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Title: Luminescence analyses as a novel proxy for tsunamigenic stratigraphies
Authors: López Cadavid, Gloria I.
Costa, Pedro J. M.
Andrade, César Freire de
Issue Date: 2019
Publisher: International Union for Quaternary Research
Citation: 20th Congress of the International Union for Quaternary Research (INQUA), 2019, P-3062
Abstract: Optically Stimulated Luminescence (OSL), now a well established chronological method, provides a direct age estimate of the last exposure of commonly occurring mineral grains to daylight. The uniqueness of this trapped charged technique is its intrinsic relation to transport mechanisms and depositional processes forming a sedimentary deposit. The extraordinary sensitivity of this signal to environmental conditions allows to differentiate degrees of sedimentary chaos: sediments incompletely bleached at burial or transported by highly turbulent regimes, such as tsunami or storms, thus the applicability of the OSL signal as a sedimentary proxy. Recent work along Southern Portugal has revealed a complex coastal stratigraphic sequence evidencing multiple potential palaeotsunami and palaeostorm events. This rugged coast of the Central Algarve has witness severe shoreline erosion over the last 300 years, nonetheless small embayments such as the infilled coastal lagoon of Almargem have demonstrated to be good event entrapment basins, allowing preservation of complex sedimentary facies. A series of large trenches dug along two N-S cross-shore profiles were excavated with their long axis both parallel and perpendicular to shore to potentialise the geometries of the different sedimentary units and structures, increasing visualization and stratigraphic correlation. The trench walls with most significant visual stratigraphic differences were sampled throughout their entire profiles. Individual small to micro (10 to 1 g) bulk samples were collected from different laminae for Portable OSL Reader (POSL) analyses, small 3 to 5 cm-diameter PVC tubes were inserted in key layers for OSL dating and Single Grain Over-Dispersion (SG-OD) OSL analyses, and multiple box-cores were retrieved throughout the trench walls for all the various other analyses (detailed sedimentology, mineralogy, petrophysics, bio-stratigraphy). So far, our research has revealed at least one massive bioclastic-rich layer and multiple heavy mineral laminae associated to extreme marine incursions. Diverse luminescence analyses were undertaken not only to establish chronologies of such events (OSL dating) but also as sedimentological characterisers: a) transport and depositional variations between individual storm-laid and tsunamigenic layers are being identified by means of SG-OD analyses; while b) luminescence profiles created from the measurement of small to micro bulk samples (e.g., individual lamina) using POSL have been useful to interpret changes in stratigraphic patterns, defining the continuity of deposition. The luminescence analyses on the multiple storm and tsunamigenic layers are detecting the variations imposed by their mechanism of formation, concurring with the other analyses done so far. Despite its massive character, the bioclastic-rich layer is most likely linked to the 1755 Lisbon Earthquake and Tsunami Event while the multiple heavy mineral laminae are most probably the result of successive storm washover events. The presence of numerous layers of different origins in such a small enclosed area makes this a challenging tsunami vs storm study site.
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Type: Presentation
Appears in Collections:Congresos, encuentros científicos y estancias de investigación

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