Browsing by Author "Madeira, J."
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- Oceanic Island forests buried by Holocene (Meghalayan) explosive eruptions: palaeobiodiversity in pre-anthropic volcanic charcoal from Faial Island (Azores, Portugal) and its palaeoecological implicationsPublication . Góis-Marques, C. A.; Rubiales, J. M.; Nascimento, L.; Sequeira, Miguel Menezes de; Fernández-Palacios, J. M.; Madeira, J.In Faial Island (Azores Archipelago, North Atlantic Ocean), charcoalified and mummified wood fossils have been reported within late Holocene (Meghalayan) pyroclastic deposits from the Caldeira Formation. Due to their re cent age, a detailed study conveys a snapshot into Azorean palaeophytodiversity and palaeovegetation, ca. 7–5 centuries before the arrival of Portuguese settlers to the Azores Islands. Here we provide the first detailed ana tomical and taxonomical study of these wood fossils. In total, 41 samples were collected from seven localities, mainly from a ~1200 yr BP ignimbrite. Field work revealed autochthonous and paraautochthonous assemblages, with tree trunks in upright position. The anatomical study of the fossil woods resulted in the identification of Juniperus brevifolia, Laurus azorica, Myrsine retusa, Morella faya, Picconia azorica, Prunus lusitanica subsp. azorica, and Vaccinium cylindraceum. Two fossil assemblages are comparable to the proposed potential natural vegetation (PNV) for the Azores. Surprisingly, P. lusitanica subsp. azorica was the second most abundant fossil wood suggest ing that this tree was more abundant in a recent past in Faial Island and probably in the archipelago. This is cor roborated by historical accounts, and its modern scarcity was certainly anthropically driven. Identifying Holocene plant macrofossils is essential to properly reconstruct oceanic islands terrestrial palaeoecosystems, especially where forests with high percentage of entomophilous taxa are underrepresented in palaeopalynological limnic record. Further work is necessary to reconstruct Faial Island and Azores archipelago palaeovegetation which is essential to provide an ecosystem base-line for restoration and management.
- Tectónica da Ponta de São Lourenço, Ilha da MadeiraPublication . PRADA, SUSANA; Madeira, J.; Fonseca, P. E.; Silveira, A.; Prada, S.; Rodrigues, C. F.The tectonics of Madeira Island is, in its generality, still quite unknown. A structural characterization togeth er with the formulation of hypothesis contributing to this problematic is, in our opinion, of extreme impor tance to relate this context with the regional and global tectonic models. The Ponta de São Lourenço, in Madeira Island, exhibited an unusual situation for studies of this nature, due to the diversity of lithologies found there, as well as the presence of excellent natural cross-sections originated by marine erosion. In terms of tectonics, the studied area is dominated by accidents of distensive nature, organized in two out standing families: E-W and NW-SE. From these structures the most important are: a dyke swarm, very dense and with an approximate WNW-ESE direction; and a fracture pattern where we highlight the Seixal Machico (N50ºW, subvertical), Pedra Furada (E-W, 70ºN) and Estreito (N50ºW, 70ºN) faults. The analysis of those structures suggests a distensive tension field with minimum compression axis (s3) oriented with NNE-SSW direction, and maximum compression axis (s1) subvertical, related to the weight of the lytho static column (gravitical origin). This tension field suggests a hotspot rift model, similar to the one proposed by WALKER (1987), for shield volcanoes of the Hawaiian kind. This tension field seems to have a constant space-temporal orientation, with probable neotectonic activity. It is also proposed a tectonic control of vol canism, confirmed by the presence of aligned volcanic structures with the same directions previously described. It is equally proposed the existence of a structural control of the coast line and the landforms in the area.