RECOGNIZING POSSIBLE TSUNAMI SEDIMENTS IN THE ULTRADISSIPATIVEENVIRONMENT OF THE TAGUS ESTUARY (PORTUGAL)

摘要

Large tsunamis may significantly disturb the permanent sedimentation regimes of coastalrnareas and landward tapering units of sand extending far inland constitute the commonestrnsignature of tsunamigenic flooding in intermediate to slightly reflective coastal environments,rnincluding sections of the Portuguese littoral fringe. These morphodynamic contexts providernaccommodation space for alien sediment that bears distinct ecological and textural signaturesrnand thus may be readily distinguished from the modal background deposits. In contrast,rndissipative contexts may effectively attenuate the incoming waves, reducing their impacts torntransient modification of the vectorial pattern of current fields and to significant addition ofrnmarine water, without remarkable change of the energy levels. In this case, sediment carriedrnand deposited by the tsunamigenic event will most probably be locally sourced and thernassociated disturbance will preferably leave a signature related with changes in compositionrnof sediment rather than in texture and with physical and chemical properties of sea water,rntranslated to geochemical proxies of salinity or unusual plankton contributions to the totalrnfossil content. This hypothesis is discussed in this study based on the geological record ofrnpaleotsunamis in the dissipative context of the Tagus estuary. Results, supported byrnsedimentological, geochemical, magnetic susceptibility and paleoecological data, suggest thatrnat least two tsunamigenic events are recorded in this area (tentatively dated A.D. 1531 andrn1755), their signature preferably associating with short-lived episodes of anomalousrndispersion paths of intra-estuarine sediment and increased marine influence, in the absence ofrnrelevant textural contrast.