High frequency sea level fluctuations recorded in the Black Sea since the LGM determined from sequence stratigraphy correlated to core analysis and dating
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Lericolais, Gilles۱; Bulois, Cedric۲; Gillet, Hervé۳; Guichard, François۴; Morigi, Caterina۵; Popescu, Irina۶; Minereau, Amelie۷; Popescu, Speranta Maria۸|
|Holding Date||11 October 2008|
The Black Sea is the largest anoxic basin in the world. During the LGM, it corresponded to a low salinity lake, disconnected and evolving regardless of the Mediterranean Sea. Its location implies that its water level depends on the Eurasian climatic fluctuations. During glacial periods, the Black Sea was isolated from the Global Ocean and was more sensitive to climate changes as the Caspian Sea which provides today a perfect paleoclimate archive. We present here results obtained from a European Project: ASSEMBLAGE (EVK3-CT-2002-00090) which provided information on the assessment of the North-western part of the Black Sea sedimentary systems, including the continental shelf and slope down to the deep sea zone. Here is presented a 3D geometric interpretation of VHR seismic Chirp profiles acquired on the Romanian shelf during this EU Project. The results provide a solid record of the Black Sea Last Glacial Maximum water level fluctuations. This pseudo-3D seismic interpretation shows that the Black Sea lacustrine shelf deposits form a significant basinward-prograding wedge system. On top of these prograding sequences is a set of sand dunes that delineates a wave cut-terrace like feature around the isobath -100 m. Landward of this dune field are small depressions containing barkhan-like bodies. The upper part of the last prograding sequence is incised by anastomosed channels which end in the Danube (Viteaz) canyon which are also built on the lacustrine prograding wedge. Ten stratigraphic units were correlated with analyses of cores retrieved from this area. The results demonstrate that the 1st 8 sequences represent lacustrine prograding wedges, the 9th sequence is the dune system itself and the 10th is a marine mud drape which covers the entire continental shelf. The lacustrine prograding wedges represent a lowstand deposit characterised by forced regression-like reflectors mapped from the pseudo-3D seismic data. Their hinge point corresponds to the wave erosion surface mapped around -100 m isobath on the multibeam mosaic. Dated cores give age control on this lowstand period, which lasted from 11 to 8.5 kyr 14C BP as implied by: (1) the continuously dry climatic conditions in the region inferred by high percentages of herbs and steppe elements determined from ASSEMBLAGE cores and, (2) the formation of dunes between 10 and 8.5 kyr 14C BP on the desiccated north-western Black Sea shelf at -100 m and (3) all of these covered by a marine mud drape confirming that the dune system is no longer active. The buried, anastomosed fluvial channels that suddenly disappear below 90 m depth, and a unique wave-cut terrace between 95 and 100 m bsl on the outer shelf are also consistent with a lowstand base-level at around -100 m. Preservation of sand dunes and the occurrence of buried incised valleys mark a rapid transgression within less than a century during which ravinement processes related to the water level rise had no time to significantly erode the seafloor.