters, conditions that are incompatible with the present-day water depth at which this structure occurs. A detailed geomorphological analysis – coupled with a correlation with a modified reference eustatic curve – allowed the formulation and discussion of a formation model for the tuff ring, which involves extrusion during a period immediately preceding a rapid relative sea-level drop, most likely at ≈43 ka. Extrusion during such a period would have allowed for the subaerial consolidation and palagonitization of the tuff ring, increasing its resistance to erosion, before being finally submerged during the Last Glacial Termination. Submersion during the Last Glacial Termination – a period characterised by extremely fast sea-level rise – also helps to explain why this tuff ring was submerged without being completely razed by marine erosion. Our study offers insights on the formation and preservation of tuff rings in coastal environments, and in relation to sea-level oscillations, suggesting that consolidation plays a crucial role in the process. Crucially, our study suggests that Santa Maria's volcanism might have extended well into the very late Pleistocene, raising important hazard implications. Future work is scheduled to confirm this hypothesis, including sampling of the volcanic products by dredging and/or by remote operated vehicle. Our study also emphasises the importance of available high-resolution bathymetric surveys to the formulation of solid volcanic hazard assessments on volcanic islands.
- A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio, Cape VerdePublication . Madeira, José; Ramalho, Ricardo Dos Santos; Hoffmann, Dirk L.; Mata, João; Moreira, Mário Augusto de AndradeIn the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large-scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high-confidence U/Th age of 78.8 +- 0.9 ka, which overlaps within error with the 73 +- 7 ka age proposed for Fogo volcano’s flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea-level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower-confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well-preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly-active volcanoes such as in Cape Verde.
- Diverse bioerosion structures in lower Pliocene deposits from a volcanic oceanic island: Baía de Nossa Senhora section, Santa Maria Island, Azores (central North Atlantic)Publication . Dávid, Árpád; Uchman, Alfred; Ramalho, Ricardo Dos Santos; Madeira, José; Melo, Carlos; Madeira, Patrícia; Rebelo, Ana Cristina; Berning, Björn; Johnson, Markes E.; Ávila, Sérgio P.Pliocene body fossils from Santa Maria Island, Azores, have been studied for decades, but only more recently have ichnofossils received their due attention. Calcareous Pliocene deposits from the Baía de Nossa Senhora section contain numerous, diverse, well-preserved natural casts of invertebrate borings. The study of this type of fossils adds to knowledge on the dispersal of benthic faunas across oceans to volcanic oceanic islands. The borings belong to seven ichnogenera and twenty-two ichnotaxa at the ichnospecies level with more than half pertaining to Entobia, which is produced by clionaid sponges. Other borings found were produced by bivalves (Gastrochaenolites), polychaete worms (Caulostrepsis and Maeandropolydora), sipunculid worms (Trypanites), phoronid worms (Talpina) and ctenostome bryozoans (Iramena). The occurrence, ichnogeny, distribution and preservational state of the borings suggest that the bearing bioclasts have been exposed for several years on the sea floor. The borings derive from different bathymetric zones on the shelf, and their formation took place during several bioerosional phases. The association of borings belongs to the Entobia ichnofacies, which is typical of carbonate rocky shores, and shows close similarity to those described from the Paratethys, Mediterranean and partly the eastern Atlantic regions. This fits the idea that most of the Neogene shallow-water marine fauna in the Azores is biogeographically related to the eastern Atlantic shores.