A greater comprehension of the recent geological past, and particularly of the paleogeographic evolution, is crucial in coastal areas affected by sea level rise phenomena, in order to evaluate future scenarios and elaborate intervention strategies. To this aim, sedimentological, geochemical and micropaleontological data from sediment cores in NW Adriatic Sea were obtained to reconstruct stratigraphic framework and paleogeographic setting during the last post-glacial sea-level rise (14,000 - 6,000 yr BP). Within this record, two marine-transitional lithostratigraphic units were identified: A) a coastal lagoon system and B) a barrier-lagoon system, which is dated back around 10,000 yr BP. These deposits represent the first phases of drowning of the distal Po plain occurring after the marine ingression due to the Late-Glacial sea level rise (<15,000 yr BP). During deposition of unit A, fine grain-size and paleontological assemblage suggest sedimentation occurred in low-energy marine environments such as very shallow protected coastal lagoon. The presence of intervals with rare, and sometimes reworked marine microfauna and geochemical indicators of sub-aerial exposure (limonite, pyrite and CaO depletion) in the study cores suggests that the lagoon could have been intermittently dried up. Metal leaching occurred during aerobic sub-aerial conditions and was followed by subsequent precipitation as metal sulphide in submerged anaerobic conditions. At that stage of paleoenvironmental evolution, a definite coastline was lacking, and normal saline areas graded landwards into large low energy brackish or marshy coastal lagoons. Under the influence of rising sea level, tidal currents penetrated into the former valleys and coastal erosion progressively affected higher parts of the submerging landscape. Generally, the unit A is separated from the overlying unit B by a sharp erosional surface. The unit B is interpreted to reflect a rapid process of landward migration of the back-barrier system induced by a transgressive pulse during the Holocene transgression. The transgression is represented by the erosional surface at the base of this unit that has the characteristic of a ravinement surface. Sandy-silt coarsening upward intervals, with marine macrofauna remains overlying the peat layers, were interpreted as bay or distributary's mouth fill, which occurred during transgression. In this phase, wave reworking of sediments, due to relative sea-level increase, led to the removal of fine fraction and mixing of biological assemblages, as can be inferred from the presence of both marine and euryhaline molluscs. Marine flooding led to the formation of wash-over levels. A brackish, marsh environment interconnected to the sea was formed, where terrestrial organic matter began to accumulate forming peat levels. Considering the dating results, the whole Unit is included in the barrier-lagoon-estuary system, which is well preserved between 25 and 45 m water depth in the northern Adriatic Sea. Finally, a stronger relative sea level rise determined erosion and accumulation of the overlaying transgressive sand sheet corresponding to the present "relict sands" of the middle Adriatic basin.
A coastal lagoon and barrier-lagoon system in NW Adriatic Sea (Italy) dating back 10.000 years ago
Alvisi F;
2009
Abstract
A greater comprehension of the recent geological past, and particularly of the paleogeographic evolution, is crucial in coastal areas affected by sea level rise phenomena, in order to evaluate future scenarios and elaborate intervention strategies. To this aim, sedimentological, geochemical and micropaleontological data from sediment cores in NW Adriatic Sea were obtained to reconstruct stratigraphic framework and paleogeographic setting during the last post-glacial sea-level rise (14,000 - 6,000 yr BP). Within this record, two marine-transitional lithostratigraphic units were identified: A) a coastal lagoon system and B) a barrier-lagoon system, which is dated back around 10,000 yr BP. These deposits represent the first phases of drowning of the distal Po plain occurring after the marine ingression due to the Late-Glacial sea level rise (<15,000 yr BP). During deposition of unit A, fine grain-size and paleontological assemblage suggest sedimentation occurred in low-energy marine environments such as very shallow protected coastal lagoon. The presence of intervals with rare, and sometimes reworked marine microfauna and geochemical indicators of sub-aerial exposure (limonite, pyrite and CaO depletion) in the study cores suggests that the lagoon could have been intermittently dried up. Metal leaching occurred during aerobic sub-aerial conditions and was followed by subsequent precipitation as metal sulphide in submerged anaerobic conditions. At that stage of paleoenvironmental evolution, a definite coastline was lacking, and normal saline areas graded landwards into large low energy brackish or marshy coastal lagoons. Under the influence of rising sea level, tidal currents penetrated into the former valleys and coastal erosion progressively affected higher parts of the submerging landscape. Generally, the unit A is separated from the overlying unit B by a sharp erosional surface. The unit B is interpreted to reflect a rapid process of landward migration of the back-barrier system induced by a transgressive pulse during the Holocene transgression. The transgression is represented by the erosional surface at the base of this unit that has the characteristic of a ravinement surface. Sandy-silt coarsening upward intervals, with marine macrofauna remains overlying the peat layers, were interpreted as bay or distributary's mouth fill, which occurred during transgression. In this phase, wave reworking of sediments, due to relative sea-level increase, led to the removal of fine fraction and mixing of biological assemblages, as can be inferred from the presence of both marine and euryhaline molluscs. Marine flooding led to the formation of wash-over levels. A brackish, marsh environment interconnected to the sea was formed, where terrestrial organic matter began to accumulate forming peat levels. Considering the dating results, the whole Unit is included in the barrier-lagoon-estuary system, which is well preserved between 25 and 45 m water depth in the northern Adriatic Sea. Finally, a stronger relative sea level rise determined erosion and accumulation of the overlaying transgressive sand sheet corresponding to the present "relict sands" of the middle Adriatic basin.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
