Natural Hazards and Earth System Sciences Recent high-energy marine events in the sediments of Lagoa de ´Obidos and Martinhal (Portugal): recognition, age and likely causes (original) (raw)
Related papers
2015
This study addresses the Holocene sedimentary succession at Martinhal coastal lowland (Algarve), a site recording the AD1755 Lisbon tsunami. Data on twenty-four short cores and sixteen trenches, retrieved from both the literature and new field work were combined to improve the stratigraphic resolution and lateral correlation of sedimentary units accumulated in this lowland. Four fining-upward depositional sequences (S1 to S4) were identified, showing spatial variation with maximum thickness - S1: 0-3 m; S2: 1.0-2.4 m; S3: 0.2-1.0 m; S4: 0.3-2.3 m. Sequences S1, S2 and S3 consist of medium to coarse yellow sands with levels rich in bioclasts that give place upsequence to brown organic silt and clay. Sequences S1 to S3 testify to the progressive infilling of a small estuary, coeval with the progressive development and changes in efficiency of a sandy barrier. The uppermost sequence (S4) is clearly different and also shows contrast in facies at seaward and landward locations. S4 compri...
Natural Hazards and Earth System Science, 2012
A key issue in coastal hazards research is the need to distinguish sediments deposited by past extreme storms from those of past tsunamis. This study contributes to this aim by investigating patterns of sedimentation associated with extreme coastal flood events, in particular, within the Lagoa deÓbidos (Portugal). The recent stratigraphy of this coastal lagoon was studied using a wide range of techniques including visual description, grain-size analysis, digital and x-ray photography, magnetic susceptibility and geochemical analysis. The sequence was dated by 14 C, 210 Pb and Optically Stimulated Luminescence. Results disclose a distinctive coarser sedimentary unit, within the top of the sequence studied, and shown in quartz sand by the enrichment of elements with marine affinity (e.g., Ca and Na) and carbonates. The unit fines upwards and inland, thins inland and presents a sharp erosive basal contact. A noticeable post-event change in the sedimentary pattern was observed. The likely agent of sedimentation is discussed here and the conceivable association with the Great Lisbon tsunami of AD 1755 is debated, while a comparison is attempted with a possibly synchronous deposit from a tsunami in Martinhal (Algarve, Portugal). The possibility of a storm origin is also discussed in the context of the storminess of the western Portuguese coast and the North Atlantic Oscillation. This study highlights certain characteristics of the sedimentology of the deposits that may have a value in the recognition of extreme marine inundation signatures elsewhere in the world.
Geosciences, 2020
The CE 1755 Lisbon tsunami was the largest historical tsunami to affect the Atlantic coasts of Europe and North Africa. This study presents the results obtained from the application of different sedimentological techniques (e.g., grain size, morphoscopy, microtextural analysis, geochemistry, radiocarbon dating) on sediments retrieved from the Alcabrichel River alluvial plain (of about 500 m far away from its mouth and approximatively 50 km northwest of Lisbon, Portugal). The results allowed the identification of a sandy layer that was associated with the CE 1755 tsunami. Furthermore, a new microtextural semi-quantitative classification was applied to enhance the identification of extreme marine inundation deposits. Based on sedimentological data, three different tsunami inundation phases were identified, including two inundations and a likely backwash. This innovative work offers physical evidence of the spatial presence of the CE 1755 tsunami event on the western coast of Europe. I...
This study addresses the Holocene sedimentary succession at Martinhal coastal lowland (Algarve), a site recording the AD1755 Lisbon tsunami. Data on twenty-four short cores and sixteen trenches, retrieved from both the literature and new field work were combined to improve the stratigraphic resolution and lateral correlation of sedimentary units accumulated in this lowland. Four fining-upward depositional sequences (S1 to S4) were identified, showing spatial variation with maximum thickness - S1: 0-3 m; S2: 1.0-2.4 m; S3: 0.2-1.0 m; S4: 0.3-2.3 m. Sequences S1, S2 and S3 consist of medium to coarse yellow sands with levels rich in bioclasts that give place upsequence to brown organic silt and clay. Sequences S1 to S3 testify to the progressive infilling of a small estuary, coeval with the progressive development and changes in efficiency of a sandy barrier. The uppermost sequence (S4) is clearly different and also shows contrast in facies at seaward and landward locations. S4 comprises: 1) seaward (ESE), a basal subunit with boulders to pebbly gravels and coarse to medium well sorted sands, interpreted as deposited by the AD1755 tsunami, covered by an upper subunit comprising an alternation of well sorted white sands and brown silts, interpreted as aeolian/washover and flood deposits; 2) upstream (WNW), alternating layers of poorly sorted very coarse to medium sands and brown silts represent flood plain accretion and channel infill within the lowland. Quartz OSL dating was done by measuring small aliquots in samples from the AD1755 event-layer and large aliquots in samples from other deposits. The coarser and more basal sediment of the AD1755 sediment yielded overestimated OSL ages due to partial bleaching; at the top, and in finer sediment, and also in the more landward locations, OSL ages are closer to the true age of this event. At this location, routine Quartz OSL dating using large aliquots can give useful data on the burial ages for a variety of sediments, provided that the burial time exceeds ca. 1 ky. In contrast, recent abrupt-event sediments should be dated by measuring small aliquots or single grains to get a better control on partial bleaching effects on OSL results.
Distinguishing tsunami and storm deposits: An example from Martinhal, SW Portugal
Sedimentary Geology, 2007
Geological identification of past tsunamis is important for risk assessment studies, especially in areas where the historical record is limited or absent. The main problem when using the geological evidence is to distinguish between tsunami and storm deposits. Both are high-energy events that may leave marine traces in coastal stratigraphic sequences. At Martinhal, SW Portugal both storm surge and tsunami deposits are present at the same site within a single stratigraphic sequence, which makes it suitable to study the differences between them, excluding variations caused by local factors. The tsunami associated with the Lisbon earthquake of November 1st 1755 AD, had a major impact on the geomorphology and sedimentology of Martinhal. It breached the barrier and laid down an extensive sheet of sand, as described in eyewitness reports. Besides the tsunami deposit the stratigraphy of Martinhal also displays evidence for storm surges that have breached and overtopped the barrier, flooding the lowland and leaving sand layers. Both marine-derived flood deposits show similar grain size characteristics and distinctive marine foraminifera. The most important differences are the rip-up clasts and boulders exclusively found in the tsunami deposit and the landward extent of the tsunami deposit that everywhere exceeds that of the storm deposits. Identification of both depositional units was only possible using a collection of different data and extensive stratigraphical information from cores as well as trenches.
Marine Geology, 2008
One of the aims of the SEDPORT project (ESF Euromargins EUROCORES program) was to investigate the depositional features on the shelf and upper slope of the Portuguese Margin — Tagus System, during the Holocene. To fulfill the objectives, two shallow box-cores and two sedimentary sequences were studied: PO287-26B and the spliced sequence of sites D13902, PO287-26G recovered on the Portuguese Continental Shelf SSW off the Tagus River mouth, and the box PO287-28B and gravity-core GeoB8903 collected towards the W of the river mouth.Magnetic susceptibility, grain-size and XRF-Fe data as well as 210Pb and AMS 14C dating of these sedimentary sequences have allowed the identification of an “instantaneous deposit” at about 20 cm in the box-cores. Downcore we could establish: a hiatus corresponding to 355 yr of sedimentation at both sites; a 39 cm “instantaneous deposit” of coarse material including carbonate broken shells on the SSW site; and a 1.5 m “instantaneous deposit” of fine material on the W site. Both hiatus and instantaneous deposits are considered to be of tsunamigenic origin. The box-cores' deposit has an estimated age of 1969 AD and it is attributed to the 1969 earthquake caused tsunami. The hiatus and deposits found downcore are, due to the age limits encountered, believed to represent the tsunami related to the 1755 Lisbon major earthquake. These data further indicate a sediment deposit similar in type and thickness on both SSW and W sites in 1969, while the 1755 AD record indicates a higher energy backwash along the SSW and a preferential deposition of fine suspended matter towards the W.
Minerals
Outer shelf sedimentary records are promising for determining the recurrence intervals of tsunamis. However, compared to onshore deposits, offshore deposits are more difficult to access, and so far, studies of outer shelf tsunami deposits are scarce. Here, an example of studying these deposits is presented to infer implications for tsunami-related signatures in similar environments and potentially contribute to pre-historic tsunami event detections. A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE tsunami in two cores, located in the southern Portuguese continental shelf at water depths of 58 and 91 m. Age models based on 14C and 210Pbxs allowed a probable correspondence with the 1755 CE tsunami event. A multi-proxy approach, including sand composition, grain-size, inorganic geochemistry, magnetic susceptibility, and microtextural features on quartz grain surfaces, yielded evidence for a tsunami depositional signature, although only a...
2nd tsunami field symposium, GI2 …, 2008
A compilation of historical testimonies of tsunami flooding of the Portuguese Algarve coast was combined with a geomorphological characterization of that coast to produce a data base on locations prone to inundation by such high energy events. Field work has been undertaken in the lowland areas identified by this methodology between Sagres and Lagos in order to characterize the correspondent geological evidences. Results indicate that some locations vulnerable to inundation failed to preserve any sedimentological trace of extreme marine inundation. However, the completeness of the data base on these events significantly increases when the historical record is added by geological information. The sedimentary signature of tsunami inundation of the studied coastal section is varied and it may have been severely damaged by the intense anthropogenic activities that characterize the Algarve coast.