MIDDLE/LATE HOLOCENE ALLUVIATION OF SW PORTUGUESE LAGOONS –CAUSES, SEDIMENTS AND RATES

摘要

The investigation of the Middle and Latte Holocene sediment infill of proximal andrndistal depocenters related with SW Portuguese coastal lagoons indicates widespreadrnalluviation that progressed at different rates. The sedimentological, geochemical andrnpaleoecological study of the sediments allowed the recognition of two overlapping deposits,rndifferent in nature and rates of accumulation and separated by an erosive surface. The earliestrnis more organic and sedimented at lower rates in fresh to brackish water environments. Thernsubsequent is essentially minerogenic and bears fluvial facies; it contributed to reduce thernflooded surface and depth of the main basins and tributaries particularly in the last fewrncenturies, when sedimentation rates significantly increased.rnThe regional character of these changes suggests that it corresponds to a threshold ofrnmajor significance that must be related with a marked increase in availability and yield ofrnterrigenous sediment from catchments in the Latte Holocene. The causal mechanismsrndetermining this change are pointed and whilst recognizing the importance of climaticrnfluctuations an anthropogenic cause is favoured and discussed.